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The research programs in our laboratory investigate hormone action and signal transduction in the mammalian neuroendocrine system. We are studying genes whose protein products act as peptide hormones in the brain and endocrine system, or as receptors for these hormones. Our goals are to elucidate the molecular and cellular mechanisms by which these genes are regulated, and understand how these hormones act to modulate key physiological processes like reproduction and growth. We work on two different hormonal systems in the laboratory: inhibin and activin synthesis and actions within the ovarian granulosa cell and GHRH and the neuroendocrine regulation of pituitary growth hormone synthesis. Major research projects of current interest in the laboratory are described in more detail below.
Actions
and Regulation of Inhibin and Activin in the Ovary
Inhibin and activin are structurally related gonadal hormones that regulate
the secretion of follicle-stimulating hormone from the anterior pituitary
gland (Figure 1). Inhibin plays a key role in modulating the reproductive
system, while activin probably exerts many of its major functions during
embryonic development by regulating cellular growth and differentiation.
The proteins are dimers (inhibin, a-b
and activin, b-b)
of subunits encoded by three distinct genes (a,
bA and bB). We
are studying the expression and regulation of these genes and the actions
of the proteins in reproductive tissues such as the ovary and pituitary.
Two major projects are currently in progress:
cAMP-Responsive Transcription
Factors in Ovarian Gene Expression
Ovarian gametogenesis and steroidogenesis are controlled in part by
the pituitary gonadotropins follicle-stimulating hormone (FSH) and luteinizing
hormone (LH), which exert their effects via the intracellular second
messenger cAMP. Because the ovarian target genes we are studying (the
inhibins and activins) are regulated by gonadotropins and cAMP, we are
investigating the intervening components of this signaling pathway.
During the rodent reproductive cycle, FSH stimulates inhibin gene expression
in growing follicles, while the preovulatory LH surge causes a repression
of inhibin gene expression in the periovulatory period, facilitating
the secondary FSH surge and recruitment of a new cohort of follicles
into the ovulatory pool. We previously found that the cAMP-responsive
transcription factor CREB is rapidly phosphorylated and activated in
granulosa cells following FSH treatment, and plays a key role in activating
inhibin subunit gene expression. In recent studies, we found that cAMP-stimulated
transcription of the inhibin subunit genes requires actions of an additional
transcription factor, the orphan nuclear receptor steroidogenic factor-1.
In collaboration with Dr. Larry Jameson (Northwestern University Medical
School) we found that SF-1 potently activates the a
subunit gene in granulosa cells, and that the effects of cAMP and SF-1
on promoter activity are synergistic. We are now investigating the mechanism
of this synergism, focusing on transcriptional co-activators known to
interact with both CREB and SF-1. We are using chromatin immunoprecipitation
approaches to investigate transcription factor and coactivator association
with the inhibin composite regulatory element following hormonal stimulation
of granulosa cells. In collaboration with Dr. Ishwar Radhakrishnan (Northwestern
University) we are using biophysical approaches to investigate structural
interactions between these regulatory proteins on the inhibin a
subunit gene promoter.
During
the periovulatory period, we demonstrated that the cAMP-inducible transcriptional
repressor ICER is rapidly induced by LH in ovarian cells. The induction
of this repressor protein occurs at a time during the reproductive cycle
when inhibin subunit gene expression is strongly down-regulated, suggesting
an involvement of ICER in this process. We found that overexpression
of ICER in granulosa cells can block cAMP-dependent inhibin gene expression,
consistent with such a model. Our most recent studies demonstrate that
of four known ICER isoforms, I and Ig most
strongly bind to the CRE and repress the gene, and that ICER likely
exerts its inhibitory actions as a passive repressor, by occupying the
CRE site in the inhibin a subunit gene promoter,
prohibiting occupancy by the positively acting transcription factor
CREB. Our studies suggest a model in which occupancy of the inhibin
promoter by transcriptional activators such as CREB and SF-1 or transcriptional
repressors such as ICER is associated with differential inhibin gene
regulation by FSH and LH during the reproductive cycle (Figure 2).