The regulation of glucose homeostasis depends on the secretion of insulin from the pancreatic beta-cell in amounts commensurate to satisfy peripheral tissue requirements. Playing a strategic role in the regulation of insulin release in the phospholipase C (PLC)/protein kinase C (PKC) transduction system. Islets of Langerhans contain the three major isozymes of PLC(beta1, gamma1 and delta1) and the calcium- dependent PKC isozyme, PKCalpha. Species differences in the content and activation of PLC and PKC have been identified and these differences play a major role in regulating the response to glucose. For example, rat islets contain 5 times as much PLCdelta1 as do mouse islets and in response to glucose stimulation inositol phosphates (IPs), a marker for PLC activation, increase 400%. The activation of PLC in rat islets is paralleled by a rising 30-fold increase in second phase release. In contrast, mouse islet responses to glucose are notable for the minimal activation of PLC and the small, flat second phase insulin secretory response. Rat and human islets, but not mouse islets, can be sensitized by prior short term exposure to glucose, a process termed time-dependent potentiation (TDP), or they can be desensitized by long term exposure to glucose, referred to as glucose toxicity or time-dependent suppression (TDS). The immunity of mouse islets to these multiple effects of glucose is a consequence of glucose's inability to activate PLC: significant increases in PLC activation, second phase insulin secretion and the induction of both TDP and TDS result when mouse islets are stimulated with carbachol, an agonist that significantly activates an isozyme of PLC distinct from that activated by glucose. These and other observations have led us to conclude that PLC/PKC activation regulates physiologic insulin secretion and that disordered PLC/PKC signaling culminates in disordered insulin secretion. Experiments described critically test this concept using rat and mouse islets. We will also treat cultured rat or mouse islets with adenoviral vectors containing the cDNAs for the isozymes of PLC and to determine if the increased expression and subsequent activation of these enzymes reestablishes sensitivity to glucose. Finally, the impact of the regulated over- expression of the major PLC isozymes and PKCalpha (using a reverse tetracycline-controlled transactivator system) on in vivo glucose homeostasis and in vitro sensitivity to stimulation will be determined in transgenic mice. These studies will define the role of PLC/PKC activation in regulating islet response patterns to glucose stimulation and they may facilitate the development of genetically-engineered cell lines designed to replace missing of defective beta-cells.
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