Effect of glucolipotoxicity and rosiglitazone upon insulin secretion.

Type 2 diabetes is characterised by elevated blood glucose and fatty acid concentrations, and aberrant expression of exocytotic soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Restoration of normoglycaemia is often accomplished through use of the thiazolidinedione drug rosiglitazone (RSG), although little is known of its actions on the pancreas. Here we report that high glucose resulted in 96.6+/-0.2% inhibition of secretagogue-stimulated insulin secretion and 44.9+/-6.2% reduction in beta-cell insulin content. High glucose and lipid resulted in altered target-SNARE expression, syntaxin 1 becoming barely detectable whilst SNAP-25 was greatly up-regulated. RSG intervention further increased the expression of SNAP-25, but did not up-regulate syntaxin 1 expression. In summary, high glucose results in almost total attenuation of stimulated insulin secretion, partial depletion of beta-cell insulin stores and dysregulation of SNARE protein expression. RSG up-regulates SNAP-25 expression, but crucially not syntaxin 1 and hence fails to enhance insulin secretion.

Calpain-10: from genome search to function.

Calpain-10 (CAPN10) is the first diabetes gene to be identified through a genome scan. Many investigators, but not all, have subsequently found associations between CAPN10 polymorphism and type 2 diabetes (T2D) as well as insulin action, insulin secretion, aspects of adipocyte biology and microvascular function. However, this has not always been with the same single nucleotide polymorphism (SNP) or haplotype or the same phenotype, suggesting that there might be more than one disease-associated CAPN10 variant and that these might vary between ethnic groups and the phenotype under study. Our understanding of calpain-10 physiological action has also been greatly augmented by our knowledge of the calpain family domain structure and function, and the relationship between calpain-10 and other calpains is discussed here. Both genetic and functional data indicates that calpain-10 has an important role in insulin resistance and intermediate phenotypes, including those associated with the adipocyte. In this regard, emerging evidence would suggest that calpain-10 facilitates GLUT4 translocation and acts in reorganization of the cytoskeleton. Calpain-10 is also an important molecule in the beta-cell. It is likely to be a determinant of fuel sensing and insulin exocytosis, with actions at the mitochondria and plasma membrane respectively. We postulate that the multiple actions of calpain-10 may relate to its different protein isoforms. In conclusion, the discovery of calpain-10 by a genetic approach has identified it as a molecule of importance to insulin signaling and secretion that may have relevance to the future development of novel therapeutic targets for the treatment of T2D.

Polymorphic variations in the neurogenic differentiation-1, neurogenin-3, and hepatocyte nuclear factor-1alpha genes contribute to glucose intolerance in a South Indian population.

The neurogenic differentiation-1 (NEUROD1), neurogenin-3 (NEUROG3), and hepatic nuclear factor-1alpha (TCF1) genes are interacting transcription factors implicated in controlling islet cell development and insulin secretion. Polymorphisms of these genes (Ala45Thr [NEUROD1], Ser199Phe [NEUROG3], and Ala98Val [TCF1]) have been postulated to influence the development of type 2 diabetes. We have investigated the role and interaction between these variants using PCR/restriction fragment-length polymorphism assays in 454 subjects recruited as part of a population survey in South India. Additionally, 97 South Indian parent-offspring trios were studied. Polymorphisms of all three genes were associated with either fasting blood glucose (FBG) and/or 2-h blood glucose (BG) in either the total dataset or when restricted to a normoglycemic population. A monotonically increasing effect, dependent on the total number of risk-associated alleles carried, was observed across the whole population (P < 0.0001 for FBG and 2-h BG), raising FBG by a mean of 2.9 mmol/l and 2-h BG by a mean of 4.3 mmol/l. Similarly, an ascending number of the same risk alleles per subject increased the likelihood of type 2 diabetes (P = 0.002). In conclusion, we observed a combined effect of variations in NEUROD1, NEUROG3, and TCF1 in contributing to overall glucose intolerance in a South Indian population.

SPINK1 is a susceptibility gene for fibrocalculous pancreatic diabetes in subjects from the Indian subcontinent.

Fibrocalculous pancreatic diabetes (FCPD) is a secondary cause of diabetes due to chronic pancreatitis. Since the N34S variant of the SPINK1 trypsin inhibitor gene has been found to partially account for genetic susceptibility to chronic pancreatitis, we used a family-based and case-control approach in two separate ethnic groups from the Indian subcontinent, to determine whether N34S was associated with susceptibility to FCPD. Clear excess transmission of SPINK1 N34S to the probands with FCPD in 69 Bangladeshi families was observed (P<.0001; 20 transmissions and 2 nontransmissions). In the total study group (Bangladeshi and southern Indian) the N34S variant was present in 33% of 180 subjects with FCPD, 4.4% of 861 nondiabetic subjects (odds ratio 10.8; P<.0001 compared with FCPD), 3.7% of 219 subjects with type 2 diabetes, and 10.6% of 354 subjects with early-onset diabetes (aged <30 years) (P=.02 compared with the ethnically matched control group). These results suggest that the N34S variant of SPINK1 is a susceptibility gene for FCPD in the Indian subcontinent, although, by itself, it is not sufficient to cause disease.

Haplotype combinations of calpain 10 gene polymorphisms associate with increased risk of impaired glucose tolerance and type 2 diabetes in South Indians.

Haplotype combination 112/121 and its intrinsic variants (UCSNP43, -19, and -63) identified within the calpain 10 gene are associated with increased risk of type 2 diabetes in Mexican-Americans. We evaluated whether this haplotype combination and its constituent haplotypes and variants contribute to increased susceptibility to impaired fasting glucose (IFG)/impaired glucose tolerance (IGT) and type 2 diabetes in a South Indian population. Two study groups were used: 95 families ascertained through a proband with type 2 diabetes and 468 subjects recruited as part of an urban survey (69.1% with normal glucose tolerance, 12.8% with IFG/IGT, and 18.2% with type 2 diabetes). The four-locus haplotype combination 1112/1121 (UCSNP44, -43, -19, and -63) in South Indians conferred both a 10.7-fold increased risk for IFG/IGT (P = 0.001) and a 5.78- to 6.52-fold increased risk for type 2 diabetes in the two study groups (families P = 0.025, urban survey P = 0.015). A combination of the 1112 haplotype with the 1221 haplotype also appeared to increase risk for both IFG/IGT and type 2 diabetes. Contrary to what might be expected, quantitative trait analysis in the families found that transmission of the disease-related 1121 and 1112 haplotypes was associated with a reduced hip size and lower waist-to-hip ratio, respectively. This study supports the paradigm that specific haplotype combinations of calpain 10 variants increase risk of both IFG/IGT and type 2 diabetes. However, the relative infrequency of the "at-risk" combinations in the South Indian population suggests that calpain 10 is not a common determinant of susceptibility to type 2 diabetes.