OBJECTIVE-Mammalian target of rapamycin (mTOR) and its downstream target S6 kinase 1 (S6K1) mediate nutrient-induced insulin resistance by downregulating insulin receptor substrate proteins with subsequent reduced Akt phosphorylation. Therefore, mTOR/S6K1 inhibition could become a therapeutic strategy in insulin-resistant states, including type 2 diabetes. We tested this hypothesis in the Psammomys obesus (P. obesus) model of nutrition-dependent type 2 diabetes, using the mTOR inhibitor rapamycin.RESEARCH DESIGN AND METHODS-Normoglycemic and diabetic P. obesus were treated with 0.2 mg ⅐ kg Ϫ1 ⅐ day Ϫ1 i.p. rapamycin or vehicle, and the effects on insulin signaling in muscle, liver and islets, and on different metabolic parameters were analyzed.RESULTS-Unexpectedly, rapamycin worsened hyperglycemia in diabetic P. obesus without affecting glycemia in normoglycemic controls. There was a 10-fold increase of serum insulin in diabetic P. obesus compared with controls; rapamycin completely abolished this increase. This was accompanied by weight loss and a robust increase of serum lipids and ketone bodies. Rapamycin decreased muscle insulin sensitivity paralleled by increased glycogen synthase kinase 3␤ activity. In diabetic animals, rapamycin reduced ␤-cell mass by 50% through increased apoptosis. Rapamycin increased the stress-responsive c-Jun NH 2 -terminal kinase pathway in muscle and islets, which could account for its effect on insulin resistance and ␤-cell apoptosis. Moreover, glucose-stimulated insulin secretion and biosynthesis were impaired in islets treated with rapamycin.CONCLUSIONS-Rapamycin induces fulminant diabetes by increasing insulin resistance and reducing ␤-cell function and mass. These findings emphasize the essential role of mTOR/S6K1 in orchestrating ␤-cell adaptation to hyperglycemia in type 2 diabetes. It is likely that treatments based on mTOR inhibition will cause exacerbation of diabetes. Diabetes 57:945-957, 2008

Insulin, synthesized by the beta cells of pancreatic islets, is of major physiological importance in metabolic homeostasis. While mature insulin consists of two polypeptide chains joined by disulphide bridges, the gene encodes for a highly conserved single chain precursor, preproinsulin [1]. In most species preproinsulin exists as a single gene, whereas in the mouse and the rat two non-allelic insulin genes are present. The human insulin gene is located on the short arm of chromosome 11 (p15.5) [2], the rat insulin I and II genes are colocalized on chromosome 1 [3] and the mouse genes are positioned on two different chromosomes, insulin I on chromosome 19 [4] and insulin II on chromosome 7 [5]. In adult islets, the nonallelic genes appear to be coordinately expressed and regulated [6, 7]. The rodent insulin II and the human genes contain three exons and two introns, whilst insulin I lacks the second intron. The organisation and structure of the insulin gene has been reviewed in detail [8]. Insulin is regulat- AbstractThe mammalian insulin gene is exclusively expressed in the beta cells of the endocrine pancreas. Two decades of intensive physiological and biochemical studies have led to the identification of regulatory sequence motifs along the insulin promoter and to the isolation of transcription factors which interact to activate gene transcription. The majority of the islet-restricted (BETA2, PDX-1, RIP3b1-Act/C1) and ubiquitous (E2A, HEB) insulin-binding proteins have been characterized. Transcriptional regulation results not only from specific combinations of these activators through DNA-protein and protein-protein interactions, but also from their relative nuclear concentrations, generating a cooperativity and transcriptional synergism unique to the insulin gene. Their DNA binding activity and their transactivating potency can be modified in response to nutrients (glucose, NEFA) or hormonal stimuli (insulin, leptin, glucagon like peptide-1, growth hormone, prolactin) through kinase-dependent signalling pathways (PI3-K, p38MAPK, PKA, CaMK) modulating their affinities for DNA and/or for each other. From the overview of the research presented, it is clear that much more study is required to fully comprehend the mechanisms involved in the regulated-expression of the insulin gene in the beta cell to prevent its impairment in diabetes. [Diabetologia (2002) 45: 309±326]

Plasma insulin concentration was measured during a standardized glucose infusion test (GIT) In 15 out of 85 healthy subjects the plasma insulin response during GIT was of the diabetic type as judged from the frequency distribution of the computer parameters (low values). The similarity was still more striking when the characteristics of the insulin curves in these 15 subjects were compared with those in patients with mild diabetes or with a decreased glucose tolerance only. It is postulated that this type of low insulin response reflects a derangement of the release of insulin into the circulation, and that it marks an alteration which probably is a prerequisite for the development of diabetes mellitus. In this sense, these subjects may be considered to be potential diabetics.

These findings suggest that in a significant proportion of type 2 diabetic patients who fail to respond to dietary measures, short-term intensive insulin treatment can effectively establish responsiveness, allowing long-term glycemic control without medication. Further studies are required to establish whether simpler treatment regimens could be equally effective. If the hypothesis offered here finds support, present approaches to the management of newly diagnosed type 2 diabetes may need to be revised.

A B ST R A CT Splanchnic and leg exchange of glucose, lactate, pyruvate, and individual plasma amino acids was studied in diabetics 24 hr after withdrawal of insulin and in healthy controls. Measurements were made in the basal postabsorptive state and during the administration of glucose at a rate of 2 mg/kg per min for 45 min.In the basal state, net splanchnic glucose production did not differ significantly between diabetics and controls. However, splanchnic uptake of alanine and other glycogenic amino acids was 11-2 times greater in the diabetics, while lactate and pyruvate uptake was increased by 65-115%. Splanchnic uptake of these glucose precursors could account for 32% of hepatic glucose output in the diabetics, as compared to 20% in the controls. This increase in precursor uptake was a consequence of a two-to threefold increment in fractional extraction of these substrates inasmuch as arterial levels of alanine, glycine, and threonine were reduced in the diabetics, while the levels of the remaining substrates were similar in the two groups. Peripheral output of alanine and other glycogenic amino acids as reflected in arterio-femoral venous differences was similar in both groups. An elevation in arterial valine, leucine, and isoleucine was observed in the diabetics, but could not be accounted for on the basis of altera- Administration of glucose (2 mg/kg per min) for 45 min resulted in an 80% reduction in splanchnic glucose output in controls, but failed to inhibit hepatic glucose release in the diabetics despite a twofold greater increment in arterial glucose levels. In both groups no consistent changes in arterial glucagon were observed during the infusion.It is concluded that in nonketotic diabetics (a) total splanchnic output of glucose is comparable to controls, but the relative contribution of gluconeogenesis may be increased by more than 50%; (b) accelerated splanchnic uptake of glucose precursors is a consequence of increased hepatic extraction of available substrates rather than a result of augmented substrate supply; and (c) the failure of glucose infusion to inhibit hepatic glucose output suggests that the exquisite sensitivity of the liver to the infusion of glucose in normal man is a consequence of glucose-induced insulin secretion. INTRODUCTIONThe contribution of altered hepatic glucose balance to the metabolic defect in diabetes mellitus has long been the subject of inquiry and debate. Numerous investigations employing the hepatic venous catheter technique (2, 3), or isotope dilution methods (4-6), have provided data on the net rate of hepatic glucose release and on systemic glucose turnover in diabetic patients. Little is known, however, about the effect of diabetes on the hepatic uptake and peripheral release of glucose precursors. Specifically, the question of whether an augmentation in substrate availability or alterations in

The protein kinase C (PKC) family consists of 11 isoenzymes. Following activation, each isoenzyme translocates and binds to a specific receptor for activated C kinase (RACK) Although PKC activation enhances insulin release, the specific function of each isoenzyme is unknown. Here we show that following stimulation with glucose, ␣PKC and ⑀PKC translocate to the cell's periphery, while ␦PKC and PKC translocate to perinuclear sites. ␤C2-4, a peptide derived from the RACK1-binding site in the C2 domain of ␤PKC, inhibits translocation of ␣PKC and reduces insulin response to glucose. Likewise, ⑀V1-2, an ⑀PKC-derived peptide containing the site for its specific RACK, inhibits translocation of ⑀PKC and reduces insulin response to glucose. Inhibition of islet-glucose metabolism with mannoheptulose blocks translocation of both ␣PKC and ⑀PKC and diminishes insulin response to glucose while calcium-free buffer inhibits translocation of ␣PKC but not ⑀PKC and lowers insulin response by 50%. These findings illustrate the unique ability of specific translocation inhibitors to elucidate the isoenzyme-specific functions of PKC in complex signal transduction pathways.(Protein kinase C is a family of 11 lipid-dependent serine/ threonine kinases involved in a wide spectrum of signal transduction (7,8). Upon activation, PKC 1 isoenzymes translocate to new cellular sites, including the plasma membrane (9, 10), cytoskeletal elements (11,12), and the nucleus (13,14), as well as other subcellular compartments (15). Many cells are known to contain several isoenzymes (16,17), each localizing to a different cellular site upon stimulation (18). The multiplicity of isoforms of a single enzyme renders the analysis of enzymefunction relationship difficult. Recent work revealed that activated PKC isoenzymes bind anchoring proteins termed RACKs (1-3), believed to be positioned in close proximity to the isoenzyme's substrate. It was further shown that the functional specificity of the PKC isoenzyme is determined, in part, by the differential localization of the isoenzyme-specific RACKs (19). The RACK for ␤PKC, RACK1, has been cloned, and at least part of its binding site on ␤PKC has been mapped to a short sequence within the C2 domain (1). ␤C2-4, a nonopeptide derived from this region, inhibits phorbol ester-induced translocation of the C2-containing isoenzymes but not the translocation of C2-less isoenzymes such as ␦-and ⑀PKC when tested in intact cells (1). A short peptide derived from the V1 region of ⑀PKC, ⑀V1-2, was similarly shown to inhibit the translocation of ⑀PKC, but not ␣-, ␤-, and ␦PKC (20). Furthermore, these isozyme-specific inhibitors blocked the specific function of individual isoenzymes; for example, ⑀V1-2, but not ␤C2-4, inhibited phorbol 12-myristate 13-acetate-induced regulation of the contraction rate in intact cardiomyocytes. Here we use these novel PKC isozyme-specific inhibitors to determine that PKC activation is part of the signals involved in the regulation of glucose-induced insulin secretion and to identify the specif...

BackgroundPalmitate is a potent inducer of endoplasmic reticulum (ER) stress in β-cells. In type 2 diabetes, glucose amplifies fatty-acid toxicity for pancreatic β-cells, leading to β-cell dysfunction and death. Why glucose exacerbates β-cell lipotoxicity is largely unknown. Glucose stimulates mTORC1, an important nutrient sensor involved in the regulation of cellular stress. Our study tested the hypothesis that glucose augments lipotoxicity by stimulating mTORC1 leading to increased β-cell ER stress.Principal FindingsWe found that glucose amplifies palmitate-induced ER stress by increasing IRE1α protein levels and activating the JNK pathway, leading to increased β-cell apoptosis. Moreover, glucose increased mTORC1 activity and its inhibition by rapamycin decreased β-cell apoptosis under conditions of glucolipotoxicity. Inhibition of mTORC1 by rapamycin did not affect proinsulin and total protein synthesis in β-cells incubated at high glucose with palmitate. However, it decreased IRE1α expression and signaling and inhibited JNK pathway activation. In TSC2-deficient mouse embryonic fibroblasts, in which mTORC1 is constitutively active, mTORC1 regulated the stimulation of JNK by ER stressors, but not in response to anisomycin, which activates JNK independent of ER stress. Finally, we found that JNK inhibition decreased β-cell apoptosis under conditions of glucolipotoxicity.Conclusions/SignificanceCollectively, our findings suggest that mTORC1 mediates glucose amplification of lipotoxicity, acting through activation of ER stress and JNK. Thus, mTORC1 is an important transducer of ER stress in β-cell glucolipotoxicity. Moreover, in stressed β-cells mTORC1 inhibition decreases IRE1α protein expression and JNK activity without affecting ER protein load, suggesting that mTORC1 regulates the β-cell stress response to glucose and fatty acids by modulating the synthesis and activity of specific proteins involved in the execution of the ER stress response. This novel paradigm may have important implications for understanding β-cell failure in type 2 diabetes.

Accumulation of misfolded proinsulin in the β-cell leads to dysfunction induced by endoplasmic reticulum (ER) stress, with diabetes as a consequence. Autophagy helps cellular adaptation to stress via clearance of misfolded proteins and damaged organelles. We studied the effects of proinsulin misfolding on autophagy and the impact of stimulating autophagy on diabetes progression in Akita mice, which carry a mutation in proinsulin, leading to its severe misfolding. Treatment of female diabetic Akita mice with rapamycin improved diabetes, increased pancreatic insulin content, and prevented β-cell apoptosis. In vitro, autophagic flux was increased in Akita β-cells. Treatment with rapamycin further stimulated autophagy, evidenced by increased autophagosome formation and enhancement of autophagosome–lysosome fusion. This was associated with attenuation of cellular stress and apoptosis. The mammalian target of rapamycin (mTOR) kinase inhibitor Torin1 mimicked the rapamycin effects on autophagy and stress, indicating that the beneficial effects of rapamycin are indeed mediated via inhibition of mTOR. Finally, inhibition of autophagy exacerbated stress and abolished the anti-ER stress effects of rapamycin. In conclusion, rapamycin reduces ER stress induced by accumulation of misfolded proinsulin, thereby improving diabetes and preventing β-cell apoptosis. The beneficial effects of rapamycin in this context strictly depend on autophagy; therefore, stimulating autophagy may become a therapeutic approach for diabetes.