Endogenous Retrovirus Activation as a Key Mechanism of Anti-Tumor Immune Response in Radiotherapy

Activating antiviral immune responses potentiates immune checkpoint inhibition in glioblastoma models

Viral mimicry refers to the activation of innate antiviral immune responses due to the induction of endogenous retroelements (REs). Viral mimicry augments antitumor immune responses and sensitizes solid tumors to immunotherapy. Here, we found that targeting what we believe to be a novel, master epigenetic regulator, Zinc Finger Protein 638 (ZNF638), induces viral mimicry in glioblastoma (GBM) preclinical models and potentiates immune checkpoint inhibition (ICI). ZNF638 recruits the HUSH complex, which precipitates repressive H3K9me3 marks on endogenous REs. In GBM, ZNF638 is associated with marked locoregional immunosuppressive transcriptional signatures, reduced endogenous RE expression, and poor immune cell infiltration. Targeting ZNF638 decreased H3K9 trimethylation, increased REs, and activated intracellular dsRNA signaling cascades. Furthermore, ZNF638 knockdown upregulated antiviral immune programs and significantly increased PD-L1 immune checkpoint expression in diverse GBM models. Importantly, targeting ZNF638 sensitized mice to ICI in syngeneic murine orthotopic models through innate IFN signaling. This response was recapitulated in recurrent GBM (rGBM) samples with radiographic responses to checkpoint inhibition with widely increased expression of dsRNA, PD-L1, and perivascular CD8 cell infiltration, suggesting that dsRNA signaling may mediate response to immunotherapy. Finally, low ZNF638 expression was a biomarker of clinical response to ICI and improved survival in patients with rGBM and patients with melanoma. Our findings suggest that ZNF638 could serve as a target to potentiate immunotherapy in gliomas.

Interferon signaling is enhanced by ATR inhibition in glioblastoma cells irradiated with X-rays, protons or carbon ions

BACKGROUND AND PURPOSE

Interferon (IFN) signaling plays an important role in antitumor immune responses. Inhibitors of the DNA damage response, such as ATR inhibitors, can increase IFN signaling upon conventional radiotherapy with X-rays. However, it is not known whether such inhibitors also enhance IFN signaling after irradiation with high linear energy transfer (LET) particles.

MATERIALS AND METHODS

Human glioblastoma U-251 and T98G cells were irradiated with X-rays, protons (LET: 4.8 and 41.9 keV/µm) and carbon ions (LET: 28 and 73 keV/µm), with and without ATR inhibitor (VE-822) or ATM inhibitor (AZD1390). DNA damage signaling and cell cycle distribution were analyzed by immunoblotting and flow cytometry, and radiosensitivity was assessed by clonogenic survival assay. IFN-β secretion was measured by ELISA, and STAT1 activation was examined by immunoblotting.

RESULTS

High-LET protons and carbon ions caused stronger activation of the DNA damage response compared to low-LET protons and X-rays at similar radiation doses. G2 checkpoint arrest was abrogated by the ATR inhibitor and prolonged by the ATM inhibitor after all radiation types. The inhibitors increased radiosensitivity, as measured after X- and carbon ion irradiation. ATR inhibition increased IFN signaling following both low-LET and high-LET irradiation. ATM inhibition also increased IFN signaling, but to a lesser extent. Notably, both cell lines secreted significantly more IFN-β when the inhibitors were combined with high-LET compared to low-LET irradiation.

CONCLUSION

These findings indicate that DNA damage response inhibitors can enhance IFN signaling following X-, proton and carbon ion irradiation, with a strong positive dependency on LET.

Prostate Luminal Cell Plasticity and Cancer

Cellular plasticity in prostate cancer promotes treatment resistance. Several independent studies have used mouse models, single-cell RNA sequencing, and genetic lineage tracing approaches to characterize cellular differentiation and plasticity during prostate organogenesis, homeostasis and androgen-mediated tissue regeneration. We review these findings and recent work using immune-competent genetically-engineered mouse models to characterize cellular plasticity and clonal dynamic changes during prostate cancer progression. Collectively these studies highlight the influence of the tumor microenvironment and the function of epigenetic regulators in promoting cellular plasticity. How the epigenetic alternations that promote cell plasticity affect tumor immunogenicity remains an active area of research with implications for disease treatment.

Multifaceted role of TRIM28 in health and disease

The TRIM (tripartite motif) family, with TRIM28 as a key member, plays a vital role in regulating health and disease. TRIM28 contains various functional domains essential for transcriptional regulation, primarily through its interaction with KRAB-ZNF proteins, which influence chromatin remodeling and gene expression. Despite extensive research, the precise mechanisms by which TRIM28 impacts health and disease remain elusive. This review delves into TRIM28's multifaceted roles in maintaining health, contributing to a variety of diseases, and influencing cancer progression. In cancers, TRIM28 exhibits a dual nature, functioning as both a tumor promoter and suppressor depending on the cellular context and cancer type. The review also explores its critical involvement in processes such as DNA repair, cell cycle regulation, epithelial-to-mesenchymal transition, and the maintenance of stem cell properties. By uncovering TRIM28's complex roles across different cancers and other diseases, this review underscores its potential as a therapeutic target. The significance of TRIM28 as a versatile regulator opens the door to innovative therapeutic strategies, particularly in cancer treatment and the management of other diseases. Ongoing research into TRIM28 may yield key insights into disease progression and novel treatment options.

The Roles of H3K9me3 Writers, Readers, and Erasers in Cancer Immunotherapy

The interplay between cancer and the immune system has captivated researchers for a long time. Recent developments in cancer immunotherapy have substantiated this interest with a significant benefit to cancer patients. Tumor and immune cells are regulated via a wide range of molecular mechanisms involving intricate transcriptional and epigenetic networks. Epigenetic processes influence chromatin structure and accessibility, thus governing gene expression, replication, and DNA damage repair. However, aberrations within epigenetic signatures are frequently observed in cancer. One of the key epigenetic marks is the trimethylation of histone 3 at lysine 9 (H3K9me3), confined mainly within constitutive heterochromatin to suppress DNA accessibility. It is deposited at repetitive elements, centromeric and telomeric loci, as well as at the promoters of various genes. Dysregulated H3K9me3 deposition disrupts multiple pathways, including immune signaling. Consequently, altered H3K9me3 dynamics may modify the efficacy of immunotherapy. Indeed, growing evidence highlights the pivotal roles of various proteins mediating H3K9me3 deposition (SETDB1/2, SUV39H1/2), erasure (KDM3, KDM4 families, KDM7B, LSD1) and interpretation (HP1 proteins, KAP1, CHD4, CDYL, UHRF1) in modulating immunotherapy effectiveness. Here, we review the existing literature to synthesize the available information on the influence of these H3K9me3 writers, erasers, and readers on the response to immunotherapy.

Targeting ZNF638 activates antiviral immune responses and potentiates immune checkpoint inhibition in glioblastoma

Viral mimicry refers to the activation of innate anti-viral immune responses due to the induction of endogenous retroelement (RE) expression. Viral mimicry has been previously described to augment anti-tumor immune responses and sensitize solid tumors to immunotherapy including colorectal cancer, melanoma, and clear renal cell carcinoma. Here, we found that targeting a novel, master epigenetic regulator, Zinc Finger Protein 638 (ZNF638), induces viral mimicry in glioblastoma (GBM) preclinical models and potentiates immune checkpoint inhibition (ICI). ZNF638 recruits the HUSH complex, which precipitates repressive H3K9me3 marks on endogenous REs. In GBM, ZNF638 is associated with marked locoregional immunosuppressive transcriptional signatures, reduced endogenous RE expression and poor immune cell infiltration (CD8 + T-cells, dendritic cells). ZNF638 knockdown decreased H3K9-trimethylation, increased cytosolic dsRNA and activated intracellular dsRNA-signaling cascades (RIG-I, MDA5 and IRF3). Furthermore, ZNF638 knockdown upregulated antiviral immune programs and significantly increased PD-L1 immune checkpoint expression in patient-derived GBM neurospheres and diverse murine models. Importantly, targeting ZNF638 sensitized mice to ICI in syngeneic murine orthotopic models through innate interferon signaling. This response was recapitulated in recurrent GBM (rGBM) samples with radiographic responses to checkpoint inhibition with widely increased expression of dsRNA, PD-L1 and perivascular CD8 cell infiltration, suggesting dsRNA-signaling may mediate response to immunotherapy. Finally, we showed that low ZNF638 expression was a biomarker of clinical response to ICI and improved survival in rGBM patients and melanoma patients. Our findings suggest that ZNF638 could serve as a target to potentiate immunotherapy in gliomas.

Histone demethylase enzymes KDM5A and KDM5B modulate immune response by suppressing transcription of endogenous retroviral elements

Epigenetic factors, including lysine-specific demethylases such as the KDM5 paralogs KDM5A and KDM5B have been implicated in cancer and the regulation of immune responses. Here, we performed a comprehensive multiomic study in cells lacking KDM5A or KDM5B to map changes in transcriptional regulation and chromatin organization. RNA-seq analysis revealed a significant decrease in the expression of Krüppel-associated box containing zinc finger (KRAB-ZNF) genes in KDM5A or KDM5B knockout cell lines, which was accompanied by changes ATAC-seq and H3K4me3 ChIP-seq. Pharmacological inhibition of KDM5A and KDM5B catalytic activity with a pan-KDM5 inhibitor, CPI-455, did not significantly change KRAB-ZNF expression, raising the possibility that regulation of KRAB-ZNF expression does not require KDM5A and KDM5B demethylase activity. KRAB-ZNF are recognized suppressors of the transcription of endogenous retroviruses (ERVs) and HAP1 cells with KDM5A or KDM5B gene inactivation showed elevated ERV expression, increased dsRNA levels and elevated levels of immune response genes. Acute degradation of KDM5A using a dTAG system in HAP1 cells led to increased ERV expression, demonstrating that de-repression of ERV genes occurs rapidly after loss of KDM5A. Co-immunoprecipitation of KDM5A revealed an interaction with the Nucleosome Remodeling and Deacetylase (NuRD) complex suggesting that KDM5A and NuRD may act together to regulate the expression of ERVs through KRAB-ZNFs. These findings reveal roles of KDM5A and KDM5B in modulating ERV expression and underscore the therapeutic potential of using degraders of KDM5A and KDM5B to modulate tumor immune responses.

TRIM28 in cancer and cancer therapy

TRIM28 (tripartite motif protein 28) was initially believed to be a transcription inhibitor that plays an important role in DNA damage repair (DDR) and in maintaining cancer cellular stemness. As research has continued to deepen, several studies have found that TRIM28 not only has ubiquitin E3 ligase activity to promote degradation of substrates, but also can promote SUMOylation of substrates. Although TRIM28 is highly expressed in various cancer tissues and has oncogenic effects, there are still a few studies indicating that TRIM28 has certain anticancer effects. Additionally, TRIM28 is subject to complex upstream regulation. In this review, we have elaborated on the structure and regulation of TRIM28. At the same time, highlighting the functional role of TRIM28 in tumor development and emphasizing its impact on cancer treatment provides a new direction for future clinical antitumor treatment.

Potential Benefits of Combining Proton or Carbon Ion Therapy with DNA Damage Repair Inhibitors

The use of charged particle radiotherapy is currently increasing, but combination therapy with DNA repair inhibitors remains to be exploited in the clinic. The high-linear energy transfer (LET) radiation delivered by charged particles causes clustered DNA damage, which is particularly effective in destroying cancer cells. Whether the DNA damage response to this type of damage is different from that elicited in response to low-LET radiation, and if and how it can be targeted to increase treatment efficacy, is not fully understood. Although several preclinical studies have reported radiosensitizing effects when proton or carbon ion irradiation is combined with inhibitors of, e.g., PARP, ATR, ATM, or DNA-PKcs, further exploration is required to determine the most effective treatments. Here, we examine what is known about repair pathway choice in response to high- versus low-LET irradiation, and we discuss the effects of inhibitors of these pathways when combined with protons and carbon ions. Additionally, we explore the potential effects of DNA repair inhibitors on antitumor immune signaling upon proton and carbon ion irradiation. Due to the reduced effect on healthy tissue and better immune preservation, particle therapy may be particularly well suited for combination with DNA repair inhibitors.

Colorectal cancer-specific IFNβ delivery overcomes dysfunctional dsRNA-mediated type I interferon signaling to increase the abscopal effect of radiotherapy

BACKGROUND

Cancer-intrinsic type I interferon (IFN-I) production triggered by radiotherapy (RT) is mainly dependent on cytosolic double-stranded DNA (dsDNA)-mediated cGAS/STING signaling and increases cancer immunogenicity and enhances the antitumor immune response to increase therapeutic efficacy. However, cGAS/STING deficiency in colorectal cancer (CRC) may suppress the RT-induced antitumor immunity. Therefore, we aimed to evaluate the importance of the dsRNA-mediated antitumor immune response induced by RT in patients with CRC.

METHODS

Cytosolic dsRNA level and its sensors were evaluated via cell-based assays (co-culture assay, confocal microscopy, pharmacological inhibition and immunofluorescent staining) and in vivo experiments. Biopsies and surgical tissues from patients with CRC who received preoperative chemoradiotherapy (neoCRT) were collected for multiplex cytokine assays, immunohistochemical analysis and SNP genotyping. We also generated a cancer-specific adenovirus-associated virus (AAV)-IFNβ1 construct to evaluate its therapeutic efficacy in combination with RT, and the immune profiles were analyzed by flow cytometry and RNA-seq.

RESULTS

Our studies revealed that RT stimulates the autonomous release of dsRNA from cancer cells to activate TLR3-mediated IFN-I signatures to facilitate antitumor immune responses. Patients harboring a dysfunctional TLR3 variant had reduced serum levels of IFN-I-related cytokines and intratumoral CD8+ immune cells and shorter disease-free survival following neoCRT treatment. The engineered cancer-targeted construct AAV-IFNβ1 significantly improved the response to RT, leading to systematic eradication of distant tumors and prolonged survival in defective TLR3 preclinical models.

CONCLUSION

Our results support that increasing cancer-intrinsic IFNβ1 expression is an immunotherapeutic strategy that enhances the RT-induced antitumor immune response in locally patients with advanced CRC with dysfunctional TLR3.

DAMPs and DAMP-sensing receptors in inflammation and diseases

Damage-associated molecular patterns (DAMPs) are endogenous danger molecules produced in cellular damage or stress, and they can activate the innate immune system. DAMPs contain multiple types of molecules, including nucleic acids, proteins, ions, glycans, and metabolites. Although these endogenous molecules do not trigger immune response under steady-state condition, they may undergo changes in distribution, physical or chemical property, or concentration upon cellular damage or stress, and then they become DAMPs that can be sensed by innate immune receptors to induce inflammatory response. Thus, DAMPs play an important role in inflammation and inflammatory diseases. In this review, we summarize the conversion of homeostatic molecules into DAMPs; the diverse nature and classification, cellular origin, and sensing of DAMPs; and their role in inflammation and related diseases. Furthermore, we discuss the clinical strategies to treat DAMP-associated diseases via targeting DAMP-sensing receptors.

Transcription of endogenous retroviruses in senescent cells contributes to the accumulation of double-stranded RNAs that trigger an anti-viral response that reinforces senescence

An important epigenetic switch marks the onset and maintenance of senescence. This allows transcription of the genetic programs that arrest the cell cycle and alter the microenvironment. Transcription of endogenous retroviruses (ERVs) is also a consequence of this epigenetic switch. In this manuscript, we have identified a group of ERVs that are epigenetically silenced in proliferating cells but are upregulated during replicative senescence or during various forms of oncogene-induced senescence, by RAS and Akt, or after HDAC4 depletion. In a HDAC4 model of senescence, removal of the repressive histone mark H3K27me3 is the plausible mechanism that allows the transcription of intergenic ERVs during senescence. We have shown that ERVs contribute to the accumulation of dsRNAs in senescence, which can initiate the antiviral response via the IFIH1-MAVS signaling pathway and thus contribute to the maintenance of senescence. This pathway, and MAVS in particular, plays an active role in shaping the microenvironment and maintaining growth arrest, two essential features of the senescence program.

The entanglement of DNA damage and pattern recognition receptor signaling

Cells are under constant pressure to suppress DNA damage originating from both exogenous and endogenous sources. Cellular responses to DNA damage help to prevent mutagenesis and cell death that arises when DNA damage is either left unrepaired or repaired inaccurately. During the "acute phase" of DNA damage signaling, lesions are recognized, processed, and repaired to restore the primary DNA sequence whilst cell cycle checkpoints delay mitotic progression, cell death and the propagation of errors to daughter cells. Increasingly, there is recognition of a "chronic phase" of DNA damage signaling, exemplified by the secretion of dozens of cytokines days after the inciting damage event. In this review, we focus on the cellular origin of these chronic responses, the molecular pathways that control them and the increasing appreciation for the interconnection between acute and chronic DNA damage responses.

Competition for H2A.Z underlies the developmental impacts of repetitive element de-repression

The histone variant H2A.Z is central to early embryonic development, determining transcriptional competency through chromatin regulation of gene promoters and enhancers. In addition to genic loci, we find that H2A.Z resides at a subset of evolutionarily young repetitive elements, including DNA transposons, long interspersed nuclear elements and long terminal repeats, during early zebrafish development. Moreover, increases in H2A.Z occur when repetitive elements become transcriptionally active. Acquisition of H2A.Z corresponds with a reduction in the levels of the repressive histone modification H3K9me3 and a moderate increase in chromatin accessibility. Notably, however, de-repression of repetitive elements also leads to a significant reduction in H2A.Z over non-repetitive genic loci. Genic loss of H2A.Z is accompanied by transcriptional silencing at adjacent coding sequences, but remarkably, these impacts are mitigated by augmentation of total H2A.Z protein via transgenic overexpression. Our study reveals that levels of H2A.Z protein determine embryonic sensitivity to de-repression of repetitive elements, that repetitive elements can function as a nuclear sink for epigenetic factors and that competition for H2A.Z greatly influences overall transcriptional output during development. These findings uncover general mechanisms in which counteractive biological processes underlie phenotypic outcomes.

Retinoic acid-inducible gene-I like receptor pathway in cancer: modification and treatment

Retinoic acid-inducible gene-I (RIG-I) like receptor (RLR) pathway is one of the most significant pathways supervising aberrant RNA in cells. In predominant conditions, the RLR pathway initiates anti-infection function via activating inflammatory effects, while recently it is discovered to be involved in cancer development as well, acting as a virus-mimicry responder. On one hand, the product IFNs induces tumor elimination. On the other hand, the NF-κB pathway is activated which may lead to tumor progression. Emerging evidence demonstrates that a wide range of modifications are involved in regulating RLR pathways in cancer, which either boost tumor suppression effect or prompt tumor development. This review summarized current epigenetic modulations including DNA methylation, histone modification, and ncRNA interference, as well as post-transcriptional modification like m6A and A-to-I editing of the upstream ligand dsRNA in cancer cells. The post-translational modulations like phosphorylation and ubiquitylation of the pathway's key components were also discussed. Ultimately, we provided an overview of the current therapeutic strategies targeting the RLR pathway in cancers.

Analysis of mitochondrial double-stranded RNAs in human cells

Human mitochondrial genome is transcribed bidirectionally, generating long complementary RNAs that can form double-stranded RNAs (mt-dsRNAs). When released to the cytosol, these mt-dsRNAs can activate antiviral signaling. Here, we present a detailed protocol for the analysis of mt-dsRNA expression. The protocol provides three approaches that can complement one another in examining mt-dsRNAs. While the described protocol is optimized for human cells, this approach can be adapted for use in other animal cell lines and tissue samples. For complete details on the use and execution of this protocol, please refer to Kim et al. (2022).1.

TRIM28 promotes luminal cell plasticity in a mouse model of prostate cancer

The Tripartite motif-containing 28 (TRIM28) transcriptional cofactor is significantly upregulated in high-grade and metastatic prostate cancers. To study the role of TRIM28 in prostate cancer progression in vivo, we generated a genetically-engineered mouse model, combining prostate-specific inactivation of Trp53, Pten and Trim28. Trim28 inactivated NPp53T mice developed an inflammatory response and necrosis in prostate lumens. By conducting single-cell RNA sequencing, we found that NPp53T prostates had fewer luminal cells resembling proximal luminal lineage cells, which are cells with progenitor activity enriched in proximal prostates and prostate invagination tips in wild-type mice with analogous populations in human prostates. However, despite increased apoptosis and reduction of cells expressing proximal luminal cell markers, we found that NPp53T mouse prostates evolved and progressed to invasive prostate carcinoma with a shortened overall survival. Altogether, our findings suggest that TRIM28 promotes expression of proximal luminal cell markers in prostate tumor cells and provides insights into TRIM28 function in prostate tumor plasticity.

Expression of Human Endogenous Retrovirus Group K (HERV-K) HML-2 Correlates with Immune Activation of Macrophages and Type I Interferon Response

Human endogenous retroviruses (HERVs) comprise about 8.3% of the human genome and are capable of producing RNA molecules that can be sensed by pattern recognition receptors, leading to the activation of innate immune response pathways. The HERV-K (HML-2) subgroup is the youngest HERV clade with the highest degree of coding competence. Its expression is associated with inflammation-related diseases. However, the precise HML-2 loci, stimuli, and signaling pathways involved in these associations are not well understood or defined. To elucidate HML-2 expression on a locus-specific level, we used the retroelement sequencing tools TEcount and Telescope to analyze publicly available transcriptome sequencing (RNA-seq) and chromatin immunoprecipitation (ChIP) sequencing data sets of macrophages treated with a wide range of agonists. We found that macrophage polarization significantly correlates with modulation of the expression of specific HML-2 proviral loci. Further analysis demonstrated that the provirus HERV-K102, located in an intergenic region of locus 1q22, constituted the majority of the HML-2 derived transcripts following pro-inflammatory (M1) polarization and was upregulated explicitly in response to interferon gamma (IFN-γ) signaling. We found that signal transducer and activator of transcription 1 and interferon regulatory factor 1 interact with a solo long terminal repeat (LTR) located upstream of HERV-K102, termed LTR12F, following IFN-γ signaling. Using reporter constructs, we demonstrated that LTR12F is critical for HERV-K102 upregulation by IFN-γ. In THP1-derived macrophages, knockdown of HML-2 or knockout of MAVS, an adaptor of RNA-sensing pathways, significantly downregulated genes containing interferon-stimulated response elements (ISREs) in their promoters, suggesting an intermediate role of HERV-K102 in the switch from IFN-γ signaling to the activation of type I interferon expression and, therefore, in a positive feedback loop to enhance pro-inflammatory signaling. IMPORTANCE The human endogenous retrovirus group K subgroup, HML-2, is known to be elevated in a long list of inflammation-associated diseases. However, a clear mechanism for HML-2 upregulation in response to inflammation has not been defined. In this study, we identify a provirus of the HML-2 subgroup, HERV-K102, which is significantly upregulated and constitutes the majority of the HML-2 derived transcripts in response to pro-inflammatory activation of macrophages. Moreover, we identify the mechanism of HERV-K102 upregulation and demonstrate that HML-2 expression enhances interferon-stimulated response element activation. We also demonstrate that this provirus is elevated in vivo and correlates with interferon gamma signaling activity in cutaneous leishmaniasis patients. This study provides key insights into the HML-2 subgroup and suggests that it may participate in enhancing pro-inflammatory signaling in macrophages and probably other immune cells.

The role of tripartite motif-containing 28 in cancer progression and its therapeutic potentials

Tripartite motif-containing 28 (TRIM28) belongs to tripartite motif (TRIM) family. TRIM28 not only binds and degrades its downstream target, but also acts as a transcription co-factor to inhibit gene expression. More and more studies have shown that TRIM28 plays a vital role in tumor genesis and progression. Here, we reviewed the role of TRIM28 in tumor proliferation, migration, invasion and cell death. Moreover, we also summarized the important role of TRIM28 in tumor stemness sustainability and immune regulation. Because of the importance of TRIM28 in tumors, TIRM28 may be a candidate target for anti-tumor therapy and play an important role in tumor diagnosis and treatment in the future.

Human Endogenous Retroviruses in Diseases

Human endogenous retroviruses (HERVs), which are conserved sequences of ancient retroviruses, are widely distributed in the human genome. Although most HERVs have been rendered inactive by evolution, some have continued to exhibit important cytological functions. HERVs in the human genome perform dual functions: on the one hand, they are involved in important physiological processes such as placental development and immune regulation; on the other hand, their aberrant expression is closely associated with the pathological processes of several diseases, such as cancers, autoimmune diseases, and viral infections. HERVs can also regulate a variety of host cellular functions, including the expression of protein-coding genes and regulatory elements that have evolved from HERVs. Here, we present recent research on the roles of HERVs in viral infections and cancers, including the dysregulation of HERVs in various viral infections, HERV-induced epigenetic modifications of histones (such as methylation and acetylation), and the potential mechanisms of HERV-mediated antiviral immunity. We also describe therapies to improve the efficacy of vaccines and medications either by directly or indirectly targeting HERVs, depending on the HERV.

SETDB1 Restrains Endogenous Retrovirus Expression and Antitumor Immunity during Radiotherapy

The type I interferon response plays a pivotal role in promoting antitumor immune activity in response to radiotherapy. The identification of approaches to boost the radiation-induced type I interferon response could help improve the efficacy of radiotherapy. Here we show that the histone methyltransferase SETDB1 is a potent suppressor of radiation-induced endogenous retrovirus expression. SETDB1 inhibition significantly enhanced the efficacy of radiotherapy by promoting radiation-induced viral mimicry to upregulate type I interferons. SETDB1 expression correlated with radiotherapy efficacy in human non-small cell carcinoma and melanoma patients. In a murine tumor model, genetic deletion of Setdb1 significantly enhanced radiotherapy efficacy, and Setdb1-deficient tumors had enhanced intratumoral lymphocyte infiltration, an observation confirmed in human cancer samples. Setdb1 deficiency led to increased basal and radiation-induced endogenous retrovirus (ERV) expression, enhanced MDA5/MAVS signaling, and upregulated type I interferons, which were essential for SETDB1 deficiency-induced radiosensitization. Taken together, these data suggest that inhibition of SETDB1 is a promising approach to enhance cancer radiotherapy efficacy by promoting radiation-induced viral mimicry and antitumor immunity through ERV induction.

SIGNIFICANCE

The identification of the SETDB1-mediated suppression of radiotherapy-induced viral mimicry reveals SETDB1 inhibition as a potential approach to sensitize tumors to radiotherapy by enhancing the type I interferon response.

Ultraviolet light induces HERV expression to activate RIG-I signalling pathway in keratinocytes

Skin inflammation and photosensitivity are common in lupus erythematosus (LE) patients, and ultraviolet (UV) light is a known trigger of skin and possibly systemic inflammation in systemic lupus erythematosus (SLE) and discoid lupus erythematosus (DLE) patients. Type I interferons (IFN) are upregulated in LE skin after UV exposure; however, the mechanisms to explain UVB-induced inflammation remain unclear. Here, we demonstrated that UVB irradiation-induced activation of human endogenous retroviruses (HERVs) plays a major role in the immune response. UVB-induced HERV-associated dsRNA transcription and subsequent activation of the innate antiviral RIG-I/MDA5/IRF7 pathway led to downstream transcription of interferon-stimulated genes, which promotes UVB-induced apoptosis and proliferation inhibition in keratinocytes through RIG-I and MDA5 pathways. Our findings indicate that UVB irradiation induces HERV-dsRNA overexpression, and the dsRNA-sensing innate immunity pathway promotes type I IFN production, which may be a potential mechanism of skin inflammatory response and skin lesion of SLE/DLE.

Influenza A Virus Infection Reactivates Human Endogenous Retroviruses Associated with Modulation of Antiviral Immunity

Human endogenous retrovirus (HERVs), normally silenced by methylation or mutations, can be reactivated by multiple environmental factors, including infections with exogenous viruses. In this work, we investigated the transcriptional activity of HERVs in human A549 cells infected by two wild-type (PR8M, SC35M) and one mutated (SC35MΔNS1) strains of Influenza A virus (IAVs). We found that the majority of differentially expressed HERVs (DEHERVS) and genes (DEGs) were up-regulated in the infected cells, with the most significantly enriched biological processes associated with the genes differentially expressed exclusively in SC35MΔNS1 being linked to the immune system. Most DEHERVs in PR8M and SC35M are mammalian apparent LTR retrotransposons, while in SC35MΔNS1, more HERV loci from the HERVW9 group were differentially expressed. Furthermore, up-regulated pairs of HERVs and genes in close chromosomal proximity to each other tended to be associated with immune responses, which implies that specific HERV groups might have the potential to trigger specific gene networks and influence host immunological pathways.

Endogenous Retroviruses (ERVs): Does RLR (RIG-I-Like Receptors)-MAVS Pathway Directly Control Senescence and Aging as a Consequence of ERV De-Repression?

Bi-directional transcription of Human Endogenous Retroviruses (hERVs) is a common feature of autoimmunity, neurodegeneration and cancer. Higher rates of cancer incidence, neurodegeneration and autoimmunity but a lower prevalence of autoimmune diseases characterize elderly people. Although the re-expression of hERVs is commonly observed in different cellular models of senescence as a result of the loss of their epigenetic transcriptional silencing, the hERVs modulation during aging is more complex, with a peak of activation in the sixties and a decline in the nineties. What is clearly accepted, instead, is the impact of the re-activation of dormant hERV on the maintenance of stemness and tissue self-renewing properties. An innate cellular immunity system, based on the RLR-MAVS circuit, controls the degradation of dsRNAs arising from the transcription of hERV elements, similarly to what happens for the accumulation of cytoplasmic DNA leading to the activation of cGAS/STING pathway. While agonists and inhibitors of the cGAS-STING pathway are considered promising immunomodulatory molecules, the effect of the RLR-MAVS pathway on innate immunity is still largely based on correlations and not on causality. Here we review the most recent evidence regarding the activation of MDA5-RIG1-MAVS pathway as a result of hERV de-repression during aging, immunosenescence, cancer and autoimmunity. We will also deal with the epigenetic mechanisms controlling hERV repression and with the strategies that can be adopted to modulate hERV expression in a therapeutic perspective. Finally, we will discuss if the RLR-MAVS signalling pathway actively modulates physiological and pathological conditions or if it is passively activated by them.

Tumor-targeted nano-delivery system of therapeutic RNA

The birth of RNAi technology has pioneered actionability at the molecular level. Compared to DNA, RNA is less stable and therefore requires more demanding delivery vehicles. With their flexible size, shape, structure, and accessible surface modification, non-viral vectors show great promise for application in RNA delivery. Different non-viral vectors have different ways of binding to RNA. Low immunotoxicity gives RNA significant advantages in tumor treatment. However, the delivery of RNA still has many limitations in vivo. This manuscript summarizes the size-targeting dependence of different organs, followed by a summary of nanovesicles currently in or undergoing clinical trials. It also reviews all RNA delivery systems involved in the current study, including natural, bionic, organic, and inorganic systems. It summarizes the advantages and disadvantages of different delivery methods, which will be helpful for future RNA vehicle design. It is hoped that this will be helpful for gene therapy of clinical tumors.

KAP1/TRIM28: Transcriptional Activator and/or Repressor of Viral and Cellular Programs?

Several transcriptional and epigenetic regulators have been functionally linked to the control of viral and cellular gene expression programs. One such regulator is Krüppel-associated box (KRAB)-associated protein 1 (KAP1: also named TRIM28 or TIF1β), which has been extensively studied in the past three decades. Here we offer an up-to date review of its various functions in a diversity of contexts. We first summarize the discovery of KAP1 repression of endogenous retroviruses during development. We then deliberate evidence in the literature suggesting KAP1 is both an activator and repressor of HIV-1 transcription and discuss experimental differences and limitations of previous studies. Finally, we discuss KAP1 regulation of DNA and RNA viruses, and then expand on KAP1 control of cellular responses and immune functions. While KAP1 positive and negative regulation of viral and cellular transcriptional programs is vastly documented, our mechanistic understanding remains narrow. We thus propose that precision genetic tools to reveal direct KAP1 functions in gene regulation will be required to not only illuminate new biology but also provide the foundation to translate the basic discoveries from the bench to the clinics.

Comprehensive profiling of the TRIpartite motif family to identify pivot genes in hepatocellular carcinoma

INTRODUCTION

TRIpartite motif (TRIM) proteins are important members of the Really Interesting New Gene-finger-containing E3 ubiquitin-conjugating enzyme and are involved in the progression of hepatocellular carcinoma (HCC). However, the diverse expression patterns of TRIMs and their roles in prognosis and immune infiltrates in HCC have yet to be analyzed.

MATERIALS

Combined with previous research, we used an Oncomine database and the Human Protein Atlas to compare TRIM family genes' transcriptional levels between tumor samples and normal liver tissues, as verified by the Gene Expression Profiling Interactive Analysis database. We investigated the patient survival data of TRIMs from the Kaplan-Meier plotter database. Clinicopathologic characteristics associations and potential diagnostic and prognostic values were validated with clinical and expressional data collected from the cancer genome atlas.

RESULTS

We identified TRIM28, TRIM37, TRIM45, and TRIM59 as high-priority members of the TRIMs family that modulates HCC. Low expression of TRIM28 was associated with shorter overall survival (OS) than high expression (log-rank p = 0.009). The same trend was identified for TRIM37 (p = 0.001), TRIM45 (p = 0.013), and TRIM59 (p = 0.011). Multivariate analysis indicated that the level of TRIM37 was a significant independent prognostic factor for both OS (p = 0.043) and progression-free interval (p = 0.044). We performed expression and mutation analysis and functional pathways and tumor immune infiltration analysis of the changes in TRIM factors.

CONCLUSION

These data suggested that TRIM28, TRIM37, TRIM45, and TRIM59 could serve as efficient prognostic biomarkers and therapeutic targets in HCC.

A Systematic Review of Expression and Immunogenicity of Human Endogenous Retroviral Proteins in Cancer and Discussion of Therapeutic Approaches

Human endogenous retroviruses (HERVs) are remnants of ancient retroviral infections that have become fixed in the human genome. While HERV genes are typically silenced in healthy somatic cells, there are numerous reports of HERV transcription and translation across a wide spectrum of cancers, while T and B cell responses against HERV proteins have been detected in cancer patients. This review systematically categorizes the published evidence on the expression of and adaptive immune response against specific HERVs in distinct cancer types. A systematic literature search was performed using Medical Search Headings (MeSH) in the PubMed/Medline database. Papers were included if they described the translational activity of HERVs. We present multiple tables that pair the protein expression of specific HERVs and cancer types with information on the quality of the evidence. We find that HERV-K is the most investigated HERV. HERV-W (syncytin-1) is the second-most investigated, while other HERVs have received less attention. From a therapeutic perspective, HERV-K and HERV-E are the only HERVs with experimental demonstration of effective targeted therapies, but unspecific approaches using antiviral and demethylating agents in combination with chemo- and immunotherapies have also been investigated.

Expression of Endogenous Retroviral RNA in Prostate Tumors has Prognostic Value and Shows Differences among Americans of African Versus European/Middle Eastern Ancestry

Simple Summary

Endogenous retroviruses (ERVs) are viral sequences that have been incorporated into the human genome over millions of years via integrations in germ-line cells. In this study, we investigated whether the expression of ERVs was associated with two different aspects of prostate cancer (PCa). First, Black American men have a higher incidence and poorer outcome of PCa compared to White men. We identified differences in ERV expression among prostate tumors between men of primarily African versus primarily European or Middle Eastern ancestry, which may be associated with differences in the mechanism of cancer progression in patients of these distinct ancestries. Second, we determined whether ERV expression might be correlated with the progression of disease, regardless of ancestry. We identified the ERV expression signatures that correlated with biochemical relapse among PCa patients of all ancestries, indicating that ERVs may be useful for identifying cancer patients at greatest risk of progression. The utility of ERV expression for studying cancer progression may extend to other cancers.

Abstract

Endogenous retroviruses (ERVs) are abundant, repetitive elements dispersed across the human genome and are implicated in various diseases. We investigated two potential roles for ERVs in prostate cancer (PCa). First, the PCa of Black Americans (BA) is diagnosed at an earlier median age and at a more advanced stage than the PCa of White Americans (WA). We used publicly available RNA-seq data from tumor-enriched samples of 27 BA and 65 WA PCa patients in order to identify 12 differentially expressed ERVs (p_adj < 0.1) and used a tissue microarray of the PCa cores from an independent set of BA and WA patients to validate the differential protein expression of one of these ERVs, ERV3-1 (p = 2.829 × 10⁻⁷). Second, we used 57 PCa tumors from patients of all ancestries from one hospital as a training set to identify the ERVs associated with time to biochemical relapse. A 29-ERV prognostic panel was then tested and validated on 35 separate PCa tumors from patients obtained in two different hospitals with a dramatic increase in prognostic power relative to clinical parameters alone (p = 7.4 × 10⁻¹¹). In summary, ERV RNA expression differences in the prostate tumors of patients of different ancestries may be associated with dissimilarities in the mechanism of cancer progression. In addition, the correlation of expression of certain ERVs in prostate tumors with the risk of biochemical relapse indicates a possible role for ERV expression in cancer progression.

DNA Damage-Induced Inflammatory Microenvironment and Adult Stem Cell Response

Adult stem cells ensure tissue homeostasis and regeneration after injury. Due to their longevity and functional requirements, throughout their life stem cells are subject to a significant amount of DNA damage. Genotoxic stress has recently been shown to trigger a cascade of cell- and non-cell autonomous inflammatory signaling pathways, leading to the release of pro-inflammatory factors and an increase in the amount of infiltrating immune cells. In this review, we discuss recent evidence of how DNA damage by affecting the microenvironment of stem cells present in adult tissues and neoplasms can affect their maintenance and long-term function. We first focus on the importance of self-DNA sensing in immunity activation, inflammation and secretion of pro-inflammatory factors mediated by activation of the cGAS-STING pathway, the ZBP1 pathogen sensor, the AIM2 and NLRP3 inflammasomes. Alongside cytosolic DNA, the emerging roles of cytosolic double-stranded RNA and mitochondrial DNA are discussed. The DNA damage response can also initiate mechanisms to limit division of damaged stem/progenitor cells by inducing a permanent state of cell cycle arrest, known as senescence. Persistent DNA damage triggers senescent cells to secrete senescence-associated secretory phenotype (SASP) factors, which can act as strong immune modulators. Altogether these DNA damage-mediated immunomodulatory responses have been shown to affect the homeostasis of tissue-specific stem cells leading to degenerative conditions. Conversely, the release of specific cytokines can also positively impact tissue-specific stem cell plasticity and regeneration in addition to enhancing the activity of cancer stem cells thereby driving tumor progression. Further mechanistic understanding of the DNA damage-induced immunomodulatory response on the stem cell microenvironment might shed light on age-related diseases and cancer, and potentially inform novel treatment strategies.

Endogenous Retroelements and the Viral Mimicry Response in Cancer Therapy and Cellular Homeostasis

Features of the cancer epigenome distinguish cancers from their respective cell of origin and establish therapeutic vulnerabilities that can be exploited through pharmacologic inhibition of DNA- or histone-modifying enzymes. Epigenetic therapies converge with cancer immunotherapies through "viral mimicry," a cellular state of active antiviral response triggered by endogenous nucleic acids often derived from aberrantly transcribed endogenous retrotransposons. This review describes the initial characterization and expansion of viral mimicry-inducing approaches as well as features that "prime" cancers for viral mimicry induction. Increased understanding of viral mimicry in therapeutic contexts suggests potential physiologic roles in cellular homeostasis. SIGNIFICANCE: Recent literature establishes elevated cytosolic double strand RNA (dsRNA) levels as a cancer-specific therapeutic vulnerability that can be elevated by viral mimicry-inducing therapies beyond tolerable thresholds to induce antiviral signaling and increase dependence on dsRNA stress responses mediated by ADAR1. Improved understanding of viral mimicry signaling and tolerance mechanisms reveals synergistic treatment combinations with epigenetic therapies that include inhibition of BCL2, ADAR1, and immune checkpoint blockade. Further characterization of viral mimicry tolerance may identify contexts that maximize efficacy of conventional cancer therapies.

Abscopal Effect and Drug-Induced Xenogenization: A Strategic Alliance in Cancer Treatment?

The current state of cancer treatment is still far from being satisfactory considering the strong impairment of patients' quality of life and the high lethality of malignant diseases. Therefore, it is critical for innovative approaches to be tested in the near future. In view of the crucial role that is played by tumor immunity, the present review provides essential information on the immune-mediated effects potentially generated by the interplay between ionizing radiation and cytotoxic antitumor agents when interacting with target malignant cells. Therefore, the radiation-dependent abscopal effect (i.e., a biological effect of ionizing radiation that occurs outside the irradiated field), the influence of cancer chemotherapy on the antigenic pattern of target neoplastic cells, and the immunogenic cell death (ICD) caused by anticancer agents are the main topics of this presentation. It is widely accepted that tumor immunity plays a fundamental role in generating an abscopal effect and that anticancer drugs can profoundly influence not only the host immune responses, but also the immunogenic pattern of malignant cells. Remarkably, several anticancer drugs impact both the abscopal effect and ICD. In addition, certain classes of anticancer agents are able to amplify already expressed tumor-associated antigens (TAA). More importantly, other drugs, especially triazenes, induce the appearance of new tumor neoantigens (TNA), a phenomenon that we termed drug-induced xenogenization (DIX). The adoption of the abscopal effect is proposed as a potential therapeutic modality when properly applied concomitantly with drug-induced increase in tumor cell immunogenicity and ICD. Although little to no preclinical or clinical studies are presently available on this subject, we discuss this issue in terms of potential mechanisms and therapeutic benefits. Upcoming investigations are aimed at evaluating how chemical anticancer drugs, radiation, and immunotherapies are interacting and cooperate in evoking the abscopal effect, tumor xenogenization and ICD, paving the way for new and possibly successful approaches in cancer therapy.

Non-canonical roles of apoptotic and DNA double-strand break repair factors in mediating cellular response to ionizing radiation

PURPOSE

To provide an updated summary of recent advances in our understanding of the non-canonical roles of apoptotic and DNA double-strand break repair factors in various biological processes, especially in the cellular response to radiotherapy.

CONCLUSION

Apoptotic caspases are usually considered as "executioners'' of unwanted or damaged cells or tissues. However, recent studies indicated they play multiple additional, often counterintuitive roles in many biological processes. Similarly, DNA double-strand break (DSB) repair factors were also found to play unexpected roles beyond repairing damaged DNA. In this review, I will summarize key findings on the non-canonical roles of apoptotic and DSB repair factors in disparate biological and pathological processes such as radiation-induced genetic instability and carcinogenesis, wound healing and tissue regeneration, induced pluripotent stem cell induction, spontaneous and stochastic generation of cancer stem cells, and cancer immunotherapy. I believe these findings will usher in more studies in this exciting and rapidly evolving field.

Switching Sides: How Endogenous Retroviruses Protect Us from Viral Infections

Long disregarded as junk DNA or genomic dark matter, endogenous retroviruses (ERVs) have turned out to represent important components of the antiviral immune response. These remnants of once-infectious retroviruses not only regulate cellular immune activation, but may even directly target invading viral pathogens. In this Gem, we summarize mechanisms by which retroviral fossils protect us from viral infections. One focus will be on recent advances in the role of ERVs as regulators of antiviral gene expression.

Endogenous retroviruses in the origins and treatment of cancer

Endogenous retroviruses (ERVs) are emerging as promising therapeutic targets in cancer. As remnants of ancient retroviral infections, ERV-derived regulatory elements coordinate expression from gene networks, including those underpinning embryogenesis and immune cell function. ERV activation can promote an interferon response, a phenomenon termed viral mimicry. Although ERV expression is associated with cancer, and provisionally with autoimmune and neurodegenerative diseases, ERV-mediated inflammation is being explored as a way to sensitize tumors to immunotherapy. Here we review ERV co-option in development and innate immunity, the aberrant contribution of ERVs to tumorigenesis, and the wider biomedical potential of therapies directed at ERVs.

Expanding roles of cell cycle checkpoint inhibitors in radiation oncology

PURPOSE

Radiation-induced activation of cell cycle checkpoints have been of long-standing interest. The WEE1, CHK1 and ATR kinases are key factors in cell cycle checkpoint regulation and are essential for the S and G2 checkpoints. Here, we review the rationale for why inhibitors of WEE1, CHK1 and ATR could be beneficial in combination with radiation.

CONCLUSIONS

Combined treatment with radiation and inhibitors of these kinases results in checkpoint abrogation and subsequent mitotic catastrophe. This might selectively radiosensitize tumor cells, as they often lack the p53-dependent G1 checkpoint and therefore rely more on the G2 checkpoint to repair DNA damage. Further affecting the repair of radiation damage, inhibition of WEE1, CHK1 or ATR also specifically suppresses the homologous recombination repair pathway. Moreover, inhibition of these kinases can induce massive replication stress during S phase of the cell cycle, likely contributing to eliminate radioresistant S phase cells. Intriguingly, recent findings suggest that cell cycle checkpoint inhibitors in combination with radiation can also enhance anti-tumor immune effects. Altogether, the expanding knowledge about the functional roles of WEE1, CHK1 and ATR inhibitors support that they are promising candidates for use in combination with radiation treatment.

Prolonged cetuximab treatment promotes p27Kip1-mediated G1 arrest and autophagy in head and neck squamous cell carcinoma

Cetuximab, an anti-epidermal growth factor receptor (EGFR) monoclonal antibody, is an efficient anti-tumor therapeutic agent that inhibits the activation of EGFR; however, data related to the cellular effects of prolonged cetuximab treatment are limited. In this study, the long-term cellular outcome of prolonged cetuximab treatment and the related molecular mechanism were explored in a head and neck squamous cell carcinoma cell line constitutively expressing a fluorescent ubiquitination-based cell cycle indicator. Fluorescent time-lapse imaging was used to assess clonal growth, cell motility, and cell-cycle progression. Western blot analysis was performed to measure the level of phosphorylation and protein-expression following cetuximab treatment. Over 5 days cetuximab treatment decreased cell motility and enhanced G1 phase cell arrest in the central region of the colonies. Significantly decreased phosphorylation of retinoblastoma, Skp2, and Akt-mTOR proteins, accumulation of p27^Kip1, and induction of type II LC3B were observed over 8 days cetuximab treatment. Results of the present study elucidate the cetuximab-dependent inhibition of cell migration, resulting in high cell density-related stress and persistent cell-cycle arrest at G1 phase culminating in autophagy. These findings provide novel molecular insights related to the anti-tumor effects of prolonged cetuximab treatment with the potential to improve future therapeutic strategy.

ATM inhibition enhances cancer immunotherapy by promoting mtDNA leakage and cGAS/STING activation

Novel approaches are needed to boost the efficacy of immune checkpoint blockade (ICB) therapy. Ataxia telangiectasia mutated (ATM) protein plays a central role in sensing DNA double-stranded breaks (DSBs) and coordinating their repair. Recent data indicated that ATM might be a promising target to enhance ICB therapy. However, the molecular mechanism involved has not been clearly elucidated. Here, we show that ATM inhibition could potentiate ICB therapy by promoting cytoplasmic leakage of mitochondrial DNA (mtDNA) and activation of the cGAS/STING pathway. We show that genetic depletion of ATM in murine cancer cells delayed tumor growth in syngeneic mouse hosts in a T cell-dependent manner. Furthermore, chemical inhibition of ATM potentiated anti-PD-1 therapy of mouse tumors. ATM inhibition potently activated the cGAS/STING pathway and enhanced lymphocyte infiltration into the tumor microenvironment by downregulating mitochondrial transcription factor A (TFAM), which led to mtDNA leakage into the cytoplasm. Moreover, our analysis of data from a large patient cohort indicated that ATM mutations, especially nonsense mutations, predicted for clinical benefits of ICB therapy. Our study therefore provides strong evidence that ATM may serve as both a therapeutic target and a biomarker to enable ICB therapy.

Signaling Through Nucleic Acid Sensors and Their Roles in Inflammatory Diseases

Recognition of pathogen-derived nucleic acids by pattern-recognition receptors (PRRs) is essential for eliciting antiviral immune responses by inducing the production of type I interferons (IFNs) and proinflammatory cytokines. Such responses are a prerequisite for mounting innate and pathogen-specific adaptive immune responses. However, host cells also use nucleic acids as carriers of genetic information, and the aberrant recognition of self-nucleic acids by PRRs is associated with the onset of autoimmune or autoinflammatory diseases. In this review, we describe the mechanisms of nucleic acid sensing by PRRs, including Toll-like receptors, RIG-I-like receptors, and DNA sensor molecules, and their signaling pathways as well as the disorders caused by uncontrolled or unnecessary activation of these PRRs.

The Impact of Radiation-Induced DNA Damage on cGAS-STING-Mediated Immune Responses to Cancer

Radiotherapy is a major modality used to combat a wide range of cancers. Classical radiobiology principles categorize ionizing radiation (IR) as a direct cytocidal therapeutic agent against cancer; however, there is an emerging appreciation for additional antitumor immune responses generated by this modality. A more nuanced understanding of the immunological pathways induced by radiation could inform optimal therapeutic combinations to harness radiation-induced antitumor immunity and improve treatment outcomes of cancers refractory to current radiotherapy regimens. Here, we summarize how radiation-induced DNA damage leads to the activation of a cytosolic DNA sensing pathway mediated by cyclic GMP-AMP (cGAMP) synthase (cGAS) and stimulator of interferon genes (STING). The activation of cGAS-STING initiates innate immune signaling that facilitates adaptive immune responses to destroy cancer. In this way, cGAS-STING signaling bridges the DNA damaging capacity of IR with the activation of CD8+ cytotoxic T cell-mediated destruction of cancer-highlighting a molecular pathway radiotherapy can exploit to induce antitumor immune responses. In the context of radiotherapy, we further report on factors that enhance or inhibit cGAS-STING signaling, deleterious effects associated with cGAS-STING activation, and promising therapeutic candidates being investigated in combination with IR to bolster immune activation through engaging STING-signaling. A clearer understanding of how IR activates cGAS-STING signaling will inform immune-based treatment strategies to maximize the antitumor efficacy of radiotherapy, improving therapeutic outcomes.

Melanoma Stem Cell-Like Phenotype and Significant Suppression of Immune Response within a Tumor Are Regulated by TRIM28 Protein

TRIM28 emerged as a guard of the intrinsic "state of cell differentiation", facilitating self-renewal of pluripotent stem cells. Recent reports imply TRIM28 engagement in cancer stem cell (CSC) maintenance, although the exact mechanism remains unresolved. TRIM28 high expression is associated with worse melanoma patient outcomes. Here, we investigated the association between TRIM28 level and melanoma stemness, and aligned it with the antitumor immune response to find the mechanism of "stemness high/immune low" melanoma phenotype acquisition. Based on the SKCM TCGA data, the TRIM28 expression profile, clinicopathological features, expression of correlated genes, and the level of stemness and immune scores were analyzed in patient samples. The biological function for differentially expressed genes was annotated with GSEA. Results were validated with additional datasets from R2: Genomics Analysis and Visualization Platform and in vitro with a panel of seven melanoma cell lines. All statistical analyses were accomplished using GraphPad Prism 8. TRIM28HIGH-expressing melanoma patients are characterized by worse outcomes and significantly different gene expression profiles than the TRIM28NORM cohort. TRIM28 high level related to higher melanoma stemness as measured with several distinct scores and TRIM28HIGH-expressing melanoma cell lines possess the greater potential of melanosphere formation. Moreover, TRIM28HIGH melanoma tumors were significantly depleted with infiltrating immune cells, especially cytotoxic T cells, helper T cells, and B cells. Furthermore, TRIM28 emerged as a good predictor of "stemness high/immune low" melanoma phenotype. Our data indicate that TRIM28 might facilitate this phenotype by direct repression of interferon signaling. TRIM28 emerged as a direct link between stem cell-like phenotype and attenuated antitumor immune response in melanoma, although further studies are needed to evaluate the direct mechanism of TRIM28-mediated stem-like phenotype acquisition.

Contribution of Human Retroviruses to Disease Development-A Focus on the HIV- and HERV-Cancer Relationships and Treatment Strategies

Animal retroviruses are known for their transforming potential, and this is also true for the ones hosted by humans, which have gathered expanding attention as one of the potent causative agents in various disease, including specific cancer types. For instance, Human T Lymphotropic virus (HTLV) is a well-studied class of oncoviruses causing T cell leukemia, while human immunodeficiency virus (HIV) leads to acquired immunodeficiency syndrome (AIDS), which is linked to a series of defining cancers including Kaposi sarcoma, certain types of non-Hodgkin lymphoma, and cervical cancer. Of note, in addition to these "modern" exogenous retroviruses, our genome harbors a staggering number of human endogenous retroviruses (HERVs). HERVs are the genetic remnants of ancient retroviral germline infection of human ancestors and are typically silenced in normal tissues due to inactivating mutations and sequence loss. While some HERV elements have been appropriated and contribute to human physiological functions, others can be reactivated through epigenetic dysregulations to express retroviral elements and promote carcinogenesis. Conversely, HERV replication intermediates or protein products can also serve as intrinsic pathogen-associated molecular patterns that cause the immune system to interpret it as an exogenous infection, thereby stimulating immune responses against tumors. As such, HERVs have also been targeted as a potential internal strategy to sensitize tumor cells for promising immunotherapies. In this review, we discuss the dynamic role of human retroviruses in cancer development, focusing on HIV and HERVs contribution. We also describe potential treatment strategies, including immunotherapeutic targeting of HERVs, inhibiting DNA methylation to expose HERV signatures, and the use of antiretroviral drugs against HIV and HERVs, which can be employed as prospective anti-cancer modalities.