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Increased endoplasmic reticulum (ER) stress and the activated unfolded protein response (UPR) signaling associated with it play key roles in physiological processes as well as under pathological conditions. The UPR normally protects cells and re-establishes cellular homeostasis, but prolonged UPR activation can lead to the development of various pathologies. These features make the UPR signaling pathway an attractive target for the treatment of diseases whose pathogenesis is characterized by chronic activation of this pathway. Here, we focus on the molecular signaling pathways of the UPR and suggest possible ways to target this response for therapeutic purposes.

Celastrol, a pentacyclic triterpene, is the most potent antiobesity agent that has been reported thus far1. The mechanism of celastrol’s leptin-sensitizing and antiobesity effects has not yet been elucidated. In this study, we identified interleukin-1 receptor 1 (IL1R1) as a mediator of celastrol’s action by using temporally resolved analysis of the hypothalamic transcriptome in celastrol-treated DIO, lean, and db/db mice. We demonstrate that IL1R1-deficient mice are completely resistant to the effects of celastrol in leptin sensitization and treatment of obesity, diabetes, and nonalcoholic steatohepatitis. Thus, we conclude that IL1R1 is a gatekeeper for celastrol’s metabolic actions.IL1R1 is a gatekeeper for celastrol’s metabolic actions.

Increased endoplasmic reticulum (ER) stress is one of the central mechanisms that lead to dysregulated metabolic homeostasis in obesity. It is thus crucial to understand the underpinnings of the mechanisms that lead to the development of ER stress. A high level of ER Ca²⁺ is imperative for maintenance of normal ER function and this high Ca²⁺ concentration of ER is maintained by sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA). Here, we show that SERCA2b protein and mRNA levels are dramatically reduced in the liver of obese mice and restoration of SERCA2b in the liver of obese and diabetic mice alleviates ER stress, increases glucose tolerance, and significantly reduces the blood glucose levels. Furthermore, overexpression of SERCA2b in the liver of obese mice significantly reduces the lipogenic gene expression and the triglyceride content in the liver. Our results document the importance of SERCA2b in dysregulated glucose and lipid homeostasis in the liver of obese mice and suggest development of drugs to increase SERCA2b activity for treatment of type 2 diabetes and nonalcoholic steatohepatitis.

Insulin dials down endogenous hepatic glucose production after a meal by deactivating the transcription factor FoxO1. In a mouse model of insulin resistance, Umut Ozcan and his colleagues now show that hepatic overexpression of Xbp-1s, a factor involved in the cell stress response, leads to the protein degradation of FoxO1, thus reducing serum glucose levels. These results suggest a way to bypass one aspect of insulin resistance. To date, the only known role of the spliced form of X-box–binding protein-1 (XBP-1s) in metabolic processes has been its ability to act as a transcription factor that regulates the expression of genes that increase the endoplasmic reticulum (ER) folding capacity, thereby improving insulin sensitivity. Here we show that XBP-1s interacts with the Forkhead box O1 (FoxO1) transcription factor and directs it toward proteasome-mediated degradation. Given this new insight, we tested modest hepatic overexpression of XBP-1s in vivo in mouse models of insulin deficiency or insulin resistance and found it improved serum glucose concentrations, even without improving insulin signaling or ER folding capacity. The notion that XBP-1s can act independently of its role in the ER stress response is further supported by our finding that in the severely insulin resistant ob/ob mouse strain a DNA-binding–defective mutant of XBP-1s, which does not have the ability to increase ER folding capacity, is still capable of reducing serum glucose concentrations and increasing glucose tolerance. Our results thus provide the first evidence to our knowledge that XBP-1s, through its interaction with FoxO1, can bypass hepatic insulin resistance independent of its effects on ER folding capacity, suggesting a new therapeutic approach for the treatment of type 2 diabetes.

Endoplasmic reticulum (ER) stress is a key link between obesity, insulin resistance, and type 2 diabetes. Here, we provide evidence that this mechanistic link can be exploited for therapeutic purposes with orally active chemical chaperones. 4-Phenyl butyric acid and taurine-conjugated ursodeoxycholic acid alleviated ER stress in cells and whole animals. Treatment of obese and diabetic mice with these compounds resulted in normalization of hyperglycemia, restoration of systemic insulin sensitivity, resolution of fatty liver disease, and enhancement of insulin action in liver, muscle, and adipose tissues. Our results demonstrate that chemical chaperones enhance the adaptive capacity of the ER and act as potent antidiabetic modalities with potential application in the treatment of type 2 diabetes.

Obesity contributes to the development of type 2 diabetes, but the underlying mechanisms are poorly understood. Using cell culture and mouse models, we show that obesity causes endoplasmic reticulum (ER) stress. This stress in turn leads to suppression of insulin receptor signaling through hyperactivation of c-Jun N-terminal kinase (JNK) and subsequent serine phosphorylation of insulin receptor substrate-1 (IRS-1). Mice deficient in X-box-binding protein-1 (XBP-1), a transcription factor that modulates the ER stress response, develop insulin resistance. These findings demonstrate that ER stress is a central feature of peripheral insulin resistance and type 2 diabetes at the molecular, cellular, and organismal levels. Pharmacologic manipulation of this pathway may offer novel opportunities for treating these common diseases.

Melanin Concentrating Hormone Is a Novel Regulator of Islet Function and Growth Pavlos Pissios 1 , Umut Ozcan 2 , Efi Kokkotou 3 , Terumasa Okada 2 , Chong Wee Liew 2 , Siming Liu 2 , Jennifer N. Peters 4 , Gabriella Dahlgren 4 , Jason Karamchandani 2 , Yogish C. Kudva 5 , Amarnath J. Kurpad 2 , Robert T. Kennedy 4 , Eleftheria Maratos-Flier 1 and Rohit N. Kulkarni 2 1 Division of Endocrinology, Beth Israel Deaconess Medical Center, Boston, Massachusetts 2 Research Division, Joslin Diabetes Center, Department of Medicine, Harvard Medical School, Boston, Massachusetts 3 Division of Gastroenterology, Department of Medicine, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 4 Department of Chemistry and Pharmacology, University of Michigan, Ann Arbor, Michigan 5 Department of Endocrinology and Metabolism, Mayo Clinic, Rochester, Minnesota Address correspondence and reprint requests to Rohit N. Kulkarni, MD, PhD, Room 602, Joslin Diabetes Center, One Joslin Place, Boston, MA 02215. E-mail: rohit.kulkarni{at}joslin.harvard.edu Abstract Melanin concentrating hormone (MCH) is a hypothalamic neuropeptide known to play a critical role in energy balance. We have previously reported that overexpression of MCH is associated with mild obesity. In addition, mice have substantial hyperinsulinemia and islet hyperplasia that is out of proportion with their degree of obesity. In this study, we further explored the role of MCH in the endocrine pancreas. Both MCH and MCHR1 are expressed in mouse and human islets and in clonal β-cell lines as assessed using quantitative real-time PCR and immunohistochemistry. Mice lacking MCH (MCH-KO) on either a C57Bl/6 or 129Sv genetic background showed a significant reduction in β-cell mass and complemented our earlier observation of increased β-cell mass in MCH -overexpressing mice. Furthermore, the compensatory islet hyperplasia secondary to a high-fat diet, which was evident in wild-type controls, was attenuated in MCH-KO. Interestingly, MCH enhanced insulin secretion in human and mouse islets and rodent β-cell lines in a dose-dependent manner. Real-time PCR analyses of islet RNA derived from MCH-KO revealed altered expression of islet-enriched genes such as glucagon, forkhead homeobox A2, hepatocyte nuclear factor (HNF)4α, and HNF1α. Together, these data provide novel evidence for an autocrine role for MCH in the regulation of β-cell mass dynamics and in islet secretory function and suggest that MCH is part of a hypothalamic-islet (pancreatic) axis. ELISA, enzyme-linked immunosorbent assay ERK, extracellular signal–related kinase HNF, hepatocyte nuclear factor MCH, melanin concentrating hormone MCHR, MCH receptor RIA, radioimmunoassay TBS, Tris-buffered solution Footnotes P.P., U.O., and E.K. contributed equally to this work. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Accepted October 19, 2006. Received May 23, 2006. DIABETES

Withaferin A is a naturally occurring compound that has potent antidiabetic and anti-obesity properties in mice. The increasing global prevalence of obesity and its associated disorders points to an urgent need for the development of novel and effective therapeutic strategies that induce healthy weight loss. Obesity is characterized by hyperleptinemia and central leptin resistance. In an attempt to identify compounds that could reverse leptin resistance and thus promote weight loss, we analyzed a library of small molecules that have mRNA expression profiles similar to that of celastrol, a naturally occurring compound that we previously identified as a leptin sensitizer. Through this process, we identified another naturally occurring compound, withaferin A, that also acts as a leptin sensitizer. We found that withaferin-A treatment of mice with diet-induced obesity (DIO) resulted in a 20–25% reduction of body weight, while also decreasing obesity-associated abnormalities, including hepatic steatosis. Withaferin-A treatment marginally affected the body weight of ob/ob and db/db mice, both of which are deficient in leptin signaling. In addition, withaferin A, unlike celastrol, has beneficial effects on glucose metabolism that occur independently of its leptin-sensitizing effect. Our results show that the metabolic abnormalities of DIO can be mitigated by sensitizing animals to endogenous leptin, and they indicate that withaferin A is a potential leptin sensitizer with additional antidiabetic actions.

Insulin can signal through phosphotidylinositol 3-kinase (PI3K) to increase cellular growth, which often results in increased protein translation and stress of the endoplasmic reticulum (ER). Umut Ozcan and his colleagues now find that insulin signaling leads to the disassociation of p85α and p85β, the heterodimeric regulatory subunits of PI3K, allowing them to interact with and increase the nuclear localization of a key transcription factor that resolves ER stress. Thus, in states of insulin resistance, such as in prediabetes, resolution of ER stress is hampered, further exacerbating the disease ( pages 374–376 and 438–445 ). Despite the fact that X-box binding protein-1 (XBP-1) is one of the main regulators of the unfolded protein response (UPR), the modulators of XBP-1 are poorly understood. Here, we show that the regulatory subunits of phosphotidyl inositol 3-kinase (PI3K), p85α (encoded by Pik3r1 ) and p85β (encoded by Pik3r2 ) form heterodimers that are disrupted by insulin treatment. This disruption of heterodimerization allows the resulting monomers of p85 to interact with, and increase the nuclear translocation of, the spliced form of XBP-1 (XBP-1s). The interaction between p85 and XBP-1s is lost in ob/ob mice, resulting in a severe defect in XBP-1s translocation to the nucleus and thus in the resolution of endoplasmic reticulum (ER) stress. These defects are ameliorated when p85α and p85β are overexpressed in the liver of ob/ob mice. Our results define a previously unknown insulin receptor signaling pathway and provide new mechanistic insight into the development of ER stress during obesity.

The activation of stress kinases, such as p38 MAPK, is believed to be detrimental to normal cellular processes. However, Umut Ozcan and his colleagues now show that p38 MAPK is actually beneficial, as in mice it increases the mRNA stability and nuclear localization of Xbp1s, a crucial factor in resolving endoplasmic reticulum stress and improving glucose homeostasis. These results suggest a possible indirect way of targeting XBP1s in the treatment of type 2 diabetes. Here we show that p38 mitogen-activated protein kinase (p38 MAPK) phosphorylates the spliced form of X-box binding protein 1 (Xbp1s) on its Thr48 and Ser61 residues and greatly enhances its nuclear migration in mice, whereas mutation of either residue to alanine substantially reduces its nuclear translocation and activity. We also show that p38 MAPK activity is markedly reduced in the livers of obese mice compared with lean mice. Further, we show that activation of p38 MAPK by expression of constitutively active MAP kinase kinase 6 (MKK6Glu) greatly enhances nuclear translocation of Xbp1s, reduces endoplasmic reticulum stress and establishes euglycemia in severely obese and diabetic mice. Hence, our results define a crucial role for phosphorylation on Thr48 and Ser61 of Xbp1s in the maintenance of glucose homeostasis in obesity, and they suggest that p38 MAPK activation in the livers of obese mice could lead to a new therapeutic approach to the treatment of type 2 diabetes.