Fetal/Postnatal Regulation of GH Receptor Expression

生长激素受体（GHR）的表达和功能对于垂体生长激素（GH）在完整动物中的作用至关重要。 这种受体蛋白的表达水平随着发育而显著变化; GHR在胎儿组织中几乎检测不到，而其表达在出生后增加，在妊娠期间达到最大水平。 GHR基因表达也受疾病状态的影响。 肝脏GHR表达的减少有助于抵抗糖尿病中GH的作用。 相比之下，在糖尿病啮齿动物模型中肾脏中GHR mRNA水平的上调与使用GH和GH拮抗剂转基因小鼠、GHR基因敲除小鼠和使用pegvisomant药理学阻断GHR的研究一致，提出了肾脏GHR表达失调可能参与糖尿病肾病发病机制的可能性。 在动物模型中的研究也表明GH/GHR轴在糖尿病增殖性视网膜病变的发病机制中起重要作用。 在哺乳动物中，营养不良导致GH抵抗状态，这是由于肝脏中GHR基因转录减少。 GHR表达减少也与创伤、脓毒症和手术相关的分解代谢状态下GH抵抗的发病机制有关。GHR基因的表达调控是复杂的。 本实验室最近的研究表明，GHR基因由多个受多个启动子控制的5 '-非翻译外显子组成。 在小鼠中，我们已经表征了两个5 '-UTR，称为L1和L2，并获得了新的5'-UTR存在的证据。 L1转录本仅在肝脏中表达，并且仅在妊娠期间表达。 L2转录本广泛表达，占非妊娠状态下GHR转录本的约50%。 本研究建议基于以下总体假设：GHR基因表达调控的关键模式是通过使用替代启动子启动和控制转录。在具体目标1中，我们提出通过使用同源重组技术工程化L2缺失小鼠来建立L2转录本的生物学作用。 具体目标2旨在鉴定和表征调节鼠GHR基因L2转录本表达的潜在新型反式作用因子。 具体目标3涉及控制最近鉴定的鼠GHR基因的L5转录物的表达的顺式元件的研究。具体目标4详细研究了糖尿病GHR基因表达异常的分子基础。这些研究的结果将为制定新的治疗策略提供基础，以改变诸如身材矮小、糖尿病肾病和视网膜病变以及与营养不良、创伤、败血症和手术相关的分解代谢状态等疾病的结果。

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.