PRMT5 Inhibition: A Novel Strategy in Cancer Treatment

What is PRMT5？

Protein arginine methyltransferase 5 (PRMT5) is a type II arginine methyltransferase, and it holds significant promise as an epigenetic target in clinical applications. The symmetrical dimethylation of arginine residues on non-histone proteins plays a pivotal role in regulating numerous physiological functions within mammalian cells. PRMT5 has emerged as a promising therapeutic target for a wide spectrum of diseases, including infectious diseases, heart conditions, and various types of cancer. Over the past decade, researchers have discovered numerous PRMT5 inhibitors that are now being utilized in the treatment of diseases, particularly in the context of tumor management.

The Structure and Function of PRMT5

The PRMT family of enzymes plays a crucial role as "writers" in post-translational modifications (PTMs), facilitating three distinct types of methylation processes. In essence, they transfer a methyl (-CH3) group from the methyl donor S-adenosylmethionine (SAM or AdoMet) to a guanidinium nitrogen atom of arginine within a target protein. This process results in the creation of a methylated guanidinium group and S-adenosylhomocysteine (SAH or AdoHcy), which is recycled for methionine biosynthesis.

Within human cells, nine PRMTs are responsible for catalyzing these methylation reactions. Type I PRMTs, such as PRMT1, 2, 3, 6, 8, and CARM1 (PRMT4), catalyze ω-NG-monomethylarginine (MMA) and asymmetric ω-NG, NG– asymmetric dimethylarginine (aDMA). Type II PRMTs, represented by PRMT5 and 9, catalyze MMA and ω-NG, NG– symmetric dimethylarginine (sDMA). A type III PRMT, PRMT7, solely catalyzes MMA. These modifications introduce steric effects and alter the hydrogen bonding interactions of the methylated side chain, consequently modifying the molecular characteristics and function of the protein being modified.

PRMT5 is a key player, forming a distinctive hetero-octameric complex comprising four PRMT5 proteins and four essential cofactors, MEP50 (methylosome protein 50)/WDR77 (WD repeat domain 77). The unique N-terminal TIM barrel structure of the PRMT5 monomer enables the formation of a PRMT5 tetramer at the center of the octameric complex. This is followed by the decoration of the PRMT5 tetramer with four MEP50 molecules[1-2]. The PRMT5/MEP50 complex exhibits a stronger affinity for SAM and the target substrate when compared to a PRMT5 homodimer, resulting in increased methylation activity of the hetero-octameric PRMT5/MEP50 complex[1].

PRMT5's C-terminal catalytic domain comprises two essential domains: the Rossam fold and β-barrel domains. These domains are responsible for binding cofactors (the methyl donor, SAM/AdoMet) and the substrate, respectively. The structural characteristics of substrate binding dictate a preference for methylation of glycine-rich sequences. This preference allows for conformational flexibility in the polypeptide chain, ultimately forming a sharp β-turn[1]. Extensive proteomic analysis of immuno-enriched arginine methylation sites for PRMT5 indicates a particular fondness for arginine residues flanked by glycines (at positions -1 and +1; e.g., GRG), as opposed to other amino acids found within substrate peptides[3-4].

Fig.1 Overall structure of PRMT5. [5]

Role of PRMT5 in Human Diseases

PRMT5's Significance in Cancer Development

An increasing body of research has firmly established PRMT5 as a pivotal contributor to tumor progression across various cancer types. PRMT5's role in cancer is deeply rooted in its methyltransferase activity, impacting crucial cellular processes such as cell signaling, DNA damage response, gene regulation, and splicing, among others[6]. Notably, the dysregulation of PRMT5 plays a critical role in the advancement of hematologic malignancies. For instance, in lymphoma cell lines, PRMT5 is overexpressed, leading to an upsurge in the expression of pro-survival proteins like cyclin D1, c-myc, and survivin. This occurs through the deposition of repressive methylation marks on H3R8 within the promoter regions of AXIN2 and WIF1, both of which act as negative regulators of the wnt/β-catenin signaling pathway[7]. Similarly, in in vivo studies, the tumorigenesis of lymphocytes driven by oncogenes such as cyclin D1 is heavily reliant on elevated PRMT5 expression. Furthermore, increased PRMT5 expression exacerbates the antagonism of p53's apoptotic function via arginine methylation[8]. In mantle cell lymphoma (MCL), diminished levels of miR-92b and miR-96 contribute to heightened PRMT5 expression, fueling cell proliferation. Another study highlights PRMT5's interaction with tripartite motif-containing protein 21 (TRIM21), an IKKβ ubiquitin ligase, which inhibits IKKβ degradation in multiple myeloma, consequently activating NF-κB signaling and fostering the growth of multiple myeloma cells[9].

Moreover, PRMT5 acts as a promoter of oncogenicity in a wide spectrum of solid cancers, including those affecting the colon, breast, prostate, lung, liver, bone, skin, ovaries, stomach, brain, and pancreas, among others. In hepatocellular carcinoma, PRMT5-catalyzed repressive dimethylation on H4R3 at the B-cell translocation gene 2 (BTG2) promoter stimulates cell proliferation via the ERK signaling pathway[10]. Similarly, in lung cancer, PRMT5 triggers the downregulation of tumor suppressor genes, such as GLI pathogenesis related 1 (GLIPR1), leprecan-like 1 (Leprel1), and BTG2, while elevating the levels of growth factors like fibroblast growth factor receptor substrate 1/2/3/4 (FGFR1/2/3/4) and human epidermal growth factor receptor 2/3 (HER2/3), thereby fostering cell growth. Particularly noteworthy is PRMT5's promotion of increased FGFR3 signaling due to the suppression of the miR-99 family, which negatively regulates FGFR3 expression in lung cancer[11]. Clinically, elevated PRMT5 levels have been linked to poorer prognoses in breast cancer, hepatocellular carcinoma, lung cancer, ovarian cancer, and gastric cancer cases[12]. Furthermore, the high nuclear expression of PRMT5 is proposed as a potential biomarker for assessing submucosal invasion in early-stage colorectal cancer (CRC). A similar prognostic potential is observed for high PRMT5 expression in the nucleus and/or cytoplasm in brain, lung, ovarian, skin, and prostate cancers[13]. Collectively, these lines of evidence underscore PRMT5's extensive oncogenic role in various cancers and its potential as a valuable clinical biomarker to enhance patient treatment strategies.

PRMT5's Impact on Diabetes

In addition to its role in cancer, there is compelling evidence indicating that PRMT5 also exerts significant influence in the realm of diabetes. Type 2 diabetes mellitus (T2DM), accounting for over 90% of diabetes cases, primarily manifests as dysfunction within pancreatic β-cells, resulting in impaired insulin release and insulin resistance[14]. This condition is further marked by associated pathophysiological factors, including hyperglycemia, hyperlipidemia, mitochondrial dysfunction, inflammation, and elevated levels of reactive oxygen species (ROS). Remarkably, PRMT5 has been implicated in metabolic pathways that contribute to the progression of T2DM.

For instance, within white adipose tissue, PRMT5 methylates sterol regulatory element-binding transcription factor 1a (SREBP1a), amplifying the formation of triacylglycerols[15]. PRMT5 also plays a role in the methylation of the transcription elongation factor SPT5, promoting the genesis of lipid droplets[15]. In a separate study, PRMT5 was found to act as a coactivator for adipogenic genes, including peroxisome proliferator-activated receptor γ2 (PPARγ2), adipocyte protein 2 (aP2), adiponectin, leptin, and resistin. This underscores PRMT5's regulatory role in fatty acid metabolism and, consequently, insulin sensitivity. Notably, increased activity of CREB and CRTC2, observed in diabetes, contributes to hyperglycemia. In summary, these PRMT5-mediated events play a pivotal role in impairing pancreatic β-cell function and influencing glucose regulation.

Contrastingly, another study reported that the conditional knockout of PRMT5 in islet cells of the pancreas led to impairments in glucose tolerance and glucose-stimulated insulin release in β-cells[16]. The proposed mechanism suggests that PRMT5 dimethylates H3R8, enhancing the binding of the brahma-related gene-1 (BRG1) chromatin remodeling enzyme to the insulin promoter, thereby increasing insulin production. This intriguing observation was attributed to a compensatory mechanism involving elevated β-cell proliferation induced by impaired insulin production in PRMT5 knockout mice[16]. Consequently, further research is needed to fully grasp the nuanced role of PRMT5 in T2DM models, shedding light on its potential as a viable therapeutic target in diabetes.

PRMT5's Involvement in Cardiovascular Diseases

Cardiovascular diseases encompass a wide range of conditions affecting the heart and blood vessels[17]. While research on the role of PRMT5 in cardiovascular diseases is relatively limited, recent reports hint at its potential as a risk indicator for certain cardiovascular conditions or as a factor influencing disease outcomes. In cellular models of cardiomyocyte hypertrophy, a condition leading to heart enlargement and ultimately heart failure, the overexpression of PRMT5 has been shown to reduce isoprenaline-induced hypertrophy by repressively methylating HOXA9, a gene implicated in the development of various cardiovascular diseases. This finding aligns with previous research demonstrating PRMT5's role in curbing the expression of hypertrophic genes in cardiomyocytes through the methylation of GATA4, a transcription factor governing cardiac remodeling[18]. Consequently, these findings suggest that low PRMT5 expression in the blood could potentially serve as a biomarker for individuals at an elevated risk of developing acute myocardial infarction (AMI). Conversely, elevated PRMT5 levels may exacerbate the pathological progression of inflammation-driven cardiovascular diseases. For example, the heightened expression of C-X-C motif chemokine ligand 10 (CXCL10), a chemokine strongly associated with atherosclerosis and coronary artery disease in endothelial cells, is partly driven by PRMT5-mediated methylation of NF-κB at R30 and R35. This phenomenon occurs in response to tumor necrosis factor-alpha (TNF-α), a potent activator of NF-κB signaling[19]. In summary, these studies suggest that PRMT5's role is context-dependent, emphasizing the need for further exploration into PRMT5's molecular activities in contributing to various cardiovascular diseases.

Signaling Pathway of PRMT5

Firstly, PRMT5 plays a pivotal role in regulating the cell cycle and apoptosis through histone methylation. In acute lymphoblastic leukemia (ALL), PRMT5 fosters lymphocyte proliferation by inhibiting apoptosis. Silencing PRMT5 and subsequently down-regulating H4R3sme2 induce the differentiation of ALL from the pre-B to immature B stage. Furthermore, the inhibition of PRMT5-mediated H3K27 methylation contributes to cell cycle progression.

Secondly, PRMT5 operates within a DNA damage-responsive co-activator complex that interacts with p53, methylating the latter. Depletion of PRMT5 results in an increase in sub-G1 cells during the DNA damage response. Additionally, compared to wild-type p53, a p53 mutant compromised in arginine methylation may affect the cell cycle. PRMT5's role in DNA damage repair involves 53BP1, and PRMT5 activity may determine whether cells undergo DNA repair or apoptosis in response to damage. Moreover, reduced PRMT5 activity leads to exon skipping and intron retention, affecting genes in the DNA repair pathway. Thus, loss of PRMT5 activity triggers endogenous DNA damage, activating p53 and inducing apoptosis. PRMT5 also inhibits cyclin D1T286A-induced apoptosis in a MEP50 phosphorylation-dependent manner, with cyclin D1T286A/CDK4 promoting p53-dependent PRMT5 methylation.

Thirdly, PRMT5 promotes cell cycle progression by up-regulating cell cycle proteins, including β-catenin, Cyclin D1, CDK4/6, and CCND1/D2/E1. Inhibition of PRMT5 leads to decreased proportions of G1 and S phases, increased G2/M phase, and apoptosis in sub-G1. Knockdown of PRMT5 increases the G1 phase of HCC cells while decreasing S and G2/M phases.

Finally, PRMT5 regulates the cell cycle and apoptosis by affecting PIPs (PRMT5-interacting proteins), either directly or indirectly. PRMT5 and PRMT1 interact with CFLARL, a key anti-apoptotic protein. They regulate the polyubiquitination and degradation of CFLARL, impacting the interaction between CFLARL and ITCH, ultimately affecting the apoptosis of non-small cell lung cancer cells. PRMT5 also has a pronounced co-localization pattern with Myc protein, primarily in the nucleus. Preventing the specific interaction between PRMT5 and Myc inhibits the growth of medulloblastoma cells and promotes the apoptosis of Myc-dependent tumors. Depleting PRMT5 impacts the binding of SRSF1 to mRNAs and proteins, leading to alternative splicing of multiple essential genes and, consequently, the demise of human acute myeloid leukemia cells. Additionally, PRMT5 interacts with Zili and Vasa, catalyzing the SDMA of Vasa and Zili. Deleting PRMT5 dysregulates the expression of Vasa and Zili, triggering germ cell apoptosis.

Fig.2 Signaling Pathway of PRMT5.[20]

PRMT5 Proteins

Recombinant Human PRMT5 Protein

Click here for more PRMT5

Synonym : HRMT1L5; IBP72; JBP1; SKB1; SKB1Hs

References:

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[20] Liang Z, Wen C, Jiang H, Ma S, Liu X. Protein Arginine Methyltransferase 5 Functions via Interacting Proteins. Front Cell Dev Biol. 2021 Aug 27;9:725301. doi: 10.3389/fcell.2021.725301