Abstract
The corpus luteum is one of the most intriguing and understudied endocrine glands in the
human body. Understanding the mechanisms behind corpora lutea development, function
and regression will enhance many major unresolved issues in human reproduction, with
important consequences on infertility, miscarriage and contraception. Formed at the endstage of folliculogenesis, the corpus luteum has the important role of being the interface
between pregnancy and menstruation. This transition involves the terminally differentiated
granulosa- and theca-lutein cells along with endothelial, fibroblast and immune cells to form
the most steroidogenically active tissue in the body. In a non-conception cycle the corpus
luteum will experience high cell turnover, intense remodelling and structural and functional
demise within a 14-day period, succinctly followed by the initiation of a new cycle.
Alternatively, during maternal recognition of pregnancy, the corpus luteum will continue to
secrete vast quantities of steroid hormones under the trophic stimulation of conceptus
derived-human chorionic gonadotrophin (hCG), subsequently maintaining structural and
functional integrity. Therefore the luteal lifespan is regulated by massive tissue and vascular
remodelling that is induced and regulated by gonadotrophins (luteinising hormone (LH) and
hCG) which elicit disparate changes in the number and function of cells that do not express
hCG/LH receptors. For example, luteolysis in women is associated with an up-regulation of
the expression and activity of matrix metalloproteinase-2 (MMP-2) which is inhibited by
hCG during maternal recognition of pregnancy. As the primary source of MMP-2 is
fibroblasts that do not express LH/hCG receptors, the scope of this thesis was to investigate
the regulation of MMP-2 by dissecting out potential paracrine signalling molecules.
Women with regular cycles having hysterectomy for non-malignant conditions and women
undergoing oocyte retrieval for assisted conception were used in the current study. Novel
primary cultures and co-cultures of luteinised granulosa cells and fibroblast-like cells
allowed the mechanistic in vivo interactions of the corpus luteum to be mimicked with an in
vitro system. Herein, activin A is identified as a regulated molecule that may promote tissue
remodelling during luteolysis. Activin A is secreted by luteal steroidogenic cells and at
physiological concentrations it up regulates MMP-2 activity and expression in luteal
fibroblast-like cells. HCG can inhibit activin A action through several mechanisms
including up-regulating activin inhibitors, inhibin A and follistatin.
If activin A is a paracrine molecule with potential roles in luteolysis, Cortisol is a paracrine
molecule with possible roles in luteal rescue. The corpus luteum expresses glucocorticoid
receptors and the Cortisol metabolising enzymes lip hydroxysteroid dehydrogenase type 1
(11 piISDl) and type 2 (11PHSD2). Both in vitro and in vivo, hCG promotes 11(3HSD1
expression and facilitates the local generation of Cortisol. Cortisol at physiological
concentrations inhibits local MMP-2 expression to inhibit the tissue remodelling associated
with luteolysis. During luteolysis activin and hCG have opposing effects and the same is
true during luteinisation. HCG promotes a luteinised granulosa cell phenotype, whilst
activin A promotes a more follicular phenotype by up-regulating granulosa cells markers
such as follicle-stimulating hormone (FSH) receptor, 11 [1HSD2 and blocking hCG upregulation of steroidogenic acute regulatory protein (StAR), 3p hydroxysteroid
dehydrogenase (3|3HSD) and 11PHSD1. Activin A appears to be an anti-luteal molecule in
nature. Activin A is not the only activin produced by the corpus luteum, activin B is also a
luteal cell product and its secretion is regulated in a similar manner to activin A with similar
endocrine effects. As activin has a fundamental role at beginning and end of the luteal
phase, it has the potential to be a major regulatory molecule in women. Indeed it can
regulate its own activity in the absence of hCG by self promoting its own expression and
down regulating its own inhibitors.
Collectively these results suggest that activin is an excellent anti-luteal molecule whose
paracrine actions are to remove or potentially inhibit luteal tissue formation, and moreover to
facilitate human luteolysis. However, the biological actions of activin A are inhibited during
maternal recognition of pregnancy by the endocrine actions of conceptus-derived hCG that
has marked and disparate changes on surrounding cell types that do not expression the
LH/hCG receptor. Consequently, luteolysis is prevented, such that luteal fibroblast-like
MMP-2 is inhibited as hCG-derived Cortisol supports promotes the maternal recognition of
pregnancy.
