Shared Metabolic Pathways in Fuel-Stimulated Insulin Secretion
Insulin secretion is a fundamental process of pancreatic beta-cells required for the maintenance of glucose homeostasis. Fuel-stimulated insulin secretion occurs in proportion to the rate of metabolism of fuel substrates, yet the signals generated by metabolism of these secretagogues are incompletely understood. The increased burden placed on the beta-cell in conditions of obesity and insulin resistance often leads to dysregulation of stimulous-secretion coupling. Therefore, better understanding of the metabolic events required for insulin release is likely to be helpful in development of more effective treatments for diabetes.
Previous work in our lab revealed a critical role for the pyruvate-isocitrate cycling pathway in glucose-stimulated insulin secretion. It has been our hypothesis that this series of reactions plays a unique role in the beta-cell, and may be responsible for the generation of second-messenger signals critical for insulin secretion in response to increased fuel metabolism. One of the intermediates in the pyruvate/isocitrate cycle is cytosolic 2-oxoglutarate (2OG). In an effort to better understand the components of the pyruvate/isocitrate cycle and the signals that it generates, we initially focused our studies on the transporter protein responsible for the return of 2OG to the mitochondria, the 2-oxoglutarate carrier (OGC).
OGC was overexpressed in rat insulinoma 832/13 beta-cells and suppressed in both 832/13 cells and islets, and effects on metabolism and insulin secretion were measured. While overexpression of the OGC failed to alter insulin secretion, its siRNA-mediated suppression resulted in decreased insulin secretion in response to glucose, glutamine + BCH, and dimethyl-2-oxoglutarate. Suppression of OGC did not affect core pathways of fuel metabolism such as glucose usage, glucose oxidation or ATP production during glucose-stimulated insulin secretion (GSIS) or glutamine oxidation or ATP production during amino acid-stimulated insulin secretion (AASIS). Similar to previous findings, glucose-induced NADPH production was determined to be decreased in response to OGC suppression, whereas NADPH production during AASIS in untreated cells was already much lower than for GSIS, and suppression of OGC failed to decrease NADPH further.
As an additional approach to studying the role of 2OG metabolism in insulin secretion, we also investigated the mitochondrial enzyme glutamate dehydrogenase (Glud1). Overexpression of wild-type Glud1 failed to alter insulin secretion in 832/13 cells or in islets; however, suppression of Glud1 decreased both GSIS and AASIS, but did not affect dimethyl-2OG-stimulated insulin secretion. The reduction in AASIS was most likely the result of reduced glutamine oxidation. In contrast, during GSIS, NADPH production was decrease by Glud1 suppression, similar to our observation with the OGC.
In summary, these data expand our understanding of the metabolic pathways necessary for insulin secretion, and support the idea of a common metabolic pathway required for fuel-stimulated insulin release, including flux through the OGC, Glud1, and ICDc. However, while these data support the hypothesis that NADPH production is necessary for robust GSIS, it plays a less-prominent role during AASIS, and most likely works in concert with additional coupling-factors and signals.
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