Histone deacetylase inhibitors potentiate vesicular stomatitis virus oncolysis in prostate cancer cells by modulating NF-κB-dependent autophagy.

Vesicular stomatitis virus (VSV) is an oncolytic virus that induces cancer cell death through activation of the apoptotic pathway. Intrinsic resistance to oncolysis is found in some cell lines and many primary tumors as a consequence of residual innate immunity to VSV. In resistant-tumor models, VSV oncolytic potential can be reversibly stimulated by combination with epigenetic modulators, such as the histone deacetylase inhibitor vorinostat. Based on this reversible effect of vorinostat, we reasoned that critical host genes involved in oncolysis may likewise be reversibly regulated by vorinostat. A transcriptome analysis in prostate cancer PC3 cells identified a subset of NF-κB target genes reversibly regulated by vorinostat, as well as a group of interferon (IFN)-stimulated genes (ISGs). Consistent with the induction of NF-κB target genes, vorinostat-mediated enhancement of VSV oncolysis increased hyperacetylation of NF-κB RELA/p65. Additional bioinformatics analysis revealed that NF-κB signaling also increased the expression of several autophagy-related genes. Kinetically, autophagy preceded apoptosis, and apoptosis was observed only when cells were treated with both VSV and vorinostat. VSV replication and cell killing were suppressed when NF-κB signaling was inhibited using pharmacological or genetic approaches. Inhibition of autophagy by 3-methyladenine (3-MA) enhanced expression of ISGs, and either 3-MA treatment or genetic ablation of the autophagic marker Atg5 decreased VSV replication and oncolysis. Together, these data demonstrate that vorinostat stimulates NF-κB activity in a reversible manner via modulation of RELA/p65 signaling, leading to induction of autophagy, suppression of the IFN-mediated response, and subsequent enhancement of VSV replication and apoptosis.

Host restriction factor SAMHD1 limits human T cell leukemia virus type 1 infection of monocytes via STING-mediated apoptosis.

Human T cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T cell leukemia and HTLV-1-associated myelopathies. In addition to T cells, HTLV-1 infects cells of the myeloid lineage, which play critical roles in the host innate response to viral infection. Investigating the monocyte depletion observed during HTLV-1 infection, we discovered that primary human monocytes infected with HTLV-1 undergo abortive infection accompanied by apoptosis dependent on SAMHD1, a host restriction factor that hydrolyzes endogenous dNTPs to below the levels required for productive reverse transcription. Reverse transcription intermediates (RTI) produced in the presence of SAMHD1 induced IRF3-mediated antiviral and apoptotic responses. Viral RTIs complexed with the DNA sensor STING to trigger formation of an IRF3-Bax complex leading to apoptosis. This study provides a mechanistic explanation for abortive HTLV-1 infection of monocytes and reports a link between SAMHD1 restriction, HTLV-1 RTI sensing by STING, and initiation of IRF3-Bax driven apoptosis.

Systems analysis of a RIG-I agonist inducing broad spectrum inhibition of virus infectivity.

The RIG-I like receptor pathway is stimulated during RNA virus infection by interaction between cytosolic RIG-I and viral RNA structures that contain short hairpin dsRNA and 5' triphosphate (5'ppp) terminal structure. In the present study, an RNA agonist of RIG-I was synthesized in vitro and shown to stimulate RIG-I-dependent antiviral responses at concentrations in the picomolar range. In human lung epithelial A549 cells, 5'pppRNA specifically stimulated multiple parameters of the innate antiviral response, including IRF3, IRF7 and STAT1 activation, and induction of inflammatory and interferon stimulated genes - hallmarks of a fully functional antiviral response. Evaluation of the magnitude and duration of gene expression by transcriptional profiling identified a robust, sustained and diversified antiviral and inflammatory response characterized by enhanced pathogen recognition and interferon (IFN) signaling. Bioinformatics analysis further identified a transcriptional signature uniquely induced by 5'pppRNA, and not by IFNα-2b, that included a constellation of IRF7 and NF-kB target genes capable of mobilizing multiple arms of the innate and adaptive immune response. Treatment of primary PBMCs or lung epithelial A549 cells with 5'pppRNA provided significant protection against a spectrum of RNA and DNA viruses. In C57Bl/6 mice, intravenous administration of 5'pppRNA protected animals from a lethal challenge with H1N1 Influenza, reduced virus titers in mouse lungs and protected animals from virus-induced pneumonia. Strikingly, the RIG-I-specific transcriptional response afforded partial protection from influenza challenge, even in the absence of type I interferon signaling. This systems approach provides transcriptional, biochemical, and in vivo analysis of the antiviral efficacy of 5'pppRNA and highlights the therapeutic potential associated with the use of RIG-I agonists as broad spectrum antiviral agents.

BCL-2 inhibitors sensitize therapy-resistant chronic lymphocytic leukemia cells to VSV oncolysis.

Many primary cancers including chronic lymphocytic leukemia (CLL) are resistant to vesicular stomatitis virus (VSV)-induced oncolysis due to overexpression of the antiapoptotic and antiautophagic members of the B-cell lymphoma-2 (BCL-2) family. In the present study, we investigated the mechanisms of CLL cell death induced as a consequence of VSV infection in the presence of BCL-2 inhibitors, obatoclax, and ABT-737 in primary ex vivo CLL patient samples. Microarray analysis of primary CD19⁺ CD5⁺ CLL cells treated with obatoclax and VSV revealed changes in expression of genes regulating apoptosis, the mechanistic target of rapamycin (mTOR) pathway, and cellular metabolism. A combined therapeutic effect was observed for VSV and BCL-2 inhibitors in cells from untreated patients and from patients unresponsive to standard of care therapy. In addition, combination treatment induced several markers of autophagy--LC3-II accumulation, p62 degradation, and staining of autophagic vacuoles. Inhibition of early stage autophagy using 3-methyladenine (3-MA) led to increased apoptosis in CLL samples. Mechanistically, a combination of BCL-2 inhibitors and VSV disrupted inhibitory interactions of Beclin-1 with BCL-2 and myeloid cell leukemia-1 (MCL-1), thus biasing cells toward autophagy. We propose a mechanism in which changes in cellular metabolism, coupled with pharmacologic disruption of the BCL-2-Beclin-1 interactions, facilitate induction of apoptosis and autophagy to mediate the cytolytic effect of VSV.

Linear ubiquitination of NEMO negatively regulates the interferon antiviral response through disruption of the MAVS-TRAF3 complex.

The RIG-I/Mda5 sensors recognize viral intracellular RNA and trigger host antiviral responses. RIG-I signals through the adaptor protein MAVS, which engages various TRAF family members and results in type I interferon (IFNs) and proinflammatory cytokine production via activation of IRFs and NF-κB, respectively. Both the IRF and NF-κB pathways also require the adaptor protein NEMO. We determined that the RIG-I pathway is differentially regulated by the linear ubiquitin assembly complex (LUBAC), which consists of the E3 ligases HOIL-1L, HOIP, and the accessory protein SHARPIN. LUBAC downregulated virus-mediated IFN induction by targeting NEMO for linear ubiquitination. Linear ubiquitinated NEMO associated with TRAF3 and disrupted the MAVS-TRAF3 complex, which inhibited IFN activation while stimulating NF-κB-dependent signaling. In SHARPIN-deficient MEFs, vesicular stomatitis virus replication was decreased due to increased IFN production. Linear ubiquitination thus switches NEMO from a positive to a negative regulator of RIG-I signaling, resulting in an attenuated IFN response.

Modulation of innate immune responses during human T-cell leukemia virus (HTLV-1) pathogenesis.

Infection with the Human T-cell Leukemia virus type I (HTLV-1) retrovirus results in a number of diverse pathologies, including the aggressive, fatal T-cell malignancy adult T-cell leukemia (ATL) and the chronic, progressive neurologic disorder termed HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Worldwide, it is estimated there are 15-20 million HTLV-1-infected individuals; although the majority of HTLV-1-infected individuals remain asymptomatic carriers (AC) during their lifetime, 2-5% of AC develops either ATL or HAM/TSP, but never both. Regardless of asymptomatic status or clinical outcome, HTLV-1 carriers are at high risk of opportunistic infection. The progression to pathological HTLV-1 disease is in part attributed to the failure of the innate and adaptive immune system to control virus spread. The innate immune response against retroviral infection requires recognition of viral pathogen-associated molecular patterns (PAMPs) through pattern-recognition receptors (PRR) dependent pathways, leading to the induction of host antiviral and inflammatory responses. Recent studies have begun to characterize the interplay between HTLV-1 infection and the innate immune response and have identified distinct gene expression profiles in patients with ATL or HAM/TSP--upregulation of growth regulatory pathways in ATL and constitutive activation of antiviral and inflammatory pathways in HAM/STP. In this review, we provide an overview of the replicative lifecycle of HTLV-1 and the distinct pathologies associated with HTLV-1 infection. We also explore the innate immune mechanisms that respond to HTLV-1 infection, the strategies used by HTLV-1 to subvert these defenses and their contribution to HTLV-1-associated diseases.

Orchestrating the interferon antiviral response through the mitochondrial antiviral signaling (MAVS) adapter.

Sensing of RNA virus infection by the RIG-I-like receptors (RLRs) engages a complex signaling cascade that utilizes the mitochondrial antiviral signaling (MAVS) adapter protein to orchestrate the innate host response to pathogen, ultimately leading to the induction of antiviral and inflammatory responses mediated by type I interferon (IFN) and NF-κB pathways. MAVS is localized to the outer mitochondrial membrane, and has been associated with peroxisomes, the endoplasmic reticulum and autophagosomes, where it coordinates signaling events downstream of RLRs. MAVS not only plays a pivotal role in the induction of antiviral and inflammatory pathways but is also involved in the coordination of apoptotic and metabolic functions. This review summarizes recent findings related to the MAVS adapter and its essential role in the innate immune response to RNA viruses.

The viral oncoprotein tax sequesters DNA damage response factors by tethering MDC1 to chromatin.

Infection with human T-cell leukemia virus induces cellular genomic instability mediated through the viral oncoprotein Tax. Here we present evidence that Tax undermines the cellular DNA damage response by sequestration of damage response factors. We show by confocal microscopy that Tax forms damage-independent nuclear foci that contain DNA-PK, BRCA1, and MDC1. Tax sequesters MDC1 to chromatin sites distinct from classic ionizing radiation-induced foci. The recruitment of MDC1 is competitive between the two foci. The N-terminal region of Tax is sufficient for foci localization, and the C-terminal half is critical for binding to MDC1 and recruitment of additional response factors. Tax expression and DNA damage response factor recruitment repressed the formation of ionizing radiation-induced Nbs1-containing foci. The Tax-induced "pseudo" DNA damage response results in phosphorylation and monoubiquitylation of H2AX, which is ablated by siRNA suppression of MDC1. These data support a model for virus-induced genomic instability in which viral oncogene-induced damage-independent foci compete with normal cellular DNA damage response.

HTLV-1 Tax oncoprotein subverts the cellular DNA damage response via binding to DNA-dependent protein kinase.

Human T-cell leukemia virus type-1 is the causative agent for adult T-cell leukemia. Previous research has established that the viral oncoprotein Tax mediates the transformation process by impairing cell cycle control and cellular response to DNA damage. We showed previously that Tax sequesters huChk2 within chromatin and impairs the response to ionizing radiation. Here we demonstrate that DNA-dependent protein kinase (DNA-PK) is a member of the Tax.Chk2 nuclear complex. The catalytic subunit, DNA-PKcs, and the regulatory subunit, Ku70, were present. Tax-containing nuclear extracts showed increased DNA-PK activity, and specific inhibition of DNA-PK prevented Tax-induced activation of Chk2 kinase activity. Expression of Tax induced foci formation and phosphorylation of H2AX. However, Tax-induced constitutive signaling of the DNA-PK pathway impaired cellular response to new damage, as reflected in suppression of ionizing radiation-induced DNA-PK phosphorylation and gammaH2AX stabilization. Tax co-localized with phospho-DNA-PK into nuclear speckles and a nuclear excluded Tax mutant sequestered endogenous phospho-DNA-PK into the cytoplasm, suggesting that Tax interaction with DNA-PK is an initiating event. We also describe a novel interaction between DNA-PK and Chk2 that requires Tax. We propose that Tax binds to and stabilizes a protein complex with DNA-PK and Chk2, resulting in a saturation of DNA-PK-mediated damage repair response.

Human LINE-1 retrotransposon induces DNA damage and apoptosis in cancer cells.

BACKGROUND: Long interspersed nuclear elements (LINEs), Alu and endogenous retroviruses (ERVs) make up some 45% of human DNA. LINE-1 also called L1, is the most common family of non-LTR retrotransposons in the human genome and comprises about 17% of the genome. L1 elements require the integration into chromosomal target sites using L1-encoded endonuclease which creates staggering DNA breaks allowing the newly transposed L1 copies to integrate into the genome. L1 expression and retrotransposition in cancer cells might cause transcriptional deregulation, insertional mutations, DNA breaks, and an increased frequency of recombinations, contributing to genome instability. There is however little evidence on the mechanism of L1-induced genetic instability and its impact on cancer cell growth and proliferation. RESULTS: We report that L1 has genome-destabilizing effects indicated by an accumulation of gamma-H2AX foci, an early response to DNA strand breaks, in association with an abnormal cell cycle progression through a G2/M accumulation and an induction of apoptosis in breast cancer cells. In addition, we found that adjuvant L1 activation may lead to supra-additive killing when combined with radiation by enhancing the radiation lethality through induction of apoptosis that we have detected through Bax activation. CONCLUSION: L1 retrotransposition is sensed as a DNA damaging event through the creation DNA breaks involving L1-encoded endonuclease. The apparent synergistic interaction between L1 activation and radiation can further be utilized for targeted induction of cancer cell death. Thus, the role of retrotransoposons in general, and of L1 in particular, in DNA damage and repair assumes larger significance both for the understanding of mutagenicity and, potentially, for the control of cell proliferation and apoptosis.