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.

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.

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Aims/hypothesis Levels of uncoupling protein-2 (UCP2) are regulated in the pancreatic beta cells and an increase in the protein level has been associated with mitochondrial uncoupling and alteration in glucose-stimulated insulin secretion. However, it is not clear whether an increase in uncoupling protein-2 per se induces mitochondrial uncoupling and affects ATP generation and insulin secretion. Materials and methods Transgenic mice with beta cell-specific overexpression of the human UCP2 gene and INS-1 cells with doxycycline-inducible overproduction of the protein were generated and the consequences of increased levels of UCP2 on glucose-induced insulin secretion and on parameters reflecting mitochondrial uncoupling were determined.Results In transgenic mice, an increase in beta cell UCP2 protein concentration did not significantly modify plasma glucose and insulin levels. Glucose-induced insulin secretion and elevation in the ATP/ADP ratio were unaltered by an increase in UCP2 level. In INS-1 cells, a similar increase in UCP2 level did not modify glucose-induced insulin secretion, cytosolic ATP and ATP/ADP ratio, or glucose oxidation. Increased levels of UCP2 did not modify the mitochondrial membrane potential and oxygen consumption. Increased UCP2 levels decreased cytokine-induced production of reactive oxygen species. Conclusion/interpretation The results obtained in transgenic mice and in the beta cell line do not support the hypothesis that an increase in UCP2 protein per se uncouples the mitochondria and decreases glucose-induced insulin secretion. In contrast, the observation that increased UCP2 levels decrease cytokine-induced production of reactive oxygen species indicates a potential protective effect of the protein on beta cells, as observed in other cell types.

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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

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.

Protein synthesis in mammalian cells is regulated through alterations in the states of phosphorylation of initiation and elongation factors (eIF's and eEF's), and of other regulatory proteins. This modulates their activities or their abilities to interact with one another. Insulin activates a number of these proteins including the guanine nucleotide-exchange factor eIF2B; the eIF4F complex, which (through eIF4E) interacts with the cap of the mRNA; p70 S6 kinase and elongation factor eEF2, which mediates the translation step of elongation. Control of the last three of these is linked to mTOR. In CHO cells, regulation of all these proteins by insulin is modulated by the presence of amino acids and/or glucose in the medium. For example, p70 S6 kinase activity declines in the absence of amino acids and cannot be stimulated by insulin under this condition. Readdition of amino acids, especially leucine, restores activity and insulin-sensitivity. In the case of eIF2B and eEF2, both amino acids and glucose must be provided in order for insulin to regulate their activities. In contrast, insulin-stirnulation of the formation of eIF4F complexes requires glucose, but not amino acids. Glucose metabolism is required for this permissive effect.Provision of substrates to mammalian tissues requires constant adaptations of metabolism to the quality and quantity of nutrients. Although it is well known that insulin has an important role in hepatic adaptations at the gene level, the molecular mechanisms involved are poorly understood. Recent findings have pointed to SREBP-lc (Sterol Regulatory Element Binding Protein -1c) as the transcription factor involved in insulin action. We have shown that SREBP-lc expression and nuclear abundance is positively regulated by insulin through a mechanism involving PL-kinase, and negatively by glucagon. Adenovirus-mediated ovcrexprcssion of a transcriptionally active form of SREBP-I c in cultured rat hepatocytes mimicks the inductive effects of insulin on glycolytic and lipogenic gene expression. A dominant negative form of SREBP-lc has the opposite effects. SREBP-lc mimicks also the negative effect of insulin since it is able to down-regulate the expression and the promoter activity of phosphoenolpyruvate carboxykinase in cultured hepatocytes. Injection of an adenovirus containing the active form of SREBP-lc to diabetic mice induces a rapid decrease of their hyperglycaemia and the predicted changes in hepatic gene expression. These studies point out to SREBP-lc as an important actor in the long term regulation of glucose homeostasis. ~1 7Insulin-Regulated Gene Expression.