Glucose/Secretagogue Metabolism in Pancreatic Islets

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DESCRIPTION (provided by applicant): The broad long-term objective is to obtain new information about the metabolic signals for insulin secretion. The immediate purpose of this project is to gain a better understanding of the role of mitochondrial biosynthetic pathways in supporting insulin secretion. Glucose, the most potent insulin secretagogue, and all other metabolizable secretagogues, stimulate insulin secretion via their metabolism in mitochondria. Our earlier work showed that one-half of glucose-derived pyruvate enters mitochondrial metabolism via carboxylation catalyzed by pyruvate carboxylase and one-half enters via decarboxylation. From the resulting oxaloacetate from carboxylation and acetyl-CoA from decarboxylation of pyruvate, any citric acid cycle intermediate can be synthesized. Recent work showed the presence of intra- and extra-mitochondrial enzymes, including succinyl- CoA: 3-ketoacid-CoA transferase (SCOT), that catalyze the synthesis and utilization of mitochondrial products. SCOT can form acetoacetate from all insulin secretagogues. Thus acetoacetate, in addition to citric acid cycle intermediates, can transfer carbon to the cytosol for the synthesis of short chain acyl-CoAs, lipids and other factors. Aim 1A is to study the pathways in mitochondria of formation of molecules that are exported to the cytosol for the synthesis of the compounds in the cytosol that support or signal insulin secretion. Aim 1B is to study the extramitochondrial utilization of these compounds. Cell lines with knocked down pyruvate carboxylase, SCOT, acetoacetyl-CoA synthetase, cytosolic malic enzyme, fatty acid synthase and other enzymes; as well as enzyme assays, including an assay for mitochondrial malic enzyme, will facilitate this work. Recent work has shown the beta cell is a lipogenic tissue. Lipids in cells and subcellular fractions are measured with gas chromatography. Mass spectrometry is used to measure short chain acyl-CoAs and to identify small molecules and lipids that change with secretagogue stimulation. Aim 2 is to discern the relevance of mitochondrial biosynthesis to normal and abnormal insulin secretion in humans. Aim 2A will further study the surprising observation that pathways of anaplerosis that are very active in rodent beta cells are far less active in human islets. We have accumulated conclusive evidence that pyruvate carboxylase, the major enzyme of anaplerosis, is 90% lower in normal human islets than in islets of rodents and clonal beta cell lines. Thus, the human beta cell may be programmed to more heavily use alternative pathways to pyruvate carboxylase for mitochondrial synthesis. The regulation of pyruvate carboxylase expression and rate of pyruvate carboxylation in human islets will be studied. Aim 2B is to continue to study intra- and extra- mitochondrial metabolic enzymes that are decreased in islets of rodents and humans with type 2 diabetes and factors that might regulate their expression.