Growth hormone (GH) is a pituitary-derived peptide hormone with potent somatogenic and metabolic actions. GH binds its receptor (GHR), a cell surface transmembrane protein enriched in liver, muscle, fat, and heart to exert profound effects on fuel metabolism, muscle mass, and energy homeostasis in humans and other vertebrates. GH sensitivity is altered in the setting of acute and chronic disease states and it is uncertain whether enhancing or inhibiting GH action would be salutary in such situations; this is important, as both agonists and antagonists now exist. My laboratory has focused on understanding physiologic and pathophysiologic determinants of GH sensitivity, which is the integrated effect of GH levels, the abundance of cell surface GHR, and the coupling of GHR activation with intracellular signaling pathways. In previous and the current VA funding cycles, we uncovered a novel mechanism modulating GH responsiveness, involving a proteolytic system composed of the metalloprotease TACE (tumor necrosis factor-a cleaving enzyme) and the TACE inhibitor TIMP3 (tissue inhibitor of metalloproteinases-3) that inducibly alters surface GHR availability by proteolytically shedding the receptor extracellular domain. However, the extent to which this metalloproteolytic system modulates GH signaling in physiological and pathophysiological states is unknown. Realizing that GH levels vary diurnally and with nutritional status, we recently examined whether such factors affect GHR abundance and GH sensitivity. Our preliminary data in mice suggest that hepatic GHR availability and corresponding GH sensitivity vary in a time-of-day-dependent fashion coordinate with the level of TIMP3. Furthermore, diet-induced obesity also affects hepatic GHR abundance. In light of recent reports, we believe understanding how these physiologic and pathophysiologic variables affect GH sensitivity will be crucial to rationally exploit GH-enhancing or -antagonistic therapeutic strategies in critically-ill individuals. In this proposal, we will test two main hypotheses: 1) Hepatic GHR abundance is metalloproteolytically modulated in a time-of-day-dependent fashion. Nutritional factors also affect hepatic GHR abundance, but at a transcriptional level. Both forms of regulation contribute to GH sensitivity and energy homeostasis in the steady state. 2) Critical illness, exemplified by sepsis, acutely desensitizes the liver to GH via TACE- mediated GHR proteolysis. Net effects of time-of-day- and diet-determined GHR abundance and timing and degree of critical illness determine acute hepatic GH responsiveness and may thus impact the course of illness. We will use novel mouse model systems and reagents and techniques we have developed to pursue two specific objectives: 1. Examine in mice potential mechanisms of diurnal variation in liver GHR levels and the impact of such time-of-day effects on hepatic GH sensitivity. 2. Determine how the time of day and feeding regimen impact the propensity of acute illness to modulate hepatic GH sensitivity, using a mouse model of sepsis (lipopolysaccharide (LPS) administration). Successful completion of these studies will definitively ascribe the response of hepatic GH sensitivity to clinically relevant acute illness, enabling rational therapeutic exploitation of the GH axis in sepsis and other forms of critical illness.
Patients who are severely ill with overwhelming infection (called sepsis) or other acute disorders are at risk of death without appropriate intensive care unit management. There is substantial debate among physicians as to whether such patients might benefit from treatment with a hormone called growth hormone (GH) that is normally made in the body and regulates growth and metabolism. Some studies have shown a benefit; in other studies, patients' risk of death was increased by GH treatment. This proposal will test in mice how the effects of GH on the liver (an organ regulated by GH) are altered in sepsis. We have evidence that GH's effects on the liver in mice may vary depending on the time of day and whether the mouse is obese, and we will test whether these issues also impact the response to sepsis and whether GH treatment can be influential. Our aim is to translate these animal studies into knowledge that directly affects the care of acutely ill veterans in the intensive care unit.
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