Marc Prentki

A genome-wide association study identifies novel risk loci for type 2 diabetes

Type 2 diabetes mellitus results from the interaction of environmental factors with a combination of genetic variants, most of which were hitherto unknown. A systematic search for these variants was recently made possible by the development of high-density arrays that permit the genotyping of hundreds of thousands of polymorphisms. We tested 392,935 single-nucleotide polymorphisms in a French case–control cohort. Markers with the most significant difference in genotype frequencies between cases of type 2 diabetes and controls were fast-tracked for testing in a second cohort. This identified four loci containing variants that confer type 2 diabetes risk, in addition to confirming the known association with the TCF7L2 gene. These loci include a non-synonymous polymorphism in the zinc transporter SLC30A8, which is expressed exclusively in insulin-producing β-cells, and two linkage disequilibrium blocks that contain genes potentially involved in β-cell development or function (IDE–KIF11–HHEX and EXT2–ALX4). These associations explain a substantial portion of disease risk and constitute proof of principle for the genome-wide approach to the elucidation of complex genetic traits.

Type 2 diabetes across generations: from pathophysiology to prevention and management

Type 2 diabetes is now a pandemic and shows no signs of abatement. In this Seminar we review the pathophysiology of this disorder, with particular attention to epidemiology, genetics, epigenetics, and molecular cell biology. Evidence is emerging that a substantial part of diabetes susceptibility is acquired early in life, probably owing to fetal or neonatal programming via epigenetic phenomena. Maternal and early childhood health might, therefore, be crucial to the development of effective prevention strategies. Diabetes develops because of inadequate islet β-cell and adipose-tissue responses to chronic fuel excess, which results in so-called nutrient spillover, insulin resistance, and metabolic stress. The latter damages multiple organs. Insulin resistance, while forcing β cells to work harder, might also have an important defensive role against nutrient-related toxic effects in tissues such as the heart. Reversal of overnutrition, healing of the β cells, and lessening of adipose tissue defects should be treatment priorities.

Are the beta-cell signaling molecules malonyl-CoA and cystolic long-chain acyl-CoA implicated in multiple tissue defects of obesity and NIDDM?

Widely held theories of the pathogenesis of obesity-associated NIDDM have implicated apparently incompatible events as seminal: 1) insulin resistance in muscle, 2) abnormal secretion of insulin, and 3) increases in intra-abdominal fat. Altered circulating or tissue lipids are characteristic features of obesity and NIDDM. The etiology of these defectss is not known. In this perspective, we propose that the same metabolic events, elevated malonyl-CoA and long-chain acyl-CoA (LC-CoA), in various tissues mediate, in part, the pleiotropic alterations characteristic of obesity and NIDDM. We review the evidence in support of the emerging concept that malonyl-CoA and LC-CoA act as metabolic coupling factors in β-cell signal transduction, linking fuel metabolism to insulin secretion. We suggest that acetyl-CoA carboxylase, which synthesizes malonyl-CoA, a “signal of plenty,” and carnitine palmitoyl transferase 1, which is regulated by it, may perform as fuel sensors in the β-cell, integrating the concentrations of all circulating fuel stimuli in the β-cell as well as in muscle, liver, and adipose tissue. The target effectors of LC-CoA may include protein kinase C sub-types, complex lipid formation, genes encoding metabolic enzymes or transduction factors, and protein acylation. We support the concept that only under conditions in which both glucose and lipids are plentiful will the metabolic abnormality, which may be termed glucolipoxia, become apparent. If our hypothesis is correct that common signaling abnormalities in the metabolism of malonyl-CoA and LC-CoA contribute to altered insulin release and sensitivity, it offers a novel explanation for the presence of variable combinations of these defects in individuals with differing genetic backgrounds and for the fact that it has been difficult to determine whether one or the other is the primary event.

Glucagon-like peptide-1 promotes DNA synthesis, activates phosphatidylinositol 3-kinase and increases transcription factor pancreatic and duodenal homeobox gene 1 (PDX-1) DNA binding activity in beta (INS-1)-cells.

Aims/hypothesis. Glucagon-like peptide-1 is a potent glucoincretin hormone and a potentially important drug in the treatment of Type II (non-insulin-dependent) diabetes mellitus. We have investigated whether it acts as a growth factor in beta (INS-1)-cells and have studied the signalling pathways and transcription factors implicated in this process. Methods. Cell proliferation was assessed by tritiated thymidine incorporation measurements. We have examined the action of glucagon-like peptide-1 on the enzymatic activity of phosphatidylinositol 3-kinase. The DNA binding activity of transcription factors was investigated by electrophoretic mobility shift assay. Measurements of mRNA were done using the northern technique. Results. Glucagon-like peptide-1 caused an increase in tritiated thymidine incorporation in beta (INS-1)-cells and phosphatidylinositol 3-kinase activity in a dose-dependent manner non-additively with glucose. The phosphatidylinositol 3-kinase inhibitors wortmannin and LY294 002 blocked the effects of glucagon-like peptide-1 on DNA synthesis. Transcription factor pancreatic and duodenal homebox gene 1 (PDX-1) DNA binding activity was increased by glucagon-like peptide-1 at 3 or 11 mmol/l glucose and the phosphatidylinositol 3-kinase inhibitor LY294 002 suppressed the action of glucagon-like peptide-1 on PDX-1 DNA binding activity. Glucagon-like peptide-1 and glucose alone did not change activating protein-1 DNA binding activity. They synergised, however, to increase the activity of activating protein-1. Glucagon-like peptide-1 also increased the expression of PDX-1, glucose transporter 2, glucokinase and insulin mRNAs. Finally, glucagon-like peptide-1 increased the incorporation of tritiated thymidine in isolated rat islets. Conclusion/interpretation. The results suggest that glucagon-like peptide-1 may act as a growth factor for the beta cell by a phosphatidylinositol 3-kinase mediated event. Glucagon-like peptide-1 could also regulate the expression of the insulin gene and genes encoding enzymes implicated in glucose transport and metabolism through the phosphatidylinositol 3-kinase/PDX-1 transduction signalling pathway. [Diabetologia (1999) 42: 856–864]

Genetic variant near IRS1 is associated with type 2 diabetes, insulin resistance and hyperinsulinemia

Genome-wide association studies have identified common variants that only partially explain the genetic risk for type 2 diabetes (T2D). Using genome-wide association data from 1,376 French individuals, we identified 16,360 SNPs nominally associated with T2D and studied these SNPs in an independent sample of 4,977 French individuals. We then selected the 28 best hits for replication in 7,698 Danish subjects and identified 4 SNPs showing strong association with T2D, one of which (rs2943641, P = 9.3 × 10−12, OR = 1.19) was located adjacent to the insulin receptor substrate 1 gene (IRS1). Unlike previously reported T2D risk loci, which predominantly associate with impaired beta cell function, the C allele of rs2943641 was associated with insulin resistance and hyperinsulinemia in 14,358 French, Danish and Finnish participants from population-based cohorts; this allele was also associated with reduced basal levels of IRS1 protein and decreased insulin induction of IRS1-associated phosphatidylinositol-3-OH kinase activity in human skeletal muscle biopsies.

AMP kinase and malonyl-CoA: targets for therapy of the metabolic syndrome.

Patients with the metabolic syndrome are characterized by insulin resistance, obesity and a predisposition to hypertension, dyslipidaemia, pancreatic β-cell dysfunction, type 2 diabetes and premature atherosclerosis. Here we review the hypothesis that a common feature linking these multiple abnormalities is dysregulation of the AMP-activated protein kinase (AMPK)/malonyl-CoA fuel-sensing and signalling network. It is proposed that such dysregulation leads to alterations in cellular fatty-acid metabolism that in turn cause ectopic lipid accumulation, cellular dysfunction and ultimately disease. Evidence is also presented that factors that activate AMP kinase and/or reduce malonyl-CoA levels might reverse these abnormalities or prevent them from occurring.

AMPK, insulin resistance, and the metabolic syndrome

Insulin resistance (IR) and hyperinsulinemia are hallmarks of the metabolic syndrome, as are central adiposity, dyslipidemia, and a predisposition to type 2 diabetes, atherosclerotic cardiovascular disease, hypertension, and certain cancers. Regular exercise and calorie restriction have long been known to increase insulin sensitivity and decrease the prevalence of these disorders. The subsequent identification of AMP-activated protein kinase (AMPK) and its activation by exercise and fuel deprivation have led to studies of the effects of AMPK on both IR and metabolic syndrome-related diseases. In this review, we evaluate this body of literature, with special emphasis on the hypothesis that dysregulation of AMPK is both a pathogenic factor for these disorders in humans and a target for their prevention and therapy.

Fatty Acid Signaling in the β-Cell and Insulin Secretion

Fatty acids (FAs) and other lipid molecules are important for many cellular functions, including vesicle exocytosis. For the pancreatic β-cell, while the presence of some FAs is essential for glucose-stimulated insulin secretion, FAs have enormous capacity to amplify glucose-stimulated insulin secretion, which is particularly operative in situations of β-cell compensation for insulin resistance. In this review, we propose that FAs do this via three interdependent processes, which we have assigned to a “trident model” of β-cell lipid signaling. The first two arms of the model implicate intracellular metabolism of FAs, whereas the third is related to membrane free fatty acid receptor (FFAR) activation. The first arm involves the AMP-activated protein kinase/malonyl-CoA/long-chain acyl-CoA (LC-CoA) signaling network in which glucose, together with other anaplerotic fuels, increases cytosolic malonyl-CoA, which inhibits FA partitioning into oxidation, thus increasing the availability of LC-CoA for signaling purposes. The second involves glucose-responsive triglyceride (TG)/free fatty acid (FFA) cycling. In this pathway, glucose promotes LC-CoA esterification to complex lipids such as TG and diacylglycerol, concomitant with glucose stimulation of lipolysis of the esterification products, with renewal of the intracellular FFA pool for reactivation to LC-CoA. The third arm involves FFA stimulation of the G-protein–coupled receptor GPR40/FFAR1, which results in enhancement of glucose-stimulated accumulation of cytosolic Ca2+ and consequently insulin secretion. It is possible that FFA released by the lipolysis arm of TG/FFA cycling is partly “secreted” and, via an autocrine/paracrine mechanism, is additive to exogenous FFAs in activating the FFAR1 pathway. Glucose-stimulated release of arachidonic acid from phospholipids by calcium-independent phospholipase A2 and/or from TG/FFA cycling may also be involved. Improved knowledge of lipid signaling in the β-cell will allow a better understanding of the mechanisms of β-cell compensation and failure in diabetes.

Coordinate regulation of malonyl-CoA decarboxylase, sn-glycerol-3-phosphate acyltransferase, and acetyl-CoA carboxylase by AMP-activated protein kinase in rat tissues in response to exercise

Changes in the concentration of malonyl-CoA in many tissues have been related to alterations in the activity of acetyl-CoA carboxylase (ACC), the rate-limiting enzyme in its formation. In contrast, little is known about the physiological role of malonyl-CoA decarboxylase (MCD), an enzyme responsible for malonyl-CoA catabolism. In this study, we examined the effects of voluntary exercise on MCD activity in rat liver, skeletal muscle, and adipose tissue. In addition, the activity ofsn-glycerol-3-phosphate acyltransferase (GPAT), which like MCD and ACC can be regulated by AMP-activated protein kinase (AMPK), was assayed. Thirty min after the completion of a treadmill run, MCD activity was increased ∼2-fold, malonyl-CoA levels were reduced, and ACC and GPAT activities were diminished by 50% in muscle and liver. These events appeared to be mediated via activation of AMPK since: 1) AMPK activity was concurrently increased by exercise in both tissues; 2) similar findings were observed after the injection of 5-amino 4 imidazole carboxamide, an AMPK activator; 3) changes in the activity of GPAT and ACC paralleled that of MCD; and 4) the increase in MCD activity in muscle was reversed in vitro by incubating immunoprecipitated enzyme from the exercised muscle with protein phosphatase 2A, and it was reproduced by incubating immunopurified MCD from resting muscle with purified AMPK. An unexpected finding was that exercise caused similar changes in the activities of ACC, MCD, GPAT, and AMPK and the concentration of malonyl-CoA in adipose tissue. In conclusion: MCD, GPAT, and ACC are coordinately regulated by AMPK in liver and adipose tissue in response to exercise, and except for GPAT, also in muscle. The results suggest that AMPK activation plays a major role in regulating lipid metabolism in many cells following exercise. They also suggest that in each of them, it acts to increase fatty acid oxidation and decrease its esterification.

Role for Activating Transcription Factor 3 in Stress-Induced β-Cell Apoptosis

ABSTRACT Activating transcription factor 3 (ATF3) is a stress-inducible gene and encodes a member of the ATF/CREB family of transcription factors. However, the physiological significance of ATF3 induction by stress signals is not clear. In this report, we describe several lines of evidence supporting a role of ATF3 in stress-induced β-cell apoptosis. First, ATF3 is induced in β cells by signals relevant to β-cell destruction: proinflammatory cytokines, nitric oxide, and high concentrations of glucose and palmitate. Second, induction of ATF3 is mediated in part by the NF-κB and Jun N-terminal kinase/stress-activated protein kinase signaling pathways, two stress-induced pathways implicated in both type 1 and type 2 diabetes. Third, transgenic mice expressing ATF3 in β cells develop abnormal islets and defects secondary to β-cell deficiency. Fourth, ATF3 knockout islets are partially protected from cytokine- or nitric oxide-induced apoptosis. Fifth, ATF3 is expressed in the islets of patients with type 1 or type 2 diabetes, and in the islets of nonobese diabetic mice that have developed insulitis or diabetes. Taken together, our results suggest ATF3 to be a novel regulator of stress-induced β-cell apoptosis.