Expression and function of the growth hormone receptor (GHR) is critical for the action of pituitary growth hormone (GH) in the intact animal. The level of expression of this receptor protein varies dramatically with development; GHR is virtually undetectable in fetal tissues while its expression increases during postnatal life with maximum levels being achieved during pregnancy. GHR gene expression is also influenced by disease states. Decreased expression of hepatic GHR contributes to the resistance to GH's actions in diabetes mellitus. In contrast, up-regulation of GHR mRNA levels in the kidney in a rodent model of diabetes mellitus in concert with studies using GH and GH antagonist transgenic mice, GHR knockout mice, and pharmacological blockade of the GHR using pegvisomant raises the possibility that dysregulation of expression of renal GHR may be involved in the pathogenesis of diabetic nephropathy. Studies in an animal model also suggest an essential role of the GH/GHR axis in the pathogenesis of diabetic proliferative retinopathy. In mammals, undernutrition causes a state of GH resistance due to decrease in GHR gene transcription in liver. Decreased expression of GHR is also implicated in the pathogenesis of GH resistance in catabolic states associated with trauma, sepsis and surgery. The regulation of expression of the GHR gene is complex. Recent studies from our laboratory have revealed that the GHR gene consists of multiple 5'-untranslated exons under the control of multiple promoters. In the mouse, we have characterized two 5'- UTRs termed L1 and L2 and have obtained evidence for the existence of novel 5'-UTRs. L1 transcripts are expresses only in the liver and only during pregnancy. L2 transcripts are ubiquitously expressed and account for approximately 50 percent of the GHR transcripts in the non-pregnant state. This research proposal is based on the overall HYPOTHESIS that the key mode of regulation of expression of the GHR gene is by the use of alternate promoters for initiation and control of transcription.
In Specific Aim 1 we propose to establish the biological role of the L2 transcript by engineering an L2 null mouse using homologous recombination technology.
Specific Aim 2 addresses the identification and characterization of potentially novel trans-acting factor(s) regulating expression of the L2 transcript of the murine GHR gene.
Specific Aim 3 relates to the investigation of the cis-elements controlling the expression of the recently identified L5 transcript of the murine GHR gene.
Specific Aim 4 details an investigation into the molecular basis for abnormalities in GHR gene expression in diabetes mellitus. The results of these studies will provide the foundation for formulating novel therapeutic strategies to alter the outcome of conditions such as short stature, diabetic nephropathy and retinopathy, and catabolic states associated with malnutrion, trauma, sepsis and surgery.
