Abstract
Inhibition of glucagon hypersecretion from pancreatic α-cells is an appealing strategy for the treatment of diabetes. Our hypothesis is that proteins that associate with glucagon within alpha cell secretory granules will regulate glucagon secretion, and may provide druggable targets for controlling abnormal glucagon secretion in diabetes. Recently, we identified a dynamic glucagon interactome within the secretory granules of the α cell line, αTC1-6, and showed that select proteins within the interactome could modulate glucagon secretion. In the present study, we show that one of these interactome proteins, the neuronal protein stathmin-2, is expressed in αTC1-6 cells and in mouse pancreatic alpha cells, and is a novel regulator of glucagon secretion. The secretion of both glucagon and Stmn2 was significantly enhanced in response to 55 mM K+, and immunofluorescence confocal microscopy showed co-localization of stathmin-2 with glucagon and the secretory granule markers chromogranin A and VAMP-2 in αTC1-6 cells. In mouse pancreatic islets, Stathmin-2 co-localized with glucagon, but not with insulin, and co-localized with secretory pathway markers. To show a function for stathmin-2 in regulating glucagon secretion, we showed that siRNA—mediated depletion of stathmin-2 in αTC1-6 cells caused glucagon secretion to become constitutive without any effect on proglucagon mRNA levels, while overexpression of stathmin-2 completely abolished both basal and K+-stimulated glucagon secretion. Overexpression of stathmin-2 increased the localization of glucagon into the endosomal-lysosomal compartment, while depletion of stathmin-2 reduced the endosomal localization of glucagon. Therefore, we describe stathmin-2 as having a novel role as an alpha cell secretory granule protein that modulates glucagon secretion via trafficking through the endosomal-lysosomal system. These findings describe a potential new pathway for the regulation of glucagon secretion, and may have implications for controlling glucagon hypersecretion in diabetes.
Highlights
Hyperglucagonemia is a characteristic sign of diabetes, causing fasting hyperglycemia and glycemic volatility
To show a function for stathmin-2 in regulating glucagon secretion, we showed that siRNA - mediated depletion of stathmin-2 in aTC1-6 cells caused glucagon secretion to become constitutive without any effect on proglucagon mRNA levels, while overexpression of stathmin-2 completely abolished both basal and
Stathmin-2 was co-secreted with glucagon in response to 55 mM K+, and Hyperglucagonemia is a characteristic sign of diabetes, causing fasting hyperglycemia and glycemic volatility
Summary
Hyperglucagonemia is a characteristic sign of diabetes, causing fasting hyperglycemia and glycemic volatility. Persistent hyperglucagonemia may exacerbate abnormal glucose metabolism in patients with type 2 diabetes and lead to metabolic disturbances in obese and prediabetic individuals (2). Controlling this excess glucagon secretion may be a potential therapeutic strategy for diabetes (3) so that glycemia and glucose metabolism may be better regulated. Such an approach has been suggested as a priority for the treatment of diabetes (1). Inhibiting glucagon secretion, rather than blocking the glucagon receptor, may be a more appropriate therapeutic approach for the treatment of hyperglucagonemia of diabetes (7)
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