ER Stress and Lipogenesis: A Slippery Slope toward Hepatic Steatosis

MASH as an emerging cause of hepatocellular carcinoma: current knowledge and future perspectives

Hepatocellular carcinoma is one of the deadliest and fastest-growing cancers. Among HCC etiologies, metabolic dysfunction-associated fatty liver disease (MAFLD) has served as a major HCC driver due to its great potential for increasing cirrhosis. The obesogenic environment fosters a positive energy balance and results in a continuous rise of obesity and metabolic syndrome. However, it is difficult to understand how metabolic complications lead to the poor prognosis of liver diseases and which molecular mechanisms are underpinning MAFLD-driven HCC development. Thus, suitable preclinical models that recapitulate human etiologies are essentially required. Numerous preclinical models have been created but not many mimicked anthropometric measures and the course of disease progression shown in the patients. Here we review the literature on adipose tissues, liver-related HCC etiologies and recently discovered genetic mutation signatures found in MAFLD-driven HCC patients. We also critically review current rodent models suggested for MAFLD-driven HCC study.

Roles of X-box binding protein 1 in liver pathogenesis

The prevalence of drug-induced liver injury (DILI) and viral liver infections presents significant challenges in modern healthcare and contributes to considerable morbidity and mortality worldwide. Concurrently, metabolic dysfunctionassociated steatotic liver disease (MASLD) has emerged as a major public health concern, reflecting the increasing rates of obesity and leading to more severe complications such as fibrosis and hepatocellular carcinoma. X-box binding protein 1 (XBP1) is a distinct transcription factor with a basic-region leucine zipper structure, whose activity is regulated by alternative splicing in response to disruptions in endoplasmic reticulum (ER) homeostasis and the unfolded protein response (UPR) activation. XBP1 interacts with a key signaling component of the highly conserved UPR and is critical in determining cell fate when responding to ER stress in liver diseases. This review aims to elucidate the emerging roles and molecular mechanisms of XBP1 in liver pathogenesis, focusing on its involvement in DILI, viral liver infections, MASLD, fibrosis/cirrhosis, and liver cancer. Understanding the multifaceted functions of XBP1 in these liver diseases offers insights into potential therapeutic strategies to restore ER homeostasis and mitigate liver damage.

TRPV1 alleviates APOE4-dependent microglial antigen presentation and T cell infiltration in Alzheimer's disease

BACKGROUND

Persistent innate and adaptive immune responses in the brain contribute to the progression of Alzheimer's disease (AD). APOE4, the most important genetic risk factor for sporadic AD, encodes apolipoprotein E4, which by itself is a potent modulator of immune response. However, little is known about the immune hub that governs the crosstalk between the nervous and the adaptive immune systems. Transient receptor potential vanilloid type 1 (TRPV1) channel is a ligand-gated, nonselective cation channel with Ca2+ permeability, which has been proposed as a neuroprotective target in AD.

METHODS

Using Ca2+-sensitive dyes, dynamic changes of Ca2+ in microglia were measured, including exogenous Ca2+ uptake and endoplasmic reticulum Ca2+ release. The mRFP-GFP-tagged LC3 plasmid was expressed in microglia to characterize the role of TRPV1 in the autophagic flux. Transcriptomic analyses and flow cytometry were performed to investigate the effects of APOE4 on brain microglia and T cells from APOE-targeted replacement mice with microglia-specific TRPV1 gene deficiency.

RESULTS

Both APOE4 microglia derived from induced pluripotent stem cells of AD patients and APOE4-related tauopathy mouse model showed significantly increased cholesterol biosynthesis and accumulation compared to their APOE3 counterparts. Further, cholesterol dysregulation was associated with persistent activation of microglia and elevation of major histocompatibility complex II-dependent antigen presentation in microglia, subsequently accompanied by T cell infiltration. In addition, TRPV1-mediated transient Ca2+ influx mitigated cholesterol biosynthesis in microglia by suppressing the transcriptional activation of sterol regulatory element-binding protein 2, promoted autophagic activity and reduced lysosomal cholesterol accumulation, which were sufficient to resolve excessive immune response and neurodegeneration in APOE4-related tauopathy mouse model. Moreover, microglia-specific deficiency of TRPV1 gene accelerated glial inflammation, T cell response and associated neurodegeneration in an APOE4-related tauopathy mouse model.

CONCLUSIONS

The findings provide new perspectives for the treatment of APOE4-dependent neurodegeneration including AD.

Interleukin-38 alleviates hepatic steatosis through AMPK/autophagy-mediated suppression of endoplasmic reticulum stress in obesity models

Interleukin-38 (IL-38), recently recognized as a cytokine with anti-inflammatory properties that mitigate type 2 diabetes, has been associated with indicators of insulin resistance and nonalcoholic fatty liver disease (NAFLD). This study investigated the impact of IL-38 on hepatic lipid metabolism and endoplasmic reticulum (ER) stress. We assessed protein expression levels using Western blot analysis, while monodansylcadaverine staining was employed to detect autophagosomes in hepatocytes. Oil red O staining was utilized to examine lipid deposition. The study revealed elevated serum IL-38 levels in high-fat diet (HFD)-fed mice and IL-38 secretion from mouse keratinocytes. IL-38 treatment attenuated lipogenic lipid accumulation and ER stress markers in hepatocytes exposed to palmitate. Furthermore, IL-38 treatment increased AMP-activated protein kinase (AMPK) phosphorylation and autophagy. The effects of IL-38 on lipogenic lipid deposition and ER stress were nullified in cultured hepatocytes by suppressing AMPK through small interfering (si) RNA or 3-methyladenine (3MA). In animal studies, IL-38 administration mitigated hepatic steatosis by suppressing the expression of lipogenic proteins and ER stress markers while reversing AMPK phosphorylation and autophagy markers in the livers of HFD-fed mice. Additionally, AMPK siRNA, but not 3MA, mitigated IL-38-enhanced fatty acid oxidation in hepatocytes. In summary, IL-38 alleviates hepatic steatosis through AMPK/autophagy signaling-dependent attenuation of ER stress and enhancement of fatty acid oxidation via the AMPK pathway, suggesting a therapeutic strategy for treating NAFLD.

Grape-Seed Procyanidin Extract (GSPE) Seasonal-Dependent Modulation of Glucose and Lipid Metabolism in the Liver of Healthy F344 Rats

Seasonality is gaining attention in the modulation of some physiological and metabolic functions in mammals. Furthermore, the consumption of natural compounds, such as GSPE, is steadily increasing. Consequently, in order to study the interaction of seasonal variations in day length over natural compounds’ molecular effects, we carried out an animal study using photo-sensitive rats which were chronically exposed for 9 weeks to three photoperiods (L6, L18, and L12) in order to mimic the day length of different seasons (winter/summer/and autumn-spring). In parallel, animals were also treated either with GSPE 25 (mg/kg) or vehicle (VH) for 4 weeks. Interestingly, a seasonal-dependent GSPE modulation on the hepatic glucose and lipid metabolism was observed. For example, some metabolic genes from the liver (SREBP-1c, Gk, Acacα) changed their expression due to seasonality. Furthermore, the metabolomic results also indicated a seasonal influence on the GSPE effects associated with glucose-6-phosphate, D-glucose, and D-ribose, among others. These differential effects, which were also reflected in some plasmatic parameters (i.e., glucose and triglycerides) and hormones (corticosterone and melatonin), were also associated with significant changes in the expression of several hepatic circadian clock genes (Bmal1, Cry1, and Nr1d1) and ER stress genes (Atf6, Grp78, and Chop). Our results point out the importance of circannual rhythms in regulating metabolic homeostasis and suggest that seasonal variations (long or short photoperiods) affect hepatic metabolism in rats. Furthermore, they suggest that procyanidin consumption could be useful for the modulation of the photoperiod-dependent changes on glucose and lipid metabolism, whose alterations could be related to metabolic diseases (e.g., diabetes, obesity, and cardiovascular disease). Furthermore, even though the GSPE effect is not restricted to a specific photoperiod, our results suggest a more significant effect in the L18 condition.

Caffeine blocks SREBP2-induced hepatic PCSK9 expression to enhance LDLR-mediated cholesterol clearance

Evidence suggests that caffeine (CF) reduces cardiovascular disease (CVD) risk. However, the mechanism by which this occurs has not yet been uncovered. Here, we investigated the effect of CF on the expression of two bona fide regulators of circulating low-density lipoprotein cholesterol (LDLc) levels; the proprotein convertase subtilisin/kexin type 9 (PCSK9) and the low-density lipoprotein receptor (LDLR). Following the observation that CF reduced circulating PCSK9 levels and increased hepatic LDLR expression, additional CF-derived analogs with increased potency for PCSK9 inhibition compared to CF itself were developed. The PCSK9-lowering effect of CF was subsequently confirmed in a cohort of healthy volunteers. Mechanistically, we demonstrate that CF increases hepatic endoplasmic reticulum (ER) Ca²⁺ levels to block transcriptional activation of the sterol regulatory element-binding protein 2 (SREBP2) responsible for the regulation of PCSK9, thereby increasing the expression of the LDLR and clearance of LDLc. Our findings highlight ER Ca²⁺ as a master regulator of cholesterol metabolism and identify a mechanism by which CF may protect against CVD.

Pt(ii) complex containing the 1R,2R enantiomer of trans-1,2-diamino-4-cyclohexene ligand effectively and selectively inhibits the viability of aggressive pancreatic adenocarcinoma cells and alters their lipid metabolism

New compounds structurally derived from oxaliplatin exhibit high potency in malignant pancreatic adenocarcinoma cells. Their mechanism of antiproliferative action in pancreatic cancer cells involves inhibition of de novo lipid synthesis.

Erythritol Improves Nonalcoholic Fatty Liver Disease by Activating Nrf2 Antioxidant Capacity

Nonalcoholic fatty liver disease (NAFLD) is a kind of serious fat disorder that has become a critical problem to human society. Therefore, finding drugs that are safe and effective has become more and more important. Erythritol (Ery) is a polyol sweetener with a variety of biological functions. However, whether Ery has a relieving effect on NAFLD has not been reported yet. Therefore, we induced HepG2 cells with oleic acid and palmitic acid as our in vitro model. Moreover, we choose wild-type mice with tyloxapol and high-fat diet and nuclear factor E2-related factor 2 (Nrf2) knockout mice with high-fat diet as our in vivo model. We found that Ery could reverse the lipid accumulation, oxidative stress, and endoplasmic reticulum stress caused by the NAFLD model. The mechanism studies showed that Ery promoted the translocation of Nrf2 from cytoplasm to nucleus, and the molecular simulation docking results of Ery and Nrf2 showed that there was a hydrogen bond between them. Moreover, Ery could promote the production of HO-1 and NQO1 antioxidant proteins and inhibit the expression of endoplasmic reticulum stress proteins GPR78, p-PERK, and CHOP. On the contrast, when Nrf2 was knocked out in mice, Ery lost its protective effect on NAFLD. In conclusion, we found that the potential mechanism of Ery's protective effect is that it plays an antioxidant role by activating the Nrf2 signaling pathway, thereby inhibiting endoplasmic reticulum stress and lipid accumulation in NAFLD.

Role of Oxidative Stress in the Pathogenesis of Non-Alcoholic Fatty Liver Disease: Implications for Prevention and Therapy

Oxidative stress (OxS) is considered a major factor in the pathophysiology of inflammatory chronic liver diseases, including non-alcoholic liver disease (NAFLD). Chronic impairment of lipid metabolism is closely related to alterations of the oxidant/antioxidant balance, which affect metabolism-related organelles, leading to cellular lipotoxicity, lipid peroxidation, chronic endoplasmic reticulum (ER) stress, and mitochondrial dysfunction. Increased OxS also triggers hepatocytes stress pathways, leading to inflammation and fibrogenesis, contributing to the progression of non-alcoholic steatohepatitis (NASH). The antioxidant response, regulated by the Nrf2/ARE pathway, is a key component in this process and counteracts oxidative stress-induced damage, contributing to the restoration of normal lipid metabolism. Therefore, modulation of the antioxidant response emerges as an interesting target to prevent NAFLD development and progression. This review highlights the link between disturbed lipid metabolism and oxidative stress in the context of NAFLD. In addition, emerging potential therapies based on antioxidant effects and their likely molecular targets are discussed.

Nifedipine Upregulates ATF6-α, Caspases -12, -3, and -7 Implicating Lipotoxicity-Associated Renal ER Stress

Nifedipine (NF) is reported to have many beneficial effects in antihypertensive therapy. Recently, we found that NF induced lipid accumulation in renal tubular cells. Palmitic acid-induced renal lipotoxicity was found to be partially mediated by endoplasmic reticular (ER) stress, while it can also be elicited by NF in kidney cells; we examined the induction of suspected pathways in both in vitro and in vivo models. NRK52E cells cultured in high-glucose medium were treated with NF (30 µM) for 24–48 h. ER stress-induced lipotoxicity was explored by staining with thioflavin T and Nile red, transmission electron microscopy, terminal uridine nick-end labeling, and Western blotting. ER stress was also investigated in rats with induced chronic kidney disease (CKD) fed NF for four weeks. NF induced the production of unfolded protein aggregates, resulting in ER stress, as evidenced by the upregulation of glucose-regulated protein, 78 kDa (GRP78), activating transcription factor 6α (ATF6α), C/EBP-homologous protein (CHOP), and caspases-12, -3, and -7. In vitro early apoptosis was more predominant than late apoptosis. Most importantly, ATF6α was confirmed to play a unique role in NF-induced ER stress in both models. CKD patients with hypertension should not undergo NF therapy. In cases where it is required, alleviation of ER stress should be considered to avoid further damaging the kidneys.

Activation of Wnt/β-catenin pathway causes insulin resistance and increases lipogenesis in HepG2 cells via regulation of endoplasmic reticulum stress

BACKGROUND

Wnt/β-catenin signaling is involved in glucose and lipid metabolism, but the mechanism is not clear yet.

AIM

The objective is to study mechanisms of Wnt/β-catenin signaling on regulating hepatocytes metabolism.

METHODS

Real-time qPCR, Western blot, and Oil-red O staining methods were used.

RESULTS

The Wnt/β-catenin signaling was activated in hepatocytes by CP21R7, and the level of phosphorylated IRS-1 (Ser307) and TRB3 were significantly increased, while the levels of phosphorylated IRS-1 (Tyr612) and phosphorylated Akt were decreased. Moreover, the expression of FGF21, FAS, SCD1, PPARγ and ADRP was significantly increased. The expression of ATF4, ATF5, eIF2α, GRP78, CHOP and phosphorylated level of PERK were also increased. The expression of FGF21 and TRB3 was significantly down-regulated, and the lipid droplets were notably reduced after the ER stress was inhibited by TUDCA. The expression of FGF21 was significantly decreased when the IRE1 pathway of the UPR was inhibited by STF-083010.

CONCLUSIONS

Activation of Wnt/β-catenin signaling pathway could cause insulin resistance and lipogenesis in hepatocytes via regulation of the IRE1 pathway of the ER stress and UPR, providing new targets for the treatment of metabolic disorders.

Hepatic triglyceride accumulation via endoplasmic reticulum stress-induced SREBP-1 activation is regulated by ceramide synthases

The endoplasmic reticulum (ER) is not only important for protein synthesis and folding but is also crucial for lipid synthesis and metabolism. In the current study, we demonstrate an important role of ceramide synthases (CerS) in ER stress and NAFLD progression. Ceramide is important in sphingolipid metabolism, and its acyl chain length is determined by a family of six CerS in mammals. CerS2 generates C22-C24 ceramides, and CerS5 or CerS6 produces C16 ceramide. To gain insight into the role of CerS in NAFLD, we used a high-fat diet (HFD)-induced NAFLD mouse model. Decreased levels of CerS2 and increased levels of CerS6 were observed in the steatotic livers of mice fed a HFD. In vitro experiments with Hep3B cells indicated the protective role of CerS2 and the detrimental role of CerS6 in the ER stress response induced by palmitate treatment. In particular, CerS6 overexpression increased sterol regulatory element-binding protein-1 (SREBP-1) cleavage with decreased levels of INSIG-1, leading to increased lipogenesis. Blocking ER stress abrogated the detrimental effects of CerS6 on palmitate-induced SREBP-1 cleavage. In accordance with the protective role of CerS2 in the palmitate-induced ER stress response, CerS2 knockdown enhanced ER stress and SREBP-1 cleavage, and CerS2 heterozygote livers exhibited a stronger ER stress response and higher triglyceride levels following HFD. Finally, treatment with a low dose of bortezomib increased hepatic CerS2 expression and protected the development of NAFLD following HFD. These results indicate that CerS and its derivatives impact hepatic ER stress and lipogenesis differently and might be therapeutic targets for NAFLD.

Promoting the activity of a protective membrane protein may help limit the development of non-alcoholic fatty liver disease (NAFLD) in obesity. Stress on a key cellular organelle, the endoplasmic reticulum (ER), contributes to NAFLD progression. Woo-Jae Park at Gachon University in Incheon, Joo-Won Park at Ewha Womans University, Seoul, and co-workers across South Korea have uncovered the role of a family of ER membrane proteins called ceramide synthases (CerS) in the regulation of ER stress during disease development. The team found increased levels of CerS6 in the livers of mouse fed a high-fat diet, while CerS2 decreased. The increased C16-ceramide by CerS6 overexpression triggered excess fat formation by increasing ER stress and SREBP-1 cleavage. However, when the team enhanced the expression of CerS2 using an existing chemotherapy drug, mice were protected from developing NAFLD.

Erythropoietin suppresses hepatic steatosis and obesity by inhibiting endoplasmic reticulum stress and upregulating fibroblast growth factor 21

Erythropoietin (EPO), known primarily for its role in erythropoiesis, was recently reported to play a beneficial role in regulating lipid metabolism; however, the underlying mechanism through which EPO decreases hepatic lipid accumulation requires further investigation. Endoplasmic reticulum (ER) stress may contribute to the progression of hepatic steatosis. The present study investigated the effects of EPO on regulating ER stress in fatty liver. It was demonstrated that EPO inhibited hepatic ER stress and steatosis in vivo and in vitro. Interestingly, these beneficial effects were abrogated in liver-specific sirtuin 1 (SIRT1)-knockout mice compared with wild-type littermates. In addition, in palmitate-treated hepatocytes, small interfering RNA-mediated SIRT1 silencing suppressed the effects of EPO on lipid-induced ER stress. Additionally, EPO stimulated hepatic fibroblast growth factor 21 (FGF21) expression and secretion in a SIRT1-dependent manner in mice. Furthermore, the sensitivity of hepatocytes from obese mice to FGF21 was restored following treatment with EPO. Collectively, the results of the present study revealed a new mechanism underlying the regulation of hepatic ER stress and FGF21 expression induced by EPO; thus, EPO may be considered as a potential therapeutic agent for the treatment of fatty liver and obesity.

Induction of Liver Steatosis in BAP31-Deficient Mice Burdened with Tunicamycin-Induced Endoplasmic Reticulum Stress

Endoplasmic reticulum (ER) stress is highly associated with liver steatosis. B-cell receptor-associated protein 31 (BAP31) has been reported to be involved in ER homeostasis, and plays key roles in hepatic lipid metabolism in high-fat diet-induced obese mice. However, whether BAP31 modulates hepatic lipid metabolism via regulating ER stress is still uncertain. In this study, wild-type and liver-specific BAP31-depleted mice were administrated with ER stress activator of Tunicamycin, the markers of ER stress, liver steatosis, and the underlying molecular mechanisms were determined. BAP31 deficiency increased Tunicamycin-induced hepatic lipid accumulation, aggravated liver dysfunction, and increased the mRNA levels of ER stress markers, including glucose-regulated protein 78 (GRP78), X-box binding protein 1 (XBP1), inositol-requiring protein-1α (IRE1α) and C/EBP homologous protein (CHOP), thus promoting ER stress in vivo and in vitro. Hepatic lipid export via very low-density lipoprotein (VLDL) secretion was impaired in BAP31-depleted mice, accompanied by reduced Apolipoprotein B (APOB) and microsomal triglyceride transfer protein (MTTP) expression. Exogenous lipid clearance was also inhibited, along with impaired gene expression related to fatty acid transportation and fatty acid β-oxidation. Finally, BAP31 deficiency increased Tunicamycin-induced hepatic inflammatory response. These results demonstrate that BAP31 deficiency increased Tunicamycin-induced ER stress, impaired VLDL secretion and exogenous lipid clearance, and reduced fatty acid β-oxidation, which eventually resulted in liver steatosis.

Pharmacologic inhibition of S1P attenuates ATF6 expression, causes ER stress and contributes to apoptotic cell death

Mammalian cells express unique transcription factors embedded in the endoplasmic reticulum (ER) membrane, such as the sterol regulatory element-binding proteins (SREBPs), that promote de novo lipogenesis. Upon their release from the ER, the SREBPs require proteolytic activation in the Golgi by site-1-protease (S1P). As such, inhibition of S1P, using compounds such as PF-429242 (PF), reduces cholesterol synthesis and may represent a new strategy for the management of dyslipidemia. In addition to the SREBPs, the unfolded protein response (UPR) transducer, known as the activating transcription factor 6 (ATF6), is another ER membrane-bound transcription factor that requires S1P-mediated activation. ATF6 regulates ER protein folding capacity by promoting the expression of ER chaperones such as the 78-kDa glucose-regulated protein (GRP78). ER-resident chaperones like GRP78 prevent and/or resolve ER polypeptide accumulation and subsequent ER stress-induced UPR activation by folding nascent polypeptides. Here we report that pharmacological inhibition of S1P reduced the expression of ATF6 and GRP78 and induced the activation of UPR transducers inositol-requiring enzyme-1α (IRE1α) and protein kinase RNA-like ER kinase (PERK). As a consequence, S1P inhibition also increased the susceptibility of cells to ER stress-induced cell death. Our findings suggest that S1P plays a crucial role in the regulation of ER folding capacity and also identifies a compensatory cross-talk between UPR transducers in order to maintain adequate ER chaperone expression and activity.

Autophagy Dysregulation and Obesity-Associated Pathologies

Autophagy is one of the major degradative mechanisms that can eliminate excessive nutrients, toxic protein aggregates, damaged organelles and invading microorganisms. In response to obesity and obesity-associated lipotoxic, proteotoxic and oxidative stresses, autophagy plays an essential role in maintaining physiological homeostasis. However, obesity and its associated stress insults can often interfere with the autophagic process through various mechanisms, which result in further aggravation of obesity-related metabolic pathologies in multiple metabolic organs. Paradoxically, inhibition of autophagy, within specific contexts, indirectly produces beneficial effects that can alleviate several detrimental consequences of obesity. In this minireview, we will provide a brief discussion about our current understanding of the impact of obesity on autophagy and the role of autophagy dysregulation in modulating obesity-associated pathological outcomes.

SIRT1/HSF1/HSP pathway is essential for exenatide-alleviated, lipid-induced hepatic endoplasmic reticulum stress

Recent studies have indicated that lipid-induced endoplasmic reticulum (ER) stress is a major contributor to the progression of hepatic steatosis. Exenatide (exendin-4), a glucagon-like peptide-1 receptor agonist, is known to improve hepatic steatosis, with accumulating evidence. In this study, we investigated whether exenatide could alleviate lipid-induced hepatic ER stress through mammal sirtuin 1 (SIRT1) and illustrated the detailed mechanisms. Male C57BL/6J mice challenged with a high-fat diet (HFD) were treated with exenatide or normal saline by intraperitoneal injection for 4 weeks. We observed that HFD feeding induced hepatic ER stress as indicated by increased expression of glucose-regulated protein 78, phosphorylated protein kinase-like ER kinase, and phosphorylated eukaryotic initiation factor 2α, while these increases were significantly inhibited by exenatide. Exenatide notably decreased the liver weight and hepatic steatosis induced by HFD challenge. Consistently, in human HepG2 cells and primary murine hepatocytes, exendin-4 also significantly alleviated the ER stress and lipid accumulation induced by palmitate. Importantly, further studies showed that exendin-4 enhanced the binding of heat shock factor 1 to the promoter of heat shock protein (HSP) genes through SIRT1-mediated deacetylation, which then increased the expression of molecular chaperones HSP70 and HSP40 to alleviate hepatic ER stress. Finally, inhibition of SIRT1 by genetic whole-body heterozygous knockout or by lentiviral short hairpin RNA knockdown greatly diminished the effect of exenatide on deacetylating heat shock factor 1, increasing HSP expression and alleviating ER stress and hepatic steatosis in HFD-fed mice.

CONCLUSION

The SIRT1/heat shock factor 1/HSP pathway is essential for exenatide-alleviated, lipid-induced ER stress and hepatic steatosis, which provides evidence for a molecular mechanism to support exenatide and incretin mimetics as promising therapeutics for obesity-induced hepatic steatosis. (Hepatology 2017;66:809-824).

Astragaloside IV attenuates free fatty acid-induced ER stress and lipid accumulation in hepatocytes via AMPK activation

Although the pathogenesis of non-alcoholic fatty liver disease (NAFLD) is not completely understood, the increased influx of free fatty acids (FFAs) into the liver and the FFA-induced hepatic endoplasmic reticulum (ER) stress are two crucial pathogenic processes in the initiation and development of NAFLD. In this study we investigated the effects of astragaloside IV (AS-IV), a bioactive compound purified from Astragali Radix, on FFA-induced lipid accumulation in hepatocytes and elucidated the underlying mechanisms. Human HepG2 cells and primary murine hepatocytes were exposed to FFAs (1 mmol/L, oleate/palmitate, 2:1 ratio) with or without AS-IV for 24 h. Exposure to FFAs induced marked lipid accumulation in hepatocytes, whereas co-treatment with AS-IV (100 μg/mL) significantly attenuated this phenomenon. Notably, AS-IV (50-200 μg/mL) concentration-dependently enhanced the phosphorylation of AMPK, acetyl-CoA carboxylase (ACC) and SREBP-1c, inhibited the accumulation and nuclear translocation of mature SREBP-1 and subsequently decreased the mRNA levels of lipogenic genes including acc1, fas and scd1. AS-IV treatment also concentration-dependently attenuated FFA-induced hepatic ER stress evidenced by the reduction of the key markers, GRP78, CHOP and p-PERK. Pretreated the cells with the AMPK inhibitor compound C (20 μmol/L) greatly diminished these beneficial effects of AS-IV. Our results demonstrate that AS-IV attenuates FFA-induced ER stress and lipid accumulation in an AMPK-dependent manner in hepatocytes, which supports its use as promising therapeutics for hepatic steatosis.

Endoplasmic reticulum stress participates in inflammation-accelerated, lipid-mediated injury of human glomerular mesangial cells

AIM

The mechanism of lipid-mediated injury of human glomerular mesangial cells (HMCs) remains unclear. We investigated the association between endoplasmic reticulum (ER) stress and lipid-mediated injury in HMCs in vitro and the potential efficacy of a therapeutic approach targeting ER stress.

METHODS

Human glomerular mesangial cells were exposed to low-density lipoprotein (LDL) and/or interleukin-1β (IL-1β). For evaluation of whether ER stress participates in lipid-mediated injury to HMCs, HMCs were pretreated with tunicamycin or treated with sodium 4-phenylbutyrate (4-PBA).

RESULTS

Incubation of HMCs with LDL + IL-1β significantly increased lipid accumulation and induced phenotypic changes. ER stress was induced in lipid-loaded HMCs, as indicated by upregulation of glucose-regulated protein 78 (GRP78) and protein kinase RNA-like ER kinase (PERK) proteins. Moreover, persistent ER stress increased expression of nuclear factor (NF)-κB p65 protein, fibronectin, and α-smooth muscle actin (α-SMA) mRNA partly through the PERK - eukaryotic initiation factor-2α (eIF2α) pathway. Preconditioning with ER stress by tunicamycin and inhibition of ER stress by 4-PBA both reversed the phenotypic changes and decreased lipid accumulation and inflammatory cytokine secretion by the PERK - eIF2α pathway.

CONCLUSION

These data provide evidence that ER stress participates in inflammation associated with lipid-induced injury of HMCs. Modulation of ER stress may be a novel therapeutic approach for combating lipid-induced injury of HMCs.

Hepatic rhythmicity of endoplasmic reticulum stress is disrupted in perinatal and adult mice models of high-fat diet-induced obesity

We investigated the regulation of hepatic ER stress in healthy liver and adult or perinatally programmed diet-induced non-alcoholic fatty liver disease (NAFLD). Female mice were fed either obesogenic or control diet before mating, during pregnancy and lactation. Post-weaning, offspring from each maternal group were divided into either obesogenic or control diet. At six months, offspring were sacrificed at 4-h intervals over 24 h. Offspring fed obesogenic diets developed NAFLD phenotype, and the combination of maternal and offspring obesogenic diets exacerbated this phenotype. UPR signalling pathways (IREα, PERK, ATF6) and their downstream regulators showed different basal rhythmicity, which was modified in offspring exposed to obesogenic diet and maternal programming. The double obesogenic hit increased liver apoptosis measured by TUNEL staining, active caspase-3 and phospho-JNK and GRP78 promoter methylation levels. This study demonstrates that hepatic UPR is rhythmically activated. The combination of maternal obesity (MO) and obesogenic diets in offspring triggered altered UPR rhythmicity, DNA methylation and cellular apoptosis.

Transcriptional regulation of hepatic lipogenesis

Fatty acid and fat synthesis in the liver is a highly regulated metabolic pathway that is important for very low-density lipoprotein (VLDL) production and thus energy distribution to other tissues. Having common features at their promoter regions, lipogenic genes are coordinately regulated at the transcriptional level. Transcription factors, such as upstream stimulatory factors (USFs), sterol regulatory element-binding protein 1C (SREBP1C), liver X receptors (LXRs) and carbohydrate-responsive element-binding protein (ChREBP) have crucial roles in this process. Recently, insights have been gained into the signalling pathways that regulate these transcription factors. After feeding, high blood glucose and insulin levels activate lipogenic genes through several pathways, including the DNA-dependent protein kinase (DNA-PK), atypical protein kinase C (aPKC) and AKT-mTOR pathways. These pathways control the post-translational modifications of transcription factors and co-regulators, such as phosphorylation, acetylation or ubiquitylation, that affect their function, stability and/or localization. Dysregulation of lipogenesis can contribute to hepatosteatosis, which is associated with obesity and insulin resistance.

Ursolic acid inhibits the development of nonalcoholic fatty liver disease by attenuating endoplasmic reticulum stress

Ursolic acid (UA) is a natural pentacyclic triterpenoid compound, which is enriched with many herbs and plants, such as apple, cranberry and olive. UA performs multiple biological activities including anti-oxidation, anti-inflammation, anti-cancer and hepatoprotection. However, the exact mechanism underlying the hepatoprotective activity of UA remains unclear. In this study, the effects of UA on the development of nonalcoholic fatty liver disease (NAFLD) were investigated. In vivo, UA treatment (0.14%, w/w) significantly decreased the liver weight, serum levels of ALT/AST and hepatic steatosis in db/db mice (a type 2 diabetic mouse model). In vitro, UA treatment (10-30 μg ml(-1)) significantly decreased palmitic acid induced intracellular lipid accumulation in L02 cells. Our results suggested that the beneficial effects of UA on NAFLD may be due to its ability to increase lipid β-oxidation and to inhibit the hepatic endoplasmic reticulum (ER) stress. Together, UA may be further considered as a natural compound for NAFLD treatment.

Gp78, an E3 ubiquitin ligase acts as a gatekeeper suppressing nonalcoholic steatohepatitis (NASH) and liver cancer

Nonalcoholic steatohepatitis (NASH) is related to metabolic dysregulation and the perturbation of endoplasmic reticulum (ER) homeostasis that frequently develops into hepatocellular carcinoma (HCC). Gp78 is E3 ligase, which regulates endoplasmic reticulum-associated degradation (ERAD) by ubiquitinylation of misfolded ER proteins. Here, we report that upon ageing (12 months), gp78^-/- mice developed obesity, recapitulating age-related human NASH. Liver histology of gp78^-/- mice revealed typical steatosis, hepatic inflammation and fibrosis, followed by progression to hepatocellular tumors. Acute ER stress revealed that loss of gp78 results in up regulation of unfolded protein response (UPR) pathways and SREBP-1 regulating de novo lipogenesis, responsible for fatty liver. Tissue array of human hepatocellular carcinoma (HCC) demonstrated that the expression of gp78 was inversely correlated with clinical grades of cancer. Here, we have described the generation of the first preclinical experimental model system which spontaneously develops age-related NASH and HCC, linking ERAD to hepatosteatosis, cirrhosis, and cancer. It suggests that gp78 is a regulator of normal liver homeostasis and a tumor suppressor in human liver.

Inhibition of CYP4A reduces hepatic endoplasmic reticulum stress and features of diabetes in mice

BACKGROUND & AIMS

Endoplasmic reticulum (ER) stress is implicated in the development of type 2 diabetes mellitus. ER stress activates the unfolded protein response pathway, which contributes to apoptosis and insulin resistance. We investigated the roles of cytochrome P450 4A (CYP4A) in the regulation of hepatic ER stress, insulin resistance, and the development of diabetes in mice.

METHODS

We used mass spectrometry to compare levels of CYP450 proteins in livers from C57BL/6J and C57BL/KsJ-db/db (db/db) mice; findings were confirmed by immunoblot and real-time PCR analyses. To create a model of diet-induced diabetes, C57BL/6J mice were placed on high-fat diets. Mice were given intraperitoneal injections of an inhibitor (HET0016) or an inducer (clofibrate) of CYP4A, or tail injections of small hairpin RNAs against CYP4A messenger RNA; liver tissues were collected and analyzed for ER stress, insulin resistance, and apoptosis. The effect of HET0016 and CYP4A knockdown also were analyzed in HepG2 cells.

RESULTS

Levels of the CYP4A isoforms were highly up-regulated in livers of db/db mice compared with C57BL/6J mice. Inhibition of CYP4A in db/db and mice on high-fat diets reduced features of diabetes such as insulin hypersecretion, hepatic steatosis, and increased glucose tolerance. CYP4A inhibition reduced levels of ER stress, insulin resistance, and apoptosis in the livers of diabetic mice; it also restored hepatic functions. Inversely, induction of CYP4A accelerated ER stress, insulin resistance, and apoptosis in livers of db/db mice.

CONCLUSIONS

CYP4A proteins are up-regulated in livers of mice with genetically induced and diet-induced diabetes. Inhibition of CYP4A in mice reduces hepatic ER stress, apoptosis, insulin resistance, and steatosis. Strategies to reduce levels or activity of CYP4A proteins in liver might be developed for treatment of patients with type 2 diabetes.

Fetal and neonatal exposure to nicotine leads to augmented hepatic and circulating triglycerides in adult male offspring due to increased expression of fatty acid synthase

While nicotine replacement therapy is assumed to be a safer alternative to smoking during pregnancy, the long-term consequences for the offspring remain elusive. Animal studies now suggest that maternal nicotine exposure during perinatal life leads to a wide range of adverse outcomes for the offspring including increased adiposity. The focus of this study was to investigate if nicotine exposure during pregnancy and lactation leads to alterations in hepatic triglyceride synthesis. Female Wistar rats were randomly assigned to receive daily subcutaneous injections of saline (vehicle) or nicotine bitartrate (1mg/kg/day) for two weeks prior to mating until weaning. At postnatal day 180 (PND 180), nicotine exposed offspring exhibited significantly elevated levels of circulating and hepatic triglycerides in the male offspring. This was concomitant with increased expression of fatty acid synthase (FAS), the critical hepatic enzyme in de novo triglyceride synthesis. Given that FAS is regulated by the nuclear receptor Liver X receptor (LXRα), we measured LXRα expression in both control and nicotine-exposed offspring. Nicotine exposure during pregnancy and lactation led to an increase in hepatic LXRα protein expression and enriched binding to the putative LXRE element on the FAS promoter in PND 180 male offspring. This was also associated with significantly enhanced acetylation of histone H3 [K9,14] surrounding the FAS promoter, a hallmark of chromatin activation. Collectively, these findings suggest that nicotine exposure during pregnancy and lactation leads to an increase in circulating and hepatic triglycerides long-term via changes in the transcriptional and epigenetic regulation of the hepatic lipogenic pathway.

Enhanced endoplasmic reticulum SERCA activity by overexpression of hepatic stimulator substance gene prevents hepatic cells from ER stress-induced apoptosis

Although the potential pathogenesis of nonalcoholic fatty liver disease (NAFLD) is unclear, increasing evidence indicates that endoplasmic reticulum (ER) stress may link free fatty acids to NAFLD. Since we previously reported that hepatic stimulator substance (HSS) could protect the liver from steatosis, this study is aimed to investigate whether HSS protection could be related with its inhibition on ER stress. The HSS gene was stably transfected into BEL-7402 hepatoma cells and effectively expressed in ER. The palmitic acid (PA)-induced heptocyte lipotoxicity was reproduced in the HSS-transfected cells, and HSS alleviation of the ER stress and apoptosis were subsequently examined. The results showed that PA treatment led to a heavy accumulation of fatty acids within the cells and a remarkable increase in reactive oxygen species (ROS). However, in the HSS-expressing cells, production of ROS was inhibited and ER stress-related marker glucose-regulated protein 78 (GRP-78), sterol regulatory element-binding protein (SREBP), anti-phospho-PRK-1ike ER kinase (p-PERK), anti-phospho-eukaryotic initiation factor 2α (p-eIF2α), and anti-C/EBP homologous protein (CHOP) were downregulated compared with the wild-type or mutant HSS-transfected cells. Furthermore, PA treatment severely impaired the activity of sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA), leading to imbalanced calcium homeostasis during ER stress, which could be rescued in the HSS-trasfected cells. The protection provided by HSS to the SERCA is identical to that observed with N-acetyl-l-cysteine (NAC) and sodium dimercaptopropane sulfonate (Na-DMPS), which are two typical free radical scavengers. As a consequence, the rate of ER stress-mediated apoptosis in the HSS-expressing cells was significantly reduced. In conclusion, the protective effect of HSS against ER stress may be associated with the removal of ROS to restore the activity of the SERCA.

CEACAM1 loss links inflammation to insulin resistance in obesity and non-alcoholic steatohepatitis (NASH)

Mounting epidemiological evidence points to an association between metabolic syndrome and non-alcoholic steatohepatitis (NASH), an increasingly recognized new epidemic. NASH pathologies include hepatocellular ballooning, lobular inflammation, hepatocellular injury, apoptosis, and hepatic fibrosis. We will review the relationship between insulin resistance and inflammation in visceral obesity and NASH in an attempt to shed more light on the pathogenesis of these major metabolic diseases. Moreover, we will identify loss of the carcinoembryonic antigen-related cell adhesion molecule 1 as a unifying mechanism linking the immunological and metabolic abnormalities in NASH.

Genome-wide association study identifies novel loci associated with concentrations of four plasma phospholipid fatty acids in the de novo lipogenesis pathway: results from the Cohorts for Heart and Aging Research in Genomic Epidemiology (CHARGE) consortium

BACKGROUND- Palmitic acid (16:0), stearic acid (18:0), palmitoleic acid (16:1n-7), and oleic acid (18:1n-9) are major saturated and monounsaturated fatty acids that affect cellular signaling and metabolic pathways. They are synthesized via de novo lipogenesis and are the main saturated and monounsaturated fatty acids in the diet. Levels of these fatty acids have been linked to diseases including type 2 diabetes mellitus and coronary heart disease. METHODS AND RESULTS- Genome-wide association studies were conducted in 5 population-based cohorts comprising 8961 participants of European ancestry to investigate the association of common genetic variation with plasma levels of these 4 fatty acids. We identified polymorphisms in 7 novel loci associated with circulating levels of ≥1 of these fatty acids. ALG14 (asparagine-linked glycosylation 14 homolog) polymorphisms were associated with higher 16:0 (P=2.7×10(-11)) and lower 18:0 (P=2.2×10(-18)). FADS1 and FADS2 (desaturases) polymorphisms were associated with higher 16:1n-7 (P=6.6×10(-13)) and 18:1n-9 (P=2.2×10(-32)) and lower 18:0 (P=1.3×10(-20)). LPGAT1 (lysophosphatidylglycerol acyltransferase) polymorphisms were associated with lower 18:0 (P=2.8×10(-9)). GCKR (glucokinase regulator; P=9.8×10(-10)) and HIF1AN (factor inhibiting hypoxia-inducible factor-1; P=5.7×10(-9)) polymorphisms were associated with higher 16:1n-7, whereas PKD2L1 (polycystic kidney disease 2-like 1; P=5.7×10(-15)) and a locus on chromosome 2 (not near known genes) were associated with lower 16:1n-7 (P=4.1×10(-8)). CONCLUSIONS- Our findings provide novel evidence that common variations in genes with diverse functions, including protein-glycosylation, polyunsaturated fatty acid metabolism, phospholipid modeling, and glucose- and oxygen-sensing pathways, are associated with circulating levels of 4 fatty acids in the de novo lipogenesis pathway. These results expand our knowledge of genetic factors relevant to de novo lipogenesis and fatty acid biology.

Do hepatitis B virus and hepatitis C virus co-infections increase hepatocellular carcinoma occurrence through synergistically modulating lipogenic gene expression?

Hepatitis B virus (HBV) and hepatitis C virus (HCV) infections cause a wide range of liver diseases including hepatocellular carcinoma (HCC). Because of the similar modes of transmission, HBV HCV co-infections are found in approximately 7-20 million people globally. Compared with HBV or HCV mono-infections, co-infections are associated with more severe liver diseases and higher risk of HCC. Abnormal lipid biosynthesis and metabolism has been increasingly recognized as a cause for cancer. While HBV infection does not seem to significantly increase the risk of developing hepatic steatosis, steatosis is a prominent feature of chronic hepatitis C (CHC). In addition, steatosis in HBV or HCV mono-infections is a significant and independent risk factor for HCC. However, whether and how HBV HCV co-infections synergistically increase the risk of HCC development through modulating lipid metabolism is not well understood. Possible mechanisms by which steatosis causes HCC include: activation of sterol regulatory element-binding protein-mediated lipogenesis through the PI3K-Akt pathway, abnormal activation of peroxisome proliferator-activated receptors and endoplasmic reticulum stress. Here, we review the potential mechanisms by which HBV HCV co-infections may increase HCC risk through modulation of lipogenic gene expression. We begin with reviewing the impact of HBV and HCV on host lipogenic gene expression and carcinogenesis. We then discuss the potential mechanisms by which HBV and HCV can increase carcinogenesis through synergistically activating lipid biosynthesis and metabolism. We end by sharing our thoughts on future research directions in this emerging paradigm with an ultimate goal of developing effective therapeutics.

Accumulation of endoplasmic reticulum stress and lipogenesis in the liver through generational effects of high fat diets

BACKGROUND & AIMS

The dramatic rise of nonalcoholic fatty liver disease (NAFLD) among children in the past decade cannot be solely explained by the increased high fat diet (HFD) intake in kids. Recent studies suggest that the offspring of HFD-fed mothers develop a worse form of NAFLD when weaned on the HFD than when weaned on the normal chow (NC), indicating that a feed-forward circle may exacerbate the syndromes throughout multiple generations. In the present study, the aforementioned feed-forward circle was investigated in mice by employing continuous HFD feeding for three generations.

METHODS

C57BL/6 mice were fed with either a HFD or NC for three consecutive generations (F0, F1, and F2). Body weight, food intake, hepatic histology; levels of insulin, leptin, and triglycerides; expression of factors involved in lipogenesis and endoplasmic reticulum (ER) stress pathways; and histone methylation status were investigated in male offspring.

RESULTS

Obesity occurred earlier, became more severe through generations (F2>F1>F0), and was accompanied by a gradual increase of histological scoring of steatosis in male mice with transgenerational HFD feeding. The highest degree of steatosis occurred in HFD-treated F2 mice and was associated with the highest levels of insulin and leptin. The latter mice were characterized by enhanced lipogenesis and ER stress with a trend of transgenerational changes was detected for LXRα, ERO1-α, histone methylations, and H3K9 histone methyltransferase. Furthermore, chromatin immunoprecipitation (CHIP) assay demonstrated a significantly reduced accumulation of methylated histones in LXRα and ERO1-α gene promoters.

CONCLUSIONS

Under HFD feeding stress, the male offspring of the F2 generation (derived from both grand-maternal and maternal obesity) are extremely susceptible to developing obesity and hepatic steatosis. This is presumably a consequence of transgenerational accumulation of epigenetic modifications leading to up-regulation of lipogenesis and ER stress pathways in the liver.

Liver-specific deletion of protein tyrosine phosphatase (PTP) 1B improves obesity- and pharmacologically induced endoplasmic reticulum stress

Obesity is associated with induction of the ER (endoplasmic reticulum)-stress response signalling and insulin resistance. PTP1B (protein tyrosine phosphatase 1B) is a major regulator of adiposity and insulin sensitivity. The aim of the present study was to investigate the role of L-PTP1B (liver-specific PTP1B) in chronically HFD (high-fat diet) and pharmacologically induced (tunicamycin and thapsigargin) ER-stress response signalling in vitro and in vivo. We assessed the effects of ER-stress response induction on hepatic PTP1B expression, and consequences of hepatic-PTP1B deficiency, in cells and mouse liver, on components of ER-stress response signalling. We found that PTP1B protein and mRNA expression levels were up-regulated in response to acute and/or chronic ER stress, in vitro and in vivo. Silencing PTP1B in hepatic cell lines or mouse liver (L-PTP1B(-/-)) protected against induction of pharmacologically induced and/or obesity-induced ER stress. The HFD-induced increase in CHOP (CCAAT/enhancer-binding protein homologous protein) and BIP (binding immunoglobulin protein) mRNA levels were partially inhibited, whereas ATF4 (activated transcription factor 4), GADD34 (growth-arrest and DNA-damage-inducible protein 34), GRP94 (glucose-regulated protein 94), ERDJ4 (ER-localized DnaJ homologue) mRNAs and ATF6 protein cleavage were completely suppressed in L-PTP1B(-/-) mice relative to control littermates. L-PTP1B(-/-) mice also had increased nuclear translocation of spliced XBP-1 (X box-binding protein-1) via increased p85α binding. We demonstrate that the ER-stress response and L-PTP1B expression are interlinked in obesity- and pharmacologically induced ER stress and this may be one of the mechanisms behind improved insulin sensitivity and lower lipid accumulation in L-PTP1B(-/-) mice.

Adiponectin suppresses endoplasmic reticulum stress in nonalcoholic steatohepatitis

In this study, we examined whether adiponectin suppresses endoplasmic reticulum (ER) stress in nonalcoholic steatohepatitis (NASH) using male transgenic mice expressing nSREBP-1c in adipose tissue, nSREBP-1c/adiponectin double-transgenic mice expressing human adiponectin in the liver, and wild-type male mice as the control. Histological findings similar to those observed in liver specimens from patients with NASH were observed in the livers from the nSREBP-1c transgenic mice at 30 weeks of age. By contrast, the NASH-like liver histology was markedly attenuated in age-matched nSREBP-1c/adiponectin double-transgenic mice. The nSREBP-1c/adiponectin double-transgenic mice showed human adiponectin production in the liver and a restored circulating human adiponectin level. Human adiponectin messenger ribonucleic acid (mRNA) expression in the liver was identified in the nSREBP-1c/adiponectin double-transgenic mice, but adiponectin receptor 1 and 2 mRNA expression in the liver was normal. TNFα mRNA was decreased in the liver of the nSREBP-1c/adiponectin double-transgenic mice compared with the nSREBP-1c transgenic mice. The protein expressions of X-box-binding protein-1, activating transcription factor 4, acetyl-CoA carboxylase, TNFα and NFκB were down-regulated in liver tissues from the nSREBP-1c/adiponectin double-transgenic mice. Mouse adiponectin and activating transcription factor 6 expressions were almost the same in the three groups. Post-load plasma glucose levels were significantly lower in the nSREBP-1c/adiponectin double-transgenic mice compared with the nSREBP-1c transgenic mice. These results indicate that adiponectin expressed in the liver suppresses ER stress and attenuates hepatic steatosis, inflammation and insulin resistance in NASH. Adiponectin may open the way to novel therapies for human NASH.

Progressive endoplasmic reticulum stress contributes to hepatocarcinogenesis in fatty acyl-CoA oxidase 1-deficient mice

Fatty acyl-coenzyme A oxidase 1 (ACOX1) knockout (ACOX1(-/-)) mice manifest hepatic metabolic derangements that lead to the development of steatohepatitis, hepatocellular regeneration, spontaneous peroxisome proliferation, and hepatocellular carcinomas. Deficiency of ACOX1 results in unmetabolized substrates of this enzyme that function as biological ligands for peroxisome proliferator-activated receptor-α (PPARα) in liver. Here we demonstrate that sustained activation of PPARα in ACOX1(-/-) mouse liver by these ACOX1 substrates results in endoplasmic reticulum (ER) stress. Overexpression of transcriptional regulator p8 and its ER stress-related effectors such as the pseudokinase tribbles homolog 3, activating transcription factor 4, and transcription factor CCAAT/-enhancer-binding protein homologous protein as well as phosphorylation of eukaryotic translation initiation factor 2α, indicate the induction of unfolded protein response signaling in the ACOX1(-/-) mouse liver. We also show here that, in the liver, p8 is a target for all three PPAR isoforms (-α, -β, and -γ), which interact with peroxisome proliferator response elements in p8 promoter. Sustained activation of p8 and unfolded protein response-associated ER stress in ACOX1(-/-) mouse liver contributes to hepatocyte apoptosis and liver cell proliferation culminating in the development of hepatocarcinogenesis. We also demonstrate that human ACOX1 transgene is functional in ACOX1(-/-) mice and effectively prevents metabolic dysfunctions that lead to ER stress and carcinogenic effects. Taken together, our data indicate that progressive PPARα- and p8-mediated ER stress contribute to the hepatocarcinogenesis in ACOX1(-/-) mice.

Constitutive role for IRE1α-XBP1 signaling pathway in the insulin-mediated hepatic lipogenic program

The role of the IRE1α-X-box-binding protein 1 (XBP1) pathway in the insulin-mediated hepatic lipogenic program and associated mechanisms were investigated in this study. We observed that phosphorylation of IRE1α (an upstream activator of XBP1) and splicing (activation) of XBP1 were elevated in the liver of the C57BL/6 mice with insulin resistance/hyperinsulinemia induced by high-fat diet. Treatment of nonobese diabetic mice with insulin activated hepatic XBP1. In cultured primary mouse hepatocytes, prolonged exposure to insulin induced IRE1α phosphorylation and XBP1 splicing significantly in the presence of insulin resistance. Overexpression of the activated XBP1 elevated the promoter activities of the sterol regulatory element-binding protein (SREBP)-1c and fatty acid synthase (FAS) genes. Knockdown of either the IRE1α or XBP1 gene by small interfering RNA prevented the insulin-stimulated promoter activities of both SREBP-1 and FAS genes. In investigating the associated mechanisms, we found a direct interaction between XBP1 and SREBP-1 promoter detected by the chromatin immunoprecipitation assays. Furthermore, the XBP1-mediated stimulation of the FAS promoter was eliminated by knocking down the SREBP-1c gene (Srebf1). Finally, we observed that insulin activation of the IRE1α-XBP1 pathway was prevented by inhibition of mammalian target of rapamycin-dependent protein synthesis. In conclusion, our results show that the IRE1α-XBP1-mediated unfolded protein response pathway is an integrated part of the insulin-induced hepatic lipogenic program and functions at an increased basal level in the presence of insulin resistance and hyperinsulinemia. Besides, the insulin-mediated protein synthesis is tightly connected with the insulin-mediated lipogenic program.

Hepatotoxicity related to antirheumatic drugs

Antirheumatic agents are among commonly used drugs associated with adverse hepatic reactions. Sulfasalazine and azathioprine are among the most important causes of acute hepatotoxicity. Because such a large number of people take NSAIDs, even the rare occurrence of hepatotoxicity from these agents might contribute substantially to the total burden of drug-induced liver disease. A wide spectrum of hepatotoxic effects is described with antirheumatic drugs. Studies investigating genetic susceptibility to diclofenac hepatotoxicity have expanded our understanding of the potential drug-specific, class-specific and general factors involved in its pathogenesis, and methotrexate-associated liver disease demonstrates the interaction between drug, host and environmental factors that determines the likelihood and magnitude of liver disease. Infliximab therapy is associated with typical drug-induced autoimmune hepatitis. Although validated causality assessment methods have been used to objectively assess the strength of the association between a drug and a clinical event, in practice the diagnosis of drug-induced liver injury (DILI) involves a clinical index of suspicion, pattern recognition, the establishment of a temporal relationship between drug exposure and the adverse event, and the exclusion of alternative explanations for the clinical presentation. Detailed understanding of genetic and environmental factors underlying an individual's susceptibility would enable risk reduction and potentially primary prevention of hepatotoxicity.

Deficiency in the extracellular signal-regulated kinase 1 (ERK1) protects leptin-deficient mice from insulin resistance without affecting obesity

AIMS/HYPOTHESIS

Extracellular signal-regulated kinase (ERK) activity is increased in adipose tissue in obesity and type 2 diabetes mellitus and strong evidences suggests that it is implicated in the downregulation of insulin signalling and action in the insulin-resistant state. To determine the role of ERK1 in obesity-associated insulin resistance in vivo, we inactivated Erk1 (also known as Mapk3) in obese leptin-deficient mice (ob/ob).

METHODS

Mice of genotype ob/ob-Erk1⁻(/)⁻ were obtained by crossing Erk1⁻(/)⁻ mice with ob/ob mice. Glucose tolerance and insulin sensitivity were studied in 12-week-old mice. Tissue-specific insulin sensitivity, insulin signalling, liver steatosis and adipose tissue inflammation were determined.

RESULTS

While ob/ob-Erk1⁻(/)⁻ and ob/ob mice exhibited comparable body weight and adiposity, ob/ob-Erk1⁻(/)⁻ mice did not develop hyperglycaemia and their glucose tolerance was improved. Hyperinsulinaemic-euglycaemic clamp studies demonstrated an increase in whole-body insulin sensitivity in the ob/ob-Erk1⁻(/)⁻ mice associated with an increase in both insulin-stimulated glucose disposal in skeletal muscles and adipose tissue insulin sensitivity. This occurred in parallel with improved insulin signalling in both tissues. The ob/ob-Erk1⁻(/)⁻ mice were also partially protected against hepatic steatosis with a strong reduction in acetyl-CoA carboxylase level. These metabolic improvements were associated with reduced expression of mRNA encoding inflammatory cytokine and T lymphocyte markers in the adipose tissue.

CONCLUSIONS/INTERPRETATION

Our results demonstrate that the targeting of ERK1 could partially protect obese mice against insulin resistance and liver steatosis by decreasing adipose tissue inflammation and by increasing muscle glucose uptake. Our results indicate that deregulation of the ERK1 pathway could be an important component in obesity-associated metabolic disorders.

Differences in betaine-homocysteine methyltransferase expression, endoplasmic reticulum stress response, and liver injury between alcohol-fed mice and rats

Chronic ethanol infusion resulted in greater serum alanine aminotransferase elevation, lipid accumulation, necroinflammation, and focal hepatic cell death in mice than rats. Mice exhibited a remarkable hyperhomocysteinemia but no increase was seen in rats. Similarly, a high-methionine low-folate diet (HMLF) induced less steatosis, serum alanine aminotransferase increase, and hyperhomocysteinemia in rats than in mice. Western blot analysis of betaine homocysteine methyltransferase (BHMT) expression showed that rats fed either ethanol or HMLF had significantly increased BHMT expression, which did not occur in mice. Nuclear factor-kappaB p65 was increased in mouse in response to alcohol feeding. The human BHMT promoter was repressed by homocysteine in mouse hepatocytes but not rat hepatocytes. BHMT induction was faster and greater in primary rat hepatocytes than mouse hepatocytes in response to exogenous homocysteine exposure. Mice fed ethanol intragastrically exhibited an increase in glucose-regulated protein 78 and inositol-requiring enzyme 1, which was not seen in the rat, and sterol regulatory element binding protein 1 was increased to a greater extent in mice than rats. Thus, rats are more resistant to ethanol-induced steatosis, endoplasmic reticulum stress, and hyperhomocysteinemia, and this correlates with induction of BHMT in rats.

CONCLUSION

These findings support the hypothesis that a critical factor in the pathogenesis of alcoholic liver injury is the enhanced ability of rat or impaired ability of mouse to up-regulate BHMT which prevents hyperhomocysteinemia, endoplasmic reticulum stress, and liver injury.

Hypothalamic lipotoxicity and the metabolic syndrome

Ectopic accumulation of lipids in peripheral tissues, such as pancreatic beta cells, liver, heart and skeletal muscle, leads to lipotoxicity, a process that contributes substantially to the pathophysiology of insulin resistance, type 2 diabetes, steatotic liver disease and heart failure. Current evidence has demonstrated that hypothalamic sensing of circulating lipids and modulation of hypothalamic endogenous fatty acid and lipid metabolism are two bona fide mechanisms modulating energy homeostasis at the whole body level. Key enzymes, such as AMP-activated protein kinase (AMPK) and fatty acid synthase (FAS), as well as intermediate metabolites, such as malonyl-CoA and long-chain fatty acids-CoA (LCFAs-CoA), play a major role in this neuronal network, integrating peripheral signals with classical neuropeptide-based mechanisms. However, one key question to be addressed is whether impairment of lipid metabolism and accumulation of specific lipid species in the hypothalamus, leading to lipotoxicity, have deleterious effects on hypothalamic neurons. In this review, we summarize what is known about hypothalamic lipid metabolism with focus on the events associated to lipotoxicity, such as endoplasmic reticulum (ER) stress in the hypothalamus. A better understanding of these molecular mechanisms will help to identify new drug targets for the treatment of obesity and metabolic syndrome.