Inhibitory effect of UDP-glucose on cAMP generation and insulin secretion.

Type-2 diabetes (T2D) is a global disease caused by the inability of pancreatic ß-cells to secrete adequate insulin. However, the molecular mechanisms underlying the failure of ß-cells to respond to glucose in T2D remains unknown. Here, we investigated the relative contribution of UDP-glucose (UDP-G), a P2Y14-specific agonist, in the regulation of insulin release using human isolated pancreatic islets and INS-1 cells. P2Y14 was expressed in both human and rodent pancreatic ß-cells. Dose-dependent activation of P2Y14 by UDP-G suppressed glucose-stimulated insulin secretion (GSIS) and knockdown of P2Y14 abolished the UDP-G effect. 12-h pretreatment of human islets with pertussis-toxin (PTX) improved GSIS and prevented the inhibitory effect of UDP-G on GSIS. UDP-G on GSIS suppression was associated with suppression of cAMP in INS-1 cells. UDP-G decreased the reductive capacity of nondiabetic human islets cultured at 5 mm glucose for 72 h and exacerbated the negative effect of 20 mm glucose on the cell viability during culture period. T2D donor islets displayed a lower reductive capacity when cultured at 5 mm glucose for 72 h that was further decreased in the presence of 20 mm glucose and UDP-G. Presence of a nonmetabolizable cAMP analog during culture period counteracted the effect of glucose and UDP-G. Islet cultures at 20 mm glucose increased apoptosis, which was further amplified when UDP-G was present. UDP-G modulated glucose-induced proliferation of INS-1 cells. The data provide intriguing evidence for P2Y14 and UDP-G's role in the regulation of pancreatic ß-cell function.

A comparative analysis of human and mouse islet G-protein coupled receptor expression.

G-protein coupled receptors (GPCRs) are essential for islet function, but most studies use rodent islets due to limited human islet availability. We have systematically compared the GPCR mRNA expression in human and mouse islets to determine to what extent mouse islets can be used as surrogates for human islets to study islet GPCR function, and we have identified species-specific expression of several GPCRs. The A3 receptor (ADORA3) was expressed only in mouse islets and the A3 agonist MRS 5698 inhibited glucose-induced insulin secretion from mouse islets, with no effect on human islets. Similarly, mRNAs encoding the galanin receptors GAL1 (GALR1), GAL2 (GALR2) and GAL3 GALR3) were abundantly expressed in mouse islets but present only at low levels in human islets, so that it reads (GALR3) and galanin inhibited insulin secretion only from mouse islets. Conversely, the sst1 receptor (SSTR1) was abundant only in human islets and its selective activation by CH 275 inhibited insulin secretion from human islets, with no effect on mouse islets. Our comprehensive human and mouse islet GPCR atlas has demonstrated that species differences do exist in islet GPCR expression and function, which are likely to impact on the translatability of mouse studies to the human context.

Absence of adenosine A1 receptors unmasks pulses of insulin release and prolongs those of glucagon and somatostatin.

AIMS: Extracellular ATP modulates pulsatile release of insulin, glucagon and somatostatin by activating P2Y(1) receptors. The present study examines if adenosine via A(1) receptors (A(1)R) interferes with pulsatile islet hormone release. MAIN METHODS: Pancreas was perfused in mice expressing or lacking the A(1) receptor and the hormones measured with radioimmunoassay. Cytoplasmic Ca(2+) was recorded in isolated beta-cells using the fura-2 indicator. KEY FINDINGS: Addition of 10 microM adenosine removed the Ca(2+) transients supposed to coordinate the insulin release pulses. This effect of adenosine was counteracted by 100 nM of the A(1)R antagonist DPCPX. In situ perfusion of the pancreas indicated two phases of islet hormone release when glucose was raised from 3.3 to 16.7 mM. The first phase was characterized by a brief dip followed by a peak, which was more pronounced for insulin and somatostatin than for glucagon. The second phase was markedly affected by knock out of A(1)R. The wild-type A(1)R (+/+) mice, usually lacked statistically verified insulin pulses but generated antisynchronous glucagon and somatostatin pulses with half-widths of 4 min. In the A(1)R (-/-) mice time-average release of insulin during the second phase was almost three times higher than in the controls and 30% of the hormone was released as distinct pulses with half-widths of 3 min. The absence of the A(1)R receptor resulted in 50% prolongation of the pulse cycles of glucagon and somatostatin and loss of their antisynchronous relationship. SIGNIFICANCE: The A(1)R receptor is important both for the amplitude (insulin) and duration (glucagon and somatostatin) of islet hormone pulses.

Uridine diphosphate (UDP) stimulates insulin secretion by activation of P2Y6 receptors.

We examined the transcriptional expression and functional effects of receptors for the extracellular pyrimidines uridine triphosphate (UTP) and uridine diphosphate (UDP), on insulin and glucagon secretion in isolated mouse pancreatic islets and purified beta-cells. Using real-time PCR, the UDP receptor P2Y(6) was found to be highly expressed in both whole islets and beta-cells purified by repeated counter-flow elutriation, whereas no mRNA expression for UTP receptors P2Y(4) and P2Y(2) could be detected. Functional in vitro experiments revealed that the P2Y(6) agonist UDPbetaS dose-dependently enhanced insulin and glucagon release during short-term incubation (1h), while P2Y(6) activation during a longer period (24h), selectively increased insulin release, especially at high glucose levels. The corresponding EC(50) value for UDPbetaS ranged from 3.2 x 10(-8)M to 1.6 x 10(-8)M for both glucose concentrations. The P2Y(6) antagonist MRS2578 inhibited the effects of UDPbetaS, supporting a P2Y(6) specific effect. In addition to negative RT-PCR results, the lack of response to UTPgammaS a selective P2Y(2/4) agonist further rule out the involvement of P2Y(2/4) receptors in the islet hormone release. Our results suggest a modulatory role for UDP via a functional active P2Y(6) receptor in the regulation of islet hormone release.