The serine/threonine kinase Akt/PKB plays key roles in the regulation of cell growth, survival, and metabolism. It remains unclear, however, whether the functions of individual Akt/PKB isoforms are distinct. To investigate the function of Akt2/PKBβ, mice lacking this isoform were generated. Both male and female Akt2/PKBβ-null mice exhibit mild growth deficiency and an age-dependent loss of adipose tissue or lipoatrophy, with all observed adipose depots dramatically reduced by 22 weeks of age. Akt2/PKBβ-deficient mice are insulin resistant with elevated plasma triglycerides. In addition, Akt2/PKBβ-deficient mice exhibit fed and fasting hyperglycemia, hyperinsulinemia, glucose intolerance, and impaired muscle glucose uptake. In males, insulin resistance progresses to a severe form of diabetes accompanied by pancreatic β cell failure. In contrast, female Akt2/PKBβ-deficient mice remain mildly hyperglycemic and hyperinsulinemic until at least one year of age. Thus, Akt2/PKBβ-deficient mice exhibit growth deficiency similar to that reported previously for mice lacking Akt1/PKBα, indicating that both Akt2/PKBβ and Akt1/PKBα participate in the regulation of growth. The marked hyperglycemia and loss of pancreatic β cells and adipose tissue in Akt2/PKBβ-deficient mice suggest that Akt2/PKBβ plays critical roles in glucose metabolism and the development or maintenance of proper adipose tissue and islet mass for which other Akt/PKB isoforms are unable to fully compensate.
Robert S. Garofalo, Stephen J. Orena, Kristina Rafidi, Anthony J. Torchia, Jeffrey L. Stock, Audrey L. Hildebrandt, Timothy Coskran, Shawn C. Black, Dominique J. Brees, Joan R. Wicks, John D. McNeish, Kevin G. Coleman
Recent evidence suggests the existence of a hepatoportal vein glucose sensor, whose activation leads to enhanced glucose use in skeletal muscle, heart, and brown adipose tissue. The mechanism leading to this increase in whole body glucose clearance is not known, but previous data suggest that it is insulin independent. Here, we sought to further determine the portal sensor signaling pathway by selectively evaluating its dependence on muscle GLUT4, insulin receptor, and the evolutionarily conserved sensor of metabolic stress, AMP-activated protein kinase (AMPK). We demonstrate that the increase in muscle glucose use was suppressed in mice lacking the expression of GLUT4 in the organ muscle. In contrast, glucose use was stimulated normally in mice with muscle-specific inactivation of the insulin receptor gene, confirming independence from insulin-signaling pathways. Most importantly, the muscle glucose use in response to activation of the hepatoportal vein glucose sensor was completely dependent on the activity of AMPK, because enhanced hexose disposal was prevented by expression of a dominant negative AMPK in muscle. These data demonstrate that the portal sensor induces glucose use and development of hypoglycemia independently of insulin action, but by a mechanism that requires activation of the AMPK and the presence of GLUT4.
Rémy Burcelin, Valerie Crivelli, Christophe Perrin, Anabela Da Costa, James Mu, Barbara B. Kahn, Morris J. Birnbaum, C. Ronald Kahn, Peter Vollenweider, Bernard Thorens
Bone marrow or hematopoietic stem cell transplantation is a potential treatment for autoimmune disease. The clinical application of this approach is, however, limited by the risks associated with allogeneic transplantation. In contrast, syngeneic transplantation would be safe and have wide clinical application. Because T cell tolerance can be induced by presenting antigen on resting antigen-presenting cells (APCs), we reasoned that hematopoietic stem cells engineered to express autoantigen in resting APCs could be used to prevent autoimmune disease. Proinsulin is a major autoantigen associated with pancreatic β cell destruction in humans with type 1 diabetes (T1D) and in autoimmune NOD mice. Here, we demonstrate that syngeneic transplantation of hematopoietic stem cells encoding proinsulin transgenically targeted to APCs totally prevents the development of spontaneous autoimmune diabetes in NOD mice. This antigen-specific immunotherapeutic strategy could be applied to prevent T1D and other autoimmune diseases in humans.
Raymond J. Steptoe, Janine M. Ritchie, Leonard C. Harrison
Insulin is a major target of the autoimmune response associated with destruction of pancreatic β cells in type 1 diabetes. A peptide that spans the junction of the insulin B chain and the connecting (C) peptide in proinsulin has been reported to stimulate T cells from humans at risk for type 1 diabetes and autoimmune diabetes–prone NOD mice. Here we show that proinsulin B24–C36 peptide binds to I-Ag7, the MHC class II molecule of the NOD mouse, and, after intranasal administration, induces regulatory CD4+ T cells that, in the absence of CD8+ T cells, block the adoptive transfer of diabetes. Curiously, however, intranasal B24–C36 did not inhibit development of spontaneous diabetes in treated mice. We then determined that B24–C36, and its core sequence B25–C34, bind to Kd, the NOD mouse MHC class I molecule, and elicit CD8+ CTLs. When the CD8+ T lymphocyte epitope was truncated at the C34 valine anchor residue for binding to Kd, the residual CD4+ T cell epitope, B24–C32/33, significantly inhibited diabetes development after a single intranasal dose. This study identifies a novel CTL epitope in proinsulin and demonstrates that the therapeutic potential of a “tolerogenic” autoantigen peptide can be compromised by the presence of an integral CTL epitope.
Nathan R. Martinez, Petra Augstein, Antonis K. Moustakas, George K. Papadopoulos, Silvia Gregori, Luciano Adorini, David C. Jackson, Leonard C. Harrison
Protein targeting to glycogen (PTG) is a scaffolding protein that targets protein phosphatase 1α (PP1α) to glycogen, and links it to enzymes involved in glycogen synthesis and degradation. We generated mice that possess a heterozygous deletion of the PTG gene. These mice have reduced glycogen stores in adipose tissue, liver, heart, and skeletal muscle, corresponding with decreased glycogen synthase activity and glycogen synthesis rate. Although young PTG heterozygous mice initially demonstrate normal glucose tolerance, progressive glucose intolerance, hyperinsulinemia, and insulin resistance develop with aging. Insulin resistance in older PTG heterozygous mice correlates with a significant increase in muscle triglyceride content, with a corresponding attenuation of insulin receptor signaling. These data suggest that PTG plays a critical role in glycogen synthesis and is necessary to maintain the appropriate metabolic balance for the partitioning of fuel substrates between glycogen and lipid.
Sean M. Crosson, Ahmir Khan, John Printen, Jeffrey E. Pessin, Alan R. Saltiel
Mice with 50% Pdx1, a homeobox gene critical for pancreatic development, had worsening glucose tolerance with age and reduced insulin release in response to glucose, KCl, and arginine from the perfused pancreas. Surprisingly, insulin secretion in perifusion or static incubation experiments in response to glucose and other secretagogues was similar in islets isolated from Pdx1+/– mice compared with Pdx1+/+ littermate controls. Glucose sensing and islet Ca2+ responses were also normal. Depolarization-evoked exocytosis and Ca2+ currents in single Pdx1+/– cells were not different from controls, arguing against a ubiquitous β cell stimulus-secretion coupling defect. However, isolated Pdx1+/– islets and dispersed β cells were significantly more susceptible to apoptosis at basal glucose concentrations than Pdx1+/+ islets. BclXL and Bcl-2 expression were reduced in Pdx1+/– islets. In vivo, increased apoptosis was associated with abnormal islet architecture, positive TUNEL, active caspase-3, and lymphocyte infiltration. Although similar in young mice, both β cell mass and islet number failed to increase with age and were approximately 50% less than controls by one year. These results suggest that an increase in apoptosis, with abnormal regulation of islet number and β cell mass, represents a key mechanism whereby partial PDX1 deficiency leads to an organ-level defect in insulin secretion and diabetes.
James D. Johnson, Noreen T. Ahmed, Dan S. Luciani, Zhiqiang Han, Hung Tran, Jun Fujita, Stanley Misler, Helena Edlund, Kenneth S. Polonsky
Adipose tissue lipolysis supplies circulating FFAs, which largely meet lipid fuel needs; however, excess FFAs, can contribute to the adverse health consequences of obesity. Because “normal” FFA release has not been well defined, average (mean of 4 days) basal FFA release and its potential regulation factors were measured in 50 lean and obese adults (25 women). Resting energy expenditure (REE), but not body composition, predicted most of the interindividual variation in FFA release. There was a significant, positive linear relationship between palmitate release and REE; however, women released approximately 40% more FFA than men relative to REE. Neither plasma palmitate concentrations nor respiratory quotient by indirect calorimetry differed between men and women. Glucose release rates were not different in men and women whether related to REE or fat free mass. These findings indicate that nonoxidative FFA clearance is greater in women than in men. This could be an advantage at times of increased fuel needs. We conclude that “normal” adipose tissue lipolysis is different in men and women and that the fuel export role of adipose tissue in obesity will need to be reassessed.
Søren Nielsen, ZengKui Guo, Jeanine B. Albu, Samuel Klein, Peter C. O’Brien, Michael D. Jensen
Bone marrow harbors cells that have the capacity to differentiate into cells of nonhematopoietic tissues of neuronal, endothelial, epithelial, and muscular phenotype. Here we demonstrate that bone marrow–derived cells populate pancreatic islets of Langerhans. Bone marrow cells from male mice that express, using a CRE-LoxP system, an enhanced green fluorescent protein (EGFP) if the insulin gene is actively transcribed were transplanted into lethally irradiated recipient female mice. Four to six weeks after transplantation, recipient mice revealed Y chromosome and EGFP double-positive cells in their pancreatic islets. Neither bone marrow cells nor circulating peripheral blood nucleated cells of donor or recipient mice had any detectable EGFP. EGFP-positive cells purified from islets express insulin, glucose transporter 2 (GLUT2), and transcription factors typically found in pancreatic β cells. Furthermore, in vitro these bone marrow–derived cells exhibit — as do pancreatic β cells — glucose-dependent and incretin-enhanced insulin secretion. These results indicate that bone marrow harbors cells that have the capacity to differentiate into functionally competent pancreatic endocrine β cells and that represent a source for cell-based treatment of diabetes mellitus. The results generated with the CRE-LoxP system also suggest that in vivo cell fusion is an unlikely explanation for the “transdifferentiation” of bone marrow–derived cells into differentiated cell phenotypes.
Andreea Ianus, George G. Holz, Neil D. Theise, Mehboob A. Hussain
To elucidate the function of PPARγ in leptin-deficient mouse (ob/ob) liver, a PPARγ liver-null mouse on an ob/ob background, ob/ob-PPARγ(fl/fl)AlbCre+, was produced using a floxed PPARγ allele, PPARγ(fl/fl), and Cre recombinase under control of the albumin promoter (AlbCre). The liver of ob/ob-PPARγ(fl/fl)AlbCre+ mice had a deletion of exon 2 and a corresponding loss of full-length PPARγ mRNA and protein. The PPARγ-deficient liver in ob/ob mice was smaller and had a dramatically decreased triglyceride (TG) content compared with equivalent mice lacking the AlbCre transgene (ob/ob-PPARγ(fl/fl)AlbCre–). Messenger RNA levels of the hepatic lipogenic genes, fatty acid synthase, acetyl-CoA carboxylase, and stearoyl-CoA desaturase-1, were reduced in ob/ob-PPARγ(fl/fl)AlbCre+ mice, and the levels of serum TG and FFA in ob/ob-PPARγ(fl/fl)AlbCre+ mice were significantly higher than in the control ob/ob-PPARγ(fl/fl)AlbCre– mice. Rosiglitazone treatment exacerbated the fatty liver in ob/ob-PPARγ(fl/fl)AlbCre– mice compared with livers from nonobese Cre– mice; there was no effect of rosiglitazone in ob/ob-PPARγ(fl/fl)AlbCre+ mice. The deficiency of hepatic PPARγ further aggravated the severity of diabetes in ob/ob mice due to decreased insulin sensitivity in muscle and fat. These data indicate that hepatic PPARγ plays a critical role in the regulation of TG content and in the homeostasis of blood glucose and insulin resistance in steatotic diabetic mice.
Kimihiko Matsusue, Martin Haluzik, Gilles Lambert, Sun-Hee Yim, Oksana Gavrilova, Jerrold M. Ward, Bryan Brewer Jr., Marc L. Reitman, Frank J. Gonzalez
We and others have suggested that insulin predominantly acts indirectly to inhibit hepatic glucose production (HGP) via suppression of gluconeogenic precursors, FFAs, and glucagon. To test that hypothesis, we performed high-dose hyperinsulinemic-euglycemic clamps using [3-3H]-glucose in liver-specific insulin receptor knockout (LIRKO) mice, LIRKO mice treated with streptozotocin (LIRKO+STZ), and controls. In LIRKO mice, fasted glucose was normal, but insulin levels were elevated tenfold. STZ treatment reduced insulinemia by 60% with resulting hyperglycemia. Interestingly, basal HGP was similar in all three groups. During the clamp, HGP was suppressed by 82 ± 17% in controls, but was not suppressed in either LIRKO or LIRKO+STZ mice. Glucose infusion and utilization were impaired (∼50%) in LIRKO and LIRKO+STZ mice versus controls. Insulin suppressed FFAs similarly in all groups (∼46%). Glucagon was not significantly suppressed during the clamp. Thus, in LIRKO mice, (a) high-dose insulin fails to suppress HGP indicating that both direct and indirect effects of insulin require an intact insulin-signaling pathway in the liver; (b) primary hepatic insulin resistance leads to hyperinsulinemia and secondary extrahepatic insulin resistance; and (c) lowering insulin levels with STZ tended to improve extrahepatic insulin sensitivity but failed to reveal the previously postulated indirect role of insulin in suppressing HGP.
Simon J. Fisher, C. Ronald Kahn