E2F transcription factors are thought to be key regulators of cell growth control. Here we use mutant mouse strains to investigate the function of E2F1 and E2F2 in vivo. E2F1/E2F2 compound-mutant mice develop nonautoimmune insulin-deficient diabetes and exocrine pancreatic dysfunction characterized by endocrine and exocrine cell dysplasia, a reduction in the number and size of acini and islets, and their replacement by ductal structures and adipose tissue. Mutant pancreatic cells exhibit increased rates of DNA replication but also of apoptosis, resulting in severe pancreatic atrophy. The expression of genes involved in DNA replication and cell cycle control was upregulated in the E2F1/E2F2 compound-mutant pancreas, suggesting that their expression is repressed by E2F1/E2F2 activities and that the inappropriate cell cycle found in the mutant pancreas is likely the result of the deregulated expression of these genes. Interestingly, the expression of ductal cell and adipocyte differentiation marker genes was also upregulated, whereas expression of pancreatic cell marker genes were downregulated. These results suggest that E2F1/E2F2 activity negatively controls growth of mature pancreatic cells and is necessary for the maintenance of differentiated pancreatic phenotypes in the adult.
Ainhoa Iglesias, Matilde Murga, Usua Laresgoiti, Anouchka Skoudy, Irantzu Bernales, Asier Fullaondo, Bernardino Moreno, José Lloreta, Seth J. Field, Francisco X. Real, Ana M. Zubiaga
We explored the effects of bile acids on triglyceride (TG) homeostasis using a combination of molecular, cellular, and animal models. Cholic acid (CA) prevents hepatic TG accumulation, VLDL secretion, and elevated serum TG in mouse models of hypertriglyceridemia. At the molecular level, CA decreases hepatic expression of SREBP-1c and its lipogenic target genes. Through the use of mouse mutants for the short heterodimer partner (SHP) and liver X receptor (LXR) α and β, we demonstrate the critical dependence of the reduction of SREBP-1c expression by either natural or synthetic farnesoid X receptor (FXR) agonists on both SHP and LXRα and LXRβ. These results suggest that strategies aimed at increasing FXR activity and the repressive effects of SHP should be explored to correct hypertriglyceridemia.
Mitsuhiro Watanabe, Sander M. Houten, Li Wang, Antonio Moschetta, David J. Mangelsdorf, Richard A. Heyman, David D. Moore, Johan Auwerx
We evaluated the effects of E2F1 on glucose homeostasis using E2F1–/– mice. E2F1–/– mice show an overall reduction in pancreatic size as the result of impaired postnatal pancreatic growth. Furthermore, these animals have dysfunctional β cells, linked to impaired PDX-1 activity. Because of the disproportionate small pancreas and dysfunctional islets, E2F1–/– mice secrete insufficient amounts of insulin in response to a glucose load, resulting in glucose intolerance. Despite this glucose intolerance, E2F1–/– mice do not develop overt diabetes mellitus because they have insulin hypersensitivity, which is secondary to a diminished adipose tissue mass and altered adipocytokine levels, which compensates for the defect in insulin secretion. These data demonstrate that factors controlling cell proliferation, such as E2F1, determine pancreatic growth and function, subsequently affecting metabolic homeostasis.
Lluis Fajas, Jean-Sébastien Annicotte, Stéphanie Miard, David Sarruf, Mitsuhiro Watanabe, Johan Auwerx
IGF-1 has been associated with the pathogenesis of diabetic retinopathy, although its role is not fully understood. Here we show that normoglycemic/normoinsulinemic transgenic mice overexpressing IGF-1 in the retina developed most alterations seen in human diabetic eye disease. A paracrine effect of IGF-1 in the retina initiated vascular alterations that progressed from nonproliferative to proliferative retinopathy and retinal detachment. Eyes from 2-month-old transgenic mice showed loss of pericytes and thickening of basement membrane of retinal capillaries. In mice 6 months and older, venule dilatation, intraretinal microvascular abnormalities, and neovascularization of the retina and vitreous cavity were observed. Neovascularization was consistent with increased IGF-1 induction of VEGF expression in retinal glial cells. In addition, IGF-1 accumulated in aqueous humor, which may have caused rubeosis iridis and subsequently adhesions between the cornea and iris that hampered aqueous humor drainage and led to neovascular glaucoma. Furthermore, all transgenic mice developed cataracts. These findings suggest a role of IGF-1 in the development of ocular complications in long-term diabetes. Thus, these transgenic mice may be used to study the mechanisms that lead to diabetes eye disease and constitute an appropriate model in which to assay new therapies.
Jesús Ruberte, Eduard Ayuso, Marc Navarro, Ana Carretero, Víctor Nacher, Virginia Haurigot, Mónica George, Cristina Llombart, Alba Casellas, Cristina Costa, Assumpció Bosch, Fatima Bosch
In the current studies we generated transgenic mice that overexpress human Insig-1 in the liver under a constitutive promoter. In cultured cells Insig-1 and Insig-2 have been shown to block lipid synthesis in a cholesterol-dependent fashion by inhibiting proteolytic processing of sterol regulatory element–binding proteins (SREBPs), membrane-bound transcription factors that activate lipid synthesis. Insig’s exert this action in the ER by binding SREBP cleavage-activating protein (SCAP) and preventing it from escorting SREBPs to the Golgi apparatus where the SREBPs are processed to their active forms. In the livers of Insig-1 transgenic mice, the content of all nuclear SREBPs (nSREBPs) was reduced and declined further upon feeding of dietary cholesterol. The nuclear content of the insulin-induced SREBP isoform, SREBP-1c, failed to increase to a normal extent upon refeeding on a high-carbohydrate diet. The nSREBP deficiency produced a marked reduction in the levels of mRNAs encoding enzymes required for synthesis of cholesterol, fatty acids, and triglycerides. Plasma cholesterol levels were strongly reduced, and plasma triglycerides did not exhibit their normal rise after refeeding. These results provide in vivo support for the hypothesis that nSREBPs are essential for high levels of lipid synthesis in the liver and indicate that Insig’s modulate nSREBP levels by binding and retaining SCAP in the ER.
Luke J. Engelking, Hiroshi Kuriyama, Robert E. Hammer, Jay D. Horton, Michael S. Brown, Joseph L. Goldstein, Guosheng Liang
Insulin resistance in skeletal muscle plays a major role in the development of type 2 diabetes and may be causally associated with increases in intramuscular fatty acid metabolites. Fatty acid transport protein 1 (FATP1) is an acyl-CoA synthetase highly expressed in skeletal muscle and modulates fatty acid uptake and metabolism by converting fatty acids into fatty acyl-CoA. To investigate the role of FATP1 in glucose homeostasis and in the pathogenesis of insulin resistance, we examined the effect of acute lipid infusion or chronic high-fat feeding on insulin action in FATP1 KO mice. Whole-body adiposity, adipose tissue expression of adiponectin, intramuscular fatty acid metabolites, and insulin sensitivity were not altered in FATP1 KO mice fed a regular chow diet. In contrast, FATP1 deletion protected the KO mice from fat-induced insulin resistance and intramuscular accumulation of fatty acyl-CoA without alteration in whole-body adiposity. These findings demonstrate an important role of intramuscular fatty acid metabolites in causing insulin resistance and suggest that FATP1 may be a novel therapeutic target for the treatment of insulin resistance and type 2 diabetes.
Jason K. Kim, Ruth E. Gimeno, Takamasa Higashimori, Hyo-Jeong Kim, Hyejeong Choi, Sandhya Punreddy, Robin L. Mozell, Guo Tan, Alain Stricker-Krongrad, David J. Hirsch, Jonathan J. Fillmore, Zhen-Xiang Liu, Jianying Dong, Gary Cline, Andreas Stahl, Harvey F. Lodish, Gerald I. Shulman
Accelerated atherosclerosis is a major cause of morbidity and death in insulin-resistant states such as obesity and the metabolic syndrome, but the underlying mechanisms are poorly understood. We show that macrophages from obese (ob/ob) mice have increased binding and uptake of oxidized LDL, in part due to a post-transcriptional increase in CD36 protein. Macrophages from ob/ob mice are also insulin resistant, as shown by reduced expression and signaling of insulin receptors. Three lines of evidence indicate that the increase in CD36 is caused by defective insulin signaling: (a) Treatment of wild-type macrophages with LY294002, an inhibitor of insulin signaling via PI3K, results in an increase in CD36; (b) insulin receptor knockout macrophages show a post-transcriptional increase in CD36 protein; and (c) administration of thiazolidinediones to intact ob/ob mice and ob/ob, LDL receptor–deficient mice results in a reversal of macrophage insulin receptor defects and decreases CD36 protein. The last finding contrasts with the increase in CD36 that results from treatment of macrophages with these drugs ex vivo. The results suggest that defective macrophage insulin signaling predisposes to foam cell formation and atherosclerosis in insulin-resistant states and that this is reversed in vivo by treatment with PPAR-γ activators.
Chien-Ping Liang, Seongah Han, Haruka Okamoto, Ronald Carnemolla, Ira Tabas, Domenico Accili, Alan R. Tall
The role of the gluco-incretin hormones GIP and GLP-1 in the control of β cell function was studied by analyzing mice with inactivation of each of these hormone receptor genes, or both. Our results demonstrate that glucose intolerance was additively increased during oral glucose absorption when both receptors were inactivated. After intraperitoneal injections, glucose intolerance was more severe in double- as compared to single-receptor KO mice, and euglycemic clamps revealed normal insulin sensitivity, suggesting a defect in insulin secretion. When assessed in vivo or in perfused pancreas, insulin secretion showed a lack of first phase in Glp-1R–/– but not in Gipr–/– mice. In perifusion experiments, however, first-phase insulin secretion was present in both types of islets. In double-KO islets, kinetics of insulin secretion was normal, but its amplitude was reduced by about 50% because of a defect distal to plasma membrane depolarization. Thus, gluco-incretin hormones control insulin secretion (a) by an acute insulinotropic effect on β cells after oral glucose absorption (b) through the regulation, by GLP-1, of in vivo first-phase insulin secretion, probably by an action on extra-islet glucose sensors, and (c) by preserving the function of the secretory pathway, as evidenced by a β cell autonomous secretion defect when both receptors are inactivated.
Frédéric Preitner, Mark Ibberson, Isobel Franklin, Christophe Binnert, Mario Pende, Asllan Gjinovci, Tanya Hansotia, Daniel J. Drucker, Claes Wollheim, Rémy Burcelin, Bernard Thorens
Lipodystrophy is characterized by the complete or partial absence of adipose tissue, insulin resistance, hepatic steatosis, and leptin deficiency. Here, we show that low-dose central leptin corrects the insulin resistance and fatty liver of lipodystrophic aP2-nSREBP-1c mice, while the same dose given peripherally does not. Central leptin also repressed stearoyl-CoA desaturase-1 (SCD-1) RNA and enzymatic activity, which were increased in livers of lipodystrophic mice. aP2-nSREBP-1c mice homozygous for an SCD-1 deletion had markedly reduced hepatic steatosis, increased saturated fatty acids, decreased acetyl-CoA carboxylase activity, and decreased malonyl-CoA levels in the liver. Despite the reduction in hepatic steatosis, these mice remained diabetic. A leptin dose-response curve showed that subcutaneous leptin improved hyperglycemia and hyperinsulinemia in aP2-nSREBP-1c mice at doses that did not substantially alter hepatic steatosis or hepatic SCD enzymatic activity. Leptin treatment at this dose improved insulin-stimulated insulin receptor and insulin receptor substrate 2 (IRS-2) phosphorylation, IRS-2–associated PI3K activity, and Akt activity in liver. Together, these data suggest that CNS-mediated repression of SCD-1 contributes to leptin’s antisteatotic actions. Intracerebroventricular leptin improves glucose homeostasis by improving insulin signal transduction in liver, but in this case the effect appears to be independent of SCD-1.
Esra Asilmaz, Paul Cohen, Makoto Miyazaki, Pawel Dobrzyn, Kohjiro Ueki, Gulnorakhon Fayzikhodjaeva, Alexander A. Soukas, C. Ronald Kahn, James M. Ntambi, Nicholas D. Socci, Jeffrey M. Friedman
Tandem mass spectrometry was applied to detect derangements in the pathways of amino acid and fatty acid metabolism in N-ethyl-N-nitrosourea–treated (ENU-treated) mice. We identified mice with marked elevation of blood branched-chain amino acids (BCAAs), ketoaciduria, and clinical features resembling human maple syrup urine disease (MSUD), a severe genetic metabolic disorder caused by the deficiency of branched-chain α-keto acid dehydrogenase (BCKD) complex. However, the BCKD genes and enzyme activity were normal. Sequencing of branched-chain aminotransferase genes (Bcat) showed no mutation in the cytoplasmic isoform (Bcat-1) but revealed a homozygous splice site mutation in the mitochondrial isoform (Bcat-2). The mutation caused a deletion of exon 2, a marked decrease in Bcat-2 mRNA, and a deficiency in both BCAT-2 protein and its enzyme activity. Affected mice responded to a BCAA-restricted diet with amelioration of the clinical symptoms and normalization of the amino acid pattern. We conclude that BCAT-2 deficiency in the mouse can cause a disease that mimics human MSUD. These mice provide an important animal model for study of BCAA metabolism and its toxicity. Metabolomics-guided screening, coupled with ENU mutagenesis, is a powerful approach in uncovering novel enzyme deficiencies and recognizing important pathways of genetic metabolic disorders.
Jer-Yuarn Wu, Hsiao-Jung Kao, Sing-Chung Li, Robert Stevens, Steven Hillman, David Millington, Yuan-Tsong Chen