Ataxia telangiectasia-mutated-Rad3-related DNA damage checkpoint signaling pathway triggered by hepatitis B virus infection

Deciphering the phospho-signature induced by hepatitis B virus in primary human hepatocytes

Phosphorylation is a major post-translation modification (PTM) of proteins which is finely tuned by the activity of several hundred kinases and phosphatases. It controls most if not all cellular pathways including anti-viral responses. Accordingly, viruses often induce important changes in the phosphorylation of host factors that can either promote or counteract viral replication. Among more than 500 kinases constituting the human kinome only few have been described as important for the hepatitis B virus (HBV) infectious cycle, and most of them intervene during early or late infectious steps by phosphorylating the viral Core (HBc) protein. In addition, little is known on the consequences of HBV infection on the activity of cellular kinases. The objective of this study was to investigate the global impact of HBV infection on the cellular phosphorylation landscape early after infection. For this, primary human hepatocytes (PHHs) were challenged or not with HBV, and a mass spectrometry (MS)-based quantitative phosphoproteomic analysis was conducted 2- and 7-days post-infection. The results indicated that while, as expected, HBV infection only minimally modified the cell proteome, significant changes were observed in the phosphorylation state of several host proteins at both time points. Gene enrichment and ontology analyses of up- and down-phosphorylated proteins revealed common and distinct signatures induced by infection. In particular, HBV infection resulted in up-phosphorylation of proteins involved in DNA damage signaling and repair, RNA metabolism, in particular splicing, and cytoplasmic cell-signaling. Down-phosphorylated proteins were mostly involved in cell signaling and communication. Validation studies carried out on selected up-phosphorylated proteins, revealed that HBV infection induced a DNA damage response characterized by the appearance of 53BP1 foci, the inactivation of which by siRNA increased cccDNA levels. In addition, among up-phosphorylated RNA binding proteins (RBPs), SRRM2, a major scaffold of nuclear speckles behaved as an antiviral factor. In accordance with these findings, kinase prediction analysis indicated that HBV infection upregulates the activity of major kinases involved in DNA repair. These results strongly suggest that HBV infection triggers an intrinsic anti-viral response involving DNA repair factors and RBPs that contribute to reduce HBV replication in cell culture models.

Mechanism of cell cycle regulation and cell proliferation during human viral infection

Over the history of the coevolution of Host viral interaction, viruses have customized the host cellular machinery into their use for viral genome replication, causing effective infection and ultimately aiming for survival. They do so by inducing subversions to the host cellular pathways like cell cycle via dysregulation of important cell cycle checkpoints by viral encoded proteins, arresting the cell cycle machinery, blocking cytokinesis as well as targeting subnuclear bodies, thus ultimately disorienting the cell proliferation. Both DNA and RNA viruses have been active participants in such manipulation resulting in serious outcomes of cancer. They achieve this by employing different mechanisms-Protein-protein interaction, protein-phosphorylation, degradation, redistribution, viral homolog, and viral regulation of APC at different stages of cell cycle events. Several DNA viruses cause the quiescent staged cells to undergo cell cycle which increases nucleotide pools logistically significantly persuading viral replication whereas few other viruses arrest a particular stage of cell cycle. This allows the latter group to sustain the infection which allows them to escape host immune response and support viral multiplication. Mechanical study of signaling such viral mediated pathways could give insight into understanding the etiology of tumorigenesis and progression. Overall this chapter highlights the possible strategies employed by DNA/RNA viral families which impact the normal cell cycle but facilitate viral infected cell replication. Such information could contribute to comprehending viral infection-associated disorders to further depth.

ATM-Dependent Phosphorylation of Hepatitis B Core Protein in Response to Genotoxic Stress

Chronic hepatitis caused by infection with the Hepatitis B virus is a life-threatening condition. In fact, 1 million people die annually due to liver cirrhosis or hepatocellular carcinoma. Recently, several studies demonstrated a molecular connection between the host DNA damage response (DDR) pathway and HBV replication and reactivation. Here, we investigated the role of Ataxia-telangiectasia-mutated (ATM) and Ataxia telangiectasia and Rad3-related (ATR) PI3-kinases in phosphorylation of the HBV core protein (HBc). We determined that treatment of HBc-expressing hepatocytes with genotoxic agents, e.g., etoposide or hydrogen peroxide, activated the host ATM-Chk2 pathway, as determined by increased phosphorylation of ATM at Ser1981 and Chk2 at Thr68. The activation of ATM led, in turn, to increased phosphorylation of cytoplasmic HBc at serine-glutamine (SQ) motifs located in its C-terminal domain. Conversely, down-regulation of ATM using ATM-specific siRNAs or inhibitor effectively reduced etoposide-induced HBc phosphorylation. Detailed mutation analysis of S-to-A HBc mutants revealed that S170 (S168 in a 183-aa HBc variant) is the primary site targeted by ATM-regulated phosphorylation. Interestingly, mutation of two major phosphorylation sites involving serines at positions 157 and 164 (S155 and S162 in a 183-aa HBc variant) resulted in decreased etoposide-induced phosphorylation, suggesting that the priming phosphorylation at these serine-proline (SP) sites is vital for efficient phosphorylation of SQ motifs. Notably, the mutation of S172 (S170 in a 183-aa HBc variant) had the opposite effect and resulted in massively up-regulated phosphorylation of HBc, particularly at S170. Etoposide treatment of HBV infected HepG2-NTCP cells led to increased levels of secreted HBe antigen and intracellular HBc protein. Together, our studies identified HBc as a substrate for ATM-mediated phosphorylation and mapped the phosphorylation sites. The increased expression of HBc and HBe antigens in response to genotoxic stress supports the idea that the ATM pathway may provide growth advantage to the replicating virus.

Breaking Bad: How Viruses Subvert the Cell Cycle

Interactions between the host and viruses during the course of their co-evolution have not only shaped cellular function and the immune system, but also the counter measures employed by viruses. Relatively small genomes and high replication rates allow viruses to accumulate mutations and continuously present the host with new challenges. It is therefore, no surprise that they either escape detection or modulate host physiology, often by redirecting normal cellular pathways to their own advantage. Viruses utilize a diverse array of strategies and molecular targets to subvert host cellular processes, while evading detection. These include cell-cycle regulation, major histocompatibility complex-restricted antigen presentation, intracellular protein transport, apoptosis, cytokine-mediated signaling, and humoral immune responses. Moreover, viruses routinely manipulate the host cell cycle to create a favorable environment for replication, largely by deregulating cell cycle checkpoints. This review focuses on our current understanding of the molecular aspects of cell cycle regulation that are often targeted by viruses. Further study of their interactions should provide fundamental insights into cell cycle regulation and improve our ability to exploit these viruses.

Role of HBx in hepatitis B virus persistence and its therapeutic implications

Chronic hepatitis B virus infection is a significant risk factor for cirrhosis and hepatocellular carcinoma. The HBx protein is required for virus replication, but the lack of robust infection models has hindered our understanding of HBx functions that could be targeted for antiviral purposes. We briefly review three properties of HBx: its binding to DDB1 and its regulation of cell survival and metabolism, to illustrate how a single viral protein can have multiple effects in a cell. We propose that different functions of HBx are needed, depending on the changing hepatocyte environment encountered during a chronic virus infection, and that these functions might serve as novel therapeutic targets for inhibiting hepatitis B virus replication and the development of associated diseases.

Past, Current, and Future Developments of Therapeutic Agents for Treatment of Chronic Hepatitis B Virus Infection

For decades, treatment of hepatitis B virus (HBV) infection has been relying on interferon (IFN)-based therapies and nucleoside/nucleotide analogues (NAs) that selectively target the viral polymerase reverse transcriptase (RT) domain and thereby disrupt HBV viral DNA synthesis. We have summarized here the key steps in the HBV viral life cycle, which could potentially be targeted by novel anti-HBV therapeutics. A wide range of next-generation direct antiviral agents (DAAs) with distinct mechanisms of actions are discussed, including entry inhibitors, transcription inhibitors, nucleoside/nucleotide analogues, inhibitors of viral ribonuclease H (RNase H), modulators of viral capsid assembly, inhibitors of HBV surface antigen (HBsAg) secretion, RNA interference (RNAi) gene silencers, antisense oligonucleotides (ASOs), and natural products. Compounds that exert their antiviral activities mainly through host factors and immunomodulation, such as host targeting agents (HTAs), programmed cell death protein 1 (PD-1)/programmed death ligand 1 (PD-L1) inhibitors, and Toll-like receptor (TLR) agonists, are also discussed. In this Perspective, we hope to provide an overview, albeit by no means being comprehensive, for the recent development of novel therapeutic agents for the treatment of chronic HBV infection, which not only are able to sustainably suppress viral DNA but also aim to achieve functional cure warranted by HBsAg loss and ultimately lead to virus eradication and cure of hepatitis B.

Cell cycle checkpoints and pathogenesis of HIV-1 infection: a brief overview

To understand HIV pathogenesis or development is no simple undertaking and neither is the cell cycle which is highly complex that requires the coordination of multiple events and machinery. It is interesting that these two processes are interrelated, intersect and interact as HIV-1 infection results in cell cycle arrest at the G2 phase which is accompanied by massive CD4+ T cell death. For its own benefit, in an impressive manner and with the overabundance of tactics, HIV maneuvers DNA damage responses and cell cycle check points for viral replication at different stages from infection, to latency and to pathogenesis. Although the cell cycle is the most critical aspect involved in both viral and cellular replication, in this review, our main focus is on recent developments, including our own observations in the field of cell cycle proteins, checkpoints and strategies utilized by the viruses to manipulate these pathways to promote their own replication and survival. We will also discuss the emerging concept of targeting the replication initiation machinery for HIV therapy.

Modulation of DNA damage and repair pathways by human tumour viruses

With between 10% and 15% of human cancers attributable to viral infection, there is great interest, from both a scientific and clinical viewpoint, as to how these pathogens modulate host cell functions. Seven human tumour viruses have been identified as being involved in the development of specific malignancies. It has long been known that the introduction of chromosomal aberrations is a common feature of viral infections. Intensive research over the past two decades has subsequently revealed that viruses specifically interact with cellular mechanisms responsible for the recognition and repair of DNA lesions, collectively known as the DNA damage response (DDR). These interactions can involve activation and deactivation of individual DDR pathways as well as the recruitment of specific proteins to sites of viral replication. Since the DDR has evolved to protect the genome from the accumulation of deleterious mutations, deregulation is inevitably associated with an increased risk of tumour formation. This review summarises the current literature regarding the complex relationship between known human tumour viruses and the DDR and aims to shed light on how these interactions can contribute to genomic instability and ultimately the development of human cancers.

Simian virus 40 large T antigen induces IFN-stimulated genes through ATR kinase

Polyomaviruses encode a large T Ag (LT), a multifunctional protein essential for the regulation of both viral and host cell gene expression and productive viral infection. Previously, we have shown that stable expression of LT protein results in upregulation of genes involved in the IFN induction and signaling pathway. In this study, we focus on the cellular signaling mechanism that leads to the induction of IFN responses by LT. Our results show that ectopic expression of SV40 LT results in the induction of IFN-stimulated genes (ISGs) in human fibroblasts and confers an antiviral state. We describe a LT-initiated DNA damage response (DDR) that activates IFN regulatory factor 1, causing IFN-β production and consequent ISG expression in human cells. This IFN-β and ISG induction is dependent on ataxia-telangiectasia mutated and Rad3-related (ATR) kinase, but independent of ATM. ATR kinase inhibition using a selective kinase inhibitor (ETP-46464) caused a decrease in IFN regulatory factor 1 stabilization and ISG expression. Furthermore, expression of a mutant LT that does not induce DDR also does not induce IFN-β and ISGs. These results show that, in the absence of viral infection, LT-initiated activation of ATR-dependent DDR is sufficient for the induction of an IFN-β-mediated innate immune response in human cells. Thus, we have uncovered a novel and critical role for ATR as a mediator of antiviral responses utilizing LT.

DNA damage sensor γ -H2AX is increased in preneoplastic lesions of hepatocellular carcinoma

Background. Phosphorylated histone H2AX (γ-H2AX) is a potential regulator of DNA repair and is a useful tool for detecting DNA damage. To evaluate the clinical usefulness of γ-H2AX in hepatocellular carcinoma (HCC), we measured the level of γ-H2AX in HCC, dysplastic nodule, and nontumorous liver diseases. Methods. The level of γ-H2AX was measured by immunohistochemistry in fifty-eight HCC, 18 chronic hepatitis, 22 liver cirrhosis, and 19 dysplastic nodules. Appropriate cases were also examined by fluorescence analysis and western blotting. Results. All cases with chronic liver disease showed increased levels of γ-H2AX expression. In 40 (69.9%) of 58 cases with HCC, the labeling index (LI) of γ-H2AX was above 50% and was inversely correlated with the histological grade. Mean γ-H2AX LI was the highest in dysplastic nodule (74.1 ± 22.1%), which was significantly higher than HCC (P < 0.005). Moreover, γ-H2AX was significantly increased in nontumorous tissues of HCC as compared with liver cirrhosis without HCC (62.5 ± 24.7%, from 5.1 to 96.0%, P < 0.005). Conclusions. γ-H2AX was increased in the preneoplastic lesions of HCC and might be a useful biomarker for predicting the risk of HCC.

The DNA damage response in viral-induced cellular transformation

The DNA damage response (DDR) has emerged as a critical tumour suppressor pathway responding to cellular DNA replicative stress downstream of aberrant oncogene over-expression. Recent studies have now implicated the DDR as a sensor of oncogenic virus infection. In this review, we discuss the mechanisms by which tumour viruses activate and also suppress the host DDR. The mechanism of tumour virus induction of the DDR is intrinsically linked to the need for these viruses to promote an S-phase environment to replicate their nucleic acid during infection. However, inappropriate expression of viral oncoproteins can also activate the DDR through various mechanisms including replicative stress, direct interaction with DDR components and induction of reactive oxygen species. Given the growth-suppressive consequences of activating the DDR, tumour viruses have also evolved mechanisms to attenuate these pathways. Aberrant expression of viral oncoproteins may therefore promote tumourigenesis through increased somatic mutation and aneuploidy due to DDR inactivation. This review will focus on the interplay between oncogenic viruses and the DDR with respect to cellular checkpoint control and transformation.

At a crossroads: human DNA tumor viruses and the host DNA damage response

Human DNA tumor viruses induce host cell proliferation in order to establish the necessary cellular milieu to replicate viral DNA. The consequence of such viral-programmed induction of proliferation coupled with the introduction of foreign replicating DNA structures makes these viruses particularly sensitive to the host DNA damage response machinery. In fact, sensors of DNA damage are often activated and modulated by DNA tumor viruses in both latent and lytic infection. This article focuses on the role of the DNA damage response during the life cycle of human DNA tumor viruses, with a particular emphasis on recent advances in our understanding of the role of the DNA damage response in EBV, Kaposi's sarcoma-associated herpesvirus and human papillomavirus infection.

Occult hepatitis B virus infection with low viremia induces DNA damage, apoptosis and oxidative stress in peripheral blood lymphocytes

Occult HBV infections (OHBI) are often associated with poor therapeutic response and increased risk of developing hepatocellular carcinoma. Despite a decade of research, OHBI still remains an intricate issue and much is yet to be defined about their possible immune implications. As HBV is known to infect peripheral blood lymphocytes, the present study aimed to explore the molecular mechanisms underlying DNA damage response triggered due to OHBI in host cells. The study was divided into three groups i.e. group A (OHBI patients n=30, viral load Collapse

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MESH Headings

• Adult
• Apoptosis
• Blood Chemical Analysis
• DNA Damage
• DNA Fragmentation
• Hepatitis B/pathology
• Hepatitis B virus/pathogenicity
• Humans
• Lymphocytes/virology
• Membrane Potential, Mitochondrial/physiology
• Middle Aged
• Nucleosomes/metabolism
• Oxidative Stress
• Signal Transduction
• Viremia

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Affiliation(s)

Arpit Bhargava Bhopal Memorial Hospital & Research Centre, Bhopal, India
Saba Khan
Hariom Panwar
Neelam Pathak
Ram P Punde
Subodh Varshney
Pradyumna K Mishra

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Impact of hepatitis B virus X protein on the DNA damage response during hepatocarcinogenesis

Hepatocellular carcinoma (HCC) is one of the most prevalent and lethal cancers worldwide. The main HCC-associated diseases are chronic infections with hepatitis B virus (HBV) and hepatitis C virus (HCV), and HBV-associated HCC is still prevalent in Asia. Many studies have suggested that HBV X protein (HBX), which is the most common ORF integrated into the host genome, plays a crucial role in hepatocarcinogenesis. However, the accumulated evidence regarding HBX-mediated signaling pathways is not concordant, and it is difficult to understand the mechanistic nature of HBX-associated hepatocarcinogenesis. For example, HBX was reported to inactivate the early responses to DNA damage via p53-dependent and -independent pathways by interacting with several DNA damage-binding proteins and was also reported to sensitize cells to p53-mediated apoptosis via ataxia-telangiectasia and Rad3-related (ATR)-dependent signaling. HBX also interferes with the centrosome replication process, resulting in rearrangement of chromosomes with micronuclei. Moreover, HBX was found to sensitize protein kinases such as Ras/Raf/mitogen-activated protein kinase (MAPK), extracellular signal-regulated kinase (ERK), stress-activated protein kinase/NH2-terminal-Jun kinase (SAPK/JNK), protein kinase B (PKB/Akt), and Janus kinase/STAT (JAK/STAT), indicating that a variety of signaling pathways may be activated by HBX. In this review, we focus on the roles of HBX in DNA damage repair during HCC development, with a view to achieving a better understanding of the significance of HBX in the early steps of hepatocarcinogenesis.

Viral manipulation of DNA repair and cell cycle checkpoints

Recognition and repair of DNA damage is critical for maintaining genomic integrity and suppressing tumorigenesis. In eukaryotic cells, the sensing and repair of DNA damage are coordinated with cell cycle progression and checkpoints, in order to prevent the propagation of damaged DNA. The carefully maintained cellular response to DNA damage is challenged by viruses, which produce a large amount of exogenous DNA during infection. Viruses also express proteins that perturb cellular DNA repair and cell cycle pathways, promoting tumorigenesis in their quest for cellular domination. This review presents an overview of strategies employed by viruses to manipulate DNA damage responses and cell cycle checkpoints as they commandeer the cell to maximize their own viral replication. Studies of viruses have identified key cellular regulators and revealed insights into molecular mechanisms governing DNA repair, cell cycle checkpoints, and transformation.