Glucose dependence of the regulated secretory pathway in alphaTC1-6 cells.
We have investigated the effects of chronically elevated glucose concentrations on the pancreatic alpha-cell line alphaTC1-6. We show that basal glucagon secretion and proglucagon gene expression were increased in response to high glucose levels. The extent of acute stimulated secretion of glucagon was also increased in response to high glucose, as was the transcription of the prohormone processing enzymes PC1/3 and PC2. The secretion of GLP-1, a proglucagon-derived peptide produced by cleavage of proglucagon by PC1/3, was also increased in response to high glucose. Gene expression profiling experiments showed that a number of components of the regulated secretory pathway were up-regulated at high glucose concentrations, including processing enzymes and exocytotic proteins. Immunoblot analysis showed that the expression of the exocytotic SNARE proteins, as well as that of PC1/3, chromogranin A, and 7B2, were all increased after chronic exposure to high glucose levels. Immunocytochemistry showed no changes in the expression of the mature alpha-cell markers glucagon and brn-4 and no induction of the immature alpha-cell marker pdx-1. We conclude that chronically elevated glucose concentrations up-regulate the regulated secretory response of the alpha-cell.