We have previously shown that hepatocyte growth factor (HGF) has beneficial effects in beta cell proliferation, function and islet transplantation. More recently, we have deciphered a dual role of HGF in beta cell survival. On one hand, HGF protects beta cells in situations of hypoxia, nutrient deprivation, streptozotocin-mediated cytotoxicity and cytokine-induced cell death. On the other hand, HGF potentiates beta cell death induced by gluco-lipoxicity in vitro. Metabolically, HGF decreases fatty acid oxidation and enhances ceramide content in beta cells. In terms of signaling, HGF-mediated activation of the beta cell pro-survival signal Akt is abolished in the presence of a gluco-lipotoxic insult. Circulating HGF levels are markedly high in obesity. Taken together, these results indicate that HGF might participate in beta cell failure in obesity/Type 2 diabetes in vivo. To address this question, in Specific Aim 1 we will characterize the functional consequences of disrupting HGF/c-met signaling in the pancreatic beta cell in vivo in obese/Type 2 diabetes conditions. Alterations in functional p53 levels often lead to increased apoptosis in many cell types. However, whether p53 has any regulatory role in directly controlling pancreatic beta cell death in obesity/Type 2 diabetes is unknown. Preliminary data from our lab clearly indicate that (i) gluco-lipotoxicity upregulates p53 expression and activation in beta cells;(ii) inhibition of p53 transactivation with the specific p53 inhibitor pifithrin-1 blocks gluco- lipotoxicity-mediated beta cell apoptosis;(iii) p53-null mouse beta cells are more resistant than wild-type beta cells to gluco-lipotoxicity-induced apoptosis;and, (iv) transgenic mice overexpressing p53 in the beta cell display glucose intolerance. Importantly, a recent comprehensive association study of obesity/type 2 diabetes and related quantitative traits has identified a single nucleotide polymorphism variant (Arg72Pro) in the TP53 gene in obese/Type 2 diabetic patients. Arg72-p53 variant has a higher apoptotic potential possibly through increased localization in the mitochondria. Taken together, these studies clearly highlight the potential link between alterations in p53 expression/activation/localization and pancreatic beta cell apoptosis, impaired insulin secretion, gluco-lipotoxicity and obesity/Type 2 diabetes. To address this point, in Specific Aim 2, we will decipher the functional consequences of gain-of-function, loss-of-function and the 72Arg variant of p53 in the beta cell in vivo under basal and obesity conditions.
In Specific Aim 3, we will analyze the relevance of p53 in mediating human beta cell apoptosis and the mechanisms involved in p53-induced beta cell apoptosis in a gluco-lipotoxic environment in vitro. The proposed studies in rodent and human beta cells in vitro and in mice in vivo will provide valuable information to identify pathways to protect beta cells against dysfunction and death in situations of obesity- mediated Type 2 diabetes.
Studies in the current proposal will decipher the role of HGF and p53 in obesity/Type 2 diabetes conditions. These studies can provide new cues on how to protect and enhance beta cell mass in diabetes and they can lead to future therapeutic strategies for the treatment of diabetic patients.
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