Oxidative and energetic stresses mediate beta-cell dysfunction induced by PGC-1α.

AIM: Alteration of functional beta-cell mass in adults can be programmed by adverse events during fetal life. Previously, it was demonstrated that high glucocorticoid (GC) levels during fetal life participate in this programming by inhibition of beta-cell development. More specifically, GC levels stimulate expression of peroxisome proliferator-activated receptor-gamma coactivator 1-alpha (PGC-1α), a transcriptional co-regulator of the GC receptor (GR), which per se impairs beta-cell mass and function when overexpressed. As PGC-1α is also a potent inducer of mitochondrial biogenesis, our study aimed to determine how PGC-1α modifies mitochondrial function in beta cells and how it might regulate insulin secretion. METHODS: Beta-cell function was studied in mice overexpressing PGC-1α specifically in beta cells and in MIN6 cells overexpressing PGC-1α in vitro. RESULTS: PGC-1α overexpression in beta cells in vivo leads to a reduced beta-cell mass early in fetal life, whereas PGC-1α overexpression in vitro stimulates mitochondrial biogenesis and respiratory activity without improving ATP production, while increasing oxidative stress and impairing insulin secretion in response to glucose. While oxidative stress with PGC-1α overexpression in beta cells activates AMPK, it has also been revealed that blocking such oxidative stress or AMPK activation restores insulin secretion. CONCLUSION: PGC-1α induces oxidative stress, which disrupts insulin secretion by AMPK activation. Thus, control of oxidative or energetic stress in beta cells may help to restore insulin secretion.

Endoplasmic reticulum stress does not mediate palmitate-induced insulin resistance in mouse and human muscle cells.

AIMS/HYPOTHESIS: Recent experiments in liver and adipocyte cell lines indicate that palmitate can induce endoplasmic reticulum (ER) stress. Since it has been shown that ER stress can interfere with insulin signalling, our hypothesis was that the deleterious action of palmitate on the insulin signalling pathway in muscle cells could also involve ER stress. METHODS: We used C2C12 and human myotubes that were treated either with palmitate or tunicamycin. Total lysates and RNA were prepared for western blotting or quantitative RT-PCR respectively. Glycogen synthesis was assessed by [¹4C]glucose incorporation. RESULTS: Incubation of myotubes with palmitate or tunicamycin inhibited insulin-stimulated protein kinase B (PKB)/ v-akt murine thymoma viral oncogene homologue 1 (Akt). In parallel, an increase in ER stress markers was observed. Pre-incubation with chemical chaperones that reduce ER stress only prevented tunicamycin but not palmitate-induced insulin resistance. We hypothesised that ER stress activation levels induced by palmitate may not be high enough to induce insulin resistance, in contrast with tunicamycin-induced ER stress. Indeed, tunicamycin induced a robust activation of the inositol-requiring enzyme 1 (IRE-1)/c-JUN NH2-terminal kinase (JNK) pathway, leading to serine phosphorylation of insulin receptor substrate 1 (IRS-1) and a decrease in IRS-1 tyrosine phosphorylation. In contrast, palmitate only induced a very weak activation of the IRE1/JNK pathway, with no IRS1 serine phosphorylation. CONCLUSIONS/INTERPRETATION: These data show that insulin resistance induced by palmitate is not related to ER stress in muscle cells.

Hepatic steatosis: a role for de novo lipogenesis and the transcription factor SREBP-1c.

Steatosis is an accumulation of triglycerides in the liver. Although an excessive availability of plasma fatty acids is an important determinant of steatosis, lipid synthesis from glucose (lipogenesis) is now also considered as an important contributing factor. Lipogenesis is an insulin- and glucose-dependent process that is under the control of specific transcription factors, sterol regulatory element binding protein 1c (SREBP-1c), activated by insulin and carbohydrate response element binding protein (ChREBP) activated by glucose. Insulin induces the maturation of SREBP-1c by a proteolytic mechanism initiated in the endoplasmic reticulum (ER). SREBP-1c in turn activates glycolytic gene expression, allowing glucose metabolism, and lipogenic genes in conjunction with ChREBP. Lipogenesis activation in the liver of obese markedly insulin-resistant steatotic rodents is then paradoxical. Recent data suggest that the activation of SREBP-1c and thus of lipogenesis is secondary in the steatotic liver to an ER stress. The ER stress activates the cleavage of SREBP-1c independent of insulin, thus explaining the paradoxical stimulation of lipogenesis in an insulin-resistant liver. Inhibition of the ER stress in obese rodents decreases SREBP-1c activation and lipogenesis and improves markedly hepatic steatosis and insulin sensitivity. ER is thus a new partner in steatosis and metabolic syndrome which is worth considering as a potential therapeutic target.

Biguanides and thiazolidinediones inhibit stimulated lipolysis in human adipocytes through activation of AMP-activated protein kinase.

AIMS/HYPOTHESIS: In rodent adipocytes, activated AMP-activated protein kinase reduces the lipolytic rate. As the hypoglycaemic drugs metformin and thiazolidinediones activate this enzyme in rodents, we tested the hypothesis that in addition to their known actions they could have an anti-lipolytic effect in human adipocytes. METHODS: Adipose tissue was obtained from individuals undergoing plastic surgery. Adipocytes were isolated and incubated with lipolytic agents (isoprenaline, atrial natriuretic peptide) and biguanides or thiazolidinediones. Lipolysis was quantified by the glycerol released in the medium. AMP-activated protein kinase activity and phosphorylation state were determined using standard procedures. RESULTS: In human adipocytes, isoprenaline and atrial natriuretic peptide stimulated the lipolytic rate three- to fourfold. Biguanides and thiazolidinediones activated AMP-activated protein kinase and inhibited lipolysis by 30-40%, at least in part by inhibiting hormone-sensitive lipase translocation to the lipid droplet. Inhibition of AMP-activated protein kinase by compound C precluded this inhibitory effect on lipolysis. Stimulation of lipolysis also induced an activation of AMP-activated protein kinase concomitant with a drop in ATP concentration. CONCLUSIONS/INTERPRETATION: We show for the first time in human adipocytes that biguanides and thiazolidinediones activate AMP-activated protein kinase, thus counteracting lipolysis induced by lipolytic agents. In addition, beta-agonist- or ANP-stimulated lipolysis increases AMP-activated protein kinase activity. This is because of an increase in the AMP/ATP ratio, linked to activation of some of the released fatty acids into acyl-CoA. AMP-activated protein kinase activation could represent a physiological means of avoiding a deleterious drain of energy during lipolysis but could be used to restrain pharmacological release of fatty acids.

AICAR and metformin, but not exercise, increase muscle glucose transport through AMPK-, ERK-, and PDK1-dependent activation of atypical PKC.

Activators of 5'-AMP-activated protein kinase (AMPK) 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR), metformin, and exercise activate atypical protein kinase C (aPKC) and ERK and stimulate glucose transport in muscle by uncertain mechanisms. Here, in cultured L6 myotubes: AICAR- and metformin-induced activation of AMPK was required for activation of aPKC and ERK; aPKC activation involved and required phosphoinositide-dependent kinase 1 (PDK1) phosphorylation of Thr410-PKC-zeta; aPKC Thr410 phosphorylation and activation also required MEK1-dependent ERK; and glucose transport effects of AICAR and metformin were inhibited by expression of dominant-negative AMPK, kinase-inactive PDK1, MEK1 inhibitors, kinase-inactive PKC-zeta, and RNA interference (RNAi)-mediated knockdown of PKC-zeta. In mice, muscle-specific aPKC (PKC-lambda) depletion by conditional gene targeting impaired AICAR-stimulated glucose disposal and stimulatory effects of both AICAR and metformin on 2-deoxyglucose/glucose uptake in muscle in vivo and AICAR stimulation of 2-[(3)H]deoxyglucose uptake in isolated extensor digitorum longus muscle; however, AMPK activation was unimpaired. In marked contrast to AICAR and metformin, treadmill exercise-induced stimulation of 2-deoxyglucose/glucose uptake was not inhibited in aPKC-knockout mice. Finally, in intact rodents, AICAR and metformin activated aPKC in muscle, but not in liver, despite activating AMPK in both tissues. The findings demonstrate that in muscle AICAR and metformin activate aPKC via sequential activation of AMPK, ERK, and PDK1 and the AMPK/ERK/PDK1/aPKC pathway is required for metformin- and AICAR-stimulated increases in glucose transport. On the other hand, although aPKC is activated by treadmill exercise, this activation is not required for exercise-induced increases in glucose transport, and therefore may be a redundant mechanism.

The polymorphism Arg585Gln in the gene of the sterol regulatory element binding protein-1 (SREBP-1) is not a determinant of ketosis prone type 2 diabetes (KPD) in Africans.

AIM: Ketosis prone type 2 diabetes (KPD) is an atypical form of diabetes described mainly in people of sub-Saharan African origin. Its pathogenesis is unknown, although we have previously described a high prevalence of glucose-6-phosphate-dehydrogenase (G6PD) deficiency in patients with KPD. However, 50% of these deficient patients lacked the G6PD gene mutation. The isoforms of the transcription factor sterol regulatory element binding protein 1 (SREBP-1) are known to stimulate G6PD gene expression, and some polymorphisms in the SREBP-1 gene (SREBF-1) have been described only in Africans. We investigated one of these, the Arg585Gln polymorphism, in a candidate gene approach for KPD. METHODS: We examined the presence of the Arg585Gln polymorphism in SREBF-1 in 217 consecutive unrelated Africans [73 patients with KPD, 80 with classical type 2 diabetes (T2D) and 64 nondiabetic subjects]. Patients underwent clinical and biochemical evaluations, and were assessed for G6PD activity and insulin secretion (glucagon test). RESULTS: There were no differences in frequency of the Arg585Gln polymorphism and the 585Gln allele among the three groups (allele frequency: KPD: 0.089, T2D: 0.031, nondiabetic group: 0.070; P=0.1). When the 585Gln allele frequency was compared separately between patients with KPD and those with T2D, it was significantly higher in the former (P=0.032). There was no difference between carriers and noncarriers of the 585Gln allele regarding G6PD activity and insulin secretion. CONCLUSION: The results of this exploratory study show that the polymorphism Arg585Gln in SREBF-1 is not associated with the KPD phenotype. Further studies in larger populations are needed to confirm our findings.

SREBP-1c transcription factor and lipid homeostasis: clinical perspective.

Insulin has long-term effects on glucose and lipid metabolism through its control on the expression of specific genes. In insulin sensitive tissues and particularly in the liver, the transcription factor sterol regulatory element binding protein-1c (SREBP-1c) transduces the insulin signal. SREBP-1c is a transcription factor which is synthetized as a precursor in the membranes of the endoplasmic reticulum and which requires post-translational modification to yield its transcriptionally active nuclear form. Insulin activates the transcription and the proteolytic maturation of SREBP-1c. SREBP-1c induces the expression of a family of genes involved in glucose utilization and fatty acid synthesis and can be considered as a thrifty gene. Since a high lipid availability is deleterious for insulin sensitivity and secretion, a role for SREBP-1c in dyslipidaemia and type 2 diabetes has been considered in genetic studies and some association demonstrated. Finally, SREBP-1c could also participate to the hepatic steatosis observed in humans and related to alcohol consumption and hyperhomocysteinaemia, two pathologies which are concomitant with a stress of the endoplasmic reticulum and an insulin-independent SREBP-1c activation.

Long chain fatty acyl-CoA synthetase 5 expression is induced by insulin and glucose: involvement of sterol regulatory element-binding protein-1c.

In a screen for sterol regulatory element-binding protein (SREBP)-1c target genes in the liver, we identified long chain fatty acyl-CoA synthetase 5 (ACS-5). Hepatic ACS-5 mRNA is poorly expressed during fasting and diabetes and strongly induced by carbohydrate refeeding and insulin treatment. In cultured hepatocytes, insulin and a high glucose concentration induce ACS-5 mRNA. Adenoviral overexpression of a nuclear form of SREBP-1c in liver of diabetic mice or in cultured hepatocytes mimics the effect of insulin to induce ACS-5. By contrast, a dominant negative form of SREBP-1c abolishes the effect of insulin on ACS-5 expression. The dietary and SREBP-1c-mediated insulin regulation of ACS-5 expression indicate that ACS-5 is involved in the anabolic fate of fatty acids.

AMP-activated protein kinase and hepatic genes involved in glucose metabolism.

Mammalian AMP-activated protein kinase presents strong structural and functional similarities with the yeast sucrose non-fermenting 1 (Snf1) kinase involved in the derepression of glucose-repressed genes. It is now clearly established that AMP-activated protein kinase in the liver decreases glycolytic/lipogenic gene expression as well as genes involved in hepatic glucose production. This is achieved through a decreased transcriptional efficiency of transcription factors such as sterol-regulatory-element-binding protein-1c, carbohydrate-response-element-binding protein, hepatocyte nuclear factor 4 alpha or forkhead-related protein. Clearly, the long-term consequences of AMP-activated protein kinase activation have to be taken into account if activators of this enzyme are to be designed as anti-diabetic drugs.

Adiponectin stimulates glucose utilization and fatty-acid oxidation by activating AMP-activated protein kinase.

Adiponectin (Ad) is a hormone secreted by adipocytes that regulates energy homeostasis and glucose and lipid metabolism. However, the signaling pathways that mediate the metabolic effects of Ad remain poorly identified. Here we show that phosphorylation and activation of the 5'-AMP-activated protein kinase (AMPK) are stimulated with globular and full-length Ad in skeletal muscle and only with full-length Ad in the liver. In parallel with its activation of AMPK, Ad stimulates phosphorylation of acetyl coenzyme A carboxylase (ACC), fatty-acid oxidation, glucose uptake and lactate production in myocytes, phosphorylation of ACC and reduction of molecules involved in gluconeogenesis in the liver, and reduction of glucose levels in vivo. Blocking AMPK activation by dominant-negative mutant inhibits each of these effects, indicating that stimulation of glucose utilization and fatty-acid oxidation by Ad occurs through activation of AMPK. Our data may provide a novel paradigm that an adipocyte-derived antidiabetic hormone, Ad, activates AMPK, thereby directly regulating glucose metabolism and insulin sensitivity in vitro and in vivo.

Adenovirus-mediated overexpression of sterol regulatory element binding protein-1c mimics insulin effects on hepatic gene expression and glucose homeostasis in diabetic mice.

In vitro, the transcription factor sterol regulatory element binding protein-1c (SREBP-1c) mimics the positive effects of insulin on hepatic genes involved in glucose utilization, such as glucokinase (GK) and enzymes of the lipogenic pathway, suggesting that it is a key factor in the control of hepatic glucose metabolism. Decreased glucose utilization and increased glucose production by the liver play an important role in the development of the hyperglycemia in diabetic states. We thus reasoned that if SREBP-1c is indeed a mediator of hepatic insulin action, a hepatic targeted overexpression of SREBP-1c should greatly improve glucose homeostasis in diabetic mice. This was achieved by injecting streptozotocin-induced diabetic mice with a recombinant adenovirus containing the cDNA of the mature, transcriptionally active form of SREBP-1c. We show here that overexpressing SREBP-1c specifically in the liver of diabetic mice induces GK and lipogenic enzyme gene expression and represses the expression of phosphoenolpyruvate carboxykinase, a key enzyme of the gluconeogenic pathway. This in turn increases glycogen and triglyceride hepatic content and leads to a marked decrease in hyperglycemia in diabetic mice. We conclude that SREBP-1c has a major role in vivo in the long-term control of glucose homeostasis by insulin.

Androgens stimulate lipogenic gene expression in prostate cancer cells by activation of the sterol regulatory element-binding protein cleavage activating protein/sterol regulatory element-binding protein pathway.

Using two independent prostate cancer cell lines (LNCaP and MDA-PCa-2a), we demonstrate that coordinated stimulation of lipogenic gene expression by androgens is a common phenomenon in androgen-responsive prostate tumor lines and involves activation of the sterol regulatory element-binding protein (SREBP) pathway. We show 1) that in both cell lines, androgens stimulate the expression of fatty acid synthase and hydroxymethylglutaryl-coenzyme A synthase, two key lipogenic genes representative for the fatty acid and the cholesterol synthesis pathway, respectively; 2) that treatment with androgens results in increased nuclear levels of active SREBP; 3) that the effects of androgens on promoter-reporter constructs derived from both lipogenic genes (fatty acid synthase and hydroxymethylglutaryl-coenzyme A synthase) depend on the presence of intact SREBP-binding sites; and 4) that cotransfection with dominant-negative forms of SREBPs abolishes the effects of androgens. Related to the mechanism underlying androgen activation of the SREBP pathway, we show that in addition to minor effects on SREBP precursor levels, androgens induce a major increase in the expression of sterol regulatory element-binding protein cleavage-activating protein (SCAP), an escort protein that transports SREBPs from their site of synthesis in the endoplasmic reticulum to their site of proteolytical activation in the Golgi. Both time course studies and overexpression experiments showing that increasing levels of SCAP enhance the production of mature SREBP and stimulate lipogenic gene expression support the contention that SCAP plays a pivotal role in the lipogenic effects of androgens in tumor cells.

Sterol-regulatory-element-binding protein 1c mediates insulin action on hepatic gene expression.

Effects of insulin on the expression of liver-specific genes are part of the adaptive mechanisms aimed at maintaining energy homeostasis in mammals. When the diet is rich in carbohydrates, secreted insulin stimulates the expression of genes for enzymes involved in glucose utilization (glucokinase, L-type pyruvate kinase and lipogenic enzymes) and inhibits genes for enzymes involved in glucose production (phosphenolpyruvate carboxykinase). The mechanisms by which insulin controls the expression of these genes have been poorly understood. Recently, the transcription factor sterol-regulatory-element-binding protein 1c has been proposed as a key mediator of insulin transcriptional effects. Here we review the evidence that has led to this proposal and the consequences for our understanding of insulin effects in physiological or pathological conditions.

Sterol regulatory element-binding protein-1c mimics the negative effect of insulin on phosphoenolpyruvate carboxykinase (GTP) gene transcription.

We have assessed the potential role of sterol regulatory element-binding protein-1c (SREBP-1c) on the transcription of the gene for the cytosolic form of phosphoenolpyruvate carboxykinase (GTP) (EC ) (PEPCK-C). SREBP-1c introduced into primary hepatocytes with an adenovirus vector caused a total loss of PEPCK-C mRNA and a marked induction of fatty acid synthase mRNA that directly coincided with the appearance of SREBP-1c in the hepatocytes. It also blocked the induction of PEPCK-C mRNA by cAMP and dexamethasone in these cells. In contrast, a dominant negative form of SREBP-1c (dnSREBP-1c) stimulated the accumulation of PEPCK-C mRNA in these cells. SREBP-1c completely blocked the induction of PEPCK-C gene transcription by the catalytic subunit of protein kinase A (PKA), and increasing concentrations of dnSREBP-1c reversed the negative effect of insulin on transcription from the PEPCK-C gene promoter in WT-IR cells. The more than 10-fold induction of PKA-stimulated PEPCK-C gene transcription caused by the co-activator CBP, was also blocked by SREBP-1c. In addition, dnSREBP-1c reversed the strong negative effect of E1A and NF1 on PKA-stimulated transcription from the PEPCK-C gene promoter. An analysis of the possible site of action of SREBP-1c using stepwise truncations of the PEPCK-C gene promoter indicated that the negative effect of SREBP-1c on transcription is exerted at a site between -355 and -277. We conclude that SREBP-1c is an intermediate in the action of insulin on PEPCK-C gene transcription in the liver and acts by blocking the stimulatory effect cAMP that is mediated via an interaction with cAMP-binding protein.

[Regulation of carbohydrate metabolism by insulin: role of transcription factor SREBP-1c in the hepatic transcriptional effects of the hormone]. / Régulation du métabolisme glucidique par l'insuline: rôle du facteur de transcription SREBP-1c dans les effets transcriptionnels hépatiques de l'hormone.

A number of tissues such as the brain must be continuously provided with glucose to meet their energy demand. In contrast, carbohydrate absorption during meals is a discontinuous process. Thus, we must store glucose when its is provided, release it or spare it when it is less abundant. Insulin, secreted by the pancreatic beta-cell is a key hormone in the adaptations of metabolic pathways linked to glucose homeostasis. It inhibits hepatic glucose production, promotes glucose storage in the liver and glucose uptake and storage in muscles and adipose tissues. This is achieved through the modifications of the activity of existing proteins (enzymes, transporters) but also through the regulation of gene expression. In the liver, when the diet is rich in carbohydrates, insulin is secreted and stimulates the expression of genes involved in glucose utilization (glucokinase, L-pyruvate kinase, lipogenic enzymes) and inhibits genes involved in glucose production (phosphenolpyruvate carboxykinase). The mechanisms by which insulin controls the expression of these genes were poorly understood. Recently, the transcription factor Sterol Regulatory Element Binding Protein-1c (SREBP-1c) has been proposed as a key mediator of insulin transcriptional effects. Insulin increases the synthesis and nuclear abundance of this factor which when overexpressed in the liver mimics the effects of insulin on insulin-sensitive genes. This suggests that SREBP-1c could be involved in pathologies such as type 2 diabetes, obesity and more generally in insulin resistance syndromes.

Study of the regulation by nutrients of the expression of genes involved in lipogenesis and obesity in humans and animals.

Dietary digestible carbohydrates are able to modulate lipogenesis, by modifying the expression of genes coding for key lipogenic enzymes, like fatty acid synthase. The overall objective of the Nutrigene project (FAIR-CT97-3011) was to study the efficiency of various carbohydrates to modulate the lipogenic capacity and relevant gene expression in rat and human species (control and obese subjects) and to understand the underlying molecular mechanisms involved in the regulation of lipogenic genes by carbohydrates. Key cellular mediators (namely SREBP-1c and 2, AMP activated protein kinase, cholesterol content) of the regulation of lipogenic gene expression by glucose and/or insulin were identified and constitute new putative targets in the development of plurimetabolic syndrome associated with obesity. In humans, hepatic lipogenesis and triglyceride synthesis, assessed in vivo by the use of stable isotopes, was promoted by a high-carbohydrate diet in non obese subjects, and in non alcoholic steatotic patients, but was not modified in the adipose tissue of obese subjects. Non digestible/fermentable carbohydrates, such as fructans, were shown to decrease hepatic lipogenesis in non obese rats, and to lessen hepatic steatosis and body weight in obese Zucker rats. If confirmed in obese humans, this would allow the development of functional food able to counteract the metabolic disturbances linked to obesity.

Stimulation of tumor-associated fatty acid synthase expression by growth factor activation of the sterol regulatory element-binding protein pathway.

Increased expression of fatty acid synthase (FAS) is observed in a clinically aggressive subset of various common cancers and interference with FAS offers promising opportunities for selective chemotherapeutic intervention. The mechanisms by which FAS expression is (up)-regulated in these tumors remain, however, largely unknown. Recently we demonstrated that in LNCaP prostate cancer cells FAS expression is markedly elevated by androgens via an indirect pathway involving sterol regulatory element-binding proteins (SREBPs). Here, we also show that growth factors such as EGF are able to stimulate FAS mRNA, protein and activity. Several observations also indicate that the effects of EGF on FAS expression are ultimately mediated by SREBPs. EGF stimulates SREBP-1c mRNA expression and induces an increase in mature nuclear SREBP-1. Moreover, in transient transfection studies EGF stimulates the transcriptional activity of a 178 bp FAS promoter fragment harboring a complex SREBP-binding site. Deletion or mutation of this binding site abolishes these effects and ectopic expression of dominant negative SREBP-1 inhibits FAS expression and induction in intact LNCaP cells. Given the frequent dysregulation of growth factor signaling in cancer and the key role of SREBP-1 in lipid homeostasis, growth factor-induced activation of the SREBP pathway is proposed as one of the mechanisms responsible for up-regulation of lipogenic gene expression in a subset of cancer cells.

Characterization of the role of AMP-activated protein kinase in the regulation of glucose-activated gene expression using constitutively active and dominant negative forms of the kinase.

In the liver, glucose induces the expression of a number of genes involved in glucose and lipid metabolism, e.g., those encoding L-type pyruvate kinase and fatty acid synthase. Recent evidence has indicated a role for the AMP-activated protein kinase (AMPK) in the inhibition of glucose-activated gene expression in hepatocytes. It remains unclear, however, whether AMPK is involved in the glucose induction of these genes. In order to study further the role of AMPK in regulating gene expression, we have generated two mutant forms of AMPK. One of these (alpha1(312)) acts as a constitutively active kinase, while the other (alpha1DN) acts as a dominant negative inhibitor of endogenous AMPK. We have used adenovirus-mediated gene transfer to express these mutants in primary rat hepatocytes in culture in order to determine their effect on AMPK activity and the transcription of glucose-activated genes. Expression of alpha1(312) increased AMPK activity in hepatocytes and blocked completely the induction of a number of glucose-activated genes in response to 25 mM glucose. This effect is similar to that observed following activation of AMPK by 5-amino-imidazolecarboxamide riboside. Expression of alpha1DN markedly inhibited both basal and stimulated activity of endogenous AMPK but had no effect on the transcription of glucose-activated genes. Our results suggest that AMPK is involved in the inhibition of glucose-activated gene expression but not in the induction pathway. This study demonstrates that the two mutants we have described will provide valuable tools for studying the wider physiological role of AMPK.

Insulin effects on sterol regulatory-element-binding protein-1c (SREBP-1c) transcriptional activity in rat hepatocytes.

The transcription factor sterol regulatory-element-binding protein-1c (SREBP-1c) plays a major role in the effect of insulin on the transcription of hepatic genes such as glucokinase and fatty acid synthase. We show here in cultured rat hepatocytes that insulin, through activation of the phosphatidylinositol 3-kinase pathway increases the abundance of the precursor form of SREBP-1c in endoplasmic reticulum. This precursor form is then rapidly cleaved, possibly irrespective of the continuous presence of insulin, leading to an increased content of the nuclear mature form of SREBP-1c. Nevertheless, the increased amount of the mature form of SREBP-1c in the nucleus is not a prerequisite for the rapid effect of insulin on the transcription of genes such as glucokinase, suggesting that additional actions of the hormone are involved, such as the activation of the nuclear form of SREBP-1c or of an unidentified SREBP-1c partner.