A role for PKD1 in insulin secretion downstream of P2Y1 receptor activation in mouse and human islets

Along with insulin, beta-cells co-secrete the neurotransmitter ATP which acts as a positive autocrine signal via P2Y(1) receptors to activate phospholipase C and increase the production of diacylglycerol (DAG). However, the downstream signaling that couples P2Y(1) activation to insulin secretion remains to be fully elucidated. Since DAG activates protein kinase D1 (PKD1) to potentiate glucose-stimulated insulin release, we hypothesized that autocrine ATP signaling activates downstream PKD1 to regulate insulin secretion. Indeed, we find that the P2Y(1) receptor agonists, MRS2365 and ATP induce, PKD1 phosphorylation at serine 916 in mouse islets. Similarly, direct depolarization of islets by KCl caused PKD1 activation, which is reduced upon P2Y(1) antagonism. Potentiation of insulin secretion by P2Y(1) activation was lost from PKD1(-/-) mouse islets, and knockdown of PKD1 reduced the ability of P2Y(1) activation to facilitate exocytosis in single mouse beta-cells. Finally, qPCR analysis confirmed PKD1 transcript (PRKD1) expression in human islets, and insulin secretion assays showed that inhibition of either P2Y(1) or PKD1 signaling impaired glucose-stimulated insulin secretion. Human islets showed donor-to-donor variation in their responses to both P2Y(1) and PKD1 inhibition, however, and we find that the P2Y(1)-PKD1 pathway contributes a substantially greater proportion of insulin secretion from islets of overweight and obese donors. Thus, PKD1 promotes increased insulin secretion, likely mediating an autocrine ATP effect via P2Y(1) receptor activation which may be more important in islets of donors who are overweight or obese.