UPR pathways combine to prevent hepatic steatosis caused by ER stress-mediated suppression of transcriptional master regulators

The chemical chaperone 4-phenylbutyrate inhibits adipogenesis by modulating the unfolded protein response

Recent studies have shown a link between obesity and endoplasmic reticulum (ER) stress. Perturbations in ER homeostasis cause ER stress and activation of the unfolded protein response (UPR). Adipocyte differentiation contributes to weight gain, and we have shown that markers of ER stress/UPR activation, including GRP78, phospho-eIF2, and spliced XBP1, are upregulated during adipogenesis. Given these findings, the objective of this study was to determine whether attenuation of UPR activation by the chemical chaperone 4-phenylbutyrate (4-PBA) inhibits adipogenesis. Exposure of 3T3-L1 preadipocytes to 4-PBA in the presence of differentiation media decreased expression of ER stress markers. Concomitant with the suppression of UPR activation, 4-PBA resulted in attenuation of adipogenesis as measured by lipid accumulation and adiponectin secretion. Consistent with these in vitro findings, female C57BL/6 mice fed a high-fat diet supplemented with 4-PBA showed a significant reduction in weight gain and had reduced fat pad mass, as compared with the high-fat diet alone group. Furthermore, 4-PBA supplementation decreased GRP78 expression in the adipose tissue and lowered plasma triglyceride, glucose, leptin, and adiponectin levels without altering food intake. Taken together, these results suggest that UPR activation contributes to adipogenesis and that blocking its activation with 4-PBA prevents adipocyte differentiation and weight gain in mice.

Circadian clock-coordinated hepatic lipid metabolism: only transcriptional regulation?

By regulating the metabolism of fatty acids, carbohydrates, and xenobiotic, the mammalian circadian clock plays a fundamental role on the liver physiology. At present, it is supposed that the circadian clock regulates metabolism mostly by regulating the expression of liver enzymes at the transcriptional level. However, recent evidences suggest that some signaling pathways synchronized by the circadian clock can also influence metabolism at a post-transcriptional level. In this context, we have recently shown that the circadian clock synchronizes the rhythmic activation of the IRE1alpha pathway in the endoplasmic reticulum. The absence of circadian clock perturbs this secondary clock, provokes deregulation of endoplasmic reticulum-localized enzymes, and leads to impaired lipid metabolism. We will describe here the additional pathways synchronized by the clock and discussed the influence of the circadian clock-controlled feeding rhythm on them.

Endoplasmic reticulum stress involved in the course of lipogenesis in fatty acids-induced hepatic steatosis

BACKGROUND AND AIMS

The aim of the present study is to elucidate whether endoplasmic reticulum stress involved in the course of lipogenesis in fatty acids induced hepatic steatosis and the potential effect of metformin on endoplasmic reticulum stress.

METHODS

HepG2 cells were exposed to different types of culture media. After incubation for 24 h, cells were harvested to evaluate cell survival rate and lipid level among different groups. Moreover, reverse transcriptase polymerase chain reaction and western blot for glucose-regulated protein-78 (GRP78), sterol response element-binding protein-1c (SREBP1c) and fatty acid synthase (FAS) were applied.

RESULTS

The levels of triglyceride (TG), mRNA of FAS, mRNA and protein of GRP78 and SREBP1c significantly increased in the free fatty acids (FFA)-induced hepatic steatosis group. Then, HepG2 cells with hepatic steatosis induced by FFA were treated by metformin, levels of TG, GRP78 mRNA, SREBP1c mRNA and FAS mRNA as well as GRP78 and SREBP1 protein levels were partially decreased but without significant differences.

CONCLUSION

Endoplasmic reticulum stress might be involved in lipogenesis in fatty acids-induced hepatic steatosis. Therefore, endoplasmic reticulum stress might serve as a novel target in the pathogenesis and therapy of non-alcoholic fatty liver disease.

A point mutation in Sec61alpha1 leads to diabetes and hepatosteatosis in mice

OBJECTIVE

Type 2 diabetes is caused by both environmental and genetic factors. To better understand the genetic factors we used forward genetics to discover genes that have not previously been implicated in the development of hyperglycemia or diabetes.

RESEARCH DESIGN AND METHODS

Offspring of ethylnitrosurea-mutagenized C57BL/6 mice were bred to homozygosity, maintained on high-fat diet, and screened for hyperglycemia. The phenotype in one diabetic family of mice was mapped among hybrid F2s with single nucleotide polymorphic markers, followed by candidate gene sequencing to identify the gene harboring the causative mutation. Subsequent analysis was done on wild-type, heterozygous, and homozygous mutant mice on a pure C57BL/6 background.

RESULTS

Diabetes mapped to a point mutation in the Sec61a1 gene that encodes a His to Tyr substitution at amino acid 344 (Y344H). Metabolic profiling, histological examination, and electron microscopy revealed that hyperglycemia was a result of insulin insufficiency due to beta-cell apoptosis brought on by endoplasmic reticulum (ER) stress. Transgenic beta-cell-specific expression of Sec61a1 in mutant mice rescued diabetes, beta-cell apoptosis, and ER stress. In vitro experiments showed that Sec61alpha1 plays a critical role in the beta-cell response to glucose.

CONCLUSIONS

Here we phenotypically characterize diabetes in mice with a novel point mutation in a basic component of the cell's ER protein translocation machinery, Sec61alpha1. Translocation by the mutant protein does not appear to be affected. Rather, ER homeostasis is perturbed leading to beta-cell death and diabetes.

Grp78 heterozygosity promotes adaptive unfolded protein response and attenuates diet-induced obesity and insulin resistance

OBJECTIVE

To investigate the role of the endoplasmic reticulum (ER) chaperone glucose-regulated protein (GRP) 78/BiP in the pathogenesis of obesity, insulin resistance, and type 2 diabetes.

RESEARCH DESIGN AND METHODS

Male Grp78(+/-) mice and their wild-type littermates were subjected to a high-fat diet (HFD) regimen. Pathogenesis of obesity and type 2 diabetes was examined by multiple approaches of metabolic phenotyping. Tissue-specific insulin sensitivity was analyzed by hyperinsulinemic-euglycemic clamps. Molecular mechanism was explored via immunoblotting and tissue culture manipulation.

RESULTS

Grp78 heterozygosity increases energy expenditure and attenuates HFD-induced obesity. Grp78(+/-) mice are resistant to diet-induced hyperinsulinemia, liver steatosis, white adipose tissue (WAT) inflammation, and hyperglycemia. Hyperinsulinemic-euglycemic clamp studies revealed that Grp78 heterozygosity improves glucose metabolism independent of adiposity and following an HFD increases insulin sensitivity predominantly in WAT. As mechanistic explanations, Grp78 heterozygosity in WAT under HFD stress promotes adaptive unfolded protein response (UPR), attenuates translational block, and upregulates ER degradation-enhancing alpha-mannosidase-like protein (EDEM) and ER chaperones, thus improving ER quality control and folding capacity. Further, overexpression of the active form of ATF6 induces protective UPR and improves insulin signaling upon ER stress.

CONCLUSIONS

HFD-induced obesity and type 2 diabetes are improved in Grp78(+/-) mice. Adaptive UPR in WAT could contribute to this improvement, linking ER homeostasis to energy balance and glucose metabolism.

Circadian clock-coordinated 12 Hr period rhythmic activation of the IRE1alpha pathway controls lipid metabolism in mouse liver

The mammalian circadian clock plays a fundamental role in the liver by regulating fatty acid, glucose, and xenobiotic metabolism. Impairment of this rhythm has been shown to lead to diverse pathologies, including metabolic syndrome. Currently, it is supposed that the circadian clock regulates metabolism mostly by regulating expression of liver enzymes at the transcriptional level. Here, we show that the circadian clock also controls hepatic metabolism by synchronizing a secondary 12 hr period rhythm characterized by rhythmic activation of the IRE1alpha pathway in the endoplasmic reticulum. The absence of circadian clock perturbs this secondary clock and provokes deregulation of endoplasmic reticulum-localized enzymes. This leads to impaired lipid metabolism, resulting in aberrant activation of the sterol-regulated SREBP transcription factors. The resulting aberrant circadian lipid metabolism in mice devoid of the circadian clock could be involved in the appearance of the associated metabolic syndrome.

Consequences of stress in the secretory pathway: The ER stress response and its role in the metabolic syndrome

The unfolded protein response (UPR) was originally identified as a signaling network coordinating adaptive and apoptotic responses to accumulation of unfolded proteins in the endoplasmic reticulum (ER). More recent work has shown that UPR signaling can be triggered by a multitude of cellular events and that the UPR plays a critical role in the prevention, and also the progression, of a wide variety of diseases. Much attention has been paid to the role of the UPR in neurodegenerative diseases in the past. More recently, important roles for the UPR in diseases associated with the metabolic syndrome have been discovered. Here we review the role of the UPR in these diseases, including type 2 diabetes, atherosclerosis, fatty liver disease, and ischemia.

Hepatitis C virus hijacks host lipid metabolism

Hepatitis C virus (HCV) enhances its replication by modulating host cell lipid metabolism. HCV circulates in the blood in association with lipoproteins. HCV infection is associated with enhanced lipogenesis, reduced secretion, and beta-oxidation of lipids. HCV-induced imbalance in lipid homeostasis leads to steatosis. Many lipids are crucial for the virus life cycle, and inhibitors of cholesterol/fatty acid biosynthetic pathways inhibit virus replication, maturation and secretion. HCV negatively modulates the synthesis and secretion of very low-density lipoproteins (VLDL). Components involved in VLDL assembly are also required for HCV morphogenesis/secretion, suggesting that HCV co-opts the VLDL secretory pathway for its own secretion. This review highlights HCV-altered lipid metabolic events that aid the virus life cycle and ultimately promote liver disease.

Hepatic Bax inhibitor-1 inhibits IRE1alpha and protects from obesity-associated insulin resistance and glucose intolerance

The unfolded protein response (UPR) or endoplasmic reticulum (ER) stress response is a physiological process enabling cells to cope with altered protein synthesis demands. However, under conditions of obesity, prolonged activation of the UPR has been shown to have deteriorating effects on different metabolic pathways. Here we identify Bax inhibitor-1 (BI-1), an evolutionary conserved ER-membrane protein, as a novel modulator of the obesity-associated alteration of the UPR. BI-1 partially inhibits the UPR by interacting with IRE1alpha and inhibiting IRE1alpha endonuclease activity as seen on the splicing of the transcription factor Xbp-1. Because we observed a down-regulation of BI-1 expression in liver and muscle of genetically obese ob/ob and db/db mice as well as in mice with diet-induced obesity in vivo, we investigated the effect of restoring BI-1 expression on metabolic processes in these mice. Importantly, BI-1 overexpression by adenoviral gene transfer dramatically improved glucose metabolism in both standard diet-fed mice as well as in mice with diet-induced obesity and, critically, reversed hyperglycemia in db/db mice. This improvement in whole body glucose metabolism and insulin sensitivity was due to dramatically reduced gluconeogenesis as shown by reduction of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase expression. Taken together, these results identify BI-1 as a critical regulator of ER stress responses in the development of obesity-associated insulin resistance and provide proof of concept evidence that gene transfer-mediated elevations in hepatic BI-1 may represent a promising approach for the treatment of type 2 diabetes.

Evolutionary conservation and adaptation in the mechanism that regulates SREBP action: what a long, strange tRIP it's been

Sterol regulatory element-binding proteins (SREBPs) are a subfamily of basic helix-loop-helix leucine zipper (bHLH-LZ) transcription factors that are conserved from fungi to humans and are defined by two key features: a signature tyrosine residue in the DNA-binding domain, and a membrane-tethering domain that is a target for regulated proteolysis. Recent studies including genome-wide and model organism approaches indicate SREBPs coordinate cellular lipid metabolism with other cellular physiologic processes. These functions are broadly related as cellular adaptation to environmental changes ranging from nutrient fluctuations to toxin exposure. This review integrates classic features of the SREBP pathway with newer information regarding the regulation and sensing mechanisms that serve to assimilate different cellular physiologic processes for optimal function and growth.

Reducing endoplasmic reticulum stress through a macrophage lipid chaperone alleviates atherosclerosis

Macrophages exhibit endoplasmic reticulum (ER) stress when exposed to lipotoxic signals associated with atherosclerosis, although the pathophysiological significance and the underlying mechanisms remain unknown. Here, we demonstrate that mitigation of ER stress with a chemical chaperone results in marked protection against lipotoxic death in macrophages and prevents macrophage fatty acid binding protein-4 (aP2) expression. Utilizing genetic and chemical models, we show that aP2 is the predominant regulator of lipid-induced macrophage ER stress. Lipid chaperone effects are mediated by the production of phospholipids rich in monounsaturated fatty acids and bioactive lipids that render macrophages resistant to lipid-induced ER stress. Furthermore, aP2’s impact on macrophage lipid metabolism and ER stress response is mediated by upregulation of key lipogenic enzymes by the liver X receptor. Our results demonstrate the central role for lipid chaperones in regulating ER homeostasis in macrophages in atherosclerosis and that ER responses can be modified, genetically or chemically, to protect the organism against the deleterious effects of hyperlipidemia.

Nutritional related liver disease: targeting the endoplasmic reticulum stress

PURPOSE OF REVIEW

Nutritional hepatic disorders are spreading worldwide associated to obesity and type 2 diabetes. The underlying mechanisms leading to the development of hepatic steatosis and its complications are not fully understood. The endoplasmic reticulum (ER) stress response has recently been proposed to play a crucial role in the setting of these pathologies. This review will evaluate the late discoveries highlighting ER stress as a major actor in the development of nutritional liver diseases.

RECENT FINDINGS

Activation of ER stress has been reported in the fatty liver of obese rodents and obese individuals. The mechanisms by which ER stress leads to the development of hepatic steatosis have been recently documented. ER stress has been shown to directly activate the lipogenic transcription factor SREBP-1c (sterol regulatory element binding protein-1c) conducting to an induction of the lipogenic pathway. ER stress activation is also associated with impaired VLDL (very low density lipoprotein) secretion. ER stress could also have a role in hepatic steatosis progression by triggering inflammation and fibrosis. In rodents, therapies aiming to reduce ER stress have fully demonstrated their efficiency in the treatment of hepatic steatosis.

SUMMARY

ER stress has been recently involved in the development of hepatic steatosis. Thus, ER stress could represent in the future an eligible therapeutic target for the treatment of nonalcoholic fatty liver disease. However, as ER stress is a fundamental mechanism involved in cell survival, any modification of this pathway must be carefully assessed.

Atf4 regulates obesity, glucose homeostasis, and energy expenditure

OBJECTIVE

We evaluate a potential role of activating transcription factor 4 (Atf4) in invertebrate and mammalian metabolism.

RESEARCH DESIGN AND METHODS

With two parallel approaches-a fat body-specific green fluorescent protein enhancer trap screen in D. melanogaster and expression profiling of developing murine fat tissues-we identified Atf4 as expressed in invertebrate and vertebrate metabolic tissues. We assessed the functional relevance of the evolutionarily conserved expression by analyzing Atf4 mutant flies and Atf4 mutant mice for possible metabolic phenotypes.

RESULTS

Flies with insertions at the Atf4 locus have reduced fat content, increased starvation sensitivity, and lower levels of circulating carbohydrate. Atf4 null mice are also lean, and they resist age-related and diet-induced obesity. Atf4 null mice have increased energy expenditure potentially accounting for the lean phenotype. Atf4 null mice are hypoglycemic, even before substantial changes in fat content, indicating that Atf4 regulates mammalian carbohydrate metabolism. In addition, the Atf4 mutation blunts diet-induced diabetes as well as hyperlipidemia and hepatosteatosis. Several aspects of the Atf4 mutant phenotype resemble mice with mutations in components of the target of rapamycin (TOR) pathway. Consistent with the phenotypic similarities, Atf4 null mice have reduced expression of genes that regulate intracellular amino acid concentrations and lower intracellular concentration of amino acids, a key TOR input. Further, Atf4 mutants have reduced S6K activity in liver and adipose tissues.

CONCLUSIONS

Atf4 regulates age-related and diet-induced obesity as well as glucose homeostasis in mammals and has conserved metabolic functions in flies.

Adaptive suppression of the ATF4-CHOP branch of the unfolded protein response by toll-like receptor signalling

The endoplasmic reticulum (ER) Unfolded Protein Response (UPR) restores equilibrium to the ER, but prolonged expression of the UPR effector CHOP (GADD153) is cytotoxic. We found that ER stress-induced CHOP expression was suppressed by prior engagement of toll-like receptor (TLR) 3 or 4 through a TRIF-dependent pathway. TLR engagement did not suppress phosphorylation of PERK or eIF-2α, which are upstream of CHOP, but phospho-eIF-2α failed to promote translation of the CHOP activator ATF4. In mice subjected to systemic ER stress, pre-treatment with low-dose lipopolysaccharide (LPS), a TLR4 ligand, suppressed CHOP expression and apoptosis in splenic macrophages, renal tubule cells, and hepatocytes, and prevented renal dysfunction and hepatosteatosis. This protective effect of LPS did not occur in Trif−/− mice nor in wild-type mice in which CHOP expression was genetically restored. Thus, TRIF-mediated signals from TLRs selectively attenuate translational activation of ATF4 and its downstream target gene CHOP. We speculate that this mechanism evolved to promote survival of TLR-expressing cells that experience prolonged levels of physiologic ER stress in the course of the host response to invading pathogens.

Fatty liver and lipotoxicity

Fatty liver disease comprises a spectrum ranging from simple steatosis to steatohepatitis which can progress to liver cirrhosis and hepatocellular cancer. Hepatic lipotoxicity may ensue when the hepatic capacity to utilize, store and export fatty acids (FA) as triglycerides is overwhelmed. Additional mechanisms of hepatic lipotoxicity include abnormal FA oxidation with formation of reactive oxygen species, disturbances in cellular membrane FA and phospholipid composition, alterations of cholesterol content and ceramide signalling. Lipotoxicity is a key factor for the progression of fatty liver disease by inducing hepatocellular death, activating Kupffer cells and an inflammatory response, impairing hepatic insulin signalling resulting in insulin resistance, and activation of a fibrogenic response in hepatic stellate cells that can ultimately lead to cirrhosis. Therefore, the concept of hepatic lipotoxicity should be considered in future therapeutic concepts for fatty liver disease.

The unfolded protein response and its relevance to connective tissue diseases

The unfolded protein response (UPR) has evolved to counter the stresses that occur in the endoplasmic reticulum (ER) as a result of misfolded proteins. This sophisticated quality control system attempts to restore homeostasis through the action of a number of different pathways that are coordinated in the first instance by the ER stress-senor proteins IRE1, ATF6 and PERK. However, prolonged ER-stress-related UPR can have detrimental effects on cell function and, in the longer term, may induce apoptosis. Connective tissue cells such as fibroblasts, osteoblasts and chondrocytes synthesise and secrete large quantities of proteins and mutations in many of these gene products give rise to heritable disorders of connective tissues. Until recently, these mutant gene products were thought to exert their effect through the assembly of a defective extracellular matrix that ultimately disrupted tissue structure and function. However, it is now becoming clear that ER stress and UPR, because of the expression of a mutant gene product, is not only a feature of, but may be a key mediator in the initiation and progression of a whole range of different connective tissue diseases. This review focuses on ER stress and the UPR that characterises an increasing number of connective tissue diseases and highlights novel therapeutic opportunities that may arise.

Targeted induction of endoplasmic reticulum stress induces cartilage pathology

Pathologies caused by mutations in extracellular matrix proteins are generally considered to result from the synthesis of extracellular matrices that are defective. Mutations in type X collagen cause metaphyseal chondrodysplasia type Schmid (MCDS), a disorder characterised by dwarfism and an expanded growth plate hypertrophic zone. We generated a knock-in mouse model of an MCDS-causing mutation (COL10A1 p.Asn617Lys) to investigate pathogenic mechanisms linking genotype and phenotype. Mice expressing the collagen X mutation had shortened limbs and an expanded hypertrophic zone. Chondrocytes in the hypertrophic zone exhibited endoplasmic reticulum (ER) stress and a robust unfolded protein response (UPR) due to intracellular retention of mutant protein. Hypertrophic chondrocyte differentiation and osteoclast recruitment were significantly reduced indicating that the hypertrophic zone was expanded due to a decreased rate of VEGF-mediated vascular invasion of the growth plate. To test directly the role of ER stress and UPR in generating the MCDS phenotype, we produced transgenic mouse lines that used the collagen X promoter to drive expression of an ER stress-inducing protein (the cog mutant of thyroglobulin) in hypertrophic chondrocytes. The hypertrophic chondrocytes in this mouse exhibited ER stress with a characteristic UPR response. In addition, the hypertrophic zone was expanded, gene expression patterns were disrupted, osteoclast recruitment to the vascular invasion front was reduced, and long bone growth decreased. Our data demonstrate that triggering ER stress per se in hypertrophic chondrocytes is sufficient to induce the essential features of the cartilage pathology associated with MCDS and confirm that ER stress is a central pathogenic factor in the disease mechanism. These findings support the contention that ER stress may play a direct role in the pathogenesis of many connective tissue disorders associated with the expression of mutant extracellular matrix proteins.

Fine-tuning of the unfolded protein response: Assembling the IRE1alpha interactome

Endoplasmic reticulum (ER) stress is a hallmark feature of secretory cells and many diseases, including cancer, neurodegeneration, and diabetes. Adaptation to protein-folding stress is mediated by the activation of an integrated signal transduction pathway known as the unfolded protein response (UPR). The UPR signals through three distinct stress sensors located at the ER membrane-IRE1alpha, ATF6, and PERK. Although PERK and IRE1alpha share functionally similar ER-luminal sensing domains and both are simultaneously activated in cellular paradigms of ER stress in vitro, they are selectively engaged in vivo by the physiological stress of unfolded proteins. The differences in terms of tissue-specific regulation of the UPR may be explained by the formation of distinct regulatory protein complexes. This concept is supported by the recent identification of adaptor and modulator proteins that directly interact with IRE1alpha. In this Review, we discuss recent evidence supporting a model where IRE1alpha signaling emerges as a highly regulated process, controlled by the formation of a dynamic scaffold onto which many regulatory components assemble.

ER stress-inducible factor CHOP affects the expression of hepcidin by modulating C/EBPalpha activity

Endoplasmic reticulum (ER) stress induces a complex network of pathways collectively termed the unfolded protein response (UPR). The clarification of these pathways has linked the UPR to the regulation of several physiological processes. However, its crosstalk with cellular iron metabolism remains unclear, which prompted us to examine whether an UPR affects the expression of relevant iron-related genes. For that purpose, the HepG2 cell line was used as model and the UPR was activated by dithiothreitol (DTT) and homocysteine (Hcys). Here, we report that hepcidin, a liver secreted hormone that shepherds iron homeostasis, exhibits a biphasic pattern of expression following UPR activation: its levels decreased in an early stage and increased with the maintenance of the stress response. Furthermore, we show that immediately after stressing the ER, the stress-inducible transcription factor CHOP depletes C/EBPalpha protein pool, which may in turn impact on the activation of hepcidin transcription. In the later period of the UPR, CHOP levels decreased progressively, enhancing C/EBPalpha-binding to the hepcidin promoter. In addition, analysis of ferroportin and ferritin H revealed that the transcript levels of these iron-genes are increased by the UPR signaling pathways. Taken together, our findings suggest that the UPR can have a broad impact on the maintenance of cellular iron homeostasis.

The constitutive androstane receptor is an anti-obesity nuclear receptor that improves insulin sensitivity

Obesity and type 2 diabetes are related metabolic disorders of high prevalence. The constitutive androstane receptor (CAR) was initially characterized as a xenobiotic receptor regulating the responses of mammals to xenotoxicants. In this study, we have uncovered an unexpected role of CAR in preventing obesity and alleviating type 2 diabetes. Using a high fat diet (HFD)-induced obesity model, we showed that treatment of wild type mice with the CAR agonist 1,4-bis[2-(3,5 dichloropyridyloxy)] benzene (TCPOBOP) efficiently prevented obesity from happening or reversed preinduced obesity. Treatment with TCPOBOP improved insulin sensitivity in both the HFD-induced type 2 diabetic model and the ob/ob mice. In contrast, CAR null mice maintained on a chow diet showed spontaneous insulin insensitivity, which cannot be relieved by TOPOBOP treatment. The hepatic steatosis in HFD-treated mice and ob/ob mice was markedly reduced by the TCPOBOP treatment. The metabolic benefits of CAR activation may have resulted from the combined effect of inhibition of lipogenesis, very low density lipoprotein secretion and export of triglycerides, and gluconeogenesis as well as increases in brown adipose tissue energy expenditure and peripheral fat mobilization. Moreover, the skeletal muscle of CAR-activated mice showed a decreased incomplete oxidation, despite having a lower expression level of peroxisome proliferator-activated receptor alpha and its target genes involved in fatty acid oxidation. In summary, our results have revealed an important metabolic function of CAR and may establish this "xenobiotic receptor" as a novel therapeutic target for the prevention and treatment of obesity and type 2 diabetes.

Intersection of the unfolded protein response and hepatic lipid metabolism

The liver plays a central role in whole-body lipid metabolism by governing the synthesis, oxidization, transport and excretion of lipids. The unfolded protein response (UPR) was identified as a signal transduction system that is activated by ER stress. Recent studies revealed a critical role of the UPR in hepatic lipid metabolism. The IRE1/XBP1 branch of the UPR is activated by high dietary carbohydrates and controls the expression of genes involved in fatty acid and cholesterol biosynthesis. PERK mediated eIF2alpha phosphorylation is also required for the expression of lipogenic genes and the development of hepatic steatosis, likely by activating C/EBP and PPARgamma transcription factors. Further studies to define the molecular pathways that lead to the activation of the UPR by nutritional cues in the liver, and their contribution to human metabolic disorders such as hepatic steatosis, atherosclerosis and type 2 diabetes that are associated with dysregulation of lipid homeostasis, are warranted.

GRP78 expression inhibits insulin and ER stress-induced SREBP-1c activation and reduces hepatic steatosis in mice

Hepatic steatosis is present in insulin-resistant obese rodents and is concomitant with active lipogenesis. Hepatic lipogenesis depends on the insulin-induced activation of the transcription factor SREBP-1c. Despite prevailing insulin resistance, SREBP-1c is activated in the livers of genetically and diet-induced obese rodents. Recent studies have reported the presence of an ER stress response in the livers of obese ob/ob mice. To assess whether ER stress promotes SREBP-1c activation and thus contributes to lipogenesis, we overexpressed the chaperone glucose-regulated protein 78 (GRP78) in the livers of ob/ob mice using an adenoviral vector. GRP78 overexpression reduced ER stress markers and inhibited SREBP-1c cleavage and the expression of SREBP-1c and SREBP-2 target genes. Furthermore, hepatic triglyceride and cholesterol contents were reduced, and insulin sensitivity improved, in GRP78-injected mice. These metabolic improvements were likely mediated by restoration of IRS-2 expression and tyrosine phosphorylation. Interestingly, GRP78 overexpression also inhibited insulin-induced SREBP-1c cleavage in cultured primary hepatocytes. These findings demonstrate that GRP78 inhibits both insulin-dependent and ER stress-dependent SREBP-1c proteolytic cleavage and explain the role of ER stress in hepatic steatosis in obese rodents.