Physiological Role of Vesicular Nucleotide Transporter in Regulating Pancreatic Beta Cell Function

Extracellular ATP is critical in regulating insulin secretion in pancreatic β cells. ATP released from insulin secretory vesicles is a major source of extracellular ATP; however, the mechanism by which ATP accumulates into these vesicles has remained elusive. We identified the expression of vesicular nucleotide transporter (VNUT) on insulin secretory granules and showed it is responsible for transporting ATP into secretory vesicles for subsequent release. Suppressing endogenous VNUT expression in β cells by small hairpin RNA (shRNA) knockdown reduced basal and glucose-induced ATP release. Importantly, reduced VNUT expression dramatically suppressed basal and glucose-stimulated insulin secretion (GSIS) in β cells. Moreover, pharmacologic blockade of VNUT also attenuated GSIS. Exogenous ATP treatment reversed these insulin secretion defects, indicating VNUT-mediated ATP release is essential for insulin secretion. Overexpressing VNUT in β cells resulted in excessive ATP release and enhanced basal and glucose-stimulated insulin secretion (GSIS) via stimulation of P2X purinergic receptors. Therefore, VNUT plays an essential role in the regulation of insulin secretion through vesicular ATP release and extracellular purinergic signaling.;In addition to insulin secretion, altered VNUT expression also affected the expression of a number of genes essential for glucose sensing and insulin secretion, including glucose transporter 2 and glucokinase. This raises the possibility that VNUT plays a critical role in regulating cellular energy homeostasis by modulating the levels of ATP in a non-diffusible compartment such as insulin granules.;Finally, β cell dysfunction is a hallmark of obesity-induced Type 2 diabetes, characterized by elevated basal insulin release and blunted GSIS. In the present study, we showed that VNUT contributes to β cell dysfunction associated with overnutrition. A high fat diet (HFD) elevated VNUT expression in β cells, inducing excessive ATP release. Elevated ATP release from HFD β cells plays a dual role in modulating insulin secretion; excessive extracellular ATP elevated basal insulin secretion through purinergic receptor signaling, but impaired GSIS by activating adenosine receptors. Blocking VNUT function restored glucose sensitivity in islets from HFD mice. Therefore, VNUT plays a critical role in maintaining normal insulin secretion, while HFD-induced overexpression of VNUT contributes to β cell dysfunction.