The long-term goal of this project is to identify factors responsible for physiological regulation of liver growth and the mechanisms through which they operate. Adult rat liver is essentially non growing, but capable of intense bursts of proliferation when activated, especially by partial hepatectomy. Short-term hepatocyte cultures have demonstrated that certain hormones and growth factors which stimulate or modulate hepatocyte proliferation in the animal exert similar effects upon these cells when isolated and maintained in culture. We hypothesize that liver growth is regulated by shifts in balance among a number of synergistically interacting hormonal signals, which may vary depending upon the metabolic status of cells. Resolution of the problem depends upon knowledge of the molecular mechanisms involved. This proposal focuses upon the mechanisms of action of certain hormones that stimulate hepatocyte proliferation: vasopressin, and related hormones whose signals operate through phosphatidylinositol (PI) breakdown, and alpha-2-and beta- adrenergic agents, and glucagon, which operate through cAMP. We are studying changes in binding of these hormones to their surface receptors in normal and regenerating hepatocytes in culture with and without synergistically acting growth factors. We will also examine modulation of receptor binding by heterologous ligands. Transduction of the hormonal signals via mechanisms involving calcium mobilization and cAMP will be compared. Calhibin, a newly discovered cytoplasmic protein which inhibits receptor binding of vasopressin, angiotensin II and alpha-2 adrenergic agonists, will receive special emphasis, being examined in relation to all of the above variants including its possible role in growth activation by both Ca2+ and cyclic AMP. In addition, the role of various gangliosides which can selectively alter the affinity of receptors for their ligands, will be assessed by analysis of normal and proliferating hepatocytes with and without addition of different gangliosides to the medium. Concurrent analysis of DNA synthesis will serve to correlate all of the results with hepatocyte proliferation. Implications of this research relate to restoration of damaged or diseased liver and in broader context to developmental disorders and neoplasia.
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