MITOGENIC SIGNAL TRANSDUCTION IN PANCREATIC BETA CELLS

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DESCRIPTION (taken from the application) Although certain nutrients and growth factors have been shown to increase Beta-cell proliferation, the signaling pathways behind regulation of Beta- cell mitogenesis are not well defined. The intention of this proposal is to better characterize mitogenic signal transduction pathways in pancreatic Beta-cells in detail, to pin point a key signaling element(s) required to evoke Beta-cell growth. Then it will be examined if adenoviral mediated expression of that key signaling element(s) in primary Beta-cells can instigate a significant expansion of Beta-cells in vitro. It is beyond the scope of a single proposal to investigate all potential mitogenic pathways in Beta-cells. Thus, the focus as indicated from preliminary studies, will be centered on glucose, IGF-1, and growth hormone (GH) induced Beta-cell growth as mediated via insulin-receptor substrate (IRS) signal transduction pathways. It has been found that glucose induces Beta- cells proliferation only in the physiologically relevant range (5-20mM), and IGF-1 and growth hormone (GH) induced Beta-cell growth are glucose- dependent. This reaffirms that most signaling pathways in Beta-cells are uniquely linked to glycolytic metabolism. As such, it will be important to elucidate what secondary signal(s) emanating from glucose metabolism provide a platform necessary for an IGF-1/GH mitogenic response. An initial study of signal transduction pathways has shown that glucose and IGF-1 can activate IRS-mediated signaling pathways in Beta-cells, but only signaling via activation of phosphatidylinositol-3-kinase (PI3'-K) and p70S6K correlated with glucose/IGF-1 induced Beta-cell growth. Glucose-dependent GH-induced Beta-cell proliferation also requires IRS- mediated activation of P13'-K, but factors independent of IGF-1 are required, since the combination of IGF-1 and GH synergistically increased Beta-cell proliferation (more than 80-fold above basal 3mM glucose). It follows that IRS-mitogenic signaling pathways in Beta-cells are likely complex, underscored by the observation that IRS-1, -2, -3, and -4 are all expressed in Beta-cells. Thus, a more detailed study is required to better define glucose-, and glucose-dependent IGF-1/GH-induced Beta-cell growth. Recently, we have generated recombinant adenoviruses to express IRS-1 and -2, are in the process of generating those to IRS-3 and -4 and have obtained via collaboration adenoviruses to express constitutively active PI3'-K. Such experimental tools will benefit the proposed studies for a in depth characterization of IRS-mediated Beta-cell mitogenesis. Moreover, it is intended to examine whether over-expression of IRS-1, -2, -3, -4, and/or PI3'-K in primary rat islet Beta-cells can induce significant Beta- cell growth. If this is achieved, then it will be important to characterize such an expanded Beta-cell population in terms of (pro)insulin production and bone fide regulated insulin secretion. It is anticipated that these studies may eventually lead to a means of inducing significant Beta-cell growth that will provide a meaningful source Beta- cell suitable for a novel Beta-cell replacement therapy for diabetes.