Pharmacodynamics describes how drug molecules produce pharmacological responses by exerting chemical influences on cellular constituents like receptors, enzymes, transporters, and ion channels. There are four main types of receptors that drugs can bind to: ligand-gated ion channels, G protein-coupled receptors, kinase-linked receptors, and nuclear receptors. G protein-coupled receptors are the largest family and mediate many hormone and neurotransmitter effects via downstream signaling pathways involving cyclic AMP, calcium, and other intracellular messengers. Drug-receptor binding causes conformational changes that can activate or inhibit the receptor and downstream signaling cascades.

1. Pharmacodynamics

2. • One of the basic tenets of pharmacology is
that drug molecules must exert some chemical
influence on one or more constituents of cells
in order to produce a pharmacological
response.

3. PROTEIN TARGETS FOR DRUG BINDING
• receptors
• enzymes
• carrier molecules (transporters)
• ion channels.

4. DRUG RECEPTORS
• The term is most often used to describe the
target molecules through which soluble
physiological mediators-hormones,
neurotransmitters, inflammatory mediators, etc.-
produce their effects.
• 'Receptor' is sometimes used to denote any
target molecule with which a drug molecule (i.e.
a foreign compound rather than an endogenous
mediator) has to combine in order to elicit its
specific effect.

5. • The term receptor is used to describe various
cell surface molecules (such as T-cell
receptors, integrins, Toll receptors, etc.)
involved in the immunological response to
foreign proteins and the interaction of cells
with each other and with the extracellular
matrix.

6. • Various carrier proteins are often referred to
as receptors, such as the low-density
lipoprotein receptor that plays a key role in
lipid metabolism and the transferrin receptor
involved in iron absorption.

7. DRUG SPECIFICITY
• it must act selectively on particular cells and
tissues.
• In other words, it must show a high degree of
binding site specificity.
• Conversely, proteins that function as drug
targets generally show a high degree of ligand
specificity; they will recognise only ligands of a
certain precise type and ignore closely related
molecules.

8. • no drug acts with complete specificity.

9. DRUG-RECEPTOR INTERACTIONS

10. • Occupation of a receptor by a drug molecule
may or may not result in activation of the
receptor.
• Agonist:
• Antagonist:
• affinity :
– The tendency of a drug to bind to the receptors
– whereas the tendency for it, once bound, to
activate the receptor is denoted by its efficacy.

11. • Drugs of high potency will generally have a high
affinity for the receptors and thus occupy a
significant proportion of the receptors even at
low concentrations.
• Agonists will also possess high efficacy, whereas
antagonists will, in the simplest case, have zero
efficacy
• Drugs with intermediate levels of efficacy, such
that even when 100% of the receptors are
occupied the tissue response is submaximal, are
known as partial agonists

12. THE BINDING OF DRUGS TO RECEPTORS
• The binding of drugs to receptors can often be
measured directly by the use of drug molecules
labelled with one or more radioactive atoms
(usually 3H, 14C or 125I).
• The main requirements are that the radioactive
ligand (which may be an agonist or antagonist)
must bind with high affinity and specificity, and
that it can be labelled to a sufficient specific
radioactivity to enable minute amounts of
binding to be measured.

13. • The usual procedure is to incubate samples of
the tissue (or membrane fragments) with
various concentrations of radioactive drug
until equilibrium is reached.
• The tissue is then removed, or the membrane
fragments separated by filtration or
centrifugation, and dissolved in scintillation
fluid for measurement of its radioactive
content.

14. TYPES OF RECEPTOR
• Receptors elicit many different types of cellular effect.
• Some of them are very rapid, such as those involved in
synaptic transmission, operating within milliseconds,
whereas other receptor-mediated effects, such as
those produced by thyroid hormone or various steroid
hormones, occur over hours or days.
• There are also many examples of intermediate
timescales; catecholamines, for example, usually act in
a matter of seconds, whereas many peptides take
rather longer to produce their effects

15. 4 types of Receptors
• Ligand Gated ion channels- Type-1
• G- Protein Coupled receptors- Type-2
• Kinase linked and related receprots- Type-3
• Nuclear receptor- Type-4

16. Ligand Gated Ion Channels
• Also know as ionotropic receptors.
• These membrane proteins are similar
structure to other ion channels but
incorporating a ligand binding site in
extracellular domain.
• Molecular Structure
• It is assembled from four different types of
subunit, termed alfa, beta, gama and delta.

18. • The oligomeric structure possesses two
acetylcholine binding sites, each lying at the
interface between one of the two alfa
subunits and its neighbour.

19. Gating Mechanism
• Receptors of this type control the fastest
synaptic events in the nervous system

23. • G Protein Coupled Receptors (GPCR)

24. Type 2
• G-protein-coupled receptors (GPCRs).
• These are also known as metabotropic receptors
or 7-transmembrane-spanning (heptahelical)
receptors.
• They are membrane receptors that are coupled to
intracellular effector systems via a G-protein.
• They constitute the largest family,5 and include
receptors for many hormones and slow
transmitters, for example the muscarinic
acetylcholine receptor, adrenergic receptors and
chemokine receptors.

25. MOLECULAR STRUCTURE
• G-protein-coupled receptors consist of a single polypeptide
chain of up to 1100 residues.
• Their characteristic structure comprises seven
transmembrane α helices, similar to those of the ion channels
discussed above, with an extracellular N-terminal domain of
varying length, and an intracellular C-terminal domain.
• GPCRs are divided into three distinct families. There is
considerable sequence homology between the members of
one family, but none between different families.
• They share the same seven-helix (heptahelical) structure, but
differ in other respects, principally in the length of the
extracellular N terminus and the location of the agonist
binding domain.

26. G-protein-coupled receptor families

27. Gating Mechanism

29. TARGETS FOR G-PROTEINS
• adenylyl cyclase, the enzyme responsible for
cAMP formation
• phospholipase C, the enzyme responsible for
inositol phosphate and diacylglycerol (DAG)
formation
• ion channels, particularly calcium and potassium
channels
• Rho A/Rho kinase, a system that controls the
activity of many signalling pathways controlling
cell growth and proliferation, smooth muscle
contraction, etc.

30. The adenylyl cyclase/cAMP system
• cAMP (cyclic 3´,5´-adenosine monophosphate)
as an intracellular mediator
• cAMP is a nucleotide synthesised within the
cell from ATP by the action of a membrane-
bound enzyme, adenylyl cyclase.
• It is produced continuously and inactivated by
hydrolysis to 5´-AMP, by the action of a family
of enzymes known as phosphodiesterases
(PDEs).

31. • Many different drugs, hormones and
neurotransmitters act on GPCRs and produce
their effects by increasing or decreasing the
catalytic activity of adenylyl cyclase, thus
raising or lowering the concentration of cAMP
within the cell.
• There are several different molecular isoforms
of the enzyme, some of which respond
selectively to Gαs or Gαi

33. The phospholipase C/inositol
phosphate system
• The phosphoinositide system, an important intracellular
second messenger system, was first discovered in the 1950s
by Hokin and Hokin.
• They found that secretion was accompanied by increased
turnover of a minor class of membrane phospholipids
known as phosphoinositides (collectively known as Pis).
• Subsequently, Michell and Berridge found that many
hormones that produce an increase in free intracellular
Ca2+s concentration (which include, for example, muscarinic
agonists and α-adrenoceptor agonists acting on smooth
muscle and salivary glands, and vasopressin acting on liver
cells) also increase PI turnover.

34. • Subsequently, it was found that one particular member of
the PI family, namely phosphatidylinositol (4,5)
bisphosphate (PIP2), which has additional phosphate groups
attached to the inositol ring, plays a key role.
• PIP2 is the substrate for a membrane-bound enzyme,
phospholipase Cβ (PLCβ), which splits it into DAG and
inositol (1,4,5) trisphosphate (IP3), both of which function
as second messengers as discussed below.
• The activation of PLCβ by various agonists is mediated
through a G-protein (Gq). After cleavage of PIP2,, DAG being
phosphorylated to form phosphatidic acid (PA), while the
IP3 is dephosphorylated and then recoupled with PA to
form PIP2 once again.

35. • Inositol phosphates and intracellular calcium
• Inositol (1,4,5) trisphosphate is a water-soluble mediator
that is released into the cytosol and acts on a specific
receptor-the IP3 receptor-which is a ligand-gated calcium
channel present on the membrane of the endoplasmic
reticulum.
• The main role of IP3 is to control the release of Ca2+ from
intracellular stores.
• Because many drug and hormone effects involve
intracellular Ca2+, this pathway is particularly important.
• IP3 is converted inside the cell to the (1,3,4,5)
tetraphosphate, IP4, by a specific kinase. The exact role of
IP4 remains unclear, but there is evidence that it too is
involved in Ca2+ signalling.

37. Diacylglycerol and protein kinase C
• Diacylglycerol is produced as well as IP3 whenever receptor-
induced PI hydrolysis occurs.
• The main effect of DAG is to activate a membrane-bound
protein kinase, protein kinase C (PKC), which catalyses the
phosphorylation of a variety of intracellular proteins.
• DAG, unlike the inositol phosphates, is highly lipophilic and
remains within the membrane. It binds to a specific site on
the PKC molecule, which migrates from the cytosol to the
cell membrane in the presence of DAG, thereby becoming
activated.
• There are 10 different mammalian PKC subtypes, which
have distinct cellular distributions and phosphorylate
different proteins. Most are activated by DAG and raised
intracellular Ca2+, both of which are produced by activation
of GPCRs.

38. • One of the subtypes is activated by the lipid mediator
arachidonic acid generated by the action of phospholipase
A2 on membrane phospholipids, so PKC activation can also
occur with agonists that activate this enzyme.
• The various PKC isoforms, like the tyrosine kinases
discussed below act on many different functional proteins,
such as ion channels, receptors, enzymes (including other
kinases) and cytoskeletal proteins.
• Kinases in general play a central role in signal transduction,
and control many different aspects of cell function. The
DAG-PKC link provides a channel whereby GPCRs can
mobilise this army of control freaks.

39. Ion channels as targets for G-proteins
• G-protein-coupled receptors can control ion channel
function directly by mechanisms that do not involve second
messengers such as cAMP or inositol phosphates.
• Early examples came from studies on potassium channels.
In cardiac muscle, for example, mAChRs are known to
enhance K+ permeability (thus hyperpolarising the cells and
inhibiting electrical activity).
• Similar mechanisms operate in neurons, where many
inhibitory drugs such as opiate analgesics reduce
excitability by opening potassium channels.
• These actions are produced by direct interaction between
the βγ subunit of G0 and the channel, without the
involvement of second messengers

40. The Rho/Rho kinase system
• This recently discovered signal transduction
pathway is activated by certain GPCRs (and also
by non-GPCR mechanisms), which couple to G-
proteins of the G12/13 type.
• The free G-protein α subunit interacts with a
guanosine nucleotide exchange factor, which
facilitates GDP-GTP exchange at another GTPase,
Rho.
• Rho-GDP, the resting form, is inactive, but when
GDP-GTP exchange occurs, Rho is activated, and
in turn activates Rho kinase.

41. • Rho kinase phosphorylates many substrate
proteins and controls a wide variety of cellular
functions, including smooth muscle contraction
and proliferation, angiogenesis and synaptic
remodelling.
• By enhancing hypoxia-induced pulmonary artery
vasoconstriction, activation of Rho kinase is
thought to be important in the pathogenesis of
pulmonary hypertension. Specific Rho kinase
inhibitors are in development for a wide range of
clinical indications-an area to watch

42. • Family A is by far the largest, comprising most
monoamine, neuropeptide and chemokine
receptors.
• Family B includes receptors for some other
peptides, such as calcitonin and glucagon.
• Family C is the smallest, its main members
being the metabotropic glutamate and GABA
receptors (Ch. 33) and the Ca2+-sensing
receptors8

43. A G-Protein-Coupled Receptor
Or G Protein-linked Receptor
7 transmembrane domains

44. The disassembly of G-Protein
upon stimulation
Spontaneous deactivation is
very fast, in minutes.
However, with the help of
RGS (regulator of G protein
signaling, a GAP for a unit),
signals can be shut off even
faster

45. The Activation
cycle of G-
Protein

46. • GPCR Signaling: cAMP

47. The visualization of cAMP in nerve cells
GPCR->Gs->adenylyl cyclase->cAMP
Gi

48. cAMP cycle: GPCR->Gs->adenylyl
cyclase->cAMP
Cyclic AMP phosphodiesterase breaks down
cAMP to 5’-AMP

49. The function of cAMP
Targeting PKA (cyclic-AMP-dependent protein kinase A)

50. The Whole Signaling
Network related to
cAMP

51. Terminology: CRE(cyclic AMP response element);
CREB: CRE binding protein; CBP: CREB binding protein

52. • GPCR Signaling: Calcium

53. Three Types of Inositol phospholipids
PI, PI(4)P, PI(4,5)P2

54. Phospholipase C-b
(PLC-b) Produces
DAG
(diacylglycerol) and
IP3 (inositol 1,4,5-
trisphosphate (IP3))
Gq->PLC-b

55. Gq signaling pathways and Calcium

56. Fertilization of an egg by a sperm triggering an increase in
cytosolic Calcium
3 major types of calcium channels:
1. Voltage dependent Ca channels on plasma membrane
2. IP3-gated Ca release channels on ER membrane
3. Ryanodine receptor on ER membrane

57. Calcium uptake and deprivation
1. Na/Ca exchanger on plasma membrane, 2. Ca pump on ER membrane, 3. Ca binding
molecules, 4. Ca pump on Mitochondia

58. Calcium Frequency
encoding signaling
strength
Local Ca blips, sparks,
puffs, reflecting local
opening of individual
channels in ER, strong
local signal induces
global activity, the
elevated Calcium trigger
calcium deprivation
system

59. Targeting molecules for Calcium
Calcium binding protein Calmodulin

60. Ca2+/calmodulin dependent protein kinase (CaM-kinase)
Memory function: 1. calmodulin dissociate after 10 sec of low calcium level; 2. remain
active after calmodulin dissociation

61. Ca2+/calmodulin dependent protein kinase (CaM-kinase)
Frequency decoder of Calcium oscillation
High frequence, CaM-kinase does not return to basal level before the second wave of
activation starts

63. Desensitization of GPCR
1. Inhibitory structural alteration of receptor; 2. receptor
internalization; 3. receptor degration
GRK (G protein-linked receptor kinase)
Arrestin takes to clathrin-coated pits and degradation

64. •GPCR Signaling Summary
• 1. G-protein types
• 2. cAMP and Calcium signaling
pathways
• 3. desensitization

65. Enzyme-linked Cell Surface Receptors
• Receptor Tyrosine kinases: phosphorylate specific tyrosines
• Tyrosine kinase associated receptors: associate with
intracellular proteins that have tyrosine kinase activity.
• Receptorlike tyrosine phosphatases: remove phosphate
group
• Receptor Serine/ Threonine kinases: phosphorylate specific
Serine/ Threonine
• Receptor guanylyl cyclases: directly catalyzes the production
of cGMP
• Histidine kinase associated receptors: kinase phoshorylates
itself on histidine and then transfers the phosphate to a
second intracellular signaling protein.

66. Receptor Tyrosine Kinases (RTKs)
• Intrinsic tyrosine kinase activity
• Soluble or membrane-bound ligands:
– Nerve growth factor, NGF
– Platelet-derived growth factor, PDGF
– Fibroblast growth factor, EGF
– Epidermal growt factor, EGF
– Insulin
• Downstream pathway activation:
– Ras-MAP kinase pathway

67. TYROSINE KINASE RECEPTORS
• these receptors traverse the membrane only once
• respond exclusively to protein stimuli
– cytokines
– mitogenic growth factors:
• platelet derived growth factor
• epidermal growth factor

68. • Functions include:
– Cell proliferation, differentiation
– Cell survival
– Cellular metabolism
• Some RTKs have been discovered in cancer research
– Her2, constitutively active form in breast cancer
– EGF-R overexpression in breast cancer
• Other RTKs have been uncovered in studies of
developmental mutations that block differentiation

69. Outline
• Activated RTKs transmit signal to Ras protein
• Ras transduces signal to downstream serine-
threonine kinases
• Ultimate activation of MAP kinase
• Activation of transcription factors

70. Ligand binding to RTKs
• Most RTKs are monomeric
• ligand binding to EC domain induces dimerization
• FGF binds to heparan sulfate enhancing its
binding to receptor: dimeric receptor-ligand
complex
• Some ligands are dimeric: direct dimerization of
receptors
• Insulin receptors occur naturally as a dimer
– Activation is due to the conformational change of the
receptor upon ligand binding

71. Substrate + ATP Substrate-P + ADP
Protein Tyrosine Kinase
Protein Tyrosine Phosphatase
(PTP)

72. Tyrosine Protein Phosphorylation
• Eukaryotic cells coordinate functions through environmental signals -
soluble factors, extracellular matrix, neighboring cells.
• Membrane receptors receive these cues and transduce signals into the
cell for appropriate response.
• Tyrosine kinase signalling is the major mechanism for receptor signal
transduction.
• Tyrosine protein phosphorylation is rare (1%) relative to
serine/threonine phosphorylation.
• TK pathways mediate cell growth, differentiation, host defense, and
metabolic regulation.
• Protein tyrosine phosphorylation is the net effect of protein tyrosine
kinases (TKs) and protein tyrosine phosphatases (PTPs).

73. Tyrosine kinase linked receptors
• Bi-functional receptor / enzyme
• Activated by hormones
• Over-expression can result in cancer

74. 4.1 Structure
N H2
C O2H
Cell membrane
Catalytic binding region
(closed in resting state)
Ligand binding region
Extracellular
N-terminal
chain
Intracellular
C-terminal
chain
Hydrophilic
transmembrane
region (a-helix)
4. Tyrosine kinase linked receptors

75. 4.2 Reaction catalysed by tyrosine kinase
N C
O
Protein Protein
OH
Tyrosine
residue
Tyrosine
kinase
Mg++
ATP ADP
N C
O
Protein Protein
O
Phosphorylated
tyrosine
residue
P
4. Tyrosine kinase linked receptors

76. 4.3 Epidermal growth factor receptor (EGF- R)
Inactive EGF-R
monomers
Cell
membrane
Binding site for EGF
EGF - protein hormone - bivalent ligand
Active site of tyrosine kinase
Induced fit
opens tyrosine kinase
active sites
Ligand binding
and dimerisation
OH
OH
OH
HO
Phosphorylation
ATP ADP
OP
OP
OP
PO
EGF
4. Tyrosine kinase linked receptors

77. • Active site on one half of dimer catalyses phosphorylation of Tyr residues on other
half
• Dimerisation of receptor is crucial
• Phosphorylated regions act as binding sites for further proteins and enzymes
• Results in activation of signalling proteins and enzymes
• Message carried into cell
4.3 Epidermal growth factor receptor (EGF- R)
4. Tyrosine kinase linked receptors

78. 4.4 Insulin receptor (tetrameric complex)
Insulin
Cell
membrane
Insulin binding site
Kinase active site
OH
OH
OH
HO
OP
Phosphorylation
ATP ADP
OP
OP
PO
Kinase active site
opened by induced fit
4. Tyrosine kinase linked receptors

79. 4.5 Growth hormone receptor
Tetrameric complex constructed in presence of growth hormone
Growth hormone binding site
Kinase active site
Kinase active site
opened by induced fit
GH
OH
OH
OH
HO
kinases
GH receptors
(no kinase activity)
GH binding
&
dimerisation
OP
OP
OP
PO
ATP ADP
Activation and
phosphorylation
OH
Binding
of kinases
OH
OH
HO
4. Tyrosine kinase linked receptors

80. P
P
P
P
P
P
P
P
P
P
Ligand
signalling protein
Ligand
4. Tyrosine kinase linked receptors
4.6 Signalling pathways

81. 1-TM Receptors
Tyrosine kinase
inherent or associated
Guanylate cyclase
Signalling proteins
cGMP
PLCg IP3
kinase
GAP Grb2 Others
PIP3
Ca++ PKC
IP3 DG
4. Tyrosine kinase linked receptors
4.6 Signalling pathways

82. Tyrosine kinase
active site
(inactive)
Receptor
binding
site
OH
HO
HO
OH
GROWTH FACTOR RECEPTOR
4. Tyrosine kinase linked receptors
4.6 Signalling pathways

83. Growth
factor
OH
HO
HO OH
1) Binding of
growth factor
2) Conformational
change
OH
HO
HO OH
Dimerisation
OH
HO
HO OH
OH
HO
HO OH
Phosphorylation
OP
PO
PO OP
OP
PO
PO OP
Grb2
OP
PO
PO OP
OP
PO
PO OP
OP
OH
Binding and
phosphorylation
of Grb2
Grb2
Binding
Ras and
GTP/GDP
exchange
OP
PO
PO OP
OP
PO
PO OP
OP
GDP
GTP
Ras
4. Tyrosine kinase linked receptors
4.6 Signalling pathways

84. 4.6 Signalling pathways
Raf (inactive) Raf (active)
Mek (inactive) Mek (active)
Map kinase (inactive) Map kinase (active)
Transcription
factor (inactive)
Transcription
factor (active)
Gene transcription
OP
PO
PO OP
OP
PO
PO OP
OP Ras
4. Tyrosine kinase linked receptors

85. Ras
• Monomeric GTPase switch protein
• Its activation is enhanced by GEF
– GDP-GTP exchange
• Deactivation of Ras-GTP complex requires
GAP, which increases intrinsic GTPase activity
100 fold
• Lifetime of Ras-GTP is higher than that of Ga
– Ras is a small protein (170 aa. Vs 300 aa of Ga)
– Ga has a domain that functions like GAP

86. • Mutant ras proteins are associated with many
cancers
• Mutant ras can bind GTP but can not
hydrolyze it, and thus remain constitutively in
“on” state
• Most oncogenic ras proteins contain a
mutation in codon 12 (Gly)
– This blocks the binding of GAP to ras, and
prevents GTP hydrolysis.

87. Linking ras to RTKs
• Experimental evidences
– Fibroblasts were induced to proliferate with FGF
and EGF
– Anti-ras antibody microinjected: cell proliferation
arrest
– Injection of mutant ras proteins allows cell to
proliferate in the absence of growth factors.
• Ligand-bound RTKs activate ras! How?

88. • Two cytosolic proteins are involved: GRB2, Sos
• SH2 domain in GRB2 binds to a P*-tyrosine
residue in the activated receptor
• Two SH3 domains of GRB2 bind to and activate
Sos
• Sos is GEF protein and convert inactive GDP-ras
into active GTP-ras
• Developmental studies elucidated the role of
GRB2 and Sos in linking RTKs to ras activation

90. TYPE 4: NUCLEAR RECEPTORS
• Receptors for steroid hormones such as
oestrogen and the glucocorticoids were present
in the cytoplasm of cells and translocated into the
nucleus after binding with their steroid partner.
• Other hormones, such as the
– thyroid hormone T3
– fat-soluble vitamins D and A (retinoic acid) and their
derivatives that regulate growth and development,
were found to act in a similar fashion.

92. Structure
• Structural design comprised of four modules .
• The N-terminal domain displays the most
heterogeneity.
• It harbours the AF1 (activation function 1) site that
binds to other cell-specific transcription factors in a
ligand-independent way and modifies the binding or
activity of the receptor itself.
• Alternative splicing of genes may yield several receptor
isoforms each with slightly different N-terminal
regions.
• The core domain of the receptor is highly conserved
and consists of the structure responsible for DNA
recognition and binding.

93. • At the molecular level, this comprises two zinc
fingers-cysteine- (or cystine/histidine) rich loops
in the amino acid chain that are held in a
particular conformation by zinc ions.
• The main function of this portion of the molecule
is to recognise and bind to the hormone response
elements located in genes that are sensitive to
regulation by this family of receptors, but it plays
a part in regulating receptor dimerisation as well.

95. ER, oestrogen receptor; FXR, farnesoid receptor; GR, glucocorticoid receptor; LXR, liver
oxysterol receptor; MR, mineralocorticoid receptor; PPAR, peroxisome proliferator receptor; PR,
prolactin receptor; RXR, retinoid receptor; TR, thyroid receptor; VDR, vitamin D receptor

96. • Steroid receptors, become mobile in the
presence of their ligand and can translocate
from the cytoplasm to the nucleus, while
others such as the RXR probably dwell mainly
within the nuclear compartment.
• They regulate many drug metabolic enzymes
and transporters and are responsible for the
biological effects of approximately 10% of all
prescription drugs.

97. • There are also many illnesses associated with
malfunctioning of the nuclear receptor
system, including inflammation, cancer,
diabetes, cardiovascular disease, obesity and
reproductive disorders

98. CLASSIFICATION OF NUCLEAR
RECEPTORS
• Class I
• consists largely of receptors for the steroid hormones,
including the glucocorticoid and mineralocorticoid
receptors (GR and MR, respectively), as well as the
oestrogen, progesterone and androgen receptors (ER,
PR, and AR, respectively).
• In the absence of their ligand, these receptors are
predominantly located in the cytoplasm, complexed
with heat shock and other proteins and possibly
reversibly attached to the cytoskeleton or other
structures.

99. • Following diffusion (or possibly transportation) of
their ligand partner into the cell and high-affinity
binding, these receptors generally form
homodimers and translocate to the nucleus.
• Here they can transactivate or transrepress genes
by binding to 'positive' or 'negative' hormone
response elements.
• Large numbers of genes can be regulated in this
way by a single ligand.
• Class I receptors generally recognise hormones
that act in a negative feedback fashion to control
biological events.

100. • Class II nuclear receptors
• Their ligands are generally lipids already present to some
extent within the cell.
• This group includes
– peroxisome proliferator-activated receptor (PPAR) that
recognises fatty acids;
– the liver oxysterol (LXR) receptor that recognises and acts as a
cholesterol sensor,
– the farnesoid (bile acid) receptor (FXR),
– a xenobiotic receptor (SXR; in rodents the PXR) that recognises a
great many foreign substances, including therapeutic drugs
– the constitutive androstane receptor (CAR), which not only
recognises the steroid androstane but also some drugs such as
phenobarbital.

101. • They induce drug-metabolising enzymes such as CYP3A
(which is responsible for metabolising about 60% of all
prescription drugs),
• They also bind some prostaglandins and non-steroidal
drugs, as well as the antidiabetic thiazolidinediones
and fibrates.
• Unlike the receptors in class I, these receptors almost
always operate as heterodimers together with the
retinoid receptor (RXR).
• They tend to mediate positive feedback effects (e.g.
occupation of the receptor amplifies rather than
inhibits a particular biological event).

102. • A third group of nuclear receptors is really a
subgroup of class II in the sense that they
form obligate heterodimers with RXR
• They too play a part in endocrine signalling.
• The group includes the thyroid hormone
receptor (TR), the vitamin D receptor (VDR)
and the retinoic acid receptor (RAR).

103. CONTROL OF GENE TRANSCRIPTION
• Hormone response elements are the short
(four or five base pairs) sequences of DNA to
which the nuclear receptors bind to modify
gene transcription.
• They are usually present symmetrically in pairs
or half sites, although these may be arranged
together in different ways.
• Each nuclear receptor exhibits a preference
for a particular sequence.

104. • In the nucleus, the ligand-bound receptor recruits
further proteins including coactivators or
corepressors to modify gene expression through
its AF1 and AF2 domains.
• Some of these coactivators are enzymes involved
in chromatin remodelling such as histone
acetylase which, together with other enzymes,
regulates the unravelling of the DNA to facilitate
access by polymerase enzymes and hence gene
transcription.