The gene encoding the growth hormone receptor is also the source for a GH-binding protein. In human and rabbit, GHBP is generated by proteolysis of GHR. In rodents, GHR and GHBP mRNAs are alternatively spliced from the GHR/GHBP gene. Both proteins are expressed in numerous tissues, but expression of GHBP is characterized by complexity not observed for GHR. Whereas GHR is an integral membrane protein, GHBP is present in soluble intracellular, membrane-associated, and secreted forms. The functions of GHBP in cells where it is expressed as a soluble or membrane- associated protein are unknown. The long-term goal of the studies of GHR and GHBP is to understand how their expression from a common gene is regulated and how GHR and the different forms of GHBP are involved in mediating GH action. The specific interest is in the up-regulation of GHR/GHBP gene expression during pregnancy in the mouse. This represents the most dramatic change in expression of this gene observed to date in any species, making it a good model for studies of GHR/GHBP gene expression. Considering the diverse metabolic and growth-promoting effects of GH, regulation of this gene is probably an important component of the complex physiological changes necessary for a successful pregnancy.
The specific aims of this revised continuation application are to: 1. Examine mechanisms regulating GHR/GHBP gene expression in the mouse. The multiple 5' noncoding exons of the GHR/GHBP gene expressed in liver and placenta will be characterized. The function of the proximal promoter regions associated with these exons will be analyzed. The contribution of transcriptional regulation to up-regulation of GHR/GHBP gene expression during pregnancy will be assessed. 2. Examine regulation of GHR/GHBP gene expression by GH and fetal/placental factors. Direct effects of GH and fetal/placental factors implicated in regulating expression in vivo will be examined in vitro. If a direct effect of fetal/placental factors is observed, the factor(s) will be characterized. 3. Characterize the GH-binding proteins present in several tissues. Studies of GHR measuring the binding of GH to cells or membrane preparations may be measuring binding to GHR and to membrane-associated GHBP. A GHR-specific RIA will be developed and used with an existing GHBP-specific RIA to determine the distribution of the proteins in membrane and soluble fractions of several tissues as a function of pregnancy.
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