Islet amyloid is a pathological characteristic of type 2 diabetes, contributing to the ?-cell loss and secretory dysfunction that characterize the disease. The unique peptide constituent of these amyloid deposits is the ?- cell secretory product islet amyloid polypeptide (IAPP), which aggregates to form amyloid resulting in ?-cell apoptosis. Human IAPP (hIAPP) aggregation is associated with mitochondrial dysfunction and oxidative stress, but the mechanism by which formation of islet amyloid results in mitochondrial dysfunction remains unclear. Cholesterol accumulation in the ?-cell is also associated with cellular loss and secretory dysfunction; however, the mechanism by which cholesterol accumulation is deleterious also remains elusive. Steroidogenic Acute Regulatory Protein (StAR) transports cholesterol from the outer to the inner mitochondrial membrane for subsequent metabolism. In classical steroidogenic tissues, cholesterol metabolism by cleavage enzymes (CYPs) results in production of glucocorticoids, mineralocorticoids and sex steroids. In non-classical steroidogenic tissues, StAR is vital to the production of bile acids, glucocorticoids, neurosteroids and oxysterols that either protect or harm the cell. We have established StAR to be present in ?-cells and specifically upregulated under conditions of amyloid formation. This upregulation of StAR in ?-cells resulted in increased mitochondrial cholesterol, decreased mitochondrial function and reduced cell viability. We also found that CYP27A1, which is involved in oxysterol production, was down regulated with islet amyloid formation, while CYP11A1 and other downstream CYPs responsible for steroid production were not present in islets. Finally, while expression of StAR is known to be regulated acutely by the transcription factor CREB (cAMP response element binding protein), we found CREB activity to be increased chronically along with StAR under amyloid forming conditions. Based on our preliminary data, we hypothesize that islet amyloid formation induces StAR expression in a CREB-dependent manner, resulting in increased transport of cholesterol into mitochondria that in turn leads to mitochondrial dysfunction, ?-cell dysfunction and ?-cell loss. This hypothesis will be addressed in the following three specific aims, each of which is intended to answer important questions.
Specific Aim 1 : To determine the role of StAR in mediating the toxic effects of increased ?-cell cholesterol in vitro.
This aim will answer the following questions: a) Does StAR knockdown under conditions of cholesterol loading protect hIAPP islets from amyloid-induced toxicity? b) Does StAR overexpression impair mitochondrial function and result in ?-cell dysfunction and loss? c) Can overexpression of CYP27A1 protect islets from the detrimental effects of StAR on mitochondrial function under amyloid-forming conditions? Specific Aim 2: To determine whether long-term reduction of StAR expression in vivo ameliorates the detrimental effects of islet amyloid deposition on ?-cell mass and function.
This aim will answer the following question: Does selective ?-cell StAR deficiency decrease ?-cell loss and dysfunction under amyloid forming conditions in vivo? Specific Aim 3: To determine whether sustained activation of CREB and CRTC2 is required for islet amyloid- induced upregulation of StAR.
This aim will answer the following questions: a) Is CREB required for amyloid- induced upregulation of StAR? b) Is CRTC2 necessary and/or sufficient for CREB?s action to increase StAR expression in islets? c) Does CREB directly bind the StAR promoter under amyloid forming conditions? These studies will provide novel data regarding the role of StAR in amyloid-induced mitochondrial dysfunction and ?-cell toxicity, and thereby potentially identify new ?-cell targets for treating type 2 diabetes.
Islet amyloid formation results in mitochondrial dysfunction leading to ?-cell death and secretory dysfunction in type 2 diabetes. Concentrations of the cholesterol transport protein Steroidogenic Acute Regulatory Protein (StAR) in the islet increase with amyloid formation, and increase cholesterol transport to mitochondria. The lack of cleavage enzymes (CYPs) in the ?-cell means that the subsequent metabolism of cholesterol will not occur, thereby leading to cholesterol accumulation, mitochondrial dysfunction and subsequently ?-cell death and secretory dysfunction. By advancing our understanding of the role of StAR in ?-cell cytotoxicity, the project endeavors to develop new approaches to preserve ?-cells and thereby prevent or treat type 2 diabetes. Given the burden of type 2 diabetes amongst Veterans, developing new approaches to reduce the morbidity and mortality of the disease is critical.
