mechanism of luteolysis in bovines with reference to recent concepts

1. Endocrine and Paracrine control of
luteolysis in bovines
Dr. Santhosh Shinde
Roll no- P 1673
Ph. D scholar

2. Overview
Historical perspective
Corpus luteum.
–
–
–
–
Where it comes from?
What it secretes?
What is its function?
How it is regulated?
Luteolysis
– Why should the corpus luteum capitulate?
– Signal for luteolysis
– Control of luteolysis
a) Endocrine control
b) Paracrine control

3. Historical perspective
Coiter (1573)- Presence of cavities filled with yellow solid on ovary
Regnier de Graaf (1641–1673)- First accurately described the structure
Marcello Malpighi (1628–1694) – Named structure as Corpora (bodies)
lutea (yellow)
Prenant (1898) - Corpus luteum acts as a gland internal secretion.
Magnus (1901)- Luteal extract has biologically active substance maintain
pregnancy
Butenaudt, Hartman and Slotta (1934)- Isolation of the pure crystalline
hormone
Slotta (1934)- Named the compound progesterone and suggested a
structural formula
Butenandt and Westphal (1934)- Compound was synthesized
Leo Loeb (1923)- Influence of uterus on regression of the corpus luteum
McCrackenit (1972)- Identified PGF2α as - luteolysis hormone

4. Formation of corpus luteum
Preovulatory surge of LH - major signal for CL development
LH induces, transformation of ovulated follicle cells into the CLLuteinization
(Richards, 1987)

5. Preovulatory LH surge
LHR
Ovulation
LH induced
Luteinization
 Theca interna cell- Small luteal cell
 Granulosa cells- Large luteal cells
(Miyanoto, 2009)

6. Cell types found in the CL
Cell type
Percentage
(%)
Volume
(%)
Large luteal
cells
4
25
Small luteal
cells
19
18
Endothelial
cells
53
17
Fibrocytes
17
11
Vascular endothelial cells - 50% of all cells in bovine CL
(Miyamoto and Shirasuna, 2009)
LLC secrete 10 times more progesterone than SLC
(Fitz, 1982; Koose, 1971)

7. Luteal development
From Day 4 to 16 of estrous cycle,
2- fold increase in size of LLC (15×106/corpus luteum),
2- fold increase in number of fibroblasts (from 21 to 50×106),
5- fold increase in number of SLC (from 10 to 50 ×106)
6.5- fold increase in number of endothelial cells (from18 to 120 ×106)
(Farin, 1986: Jablonka-shariff, 1993)
Factors regulating proliferation of SLC and fibroblasts
- Fibroblast growth factors
- insulin growth factors
- Growth hormone
- LH
(Redmer, 1996: Juengel, 1995)

8. Cont…
Proliferation of endothelial cells
neo-vascularization
(Reynolds,1992)
Vascular endothelial growth factor (VEGF)- mitogen
Capillary lumina - 22% of the total volume of the corpus luteum
(Dharmarajan, 1985)
Rate of blood flow (6–10 ml/g/min) to the corpus luteum
Consume 2-6 times more oxygen per unit weight than does the
liver, kidney, or heart
(Swann and bruce, 1986)

9. Regulation of luteal function
Matured CL secrete progesterone – Maintenance of
pregnancy
Hormones that support the growth and/or function of the
corpus luteum are termed luteotropic hormones
(Acosta et al., 2002)
Hormones includes,
LH, GH, prolactin, IGF-I, PGE2, and PGI2
(Gordon et al., 2000)
How does the progesterone secretion maintained ?

10. Molecular Mechanism of LH on Luteal Cell
(cAMP second messenger system)
LH
Receptor
Gs
Plasma Membrane
Adenylate Cyclase
cAMP
PKA
Progesterone
ATP
R
cAMP
S-ER
C
Steroid Synthesis
(+ PO4)
Mitochondria
Cholesterol
Pregnenolone
Histones
Cholesterol
Protein
Synthesis
(Enzymes)
Protein Synthesis
R-ER
mRNA
Nucleus
DNA

11. Acute effects of GH and IGF-I on progesterone secretion
Direct effect – Activation tyrosine kinase JAK2
Indirect effect- expression of IGF-I - but how does it secrete
pregesterone is unknown ?
(Parmer et al., 1991; Talawera, 1992)
Binding of IGF-I
phosphorylation of insulin receptor substrate 1
Phosphoinositide 3-kinase
Phosphoinositide phosphatidylinositol 3-phosphate
modification of the cytoskeleton
prevention of cellular death
(Davis et al., 1996; Lucy,1993)

12. Effects of prostaglandins on progesterone secretion
Prostaglandins E and I - normal luteal function
Prostaglandins E and I - higher amounts in the early luteal
phase
(Mitchell et al.,1991)
Binding sites for PGI2 reside on both small and LLC Elevation of cAMP levels
(Huggins et al., 1983; Fitz, 1984)
High-affinity receptors for PGE2
LLC
(Fitz et al., 1982)
Bovine luteal tissue
addition of PGE2- levels of cAMP
and presumably activated PKA
(Miyashita et al., 1998)

13. Luteolysis
Luteolysis is defined as lysis or structural demise of the corpus
luteum.
Two events occur during luteolysis,
a. Functional luteolysis- loss of capacity to synthesize and
secrete progesterone
b. Structural luteolysis- loss of the cells
16-17 day of estrous cycle- concentrations of mRNA encoding 3
β-HSD and StAR decreased
(Mcguire et al., 1999)
secretion of progesterone - luteal blood flow and
steroidogenic capacity of luteal cell
(Shirasuna et al., 2004)
Why mammals have developed mechanisms to curtail luteal function?

14. A New Beginning
Inhibitory effect of Progesterone on FSH Secretion will be
nullified
Who is responsible for this decrease in CL function?

15. The Uterus Regulates the Life Span of the Corpus
Luteum
Progesterone
Progesterone
Hysterectomy
1
10
Luteal Phase
Intact Animal
20
1
10
20
Luteal Phase
Hysterectomized Animal
The substance responsible – Found to be PGF2α
(McCrackenit, 1972)

16. What is the signal that initiates the release of PGF2a ?
McCracken Model of Oxytocin control on CL Regression
Posterior
Pituitary
Anterior
Pituitary
CL
Oxytocin
PGF2
Ovary
Uterus
 Estradiol from the developing preovulatory follicle triggers the
release of hypophysial oxytocin
(McCracken et al., 1981)

17. Countercurrent mechanism of luteolysis
(McMillan, 1972)
Ovarian
Artery
This mechanism is specific to certain species;
How does PGF2a cause luteolysis ?

18. Blood flow and vascular changes
PGF 2α - Acute increase (from 30 min to 2 h) in blood flow at the
periphery of the CL
(Acosta et al., 2002)
Additional evidence - blood flow to CL increased with each PGFM
pulse - spontaneous luteolysis in the heifer
(Ginther and beg, 2012)
Nitric oxide (NO) and Apelin (APJ) pathway - Potent vasorelaxant
Administration of the NO donor (SNAP) into CL - Mimicked the
actions of PGF 2α
(Shirasuna et al., 2008)
NOS inhibitor (L-NAME) - suppressed acute increase in luteal blood
flow
Need of initial increase in luteal blood flow?

19. Luteolysis accelerators within the CL (Paracrine
control)
Endothelin-1
The EDN1 – 21 a.a peptide produced by endothelial cells
First isolated from porcine vascular endothelial cells
(Yanagisawa et al., 1988)
Cause luteolysis by,
Reduce blood flow during early luteolysis - Arteriole constriction
(Granstrom and Kindahl, 1986)
Inhibits the steroidogenic activity
(Fraser et al., 1997)
Antiluteolytic property of PGE2-- attenuate the vasoconstrictive
actions of endothelin-1
(Irvin, 1999)

20. Angiotensin II
Ang II is a strong vasoconstrictive peptide
(Hayashi and Miyamoto 1999; Speth et al., 1999)
Ang II - inhibit LH-stimulated P4 release in bovine luteal cells
(Stirling et al., 1990)
Spontaneous luteolysis in the cow- PGF2α and Ang II also have
positive feedback during luteolysis
(Shirasuna et al., 2004)

21. Luteal PGF2aPGF2a from peripheral sources - Synthesis of luteal PGF2a
(Ellendorff, 1998)
PGF2a autoregulate its synthesis - stimulating the liberation of
arachidonic acid by hydrolysis of membrane phospholipids
(Watanabe et al., 1995)

22. Intracellular Signaling
PGF2a acts by binding to specific receptors – LLC
day 13 to 20- Binding affinity of the PGF2a to receptor
increases 203 time
(Rao et al., 1979)
Gordon et al., 2000)

23. Activation of PKC in LLC
- post translational modification of cellular proteins
- cholesterol availability
- maintenance of the extracellular matrix
- activation of proteins involved in apoptosis
(Pate and Keyes, 2001)

24. Involvement of cell adhesion systems within the CL
Two types of intercellular communications
Gap junction- exchange of molecules of less than about 1 kDa
(calcium ions, cAMP and IP3) between adjacent cells
(Yamasaki and Naus, 1996)
Gap junctions are formed by connexin (Cx) proteins such as Cx43
(Grazul-Bilska et al., 1997)
Cadherin family- vascular endothelial cell cadherin (VEcadherin), epithelial cadherin (E-cadherin)
Cell adhesion and gap junction - necessary for transferring the
luteolytic signal between luteal cells and endothelial cells
(Grazul-Bilska et al., 1997; Borowczyk et al., 2006)

25. Immune-Mediated Events
Immune cells infiltrate the CL during luteolysis - chemotaxic
factor
(Murdoch, 1987)
Role of Macrophages during early stages of luteolysis:
1) phagocytosis of degenerative luteal cells,
2) cytokine mediated inhibition of steroidogenesis
3) Stimulation of PGF2a secretion by the corpus luteum.
4) degradation of the extracellular matrix
(Parker, 1991)
Infiltration of leukocytes - increased local production of
cytokines (particularly IL-1, TNF-a,and IFN-γ)- stimulate
luteal PGF2a synthesis
(Vallet et al., 1993)

26. Apoptosis
Apoptosis - active, energy-dependent process by which
nonessential populations of cells delete themselves from a tissue
(Kerr, 1992)
Biochemical and morphological changes- characteristics of
apoptotic cells
- Nuclear fragmentation,
- Appearance of membrane-bound vesicles,
- Ladder like fragmentation of genomic DNA and
- Changes in gene expression (Bas, caspase)
(Sawyer, 1991)
PGF2a - promotes apoptosis in cells of corpus luteum
Cell fragments, or apoptotic bodies- targets for Phagocytosis
(Gemell et al.,1986)

27. Concept of luteolysis in cow
(Miyamoto and Shirasuna,2009)

28. Conclusions
Formation and regression of the CL involve a wide variety complex
multifaceted biological processes
Cow- PGF2α induced increase in luteal blood flow is one of the
earliest signals of the luteolytic cascade
Endocrine mediated luteolytic cascade- Endometrial PGF 2α and luteal
OXT
Luteolysis accelerators within the CL are responsible for Paracrine
control of luteolysis
Gap and adherence junctions composed of cell adhesion molecules
(CAMs) involved transport of luteolytic
Immune mediated cells are involved in luteolysis- inflammatory process