Vesicular Nucleotide Transporter-Mediated ATP Release Regulates Insulin Secretion

Extracellular ATP plays a critical role in regulating insulin secretion in pancreatic beta cells. The ATP released from insulin secretory vesicles has been proposed to be a major source of extracellular ATP. Currently, the mechanism by which ATP accumulates into insulin secretory granules remains elusive. In this study, the authors identified the expression of a vesicular nucleotide transporter (VNUT) in mouse pancreas, isolated mouse islets, and MIN6 cells, a mouse beta cell line. Immunohistochemistry and immunofluorescence revealed that VNUT colocalized extensively with insulin secretory granules. Functional studies showed that suppressing endogenous VNUT expression in beta cells by small hairpin RNA knockdown greatly reduced basal- and glucose-induced ATP release. Importantly, knockingdownVNUTexpression by VNUT small hairpin RNA in MIN6 cells and isolated mouse islets dramatically suppressed basal insulin release and glucose-stimulated insulin secretion (GSIS). Moreover, acute pharmacologic blockade of VNUT with Evans blue, a VNUT antagonist, greatly attenuated GSIS in a dose-dependent manner. Exogenous ATP treatment effectively reversed the insulin secretion defect induced by both VNUT knockdown and functional inhibition, indicating that VNUT-mediated ATP release is essential for maintaining normal insulin secretion. In contrast to VNUT knockdown, overexpression of VNUT in beta cells resulted in excessive ATP release and enhanced basal insulin secretion and GSIS. Elevated insulin secretion induced by VNUT overexpression was reversed by pharmacologic inhibition of P2X but not P2Y purinergic receptors. This study reveals VNUT is expressed in pancreatic beta cells and plays an essential and novel role in regulating insulin secretion through vesicular ATP release and extracellular purinergic signaling. (Endocrinology 154: 675-684, 2013)