The protein tyrosine phosphatase SHP-1 is a well-known inhibitor of activation-promoting signaling cascades in hematopoietic cells but its potential role in insulin target tissues is unknown. Here we show that Ptpn6(me-v/me-v) (also known as viable motheaten) mice bearing a functionally deficient SHP-1 protein are markedly glucose tolerant and insulin sensitive as compared to wild-type littermates, as a result of enhanced insulin receptor signaling to IRS-PI3K-Akt in liver and muscle. Downregulation of SHP-1 activity in liver of normal mice by adenoviral expression of a catalytically inert mutant of SHP-1, or after small hairpin RNA-mediated SHP-1 silencing, further confirmed this phenotype. Tyrosine phosphorylation of CEACAM1, a modulator of hepatic insulin clearance, and clearance of serum [125I]-insulin were markedly increased in SHP-1-deficient mice or SHP-1-deficient hepatic cells in vitro. These findings show a novel role for SHP-1 in the regulation of glucose homeostasis through modulation of insulin signaling in liver and muscle as well as hepatic insulin clearance.

We found that peroxisome proliferator-activated receptor gamma is highly expressed in human islet endocrine cells, both at the mRNA and protein levels. These results support the hypothesis of a direct influence of peroxisome proliferator-activated receptor gamma agonist on human pancreatic endocrine cells.

Transplantation of islets of Langerhans is a potential cure for type 1 diabetes, but its success is hampered by destruction of the islets. The data presented herein suggest that the active metabolite of vitamin D3 [1,25-(OH)2D3] may promote islet cell survival by modulating the effects of inflammatory cytokines, which contribute to beta-cell demise. We investigated some of the mechanisms triggering the apoptotic machinery in rat insulinoma RINm5F cells and human islets treated with IL-1beta plus interferon-gamma plus TNFalpha and assessed the effects of 1,25-(OH)2D3 in these processes. Mitochondrial transmembrane permeability and apoptotic features, determined by percentage of sub-G1 cells, quantitation of DNA strand breaks, and Hoechst staining, were significantly increased by cytokines and reverted toward control values by 1,25-(OH)2D3 cotreatment. The cytoprotection of cells correlated with the abrogation of cytokine-induced nitric oxide production. The activation of nuclear factor-kappaB plays a key role in the different pathways implicated in nitric oxide generation. We demonstrated for the first time, in both RINm5F cells and human islets, that 1,25-(OH)2D3 was able to induce and maintain high levels of A20, an antiapoptotic protein known to block nuclear factor-kappaB activation. Our study showed a clear efficiency of 1,25-(OH)2D3 on the apoptotic machinery triggered by cytokines in beta-cells and suggests that 1,25-(OH)2D3 could help overcome a major obstacle encountered in the cellular therapy of diabetes, such as nonfunction in the immediate posttransplantation period.

Aims/hypothesis Glucose and incretins regulate beta cell function, gene expression and insulin exocytosis via calcium and cAMP. Prolonged exposure to elevated glucose (also termed glucotoxicity) disturbs calcium homeostasis, but little is known about cAMP signalling. We therefore investigated long-term effects of glucose on this pathway with special regard to the incretin glucagon-like peptide 1 (GLP-1). Methods We exposed INS-1E cells and rat or human islets to different levels of glucose for 3 days and determined functional responses in terms of second messengers (cAMP, Ca 2+ ), transcription profiles, activation of cAMP-responsive element (CRE) and secretion by measuring membrane capacitance. Moreover, we modulated directly the abundance of a calcium-sensitive adenylyl cyclase (ADCY8) and GLP-1 receptor (GLP1R).Results GLP-1-or forskolin-mediated increases in cytosolic calcium, cAMP-levels or insulin secretion were largely reduced in INS-1E cells cultured at elevated glucose (>5.5 mmol/l). Statistical analysis of transcription profiles identified cAMP pathways as major targets regulated by glucose. Quantitative PCR confirmed these findings and unravelled marked downregulation of the calcium-sensitive adenylyl cyclase ADCY8 also in rat and in human islets. Re-expression of ADCY8, but not of the GLP1R, recovered GLP-1 signalling in glucotoxicity in INS-1E cells and in rat islets. Moreover, knockdown of this adenylyl cyclase showed that GLP-1-induced cAMP generation, calcium signalling, activation of the downstream target CRE and direct amplification of exocytosis by cAMP-raising agents (evaluated by capacitance measurement) proceeds via ADCY8. Conclusions/interpretation cAMP-mediated pathways are modelled by glucose, and downregulation of the calcium-B. Roger and J. Papin contributed equally to this study. Electronic supplementary material The online version of this article