Inhibition of Na/Ca exchange stimulates insulin release from isolated rat pancreatic islets

IL-1beta caused pancreatic beta-cells apoptosis is mediated in part by endoplasmic reticulum stress via the induction of endoplasmic reticulum Ca2+ release through the c-Jun N-terminal kinase pathway

Endoplasmic reticulum (ER) homeostasis is crucial for beta-cell function and survival. Direct as well as indirect evidence has pointed toward Ca(2+) as an important determinant of interleukin-1beta (IL-1beta)-induced beta-cell dysfunction and apoptosis. In the present study, we show that IL-1beta-induced apoptosis and necrosis in primary rat beta-cells and MIN6 cells largely depends on ER stress, ER Ca(2+) release, and c-Jun N-terminal kinase (JNK) activation. beta-cells also showed marked sensitivity to apoptosis induced by sarcoendoplasmic reticulum Ca(2+) ATPase (SERCA) blockers, thapsigargin and cyclopiazonic acid (CPA). IL-1beta induced ER Ca(2+) release, which was paralleled by an IL-1beta-dependent induction of JNK activation and the ER stress response, including activation of PRK (RNA-dependent protein kinase)-like ER kinase (PERK). Furthermore, reduced activation of JNK, utilizing JNK inhibitor SP600125, resulted in significant protection from IL-1beta- or thapsigargin-induced apoptosis via ER stress. In conclusion, our results suggest that the IL-1beta-induced depletion of ER Ca(2+) and activation of the ER stress via JNK pathway are potential contributory mechanisms to beta-cell death.

Cytokines downregulate the sarcoendoplasmic reticulum pump Ca2+ ATPase 2b and deplete endoplasmic reticulum Ca2+, leading to induction of endoplasmic reticulum stress in pancreatic beta-cells

Cytokines and free radicals are mediators of beta-cell death in type 1 diabetes. Under in vitro conditions, interleukin-1beta (IL-1beta) + gamma-interferon (IFN-gamma) induce nitric oxide (NO) production and apoptosis in rodent and human pancreatic beta-cells. We have previously shown, by microarray analysis of primary beta-cells, that IL-1beta + IFN-gamma decrease expression of the mRNA encoding for the sarcoendoplasmic reticulum pump Ca(2+) ATPase 2b (SERCA2b) while inducing expression of the endoplasmic reticulum stress-related and proapoptotic gene CHOP (C/EBP [CCAAT/enhancer binding protein] homologous protein). In the present study we show that cytokine-induced apoptosis and necrosis in primary rat beta-cells and INS-1E cells largely depends on NO production. IL-1beta + IFN-gamma, via NO synthesis, markedly decreased SERCA2b protein expression and depleted ER Ca(2+) stores. Of note, beta-cells showed marked sensitivity to apoptosis induced by SERCA blockers, as compared with fibroblasts. Cytokine-induced ER Ca(2+) depletion was paralleled by an NO-dependent induction of CHOP protein and activation of diverse components of the ER stress response, including activation of inositol-requiring ER-to-nucleus signal kinase 1alpha (IRE1alpha) and PRK (RNA-dependent protein kinase)-like ER kinase (PERK)/activating transcription factor 4 (ATF4), but not ATF6. In contrast, the ER stress-inducing agent thapsigargin triggered these four pathways in parallel. In conclusion, our results suggest that the IL-1beta + IFN-gamma-induced decrease in SERCA2b expression, with subsequent depletion of ER Ca(2+) and activation of the ER stress pathway, is a potential contributory mechanism to beta-cell death.

Na/Ca exchange in function, growth, and demise of beta-cells

Recent knowledge concerning the Na/Ca exchanger (NCX) in the pancreatic beta-cell is reviewed. The beta-cell expresses various NCX1 splice variants in a species-specific pattern (NCX1.3 and 1.7 in the rat; NCX1.2, 1.3, and 1.7 in the mouse) and in variable and different proportions. In the rat beta-cell, the exchanger displays a high capacity, accounts for about 70% of Ca(2+) extrusion, and participates in Ca(2+) inflow during membrane depolarization. In the mouse, however, the contribution of the exchanger to Ca(2+) extrusion is more modest, and to Ca(2+) inflow, less evident. The exchanger has a stoichiometry of 3 Na(+) for 1 Ca(2+), is electrogenic, and displays a reversal potential at -20 mV. Although being of low magnitude, the current generated by the exchanger shapes glucose-induced beta-cell electrical activity and intracellular Ca(2+) oscillations. Intracellular Ca(2+) may also trigger apoptosis. For instance, overexpression of the exchanger increases Ca(2+)-dependent and Ca(2+)-independent beta-cell death by apoptosis, a phenomenon resulting from the depletion of ER Ca(2+) stores with subsequent activation of caspase-12. Na/Ca exchange overexpression also reduces beta-cell growth. Hence, the Na/Ca exchanger is a versatile system that appears to play an important role in the function, growth, and demise of the beta-cell.

Na/Ca exchanger overexpression induces endoplasmic reticulum-related apoptosis and caspase-12 activation in insulin-releasing BRIN-BD11 cells

Ca(2+) may trigger programmed cell death (apoptosis) and regulate death-specific enzymes. Therefore, the development of strategies to control Ca(2+) homeostasis may represent a potential approach to prevent or enhance cell apoptosis. To test this hypothesis, the plasma membrane Na/Ca exchanger (NCX1.7 isoform) was stably overexpressed in insulin-secreting tumoral cells. NCX1.7 overexpression increased apoptosis induced by endoplasmic reticulum (ER) Ca(2+)-ATPase inhibitors, but not by agents increasing intracellular calcium concentration ([Ca(2+)](i)), through the opening of plasma membrane Ca(2+)-channels. NCX1.7 overexpression reduced the rise in [Ca(2+)](i) induced by all agents, depleted ER Ca(2+) stores, sensitized the cells to Ca(2+)-independent proapoptotic signaling pathways, and reduced cell proliferation by approximately 40%. ER Ca(2+) stores depletion was accompanied by the activation of the ER-specific caspase (caspase-12), and the activation was enhanced by ER Ca(2+)-ATPase inhibitors. Hence, Na/Ca exchanger overexpression, by depleting ER Ca(2+) stores, triggers the activation of caspase-12 and increases apoptotic cell death. By increasing apoptosis and decreasing cell proliferation, overexpression of Na/Ca exchanger may represent a new potential approach in cancer gene therapy. On the other hand, our results open the way to the development of new strategies to control cellular Ca(2+) homeostasis that could, on the contrary, prevent the process of apoptosis that mediates, in part, beta-cell autoimmune destruction in type 1 diabetes.

Overexpression of the Na/Ca exchanger shapes stimulus-induced cytosolic Ca(2+) oscillations in insulin-producing BRIN-BD11 cells

In response to glucose, mouse beta-cells display slow oscillations of the membrane potential and cytosolic free Ca(2+) concentration ([Ca(2+)](i)), whereas rat beta-cells display a staircase increase in these parameters. Mouse and rat islet cells differ also by their level of Na/Ca exchanger (NCX) activity. The view that the inward current generated by Na/Ca exchange shapes stimulus-induced electrical activity and [Ca(2+)](i) oscillations in pancreatic beta-cells was examined in insulin-producing BRIN-BD11 cells overexpressing the Na/Ca exchanger. BRIN-BD11 cells were stably transfected with NCX1.7, one of the exchanger isoforms identified in the beta-cell. Overexpression could be assessed at the mRNA and protein level. Appropriate targeting to the plasma membrane could be assessed by microfluorescence and the increase in Na/Ca exchange activity. In response to K(+), overexpressing cells showed a more rapid increase in [Ca(2+)](i) on membrane depolarization as well as a more rapid decrease of [Ca(2+)](i) on membrane repolarization. In response to glucose and tolbutamide, control BRIN cells showed large amplitude [Ca(2+)](i) oscillations. In contrast, overexpressing cells showed a staircase increase in [Ca(2+)](i) without such large oscillations. Diazoxide-induced membrane hyperpolarization restored large amplitude [Ca(2+)](i) oscillations in overexpressing cells. The present data confirm that Na/Ca exchange plays a significant role in the rat beta-cell [Ca(2+)](i) homeostasis, the exchanger being a versatile system allowing both Ca(2+) entry and outflow. Our data suggest that the current generated by the exchanger shapes stimulus-induced membrane potential and [Ca(2+)](i) oscillations in insulin-secreting cells, with the difference in electrical activity and [Ca(2+)](i) behavior seen in mouse and rat beta-cells resulting in part from a difference in Na/Ca exchange activity between these two cells.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Identification, expression pattern and potential activity of Na/Ca exchanger isoforms in rat pancreatic B-cells

In the pancreatic B-cell, Na/Ca exchange displays a quite high capacity and participates in the control of cytosolic free Ca2+ concentration. The Na/Ca exchanger was recently cloned in various tissues. Two genes coding for two different exchangers (NCX1 and NCX2) have been identified and evidence for several isoforms for NCX1 shown. To characterize the isoform(s) expressed in pancreatic B-cells, a RT-PCR analysis was performed on mRNA from rat pancreatic islets, purified B-cells and insulinoma B-cells (RINm5F cells). PCR amplification did not yield the expected NCX2 DNA fragment but yielded 2 NCX1 bands, corresponding to NaCa3 and NaCa7, in the three preparations. NaCa3 and NaCa7 were equally expressed in pancreatic islets and purified B-cells. In RINm5F cells, NaCa3 expression did not differ from that in islet and purified B-cells but NaCa7 was 3 times less expressed. This lower expression was accompanied by a 3 times lower Na/Ca exchange activity in RINm5F cells compared to islet cells. Our data indicate the existence of 2 NCX1 isoforms but not of NCX2 in pancreatic B-cells. The difference in both the expression patterns of NCX1 isoforms and the activity of Na/Ca exchange in islet cells and RINm5F cells is compatible with a difference in activity between NaCa3 and NaCa7.