Triiodothyronine (T3)-mediated toxicity and induction of apoptosis in insulin-producing INS-1 cells.

Thyroid hormones reduce glucose tolerance in humans and animals. This effect is related to a decrease of glucose-induced insulin secretion following a reduction of pancreatic beta cell mass due to beta cell loss. The aim of this study was to analyze in vitro the mechanisms underlying the effects of triiodothyronine (T(3)) on the cell viability and cell cycle caused by changes of cell death or proliferation rate of insulin-producing INS-1 cells. 72-h Exposure of INS-1 cells to increasing T(3) concentrations up to 500 microM resulted in a significant viability reduction. This T(3) toxicity was caused by an increased apoptotic cell death rate, which was accompanied by a decreased proliferation rate. Inhibitory effects of T(3) on glucose-induced insulin secretion were already seen after 24 h of incubation, indicating that the deleterious effects of T(3) were time-dependent, changing from specific cellular dysfunctions to a severe and extended disturbance of the cellular survival program. Only T(3) concentrations higher than 250 microM were able to decrease cell viability and proliferation rate, to increase the rate of apoptosis and to reduce glucose-induced insulin secretion. These micromolar T(3) concentrations were significantly higher than the concentration range of T(3) receptor binding, indicating that other non-receptor-mediated mechanisms beyond the receptor level must be responsible for the observed toxic effects of T(3) in vitro.

Role of Na/Ca exchange and the plasma membrane Ca2+-ATPase in beta cell function and death.

Recent progresses concerning the Na/Ca exchanger (NCX) and the plasma membrane Ca2+-ATPase (PMCA) in the pancreatic beta cell are reviewed. The rat beta cell expresses two splice variants of NCX1 and six splice variants of the 4 PMCA isoforms. At the protein level, the most abundant forms are PMCA2 and PMCA3, providing the first evidence for the presence of these two isoforms in a non-neuronal tissue. Overexpression of NCX1 in an insulinoma cell line altered the initial rise in cytosolic-free Ca2+ concentration ([Ca2+]i) induced by membrane depolarization and the return of the [Ca2+]i to the baseline value on membrane repolarization, indicating that NCX contributes to both Ca2+ inflow and outflow in the beta cell. In contrast, overexpression of the PMCA markedly reduced the global rise in Ca2+ induced by membrane depolarization, indicating that the PMCA has a capacity higher than expected to extrude Ca2+. Glucose, the main physiological stimulus of insulin release from the beta cell, has opposite effect on NCX and PMCA transcription, expression and activity, inducing an increase in the case of NCX and a decrease in the case of the PMCA. This indicates that when exposed to glucose, the beta cell switches from a low-efficiency Ca2+ extruding mechanism, the PMCA, to a high-capacity system, the NCX, in order to better face the increase in Ca2+ inflow induced by the sugar. To our knowledge, this is the first demonstration of a reciprocal change in PMCA and NCX1 expression and activity in response to a given stimulus in any tissue.

Propionate inhibits glucose-induced insulin secretion in isolated rat pancreatic islets.

Dietary fibers, probably by generating short chain fatty acids (SCFA) through enterobacterial fermentation, have a beneficial effect on the control of glycemia in patients with peripheral insulin resistance. We studied the effect of propionate on glucose-induced insulin secretion in isolated rat pancreatic islets. Evidence is presented that propionate, one of the major SCFA produced in the gut, inhibits insulin secretion induced by high glucose concentrations (11.1 and 16.7 mM) in incubated and perfused pancreatic islets. This short chain fatty acid reduces [U-(14)C]-glucose decarboxylation and raises the conversion of glucose to lactate. Propionate causes a significant decrease of both [1-(14)C]- (84%) and [2-(14)C]-pyruvate (49%) decarboxylation. These findings indicate pyruvate dehydrogenase as the major site for the propionate effect. These observations led us to postulate that the reduction in glucose oxidation and the consequent decrease in the ATP/ADP ratio may be the major mechanism for the lower insulin secretion to glucose stimulus induced by propionate.

Opposite effects of glucose on plasma membrane Ca2+-ATPase and Na/Ca exchanger transcription, expression, and activity in rat pancreatic beta-cells.

When stimulated by glucose the pancreatic beta-cell displays large oscillations of the intracellular free Ca2+concentration, resulting from intermittent Ca2+ entry from the outside and outflow from the inside, the latter process being mediated by the plasma membrane Ca2+-ATPase (PMCA) and the Na+/Ca2+ exchanger (NCX). To understand the respective role of these two mechanisms, we studied the effect of glucose on PMCA and NCX transcription, expression, and activity in rat pancreatic islet cells. Glucose (11.1 and 22.2 mm) induced a parallel decrease in PMCA transcription, expression, and activity. In contrast the sugar induced a parallel increase in NCX transcription, expression, and activity. The effects of the sugar were mimicked by the metabolizable insulin secretagogue alpha-ketoisocaproate and persisted in the presence of the Ca2+-channel blocker nifedipine. The above results are compatible with the view that, when stimulated, the beta-cell switches from a low efficiency Ca2+-extruding mechanism, the PMCA, to a high capacity system, the Na/Ca exchanger, to better face the increase in Ca2+ inflow. These effects of glucose do not result from a direct effect of the sugar itself and are not mediated by the increase in intracellular free Ca2+ concentration induced by the sugar.