The arginine methyltransferase PRMT5 regulates CIITA-dependent MHC II transcription

PRMT5 inhibition induces pro-inflammatory macrophage polarization and increased hepatic triglyceride levels without affecting atherosclerosis in mice

Protein arginine methyltransferase 5 (PRMT5) controls inflammation and metabolism through modulation of histone methylation and gene transcription. Given the important role of inflammation and metabolism in atherosclerotic cardiovascular disease, here we examined the role of PRMT5 in atherosclerosis using the specific PRMT5 inhibitor GSK3326595. Cultured thioglycollate-elicited peritoneal macrophages were exposed to GSK3326595 or DMSO control and stimulated with either 1 ng/mL LPS or 100 ng/mL interferon-gamma for 24 h. Furthermore, male low-density lipoprotein (LDL) receptor knockout mice were fed an atherogenic Western-type diet and injected intraperitoneally 3×/week with a low dose of 5 mg/kg GSK3326595 or solvent control for 9 weeks. In vitro, GSK3326595 primed peritoneal macrophages to interferon-gamma-induced M1 polarization, as evidenced by an increased M1/M2 gene marker ratio. In contrast, no difference was found in the protein expression of iNOS (M1 marker) and ARG1 (M2 marker) in peritoneal macrophages of GSK3326595-treated mice. Also no change in the T cell activation state or the susceptibility to atherosclerosis was detected. However, chronic GSK3326595 treatment did activate genes involved in hepatic fatty acid acquisition, i.e. SREBF1, FASN, and CD36 (+59%, +124%, and +67%, respectively; p < 0.05) and significantly increased hepatic triglyceride levels (+50%; p < 0.05). PRMT5 inhibition by low-dose GSK3326595 treatment does not affect the inflammatory state or atherosclerosis susceptibility of Western-type diet-fed LDL receptor knockout mice, while it induces hepatic triglyceride accumulation. Severe side effects in liver, i.e. development of non-alcoholic fatty liver disease, should thus be taken into account upon chronic treatment with this PRMT5 inhibitor.

MTAP deficiency contributes to immune landscape remodelling and tumour evasion

Methylthioadenosine phosphorylase (MTAP) deficiency occurs in various malignancies and is associated with poor survival in cancer patients. However, the mechanisms underlying tumour progression due to MTAP loss are yet to be elucidated. Utilizing integrated analyses of the transcriptome, proteome and secretome, we demonstrated that MTAP deficiency alters tumour-intrinsic, immune-related pathways and reprograms cytokine profiles towards a tumour-favourable environment. Additionally, MTAP-knockout cells exhibited a marked increase in the immune checkpoint protein PD-L1. Upon co-culturing primary T cells with cancer cells, MTAP loss-mediated PD-L1 upregulation inhibited T cell-mediated killing activity and induced several T cell exhaustion markers. In two xenograft tumour models, we showed a modest increase in average volume of tumours derived from MTAP-deficient cells than that of MTAP-proficient tumours. Surprisingly, a remarkable increase in tumour size was observed in humanized mice bearing MTAP-deficient tumours, as compared to their MTAP-expressing counterparts. Following immunophenotypic characterization of tumour-infiltrating leukocytes by mass cytometry analysis, MTAP-deficient tumours were found to display decreased immune infiltrates with lower proportions of both T lymphocytes and natural killer cells and higher proportions of immunosuppressive cells as compared to MTAP-expressing tumour xenografts. Taken together, our results suggest that MTAP deficiency restructures the tumour immune microenvironment, promoting tumour progression and immune evasion.

Targeting epigenetic regulators for inflammation: Mechanisms and intervention therapy

Emerging evidence indicates that resolution of inflammation is a critical and dynamic endogenous process for host tissues defending against external invasive pathogens or internal tissue injury. It has long been known that autoimmune diseases and chronic inflammatory disorders are characterized by dysregulated immune responses, leading to excessive and uncontrol tissue inflammation. The dysregulation of epigenetic alterations including DNA methylation, posttranslational modifications to histone proteins, and noncoding RNA expression has been implicated in a host of inflammatory disorders and the immune system. The inflammatory response is considered as a critical trigger of epigenetic alterations that in turn intercede inflammatory actions. Thus, understanding the molecular mechanism that dictates the outcome of targeting epigenetic regulators for inflammatory disease is required for inflammation resolution. In this article, we elucidate the critical role of the nuclear factor-κB signaling pathway, JAK/STAT signaling pathway, and the NLRP3 inflammasome in chronic inflammatory diseases. And we formulate the relationship between inflammation, coronavirus disease 2019, and human cancers. Additionally, we review the mechanism of epigenetic modifications involved in inflammation and innate immune cells. All that matters is that we propose and discuss the rejuvenation potential of interventions that target epigenetic regulators and regulatory mechanisms for chronic inflammation-associated diseases to improve therapeutic outcomes.

Protein Arginine Methylation: An Emerging Modification in Cancer Immunity and Immunotherapy

In recent years, protein arginine methyltransferases (PRMTs) have emerged as new members of a gene expression regulator family in eukaryotes, and are associated with cancer pathogenesis and progression. Cancer immunotherapy has significantly improved cancer treatment in terms of overall survival and quality of life. Protein arginine methylation is an epigenetic modification function not only in transcription, RNA processing, and signal transduction cascades, but also in many cancer-immunity cycle processes. Arginine methylation is involved in the activation of anti-cancer immunity and the regulation of immunotherapy efficacy. In this review, we summarize the most up-to-date information on regulatory molecular mechanisms and different underlying arginine methylation signaling pathways in innate and adaptive immune responses during cancer. We also outline the potential of PRMT-inhibitors as effective combinatorial treatments with immunotherapy.

Protein arginine methyltransferase 5 mediates THP-1-derived macrophage activation dependent on NF-κB in endometriosis

Macrophage-induced inflammation is a major factor in the pathogenesis of endometriosis. The underlying mechanisms, however, remain largely unknown. TNF-α, IL-6, IL-10 and C-C motif chemokine 20 (CCL20) levels in endometrial extracts were determined using Luminex cytokine kits. Additionally, protein arginine methyltransferase 5 (PRMT5) levels were measured using reverse transcription-quantitative PCR and western blotting. IL-6 and IP-10 levels in cells were measured using ELISA kits. In the present study, it was revealed that PRMT5 expression at both the mRNA and protein levels in THP-1-derived macrophages was significantly decreased following treatment with serum or extracts of endometrium from patients with endometriosis in the presence of lipopolysaccharide, compared with that in control cells, suggesting a possible role for macrophage-derived PRMT5 in mediating the interaction between macrophages and endometrium in endometriosis. Mechanistically, macrophage PRMT5 expression was regulated in an NF-κB-dependent and Smad2/3-independent manner, indicating that PRMT5 is a downstream target of NF-κB. Importantly, macrophage-derived PRMT5 was required for macrophage activation in endometriosis, as evidenced by the PRMT5-dependent secretion of IL-6 and IFN-γ-induced protein 10 from THP-1-derived macrophages. The present study identified NF-κB-dependent PRMT5 as a novel regulator of macrophage activation in endometriosis. Targeting PRMT5 in macrophages may be a potential therapeutic strategy against endometriosis.

PRMT5: a putative oncogene and therapeutic target in prostate cancer

Protein arginine methyltransferase 5 (PRMT5) was discovered two decades ago. The first decade focused on the biochemical characterization of PRMT5 as a regulator of many cellular processes in a healthy organism. However, over the past decade, evidence has accumulated to suggest that PRMT5 may function as an oncogene in multiple cancers via both epigenetic and non-epigenetic mechanisms. In this review, we focus on recent progress made in prostate cancer, including the role of PRMT5 in the androgen receptor (AR) expression and signaling and DNA damage response, particularly DNA double-strand break repair. We also discuss how PRMT5-interacting proteins that are considered PRMT5 cofactors may cooperate with PRMT5 to regulate PRMT5 activity and target gene expression, and how PRMT5 can interact with other epigenetic regulators implicated in prostate cancer development and progression. Finally, we suggest that targeting PRMT5 may be employed to develop multiple therapeutic approaches to enhance the treatment of prostate cancer.

Protein Arginine Methyltransferase 5 Promotes pICln-Dependent Androgen Receptor Transcription in Castration-Resistant Prostate Cancer

The majority of advanced prostate cancer therapies aim to inhibit androgen receptor (AR) signaling. However, AR reactivation inevitably drives disease progression to castration-resistant prostate cancer (CRPC). Here we demonstrate that protein arginine methyltransferase 5 (PRMT5) functions as an epigenetic activator of AR transcription in CRPC, requiring cooperation with a methylosome subunit pICln. In vitro and in xenograft tumors in mice, targeting PRMT5 or pICln suppressed growth of CRPC cells. Full-length AR and AR-V7 transcription activation required both PRMT5 and pICln but not MEP50. This activation of transcription was accompanied by PRMT5-mediated symmetric dimethylation of H4R3 at the proximal AR promoter. Further, knockdown of PRMT5 abolished the binding of pICln (but not vice versa) to the AR proximal promoter region, suggesting that PRMT5 recruits pICln to the AR promoter to activate AR transcription. Differential gene expression analysis in 22Rv1 cells confirmed that PRMT5 and pICln both regulate the androgen signaling pathway. In addition, PRMT5 and pICln protein expression positively correlated with AR and AR-V7 protein expression in CRPC tissues and their expression was highly correlated at the mRNA level across multiple publicly available CRPC datasets. Our results suggest that targeting PRMT5 or pICln may be explored as a novel therapy for CRPC treatment by suppressing expression of AR and AR splice variants to circumvent AR reactivation. SIGNIFICANCE: This study provides evidence that targeting PRMT5 can eliminate expression of AR and can be explored as a novel therapeutic approach to treat metastatic hormone-naïve and castration-resistant prostate cancer.

PRMT2 accelerates tumorigenesis of hepatocellular carcinoma by activating Bcl2 via histone H3R8 methylation

Protein arginine methyltransferases (PRMTs) have been implicated in the development of various cancers. PRMT2, a member of the type I PRMT family, is overexpressed in multiple tumors. However, the expression and role of PRMT2 in hepatocellular carcinoma (HCC) have not been studied. Here, we discovered that PRMT2 expression is elevated in HCC tissues compared to the corresponding non-tumor tissues, and PRMT2 overexpression is an independent predictor of poor prognosis in HCC patients. Depletion of PRMT2 in HCC cell lines inhibited their cell growth and induced apoptosis. Mechanistic investigations showed that PRMT2 is responsible for H3R8 asymmetric methylation (H3R8me2a). H3R8me2a enrichment at the Bcl2 promoter increases its accessibility to STAT3, promoting Bcl2 gene expression. In addition, our results confirmed that the catalytically inactive mutant of PRMT2 or the type I PRMT inhibitor MS023 impaired the pro-tumorigenic functions of PRMT2 in HCC cells. Overall, our findings showed that PRMT2 functions as an oncogenic gene in HCC, revealing its potential as a novel therapeutic target in HCC.

PRMT5-dependent transcriptional repression of c-Myc target genes promotes gastric cancer progression

The proto-oncogene c-Myc regulates multiple biological processes mainly through selectively activating gene expression. However, the mechanisms underlying c-Myc-mediated gene repression in the context of cancer remain less clear. This study aimed to clarify the role of PRMT5 in the transcriptional repression of c-Myc target genes in gastric cancer.

Methods: Immunohistochemistry was used to evaluate the expression of PRMT5, c-Myc and target genes in gastric cancer patients. PRMT5 and c-Myc interaction was assessed by immunofluorescence, co-immunoprecipitation and GST pull-down assays. Bioinformatics analysis, immunoblotting, real-time PCR, chromatin immunoprecipitation, and rescue experiments were used to evaluate the mechanism.

Results: We found that c-Myc directly interacts with protein arginine methyltransferase 5 (PRMT5) to transcriptionally repress the expression of a cohort of genes, including PTEN, CDKN2C (p18^INK4C), CDKN1A (p21^CIP1/WAF1), CDKN1C (p57^KIP2) and p63, to promote gastric cancer cell growth. Specifically, we found that PRMT5 was required to promote gastric cancer cell growth in vitro and in vivo, and for transcriptional repression of this cohort of genes, which was dependent on its methyltransferase activity. Consistently, the promoters of this gene cohort were enriched for both PRMT5-mediated symmetric di-methylation of histone H4 on Arg 3 (H4R3me2s) and c-Myc, and c-Myc depletion also upregulated their expression. H4R3me2s also colocalized with the c-Myc-binding E-box motif (CANNTG) on these genes. We show that PRMT5 directly binds to c-Myc, and this binding is required for transcriptional repression of the target genes. Both c-Myc and PRMT5 expression levels were upregulated in primary human gastric cancer tissues, and their expression levels inversely correlated with clinical outcomes.

Conclusions: Taken together, our study reveals a novel mechanism by which PRMT5-dependent transcriptional repression of c-Myc target genes is required for gastric cancer progression, and provides a potential new strategy for therapeutic targeting of gastric cancer.

Forkhead transcription factor FOXO3a mediates interferon-γ-induced MHC II transcription in macrophages

Macrophages are professional antigen-presenting cells relying on the expression of class II major histocompatibility complex (MHC II) genes. Interferon-γ (IFN-γ) activates MHC II transcription via the assembly of an enhanceosome centred on class II trans-activator (CIITA). In the present study, we investigated the role of the forkhead transcription factor FOXO3a in IFN- γ-induced MHC II transcription in macrophages. Knockdown of FOXO3a, but not FOXO1 or FOXO4, diminished IFN-γ-induced MHC II expression in RAW cells. On the contrary, over-expression of FOXO3a, but neither FOXO1 nor FOXO4, enhanced CIITA-mediated trans-activation of the MHC II promoter. IFN-γ treatment promoted the recruitment of FOXO3a to the MHC II promoter. Co-immunoprecipitation and RE-ChIP assays showed that FOXO3a was a component of the MHC II enhanceosome forming interactions with CIITA, RFX5, RFXB and RFXAP. FOXO3a contributed to MHC II transcription by altering histone modifications surrounding the MHC II promoter. Of interest, FOXO3a was recruited to the type IV CIITA promoter and directly activated CIITA transcription by interacting with signal transducer of activation and transcription 1 in response to IFN-γ stimulation. In conclusion, our data unveil a novel role for FOXO3a in the regulation of MHC II transcription in macrophages.

MKL1 mediates TNF-α induced pro-inflammatory transcription by bridging the crosstalk between BRG1 and WDR5

Tumor necrosis factor alpha (TNF-α) is a cytokine that can potently stimulate the synthesis of a range of pro-inflammatory mediators in macrophages. The underlying epigenetic mechanism, however, is underexplored. Here we report that the transcriptional modulator megakaryocytic leukemia 1 (MKL1) is associated with a histone H3K4 methyltransferase activity. Re-ChIP assay suggests that MKL1 interacts with and recruits WDR5, a component of the COMPASS complex responsible for H3K4 methylation, to the promoter regions of pro-inflammatory genes in macrophages treated with TNF-α. WDR5 enhances the ability of MKL1 to stimulate the promoter activities of pro-inflammatory genes. In contrast, silencing of WDR5 attenuates TNF-α induced production of pro-inflammatory mediators and erases the H3K4 methylation from the gene promoters. Of interest, the chromatin remodeling protein BRG1 also plays an essential role in maintaining H3K4 methylation on MKL1 target promoters by interacting with WDR5. MKL1 knockdown disrupts the interaction between BRG1 and WDR5. Together, our data illustrate a role for MKL1 in moderating the crosstalk between BRG1 and WDR5 to activate TNF-α induced pro-inflammatory transcription in macrophages.

Epigenetic activation of PERP transcription by MKL1 contributes to ROS-induced apoptosis in skeletal muscle cells

Excessive reactive oxygen species (ROS) causes irreparable damages to cells and commit cells to programmed cell death or apoptosis. A panel of well-documented pro-apoptotic genes, including p53 apoptosis effector related to PMP-22 (PERP), are up-regulated and collectively mediate ROS induced apoptosis. The epigenetic mechanism whereby ROS stimulates PERP transcription, however, lacks in-depth characterization. Here we report that the transcriptional modulator megakaryocytic leukemia 1 (MKL1) is activated by H2O2 treatment in skeletal muscle cells (C2C12). Small interfering RNA (siRNA) mediated silencing or small-molecule compound (CCG-1423) mediated inhibition of MKL1 attenuated H2O2 induced apoptosis of C2C12 cells. Over-expression of MKL1 potentiated trans-activation of PERP whereas MKL1 ablation/inhibition abrogated the induction of PERP by H2O2 in C2C12 cells. Mechanistically, MKL1 interacted with and was recruited to the PERP promoter by the transcription factor E2F1. Once bound to the PERP promoter, MKL1 engaged the histone demethylase KDM3A to modulate the chromatin structure surrounding the PERP promoter thereby leading to PERP trans-activation. Depletion of either E2F1 or KDM3A blocked the induction of PERP by H2O2. In conclusion, our data illustrate a novel epigenetic pathway that links PERP transcription to ROS-induced apoptosis in skeletal muscle cells.

Hepatocyte-specific deletion of Brg1 alleviates methionine-and-choline-deficient diet (MCD) induced non-alcoholic steatohepatitis in mice

Uncontrolled inflammatory response and augmented lipid accumulation represent two key pathophysiological events in the pathogenesis of non-alcoholic steatohepatitis (NASH). NF-κB and SREBP1c program transcriptional regulation of cellular inflammatory response and lipid metabolism, respectively. The epigenetic mechanism underlying NF-κB-dependent pro-inflammatory transcription and SREBP1c-dependent pro-lipogenic transcription remains incompletely understood. In the present study we investigated the involvement of Brg1, a chromatin remodeling protein, in NASH pathogenesis in a methionine-and-choline deficient diet (MCD) induced mouse model. Brg1 expression was up-regulated in the liver in mice fed on the MCD diet and in primary hepatocytes exposed to free fatty acids. Liver injury and hepatic inflammation attenuated in hepatocyte-specific Brg1 knockout (CKO) mice fed on the MCD diet compared to the wild type (WT) littermates. Likewise, synthesis of pro-inflammatory mediators was down-regulated in primary hepatocytes isolated from CKO mice compared to WT mice, which resulted in reduced macrophage chemotaxis. Brg1 contributed to the transcription of pro-inflammatory mediators possibly by regulating the interaction between NF-κB and its co-factor MRTF-A. On the other hand, accumulation of triglyceride and cholesterol was ameliorated in MCD-fed CKO mice with a concomitant reduction of SREBP1c target genes. Brg1 interacted with SREBP1c and modulated the transcription of SREB1c target genes in the liver in response to MCD feeding by influencing active histone modifications. In conclusion, targeting Brg1 may yield novel anti-NASH therapeutics by simultaneously normalizing hepatic inflammatory status and metabolic profile in NASH patients.

Histone methylation and acetylation in macrophages as a mechanism for regulation of inflammatory responses

Macrophages respond to noxious stimuli and contribute to inflammatory responses by eliminating pathogens or damaged tissue and maintaining homeostasis. Response to activation signals and maintenance of homeostasis require tight regulation of genes involved in macrophage activation and inactivation processes, as well as genes involved in determining their polarization state. Recent evidence has revealed that such regulation occurs through histone modifications that render inflammatory or polarizing gene promoters accessible to transcriptional complexes. Thus, inflammatory and anti-inflammatory genes are regulated by histone acetylation and methylation, determining their activation state. Herein, we review the current knowledge on the role of histone modifying enzymes (acetyltransferases, deacetylases, methyltransferases, and demethylases) in determining the responsiveness and M1 or M2 polarization of macrophages. The contribution of these enzymes in the development of inflammatory diseases is also presented.

Chemical probes targeting epigenetic proteins: Applications beyond oncology

Epigenetic chemical probes are potent, cell-active, small molecule inhibitors or antagonists of specific domains in a protein; they have been indispensable for studying bromodomains and protein methyltransferases. The Structural Genomics Consortium (SGC), comprising scientists from academic and pharmaceutical laboratories, has generated most of the current epigenetic chemical probes. Moreover, the SGC has shared about 4 thousand aliquots of these probes, which have been used primarily for phenotypic profiling or to validate targets in cell lines or primary patient samples cultured in vitro. Epigenetic chemical probes have been critical tools in oncology research and have uncovered mechanistic insights into well-established targets, as well as identify new therapeutic starting points. Indeed, the literature primarily links epigenetic proteins to oncology, but applications in inflammation, viral, metabolic and neurodegenerative diseases are now being reported. We summarize the literature of these emerging applications and provide examples where existing probes might be used.