Na+/Ca(2+ )Exchanger a Druggable Target to Promote beta -Cell Proliferation and Function

An important feature of type 2 diabetes is a decrease in <i>β</i> -cell mass. Therefore, it is essential to find new approaches to stimulate <i>β</i> -cell proliferation. We have previously shown that heterozygous inactivation of the Na <sup>+</sup> /Ca <sup>2+</sup> exchanger (isoform 1; NCX1), a protein responsible for Ca <sup>2+</sup> extrusion from cells, increases <i>β</i> -cell proliferation, mass, and function in mice. Here, we show that <i>Ncx</i> 1 inactivation also increases <i>β</i> -cell proliferation in 2-year-old mice and that NCX1 inhibition in adult mice by four small molecules of the benzoxyphenyl family stimulates <i>β</i> -cell proliferation both <i>in vitro</i> and <i>in vivo</i> . NCX1 inhibition by small interfering RNA or small molecules activates the calcineurin/nuclear factor of activated T cells (NFAT) pathway and inhibits apoptosis induced by the immunosuppressors cyclosporine A (CsA) and tacrolimus in insulin-producing cell. Moreover, NCX1 inhibition increases the expression of <i>β</i> -cell-specific genes, such as <i>Ins1, Ins2,</i> and <i>Pdx</i> 1, and inactivates/downregulates the tumor suppressors retinoblastoma protein (pRb) and miR-193a and the cell cycle inhibitor p53. Our data show that Na <sup>+</sup> /Ca <sup>2+</sup> exchange is a druggable target to stimulate <i>β</i> -cell function and proliferation. Specific <i>β</i> -cell inhibition of Na <sup>+</sup> /Ca <sup>2+</sup> exchange by phenoxybenzamyl derivatives may represent an innovative approach to promote <i>β</i> -cell regeneration in diabetes and improve the efficiency of pancreatic islet transplantation for the treatment of the disease

Emergent global oscillations in heterogeneous excitable media: The example of pancreatic beta cells

Using the standard van der Pol-FitzHugh-Nagumo excitable medium model I demonstrate a novel generic mechanism, diversity, that provokes the emergence of global oscillations from individually quiescent elements in heterogeneous excitable media. This mechanism may be operating in the mammalian pancreas, where excitable beta cells, quiescent when isolated, are found to oscillate when coupled despite the absence of a pacemaker region.Comment: See home page http://lec.ugr.es/~julya

Anti-diabetic effect of a preparation of vitamins, minerals and trace elements in diabetic rats: a gender difference

BACKGROUND: Although multivitamin products are widely used as dietary supplements to maintain health or as special medical food in certain diseases, the effects of these products were not investigated in diabetes mellitus, a major cardiovascular risk factor. Therefore, here we investigated if a preparation of different minerals, vitamins, and trace elements (MVT) for human use affects the severity of experimental diabetes. METHODS: Two days old neonatal Wistar rats from both genders were injected with 100 mg/kg of streptozotocin or its vehicle to induce diabetes. At week 4, rats were fed with an MVT preparation or vehicle for 8 weeks. Well established diagnostic parameters of diabetes, i.e. fasting blood glucose and oral glucose tolerance test were performed at week 4, 8 and 12. Moreover, serum insulin and blood HbA1c were measured at week 12. RESULTS: An impaired glucose tolerance has been found in streptozotocin-treated rats in both genders at week 4. In males, fasting blood glucose and HbA1c were significantly increased and glucose tolerance and serum insulin was decreased at week 12 in the vehicle-treated diabetic group as compared to the vehicle-treated non-diabetic group. All of the diagnostic parameters of diabetes were significantly improved by MVT treatment in male rats. In females, streptozotocin treatment resulted in a less severe prediabetic-like phenotype as only glucose tolerance and HbA1c were altered by the end of the study in the vehicle-treated diabetic group as compared to the vehicle-treated non-diabetic group. MVT treatment failed to improve the diagnostic parameters of diabetes in female streptozotocin-treated rats. CONCLUSION: This is the first demonstration that MVT significantly attenuates the progression of diabetes in male rats with chronic experimental diabetes. Moreover, we have confirmed that females are less sensitive to STZ-induced diabetes and MVT preparation did not show protection against prediabetic state. This may suggest a gender difference in the pathogenesis of diabetes

Regulation of Na+/Ca2+ exchange in the rat pancreatic B cell.

Na+/Ca2+ exchange in the B cell was recently characterized by measuring intracellular-Na(+)-dependent 45Ca2+ uptake in isolated rat pancreatic islet cells. The aim of the present study was to investigate the regulation of this process. Extracellular pH (pHo) and intracellular pH (pHi) markedly affected Na+/Ca2+ exchange. A fall of 0.04 unit in pHi decreased the exchange by 45%, whereas a rise of 0.13 unit increased the uptake by 70%. Mitochondrial poisons (oligomycin, antimycin A and 2,4-dinitrophenol) inhibited reverse Na+/Ca2+ exchange by about 25-50%. The exchanger displayed a low Q10 (temperature coefficient), indicating that it is only indirectly dependent on metabolic energy. The phorbol ester phorbol 12-myristate 13-acetate did not affect Na+/Ca2+ exchange. Likewise, lowering the extracellular K+ concentration did not inhibit 45Ca2+ uptake. In conclusion, the pHi and the metabolic state of the cell may represent important modulatory signals by which insulin secretagogues such as glucose could regulate reverse Na+/Ca2+ exchange in the B cell. The process does not appear to co-transport K+ nor to be influenced by protein kinase C

Regulation of calcium fluxes in rat pancreatic islets: dissimilar effects of glucose and of sodium ion accumulation.

1. Removal of extracellular K+ provoked a dramatic increase in 43Ca efflux from pancreatic islets prelabelled in the presence of glucose. Such an increase in 45Ca efflux was preceded by a modest and transient reduction in 45Ca efflux. The increase in 45Ca efflux was reduced when the islets were perifused either in the presence of glucose 16 . 7 mM or in the absence of extracellular Ca2+. It was abolished when the islets were perifused in the absence of extracellular Na+. 2. When the islets had been prelabelled in the absence as distinct from presence of glucose, the increase in 45Ca efflux observed on removal of extracellular K+ was of smaller amplitude. It was completely abolished when, in addition, Ca2+ was replaced by Co2+ in the perifusate. 3. Veratridine also provoked a dramatic increase in 45Ca efflux. This increase was slightly reduced when the perifusate contained glucose 5 . 6 mM and markedly reduced in the absence of extracellular Ca2+. 4. Both the removal of extracellular K+ or addition of veratridine had little or no effect on insulin release in the absence of glucose. A significant increase in insulin release was observed, however, in the presence of glucose. The increase in insulin release, due to removal of extracellular K+, was completely abolished at low extracellular Na+ concentration. Such as ionic manipulation failed to affect veratridine-induced insulin release. Tetrodotoxin failed to inhibit glucose-stimulated insulin release. 5. It is concluded that Na+ accumulation in islet cells due to either veratridine or removal of extracellular K+ provokes the release of Ca2+ from intracellular stores but fails to reproduce the effect of glucose to reduce 45Ca efflux and to stimulate insulin release

Regulation of calcium fluxes in pancreatic islets: dissociation between calcium and insulin release.

1. The release of 45calcium from prelabelled pancreatic islets is rapidly and almost totally inhibited by lanthanum. 2. Glucose provokes an intitial fall followed by a secondary rise in 45calcium efflux. The latter rise occurs concomitantly with insulin release. Its magnitude is reduced whenever the secretory response to glucose is inhibited, e.g. in the absence of extracellular calcium, presence of Verapamil, or at high magnesium concentration. 3. However, under suitable conditions, the glucose-induced secondary rise in 45calcium efflux is not totally suppressed whilst insulin release is totally abolished. 4. Inversely, when calcium is replaced by barium in the perifusate, glucose increases insulin output without causing any obvious secondary rise in 45calcium efflux. 5. It is concluded that this secondary rise, which originates from a lanthanum-nondisplaceable calcium pool, does not correspond solely to an exocytotic release of 45calcium. It could represent, in part at least, a displacement of 45calcium from cellular sites and reflect a glucose-induced increase in the rate of calcium entry in islet cells

Characterization of the 70 kDa polypeptide of the Na/Ca exchanger.

The Na/Ca exchanger is associated with 160, 120 and 70 kDa polypeptides whose nature is poorly understood. We have purified and characterized the Na/Ca exchanger from bovine cardiac sarcolemmal vesicles (SLVs) by using ion-exchange and affinity chromatographies. The Na/Ca exchanger-enriched fraction was reconstituted into asolectin liposomes [lipid to protein ratio 10:1 (w/w)] that showed Na/Ca exchange activity. Under non-reducing conditions, SDS/PAGE showed a single 70 kDa polypeptide, which was further characterized by immunoblots with different antibodies: SWant, raised against the purified exchanger protein; NH2-terminus, residues 1-21; NCX1, residues 393-406; and Exon F, residues 622-644. Immunoblots under reducing conditions with SWant, NH2-terminus and NCX1 showed three bands migrating at 160, 120 and 70 kDa for SLV preparations, whereas Exon F reacted only with the 160 and 120 kDa bands. Under non-reducing conditions, immunoblots with purified reconstituted Na/Ca exchanger showed a single band at 70 kDa reacting with SWant, NH2-terminus and NCX1 but not with Exon F. We conclude that the 70 kDa protein is associated with Na/Ca exchange activity, has the same N-terminal sequence as the cloned bovine cardiac exchanger, and has its length decreased by at least 35% from its C-terminal portion as compared with that of the wild-type exchanger