Misrouting of glucagon and stathmin-2 towards lysosomal system of α-cells in glucagon hypersecretion of diabetes.

Abstract

Glucagon hypersecretion from the pancreatic α-cell is a characteristic sign of diabetes, which exacerbates fasting hyperglycemia. Thus, targeting glucagon secretion from α-cells may be a promising approach for combating hyperglucagonemia. We have recently identified stathmin-2 as a protein that resides in α-cell secretory granules, and showed that it regulates glucagon secretion by directing glucagon towards the endolysosomal system in αTC1-6 cells. Here, we hypothesized that disruption of Stmn2-mediated trafficking of glucagon to the endolysosomes contributes to hyperglucagonemia. In isolated islets from male mice treated with streptozotocin (STZ) to induce diabetes, Arg-stimulated secretion of glucagon and Stmn2 was augmented. However, cell glucagon content was significantly increased (p<0.001), but Stmn2 levels were reduced (p<0.01) in STZ-treated mice, as measured by both ELISA and immunofluorescence intensity. Expression of Gcg mRNA increased ~4.5 times, while Stmn2 mRNA levels did not change. Using confocal immunofluorescence microscopy, the colocalization of glucagon and Stmn2 in Lamp2A+ lysosomes was dramatically reduced (p<0.001) in islets from diabetic mice, and the colocalization of Stmn2, but not glucagon, with the late endosome marker, Rab7, significantly (p<0.01) increased. Further studies were conducted in αTC1-6 cells cultured in media containing high glucose (16.7 mM) for two weeks to mimic glucagon hypersecretion of diabetes. Surprisingly, treatment of αTC1-6 cells with the lysosomal inhibitor bafilomycin A1 reduced K+-induced glucagon secretion, suggesting that high glucose may induce glucagon secretion from another lysosomal compartment. Both glucagon and Stmn2 co-localized with Lamp1, which marks secretory lysosomes, in cells cultured in high glucose. We propose that, in addition to enhanced trafficking and secretion through the regulated secretory pathway, the hyperglucagonemia of diabetes may also be due to re-routing of glucagon from the degradative Lamp2A+ lysosome towards the secretory Lamp1+lysosome.