Vesicular Nucleotide Transporter-Mediated ATP Release Regulates Insulin Secretion

Abstract

Extracellular ATP plays a critical role in regulating insulin secretion in pancreatic β 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 β cell line. Immunohistochemistry and immunofluorescence revealed that VNUT colocalized extensively with insulin secretory granules. Functional studies showed that suppressing endogenous VNUT expression in β cells by small hairpin RNA knockdown greatly reduced basal- and glucose-induced ATP release. Importantly, knocking down VNUT expression 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 β 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 β cells and plays an essential and novel role in regulating insulin secretion through vesicular ATP release and extracellular purinergic signaling.

SLC17A9 Protein Functions as a Lysosomal ATP Transporter and Regulates Cell Viability

“…SLC17A9 Localization in Lysosomes-SLC17A9 reportedly localizes in secretory vesicles/granules (3)(4)(5). Our pharmacology data obtained using isolated lysosomes suggest that lysosomal membranes contain substantial amounts of SLC17A9 protein.…”

“…SLC17A9 reportedly functions in insulin secretion through vesicular ATP release in ␤ cells (5), in exocytosis of ATP-enriched vesicles in biliary epithelial cells (25), and in exocytosis of ATP-enriched vesicles during T cell activation (4). However, the identities of the ATP-containing vesicles in these studies are unclear.…”

“…Among these, the SLC17 family mediates activities requiring transmembrane transport of organic anions, including urate secretion (SLC17A1 and SLC17A3), lysosomal export of sialic acid (SLC17A5), and vesicular accumulation of aspartate (SLC17A5) and glutamate (SLC17A5-8). The most recently described family member, SLC17A9, represents a vesicular nucleotide transporter (VNUT) and loads ATP into secretory vesicles in adrenal chromaffin cells (3), T cells (4), and zymogen granules of pancreatic cells (5).…”

Autocrine activation of P2Y1 receptors couples Ca2+ influx to Ca2+ release in human pancreatic beta cells

“…However, it has been reported that insulin granules contain similar concentrations of ATP and ADP [3], suggesting that ] i , and the magnitude of ATP-release events was increased by glucose. This could result from intragranular ATP accumulation via granule-resident vesicular nucleotide transporter [25]. It should be noted, however, that in intact rodent islets extracellular ATP plays an important role in synchronising the electrical and Ca 2+ responses among beta cells within and between islets through the induction of Ca 2+ release from InsP 3 -sensitive stores [26][27][28], in addition to stimulating exocytosis.…”

Hyperosmotic stress induces ATP release and changes in P2X7 receptor levels in human corneal and conjunctival epithelial cells

“…Exocytotic mechanisms involve ATP storage and Ca 2+ -regulated exocytotic release and SLC17A9 protein has been identified as a vesicular nucleotide transporter (VNUT) that plays a key role in ATP exocytosis [13]. VNUT expression has been detected in different types of cells [13][14][15][16][17][18][19]; however, its presence in corneal and conjunctival epithelial cells remains elusive.…”