Repression of PKR mediates palmitate-induced apoptosis in HepG2 cells through regulation of Bcl-2

Eukaryotic translation initiation factor 2α kinase 2 in pancreatic cancer: An approach towards managing clinical prognosis and molecular immunological characterization

Most patients with pancreatic cancer are already in the late stages of the disease when they are diagnosed, and pancreatic cancer is a deadly disease with a poor prognosis. With the advancement of research, immunotherapy has become a new focus in the treatment of tumors. To the best of our knowledge, there is currently no reliable diagnostic or prognostic marker for pancreatic cancer; therefore, the present study investigated the potential of eukaryotic translation initiation factor 2α kinase 2 (EIF2AK2) as a predictive and diagnostic marker for pancreatic cancer. Immunohistochemical staining of clinical samples independently verified that EIF2AK2 expression was significantly higher in clinically operated pancreatic cancer tissues than in adjacent pancreatic tissues., and EIF2AK2 expression and differentially expressed genes (DEGs) were identified using downloadable RNA sequencing data from The Cancer Genome Atlas and Genomic Tumor Expression Atlas. In addition, Gene Ontology/Kyoto Encyclopedia of Genes and Genomes analyses and immune cell infiltration were used for functional enrichment analysis of EIF2AK2-associated DEGs. The clinical importance of EIF2AK2 was also determined using Kaplan-Meier survival, Cox regression and time-dependent survival receiver operating characteristic curve analyses, and a predictive nomogram model was generated. Finally, the functional role of EIF2AK2 was assessed in PANC-1 cells using a short hairpin RNA-EIF2AK2 knockdown approach, including CCK-8, wound healing assay, cell cycle and apoptosis assays. The findings suggested that EIF2AK2 may have potential as a diagnostic and prognostic biomarker for patients with pancreatic cancer. Furthermore, EIF2AK2 may provide a new therapeutic target for patients with pancreatic cancer.

GRP78/BiP alleviates oxLDL-induced hepatotoxicity in familial hypercholesterolemia caused by missense variants of LDLR in a HepG2 cellular model

BACKGROUND AND AIMS

The accumulation of misfolded proteins, encoded by genetic variants of functional genes leads to Endoplasmic Reticulum (ER) stress, which is a critical consequence in human disorders such as familial hypercholesterolemia, cardiovascular and hepatic diseases. In addition to the identification of ER stress as a contributing factor to pathogenicity, extensive studies on the role of oxidized Low-Density Lipoprotein (oxLDL) and its ill effects in expediting cardiovascular diseases and other metabolic comorbidities are well documented. However, the current understanding of its role in hepatic insults needs to be revised. This study elucidates the molecular mechanisms underlying the progression of oxLDL and ER stress-induced cytotoxicity in HepG2.

METHODS

HepG2 cells stably expressing wild-type Low-Density lipoprotein receptor (WT-LDLR) and missense variants of LDLR that are pathogenically associated with familial hypercholesterolemia were used as the in vitro models. The relative mRNA expression and protein profiles of ER stress sensors, inflammatory and apoptotic markers, together with cytotoxic assays and measurement of mitochondrial membrane potential, were carried out in HepG2 cells treated with 100 µg per ml oxLDL for 24 to 48 h. 1-way or 2-way ANOVA was used for statistical analyses of datasets.

RESULTS

ER stress responses are elicited along all three arms of the unfolded protein response (UPR), with adverse cytotoxic and inflammatory responses in oxLDL-treated conditions. Interestingly, oxLDL-treated ER-stressed HepG2 cells manifested intriguingly low expression of BiP- the master regulator of ER stress, as observed earlier by various researchers in liver biopsies of Non-Alcoholic Steatohepatitis (NASH) patients. This study shows that overexpression of BiP rescues hepatic cells from cytotoxic and inflammatory mechanisms instigated by ER stress in combination with oxLDL, along the ER and mitochondrial membrane and restores cellular homeostasis.

CONCLUSION

The data provide interesting leads that identify patients with familial hypercholesterolemia conditions and potentially other Endoplasmic Reticulum Associated Degradation (ERAD) diseases as highly susceptible to developing hepatic insults with molecular signatures like those manifested in Non-Alcoholic Fatty Liver Disease (NAFLD) and NASH.

LIMITATIONS AND FUTURE PERSPECTIVES

Although the use of HepG2 cells as the model is a major caveat of the study, the findings of this research may be used as the pilot study to expand further investigations in primary hepatocytes or iPSC- derived cellular models.

Ceramide de novo synthesis in non-alcoholic fatty liver disease: Pathogenic mechanisms and therapeutic perspectives

Non-alcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, and its advanced form non-alcoholic steatohepatitis (NASH) may progress to cirrhosis and hepatocellular carcinoma. Ceramides have been shown to exacerbate NAFLD development through enhancing insulin resistance, reactive oxygen species production, liver steatosis, lipotoxicity and hepatocyte apoptosis, and eventually causing hepatic inflammation and fibrosis. Emerging evidence indicates that ceramide production in NAFLD is predominantly attributed to activation of the de novo synthesis pathway of ceramides in hepatocytes. More importantly, pharmacological modulation of ceramide de novo synthesis in preclinical studies seems efficacious for the treatment of NAFLD. In this review, we provide an overview of the pathogenic mechanisms of ceramides in NAFLD, discuss recent advances and challenges in pharmacological interventions targeting ceramide de novo synthesis, and propose some research directions in the field.

Al2O3 nanoparticles trigger the embryonic hepatotoxic response and potentiate TNF-α-induced apoptosis-modulatory effect of p38 MAPK and JNK inhibitors

Recent evidences illustrated that the release of aluminum oxide nanoparticles (Al₂O₃-NPs) into the biosphere may pose risk to the environment and cause adverse effects on living organisms including humans. The current study assessed the hepatotoxic effects of Al₂O₃-NPs on developing chicken embryo and cell culture models. Results demonstrated that Al₂O₃-NPs exposure causes histological abnormalities and increased the level of tissue damage markers (ALP, AST, and ALT) in the embryonic liver. Furthermore, increased oxidative stress (TBARS) and impaired function of antioxidant enzymes (SOD, CAT, and GPx) were also observed. Moreover, it adversely affects red blood cells (RBC) morphology, liver metabolism, and stress response gene expression (HO-1 and NQO-1). Dose-dependent ROS generation and cytotoxic response in addition to potentiating effect on tumor necrosis factor alpha (TNF-α)-induced apoptosis (caspase-3 activity) were also observed. Inhibition of p38 mitogen-activated protein kinase (p38 MAPK) and c-Jun N-terminal kinase (JNK) pathways modulates Al₂O₃-NPs-induced apoptosis in HepG2 cells. Novel mechanisms behind embryonic hepatotoxicity, cytotoxic potentiating effects, and possible prevention strategies have been explored.

Phosphorylation of BCL2 at the Ser70 site mediates RANKL-induced osteoclast precursor autophagy and osteoclastogenesis

Background

Phosphorylation modification of BCL2 is involved in receptor activator of nuclear factor-κB ligand (RANKL)-induced autophagy of osteoclast precursors (OCPs) and osteoclastogenesis. As an antiapoptotic molecule, the role of BCL2 phosphorylation in osteoclastogenesis is unknown. This study aimed to explore how BCL2 phosphorylation at specific sites regulates osteoclastogenesis.

Methods

We first examined the effects of RANKL on BCL2 phosphorylation at different sites (Ser70 and Ser87) in OCPs. In vivo, transgenic mice overexpressing RANKL (Tg-hRANKL mice) were used to observe the effects of RANKL on phosphorylated BCL2 at different sites in OCPs of trabecular bone. Subsequently, using site-directed mutagenesis, we observed the respective effect of BCL2 mutations at different phosphorylation sites in OCPs on osteoclastogenesis, apoptosis, autophagy and the affinity between BCL2 and Beclin1/BAX under RANKL intervention.

Results

RANKL promoted BCL2 phosphorylation at the Ser70 (S70) site, but not the Ser87 (S87) site, in OCPs. Moreover, Tg-hRANKL mice had stronger BCL2 phosphorylation capacity at S70, not S87, in the OCPs of trabecular bone than wild-type mice in the same nest. Furthermore, BCL2 mutation at S70, not S87, inhibited RANKL-induced osteoclast differentiation and bone resorption activity. In addition, BCL2 mutation at S70 promoted OCP apoptosis, while BCL2 mutation at S87 showed the opposite effect. Remarkably, the BCL2 mutation at S70, not S87, inhibited OCP autophagic activity. Furthermore, BCL2 mutation at S70 enhanced the coimmunoprecipitation of BCL2 and Beclin1, whereas BCL2 mutation at S87 enhanced the coimmunoprecipitation of BCL2 and BAX in OCPs. More importantly, OCP autophagy, osteoclast differentiation and resorption pits inhibited by BCL2 mutation at S70 could be reversed by Beclin1 upregulation with TAT-Beclin1.

Conclusion

RANKL activates OCP autophagy through BCL2 phosphorylation at S70, thereby promoting osteoclastogenesis, which indicates that the inactivation of BCL2 at S70 in OCPs may be a therapeutic strategy for pathological bone loss.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10020-022-00449-w.

The importance and future of biochemical engineering

Today's Biochemical Engineer may contribute to advances in a wide range of technical areas. The recent Biochemical and Molecular Engineering XXI conference focused on "The Next Generation of Biochemical and Molecular Engineering: The role of emerging technologies in tomorrow's products and processes". On the basis of topical discussions at this conference, this perspective synthesizes one vision on where investment in research areas is needed for biotechnology to continue contributing to some of the world's grand challenges.

Novel role of PKR in palmitate-induced Sirt1 inactivation and endothelial cell senescence

Endothelial cell senescence is regarded as a vital characteristic of cardiovascular diseases. Elevated palmitate (PA) is an independent risk factor of cardiovascular diseases, but its role in endothelial cell senescence is currently unknown. During the course of studying the prosenescent role of PA, we discovered a key role of dsRNA-dependent protein kinase [protein kinase R (PKR)] in endothelial senescence. Exposure of human umbilical vein endothelial cells (HUVECs) to PA-induced cell senescence is characterized by increased levels of senescence-associated β-galactose glucosidase activity, excessive production of reactive oxygen species production, impaired cellular proliferation, and G1 phase arrest. This phenomenon is associated with an increase of PKR autophosphorylation and decreased activity of sirtuin 1 (Sirt1), a pivotal antisenescent factor. PKR inactivation by PKR siRNA or its phosphorylation inhibitor 2-aminopurine significantly attenuated PA-induced HUVEC senescence by reversing Sirt1 activity and its downstream signaling. Moreover, to study the regulatory mechanism between PKR and Sirt1, we found that PKR promotes JNK activation to inhibit Sirt1 activity and that this effect could be reversed by the JNK inhibitor SP600125. These findings provide evidence that PKR mediates PA-induced HUVEC senescence by inhibiting Sirt1 signaling. Our study provides novel insights into the actions and mechanisms of PKR in endothelial senescence.

NEW & NOTEWORTHY This study first provides a novel observation that dsRNA-dependent protein kinase (PKR) mediates palmitate-induced sirtuin 1 inactivation and subsequent human umbilical vein endothelial cell senescence. Most importantly, these new findings will provide a potential therapeutic strategy to improve free fatty acid-induced endothelial senescence by targeting PKR in cardiovascular diseases.

A secretome profile indicative of oleate-induced proliferation of HepG2 hepatocellular carcinoma cells

Increased fatty acid (FA) is often observed in highly proliferative tumors. FAs have been shown to modulate the secretion of proteins from tumor cells, contributing to tumor survival. However, the secreted factors affected by FA have not been systematically explored. Here, we found that treatment of oleate, a monounsaturated omega-9 FA, promoted the proliferation of HepG2 cells. To examine the secreted factors associated with oleate-induced cell proliferation, we performed a comprehensive secretome profiling of oleate-treated and untreated HepG2 cells. A comparison of the secretomes identified 349 differentially secreted proteins (DSPs; 145 upregulated and 192 downregulated) in oleate-treated samples, compared to untreated samples. The functional enrichment and network analyses of the DSPs revealed that the 145 upregulated secreted proteins by oleate treatment were mainly associated with cell proliferation-related processes, such as lipid metabolism, inflammatory response, and ER stress. Based on the network models of the DSPs, we selected six DSPs (MIF, THBS1, PDIA3, APOA1, FASN, and EEF2) that can represent such processes related to cell proliferation. Thus, our results provided a secretome profile indicative of an oleate-induced proliferation of HepG2 cells.

By exposing liver cancer cells to oleate, an unsaturated fatty acid, researchers have discovered a group of secreted proteins that may help explain why fatty acids increase proliferative capacity in tumors. Soyeon Park from Pohang University of Science and Technology in South Korea and coworkers treated liver cancer cells with oleate and then measured all the proteins released from the cells. Comparison with untreated cells revealed 145 proteins secreted at elevated levels—most of which were involved in metabolism, stress responses and other proliferation-related processes—and another 192 proteins secreted at reduced levels. The researchers ran additional biochemical analyses on six secreted proteins to validate the changes following exposure to oleate. The authors suggest that these validated proteins could now serve as biomarkers of tumor aggressiveness or as future drug targets.

Sphingolipid metabolism in non-alcoholic fatty liver diseases

Non-alcoholic fatty liver disease (NAFLD) involves a panel of pathologies starting with hepatic steatosis and continuing to irreversible and serious conditions like steatohepatitis (NASH) and hepatocarcinoma. NAFLD is multifactorial in origin and corresponds to abnormal fat deposition in liver. Even if triglycerides are mostly associated with these pathologies, other lipid moieties seem to be involved in the development and severity of NAFLD. That is the case with sphingolipids and more particularly ceramides. In this review, we explore the relationship between NAFLD and sphingolipid metabolism. After providing an analysis of complex sphingolipid metabolism, we focus on the potential involvement of sphingolipids in the different pathologies associated with NAFLD. An unbalanced ratio between ceramides and terminal metabolic products in the liver and plasma promotes weight gain, inflammation, and insulin resistance. In the etiology of NAFLD, some sphingolipid species such as ceramides may be potential biomarkers for NAFLD. We review the clinical relevance of sphingolipids in liver diseases.

Interferon-α inhibits cell migration and invasion and induces the expression of antiviral proteins in Huh-7 cells transfected with hepatitis B virus X gene-expressing lentivirus

Hepatitis B virus (HBV) X protein (HBx) serves an important role in HBV infection and the development of HBV-related liver cancer. Interferon-α (IFN-α) is used to treat patients with HBV; however, the role of IFN-α in the development of HBV-related liver cancer remains unclear. The present study established a new HBV-related liver cancer model (Huh-7-HBx) by transfecting the hepatoma cell line Huh-7, with HBx-expressing lentivirus. Following IFN-α treatment, cell viability, migration and invasion, as well as the expression of antiviral proteins in Huh-7-HBx, were subsequently determined. The results demonstrated that HBx-expressing lentivirus had no significant effect on cell viability but promoted the migration and invasion of Huh-7 cells. The expression of the antiviral genes IFN α and β receptor subunit 1 (IFNAR1), IFNAR2, IFN-stimulated gene factor 3, double-stranded RNA-activated protein kinase and ribonuclease L, was also increased. Following treatment of Huh-7-HBx cells with IFN-α, the expression of antiviral genes was increased at the level of transcription and translation, whereas cell migration and invasion was decreased. The present study suggests that IFN-α may attenuate the development of HBV-related liver cancer by reducing cell migration and invasion and promoting the expression of antiviral proteins.

Contribution of C1485T mutation in the HBx gene to human and murine hepatocarcinogenesis

Although Hepatitis B virus (HBV) X gene mutations are frequently detected in HBV-related human hepatocellular carcinoma (HCC) patients, causative HBx mutations in the development of HCC have not yet been determined. We herein identified C1485T and C1653T mutations in the HBx gene as independent risk of HCC for HBV through the analysis using serum from chronic hepatitis B patients. We generated transgenic mice expressing wild-type (WT-HBxTg) and mutant (C1485T-HBxTg) HBx to assess the carcinogenic potential of mutated HBx. C1485T-HBxTg mice were more susceptible to diethylnitrosamine-induced hepatocarcinogenesis than WT-HBxTg mice and control non-Tg mice. The promotion of hepatocarcinogenesis in C1485T-HBxTg mice was accompanied by the activation of β-catenin and Jun N-terminal kinase (JNK) signaling pathways as well as the production of reactive oxygen species, whereas the activation of nuclear factor-kappa B in the livers of C1485T-HBxTg mice was attenuated. These results demonstrate that the HBx C1485T mutation contributes to human and murine hepatocarcinogenesis.

Effects of a Sublethal and Transient Stress of the Endoplasmic Reticulum on the Mitochondrial Population

Endoplasmic reticulum (ER) and mitochondria are not discrete intracellular organelles but establish close physical and functional interactions involved in several biological processes including mitochondrial bioenergetics, calcium homeostasis, lipid synthesis, and the regulation of apoptotic cell death pathways. As many cell types might face a transient and sublethal ER stress during their lifetime, it is thus likely that the adaptive UPR response might affect the mitochondrial population. The aim of this work was to study the putative effects of a non-lethal and transient endoplasmic reticulum stress on the mitochondrial population in HepG2 cells. The results show that thapsigargin and brefeldin A, used to induce a transient and sublethal ER stress, rapidly lead to the fragmentation of the mitochondrial network associated with a decrease in mitochondrial membrane potential, O2 (•-) production and less efficient respiration. These changes in mitochondrial function are transient and preceded by the phosphorylation of JNK. Inhibition of JNK activation by SP600125 prevents the decrease in O2 (•-) production and the mitochondrial network fragmentation observed in cells exposed to the ER stress but has no impact on the reduction of the mitochondrial membrane potential. In conclusion, our data show that a non-lethal and transient ER stress triggers a rapid activation of JNK without inducing apoptosis, leading to the fragmentation of the mitochondrial network and a reduction of O2 (•-) production. J. Cell. Physiol. 231: 1913-1931, 2016. © 2015 Wiley Periodicals, Inc.

Sustained Action of Ceramide on the Insulin Signaling Pathway in Muscle Cells: IMPLICATION OF THE DOUBLE-STRANDED RNA-ACTIVATED PROTEIN KINASE

In vivo, ectopic accumulation of fatty acids in muscles leads to alterations in insulin signaling at both the IRS1 and Akt steps. However, in vitro treatments with saturated fatty acids or their derivative ceramide demonstrate an effect only at the Akt step. In this study, we adapted our experimental procedures to mimic the in vivo situation and show that the double-stranded RNA-dependent protein kinase (PKR) is involved in the long-term effects of saturated fatty acids on IRS1. C2C12 or human muscle cells were incubated with palmitate or directly with ceramide for short or long periods, and insulin signaling pathway activity was evaluated. PKR involvement was assessed through pharmacological and genetic studies. Short-term treatments of myotubes with palmitate, a ceramide precursor, or directly with ceramide induce an inhibition of Akt, whereas prolonged periods of treatment show an additive inhibition of insulin signaling through increased IRS1 serine 307 phosphorylation. PKR mRNA, protein, and phosphorylation are increased in insulin-resistant muscles. When PKR activity is reduced (siRNA or a pharmacological inhibitor), serine phosphorylation of IRS1 is reduced, and insulin-induced phosphorylation of Akt is improved. Finally, we show that JNK mediates ceramide-activated PKR inhibitory action on IRS1. Together, in the long term, our results show that ceramide acts at two distinct levels of the insulin signaling pathway (IRS1 and Akt). PKR, which is induced by both inflammation signals and ceramide, could play a major role in the development of insulin resistance in muscle cells.

HIF-1α expression as a protective strategy of HepG2 cells against fatty acid-induced toxicity

Free fatty acid-induced lipotoxicity via increased endoplasmic reticulum (ER) stress and hepatocyte apoptosis is a key pathological mechanism of non-alcoholic steatohepatitis. A role of hypoxia-inducible factor 1α (HIF-1α) in this process has been suggested, but direct evidence is lacking. Here, we used HepG2 cells as a model to study whether HIF-1α can reduce palmitic acid-induced lipotoxicity and ER stress. In HepG2 cells treated with 500 µM palmitic acid, HIF-1α expression increased transiently, the decline was associated with increased cleaved caspase-3 expression. Overexpression and knockdown of HIF-1α decreased and exacerbated, respectively, palmitic acid-induced lipoapoptosis. The overexpression also blunted upregulation of the ER stress markers, C/EBP homologous protein (CHOP) and chaperone immunoglobulin heavy chain binding protein (Bip), while the knockdown increased the level of CHOP. In line with this, CHOP promoter activity decreased following HIF-1α binding to the CHOP promoter hypoxia response element. These results indicate that hepatocyte lipotoxicity is associated with decreased HIF-1α expression. It also suggests that upregulation of HIF-1α can be a possible strategy to reduce lipotoxicity in non-alcoholic fatty liver disease.

Double stranded RNA-dependent protein kinase promotes the tumorigenic phenotype in HepG2 hepatocellular carcinoma cells by activating STAT3

Previously known as a first-response protein upon viral infection and other stress signals, double-stranded RNA-dependent protein kinase (PKR, also termed EIF2AK2) has been found to be differentially expressed in multiple types of tumor, including hepatocellular carcinoma, suggesting that PKR may be involved in tumor initiation and development. However, whether and how PKR promotes or suppresses the development of hepatocellular carcinoma remains controversial. In the present study, PKR expression was investigated using qPCR and western blot analysis, which revealed that PKR expression was upregulated in liver tumor tissues, when compared to that of adjacent normal tissues, which were obtained from four primary liver cancer patients. Furthermore, in vitro cellular assays revealed that PKR exerts a key role in maintaining the proliferation and migration of HepG2 human hepatocellular carcinoma cells. Mouse models with xenograft transplantations also confirmed a tumorigenic role of PKR in HepG2 cells. Furthermore, a transcription factor, signal transducer and activator of transcription 3 (STAT3), was revealed to mediate the tumor-promoting function of PKR in HepG2 cells, as shown by in vitro cellular proliferation and migration assays. In conclusion, the results suggested a tumorigenic role of PKR in liver cancer and a detailed mechanism involving an oncogenic transcription factor, STAT3, is described. Therefore, PKR may present a potential novel therapeutic target for the treatment of liver cancer.

Dose-dependent differential response of mammalian cells to cytoplasmic stress is mediated through the heme-regulated eIF2α kinase

The heme-regulated inhibitor (HRI), a regulator of translation initiation, is known to be activated and upregulated, and it acts as either a cytoprotective player promoting cell survival or as an inducer of apoptosis during stress. However, the exact role of HRI in these two responses has not been elucidated. In the present investigation, using human cell lines, we attempted to unravel the molecular mechanism(s) of HRI-mediated differential response and the involved signaling pathways. While during low dose (5 μM) lead acetate treatment, cells did not show any diminished cell survival, significant level of apoptosis was observed at high dose (100 μM) lead acetate. Based on the results of an interactome analysis, we determined the interaction of HRI with PI-3-Kca, only at a low dose stress, which is followed by phosphorylation and activation of its downstream target, AKT. Interestingly, such an interaction and AKT activation was not observed at a high dose stress. On the other hand, an increased level of APAF-1 and activation of caspases were observed. These results indicate a critical role of HRI in cell survival during low dose stress, and in apoptosis at high dose stress. Furthermore, HRI knockdown cells are sensitized even to 5 μM lead treatment leading to caspase activation and apoptosis. Our results taken together thus elucidate for the first time the molecular mechanism and the involved signaling pathways for dose-dependent differential response of mammalian cells to lead exposure. These findings thus suggest the possibility of using HRI downregulation as a therapeutic strategy to sensitize cancer cells subjected to apoptogenic drugs.

Improved asymmetry prediction for short interfering RNAs

In the development of RNA interference therapeutics, merely selecting short interfering RNA (siRNA) sequences that are complementary to the mRNA target does not guarantee target silencing. Current algorithms for selecting siRNAs rely on many parameters, one of which is asymmetry, often predicted through calculation of the relative thermodynamic stabilities of the two ends of the siRNA. However, we have previously shown that highly active siRNA sequences are likely to have particular nucleotides at each 5'-end, independently of their thermodynamic asymmetry. Here, we describe an algorithm for predicting highly active siRNA sequences based only on these two asymmetry parameters. The algorithm uses end-sequence nucleotide preferences and predicted thermodynamic stabilities, each weighted on the basis of training data from the literature, to rank the probability that an siRNA sequence will have high or low activity. The algorithm successfully predicts weakly and highly active sequences for enhanced green fluorescent protein and protein kinase R. Use of these two parameters in combination improves the prediction of siRNA activity over current approaches for predicting asymmetry. Going forward, we anticipate that this approach to siRNA asymmetry prediction will be incorporated into the next generation of siRNA selection algorithms.

DNA methyltransferase inhibitor zebularine inhibits human hepatic carcinoma cells proliferation and induces apoptosis

Hepatocellular carcinoma is one of the most common cancers worldwide. During tumorigenesis, tumor suppressor and cancer-related genes are commonly silenced by aberrant DNA methylation in their promoter regions. Zebularine (1-(β-(D)-ribofuranosyl)-1,2-dihydropyrimidin-2-one) acts as an inhibitor of DNA methylation and exhibits chemical stability and minimal cytotoxicity both in vitro and in vivo. In this study, we explore the effect and possible mechanism of action of zebularine on hepatocellular carcinoma cell line HepG2. We demonstrate that zebularine exhibits antitumor activity on HepG2 cells by inhibiting cell proliferation and inducing apoptosis, however, it has little effect on DNA methylation in HepG2 cells. On the other hand, zebularine treatment downregulated CDK2 and the phosphorylation of retinoblastoma protein (Rb), and upregulated p21(WAF/CIP1) and p53. We also found that zebularine treatment upregulated the phosphorylation of p44/42 mitogen-activated protein kinase (MAPK). These results suggest that the p44/42 MAPK pathway plays a role in zebularine-induced cell-cycle arrest by regulating the activity of p21(WAF/CIP1) and Rb. Furthermore, although the proapoptotic protein Bax levels were not affected, the antiapoptotic protein Bcl-2 level was downregulated with zebularine treatment. In addition, the data in the present study indicate that inhibition of the double-stranded RNA-dependent protein kinase (PKR) is involved in inducing apoptosis with zebularine. These results suggest a novel mechanism of zebularine-induced cell growth arrest and apoptosis via a DNA methylation-independent pathway in hepatocellular carcinoma.

Endoplasmic reticulum stress in nonalcoholic fatty liver disease

The underlying causes of nonalcoholic fatty liver disease are unclear, although recent evidence has implicated the endoplasmic reticulum in both the development of steatosis and progression to nonalcoholic steatohepatitis. Disruption of endoplasmic reticulum homeostasis, often termed ER stress, has been observed in liver and adipose tissue of humans with nonalcoholic fatty liver disease and/or obesity. Importantly, the signaling pathway activated by disruption of endoplasmic reticulum homeostasis, the unfolded protein response, has been linked to lipid and membrane biosynthesis, insulin action, inflammation, and apoptosis. Therefore, understanding the mechanisms that disrupt endoplasmic reticulum homeostasis in nonalcoholic fatty liver disease and the role of the unfolded protein response in the broader context of chronic, metabolic diseases have become topics of intense investigation. The present review examines the endoplasmic reticulum and the unfolded protein response in the context of nonalcoholic fatty liver disease.

Endoplasmic reticulum stress and the unfolded protein response in nonalcoholic fatty liver disease

The underlying causes of nonalcoholic fatty liver disease (NAFLD) are unclear, although recent evidence has implicated the endoplasmic reticulum (ER) in both the development of steatosis and progression to nonalcoholic steatohepatitis. Disruption of ER homeostasis, often termed "ER stress," has been observed in liver and adipose tissue of humans with NAFLD and/or obesity. Importantly, the signaling pathway activated by disruption of ER homeostasis, the unfolded protein response, has been linked to lipid biosynthesis, insulin action, inflammation, and apoptosis. Therefore, understanding the mechanisms that disrupt ER homeostasis in NAFLD and the role of ER-mediated signaling have become topics of intense investigation. The present review will examine the ER and the unfolded protein response in the context of NAFLD.

Molecular mechanism by which palmitate inhibits PKR autophosphorylation

PKR (double-stranded RNA-activated protein kinase) is an important component of the innate immunity, antiviral, and apoptotic pathways. Recently, our group found that palmitate, a saturated fatty acid, is involved in apoptosis by reducing the autophosphorylation of PKR at the Thr451 residue; however, the molecular mechanism by which palmitate reduces PKR autophosphorylation is not known. Thus, we investigated how palmitate affects the phosphorylation of the PKR protein at the molecular and biophysical levels. Biochemical and computational studies show that palmitate binds to PKR, near the ATP-binding site, thereby inhibiting its autophosphorylation at Thr451 and Thr446. Mutation studies suggest that Lys296 and Asp432 in the ATP-binding site on the PKR protein are important for palmitate binding. We further confirmed that palmitate also interacts with other kinases, due to the conserved ATP-binding site. A better understanding of how palmitate interacts with the PKR protein, as well as other kinases, could shed light onto possible mechanisms by which palmitate mediates kinase signaling pathways that could have implications on the efficacy of current drug therapies that target kinases.

Bip overexpression, but not CHOP inhibition, attenuates fatty-acid-induced endoplasmic reticulum stress and apoptosis in HepG2 liver cells

AIMS

In this study we investigated whether attenuation of endoplasmic reticulum stress (ER stress) could protect HepG2 cells from free fatty acid (FFA)-induced apoptosis.

MAIN METHODS

Human liver cell line HepG2 cells were exposed to Sodium Palmitate (Pa) or Sodium Oleate (Ol). Apoptosis and ER stress of HepG2 cells were analyzed with flow cytometry, real-time RT-PCR and Western Blotting. An expression plasmid encoding for the ER chaperone immunoglobulin heavy chain-binding protein (Bip) was transfected into HepG2 cells to attenuate ER stress. Small interfering RNA siCHOP was used to knockdown the expression of C/EBP Homologous Protein (CHOP) in HepG2.

KEY FINDINGS

Pa led to cytotoxicity and apoptosis in HepG2 cells in a dose-dependent pattern and also induced ER stress indicated by increased phosphorylation of eIF2α, upregulation of IRE1α and CHOP. Bip expression levels were slightly down regulated after Pa treatment. The unsaturated fatty acid, Ol, induced neither apoptosis nor ER stress in HepG2 cells. Overexpression of Bip attenuated Pa-induced ER stress and led to a significant reduction in Pa-mediated apoptosis, indicating a requirement of ER stress for lipotoxicity in liver cells. siRNA-mediated reduction of CHOP did not protect against Pa-induced apoptosis.

SIGNIFICANCE

While ER stress makes a necessary contribution to palmitate cytotoxicity, inhibition of CHOP alone is not sufficient to prevent palmitate-induced apoptosis. Our findings could advance the detailed understanding on the mechanism of high fatty acid (FFA)-induced apoptosis.

The double-stranded RNA-dependent protein kinase differentially regulates insulin receptor substrates 1 and 2 in HepG2 cells

The RNA-dependent protein kinase (PKR), initially known as a virus infection response protein, is found to differentially regulate two major players in the insulin signaling pathway, IRS1 and IRS2. PKR up-regulates the inhibitory phosphorylation of IRS1 and the expression of IRS2 at the transcriptional level.

Initially identified to be activated upon virus infection, the double-stranded RNA–dependent protein kinase (PKR) is best known for triggering cell defense responses by phosphorylating eIF-2α, thus suppressing RNA translation. We as well as others showed that the phosphorylation of PKR is down-regulated by insulin. In the present study, we further uncovered a novel function of PKR in regulating the IRS proteins. We found that PKR up-regulates the inhibitory phosphorylation of IRS1 at Ser312, which suppresses the tyrosine phosphorylation of IRS1. This effect of PKR on the phosphorylation of IRS1 is mediated by two other protein kinases, JNK and IKK. In contrast, PKR regulates IRS2, another major IRS family protein in the liver, at the transcriptional rather than the posttranslational level, and this effect is mediated by the transcription factor, FoxO1, which has been previously shown to be regulated by insulin and plays a significant role in glucose homeostasis and energy metabolism. In summary, we found for the first time that initially known as a virus infection response gene, PKR regulates the upstream central transmitters of insulin signaling, IRS1 and IRS2, through different mechanisms.

DNA damage and DNA damage responses in THP-1 monocytes after exposure to spores of either Stachybotrys chartarum or Aspergillus versicolor or to T-2 toxin

We have characterized cell death in THP-1 cells after exposure to heat-treated spores from satratoxin G-producing Stachybotrys chartarum isolate IBT 9631, atranone-producing S. chartarum isolate IBT 9634, and sterigmatocystin-producing Aspergillus versicolor isolate IBT 3781, as well as the trichothecenes T-2 and satratoxin G. Spores induced cell death within 3-6 h, with Stachybotrys appearing most potent. IBT 9631 induced both apoptosis and necrosis, while IBT 9634 and IBT 3781 induced mostly necrosis. T-2 toxin and satratoxin G caused mainly apoptosis. Comet assay +/- formamidopyrimidine DNA glycosylase showed that only the spore exposures induced early (3h) oxidative DNA damage. Likewise, only the spores increased the formation of reactive oxygen species (ROS), suggesting that spores as particles may induce ROS formation and oxidative DNA damage. Increased Ataxia Telangiectasia Mutated (ATM) phosphorylation, indicating DNA damage, was observed after all exposures. The DNA damage response induced by IBT 9631 as well as satratoxin G was characterized by rapid (15 min) activation of p38 and H2AX. The p38 inhibitor SB 202190 reduced IBT 9631-induced H2AX activation. Both IBT 9631 and T-2 induced activation of Chk2 and H2AX after 3 h. The ATM inhibitor KU 55933, as well as transfection of cells with ATM siRNA, reduced this activation, suggesting a partial role for ATM as upstream activator for Chk2 and H2AX. In conclusion, activation of Chk2 and H2AX correlated with spore- and toxin-induced apoptosis. For IBT 9631 and satratoxin G, additional factors may be involved in triggering apoptosis, most notably p38 activation.

A dynamic analysis of IRS-PKR signaling in liver cells: a discrete modeling approach

A major challenge in systems biology is to develop a detailed dynamic understanding of the functions and behaviors in a particular cellular system, which depends on the elements and their inter-relationships in a specific network. Computational modeling plays an integral part in the study of network dynamics and uncovering the underlying mechanisms. Here we proposed a systematic approach that incorporates discrete dynamic modeling and experimental data to reconstruct a phenotype-specific network of cell signaling. A dynamic analysis of the insulin signaling system in liver cells provides a proof-of-concept application of the proposed methodology. Our group recently identified that double-stranded RNA-dependent protein kinase (PKR) plays an important role in the insulin signaling network. The dynamic behavior of the insulin signaling network is tuned by a variety of feedback pathways, many of which have the potential to cross talk with PKR. Given the complexity of insulin signaling, it is inefficient to experimentally test all possible interactions in the network to determine which pathways are functioning in our cell system. Our discrete dynamic model provides an in silico model framework that integrates potential interactions and assesses the contributions of the various interactions on the dynamic behavior of the signaling network. Simulations with the model generated testable hypothesis on the response of the network upon perturbation, which were experimentally evaluated to identify the pathways that function in our particular liver cell system. The modeling in combination with the experimental results enhanced our understanding of the insulin signaling dynamics and aided in generating a context-specific signaling network.

Recent advances in understanding the pathogenesis of nonalcoholic steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a pathological condition characterized by macrovesicular steatosis, necroinflammation, loss of hepatocytes and fibrosis. NASH is often associated with type 2 diabetes mellitus, hypertension, hyperlipoproteinemia and obesity. Recent studies emphasize the role of insulin resistance, oxidative stress and subsequent lipid peroxidation, proinflammatory cytokines, adipokines and mitochondrial dysfunction in the development and progression of NASH. In this article, we will review the role of insulin resistance, oxidative stress and subsequent lipid peroxidation, mitochondrial dysfunction, proinflammatory cytokines, adipokines such as resistin, leptin, adiponectin and PPAR-α, apoptosis, NF kappa B, SREBP-1c, endotoxaemia, and iron overload in the pathogenesis of NASH. The pathogenesis of NASH is thought to be related mainly with insulin resistance and oxidative stress and subsequent lipid peroxidation. Adipocytokines also play an important role in the pathogenesis of NASH through complex and interactive paracrine and endocrine mechanisms. Understanding of the mechanisms responsible for the pathogenesis of NASH has important implications for the treatment of NASH.

Reconstruct modular phenotype-specific gene networks by knowledge-driven matrix factorization

MOTIVATION

Reconstructing gene networks from microarray data has provided mechanistic information on cellular processes. A popular structure learning method, Bayesian network inference, has been used to determine network topology despite its shortcomings, i.e. the high-computational cost when analyzing a large number of genes and the inefficiency in exploiting prior knowledge, such as the co-regulation information of the genes. To address these limitations, we are introducing an alternative method, knowledge-driven matrix factorization (KMF) framework, to reconstruct phenotype-specific modular gene networks.

RESULTS

Considering the reconstruction of gene network as a matrix factorization problem, we first use the gene expression data to estimate a correlation matrix, and then factorize the correlation matrix to recover the gene modules and the interactions between them. Prior knowledge from Gene Ontology is integrated into the matrix factorization. We applied this KMF algorithm to hepatocellular carcinoma (HepG2) cells treated with free fatty acids (FFAs). By comparing the module networks for the different conditions, we identified the specific modules that are involved in conferring the cytotoxic phenotype induced by palmitate. Further analysis of the gene modules of the different conditions suggested individual genes that play important roles in palmitate-induced cytotoxicity. In summary, KMF can efficiently integrate gene expression data with prior knowledge, thereby providing a powerful method of reconstructing phenotype-specific gene networks and valuable insights into the mechanisms that govern the phenotype.

The role of fatty acid unsaturation in minimizing biophysical changes on the structure and local effects of bilayer membranes

Studying the effects of saturated and unsaturated fatty acids on biological and model (liposomes) membranes could provide insight into the contribution of biophysical effects on the cytotoxicity observed with saturated fatty acids. In vitro experiments suggest that unsaturated fatty acids, such as oleate and linoleate, are less toxic, and have less impact on the membrane fluidity. To understand and assess the biophysical changes in the presence of the different fatty acids, we performed computational analyses of model liposomes with palmitate, oleate, and linoleate. The computational results indicate that the unsaturated fatty acid chain serves as a membrane stabilizer by preventing changes to the membrane fluidity. Based on a Voronoi tessellation analysis, unsaturated fatty acids have structural properties that can reduce the lipid ordering within the model membranes. In addition, hydrogen bond analysis indicates a more uniform level of membrane hydration in the presence of oleate and linoleate as compared to palmitate. Altogether, these observations from the computational studies provide a possible mechanism by which unsaturated fatty acids minimize biophysical changes and protect the cellular membrane and structure. To corroborate our findings, we also performed a liposomal leakage study to assess how the different fatty acids alter the membrane integrity of liposomes. This showed that palmitate, a saturated fatty acid, caused greater destabilization of liposomes (more "leaky") than oleate, an unsaturated fatty acid.