TRPV2 channels mediate cell swelling-induced insulin secretion in mouse pancreatic β-cells

抄録

Membrane depolarization of β-cells induced by glucose, which is a consequence of glucose metabolism, ATP generation, and closure of ATP-sensitive K⁺ (K_ATP) channels, is of prime importance for glucose-stimulated insulin secretion. However, the involvement of K_ATP channel-independent mechanisms in glucose-stimulated insulin secretion has also been suggested. Swelling of β-cells has been proposed to be a candidate for one of the K_ATP channel-independent mechanisms at high glucose concentrations, since exposure of pancreatic β-cells to high-concentration glucose leads to an increase in β-cell volume and hypotonic β-cell swelling induces insulin secretion. Our previous study has suggested that nonselective cation channels are involved in the insulin secretion induced by hypotonicity in rat pancreatic β-cells. Thus, in the present study, we aimed to identify the molecular entity of the cation channels involved in the cell swelling-induced insulin secretion and further investigated the mechanism for the response in mouse pancreatic β-cells. In isolated mouse β-cells, 30% hypotonic stimulation caused cell swelling and increased intercellular Ca²⁺ concentration ([Ca²⁺]_i) at 2.8 mM glucose. The hypotonicity-induced [Ca²⁺]_i elevation was partially inhibited by ruthenium red, which blocks TRPV channels, and by nicardipine, a voltage-dependent Ca²⁺ channel (VDCC) blocker. Furthermore, the hypotonic stimulation induced a significant increase in insulin secretion from isolated mouse islets at 2.8 mM glucose, which was significantly reduced by ruthenium red. Among TRPV isoforms, expression of TRPV2 and TRPV4 was confirmed in mouse β-cell and in the β-cell line MIN6 by RT-PCR, western blotting, and immunohistochemistry. 4α-Phorbol-12,13 didecanoate or GSK1016790A, TRPV4 channel activators, had no apparent effect on [Ca2+]_i at 2.8 mM glucose in mouse β-cells as well as in MIN6 cells. In contrast, probenecid, which activates TRPV2 channels, induced an increase in [Ca²⁺]_i at 2.8 mM glucose in MIN6 cells, which was attenuated by ruthenium red. Moreover, the [Ca²⁺]_i elevation induced by the hypotonic stimulation was significantly reduced by tranilast, which blocks TRPV2 channels, and by knockdown of TRPV2 with siRNA. These results suggest that in mouse β-cells, TRPV2 channels are activated by hypotonic stimulation and, at least in part, mediate cell swelling-induced membrane depolarization, thereby causing insulin secretion.