Casitas b-lineage lymphoma (c-Cbl) is an E3 ubiquitin ligase that has an important role in regulating the degradation of cell surface receptors. In the present study we have examined the role of c-Cbl in whole-body energy homeostasis. c-Cbl–/– mice exhibited a profound increase in whole-body energy expenditure as determined by increased core temperature and whole-body oxygen consumption. As a consequence, these mice displayed a decrease in adiposity, primarily due to a reduction in cell size despite an increase in food intake. These changes were accompanied by a significant increase in activity (2- to 3-fold). In addition, c-Cbl–/– mice displayed a marked improvement in whole-body insulin action, primarily due to changes in muscle metabolism. We observed increased protein levels of the insulin receptor (4-fold) and uncoupling protein-3 (2-fold) in skeletal muscle and a significant increase in the phosphorylation of AMP-activated protein kinase and acetyl-CoA carboxylase. These findings suggest that c-Cbl plays an integral role in whole-body fuel homeostasis by regulating whole-body energy expenditure and insulin action.
Juan C. Molero, Thomas E. Jensen, Phil C. Withers, Michelle Couzens, Herbert Herzog, Christine B.F. Thien, Wallace Y. Langdon, Ken Walder, Maria A. Murphy, David D.L. Bowtell, David E. James, Gregory J. Cooney
The accurate matching of caloric intake to caloric expenditure involves a complex system of peripheral signals and numerous CNS neurotransmitter systems. Syndecans are a family of membrane-bound heparan sulfate proteoglycans that modulate ligand-receptor interactions. Syndecan-3 is heavily expressed in several areas of the brain, including hypothalamic nuclei, which are known to regulate energy balance. In particular, syndecans have been implicated in modulation of the activity of the melanocortin system, which potently regulates energy intake, energy expenditure, and peripheral glucose metabolism. Our data demonstrate that syndecan-3–null mice have reduced adipose content compared with wild-type mice. On a high-fat diet, syndecan-3–null male and female mice exhibited a partial resistance to obesity due to reduced food intake in males and increased energy expenditure in females relative to that of wild-type mice. As a result, syndecan-3–null mice on a high-fat diet accumulated less adipose mass and showed improved glucose tolerance compared with wild-type controls. The data implicate syndecan-3 in the regulation of body weight and suggest that inhibition of syndecan-3 may provide a therapeutic approach for the treatment of obesity resulting from exposure to high-fat diets.
April D. Strader, Ofer Reizes, Stephen C. Woods, Stephen C. Benoit, Randy J. Seeley
We found that when a site-specific binding protein interacts with the “handle” region of the prorenin prosegment, the prorenin molecule undergoes a conformational change to its enzymatically active state. This nonproteolytic activation is completely blocked by a decoy peptide with the handle region structure, which competitively binds to such a binding protein. Given increased plasma prorenin in diabetes, we examined the hypothesis that the nonproteolytic activation of prorenin plays a significant role in diabetic organ damage. Streptozotocin-induced diabetic rats were treated with subcutaneous administration of handle region peptide. Metabolic and renal histological changes and the renin-Ang system components in the plasma and kidneys were determined at 8, 16, and 24 weeks following streptozotocin treatment. Kidneys of diabetic rats contained increased Ang I and II without any changes in renin, Ang-converting enzyme, or angiotensinogen synthesis. Treatment with the handle region peptide decreased the renal content of Ang I and II, however, and completely inhibited the development of diabetic nephropathy without affecting hyperglycemia. We propose that the nonproteolytic activation of prorenin may be a significant mechanism of diabetic nephropathy. The mechanism and substances causing nonproteolytic activation of prorenin may serve as important therapeutic targets for the prevention of diabetic organ damage.
Atsuhiro Ichihara, Matsuhiko Hayashi, Yuki Kaneshiro, Fumiaki Suzuki, Tsutomu Nakagawa, Yuko Tada, Yukako Koura, Akira Nishiyama, Hirokazu Okada, M. Nasir Uddin, A.H.M. Nurun Nabi, Yuichi Ishida, Tadashi Inagami, Takao Saruta
The melanocortin-4 receptor (MC4R), a centrally expressed G protein–coupled receptor (GPCR), is essential for the maintenance of long-term energy balance in humans. Mutations in MC4R are the most common genetic cause of obesity. Since activation of this receptor leads to a decrease in food intake, MC4R is also a major therapeutic target for the treatment of obesity. Control of MC4R activity in vivo is modulated by the opposing effects of the anorexigenic agonist α–melanocyte-stimulating hormone (α-MSH) and the orexigenic antagonist agouti-related protein (AGRP). In addition, experiments in vitro have demonstrated that the human MC4R has an intrinsic constitutive activity on which AGRP also acts as an inverse agonist. The physiological role of this constitutive activity in the control of energy balance as well as the domain of the protein implicated in its maintenance are unknown. By systematically studying functional defects in naturally occurring MC4R mutations from obese patients, we defined a cluster of N-terminal mutations that selectively impair the constitutive activity of the receptor. Further characterization of this domain demonstrated that it functions as a tethered intramolecular ligand that maintains the constitutive activity of MC4R and may provide novel avenues for the design of drugs targeting this receptor. Our results also suggest that the tonic satiety signal provided by the constitutive activity of MC4R may be required for maintaining long-term energy homeostasis in humans.
Supriya Srinivasan, Cecile Lubrano-Berthelier, Cedric Govaerts, Franck Picard, Pamela Santiago, Bruce R. Conklin, Christian Vaisse
The molecular link between obesity and β cell failure that causes diabetes is difficult to establish. Here we show that a conditional knockout of insulin receptor substrate 2 (Irs2) in mouse pancreas β cells and parts of the brain — including the hypothalamus —increased appetite, lean and fat body mass, linear growth, and insulin resistance that progressed to diabetes. Diabetes resolved when the mice were between 6 and 10 months of age: functional β cells expressing Irs2 repopulated the pancreas, restoring sufficient β cell function to compensate for insulin resistance in the obese mice. Thus, Irs2 signaling promotes regeneration of adult β cells and central control of nutrient homeostasis, which can prevent obesity and diabetes in mice.
Xueying Lin, Akiko Taguchi, Sunmin Park, Jake A. Kushner, Fan Li, Yedan Li, Morris F. White
We previously demonstrated that insulin receptor substrate 2 (Irs2) KO mice develop diabetes associated with hepatic insulin resistance, lack of compensatory β cell hyperplasia, and leptin resistance. To more precisely determine the roles of Irs2 in β cells and the hypothalamus, we generated β cell–specific Irs2 KO and hypothalamus-specific Irs2 knockdown (βHT-IRS2) mice. Expression of Irs2 mRNA was reduced by approximately 90% in pancreatic islets and was markedly reduced in the arcuate nucleus of the hypothalamus. By contrast, Irs2 expression in liver, muscle, and adipose tissue of βHT-IRS2 mice was indistinguishable from that of control mice. The βHT-IRS2 mice displayed obesity and leptin resistance. At 4 weeks of age, the βHT-IRS2 mice showed normal insulin sensitivity, but at 8 and 12 weeks, they were insulin resistant with progressive obesity. Despite their normal insulin sensitivity at 8 weeks with caloric restriction, the βHT-IRS2 mice exhibited glucose intolerance and impaired glucose-induced insulin secretion. β Cell mass and β cell proliferation in the βHT-IRS2 mice were reduced significantly at 8 and 12 weeks but not at 10 days. Insulin secretion, normalized by cell number per islet, was significantly increased at high glucose concentrations in the βHT-IRS2 mice. We conclude that, in β cells and the hypothalamus, Irs2 is crucially involved in the regulation of β cell mass and leptin sensitivity.
Naoto Kubota, Yasuo Terauchi, Kazuyuki Tobe, Wataru Yano, Ryo Suzuki, Kohjiro Ueki, Iseki Takamoto, Hidemi Satoh, Toshiyuki Maki, Tetsuya Kubota, Masao Moroi, Miki Okada-Iwabu, Osamu Ezaki, Ryozo Nagai, Yoichi Ueta, Takashi Kadowaki, Tetsuo Noda
The insulin and IGF signaling pathways are critical for development and maintenance of pancreatic β cell mass and function. The serine-threonine kinase Akt is one of several mediators regulated by these pathways. We have studied the role of Akt in pancreatic β cell physiology by generating transgenic mice expressing a kinase-dead mutant of this enzyme in β cells. Reduction of Akt activity in transgenic animals resulted in impaired glucose tolerance due to defective insulin secretion. The mechanisms involved in dysregulation of secretion in these mice lie at the level of insulin exocytosis and are not the result of abnormalities in glucose signaling or function of voltage-gated Ca2+ channels. Therefore, transgenic mice showed increased susceptibility to developing glucose intolerance and diabetes following fat feeding. These observations suggest that Akt plays a novel and important role in the regulation of distal components of the secretory pathway and that this enzyme represents a therapeutic target for improvement of β cell function in diabetes.
Ernesto Bernal-Mizrachi, Szabolcs Fatrai, James D. Johnson, Mitsuru Ohsugi, Kenichi Otani, Zhiqiang Han, Kenneth S. Polonsky, M. Alan Permutt
The endocrine pancreas undergoes major remodeling during neonatal development when replication of differentiated β cells is the major mechanism by which β cell mass is regulated. The molecular mechanisms that govern the replication of terminally differentiated β cells are unclear. We show that during neonatal development, cyclin D2 expression in the endocrine pancreas coincides with the replication of endocrine cells and a massive increase in islet mass. Using cyclin D2–/– mice, we demonstrate that cyclin D2 is required for the replication of endocrine cells but is expendable for exocrine and ductal cell replication. As a result, 14-day-old cyclin D2–/– mice display dramatically smaller islets and a 4-fold reduction in β cell mass in comparison to their WT littermates. Consistent with these morphological findings, the cyclin D2–/– mice are glucose intolerant. These results suggest that cyclin D2 plays a key role in regulating the transition of β cells from quiescence to replication and may provide a target for the development of therapeutic strategies to induce expansion and/or regeneration of β cells.
Senta Georgia, Anil Bhushan
The autoimmune disease type 1 diabetes in humans and NOD mice is determined by multiple genetic factors, among the strongest of which is the inheritance of diabetes-permissive MHC class II alleles associated with susceptibility to disease. Here we examined whether expression of MHC class II alleles associated with resistance to disease could be used to prevent the occurrence of diabetes. Expression of diabetes-resistant MHC class II I-Aβ chain molecules in NOD mice following retroviral transduction of autologous bone marrow hematopoietic stem cells prevented the development of autoreactive T cells by intrathymic deletion and protected the mice from the development of insulitis and diabetes. These data suggest that type 1 diabetes could be prevented in individuals expressing MHC alleles associated with susceptibility to disease by restoration of protective MHC class II expression through genetic engineering of hematopoietic stem cells.
Chaorui Tian, Jessamyn Bagley, Nathalie Cretin, Nilufer Seth, Kai W. Wucherpfennig, John Iacomini
The ABC transporters ABCG5 and ABCG8 limit absorption and promote excretion of dietary plant sterols. It is not known why plant sterols are so assiduously excluded from the body. Here we show that accumulation of plant sterols in mice lacking ABCG5 and ABCG8 (G5G8–/– mice) profoundly perturbs cholesterol homeostasis in the adrenal gland. The adrenal glands of the G5G8–/– mice were grossly abnormal in appearance (brown, not white) due to a 91% reduction in cholesterol content. Despite the very low cholesterol levels, there was no compensatory increase in cholesterol synthesis or in lipoprotein receptor expression. Moreover, levels of ABCA1, which mediates sterol efflux, were increased 10-fold in the G5G8–/– adrenals. Adrenal cholesterol levels returned to near-normal levels in mice treated with ezetimibe, which blocks phytosterol absorption. To determine which plant sterol(s) caused the metabolic changes, we examined the effects of individual plant sterols on cholesterol metabolism in cultured adrenal cells. Addition of stigmasterol, but not sitosterol, inhibited SREBP-2 processing and reduced cholesterol synthesis. Stigmasterol also activated the liver X receptor in a cell-based reporter assay. These data indicate that selected dietary plant sterols disrupt cholesterol homeostasis by affecting two critical regulatory pathways of lipid metabolism.
Chendong Yang, Liqing Yu, Weiping Li, Fang Xu, Jonathan C. Cohen, Helen H. Hobbs