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International Textbook of Diabetes Mellitus, 4th Ed., Excerpt #27: Insulin Gene Expression and Biosynthesis Part 3 of 6

DeFronzoCoverDysregulation of the insulin gene

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 consist of rare control region mutations, while insulin gene expression appears to be reduced by metabolic conditions associated with the diabetic state.

Polymorphisms of the insulin gene

The diabetes susceptibility gene IDDM2 has been mapped to the insulin variable number 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 number of tandem repeats. Whereas the short class I VNTR gene predisposes to type 1 diabetes, the long class III allele is protective. Precisely how VNTR polymorphisms confer susceptibility to or protection from diabetes remains uncertain, although recent evidence clearly suggest that the VNTR determines expression levels of insulin in the thymus and, in turn, the numbers of insulin-specific 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 adverse effects of chronically elevated glucose levels include, but are not limited to, impairment of insulin gene expression in insulin-secreting cells, isolated rat and human islets, and animal models of diabetes [58]. The molecular mechanisms underlying glucotoxicity at the insulin gene involve decreased expression of Pdx-1 and MafA (Figure 6.2), as well as increased expression of C/EBPβ which directly binds the NeuroD1/Beta2, thereby preventing formation of the NeuroD1/Beta2:E47 activator complex required 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 required for normal glucose stimulation of insulin transcription [60].

ITDMFig6.2

The biochemical mechanisms whereby chronically elevated glucose impairs insulin gene expression have received considerable attention in the past few years. The prevailing hypothesis is that high glucose induces 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 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 prevented 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 elevated glucose levels may lead to dedifferentiation with loss of genes associated with β-cell function and overexpression of genes normally repressed in differentiated β cells [66,67]. For instance, the c-myc transcription factor is upregulated in diabetic islets [68] and is induced by high glucose in normal islets [69]. In turn, c-myc can inhibit insulin gene transcription [70] by competing for NeuroD1/Beta2 binding at the E-box [71].

Endoplasmic reticulum (ER) stress has also been proposed to contribute to the mechanisms of glucotoxicity independently from oxidative stress [72]. However, alleviation of ER stress by chemical chaperones in glucose-cultured islets improves insulin secretion but not intracellular insulin content, suggesting that ER stress may be involved in defective insulin secretion but not impaired insulin biosynthesis under glucotoxic conditions [73].

Glucolipotoxicity and the insulin gene

Like chronic hyperglycemia, hyperlipidemia has been proposed to contribute to β-cell dysfunction in type 2 diabetes [74]. Most of the deleterious effects of chronically elevated lipid levels on the β cell require the concomitant presence of hyperglycemia, a phenomenon referred to as glucolipotoxicity [75]. Amongst its many functional consequences, glucolipotoxicity impairs insulin gene expression via a transcriptional mechanism that involves 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 also observed in an in vivo model of glucolipotoxicity in rats following a 72-h infusion of glucose and Intralipid, a lipid emulsion which raises circulating fatty acid levels when co-infused with heparin [78,79]. It is interesting that glucotoxicity and glucolipotoxicity both affect Pdx-1 and MafA function, albeit by different 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 reduced [77] (Figure 6.2).

How de novo ceramide synthesis from palmitate, in the presence of elevated glucose, alters the function of Pdx-1 and MafA and leads to defective insulin gene expression remains unknown. One possible 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 support of this possibility, palmitate was shown to activate JNK in β cells which results in a decrease in insulin gene transcription [84].

Relevance to human type 2 diabetes

Recent studies in human islets support the notion that defective insulin gene expression may play a role in human type 2 diabetes. First, in islets isolated from pancreata of 13 type 2 diabetic cadaveric organ donors high levels of oxidative stress markers as well as low levels of glucose-induced insulin secretion, reduced insulin mRNA, but increased levels of Pdx-1 and FOXO1 mRNAs have been observed [85]. Second, nuclear expression of MafA is decreased in human diabetic islets [86]. Third, DNA methylation of the insulin promoter is increased in type 2 diabetic patients and correlates negatively with insulin gene expression and positively with hemoglobin A1c levels [87].

 

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