Figure 5. Potential role of methylation on CD4
+ cells. Methyltransferase like PRMT1, PRMT5, G9a, EHMT1, SETDB1, SMYDs, and EZH2 exert their influence on CD4
+ T cells, impacting subsequent T cell differentiation and maturation. This, in turn, leads to alterations in the expression of inflammatory factors such as IFN and ILs.
Retinoic acid-related orphan receptor γt (RORγt) is a key transcription factor that mediates the differentiation of Th17 cells
[38,63][26][51]. PRMT1 has been demonstrated to be related to RORγt and regulated mouse Th17 differentiation, which is promoted by PRMT1 overexpression. However, the knockdown of PRMT1 by shRNA and inactivation by specific PRMT1 inhibitors limits Th17 differentiation. The use of a specific inhibitor of PRMT1 damages the production of Th17 cells and alleviates activation of experimental autoimmune encephalomyelitis in mice
[38][26]. Sen et al. have suggested that PRMT1 could be a new target for reducing Th17-mediated autoimmunity by decreasing the generation of pathogenic Th17 cells
[38][26]. Vav1, a Rac/Rho guanine nucleotide exchange factor, plays a crucial role in cytokine secretion, T cell activation, and proliferation
[64,65][52][53]. Methylation of Vav1 is promoted in both human and mouse T cells and occurs in the nucleus
[39,40][27][28]. The inhibition of cellular transmethylation of PRMT1 reduces methylation of Vav1 and IL-2 production, indicating potentially crucial roles for PRMT1 in T cell-mediated disorders.
3. CARM1
CARM1 (PRMT4) is a transcriptional coactivator related to the p160 family in nuclear receptor-mediated transcription
[66][54]. In addition to the p160 family, CARM1 synergistically activates NF-κB-mediated transactivation with P300/CREB-binding protein
[41,67,68][29][55][56]. As reported, CARM1 bound to p300 in vivo and interacted with p65, a NF-κB subunit, in vitro
[43][31]. During TNFα or LPS stimulation, CARM1
−/− mouse embryonic fibroblasts exhibited dampened expression of a group of NF-κB target genes
[43,69][31][57]. The RNA-binding protein HuR, a novel substrate of CARM1, is methylated by CARM1 at arginine 217
[70][58]. Methylation of endogenous HuR and stabilization of TNF-α mRNA have been observed in lipopolysaccharide-stimulated macrophages. The methylation of HuR was similarly increased in cells overexpressing CARM1, and methylated HuR is related to the stability of HuR-dependent mRNA
[44][32]. Previous research has shown that CARM1 is associated with survival of thymocytes. Thymocytes isolated from CARM1-deleted embryos are stagnated between CD4
− CD8
− double-negative stage 1 and double-negative stage 2
[45,66][33][54]. A significant reduction in the number of thymocytes has been observed. Therefore, the methylation of arginine residues by CARM1 in inflammation suggests CARM1 as a therapeutic target for inflammatory diseases
[66][54].
4. PRMT5
Acute graft-versus-host disease (aGVHD) is a T cell-mediated immune dysfunction in which T cells in donated tissue recognize the recipient as foreign
[66][54]. PRMT5 has been identified to play a role in T cell responses in aGVHD, suggesting it as a target of this disease
[46][34]. PRMT5 is a mediator in experimental autoimmune encephalomyelitis (EAE), a well-developed animal model of autoimmune disease multiple sclerosis
[71][59]. In vivo EAE mouse models have shown that PRMT5 inhibition potently repressed memory T cell responses. Delayed-type hypersensitivity and inflammation in clinical disease were also decreased. The inhibition of PRMT5 by specific inhibitors downregulates IL-2 production and proliferation of recall Th cells. These results demonstrate the importance of PRMT5 as a regulator in adaptive memory Th cell responses
[47][35]. In lymphoma cells, deceased PRMT5 represses TP53K372 methylation, cyclin D1 transcriptional activation, and BCL3 production and promotes NF-κB p52–HDAC1 repressor complexes to the cyclin D1 promoter
[48][36].
As Nagai et al. have reported, PRMT5 forms a complex with FOXP3 homomer in Tregs
[49][37]. Therefore, a specific blockade of PRMT5 decreases methylation of FOXP3 and arrests human Treg functions. Mice with conditional knockout of PRMT5 expression in Tregs develop severe scurfy-like autoimmunity and display a limited suppressive function. This may also explain the reduced numbers of Tregs in the spleen in PRMT5 cKO mice
[49][37]. Additionally, PRMT5 has been found to regulate T cell survival and proliferation through analysis of T cell-specific PRMT5 conditional knockout mice
[50][38]. PRMT5 is essential for natural killer T cell development, optimal peripheral T cell maintenance, and early T cell development
[50][38]. Consistently, deficient IL-7-dependent survival and TCR-induced proliferation in T cells were caused by deletion of PRMT5 in vitro
[50][38]. Separately, PRMT5 has been shown to be important for antibody responses and plays essential but distinct roles in all proliferative B cell stages in mice
[51][39]. PRMT5 prevents p53-mediated suppression in pro-B and pre-B cells and inhibits apoptosis of mature B cells during simultaneous activation via p53-independent pathways
[51][39]. The inhibition of PRMT5 markedly decreases phosphorylation of STAT1 and transcription of pro-inflammatory genes, including IL-17 and IFN-γ. Additionally, PRMT5 inhibition disrupts signaling transduction by regulating phosphorylation of ERK1/2, which leads to dysregulation of the cell cycle in activated T cells
[46][34]. The data above indicate PRMT5 inhibitors as a novel method to treat T cell-dependent inflammatory disease.
5. PRMT6
Utilizing proteomics-based methods, a protein–protein interaction has been discerned between PRMT6 and interleukin-enhancer binding protein 2 (ILF2). Moreover, macrophage migration inhibitory factor has been shown to play a role in mediating alternative activation of tumor-associated macrophages. Avasarala et al. have identified the macrophage migration inhibitory factor as an important downstream molecule of PRMT6–ILF2 signaling
[52][40]. HIV-1 Tat protein is a key player in HIV replication by increasing gene transcription efficiency. HIV-1 is a specific substrate of PRMT6 in vivo and in vitro that targets Tat R52 and R53 residues for arginine methylation
[53][41]. The overexpression of PRMT6 decreases the level of Tat transactivation of HIV-1 long terminal repeat chloramphenicol acetyltransferase and luciferase reporter plasmids in a dose-dependent manner, while the knockdown of PRMT6 enhances HIV-1 production and the speed of viral replication
[53][41]. Thus, PRMT6 disrupts the transcriptional activation of Tat and may represent an innate cellular immune form of HIV-1 replication.
PRMT6 also plays a role in immunity by targeting a series of signaling pathways. PRMT6 has been identified as an NF-κB coactivator because it can generate transgenic mice that express PRMT6 fused to the hormone-binding portion of the estrogen receptor
[54][42]. PRMT6 engages in a direct interaction with RelA, whereby its overexpression amplifies the transcriptional activity of an ectopic NF-κB reporter and intrinsically regulates NF-κB target genes
[54][42]. In response to TNF-α stimulation, RelA recruits PRMT6 to specific NF-κB target promoters. Phosphatase and tensin homolog (PTEN) is recognized as a tumor-suppressor gene, and its mutation has implications in the progression of various cancers
[70][58]. PRMT6 interacts with PTEN and methylated PTEN R159, weakening the PI3K–AKT cascade
[72][60]. G protein pathway suppressor 2 (GPS2) cytoplasmic actions and anti-inflammatory roles are linked with the regulation of JNK activation as well as TNF-α target genes in macrophages
[73][61]. Interaction with the exchange factor TBL1 is helpful to protect GPS2 from degradation. The methylation of GPS2 by PRMT6 modulates the interaction with TBL1 and suppresses proteasome-dependent degradation
[74][62]. PRMT6 also attenuates anti-viral innate immunity by blocking TBK1–IRF3 signaling
[55][43]. In PRMT6-deficient mice, the TBK1–IRF3 interaction is enhanced and activates IRF3 as well as increases the production of type I IFN. A Viral infection not only upregulates PRMT6 protein levels, but also promotes the binding between PRMT6 and IRF3 and dampens the interaction between IRF3 and TBK1
[55][43].