The sarcoendoplasmic reticulum (ER) Ca2+ ATPase 2 (SERCA2) pump is a P-type ATPase tasked with the maintenance of ER Ca2+ stores. Whereas β-cell SERCA2 expression is reduced in diabetes, the role of SERCA2 in the regulation of whole-body glucose homeostasis has remained uncharacterized. To this end, SERCA2 heterozygous mice (S2HET) were challenged with a high-fat diet (HFD) containing 45% of kilocalories from fat. After 16 weeks of the HFD, S2HET mice were hyperglycemic and glucose intolerant, but adiposity and insulin sensitivity were not different between HFD-fed S2HET mice and HFD-fed wild-type controls. Consistent with a defect in β-cell function, insulin secretion, glucose-induced cytosolic Ca2+ mobilization, and the onset of steady-state glucose-induced Ca2+ oscillations were impaired in HFD-fed S2HET islets. Moreover, HFD-fed S2HET mice exhibited reduced β-cell mass and proliferation, altered insulin production and proinsulin processing, and increased islet ER stress and death. In contrast, SERCA2 activation with a small molecule allosteric activator increased ER Ca2+ storage and rescued tunicamycin-induced β-cell death. In aggregate, these data suggest a critical role for SERCA2 and the regulation of ER Ca2+ homeostasis in the β-cell compensatory response to diet-induced obesity.

Edited by Jeffrey E. Pessin Alterations in endoplasmic reticulum (ER) calcium (Ca 2؉) levels diminish insulin secretion and reduce ␤-cell survival in both major forms of diabetes. The mechanisms responsible for ER Ca 2؉ loss in ␤ cells remain incompletely understood. Moreover, a specific role for either ryanodine receptor (RyR) or inositol 1,4,5-triphosphate receptor (IP 3 R) dysfunction in the pathophysiology of diabetes remains largely untested. To this end, here we applied intracellular and ER Ca 2؉ imaging techniques in INS-1 ␤ cells and isolated islets to determine whether diabetogenic stressors alter RyR or IP 3 R function. Our results revealed that the RyR is sensitive mainly to ER stress-induced dysfunction, whereas cytokine stress specifically alters IP 3 R activity. Consistent with this observation, pharmacological inhibition of the RyR with ryanodine and inhibition of the IP 3 R with xestospongin C prevented ER Ca 2؉ loss under ER and cytokine stress conditions, respectively. However, RyR blockade distinctly prevented ␤-cell death, propagation of the unfolded protein response (UPR), and dysfunctional glucose-induced Ca 2؉ oscillations in tunicamycin-treated INS-1 ␤ cells and mouse islets and Akita islets. Monitoring at the single-cell level revealed that ER stress acutely increases the frequency of intracellular Ca 2؉ transients that depend on both ER Ca 2؉ leakage from the RyR and plasma membrane depolarization. Collectively, these findings indicate that RyR dysfunction shapes ER Ca 2؉ dynamics in ␤ cells and regulates both UPR activation and cell death, suggesting that RyR-mediated loss of ER Ca 2؉ may be an early pathogenic event in diabetes. This work was supported by National Institutes of Health Grants R01 DK093954 and UC4 DK 104166 (to C. E-M.); Department of Veterans Affairs Merit Award I01BX001733 (to C. E-M.); and Sigma Beta Sorority, Ball Brothers Foundation, and George and Frances Ball Foundation gifts (to C. E-M.).

Adipose-derived stromal/stem cells (ASCs) ameliorate hyperglycemia in rodent models of islet transplantation and autoimmune diabetes, yet the precise human ASC (hASC)-derived factors responsible for these effects remain largely unexplored. Here, we show that systemic administration of hASCs improved glucose tolerance, preserved β cell mass, and increased β cell proliferation in STZ-treated NOD-SCID mice. Co-culture experiments combining mouse or human islets with hASCs demonstrated that islet viability and function were improved by hASCs following prolonged culture or treatment with pro-inflammatory cytokines. Analysis of hASC-derived factors revealed VEGF and TIMP-1 to be highly abundant factors secreted by hASCs. Notably, TIMP-1 secretion increased in the presence of islet stress from cytokine treatment, while TIMP-1 blockade was able to abrogate in vitro pro-survival effects of hASCs. Following systemic administration by tail vein injection, hASCs were detected in the pancreas and human TIMP-1 was increased in the serum of injected mice, while recombinant TIMP-1 increased viability in INS-1 cells treated with IL-1β, IFN-γ and TNF-α. In aggregate, our data support a model whereby factors secreted by hASCs, such as TIMP-1, are able to mitigate against β cell death in rodent and in vitro models of Type 1 diabetes through a combination of local paracrine as well as systemic effects.

Background: Altered sarco-endoplasmic reticulum Ca 2ϩ ATPase 2b (SERCA2b) expression and activity contributes to ␤ cell dysfunction in diabetes. Results: SERCA2b deficiency occurs secondary to loss of pancreatic and duodenal homeobox 1 (Pdx-1)-mediated transcriptional regulation. Conclusion: Pdx-1 maintains SERCA2b expression and endoplasmic reticulum (ER) calcium levels in the ␤ cell. Significance: These findings elucidate a novel pathway that contributes to ␤ cell ER stress.

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