3. • Mostly cause long-term effects
– like growth, change in behavior, etc.
• do not catalyze any reactions
• stimulate or inhibit the target organs.
4. General characteristics
• a chemical messenger
• transports a signal from one cell to another.
• only a little amount of hormone is required
5. • All multicellular organisms produce hormones
• plant hormones: phytohormones.
• Hormones: often transported in the blood.
6. • Cells respond to a hormone when they
express a specific receptor for that hormone.
• The hormone binds to the receptor protein,
resulting in the activation of a signal
• that ultimately leads to cell type-specific
7. • Endocrine hormone: released directly into the
• paracrine signalling: simply diffuse through
the interstitial spaces to nearby target tissues
8. • A variety of exogenous chemical compounds,
both natural and synthetic, have hormone-like
effects on both humans and wildlife.
• Their interference with the synthesis, secretion,
transport, binding, action, or elimination of
natural hormones in the body can change the
homeostasis, reproduction, development, and/or
behavior, just as endogenously produced
• secreted by endocrine cells,
• chemical signal,
• circulates in bodily fluids,
• regulates behavior of other cells.
10. Hormonal signaling
• Biosynthesis: in a particular tissue
• Storage and secretion
• Transport: to the target cell(s)
• Recognition: by an associated cell membrane
or intracellular receptor protein
11. • Relay and amplification of the received hormonal
signal via a signal transduction process:
• This then leads to a cellular response.
• The reaction of the target cells may then be recognized
by the original hormone-producing cells, leading to a
down-regulation in hormone production. (This is an
example of a homeostatic negative feedback loop).
• Degradation of the hormone.
14. local hormones
• limited distribution
• stay in interstitial fluids
• or very localized blood vessels.
15. • paracrine
• Endocrine and target cells are near each other
• endocrine cells has receptors for its own
hormone self regulatory
17. Interactions with receptor
• initiate a cellular response : combining with
either a specific intracellular or cell membrane
associated receptor protein.
– 1. different receptors: on a cell
• recognize the same hormone:
• activate different signal transduction pathways
– 2. different receptors: on a cell
• recognize different hormones:
• activate the same biochemical pathway.
19. • Receptor location
• embedded in the plasma membrane at the
surface of the cell.
• intracellular receptors located in the
cytoplasm or nucleus.
20. The interaction of hormone and
• triggers a cascade of secondary effects within the
cytoplasm of the cell
– phosphorylation or dephosphorylation of cytoplasmic
– ion channel permeability,
– concentrations of intracellular molecules that may act
as secondary messengers
21. For hormones such as steroid or
• Receptors: intracellular.
• 1. cross the cell membrane.
• They can do so because they are lipid-soluble.
• 2. Combine with receptor
• 3. moves into the nucleus
• 4. binds to specific DNA sequences
• 5. amplify or suppress the action of certain genes
• Receptor for Some steroid hormones are associated
with the plasma membrane.
22. Strength of the signal
• Depends on the effective concentration of hormone-
receptor complexes that in turn depends on three
• 1. The number of hormone molecules available for
• 2. The number of receptor molecules available for
• 3. The binding affinity between hormone and receptor.
23. effects of hormonal action
• Changes in
• ion channel
• membrane potential
• behavior of enzymes
• cell metabolism
• genetic expression
• Concentration of a chemical
24. • The rate of hormone biosynthesis and secretion:
regulated by negative feedback control
• higher hormone concentration alone cannot
trigger the negative feedback mechanism.
• Negative feedback must be triggered by
overproduction of an "effect" of the hormone.
25. Hormone secretion can be stimulated
and inhibited by
• Other hormones
• Plasma concentrations of ions or nutrients
• binding globulins
• Neurons and mental activity
• Environmental changes, e.g., of light or
26. tropic hormones
• stimulate the hormone production of other
– For example, thyroid-stimulating hormone (TSH)
causes growth and increased activity of another
endocrine gland, the thyroid, which increases
output of thyroid hormones.
27. • To release active hormones quickly into the
circulation, hormone biosynthetic cells may
produce and store biologically inactive
hormones in the form of pre- or
• These can then be quickly converted into their
active hormone form in response to a
pre- or prohormones.
• apoptosis (programmed cell death)
• immune system
• preparation of the body for
• preparation of the body for a new phase of life,
• reproductive cycle
• hunger cravings
• sexual arousal
• other hormones
29. Classes of vertebrate hormones
• Peptide hormones: chains of amino acids.
– Small peptide
• TRH and vasopressin.
• insulin and growth hormone.
– Glycoprotein hormones.
• Luteinizing hormone, follicle-stimulating hormone and thyroid-stimulating hormone
• Lipid and phospholipid-derived hormones: derived from lipids
– steroid hormones that derive from cholesterol
• testosterone and cortisol, calcitriol
• Monoamines: from aromatic amino acids like phenylalanine, tyrosine,
tryptophan by the action of aromatic amino acid decarboxylase enzymes.
35. Steroid Hormones
• Steroid hormones: produced in the adrenal
cortex, testis, ovary, and some peripheral tissues
(adipose tissue, the brain!)
• All steroid hormones share a typical (but not
identical) ring structure.
36. Steroid hormones
All steroid hormones are derived from
cholesterol and differ only in the ring structure
and side chains attached to it.
All steroid hormones are lipid soluble
37. Types of steroid hormones
• Glucocorticoids; cortisol is the major
representative in most mammals
• Mineralocorticoids; aldosterone being most
• Androgens such as testosterone
• Estrogens, including estradiol and estrone
• Progestogens (also known a progestins) such
38. Steroid hormones
• Are not packaged, but synthesized and immediately
• Are all derived from the same parent compound:
• Enzymes which produce steroid hormones from
cholesterol are located in mitochondria and smooth
• Steroids are lipid soluble and thus are freely
permeable to membranes so are not stored in cells
39. Steroid hormones
• Steroid hormones are not water soluble so have to
be carried in the blood complexed to specific binding
• Corticosteroid binding globulin carries cortisol
• Sex steroid binding globulin carries testosterone and
• In some cases a steroid is secreted by one cell and is
converted to the active steroid by the target cell: an
example is androgen which secreted by the gonad
and converted into estrogen in the brain
40. Steroid Hormones
• Steroid hormones are nonpolar (no net charge), and
can thus diffuse across lipid membranes (such as the
plasma membrane). They leave cells shortly after
Polar substances are water soluble (dissolve in water),
nonpolar substances are lipid soluble.
41. Functions of Steroid Hormones
• Steroid hormones play important roles in:
- carbohydrate regulation (glucocorticoids)
- mineral balance (mineralocorticoids)
- reproductive functions (gonadal steroids)
• Steroids also play roles in inflammatory
responses, stress responses, bone metabolism,
cardiovascular fitness, behavior, cognition, and
42. How does the synthesis of steroids differ
from that of peptide hormones?
• While peptide hormones are encoded by specific genes, steroid
hormones are synthesized from the enzymatic modification of
• Thus, there is no gene which encodes aldosterone, for example.
• As a result:
- There are far fewer different types of steroid
hormones than peptide hormones.
- Steroid structures are the same from species to species
- The regulation of steroidogenesis involves control of the
enzymes which modify cholesterol into the steroid hormone of
43. The Role of Cholesterol in Steroid Synthesis
• The first enzymatic step in the production of ANY
steroid hormone begins with enzymatic
modification of cholesterol
44. Sources of Cholesterol for Steroid Synthesis
• Cholesterol can be made within the cell from acetyl CoA
(de novo synthesis).
• This is a multistep process, involving many enzymatic
• A key rate-limiting enzyme is HMG-CoA reductase ( 3-
• There is negative feedback regulation of HMG-CoA
reductase activity by cholesterol, so that high
intracellular cholesterol inhibits de novo synthesis.
acetyl CoA HMG-CoA mevalonate cholesterol
45. Sources of Cholesterol for Steroid Synthesis
• Cholesterol is also taken up by the cell in the form of
low density lipoprotein (LDL).
- LDL is a complex composed of cholesterol,
phospholipids, triglycerides, and proteins (proteins
and phospholipids make LDL soluble in blood).
- LDL is taken into cells via LDL receptors, and broken
down into esterified cholesterol, and then free
LDL esterified cholesterol free cholesterol
46. • The amount of free cholesterol in the cell is
maintained relatively constant:
Source of Cholesterol for Steroid Synthesis
esterified cholesterol level
47. Cellular Localization of Cholesterol
Metabolism for Steroid Production
• The first enzymatic step in steroid synthesis is the
conversion of cholesterol into pregnenolone.
• The enzyme that catalyzes this reaction is located
in the inner mitochondrial membrane.
48. Steroidogenic Enzymes
Common name "Old" name Current name
Side-chain cleavage enzyme;
3 beta-HSD 3 beta-HSD
17 alpha-hydroxylase/17,20 lyase P450C17 CYP17
21-hydroxylase P450C21 CYP21A2
11 beta-hydroxylase P450C11 CYP11B1
Aldosterone synthase P450C11AS CYP11B2
Aromatase P450aro CYP19
• peptide hormone,
• produced by beta cells of the pancreas,
• regulate carbohydrate and fat metabolism in the body.
• causes cells to take up glucose from the blood
– the liver,
– skeletal muscles, and
– fat tissue.
54. In the liver and skeletal muscles
• glucose is stored as
56. • Insulin inhibits the release of glucagon
– stops the use of fat as an energy source
• insulin is provided within the body in a
constant proportion to remove excess glucose
from the blood, which otherwise would be
57. • When blood glucose levels fall below a certain
level, the body begins to use stored sugar as
an energy source through glycogenolysis,
• which breaks down the glycogen stored in the
liver and muscles into glucose, which can then
be utilized as an energy source.
58. • When control of insulin levels fails, diabetes
mellitus can result.
• As a consequence, insulin is used medically to
treat some forms of diabetes mellitus.
59. • The human insulin protein is composed of 51
amino acids, and has a molecular weight of
• It is a dimer of an A-chain and a B-chain,
which are linked together by disulfide bonds.
• Several regulatory sequences in the promoter
region of the human insulin gene bind to
• There are also silencers that inhibit
61. • Insulin is produced and stored in the body as a
hexamer (a unit of six insulin molecules),
• while the active form is the monomer.
• The hexamer is an inactive form with long-
term stability, which serves as a way to keep
the highly reactive insulin protected, yet
• Insulin is produced in the pancreas and
released when any of several stimuli are
• These stimuli include ingested protein and
glucose in the blood produced from digested
63. • glucose is absorbed into the bloodstream
• blood glucose level will begin to rise.
• In target cells, insulin initiates a signal
transduction, which has the effect of increasing
glucose uptake and storage.
• Finally, insulin is degraded, terminating the
64. • Synthesized: pancreas: β-cells of the islets of
• Initially as preproinsulin
• After 5–10 min: endoplasmic reticulum, to
• to the trans-Golgi network: immature granules
• A and B chain
– linked together by disulfide bonds
– and by a C-peptide bridge,
• undergoes maturation into active insulin
– through the action of cellular endopeptidases.
66. In β-cells, insulin is synthesized from the proinsulin precursor
molecule by the action of
• proteolytic enzymes,
– known as prohormone convertases (PC1 and PC2),
– as well as the exoprotease carboxypeptidase E.
• Beta cells release insulin in two phases.
– rapidly release in response to increased blood
– sustained, slow release of newly formed vesicles
triggered independently of sugar.
• a peptide hormone
• secreted by the pancreas,
• raises blood glucose levels.
• Its effect is opposite that of insulin,
69. • releases when blood sugar levels fall too low.
• causes the liver to convert stored glycogen into glucose,
– High blood glucose levels stimulate the release of insulin.
• Insulin allows glucose to be taken up and used by insulin-
– glucagon and insulin are part of a feedback system that keeps
blood glucose levels at a stable level.
• Synthesized: alpha cells (α-cells) of the islets
• Human islet structure is much less segregated,
and alpha cells are distributed throughout the
74. • Glucose: in liver: glycogen,
• Liver cells (hepatocytes): glucagon receptors.
• glucagon binds to receptors
• liver cells convert glycogen into glucose
• release them into the bloodstream,
Glycogenolysis through glucagon
75. When glycogen stores become
• glucagon then encourages the liver and kidney to
synthesize additional glucose by gluconeogenesis.
• Glucagon turns off glycolysis in the liver,
• Glucagon also regulates the rate of glucose production
• Glucagon production appears to be dependent on the
central nervous system through pathways yet to be
• Glucagon binds to the glucagon receptor,
– a G protein-coupled receptor,
– located in the plasma membrane.
• The conformation change in the receptor
activates G proteins,
– a heterotrimeric protein with α, β, and γ subunits.
77. • When the G protein interacts with the
• it undergoes a conformational change that
results in the replacement of the GDP
molecule that was bound to the α subunit
with a GTP molecule
78. • This substitution results in the releasing of the
α subunit from the β and γ subunits.
• The alpha subunit specifically activates the
next enzyme in the cascade, adenylate
79. Adenylate cyclase manufactures cyclic adenosine
monophosphate (cyclic AMP or cAMP)
• which activates protein kinase A (cAMP-dependent protein
• This enzyme, in turn, activates phosphorylase kinase,
• which, in turn, phosphorylates glycogen phosphorylase,
converting into the active form called phosphorylase A.
• Phosphorylase A is the enzyme responsible for the release
of glucose-1-phosphate from glycogen polymers.
• 29-amino acid polypeptide.
• in humans:
• molecular weight of 3485 daltons.
• generated from the cleavage of proglucagon
• pancreatic islet α cells.
82. Uses and effects
• An injectable form of glucagon is vital first aid in
cases of severe hypoglycemia
• when the victim is unconscious or for other
reasons cannot take glucose orally.
• The dose for an adult is typically 1 milligram, and
the glucagon is given by intramuscular,
intravenous or subcutaneous injection, and
quickly raises blood glucose levels.
83. • Glucagon acts very quickly;
• common side-effects include headache and
• also known as growth hormone-inhibiting hormone
• peptide hormone
• regulates the endocrine system
• affects neurotransmission and
• Cell proliferation
• via interaction with G-protein-coupled somatostatin
85. Somatostatin has two active forms
• produced by alternative cleavage of a single
– one of 14 amino acids,
– the other of 28 amino acids
86. somatostatin to possess a large range
of functions because
• In all vertebrates, there exists six different
• SS1, SS2, SS3, SS4, SS5, and SS6.
• five different somatostatin receptors
• Humans have only one somatostatin gene, SST
• in several locations in the digestive system:
• delta cells of the pancreas
88. • produced by neuroendocrine neurons of the periventricular nucleus
of the hypothalamus.
• These neurons project to the median eminence,
• where somatostatin is released from neurosecretory nerve endings
• into the hypothalamo-hypophysial system through neuron axons.
• Somatostatin is then carried to the anterior pituitary gland, where it
inhibits the secretion of growth hormone from somatotrope cells.
89. • The somatostatin neurons in the periventricular
nucleus mediate negative feedback effects of
growth hormone on its own release;
• the somatostatin neurons respond to high
circulating concentrations of growth hormone
and somatomedins by increasing the release of
somatostatin, so reducing the rate of secretion of
• It is an inhibitory hormone
• Its actions are spread to different parts of the
91. Anterior pituitary
• Inhibit the release of growth hormone
• Inhibit the release of thyroid-stimulating
• Inhibit adenylyl cyclase in parietal cells.
92. – suppresses the release of gastrointestinal hormones
• Vasoactive intestinal peptide
• Gastric inhibitory polypeptide
• Decrease rate of gastric emptying, and reduces smooth
muscle contractions and blood flow within the intestine
• Suppresses the release of pancreatic hormones
• Inhibits insulin release
• Inhibits the release of glucagon
93. Growth hormone
• peptide hormone
– cell reproduction
– regeneration in humans and other animals.
• 191-amino acid, single-chain polypeptide that is
synthesized, stored, and secreted by somatotropic cells
within the lateral wings of the anterior pituitary gland.
• use in raising livestock more efficiently in industrial
• for increasing milk production in dairy cows.
94. Gene locus
– Growth hormone 1
– growth hormone 2
• localized in the q22-24 region of chromosome
• 191 amino
• molecular weight of 22,124 daltons.
• four helices necessary for functional interaction
with the GH receptor.
• Several molecular isoforms of GH exist in the
pituitary gland and are released to blood.
96. Biological regulation
• regulated by the neurosecretory nuclei of the hypothalamus.
• These cells release the peptides
– Growth hormone-releasing hormone (GHRH or somatocrinin)
– Growth hormone-inhibiting hormone (GHIH or somatostatin) into the
hypophyseal portal venous blood surrounding the pituitary.
• GH release is primarily determined by the balance of these two
• which in turn is affected by many physiological stimulators (e.g.,
exercise, nutrition, sleep) and inhibitors (e.g., free fatty acids) of GH
97. • Somatotropic cells in the anterior pituitary
gland then synthesize and secrete GH in a
pulsatile manner, in response to these stimuli
by the hypothalamus.
100. • sex hormones
– increased androgen secretion during puberty
• deep sleep
• niacin as nicotinic acid (Vitamin B3)
• vigorous exercise
101. Inhibitors of GH secretion include:
• GHIH (somatostatin)
• circulating concentrations of GH
• anabolic (building up)
• Increased height during childhood
• Increases calcium retention,
• strengthens and increases the mineralization of bone
• Increases muscle mass through sarcomere hyperplasia
• Promotes lipolysis
• Increases protein synthesis
103. • Stimulates the growth of all internal organs excluding the brain
• Plays a role in homeostasis
• Reduces liver uptake of glucose
• Promotes gluconeogenesis in the liver
• Contributes to the maintenance and function of pancreatic islets
• Stimulates the immune system
104. Problems caused when the body
produces too much GH
• pituitary tumor
• thickens the bones of the jaw, fingers and toes.
• increased size of digits is referred to as acromegaly.
• sweating, pressure on nerves, muscle weakness, excess
sex hormone-binding globulin (SHBG), insulin
resistance or even a rare form of type 2 diabetes, and
reduced sexual function.
105. Problems caused when the body
produces too little GH
• growth failure and short stature
• delayed sexual maturity.
• pituitary adenoma,
• fat mass
• decrease in muscle mass
• a neurotransmitter
• functions in the body,
– regulating heart rate,
– blood vessel and air passage diameters,
– metabolic shifts;
– epinephrine release is a crucial component of the
fight-or-flight response of the sympathetic nervous
108. Mechanism action
• acts on nearly all body tissues.
• Its actions vary by tissue type and tissue
expression of adrenergic receptors.
– For example, high levels of adrenaline causes
smooth muscle relaxation in the airways but
causes contraction of the smooth muscle that
lines most arterioles.
• binds to a variety of adrenergic receptors.
• nonselective agonist of all adrenergic receptors
• Epinephrine's binding to these receptors triggers a number of metabolic
– Binding to α-adrenergic receptors
• inhibits insulin secretion by the pancreas,
• stimulates glycogenolysis in the liver and muscle
• stimulates glycolysis in muscle
– β-Adrenergic receptor binding
• triggers glucagon secretion in the pancreas,
• increased adrenocorticotropic hormone (ACTH) secretion by the pituitary gland,
• increased lipolysis by adipose tissue.
• Together, these effects lead to
– increased blood glucose
– fatty acids
• cardiac arrest,
• superficial bleeding
• administration of adrenaline may
• raise or lower blood pressure
111. • Asthma
• In local anesthetics: as a vasoconstrictor to
slow the absorption and, therefore, prolong
the action of the anesthetic agent.
113. Biosynthesis regulation
• in the medulla of the adrenal gland in an
• Tyrosine oxidized to ------L-DOPA
decarboxylated to -----dopamine Oxidize to----
--norepinephrine methylated to ------
115. • Adrenocorticotropic hormone (ACTH) and the
sympathetic nervous system stimulate the
synthesis of adrenaline precursors by
enhancing the activity of tyrosine hydroxylase
and dopamine-β-hydroxylase, two key
enzymes involved in catecholamine synthesis.
117. As a stress hormone
• affects parts of the brain,
– amygdala (attention and responses are controlled)
• fight-or-flight response,
– increasing heart rate,
– triggering the release of glucose from stores
– increasing blood flow to skeletal muscle.
– It increases the brain's oxygen supply
118. as a drug,
– increases blood pressure by increasing vascular
tone (tension of vascular smooth muscle)
• nor- shows that it is the next-lower homolog
– Epinephrine: methyl group attached
– Norepinephrine: methyl group replaced by a
121. Role in Decision Making
• Cortical norepinephrine (NE) release during
attention patterns and thereby enhance
• Decision making
• Norepinephrine is synthesized from tyrosine
– as a precursor,
• releases into the synaptic cleft,
• acts on adrenergic receptors,
• followed by the signal termination,
– either by degradation of norepinephrine or
– by uptake by surrounding cells.
• Norepinephrine is synthesized by a series of
• in the adrenal medulla
• in postganglionic neurons of the sympathetic nervous
– From: tyrosine.
124. • performs its actions on the target cell
– by binding to and activating adrenergic receptors.
• The target cell expression of different types of
receptors determines the ultimate cellular
– and thus norepinephrine has different actions on
different cell types.
• In mammals, The principal metabolites are:
• 3,4-Dihydroxymandelic acid
• Vanillylmandelic acid
• 3-Methoxy-4-hydroxyphenylethylene glycol, "MHPG" or
• Norepinephrine is used
– as a vasopressor medication
– for patients with critical hypotension.
• It is given
– intravenously and
• acts on both
– α1 and α2 adrenergic receptors
– to cause vasoconstriction.
• effects are limited to the
– increasing of blood
– meat, nuts, and eggs.
– Dairy products such as
• body can synthesise tyrosine from phenylalanine
• Tyrosine is the precursor to dopamine,
– which in turn is a precursor to epinephrine and
129. The thyroid hormones
• Triiodothyronine (T3)
• Thyroxine (T4),
– Tyrosine-based hormones
– produced by: thyroid gland
– responsible for: regulation of metabolism
130. • Iodine is necessary for the production of T3 and T4.
• selenium is essential for T3 production.
• A deficiency of iodine leads to decreased production of T3 and T4,
– enlarges the thyroid tissue
– disease known as goitre.
• The major form of thyroid hormone in the blood is thyroxine (T4),
which has a longer half-life than T3.
• The ratio of T4 to T3 released into the blood is roughly 20 to 1.
• T4 is converted to the active T3 (three to four times more potent
• Both T3 and T4 are used to treat thyroid hormone
• They are both absorbed well by the gut, so can be
• Levothyroxine is the pharmaceutical name of
physiological thyroxine (T4),
• which is metabolised more slowly than T3 and hence
usually only needs once-daily administration.
• Mostly bound to transport proteins in blood.
• fraction is free (unbound) and biologically active,
– hence measuring concentrations of free thyroid
hormones is of great diagnostic value.
• Bound: not active,
• so the amount of free T3/T4 is important.
133. Bound and unbound
bound to thyroxine-binding globulin (TBG) 70%
bound to transthyretin or "thyroxine-binding
prealbumin" (TTR or TBPA)
unbound T4 (fT4) 0.03%
unbound T3 (fT3) 0.3%
134. Membrane transport
• Despite being lipophilic,
– T3 and T4 cross the cell membrane via carrier-
mediated transport, which is ATP-dependent.
• The thyroid hormones function via a well-
studied set of nuclear receptors in the nucleus
of the cell, the thyroid hormone receptors.
• act on nearly every cell in the body.
– increase the basal metabolic rate,
– affect protein synthesis,
– regulate long bone growth
– neuronal maturation,
– increase the body's sensitivity to catecholamines
137. • essential to proper development and
differentiation of all cells of the human body.
• These hormones also
– regulate protein, fat, and carbohydrate metabolism.
• They also stimulate vitamin metabolism.
• Thyroid hormone leads to heat generation in
138. Related diseases
• Both excess and deficiency of thyroxine can
– Hyperthyroidism: excess of circulating free
thyroxine, free triiodothyronine, or both.
– Hypothyroidism: deficiency of thyroxine,
triiodiothyronine, or both.
• Clinical depression can sometimes be caused by
• Thyroxine and triiodothyronine can be
– as free thyroxine and free triiodothyronine
• They can also be measured
– as total thyroxine and total triiodothyronine,
141. The steps in this process are as follows
• The Na+/I- symporter transports two sodium ions
across the basement membrane of the follicular
cells along with an iodine ion.
• I- is moved into the colloid of the follicle.
• Thyroperoxidase oxidises two I- to form I2.
• The thyroperoxidase iodinates the tyrosyl
residues of the thyroglobulin within the colloid.
142. • The thyroglobulin is synthesised in the ER of the follicular cell and secreted
into the colloid.
• Thyroid-stimulating hormone (TSH) released from the pituitary gland
binds the TSH receptor ( a Gs protein-coupled receptor) on the basolateral
membrane of the cell and stimulates the endocytosis of the colloid.
• The endocytosed vesicles fuse with the lysosomes of the follicular cell.
• The lysosomal enzymes cleave the T4 from the iodinated thyroglobulin.
• These vesicles are then exocytosed, releasing the thyroid hormones.
143. • Thyroxine is produced by attaching iodine
atoms to the ring structures of tyrosine
• Thyroxine (T4) contains four iodine atoms.
• Triiodothyronine (T3) is identical to T4, but it
has one less iodine atom per molecule.
144. • iodide trapping:
– Iodide is actively absorbed from the bloodstream
• iodine is bound to tyrosine residues in the thyroglobulin
– monoiodotyrosine (MIT)
– diiodotyrosine (DIT).
– MIT + DIT → T3
– DIT + DIT → T4
145. Effect of iodine deficiency on thyroid
• the thyroid cannot make thyroid hormone.
– leads to decreased negative feedback on the
• leading to increased production of thyroid-stimulating
• which causes the thyroid to enlarge (goiter)endemic
146. Effects of thyroxine
• Increases cardiac output
• Increases heart rate
• Increases ventilation rate
• Increases basal metabolic rate
• Potentiates the effects of catecholamines (i.e. increases
• Potentiates brain development
• Thickens endometrium in females
• increase metabolism of proteins and carbohydrates
147. A steroid hormone
• Classification on the basis of the receptors type to
which they bind:
• Vitamin D derivatives are a sixth closely related
hormone system with homologous receptors, though
they are technically sterols rather than steroids.
149. • They bind to steroid hormone receptors to bring about changes within the cell.
Receptors may be
– or cytosolic
• generally carried in the blood
– bound to specific carrier proteins
• such as sex hormone-binding globulin
• or corticosteroid-binding globulin.
• Further conversions and catabolism occurs
– in the liver,
– in other "peripheral" tissues,
– and in the target tissues.
150. Some examples of synthetic steroid
• Glucocorticoids: prednisone, dexamethasone,
• Mineralocorticoid: fludrocortisone
• Vitamin D: dihydrotachysterol
• Androgens: oxandrolone, testosterone,
nandrolone (also known as anabolic steroids)
• Oestrogens: diethylstilbestrol (DES)
• Progestins: norethindrone, medroxyprogesterone
• Steroids exert a wide variety of effects mediated by
slow genomic as well as by rapid nongenomic
• hey bind to nuclear receptors in the cell nucleus for
• Membrane-associated steroid receptors activate
intracellular signaling cascades involved in nongenomic
152. • Because steroids and sterols are lipid-soluble,
they can diffuse fairly freely from the blood
through the cell membrane and into the
cytoplasm of target cells.
• In the cytoplasm, the steroid may or may not
undergo an enzyme-mediated alteration such
as reduction, hydroxylation, or aromatization.
153. • In the cytoplasm, the steroid binds to the specific receptor, a large
• Upon steroid binding, many kinds of steroid receptor dimerize:
– Two receptor subunits join together to form one functional DNA-
binding unit that can enter the cell nucleus.
• In some of the hormone systems known, the receptor is associated
with a heat shock protein, which is released on the binding of the
ligand, the hormone.
• Once in the nucleus, the steroid-receptor ligand complex binds to
specific DNA sequences and induces transcription of its target
• six distinct classes
– all catalyzing the same reaction
– but representing unrelated gene families
with no known sequence or structural homology.
156. • The best known AC: class III
– occurs widely in eukaryotes and has important roles in many human
• All classes of AC catalyze the conversion
– ATP ----------3',5'-cyclic AMP (cAMP) + pyrophosphate.
• Divalent cations (usually Mg) are generally required
• The cAMP produced by AC then serves as a regulatory signal
– via specific cAMP-binding proteins, either transcription factors or
other enzymes (e.g., cAMP-dependent kinases).
157. Class I AC: the first class of AC to be
• large cytosolic enzymes
• Occurrence: many bacteria including E. coli.
• When deprived of glucose E. coli produce cAMP
– that serves as an internal signal
– to activate expression of genes
– for importing and metabolizing other sugars.
• cAMP exerts this effect by binding the transcription factor CRP, also
known as CAP.
158. Class II AC
• Nature: toxins
• secreted by: pathogenic bacteria
– Bacillus anthracis and Bordetella pertussis during
• These bacteria also secrete proteins that
enable the AC-II to enter host cells
159. Class III AC
• in some bacteria,
– Mycobacterium tuberculosis: key role in
• Most AC-III's are integral membrane proteins
• transduce extracellular signals into
160. in human liver
• adrenaline indirectly stimulates AC
• to mobilize stored energy in the "fight or flight"
• The effect of adrenaline is via a G protein
• which transmits chemical signals from outside
the cell across the membrane to the inside of the
161. • The outside signal i.e. adrenaline
• binds to a receptor,
– which transmits a signal to the G protein,
• G protein transmits a signal to adenylate cyclase,
• Adenylate cyclase transmits a signal
– by converting ATP to cAMP
162. cAMP: so-called second messenger
• Adenylate cyclases are often activated or
inhibited by G proteins,
• which are coupled to membrane receptors and
thus can respond to hormonal or other stimuli.
Following activation of adenylate cyclase, the
resulting cAMP acts as a second messenger by
interacting with and regulating other proteins
such as protein kinase A and cyclic nucleotide-
gated ion channels.
163. • Most class III adenylyl cyclases are
• transmembrane proteins
• with 12 transmembrane segments.
164. Types of AC-III
• There are ten known isoforms of adenylate cyclases in
165. Class IV
• first reported in: Aeromonas hydrophila,
• smallest of the AC enzyme classes;
• the AC-IV from Yersinia is a dimer of 19 kDa
subunits with no known regulatory
166. Class V and IV
• reported in:
– Prevotella ruminicola
– Rhizobium etti
– have not been extensively characterized.
169. cyclic nucleotide phosphodiesterases
• a group of enzymes
• that degrade the phosphodiester bond
– in cAMP and cGMP.
• They regulate
– of cyclic nucleotide signaling within subcellular domains.
• PDEs are therefore important regulators of signal transduction
170. Phosphodiesterase enzymes are often
targets for pharmacological inhibition
• due to their unique
– tissue distribution,
– structural properties,
– functional properties
171. PDE inhibitors
• new potential therapeutics in areas such as
– pulmonary arterial hypertension,
– coronary heart disease,
172. Cilostazol (Pletal)
• inhibits PDE3.
• This inhibition allows red blood cells to be
more able to bend.
• This is useful in conditions such as
intermittent claudication, as the cells can
maneuver through constricted veins and
arteries more easily.
174. A prostaglandin
• lipid compounds
• derived from fatty acids
• contains 20 carbon atoms, including a 5-
175. • They are mediators
• strong physiological effects,
– such as regulating the contraction and relaxation of
smooth muscle tissue.
• Nature: autocrine or paracrine,
• Production: in many places throughout the
• There are currently ten known prostaglandin receptors on various cell types.
• The diversity of receptors means that prostaglandins act on an array of cells and have a wide variety
of effects such as:
• cause constriction or dilation in vascular smooth muscle cells
• cause aggregation or disaggregation of platelets
• sensitize spinal neurons to pain
• induce labor
• decrease intraocular pressure
• regulate inflammatory mediation
177. • regulate calcium movement
• control hormone regulation
• control cell growth
• acts on thermoregulatory center of hypothalamus to produce fever
• acts on mesangial cells in the glomerulus of the kidney to increase
glomerular filtration rate
• acts on parietal cells in the stomach wall to inhibit acid secretion
• lipids known as eicosanoids.
• The two major thromboxanes
– thromboxane A2
– thromboxane B2.
Thromboxane is named for its role in clot formation
• Thromboxane is a vasoconstrictor and a
potent hypertensive agent, and it facilitates
• It is in homeostatic balance in the circulatory
system with prostacyclin, a related compound.
The mechanism of secretion of thromboxanes
from platelets is still unclear.
182. Role of A2 in platelet aggregation
• Thromboxane A2 (TXA2), produced by activated
platelets, has prothrombotic properties,
stimulating activation of new platelets as well as
increasing platelet aggregation.
• Platelet aggregation is achieved by mediating
expression of the glycoprotein complex GP IIb/IIIa
in the cell membrane of platelets. Circulating
fibrinogen binds these receptors on adjacent
platelets, further strengthening the clot.
• Thromboxane inhibitors are broadly classified as either those that inhibit
the synthesis of thromboxane, or those that inhibit the target effect of it.
• Thromboxane synthesis inhibitors, in turn, can be classified regarding
which step in the synthesis they inhibit:
• The widely used drug aspirin acts by inhibiting the ability of the COX
enzyme to synthesize the precursors of thromboxane within platelets.
Low-dose, long-term aspirin use irreversibly blocks the formation of
thromboxane A2 in platelets, producing an inhibitory effect on platelet
aggregation. This anticoagulant property makes aspirin useful for reducing
the incidence of heart attacks. 40 mg of aspirin a day is able to inhibit a
large proportion of maximum thromboxane A2 release provoked acutely,
with the prostaglandin I2 synthesis being little affected; however, higher
doses of aspirin are required to attain further inhibition.
184. • Thromboxane synthase inhibitors inhibit the final
enzyme (thromboxane synthase) in the synthesis of
thromboxane. Ifetroban is a potent and selective
thromboxane receptor antagonist. Dipyridamole
antagonizes this receptor too, but has various other
mechanisms of antiplatelet activity as well.
• The inhibitors of the target effects of thromboxane are
the thromboxane receptor antagonist, including
• Picotamide has activity both as a thromboxane
synthase inhibitor and as a thromboxane receptor
186. Signal transduction
• Ligand + cell-surface receptors change in
the conformation of the receptor(receptor
activation)trigger events inside the cell
187. Environmental stimuli
• Sensing of environments at the cellular level
relies on signal transduction
• many disease processes, such as diabetes and
heart disease arise from defects in these
pathways, highlighting the importance of this
process in biology and medicine.
188. Examples of env stimuli
• photons hitting cells in the retina of the eye
• odorants binding to odorant receptors in the
• Certain microbial molecules, such as viral
nucleotides and protein antigens, can elicit an
immune system response against invading
pathogens mediated by signal transduction
192. • ligand + receptorchange in conformation of
inside part of the receptoractivation of an
enzyme in the receptor or the exposure of a
binding site for other intracellular signaling
proteins within the cell
• eventually propagating the signal through the
193. G protein-coupled receptors (GPCRs)
• integral transmembrane proteins
• possess seven transmembrane domains
• linked to a heterotrimeric G protein.
• GPCR = Protein+G protein(Gα, Gβ, and Gγ)
– adrenergic receptors
– chemokine receptors.
194. Signal transduction by a GPCR
• G prottein(Gα, Gβ, and Gγ)<> protein + Ligand
receptor conformation changes Gα binds to GTP
• In result sites on the subunits exposes that can interact
with other molecules.
• The activated G protein subunits detach from the
• initiate signaling from many downstream effector
– such as phospholipases and ion channels
196. To perform signal transduction
• RTKs form dimers in the plasma membrane
• dimer is stabilized by ligands binding to the receptor.
• causing conformational changes.
• The receptors' kinase domains are subsequently activated,
• initiating phosphorylation signaling cascades of downstream
• that facilitate various cellular processes such as cell differentiation
• produced by a variety of cells
• play a role in cell attachment to other cells
and the extracellular matrix
• transduction of signals from extracellular
matrix components such as fibronectin and
198. • Integrins lack kinase activity;
• hence integrin-mediated signal transduction is
achieved through a variety of
• intracellular protein kinases and adaptor
199. differences between integrin-signalling in
circulating blood cells and non-circulating cells
• integrins of circulating cells: inactive.
– For example, cell membrane integrins on circulating leukocytes
are maintained in an inactive state to avoid epithelial cell
– they are only activated in response to stimuli such as those
received at the site of an inflammatory response.
• In a similar manner, integrins at the cell membrane of
circulating platelets are normally kept inactive to avoid
200. • Epithelial cells (which are non-circulating):
• helping maintain their stable adhesion to
underlying stromal cells that provide signals to
maintain normal functioning.
201. Ligand-gated ion channel
• A ligand-gated ion channel, upon binding with
– changes conformation
– to open a channel in the cell membrane
– through which ions relaying signals can pass.
• nuclear receptors
• cytoplasmic receptors,
• The typical ligands for nuclear receptors are
– like the steroid hormones testosterone and
progesterone and derivatives of vitamins A and D.
203. • To initiate signal transduction,
– the ligand pass through the plasma membrane by
• binds with the receptor,
• pass through the nuclear membrane into the
• enables gene transcription and protein
204. • Activated nuclear receptors attach to the DNA
• Where: at receptor-specific hormone-
responsive element (HRE) sequences,
• Location of HRE: in the promoter region of the
genes activated by the hormone-receptor
205. • All hormones that act by regulation of gene
expression have two consequences in their
mechanism of action
• effects are produced after a long period of
• effects persist for another long period of time
206. Nucleic receptors
• DNA-binding domains (containing zinc fingers)
• + ligand-binding domain
• zinc fingers stabilize DNA binding
• by holding its phosphate backbone.
• DNA sequences that match the receptor are
usually hexameric repeats of any kind
• sequences are similar but their orientation and
distance differentiate them.
207. Steroid receptors
• subclass of nuclear receptors
• located primarily within the cytosol
• heatshock proteins (HSPs) are necessary to
activate the receptor
• by assisting the protein to fold in a way such
that the signal sequence enabling its passage
into the nucleus is accessible.
208. Retinoic acid receptors
• subset of nuclear receptors.
• activated by an endocrine-synthesized ligand
• The ligand is synthesised from a precursor like retinol
• Retinol is brought to the cell through the bloodstream or a
completely intracellularly synthesised ligand like prostaglandin.
• These receptors are located in the nucleus
• They are not accompanied by HSPs
• They repress their gene by binding to their specific DNA sequence
209. First messengers
• intracellular chemical messengers
– paracrine/autocrine agents
– They reach the cell from the extracellular fluid and
bind to their specific receptors.
210. Second messengers
• act within the cell to trigger a response.
• Second messengers essentially serve as
chemical relays from the plasma membrane to
• they carry out intracellular signal
211. Lipophilic second messenger
• derived from lipids
• resides in cellular membranes
• enzymes stimulated by activated receptors
activate the lipids by modifying them.
• Examples include
212. Nitric oxide
• second messenger
• it is a free radical
• can diffuse through the plasma membrane
• affect nearby cells
• synthesised from arginine and oxygen by the NO synthase
• works through activation of soluble guanylyl cyclase
• which when activated produces another second messenger, cGMP.
213. Redox Signaling
• In addition to nitric oxide, other electronically-
activated species are also signal-transducing
agents in a process called redox signaling.
• Examples include
– hydrogen peroxide
– carbon monoxide
– hydrogen sulfide
215. Gene activation
• leads to further expression of a large number
– increased uptake of glucose from the blood
– migration of neutrophils to sites of infection.
216. Major pathways
• MAPK/ERK pathway:
– couples intracellular responses to the binding of
growth factors to cell surface receptors.
– very complex and includes many protein
– activation of this pathway promotes cell division,
– many forms of cancer are associated with
aberrations in it.
217. phosphoinositide cascade
• A series of events that is initiated extracellularly
• activation of phospholipase C
• liberation of
– inositol 1,4,5-trisphosphate
• inositol 1,4,5-trisphosphate increase cytosolic calcium
• diacylglycerol activate protein kinase C respectively.
218. Second messengers
• relay signals from receptors on the cell surface to target molecules
inside the cell
• relay the signals of hormones, growth factors, and others,
• cause some kind of change in the activity of the cell.
• amplify the strength of the signal.
• Secondary messengers are a component of signal transduction
219. Discovery of SM
• Earl Wilbur Sutherland, Jr.,
• Nobel Prize in Physiology or Medicine: 1971.
• Sutherland saw that
– epinephrine stimulate the liver
• to convert glycogen to glucose
– but epinephrine alone would not convert glycogen to glucose.
• He found that epinephrine had to trigger a second
messenger, cyclic AMP, for the liver to convert glycogen to
220. Types of SM
• Hydrophobic molecules:
– water-insoluble molecules,
• Examples: diacylglycerol, and phosphatidylinositols,
• Hydrophilic molecules:
– water-soluble molecules,
• Examples: cAMP, cGMP, IP3, and Ca2+,
– located within the cytosol
• Examples: nitric oxide (NO), carbon monoxide (CO) and hydrogen
– can diffuse both through cytosol and across cellular membranes.
221. Common properties of SM
• can be synthesized/released and broken down
• can be stored in special organelles
• quickly release when needed.
• production/release and destruction can be
localized, enabling the cell to limit space and time
of signal activity.
• There are several different secondary messenger systems (cAMP
system, phosphoinositol system, and arachidonic acid system),
• all are quite similar in overall mechanism, though the substances
involved in those mechanisms and effects are different.
• neurotransmitter binds to a membrane-spanning receptor protein
• changes the receptor
• causes it to expose a binding site for a G-protein.
• G-protein (named for the GDP and GTP molecules that bind to it) is
bound to the inner membrane of the cell and consists of three
– alpha, beta and gamma.
223. • When the G-protein binds to the receptor, it becomes able
to exchange a GDP (guanosine diphosphate) molecule on
its alpha subunit for a GTP (guanosine triphosphate)
• alpha subunit of the G-protein transducer breaks free from
the beta and gamma subunits,
• all parts remaining membrane-bound.
• The alpha subunit, now free to move along the inner
membrane, eventually contacts another membrane-bound
protein - the "primary effector."
224. • It creates a signal that can diffuse within the
• This signal is called the "secondary
• The secondary messenger may then activate a
"secondary effector" whose effects depend on
the particular secondary messenger system.
225. Calcium ions
• responsible for many important physiological
– such as in muscle contraction, fertilization, and
• normally bound to intracellular components,
• Calcium regulates the protein calmodulin,
• CALcium-MODULated proteIN
• calcium-binding messenger protein
• expressed in all eukaryotic cells.
• transduces calcium signals by binding calcium ions and
then modifying its interactions with various target
• mediates crucial processes such as
– smooth muscle contraction,
– intracellular movement,
– short-term and long-term memory,
– immune response.
228. • CaM is expressed in many cell types and can
have different subcellular locations,
– within organelles,
– associated with the plasma or organelle
229. • Many of the proteins that CaM binds are unable to
bind calcium themselves, and use CaM as a calcium
sensor and signal transducer.
• CaM can also make use of the calcium stores in the
endoplasmic reticulum, and the sarcoplasmic
• CaM can undergo post-translational modifications,
such as phosphorylation, acetylation, methylation and
proteolytic cleavage, each of which has potential to
modulate its actions.
• highly conserved protein
• 148 amino acids long
• 16706 Daltons
• four motifs, each of which binds a Ca2+ ion.
• two approximately symmetrical globular
domains (the N- and C-domain), separated by
a flexible linker region
• After calcium binding, hydrophobic methyl groups from
methionine residues become exposed on the protein via
• This presents hydrophobic surfaces, which can in turn bind
to Basic Amphiphilic Helices (BAA helices) on the target
• These helices contain complementary hydrophobic regions.
• The flexibility of Calmodulin's hinged region allows the
molecule to "wrap around" its target. This property allows
it to tightly bind to a wide range of different target proteins.
233. The insulin receptor
• (IR) is a transmembrane receptor that is activated by insulin, IGF-I,
IGF-II and belongs to the large class of tyrosine kinase receptors.
Metabolically, the insulin receptor plays a key role in the regulation
of glucose homeostasis, a functional process that under degenerate
conditions may result in a range of clinical manifestations including
diabetes and cancer. Biochemically, the insulin receptor is
encoded by a single gene INSR, from which alternate splicing during
transcription results in either IR-A or IR-B isoforms. Downstream
post-translational events of either isoform result in the formation of
a proteolytically cleaved α and β subunit, which upon combination
are ultimately capable of homo or hetero-dimerisation to produce
the ≈320 kDa disulfide-linked transmembrane insulin receptor.
234. • Initially, transcription of alternative splice
variants derived from the INSR gene are
translated to form one of two monomeric
isomers; IR-A in which exon 11 is omitted, and
IR-B in which exon 11 is included. Inclusion of
exon 11 results in the addition of 12 amino
acids upstream of the inherit furin protyolytic
235. Colour coded schematic of the insulin
• Upon receptor dimerisation, after protyolytic cleavage into the α- and β-
chains, the additional 12 amino acids remain present at the C-terminus of
the α-chain (designated αCT) where they are predicted to influence
• Each isometric monomer is structurally organized into 8 distinct domains
consists of; a leucine-rich repeat domain (L1, residues 1-157), a cyctine
rich region (CR, residues 158-310), an additional leucine rich repeat
domain (L2, residues 311-470), three fibronectin type III domains; FnIII-1
(residues 471-595), FnIII-2 (residues 596-808) and FnIII-3 (residues 809-
906). Additionally, an insert domain (ID, residues 638-756) resides within
FnIII-2, containing the α/β furin cleavage site, from which proteolysis
results in both IDα and IDβ domains. Within the β-chain, downstream of
the FnIII-3 domain lies a transmembrane helix (TH) and intracellular
juxtamembrane (JM) region, just upstream of the intracellular tyrosine
kinase (TK) catalytic domain, responsible for subsequent intracellular
• Upon cleavage of the monomer to its respective α- and β-chains,
236. • receptor hetero or homo-dimerisation is
maintained covalently between chains by a single
disulphide link and between monomers in the
dimer by two disulphide links extending from
each α-chain. The overall 3D ectodomain
structure, possessing four ligand binding sites,
resembles an inverted ‘V’, with the each
monomer rotated approximately 2-fold about an
axis running parallel to the inverted 'V' and L2
and FnIII-1 domains from each monomer forming
the inverted 'V's apex.
237. Ligand binding
• The insulin receptors endogenous ligands include
insulin, IGF-I and IGF-II. The binding of ligand to the α-
chains of the IR ectodomain induces a structural
changes within the receptor leading to
autophosphorylation of various tyrosine residues
within the intracellular TK domain of the β-chain.
These changes facilitate the recruitment of specific
adapter proteins such as the insulin receptor substrate
proteins (IRS) in addition to Src, Src Homology 2 - B
(SH2-B), APS and proteins phosphatases including
PTP1B, eventually promoting those downstream
processes involving blood glucose homeostasis.
238. • Strictly speaking the relationship between IR and ligand shows complex allosteric properties. This was indicated with the use of a Scatchard plots
which identified that the measurement of the ratio of IR bound ligand to unbound ligand does not follow a linear relationship with respect to changes
in the concentration of IR bound ligand, suggesting that the IR and its respective ligand share a relationship of cooperative binding. Furthermore,
the observation that the rate of IR-ligand dissociation is accelerated upon addition of unbound ligand implies that the nature of this cooperation is
negative; said differently, that the initial binding of ligand to the IR inhibits further binding to its second active site - exhibition of allosteric
• Although as of yet the precise binding mechanism of IR and its lignad has not been elucidated structurally, as identified using a systems biology
approach, biologically relevant prediction of the IR-ligand kinetics (insulin/IGF-I) has been identified in the context of the currently available IR
• These models state that each IR monomer possesses 2 insulin binding sites; site 1, which binds to the 'classical' binding surface of insulin: consisting
of L1 plus αCT domains and site 2, consisting of loops at the junction of FnIII-1 and FnIII-2 predicted to bind to the 'novel' hexamer face binding site of
insulin. As each monomer contributing to the IR ectodomain exhibits 3D 'mirrored' complementarity, N-terminal site 1 of one monomer ultimately
faces C-terminal site 2 of the second monomer, where this is also true for each monomers mirrored complement (the opposite side of the
ectodomain structure). Current literature distinguishes the complement binding sites by designating the second monomer's site 1 and site 2
nomenclature as either site 3 and site 4 or as site 1' and site 2' respectively.
• As such, these models state that each IR may bind to an insulin molecule (which has two binding surfaces) via 4 locations, being site 1, 2, (3/1') or
(4/2'). As each site 1 proximally faces site 2, upon insulin binding to a specific site, 'crosslinking' via ligand between monomers is predicted to occur
(i.e. as [monomer 1 Site 1 - Insulin - monomer 2 Site (4/2')] or as [monomer 1 Site 2 - Insulin - monomer 2 site (3/1')]). In accordance with current
mathematical modelling of IR-insulin kinetics, there are two important consequences to the events of insulin crosslinking; 1. that by the
aforementioned observation of negative cooperation between IR and its ligand that subsequent binding of ligand to the IR is reduced and 2. that the
physical action of crosslinking brings the ectodomain into such a conformation that is required for intracellular tyrosine phosphorylation events to
ensue (i.e. these events serve as the requirements for receptor activation and eventual maintenance of blood glucose homeostasis).
239. Regulation of gene expression
• The activated IRS-1 acts as a secondary messenger within the cell to stimulate the transcription of
insulin-regulated genes. First, the protein Grb2 binds the P-Tyr residue of IRS-1 in its SH2 domain.
Grb2 is then able to bind SOS, which in turn catalyzes the replacement of bound GDP with GTP on
Ras, a G protein. This protein then begins a phosphorylation cascade, culminating in the activation
of mitogen-activated protein kinase (MAPK), which enters the nucleus and phosphorylates various
nuclear transcription factors (such as Elk1).
• Stimulation of glycogen synthesis
• Glycogen synthesis is also stimulated by the insulin receptor via IRS-1. In this case, it is the SH2
domain of PI-3 kinase (PI-3K) that binds the P-Tyr of IRS-1. Now activated, PI-3K can convert the
membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-
triphosphate (PIP3). This indirectly activates a protein kinase, PKB (Akt), via phosphorylation. PKB
then phosphorylates several target proteins, including glycogen synthase kinase 3 (GSK-3). GSK-3 is
responsible for phosphorylating (and thus deactivating) glycogen synthase. When GSK-3 is
phosphorylated, it is deactivated, and prevented from deactivating glycogen synthase. In this
roundabout manner, insulin increases glycogen synthesis.
240. Degradation of insulin
• Once an insulin molecule has docked onto the
receptor and effected its action, it may be
released back into the extracellular environment
or it may be degraded by the cell. Degradation
normally involves endocytosis of the insulin-
receptor complex followed by the action of
insulin degrading enzyme. Most insulin molecules
are degraded by liver cells. It has been estimated
that a typical insulin molecule is finally degraded
about 71 minutes after its initial release into
241. Defects in Signaling Pathways Can Lead
to Cancer and Other Diseases
• In light of their complexity, it comes as no surprise that signal-transduction pathways occasionally
fail, leading to pathological or disease states. Cancer, a set of diseases characterized by
uncontrolled or inappropriate cell growth, is strongly associated with defects in signal-transduction
proteins. Indeed, the study of cancer, particularly cancer caused by certain viruses, has contributed
greatly to our understanding of signal-transduction proteins and pathways.
• For example, Rous sarcoma virus is a retrovirus that causes sarcoma (a cancer of tissues of
mesodermal origin such as muscle or connective tissue) in chickens. In addition to the genes
necessary for viral replication, this virus carries a gene termed v-src. The v-src gene is an oncogene;
it leads to the transformation of susceptible cell types. The protein encoded by v-src is a protein
tyrosine kinase that includes SH2 and SH3 domains (Figure 15.35). Indeed, the names of these
domains derive from the fact that they are Src homology domains. The v-Src protein is similar in
amino acid sequence to a protein normally found in chicken muscle cells referred to as c-Src (for
cellular Src). The c-src gene does not induce cell transformation and is termed a proto-oncogene.
The protein that it encodes is a signal-transduction protein that regulates cell growth. As we shall
see, small differences in the amino acid sequences between the proteins encoded by the proto-
oncogene and the oncogene are responsible for the oncogene product being trapped in the “on”
242. • an examination of the structure of c-Src in an inactive conformation reveals an
intricate relation between the three major domains. The SH3 domain lies nearest
the amino terminus, followed by the SH2 domain and then the kinase domain.
There is also an extended carboxyl-terminal stretch that includes a
phosphotyrosine residue. The phosphotyrosine residue is bound within the SH2
domain, whereas the linker between the SH2 domain and the kinase domain is
bound by the SH3 domain. These interactions hold the kinase domain in an
inactive conformation. The Src protein in this form can be activated by three
distinct processes (Figure 15.36): (1) the phosphotyrosine residue bound in the
SH2 pocket can be displaced by another phosphotyrosine-containing polypeptide
with a higher affinity for this SH2 domain, (2) the phosphoryl group on the tyrosine
residue can be removed by a phosphatase, and (3) the linker can be displaced
from the SH3 domain by a polypeptide with a higher affinity for this SH3 domain.
Thus, Src responds to the presence of one of a set of distinct signals. The amino
acid sequence of the viral oncogene is more than 90% identical with its cellular
counterpart. Why does it have such a different biological activity? The C-terminal
19 amino acids of c-Src are replaced by a completely different stretch of 11 amino
acids, and this region lacks the key tyrosine residue that is phosphorylated in
inactive c-Src. Since the discovery of Src, many other mutated protein kinases have
been identified as oncogenes.
243. • How did the Rous sarcoma virus acquire the mutated version of src? In an infection, a viral genome may pick up a gene from its host in such a way
that the region encoding the last few amino acids is missing. Such a modified gene may have given the Rous sarcoma virus a selective advantage
because it will have favored viral growth when introduced with the virus into a host cell.
• Impaired GTPase activity in a regulatory protein also can lead to cancer. Indeed, ras is one of the genes most commonly mutated in human tumors.
Mammalian cells contain three 21-kd Ras proteins (H-, K-, and N-Ras) that cycle between GTP and GDP forms. The most common mutations in tumors
lead to a loss of the ability to hydrolyze GTP. Thus, the Ras protein is trapped in the “on” position and continues to stimulate cell growth.
• Go to:
• 15.5.1 Protein Kinase Inhibitors May Be Effective Anticancer Drugs
• The widespread occurrence of over active protein kinases in cancer cells suggests that molecules that inhibit these enzymes might act as antitumor
agents. Recent results have dramatically supported this concept. More than 90% of patients with chronic myologenous leukemia (CML) show a
specific chromosomal defect in affected cells (Figure 15.37). The translocation of genetic material between chromosomes 9 and 22 causes the c-abl
gene, which encodes a tyrosine kinase, to be inserted into the bcr gene on chromosome 22. The result is the production of a fusion protein called
Bcr-Abl that consists primarily of sequences for the c-Abl kinase. However, the bcr-abl gene is not regulated appropriately; it is expressed at higher
levels than the gene encoding the normal c-Abl kinase. In addition, the Bcr-Abl protein may have regulatory properties that are subtly different from
those of the c-Abl kinase itself. Thus, leukemia cells express a unique target for chemotherapy. Recent clinical trials of a specific inhibitor of the Bcr-
Abl kinase have shown dramatic results; more than 90% of patients responded well to the treatment. This approach to cancer chemotherapy is
fundamentally distinct from most approaches, which target cancer cells solely on the basis of their rapid growth, leading to side effects because
normal rapidly growing cells also are affected. Thus, our understanding of signal-transduction pathways is leading to conceptually new disease
244. • Monoclonal antibody drugs are a relatively new innovation in cancer treatment. While several
monoclonal antibody drugs are available for treating certain cancers, the best way to use these new
drugs isn't always clear.
• If you and your doctor are considering using a monoclonal antibody as part of your cancer
treatment, find out what to expect from this therapy. Together you and your doctor can decide
whether a monoclonal antibody treatment may be right for you.
• What is a monoclonal antibody?
• A monoclonal antibody is a laboratory-produced molecule that's carefully engineered to attach to
specific defects in your cancer cells. Monoclonal antibodies mimic the antibodies your body
naturally produces as part of your immune system's response to germs, vaccines and other
• How do monoclonal antibody drugs work?
• When a monoclonal antibody attaches to a cancer cell, it can:
245. • Make the cancer cell more visible to the immune system. The immune system attacks foreign
invaders in your body, but it doesn't always recognize cancer cells as enemies. A monoclonal
antibody can be directed to attach to certain parts of a cancer cell. In this way, the antibody marks
the cancer cell and makes it easier for the immune system to find.
• The monoclonal antibody drug rituximab (Rituxan) attaches to a specific protein (CD20) found only
on B cells, one type of white blood cell. Certain types of lymphomas arise from these same B cells.
When rituximab attaches to this protein on the B cells, it makes the cells more visible to the
immune system, which can then attack. Rituximab lowers the number of B cells, including your
healthy B cells, but your body produces new healthy B cells to replace these. The cancerous B cells
are less likely to recur.
• Block growth signals. Chemicals called growth factors attach to receptors on the surface of normal
cells and cancer cells, signaling the cells to grow. Certain cancer cells make extra copies of the
growth factor receptor. This makes them grow faster than the normal cells. Monoclonal antibodies
can block these receptors and prevent the growth signal from getting through.
• Cetuximab (Erbitux), a monoclonal antibody approved to treat colon cancer and head and neck
cancers, attaches to receptors on cancer cells that accept a certain growth signal (epidermal growth
factor). Cancer cells and some healthy cells rely on this signal to tell them to divide and multiply.
Blocking this signal from reaching its target on the cancer cells may slow or stop the cancer from
246. • Stop new blood vessels from forming. Cancer cells rely on blood vessels to bring them the oxygen and nutrients
they need to grow. To attract blood vessels, cancer cells send out growth signals. Monoclonal antibodies that block
these growth signals may help prevent a tumor from developing a blood supply, so that it remains small. Or in the
case of a tumor with an already-established network of blood vessels, blocking the growth signals could cause the
blood vessels to die and the tumor to shrink.
• The monoclonal antibody bevacizumab (Avastin) is approved to treat a number of cancers, not including breast
cancer. Bevacizumab targets a growth signal called vascular endothelial growth factor (VEGF) that cancer cells
send out to attract new blood vessels. Bevacizumab intercepts a tumor's VEGF signals and stops them from
connecting with their targets.
• Deliver radiation to cancer cells. By combining a radioactive particle with a monoclonal antibody, doctors can
deliver radiation directly to the cancer cells. This way, most of the surrounding healthy cells aren't damaged.
Radiation-linked monoclonal antibodies deliver a low level of radiation over a longer period of time, which
researchers believe is as effective as the more conventional high-dose external beam radiation.
• Ibritumomab (Zevalin), approved for non-Hodgkin's lymphoma, combines a monoclonal antibody with radioactive
particles. The ibritumomab monoclonal antibody attaches to receptors on cancerous blood cells and delivers the
• A number of monoclonal antibody drugs are available to treat various types of cancer. Clinical trials are studying
monoclonal antibody drugs in treating nearly every type of cancer.
247. How are monoclonal antibody drugs
used in cancer treatment?
• Monoclonal antibodies are administered through a vein (intravenously). How often
you undergo monoclonal antibody treatment depends on your cancer and what
drug you're receiving. Some monoclonal antibody drugs may be used in
combination with other treatments, such as chemotherapy and hormone therapy.
Others are administered alone.
• Monoclonal antibody drugs were initially used to treat advanced cancers that
hadn't responded to chemotherapy or cancers that had returned despite
treatment. However, because these treatments have proved to be effective,
certain monoclonal antibody treatments are being used earlier in the course of the
disease. For instance, rituximab can be used as an initial treatment in some types
of non-Hodgkin's lymphoma, and trastuzumab (Herceptin) is used in the treatment
of some forms of early breast cancer.
• Many of the monoclonal antibody therapies are still considered experimental. For
this reason, these treatments are usually reserved for advanced cancers that aren't
responding to standard, proven treatments.
248. • Monoclonal antibodies (mAb or moAb) are monospecific
antibodies that are the same because they are made by
identical immune cells that are all clones of a unique parent
cell, in contrast to polyclonal antibodies which are made
from several different immune cells. Monoclonal antibodies
have monovalent affinity, in that they bind to the same
• Given almost any substance, it is possible to produce
monoclonal antibodies that specifically bind to that
substance; they can then serve to detect or purify that
substance. This has become an important tool in
biochemistry, molecular biology and medicine. When used
as medications, the non-proprietary drug name ends in -
mab (see "Nomenclature of monoclonal antibodies").
249. Cancer treatment
• One possible treatment for cancer involves
monoclonal antibodies that bind only to cancer
cell-specific antigens and induce an
immunological response against the target cancer
cell. Such mAb could also be modified for delivery
of a toxin, radioisotope, cytokine or other active
conjugate; it is also possible to design bispecific
antibodies that can bind with their Fab regions
both to target antigen and to a conjugate or
effector cell. In fact, every intact antibody can
bind to cell receptors or other proteins with its Fc
250. • The illustration below shows all these
• MAbs approved by the FDA include
251. Autoimmune diseases
• Monoclonal antibodies used for autoimmune
diseases include infliximab and adalimumab,
which are effective in rheumatoid arthritis,
Crohn's disease and ulcerative Colitis by their
ability to bind to and inhibit TNF-α.
Basiliximab and daclizumab inhibit IL-2 on
activated T cells and thereby help prevent acute
rejection of kidney transplants. Omalizumab
inhibits human immunoglobulin E (IgE) and is
useful in moderate-to-severe allergic asthma.