There is convincing evidence that abnormalities in insulin gene sequence or function play a role in pancreatic β-cell dysfunction in type 2 diabetes. Abnormalities in the insulin gene structure include rare manage region mutations, while insulin gene expression appears to be low by metabolic conditions associated along with the diabetic state.
Polymorphisms of the insulin gene
The diabetes susceptibility gene IDDM2 has actually been mapped to the insulin variable lot of tandem repeats (VNTR), a highly polymorphic region located 360bp upstream of the transcription initiation site in the human insulin gene [56]. VNTRs are classified as class I, II, or III, depending on the lot of tandem repeats. Whereas the short class I VNTR gene predisposes to type 1 diabetes, the long class III allele is protective. Exactly exactly how VNTR polymorphisms confer susceptibility to or protection from diabetes remains uncertain, despite the fact that recent evidence clearly suggest that the VNTR determines expression levels of insulin in the thymus and, in turn, the numbers of insulin-individual autoreactive T cells [56].
Glucotoxicity and the insulin gene
The glucotoxicity hypothesis proposes that chronic hyperglycemia is deleterious to β-cell function by contributing to the deterioration of insulin secretion [57]. These side effects of chronically raised glucose levels include, however are not restricted to, impairment of insulin gene expression in insulin-secreting cells, isolated rat and human islets, and pet dog models of diabetes [58]. The molecular mechanisms underlying glucotoxicity at the insulin gene involve reduced expression of Pdx-1 and MafA (Figure 6.2), too as increased expression of C/EBPβ which straight binds the NeuroD1/Beta2, thereby preventing formation of the NeuroD1/Beta2:E47 activator complex needed for insulin E1 stimulation [59]. In addition, binding of a C/EBPβ-NFAT complex at the A2C1 element of the insulin promoter under glucotoxicity prevents the formation of the MafA-NFAT complex at that site needed for typical glucose stimulation of insulin transcription [60].
The biochemical mechanisms whereby chronically raised glucose impairs insulin gene expression have actually received substantial focus in the past couple of years. The prevailing hypothesis is that higher glucose triggers the excessive production of reactive oxygen species (ROS) and the formation of advanced glycation end-products (AGE) [61,62]. This hypothesis is supported by in vivo observations. For example, treatment of Zucker diabetic fatty (ZDF) rats along with the antioxidant N-acetylcysteine normalizes plasma glucose levels and restores insulin secretion, insulin content, and preproinsulin mRNA levels [63]. Similarly, over-expression of glutathione peroxidase-1 in db/db mice reversed hyperglycemia and restored MafA nuclear localization [64].
Oxidative stress-mediated impairment in Pdx-1 binding activity is avoided by overexpression of a dominant-negative c-jun N-terminal kinase (JNK), and is mimicked by overexpression of wild-type JNK [65]. In addition, chronic exposure to raised glucose levels could bring about dedifferentiation along with loss of genes associated along with β-cell function and overexpression of genes usually repressed in differentiated β cells [66,67]. For instance, the c-myc transcription factor is upregulated in diabetic islets [68] and is induced by higher glucose in typical islets [69]. In turn, c-myc can easily inhibit insulin gene transcription [70] by competing for NeuroD1/Beta2 binding at the E-box [71].
Endoplasmic reticulum (ER) pressure has actually additionally been proposed to contribute to the mechanisms of glucotoxicity independently from oxidative pressure [72]. However, alleviation of ER pressure by chemical chaperones in glucose-cultured islets enhances insulin secretion however not intracellular insulin content, suggesting that ER pressure might be associated with defective insulin secretion however not impaired insulin biosynthesis under glucotoxic conditions [73].
Glucolipotoxicity and the insulin gene
Like chronic hyperglycemia, hyperlipidemia has actually been proposed to contribute to β-cell dysfunction in type 2 diabetes [74]. The majority of of the deleterious effects of chronically raised lipid levels on the β cell require the concomitant presence of hyperglycemia, a phenomenon called glucolipotoxicity [75]. Amongst its numerous functional consequences, glucolipotoxicity impairs insulin gene expression via a transcriptional mechanism that includes de novo synthesis of ceramide and defective function of Pdx-1 and MafA [76,77]. Importantly, defective Pdx-1 function and insulin gene expression are additionally observed in an in vivo model of glucolipotoxicity in rats complying with a 72-h infusion of glucose and Intralipid, a lipid emulsion which improves circulating fatty acid levels as quickly as co-infused along with heparin [78,79]. It is interesting that glucotoxicity and glucolipotoxicity the two affect Pdx-1 and MafA function, albeit by various mechanisms: glucotoxicity alters Pdx-1 mRNA expression [80] and MafA nuclear localization [64], while in glucolipotoxicity Pdx-1 is retained in the cytosolic compartment while MafA mRNA expression is low [77] (Figure 6.2).
How de novo ceramide synthesis from palmitate, in the presence of raised glucose, alters the function of Pdx-1 and MafA and leads to defective insulin gene expression remains unknown. One feasible candidate is the serine/threonine kinase PAS kinase, which regulates glucose-induced insulin gene transcription [81]. In insulin-secreting cells and isolated islets, we observed that overexpression of PAS kinase protects from the negative effects of palmitate on the insulin gene [82]. Recent data suggest that this could be mediated by PAS kinase inactivation of GSK3β (via phosphorylation at Ser9) and alleviation of GSK3β-mediated serine phosphorylation of Pdx-1 and proteasomal degradation ([83] and M. Semache, G. Fontés, S. Fogarty, C. Kikani, M. B. Chawki, J. Rutter and V. Poitout, unpublished data). A second candidate mediator of ceramide inhibition of the insulin gene is c-jun N-terminal kinase (JNK). In sustain of this possibility, palmitate was revealed to activate JNK in β cells which outcomes in a lower in insulin gene transcription [84].
Relevance to human type 2 diabetes
Recent studies in human islets sustain the notion that defective insulin gene expression could play a role in human type 2 diabetes. First, in islets isolated from pancreata of 13 type 2 diabetic cadaveric organ donors higher levels of oxidative pressure markers too as reasonable levels of glucose-induced insulin secretion, low insulin mRNA, however increased levels of Pdx-1 and FOXO1 mRNAs have actually been observed [85]. Second, nuclear expression of MafA is reduced in human diabetic islets [86]. Third, DNA methylation of the insulin promoter is increased in type 2 diabetic patients and correlates negatively along with insulin gene expression and positively along with hemoglobin A1c levels [87].
