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Models
of Inhibin and Activin Action and Reproductive Disease
As an important hormonal regulator of reproduction, inhibin has the
potential to be involved in reproductive diseases or disorders. Elevated
inhibin levels have been correlated with infertility, polycystic ovarian
disease, and ovarian cancer, at least in some studies. We developed
a transgenic mouse model to explore the potential effects of excess
inhibin on the reproductive axis. The inhibin a
subunit protein was overexpressed in transgenic mice from a heterologous
metallothionein-I promoter. The transgene is expressed in numerous tissues
and levels of inhibin are highly elevated compared to control mice,
leading to a decrease in serum FSH, and an increase in serum LH and
testosterone. Activin levels are also somewhat depressed. The female
mice are subfertile and have very small litters. This is a consequence
of decreased ovulation, probably secondary to alterations in FSH and
LH. The male mice are fertile, although they have reduced sperm counts
and testis size. Most interestingly, female mice that carry this transgene
develop several unique ovarian pathologies, including distension of
the bursal sac, the presence of large fluid-filled cysts, and the presence
of atypical follicles that contain multiple oocytes (Figure 3). Using
pharmacological agents to suppress synthesis of the gonadotropins FSH
and LH in these mice, we found that cyst formation is independent of
alterations in the gonadotropin ratio. We continue to investigate the
cyst phenotype in these mice and to relate the ovarian pathologies to
those observed in polycystic ovarian disease in women. In collaboration
with Dr. Teresa Woodruff (Northwestern University), we are also investigating
the mechanisms that lead to aberrant follicle formation and multi-oocytic
follicles, focusing on the potential involvement of activin in follicle
development.
GHRH
and its Receptor in the Neuroendocrine Regulation of Growth
Growth hormone-releasing hormone (GHRH) is a brain peptide that regulates
pituitary growth hormone synthesis and secretion. It also is critical
for the appropriate proliferation and differentiation of the pituitary
somatotroph (growth hormone-secreting) cell (Figure 4). Some years ago,
we identified a pituitary G protein coupled receptor that is a specific
receptor for GHRH. Our current work involves investigating the role
of this receptor in mediating GHRH actions, understanding the control
of receptor gene expression and exploring the involvement of this receptor
in diseases of growth hormone secretion. Two major projects are currently
in progress:
Expression and Regulation of the GHRH Receptor Gene in Pituitary
Cells
Pituitary somatotroph cells differentiate from a bipotential
somatotroph/blacktop precursor, and expression of the GHRH receptor
and acquisition of responsiveness to GHRH is likely a defining event
in the establishment of this cell lineage (Figure 5). We are therefore
interested in understanding the tissue- and cell-specific expression
and hormonal regulation of the GHRH receptor gene. We have cloned the
rat GHRH receptor gene and characterized the promoter. This analysis
revealed multiple potential binding sites for the POU domain protein
Pit-1, a protein previously implicated in growth hormone gene expression.
Using transient transfection approaches in pituitary tumor cell lines,
we have shown that Pit-1 positively regulates GHRH receptor promoter
activity. Pit-1 regulation is associated with cell-specific expression,
in that the GHRH receptor promoter is active in cells that express endogenous
Pit-1 (pituitary cells), but is inactive in other cell types. Co-transfection
of Pit-1 can activate the GHRH receptor promoter in non-pituitary cells.
Current studies are aimed at identifying the promoter elements and transcription
factors, in addition to Pit-1, that mediate cell-specific expression.
In related studies, we found that glucocorticoid hormones stimulate
expression of the GHRH receptor gene, both in pituitary cells and in
the intact adult pituitary gland. This is a direct transcriptional response
that can be observed using the GHRH receptor promoter in cell transfection
studies. Glucocorticoids also enhance GHRH receptor gene expression
during development, and can advance the onset of GHRH receptor gene
expression in the fetus. The timing of initial receptor gene expression
is correlated with a rise in maternal glucocorticoids, suggesting that
glucocorticoids may play a role in developmental activation of the receptor
gene. We are also interested in the role of gonadal steroids in regulation
of the pituitary GHRH receptor gene and establishment of sexually dimorphic
patterns of growth hormone gene expression.