Caspase-dependent alterations of Ca2+ signaling in the induction of apoptosis by hepatitis B virus X protein

The intersection between cysteine proteases, Ca2+ signalling and cancer cell apoptosis

Apoptosis is a highly complex and regulated cell death pathway that safeguards the physiological balance between life and death. Over the past decade, the role of Ca2+ signalling in apoptosis and the mechanisms involved have become clearer. The initiation and execution of apoptosis is coordinated by three distinct groups of cysteines proteases: the caspase, calpain and cathepsin families. Beyond its physiological importance, the ability to evade apoptosis is a prominent hallmark of cancer cells. In this review, we will explore the involvement of Ca2+ in the regulation of caspase, calpain and cathepsin activity, and how the actions of these cysteine proteases alter intracellular Ca2+ handling during apoptosis. We will also explore how apoptosis resistance can be achieved in cancer cells through deregulation of cysteine proteases and remodelling of the Ca2+ signalling toolkit.

Modulation of mitochondria by viral proteins

Mitochondria are dynamic cellular organelles with diverse functions including energy production, calcium homeostasis, apoptosis, host innate immune signaling, and disease progression. Several viral proteins specifically target mitochondria to subvert host defense as mitochondria stand out as the most suitable target for the invading viruses. They have acquired the capability to control apoptosis, metabolic state, and evade immune responses in host cells, by targeting mitochondria. In this way, the viruses successfully allow the spread of viral progeny and thus the infection. Viruses employ their proteins to alter mitochondrial dynamics and their specific functions by a modulation of membrane potential, reactive oxygen species, calcium homeostasis, and mitochondrial bioenergetics to help them achieve a state of persistent infection. A better understanding of such viral proteins and their impact on mitochondrial forms and functions is the main focus of this review. We also attempt to emphasize the importance of exploring the role of mitochondria in the context of SARS-CoV2 pathogenesis and identify host-virus protein interactions.

Calcium Ions Signaling: Targets for Attack and Utilization by Viruses

Calcium, as a second intracellular messenger, participate in various physiological and biochemical processes, including cell growth and proliferation, energy metabolism, information transfer, cell death, and immune response. Ca2+ channels or pumps in plasma and organelle membranes and Ca2+-related proteins maintain Ca2+ homeostasis by regulating Ca2+ inflow, outflow and buffering to avoid any adverse effects caused by Ca2+ overload or depletion. Thus, Ca2+ signaling also provides a target for virus invasion, replication, proliferation and release. After hijacking the host cell, viruses exploit Ca2+ signaling to regulate apoptosis and resist host immunity to establish persistent infection. In this review, we discuss cellular Ca2+ signaling and channels, interaction of calcium-associated proteins with viruses, and host cell fate, as well as the role of Ca2+ in cell death and antiviral response during viral infection.

Dysregulation of host cell calcium signaling during viral infections: Emerging paradigm with high clinical relevance

Viral infections are one of the leading causes of human illness. Viruses take over host cell signaling cascades for their replication and infection. Calcium (Ca2+) is a versatile and ubiquitous second messenger that modulates plethora of cellular functions. In last two decades, a critical role of host cell Ca2+ signaling in modulating viral infections has emerged. Furthermore, recent literature clearly implicates a vital role for the organellar Ca2+ dynamics (influx and efflux across organelles) in regulating virus entry, replication and severity of the infection. Therefore, it is not surprising that a number of viral infections including current SARS-CoV-2 driven COVID-19 pandemic are associated with dysregulated Ca2+ homeostasis. The focus of this review is to first discuss the role of host cell Ca2+ signaling in viral entry, replication and egress. We further deliberate on emerging literature demonstrating hijacking of the host cell Ca2+ dynamics by viruses. In particular, a variety of viruses including SARS-CoV-2 modulate lysosomal and cytosolic Ca2+ signaling for host cell entry and replication. Moreover, we delve into the recent studies, which have demonstrated the potential of several FDA-approved drugs targeting Ca2+ handling machinery in inhibiting viral infections. Importantly, we discuss the prospective of targeting intracellular Ca2+ signaling for better management and treatment of viral pathogenesis including COVID-19. Finally, we highlight the key outstanding questions in the field that demand critical and timely attention.

Are Humanized Mouse Models Useful for Basic Research of Hepatocarcinogenesis through Chronic Hepatitis B Virus Infection?

Chronic hepatitis B virus (HBV) infection is a global health problem that can lead to liver dysfunction, including liver cirrhosis and hepatocellular carcinoma (HCC). Current antiviral therapies can control viral replication in patients with chronic HBV infection; however, there is a risk of HCC development. HBV-related proteins may be produced in hepatocytes regardless of antiviral therapies and influence intracellular metabolism and signaling pathways, resulting in liver carcinogenesis. To understand the mechanisms of liver carcinogenesis, the effect of HBV infection in human hepatocytes should be analyzed. HBV infects human hepatocytes through transfer to the sodium taurocholate co-transporting polypeptide (NTCP). Although the NTCP is expressed on the hepatocyte surface in several animals, including mice, HBV infection is limited to human primates. Due to this species-specific liver tropism, suitable animal models for analyzing HBV replication and developing antivirals have been lacking since the discovery of the virus. Recently, a humanized mouse model carrying human hepatocytes in the liver was developed based on several immunodeficient mice; this is useful for analyzing the HBV life cycle, antiviral effects of existing/novel antivirals, and intracellular signaling pathways under HBV infection. Herein, the usefulness of human hepatocyte chimeric mouse models in the analysis of HBV-associated hepatocarcinogenesis is discussed.

Calcium signaling in hepatitis B virus infection and its potential as a therapeutic target

As a ubiquitous second messenger, calcium (Ca2+) can interact with numerous cellular proteins to regulate multiple physiological processes and participate in a variety of diseases, including hepatitis B virus (HBV) infection, which is a major cause of hepatitis, fibrosis, cirrhosis, and hepatocellular carcinoma. In recent years, several studies have demonstrated that depends on the distinct Ca2+ channels on the plasma membrane, endoplasmic reticulum, as well as mitochondria, HBV can elevate cytosolic Ca2+ levels. Moreover, within HBV-infected cells, the activation of intracellular Ca2+ signaling contributes to viral replication via multiple molecular mechanisms. Besides, the available evidence indicates that targeting Ca2+ signaling by suitable pharmaceuticals is a potent approach for the treatment of HBV infection. In the present review, we summarized the molecular mechanisms related to the elevation of Ca2+ signaling induced by HBV to modulate viral propagation and the recent advances in Ca2+ signaling as a potential therapeutic target for HBV infection. Video Abstract.

Endoplasmic reticulum & mitochondrial calcium homeostasis: The interplay with viruses

Calcium ions (Ca²⁺) act as secondary messengers in a plethora of cellular processes and play crucial role in cellular organelle function and homeostasis. The average resting concentration of Ca²⁺ is nearly 100 nM and in certain cells it can reach up to 1 µM. The high range of Ca²⁺ concentration across the plasma membrane and intracellular Ca²⁺ stores demands a well-coordinated maintenance of free Ca²⁺ via influx, efflux, buffering and storage. Endoplasmic Reticulum (ER) and Mitochondria depend on Ca²⁺ for their function and also serve as major players in intracellular Ca²⁺ homeostasis. The ER-mitochondria interplay helps in orchestrating cellular calcium homeostasis to avoid any detrimental effect resulting from Ca²⁺ overload or depletion. Since Ca²⁺ plays a central role in many biological processes it is an essential component of the virus-host interactions. The large gradient across membranes enable the viruses to easily modulate this buffered environment to meet their needs. Viruses exploit Ca²⁺ signaling to establish productive infection and evade the host immune defense. In this review we will detail the interplay between the viruses and cellular & ER-mitochondrial calcium signaling and the significance of these events on viral life cycle and disease pathogenesis.

A Calcium Sensor Discovered in Bluetongue Virus Nonstructural Protein 2 Is Critical for Virus Replication

After entering the host cells, viruses use cellular host factors to ensure a successful virus replication process. For replication in infected cells, members of the Reoviridae family form inclusion body-like structures known as viral inclusion bodies (VIB) or viral factories. Bluetongue virus (BTV) forms VIBs in infected cells through nonstructural protein 2 (NS2), a phosphoprotein. An important regulatory factor critical for VIB formation is phosphorylation of NS2. In our study, we discovered a characteristic calcium-binding EF-hand-like motif in NS2 and found that the calcium binding preferentially affects phosphorylation level of the NS2 and has a role in regulating VIB assembly.

Many viruses use specific viral proteins to bind calcium ions (Ca²⁺) for stability or to modify host cell pathways; however, to date, no Ca²⁺ binding protein has been reported in bluetongue virus (BTV), the causative agent of bluetongue disease in livestock. Here, using a comprehensive bioinformatics screening, we identified a putative EF-hand-like Ca²⁺ binding motif in the carboxyl terminal region of BTV nonstructural phosphoprotein 2 (NS2). Subsequently, using a recombinant NS2, we demonstrated that NS2 binds Ca²⁺ efficiently and that Ca²⁺ binding was perturbed when the Asp and Glu residues in the motif were substituted by alanine. Using circular dichroism analysis, we found that Ca²⁺ binding by NS2 triggered a helix-to-coil secondary structure transition. Further, cryo-electron microscopy in the presence of Ca²⁺ revealed that NS2 forms helical oligomers which, when aligned with the N-terminal domain crystal structure, suggest an N-terminal domain that wraps around the C-terminal domain in the oligomer. Further, an in vitro kinase assay demonstrated that Ca²⁺ enhanced the phosphorylation of NS2 significantly. Importantly, mutations introduced at the Ca²⁺ binding site in the viral genome by reverse genetics failed to allow recovery of viable virus, and the NS2 phosphorylation level and assembly of viral inclusion bodies (VIBs) were reduced. Together, our data suggest that NS2 is a dedicated Ca²⁺ binding protein and that calcium sensing acts as a trigger for VIB assembly, which in turn facilitates virus replication and assembly.

IMPORTANCE After entering the host cells, viruses use cellular host factors to ensure a successful virus replication process. For replication in infected cells, members of the Reoviridae family form inclusion body-like structures known as viral inclusion bodies (VIB) or viral factories. Bluetongue virus (BTV) forms VIBs in infected cells through nonstructural protein 2 (NS2), a phosphoprotein. An important regulatory factor critical for VIB formation is phosphorylation of NS2. In our study, we discovered a characteristic calcium-binding EF-hand-like motif in NS2 and found that the calcium binding preferentially affects phosphorylation level of the NS2 and has a role in regulating VIB assembly.

HBV infection increases the risk of macular degeneration: the roles of HBx-mediated sensitization of retinal pigment epithelial cells to UV and blue light irradiation

Background

Hepatitis B virus (HBV) infection is strongly associated with hepatocellular carcinoma due to the main pathogenic X protein of HBV (HBx). Whether HBV infection and the HBx protein could result in macular degeneration (MD) is not known. The aim of this study is to assess the association and underlying mechanisms between HBV infection and MD.

Methods

The National Health Research Institutes in Taiwan built a large database, the National Health Insurance Research Database (NHIRD), which includes the claims data from the Taiwan National Health Insurance (NHI) program. The Taiwan NHI is a single-payer, compulsory health insurance program for Taiwan citizens. The data for the present study were derived from the Longitudinal Health Insurance Database, which contains the claims data of 1 million insured people within the NHIRD, including beneficiary registration, inpatient and outpatient files, drug use, and other medical services. In this study, we first investigated the association of HBV infection and the risk of MD by a population-based cohorts study enrolling 39,796 HBV-infected patients and 159,184 non-HBV-infected patients.

Results

After adjustment of age, sex, and comorbidities, the risk of MD was significantly higher in the HBV-infected cohort than in the non-HBV-infected cohort (adjusted HR = 1.31; 95% CI = 1.17–1.46). In vitro, we provided evidence to demonstrate that overexpression of HBx in the human retinal pigment epithelial (RPE) cell line, ARPE19, significantly reduced cell viability and clonogenic survival upon UV and blue light irradiation. By gene microarray analysis, we further showed that almost all genes in DNA repair pathways including base excision repair, nucleotide excision repair, mismatch repair, and homologous recombination were significantly down-regulated in the UV-induced cell death of HBx-transfected ARPE19 cells.

Conclusions

The HBx protein may sensitize RPE cells to UV and blue light irradiation and increase the risk of HBV-infection-associated MD through down-regulation of multiple DNA repair pathways.

Electronic supplementary material

The online version of this article (10.1186/s12967-018-1594-4) contains supplementary material, which is available to authorized users.

Hepatitis B Virus X Protein Upregulates Intracellular Calcium Signaling by Binding C-terminal of Orail Protein

Hepatitis B virus X (HBx) protein plays a pivotal role in the development of hepatitis B virus (HBV)-associated hepatocellular carcinoma. Although regulation of cytosolic calcium is essential for HBV replication and is mediated by HBx protein, the mechanism of HBx protein regulating intracellular calcium level remains poorly understood. The present study examined whether HBx protein elevated the intracellular calcium through interacting with storeoperated calcium entry (SOCE) components, Orail and stromal interaction molecule 1, and then identified the targets of HBx protein, with an attempt to understand the mechanism of HBx protein upsetting intracellular calcium homeostasis. By employing co-immunoprecipitation and GST-pull-down assay, we found that Orail protein interacted with HBx protein, and the C-terminus of Orail was implicated in the interaction. Confocal microscopy also revealed that HBx protein could co-localize with full-length Orail protein in HEK293 cells. Moreover, live cell calcium imaging exhibited that HBx protein elevated intracellular calcium, possibly by binding to SOCE components. Our results suggest that HBx protein binds to STIM1-Orail complexes to positively regulate the activity of plasma membrane store-operated calcium channels.

Mitochondrial Proteins Coded by Human Tumor Viruses

Viruses must exploit the cellular biosynthetic machinery and evade cellular defense systems to complete their life cycles. Due to their crucial roles in cellular bioenergetics, apoptosis, innate immunity and redox balance, mitochondria are important functional targets of many viruses, including tumor viruses. The present review describes the interactions between mitochondria and proteins coded by the human tumor viruses human T-cell leukemia virus type 1, Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus, human hepatitis viruses B and C, and human papillomavirus, and highlights how these interactions contribute to viral replication, persistence and transformation.

Hepatitis B virus X protein modulates cytosolic Ca2+ signaling in primary human hepatocytes

Worldwide, approximately 240 million people are chronically infected with the hepatitis B virus (HBV); chronic HBV infection is associated with the development of life-threatening liver diseases. The HBV HBx protein alters hepatocyte physiology to promote HBV replication. We previously reported that HBx modulates calcium signaling to stimulate HBV replication in human hepatoblastoma HepG2 cells and primary rat hepatocytes. Whether HBx modulates calcium signaling in a primary human hepatocyte, the natural site of an HBV infection, has not been determined. Here, we report the effect of HBx on calcium signaling in primary human hepatocytes and show that HBx modulates calcium signaling via enhanced calcium entry through store-operated calcium channels and elevated mitochondrial calcium, similar to HBx effects in HepG2 cells and primary rat hepatocytes. In addition to demonstrating that HBV and HBx affect calcium signaling in human hepatocytes, these studies also show that HBV and HBx regulation of calcium signaling is identical in primary human and rat hepatocytes, further validating the use of cultured primary rat hepatocytes for HBV studies.

X protein variants of the autochthonous Latin American hepatitis B virus F genotype promotes human hepatocyte death by the induction of apoptosis and autophagy

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The impact of BCP mutations on HBV-X biologic activity was analyzed.

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Genotype F wild type and mutant HBV-X induce apoptosis of human hepatocytes.

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HBV-X variants modulate the expression of Bcl-2 family proteins.

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Subgenotypes F1b and F4 HBV-X and variants induce autophagy of human hepatocytes.

The hepatitis B virus X protein (HBV-X) is a multifunctional regulatory protein associated with the pathogenesis of liver disease in chronic HBV infection. Basal core promoter mutations (BCP), associated with the clinical course of chronic HBV infection, affect HBV-X at 130–131 positions. The role of these mutations on HBV-X biological activity remains largely unknown. The aim of this study was to analyze the impact of the presence of different amino acids at 130–131 positions of HBV-X on the biological activity of the protein. Transient expression of wild type and mutant F1b and F4 HBV-X increased cell mortality by the induction of apoptosis in human hepatoma cells. The wild type and mutant HBV-X differentially modulate the expression of pro-apoptotic (Bax) and anti-apoptotic (Bcl-2 and Bcl-X) regulatory proteins of the Bcl-2 family. Furthermore, the expression of HBV-X variants of both subgenotypes induced autophagy of human tumoral hepatocytes. In conclusion, HBV-X variants of the Latin American HBV F genotype promotes human hepatocytes death by the induction of apoptosis and autophagy. The results of this work describe some of the molecular mechanisms by which HBV-X variants contribute to the pathogenesis of liver diseases in the infected liver and help to the biological characterization of genotype F, responsible of the majority of HBV infections in Argentina.

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

Interference of Apoptosis by Hepatitis B Virus

Hepatitis B virus (HBV) causes liver diseases that have been a consistent problem for human health, leading to more than one million deaths every year worldwide. A large proportion of hepatocellular carcinoma (HCC) cases across the world are closely associated with chronic HBV infection. Apoptosis is a programmed cell death and is frequently altered in cancer development. HBV infection interferes with the apoptosis signaling to promote HCC progression and viral proliferation. The HBV-mediated alteration of apoptosis is achieved via interference with cellular signaling pathways and regulation of epigenetics. HBV X protein (HBX) plays a major role in the interference of apoptosis. There are conflicting reports on the HBV interference of apoptosis with the majority showing inhibition of and the rest reporting induction of apoptosis. In this review, we described recent studies on the mechanisms of the HBV interference with the apoptosis signaling during the virus infection and provided perspective.

Metabolic regulation of the PMCA: Role in cell death and survival

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The PMCA is an ATP-driven Ca²⁺ pump critical for the maintenance of low cytosolic calcium.

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The PMCA has an important but paradoxical role in cell death and survival.

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The PMCA can be differentially regulated by caspase/calpain cleavage.

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Glycolytic ATP supply may be sufficient to fuel the PMCA during metabolic stress.

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The ATP sensitivity of the PMCA can be regulated by acidic phospholipids.

The plasma membrane Ca²⁺-ATPase (PMCA) is a ubiquitously expressed, ATP-driven Ca²⁺ pump that is critical for maintaining low resting cytosolic Ca²⁺ ([Ca²⁺]_i) in all eukaryotic cells. Since cytotoxic Ca²⁺ overload has such a central role in cell death, the PMCA represents an essential “linchpin” for the delicate balance between cell survival and cell death. In general, impaired PMCA activity and reduced PMCA expression leads to cytotoxic Ca²⁺ overload and Ca²⁺ dependent cell death, both apoptosis and necrosis, whereas maintenance of PMCA activity or PMCA overexpression is generally accepted as being cytoprotective. However, the PMCA has a paradoxical role in cell death depending on the cell type and cellular context. The PMCA can be differentially regulated by Ca²⁺-dependent proteolysis, can be maintained by a localised glycolytic ATP supply, even in the face of global ATP depletion, and can be profoundly affected by the specific phospholipid environment that it sits within the membrane. The major focus of this review is to highlight some of the controversies surrounding the paradoxical role of the PMCA in cell death and survival, challenging the conventional view of ATP-dependent regulation of the PMCA and how this might influence cell fate.

Hepatitis B virus X protein stimulates high mobility group box 1 secretion and enhances hepatocellular carcinoma metastasis

Hepatitis B virus X protein (HBx) plays an important role in the progression of hepatocellular carcinoma. Here we reported that overexpression of HBx in hepatocellular carcinoma (HCC) cells could induce the secretion of high-mobility group box 1 (HMGB1) to promote invasion and metastasis of HCC in an autocrine/paracrine manner. HBx triggered an increase of cytoplasmic calcium and activated CAMKK/CAMKIV pathway, leading to subsequent translocation and release of HMGB1. HMGB1 neutralizing antibody, as well as calcium chelator or inhibitors of CAMKK/CAMKIV, could remarkably reduce invasion and metastasis of HCC cells in vitro and in a murine HCC metastasis model in vivo. Furthermore, the level of HMGB1 in patient serum and tumor tissues was positively correlated with HBV DNA load. We demonstrate an inverse relationship between HMGB1 in tumor cytoplasm and overall prognosis of HCC patients.

CONCLUSION

HBx promotes the progression of HCC through translocation and secretion of HMGB1 from tumor cells via calcium dependent cascades. These data indicates that HMGB1 could serve as a novel prognostic biomarker and potential therapeutic target for HBV-related HCC.

Hepatitis B virus modulates store-operated calcium entry to enhance viral replication in primary hepatocytes

Many viruses modulate calcium (Ca²⁺) signaling to create a cellular environment that is more permissive to viral replication, but for most viruses that regulate Ca²⁺ signaling, the mechanism underlying this regulation is not well understood. The hepatitis B virus (HBV) HBx protein modulates cytosolic Ca²⁺ levels to stimulate HBV replication in some liver cell lines. A chronic HBV infection is associated with life-threatening liver diseases, including hepatocellular carcinoma (HCC), and HBx modulation of cytosolic Ca²⁺ levels could have an important role in HBV pathogenesis. Whether HBx affects cytosolic Ca²⁺ in a normal hepatocyte, the natural site of an HBV infection, has not been addressed. Here, we report that HBx alters cytosolic Ca²⁺ signaling in cultured primary hepatocytes. We used single cell Ca²⁺ imaging of cultured primary rat hepatocytes to demonstrate that HBx elevates the cytosolic Ca²⁺ level in hepatocytes following an IP₃-linked Ca²⁺ response; HBx effects were similar when expressed alone or in the context of replicating HBV. HBx elevation of the cytosolic Ca²⁺ level required extracellular Ca²⁺ influx and store-operated Ca²⁺ (SOC) entry and stimulated HBV replication in hepatocytes. We used both targeted RT-qPCR and transcriptome-wide RNAseq analyses to compare levels of SOC channel components and other Ca²⁺ signaling regulators in HBV-expressing and control hepatocytes and show that the transcript levels of these various proteins are not affected by HBV. We also show that HBx regulation of SOC-regulated Ca²⁺ accumulation is likely the consequence of HBV modulation of a SOC channel regulatory mechanism. In support of this, we link HBx enhancement of SOC-regulated Ca²⁺ accumulation to Ca²⁺ uptake by mitochondria and demonstrate that HBx stimulates mitochondrial Ca²⁺ uptake in primary hepatocytes. The results of our study may provide insights into viral mechanisms that affect Ca²⁺ signaling to regulate viral replication and virus-associated diseases.

Mechanism of Hepatocyte Apoptosis

Hepatocyte apoptosis plays important roles in both the removal of external microorganisms and the occurrence and development of liver diseases. Different conditions, such as virus infection, fatty liver disease, hepatic ischemia reperfusion, and drug-induced liver injury, are accompanied by hepatocyte apoptosis. This review summarizes recent research on the mechanism of hepatocyte apoptosis involving the classical extrinsic and intrinsic apoptotic pathways, endoplasmic reticulum stress, and oxidative stress-induced apoptosis. We emphasized the major causes of apoptosis according to the characteristics of different liver diseases. Several concerns regarding future research and clinical application are also raised.

Hepatitis B Virus X and Regulation of Viral Gene Expression

The efficient replication of hepatitis B virus (HBV) requires the HBV regulatory hepatitis B virus X (HBx) protein. The exact contributions of HBx are not fully understood, in part because of the limitations of the assays used for its study. When HBV replication is driven from a plasmid DNA, the contribution of HBx is modest. However, there is an absolute requirement for HBx in assays that recapitulate the infectious virus life cycle. There is much evidence that HBx can contribute directly to HBV replication by acting on viral promoters embedded within protein coding sequences. In addition, HBx may also contribute indirectly by modulating cellular pathways to benefit virus replication. Understanding the mechanism(s) of HBx action during virus replication may provide insight into novel ways to disrupt chronic HBV replication.

Hepatitis B and C virus-induced hepatitis: Apoptosis, autophagy, and unfolded protein response

AIM

To investigate the co-incidence of apoptosis, autophagy, and unfolded protein response (UPR) in hepatitis B (HBV) and C (HCV) infected hepatocytes.

METHODS

We performed immunofluorescence confocal microscopy on 10 liver biopsies from HBV and HCV patients and tissue microarrays of HBV positive liver samples. We used specific antibodies for LC3β, cleaved caspase-3, BIP (GRP78), and XBP1 to detect autophagy, apoptosis and UPR, respectively. Anti-HCV NS3 and anti-HBs antibodies were also used to confirm infection. We performed triple blind counting of events to determine the co-incidence of autophagy (LC3β punctuate), apoptosis (cleaved caspase-3), and unfolded protein response (GRP78) with HBV and HCV infection in hepatocytes. All statistical analyses were performed using SPSS software for Windows (Version 16 SPSS Inc, Chicago, IL, United States). P-values < 0.05 were considered statistically significant. Statistical analyses were performed with Mann-Whitney test to compare incidence rates for autophagy, apoptosis, and UPR in HBV- and HCV-infected cells and adjacent non-infected cells.

RESULTS

Our results showed that infection of hepatocytes with either HBV and HCV induces significant increase (P < 0.001) in apoptosis (cleavage of caspase-3), autophagy (LC3β punctate), and UPR (increase in GRP78 expression) in the HCV- and HBV-infected cells, as compared to non-infected cells of the same biopsy sections. Our tissue microarray immunohistochemical expression analysis of LC3β in HBV(Neg) and HBV(Pos) revealed that majority of HBV-infected hepatocytes display strong positive staining for LC3β. Interestingly, although XBP splicing in HBV-infected cells was significantly higher (P < 0.05), our analyses show a slight increase of XBP splicing was in HCV-infected cells (P > 0.05). Furthermore, our evaluation of patients with HBV and HCV infection based on stage and grade of the liver diseases revealed no correlation between these pathological findings and induction of apoptosis, autophagy, and UPR.

CONCLUSION

The results of this study indicate that HCV and HBV infection activates apoptosis, autophagy and UPR, but slightly differently by each virus. Further studies are warranted to elucidate the interconnections between these pathways in relation to pathology of HCV and HBV in the liver tissue.

Arsenic trioxide reduces chemo-resistance to 5-fluorouracil and cisplatin in HBx-HepG2 cells via complex mechanisms

Background

Multidrug resistance is one of the major reasons chemotherapy-based treatments failed in hepatitis B virus (HBV) related hepatocellular carcinoma (HCC). Hypoxia is generally associated with tumor chemo-resistance. The aim of the study was to investigate the effect of Arsenic trioxide (As₂O₃) on the hypoxia-induced chemo-resistance to 5-FU or cisplatin and explored its underlying mechanism in the HBx-HepG2 cells.

Methods

MTT assay was used to examine the cell viability. Mitochondrial membrane potential (MMP) and cell cycle was examined by flow cytometry. qRT-PCR was employed to observe the mRNA expression level; and western blot assay was used to determine the protein expression level.

Results

Our results showed that transfection of HBx plasmid established the HBx-HepG2 cells expressing HBx, and the expression of HBx was confirmed by qRT-PCR and western blot. Exposure of HBx-HepG2 cells to hypoxia (5 % O₂, 3 % O₂, 1 % O₂) for 48 h increased the chemo-resistance to 5-fluorouracil (5-FU) (50–1600 µM) and cisplatin (25–800 µM), reduced MMP, and caused the cell cycle arrest at G₀/G₁ phase in a concentration-dependent manner. Hypoxia also concentration-dependently (5 % O₂, 3 % O₂, 1 % O₂) reduced mRNA expression level of P-glycoprotein (P-gp), multidrug resistance protein (MRP1), lung resistance protein (LRP), and decreased the protein expression level of hypoxia-inducible factor-1α (HIF-1α), P-gp MRP1, and LRP. Following pretreatment with As₂O₃ at a non-cytotoxic concentration re-sensitized the hypoxia (1 % O₂)-induced chemo-resistance to 5-FU and cisplatin in HBx-HepG2 cells. As₂O₃ pretreatment also prevented MMP reduction and G₀/G₁ arrest induced by hypoxia. Meanwhile, As₂O₃ antagonized increase of HIF-1α protein induced by hypoxia, and it also suppresses the increase in expression levels of P-gp, MRP1, and LRP mRNA and proteins. In addition, As₂O₃ in combination with 5-FU treatment caused up-regulation of DR5, caspase 3, caspase 8, and caspase 9, and down-regulation of BCL-2, but had no effect of DR4.

Conclusions

Our results may suggest that As₂O₃ re-sensitizes hypoxia-induced chemo-resistance in HBx-HepG2 via complex pathways, and As₂O₃ may be a potential agent that given in combination with other anti-drugs for the treatment of HBV related HCC, which is resistant to chemotherapy.

Wild type HBx and truncated HBx: Pleiotropic regulators driving sequential genetic and epigenetic steps of hepatocarcinogenesis and progression of HBV-associated neoplasms

Hepatitis B virus (HBV) is one of the causative agents of hepatocellular carcinoma. The molecular mechanisms of tumorigenesis are complex. One of the host factors involved is apparently the long-lasting inflammatory reaction which accompanies chronic HBV infection. Although HBV lacks a typical viral oncogene, the HBx gene encoding a pleiotropic regulatory protein emerged as a major player in liver carcinogenesis. Here we review the tumorigenic functions of HBx with an emphasis on wild type and truncated HBx variants, and their role in the transcriptional dysregulation and epigenetic reprogramming of the host cell genome. We suggest that HBx acquired by the HBV genome during evolution acts like a cellular proto-onc gene that is activated by deletion during hepatocarcinogenesis. The resulting viral oncogene (v-onc gene) codes for a truncated HBx protein that facilitates tumor progression. Copyright © 2015 John Wiley & Sons, Ltd.

Epidermal TRPM8 channel isoform controls the balance between keratinocyte proliferation and differentiation in a cold-dependent manner

Deviation of the ambient temperature is one of the most ubiquitous stimuli that continuously affect mammals' skin. Although the role of the warmth receptors in epidermal homeostasis (EH) was elucidated in recent years, the mystery of the keratinocyte mild-cold sensor remains unsolved. Here we report the cloning and characterization of a new functional epidermal isoform of the transient receptor potential M8 (TRPM8) mild-cold receptor, dubbed epidermal TRPM8 (eTRPM8), which is localized in the keratinocyte endoplasmic reticulum membrane and controls mitochondrial Ca(2+) concentration ([Ca(2+)]m). In turn, [Ca(2+)]m modulates ATP and superoxide (O2(·-)) synthesis in a cold-dependent manner. We report that this fine tuning of ATP and O2(·-) levels by cooling controls the balance between keratinocyte proliferation and differentiation. Finally, to ascertain eTRPM8's role in EH in vivo we developed a new functional knockout mouse strain by deleting the pore domain of TRPM8 and demonstrated that eTRPM8 knockout impairs adaptation of the epidermis to low temperatures.

Plasma membrane calcium ATPase regulates bone mass by fine-tuning osteoclast differentiation and survival

Plasma membrane calcium ATPases PMCA1 and PMCA4 regulate osteoclast differentiation and survival by regulating NFATc1 and NO.

The precise regulation of Ca²⁺ dynamics is crucial for proper differentiation and function of osteoclasts. Here we show the involvement of plasma membrane Ca²⁺ ATPase (PMCA) isoforms 1 and 4 in osteoclastogenesis. In immature/undifferentiated cells, PMCAs inhibited receptor activator of NF-κB ligand–induced Ca²⁺ oscillations and osteoclast differentiation in vitro. Interestingly, nuclear factor of activated T cell c1 (NFATc1) directly stimulated PMCA transcription, whereas the PMCA-mediated Ca²⁺ efflux prevented NFATc1 activation, forming a negative regulatory loop. PMCA4 also had an anti-osteoclastogenic effect by reducing NO, which facilitates preosteoclast fusion. In addition to their role in immature cells, increased expression of PMCAs in mature osteoclasts prevented osteoclast apoptosis both in vitro and in vivo. Mice heterozygous for PMCA1 or null for PMCA4 showed an osteopenic phenotype with more osteoclasts on bone surface. Furthermore, PMCA4 expression levels correlated with peak bone mass in premenopausal women. Thus, our results suggest that PMCAs play important roles for the regulation of bone homeostasis in both mice and humans by modulating Ca²⁺ signaling in osteoclasts.

Modulation of apoptotic signaling by the hepatitis B virus X protein

Worldwide, an estimated 350 million people are chronically infected with the Hepatitis B Virus (HBV); chronic infection with HBV is associated with the development of severe liver diseases including hepatitis and cirrhosis. Individuals who are chronically infected with HBV also have a significantly higher risk of developing hepatocellular carcinoma (HCC) than uninfected individuals. The HBV X protein (HBx) is a key regulatory HBV protein that is important for HBV replication, and likely plays a cofactor role in the development of HCC in chronically HBV-infected individuals. Although some of the functions of HBx that may contribute to the development of HCC have been characterized, many HBx activities, and their putative roles during the development of HBV-associated HCC, remain incompletely understood. HBx is a multifunctional protein that localizes to the cytoplasm, nucleus, and mitochondria of HBV‑infected hepatocytes. HBx regulates numerous cellular signal transduction pathways and transcription factors as well as cell cycle progression and apoptosis. In this review, we will summarize reports in which the impact of HBx expression on cellular apoptotic pathways has been analyzed. Although various effects of HBx on apoptotic pathways have been observed in different model systems, studies of HBx activities in biologically relevant hepatocyte systems have begun to clarify apoptotic effects of HBx and suggest mechanisms that could link HBx modulation of apoptotic pathways to the development of HBV-associated HCC.

Hepatitis B virus X protein targets the Bcl-2 protein CED-9 to induce intracellular Ca2+ increase and cell death in Caenorhabditis elegans

HBx is a multifunctional hepatitis B virus (HBV) protein that is crucial for HBV infection and pathogenesis and a contributing cause of hepatocyte carcinogenesis. However, the host targets and mechanisms of action of HBx are poorly characterized. We show here that expression of HBx in Caenorhabditis elegans induces both necrotic and apoptotic cell death, mimicking an early event of liver infection by HBV. Genetic and biochemical analyses indicate that HBx interacts directly with the B-cell lymphoma 2 (Bcl-2) homolog CED-9 (cell death abnormal) through a Bcl-2 homology 3 (BH3)-like motif to trigger both cytosolic Ca(2+) increase and cell death. Importantly, Bcl-2 can substitute for CED-9 in mediating HBx-induced cell killing in C. elegans, suggesting that CED-9 and Bcl-2 are conserved cellular targets of HBx. A genetic suppressor screen of HBx-induced cell death has produced many mutations, including mutations in key regulators from both apoptosis and necrosis pathways, indicating that this screen can identify new apoptosis and necrosis genes. Our results suggest that C. elegans could serve as an animal model for identifying crucial host factors and signaling pathways of HBx and aid in development of strategies to treat HBV-induced liver disorders.

Hepatitis B virus X protein targets the Bcl-2 protein CED-9 to induce intracellular Ca2+ increase and cell death in Caenorhabditis elegans

HBx is a multifunctional hepatitis B virus (HBV) protein that is crucial for HBV infection and pathogenesis and a contributing cause of hepatocyte carcinogenesis. However, the host targets and mechanisms of action of HBx are poorly characterized. We show here that expression of HBx in Caenorhabditis elegans induces both necrotic and apoptotic cell death, mimicking an early event of liver infection by HBV. Genetic and biochemical analyses indicate that HBx interacts directly with the B-cell lymphoma 2 (Bcl-2) homolog CED-9 (cell death abnormal) through a Bcl-2 homology 3 (BH3)-like motif to trigger both cytosolic Ca(2+) increase and cell death. Importantly, Bcl-2 can substitute for CED-9 in mediating HBx-induced cell killing in C. elegans, suggesting that CED-9 and Bcl-2 are conserved cellular targets of HBx. A genetic suppressor screen of HBx-induced cell death has produced many mutations, including mutations in key regulators from both apoptosis and necrosis pathways, indicating that this screen can identify new apoptosis and necrosis genes. Our results suggest that C. elegans could serve as an animal model for identifying crucial host factors and signaling pathways of HBx and aid in development of strategies to treat HBV-induced liver disorders.

Large hepatitis delta antigen activates STAT-3 and NF-κB via oxidative stress

Hepatitis delta virus (HDV) coinfection or superinfection in hepatitis B virus (HBV)-infected patients results in a more aggressive liver disease, with more often fulminant forms and more rapid progression to cirrhosis and hepatocellular carcinoma. The mechanism(s) for this pejorative evolution remains unclear. To explore a specific HDV pathogenesis, we used a model of transient transfection of plasmids expressing the small (sHDAg or p24) or the large (LHDAg or p27) delta antigen in hepatocyte cell lines. We found that the production of reactive oxygen species was significantly higher in cells expressing p27. Consequently, p27 activated the signal transducer and activator of transcription-3 (STAT-3) and the nuclear factor kappa B (NF-κB) via the oxidative stress pathway. Moreover in the presence of antioxidants (PDTC, NAC) or calcium inhibitors (TMB-8, BAPTA-AM, Ruthenium Red), p27-induced activation of STAT-3 and NF-κB was dramatically reduced. Similarly, using a mutated form of p27, where the cysteine 211-isoprenylation residue was replaced by a serine, a significant reduction of STAT-3 and NF-κB activation was seen, suggesting the involvement of isoprenylation in this process. Additionally, we show that p27 is able to induce oxidative stress through activation of NADPH oxidase-4. These results provide insight into the mechanisms by which p27 can alter intracellular events relevant to HDV-related liver pathogenesis.

Mitochondria as sensors and regulators of calcium signalling

During the past two decades calcium (Ca(2+)) accumulation in energized mitochondria has emerged as a biological process of utmost physiological relevance. Mitochondrial Ca(2+) uptake was shown to control intracellular Ca(2+) signalling, cell metabolism, cell survival and other cell-type specific functions by buffering cytosolic Ca(2+) levels and regulating mitochondrial effectors. Recently, the identity of mitochondrial Ca(2+) transporters has been revealed, opening new perspectives for investigation and molecular intervention.

Viroporins: structure and biological functions

Viroporins are small virally encoded hydrophobic proteins that oligomerize in the membrane of host cells, leading to the formation of hydrophilic pores. This activity modifies several cellular functions, including membrane permeability, Ca²⁺ homeostasis, membrane remodelling and glycoprotein trafficking.

A classification scheme for viroporins is proposed on the basis of their structure and membrane topology. Thus, class I and class II viroporins are defined according to the number of transmembrane domains in the protein (one and two, respectively), and subclasses are defined according to their orientation in the membrane.

The main function of viroporins during viral replication is to participate in virion morphogenesis and release from host cells. In addition, some viroporins are involved in viral entry and genome replication.

The structure and activity of several viroporins, such as picornavirus protein 2B (P2B), influenza A virus matrix protein 2 (M2), hepatitis C virus p7 and HIV-1 viral protein U (Vpu), have been analysed in detail.

New members of this expanding family of viral proteins have been described, from both RNA and DNA viruses. In addition to having a common general structure, all of these new viroporins have the ability to increase membrane permeability.

Viroporins represent ideal targets to block viral replication and the spread of infection. Although a number of selective inhibitors of viroporin ion channels have been analysed in detail, optimized screening systems promise to provide new and more potent antiviral compounds in the near future.

Viroporins belong to a growing family of virally encoded proteins that form aqueous channels in the membranes of host cells. Here, Carrasco and colleagues review the structure and diverse biological functions of these proteins during the viral life cycle, as well as their potential as antiviral therapeutic targets.

Viroporins are small, hydrophobic proteins that are encoded by a wide range of clinically relevant animal viruses. When these proteins oligomerize in host cell membranes, they form hydrophilic pores that disrupt a number of physiological properties of the cell. Viroporins are crucial for viral pathogenicity owing to their involvement in several diverse steps of the viral life cycle. Thus, these viral proteins, which include influenza A virus matrix protein 2 (M2), HIV-1 viral protein U (Vpu) and hepatitis C virus p7, represent ideal targets for therapeutic intervention, and several compounds that block their pore-forming activity have been identified. Here, we review recent studies in the field that have advanced our knowledge of the structure and function of this expanding family of viral proteins.

The hepatitis B virus X protein elevates cytosolic calcium signals by modulating mitochondrial calcium uptake

Chronic hepatitis B virus (HBV) infections are associated with the development of hepatocellular carcinoma (HCC). The HBV X protein (HBx) is thought to play an important role in the development of HBV-associated HCC. One fundamental HBx function is elevation of cytosolic calcium signals; this HBx activity has been linked to HBx stimulation of cell proliferation and transcription pathways, as well as HBV replication. Exactly how HBx elevates cytosolic calcium signals is not clear. The studies described here show that HBx stimulates calcium entry into cells, resulting in an increased plateau level of inositol 1,4,5-triphosphate (IP3)-linked calcium signals. This increased calcium plateau can be inhibited by blocking mitochondrial calcium uptake and store-operated calcium entry (SOCE). Blocking SOCE also reduced HBV replication. Finally, these studies also demonstrate that there is increased mitochondrial calcium uptake in HBx-expressing cells. Cumulatively, these studies suggest that HBx can increase mitochondrial calcium uptake and promote increased SOCE to sustain higher cytosolic calcium and stimulate HBV replication.

The role of hepatitis B virus X protein is related to its differential intracellular localization

Chronic hepatitis B virus (HBV) infection has been strongly associated with hepatocellular carcinoma. HBV encodes an oncogenic hepatitis B virus X protein (HBx), which is a multifunctional regulator that modulates signal transduction, transcription, cell cycle progress, protein degradation, apoptosis, and genetic stability through direct and indirect interaction with host factors. The subcellular localization of HBx is primarily cytoplasmic, with a small fraction in the nucleus. In addition, high levels of HBx expression lead to an abnormal mitochondrial distribution. The dynamic distribution of HBx could be important to the multiple functions of HBx at different stages of the HBV life cycle. This short review presents an overview of the differential roles of HBx as a function of its intracellular localization.

Hepatitis B and C virus hepatocarcinogenesis: lessons learned and future challenges

Worldwide, hepatocellular carcinoma (HCC) is one of the most common cancers. It is thought that 80% of hepatocellular carcinomas are linked to chronic infections with the hepatitis B (HBV) or hepatitis C (HCV) viruses. Chronic HBV and HCV infections can alter hepatocyte physiology in similar ways and may utilize similar mechanisms to influence the development of HCC. There has been significant progress towards understanding the molecular biology of HBV and HCV and identifying the cellular signal transduction pathways that are altered by HBV and HCV infections. Although the precise molecular mechanisms that link HBV and HCV infections to the development of HCC are not entirely understood, there is considerable evidence that both inflammatory responses to infections with these viruses, and associated destruction and regeneration of hepatocytes, as well as activities of HBV- or HCV-encoded proteins, contribute to hepatocyte transformation. In this review, we summarize progress in defining mechanisms that may link HBV and HCV infections to the development of HCC, discuss the challenges of directly defining the processes that underlie HBV- and HCV-associated HCC, and describe areas that remain to be explored.

Epidemiology and management of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a major world health problem because of the high incidence and case fatality rate. In most patients, the diagnosis of HCC is made at an advanced stage, which limits the application of curative treatments. Most HCCs develop in patients with underlying chronic liver disease. Chronic viral hepatitis B and C are the major causes of liver cirrhosis and HCC. Recent improvements in treatment of viral hepatitis and in methods for surveillance and therapy for HCC have contributed to better survival of patients with HCC. This article reviews the epidemiology, cause, prevention, clinical manifestations, surveillance, diagnosis, and treatment approach for HCC.

The X protein of hepatitis B virus inhibits apoptosis in hepatoma cells through enhancing the methionine adenosyltransferase 2A gene expression and reducing S-adenosylmethionine production

The X protein (HBx) of hepatitis B virus (HBV) is involved in the development of hepatocellular carcinoma (HCC), and methionine adenosyltransferase 2A (MAT2A) promotes the growth of liver cancer cells through altering S-adenosylmethionine homeostasis. Thus, we speculated that a link between HBx and MAT2A may contribute to HCC development. In this study, the effects of HBx on MAT2A expression and cell apoptosis were investigated, and the molecular mechanism by which HBx and MAT2A regulate tumorigenesis was evaluated. Results from immunohistochemistry analyses of 37 pairs of HBV-associated liver cancer tissues/corresponding peritumor tissues showed that HBx and MAT2A are highly expressed in most liver tumor tissues. Our in vitro results revealed that HBx activates MAT2A expression in a dose-dependent manner in hepatoma cells, and such regulation requires the cis-regulatory elements NF-κB and CREB on the MAT2A gene promoter. Electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP) further demonstrated that HBx facilitates the binding of NF-κB and CREB to MAT2A gene promoter. In addition, overexpression of HBx or MAT2A inhibits cell apoptosis, whereas knockdown of MAT2A expression stimulates apoptosis in hepatoma cells. Furthermore, we demonstrated that HBx reduces MAT1A expression and AdoMet production but enhances MAT2β expression. Thus, we proposed that HBx activates MAT2A expression through NF-κB and CREB signaling pathways to reduce AdoMet production, inhibit hepatoma cell apoptosis, and perhaps enhance HCC development. These findings should provide new insights into our understanding how the molecular mechanisms underline the effects of HBV infection on the production of MAT2A and the development of HCC.

Hepatitis B virus X protein enhances cisplatin-induced hepatotoxicity via a mechanism involving degradation of Mcl-1

Hepatitis B virus X protein (HBx) is implicated in the pathogenesis of hepatitis B virus (HBV)-associated liver diseases. However, whether HBx has the ability to disturb the susceptibility of hepatocytes to common chemotherapeutic agents remains incompletely understood. Here we demonstrate that HBx enhances cisplatin-induced hepatotoxicity by a mechanism involving degradation of Mcl-1, an antiapoptotic member of the Bcl-2 family. Ectopic expression of HBx sensitized hepatocytes to cisplatin-induced apoptosis, which was accompanied by a marked downregulation of Mcl-1 but not of Bcl-2 or Bcl-xL. Overexpression of Mcl-1 prevented HBx-induced proapoptotic and proinflammatory effects during cisplatin treatment both in vitro and in vivo. HBx-induced dysregulation of Mcl-1 resulted mainly from posttranslational degradation rather than transcription repression. Moreover, a caspase-3 inhibitor effectively abrogated HBx-enhanced Mcl-1 degradation and cell death. Importantly, antioxidants blocked activation of caspase-3 and acceleration of Mcl-1 loss, as well as cell death, in HBx-expressing hepatocytes upon cisplatin exposure in vitro and in vivo. Collectively, these data implicate oxidative stress-dependent caspase-3-mediated degradation of Mcl-1 as a mechanism contributing to HBx-mediated sensitization of cisplatin-induced hepatotoxicity. A combination of cisplatin and antioxidants might provide more advantage than cisplatin alone in the treatment of cancer patients with chronic HBV infection.

Hepatitis B virus X protein stimulates IL-6 expression in hepatocytes via a MyD88-dependent pathway

BACKGROUND & AIMS

Hepatitis B virus (HBV) X protein (HBx) has been implicated in HBV-associated carcinogenesis by activating signal transduction pathways and influencing gene transcription in liver cells. We aimed to investigate the underlying mechanisms for HBx-induced production of interleukin-6 (IL-6), one of the major inflammatory mediators that stimulate hepatocellular carcinoma development.

METHODS

HBx was overexpressed in hepatic and hepatoma cell lines and IL-6 expression levels were measured by quantitative RT-PCR and ELISA. The activation of IRAK-1, ERKs/p38, and NF-κB was determined by Western blotting using specific anti-phosphoprotein antibodies. The role of MyD88 in these processes was analyzed by MyD88 RNAi and expression of an inactive MyD88 mutant.

RESULTS

Expression of HBx in hepatic and hepatoma cells led to a dramatic enhancement of IL-6 synthesis and secretion. Dysfunction of MyD88 in these cells prevented the HBx-triggered IL-6 production. HBx expression also activated downstream signaling proteins of MyD88 including IRAK-1, ERKs/p38, and NF-κB. Inactivation of these signaling molecules blocked IL-6 synthesis as well. HBx-stimulated the expression of MyD88.

CONCLUSIONS

In hepatocytes and hepatoma cells, HBx stimulates the production of IL-6 in a MyD88-dependent manner, indicating that parenchymal liver cells are an additional source of high levels of IL-6 in the HBV-infected liver microenvironment. HBx could be involved in HBV-mediated liver carcinogenesis, through this mechanism of action.

Replication of the hepatitis B virus requires a calcium-dependent HBx-induced G1 phase arrest of hepatocytes

Chronic HBV infections cause hepatocellular carcinoma (HCC). Activities of the HBV HBx protein regulate HBV replication and may contribute to the development of HCC. We previously reported that HBx causes primary rat hepatocytes to exit G0 but stall in G1 phase of the cell cycle; entry into G1 stimulated HBV replication. We now report that the activity of the mitochondria permeability transition pore is required for HBx regulation of cell cycle proteins and HBV replication in primary rat hepatocytes, that progression from G0 to G1 stimulates HBV polymerase activity, and that HBV replication is facilitated by the HBx-induced G1 arrest. HBx stimulation of HBV replication was linked to elevation of the R2 subunit of ribonucleotide reductase. Our studies suggest that HBx uses mitochondrial-dependent calcium signaling to cause hepatocytes to exit G0 but stall in G1 and that this HBx activity alters the cellular environment and stimulates HBV replication.

[Pro-apoptotic function of hepatitis B virus X protein]

Infection of hepatitis B virus (HBV) is a main cause of liver diseases including hepatitis, cirrhosis and hepatocellular carcinoma (HCC). Among the HBV-encoded proteins, the HBV X protein (HBx) has been suspected to be strongly involved in HBV-associated liver pathogenesis. HBx, a virally encoded multifunctional regulator, has been shown to induce apoptosis, anti-apoptosis, proliferation, and transformation of cells depending on the cell lines, model systems used, assay protocols, and research groups. Among the several activities of HBx, the pro-apoptotic function of HBx will be discussed in this review. Given that the disruption of apoptosis pathway by HBx contributes to the liver pathogenesis, a better understanding of the molecular interference in the cellular pro-apoptotic networks by HBx will provide useful clues for the intervention in HBV-mediated liver diseases.

Hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC): molecular mechanisms and novel paradigms

Chronic hepatitis B (HBV) infection is a major risk factor for hepatocellular carcinoma (HCC). Most HCCs complicate the evolution of an active or inactive cirrhosis. However, some tumors occur on livers with minimal histological changes; the prevalence of such cases varies from one geographical region to the other, being much higher in the Southern half of Africa (around 40% of HCCs) than in Asia, America and Europe, where at least 90% of HCCs are associated in the cirrhosis. This heterogeneity is probably a reflection of different environmental and genetic factors. This review will summarise the current knowledge on the mechanisms involved in HBV-related liver carcinogenesis. It will show in particular how viruses can be viewed as tools to discover and dissect new cellular pathways involved in cancer development and emphasize the potential synergistic effects between HBV and hepatitis C virus (HCV), as well as between viral infections and other environmental factors, such as alcohol.

Plasma membrane calcium pump regulation by metabolic stress

The plasma membrane Ca²⁺-ATPase (PMCA) is an ATP-driven pump that is critical for the maintenance of low resting [Ca²⁺]_i in all eukaryotic cells. Metabolic stress, either due to inhibition of mitochondrial or glycolytic metabolism, has the capacity to cause ATP depletion and thus inhibit PMCA activity. This has potentially fatal consequences, particularly for non-excitable cells in which the PMCA is the major Ca²⁺ efflux pathway. This is because inhibition of the PMCA inevitably leads to cytosolic Ca²⁺ overload and the consequent cell death. However, the relationship between metabolic stress, ATP depletion and inhibition of the PMCA is not as simple as one would have originally predicted. There is increasing evidence that metabolic stress can lead to the inhibition of PMCA activity independent of ATP or prior to substantial ATP depletion. In particular, there is evidence that the PMCA has its own glycolytic ATP supply that can fuel the PMCA in the face of impaired mitochondrial function. Moreover, membrane phospholipids, mitochondrial membrane potential, caspase/calpain cleavage and oxidative stress have all been implicated in metabolic stress-induced inhibition of the PMCA. The major focus of this review is to challenge the conventional view of ATP-dependent regulation of the PMCA and bring together some of the alternative or additional mechanisms by which metabolic stress impairs PMCA activity resulting in cytosolic Ca²⁺ overload and cytotoxicity.

Redox regulation of T-cell turnover by the p13 protein of human T-cell leukemia virus type 1: distinct effects in primary versus transformed cells

The present study investigated the function of p13, a mitochondrial protein of human T-cell leukemia virus type 1 (HTLV-1). Although necessary for viral propagation in vivo, the mechanism of function of p13 is incompletely understood. Drawing from studies in isolated mitochondria, we analyzed the effects of p13 on mitochondrial reactive oxygen species (ROS) in transformed and primary T cells. In transformed cells (Jurkat, HeLa), p13 did not affect ROS unless the cells were subjected to glucose deprivation, which led to a p13-dependent increase in ROS and cell death. Using RNA interference we confirmed that expression of p13 also influences glucose starvation-induced cell death in the context of HTLV-1–infected cells. ROS measurements showed an increasing gradient from resting to mitogen-activated primary T cells to transformed T cells (Jurkat). Expression of p13 in primary T cells resulted in their activation, an effect that was abrogated by ROS scavengers. These findings suggest that p13 may have a distinct impact on cell turnover depending on the inherent ROS levels; in the context of the HTLV-1 propagation strategy, p13 could increase the pool of “normal” infected cells while culling cells acquiring a transformed phenotype, thus favoring lifelong persistence of the virus in the host.

Calcium signalling-dependent mitochondrial dysfunction and bioenergetics regulation in respiratory chain Complex II deficiency

Despite advanced knowledge on the genetic basis of oxidative phosphorylation-related diseases, the molecular and/or cellular determinants for tissue-specific dysfunction are not completely understood. Here, we report the cellular events associated with mitochondrial respiratory Complex II deficiency occurring before cell death. Mutation or chronic inhibition of Complex II determined a large increase of basal and agonist-evoked Ca(2+) signals in the cytosol and the mitochondria, in parallel with mitochondrial dysfunction characterized by membrane potential (Δψ(mit)) loss, [ATP] reduction and increased reactive oxygen species production. Cytosolic and mitochondrial Ca(2+) overload are linked to increased endoplasmic reticulum (ER) Ca(2+) leakage, and to SERCA2b and PMCA proteasome-dependent degradation. Increased [Ca(2+)](mit) is also contributed by decreased mitochondrial motility and increased ER-mitochondria contact sites. Interestingly, increased intracellular [Ca(2+)] activated on the one hand a compensatory Ca(2+)-dependent glycolytic ATP production and determined on the second hand mitochondrial pathology. These results revealed the primary function for Ca(2+) signalling in the control of mitochondrial dysfunction and cellular bioenergetics outcomes linked to respiratory chain Complex II deficiency.

Intramitochondrial calcium regulation by the FHIT gene product sensitizes to apoptosis

Despite the growing interest in the Fhit tumor suppressor protein, frequently deleted in human cancers, the mechanism of its powerful proapoptotic activity has remained elusive. We here demonstrate that Fhit sensitizes the low-affinity Ca(2+) transporters of mitochondria, enhancing Ca(2+) uptake into the organelle both in intact and in permabilized cells, and potentiating the effect of apoptotic agents. This effect can be attributed to the fraction of Fhit sorted to mitochondria, as a fully mitochondrial Fhit (a chimeric protein including a mitochondrial targeting sequence) retains the Ca(2+) signaling properties of Fhit and the proapoptotic activity of the native protein (whereas the effects on the cell cycle are lost). Thus, the partial sorting of Fhit to mitochondria allows to finely tune the sensitivity of the organelle to the highly pleiomorphic Ca(2+) signals, synergizing with apoptotic challenges. This concept, and the identification of the molecular machinery, may provide ways to act on apoptotic cell death and its derangement in cancer.

How many Ca2+ATPase isoforms are expressed in a cell type? A growing family of membrane proteins illustrated by studies in platelets

Ca(2+) signaling plays a key role in normal and abnormal platelet functions. Understanding platelet Ca(2+) signaling requires the knowledge of proteins involved in this process. Among these proteins are Ca(2+)ATPases or Ca(2+) pumps that deplete the cytosol of Ca(2+) ions. Here, we will particularly focus on two Ca(2+) pump families: the plasma membrane Ca(2+)ATPases (PMCAs) that extrude cytosolic Ca(2+) towards the extracellular medium and the sarco/endoplasmic reticulum Ca(2+)ATPases (SERCAs) that pump Ca(2+) into the endoplasmic reticulum (ER). In the present review, we will summarize data on platelet Ca(2+)ATPases including their identification and biogenesis. First of all, we will present the Ca(2+)ATPase genes and their isoforms expressed in platelets. We will especially focus on a member of the SERCA family, SERCA3, recently found to give rise to a number of species-specific isoforms. Next, we will describe the differences in Ca(2+)ATPase patterns observed in human and rat platelets. Last, we will analyze how the expression of Ca(2+)ATPase isoforms changes during megakaryocytic maturation and show that megakaryocytopoiesis is associated with a profound reorganization of the expression and/or activity of Ca(2+)ATPases. Taken together, these data provide new aspects of investigations to better understand normal and abnormal platelet Ca(2+) signaling.

Viral calciomics: interplays between Ca2+ and virus

Ca²⁺ is one of the most universal and versatile signaling molecules and is involved in almost every aspect of cellular processes. Viruses are adept at utilizing the universal Ca²⁺ signal to create a tailored cellular environment that meets their own demands. This review summarizes most of the known mechanisms by which viruses perturb Ca²⁺ homeostasis and utilize Ca²⁺ and cellular Ca²⁺-binding proteins to their benefit in their replication cycles. Ca²⁺ plays important roles in virion structure formation, virus entry, viral gene expression, posttranslational processing of viral proteins and virion maturation and release. As part of the review, we introduce an algorithm to identify linear “EF-hand” Ca²⁺-binding motifs which resulted in the prediction of a total of 93 previously unrecognized Ca²⁺-binding motifs in virus proteins. Many of these proteins are nonstructural proteins, a class of proteins among which Ca²⁺ interactions had not been formerly appreciated. The presence of linear Ca²⁺-binding motifs in viral proteins enlarges the spectrum of Ca²⁺–virus interplay and expands the total scenario of viral calciomics.