Roles of protein arginine methylation in DNA damage signaling pathways

Cell growth and division is regulated by many mechanisms to preserve genomic integrity in response to genotoxic stresses such as chemicals, UV and ionizing radiation, and viruses. Cell growth can be arrested temporarily to allow repair of damaged DNA. Cell cycle checkpoints mostly occur at the G₁/S and G₂/M transition points and are initiated by ATM/ATR kinase activation, resulting in activation of p53, BRCA1, and other parts of the DNA damage response pathway (Fig. 1). BRCA1, 53BP1 and other protein factors are recruited to damaged DNA sites as part of the repair process. In response to DNA damage, severely damaged cells can be removed by apoptosis, programmed cell death. Failure of cell cycle arrest, DNA damage repair and apoptosis frequently contributes to cellular malignancies.¹ Thus, the DNA damage response pathway is branched, and a decision is made as to whether cells will be repaired or destroyed. However, the control of the decision is still poorly understood.

Figure 1.

The roles of CARM1 and coactivator methylation in DNA damage signaling pathway. DNA damage activates ATM kinase and promotes the activity of tumor suppressors p53 and BRCA1. CARM1 methylates coactivator p300 and histones and induces coactivator complex formation with BRCA1 and other BRCT domain proteins for expression of p21, Gadd45 and other CDKIs. This leads to cell cycle arrest and DNA damage repair in response to genotoxic stress. CARM1, p300 and BRCA1 are not required for expression of genes involved in the apoptosis branch of the DNA damage response pathway. CARM1 may be an essential factor inducing cell cycle arrest and cell survival rather than apoptosis in the DNA damage signaling pathway.

DNA damage response processes are coordinated by tumor suppressor p53, a transcription regulator involved in both branches of the DNA damage response pathway by regulating expression of cell cycle and apoptosis regulators.² The association of p53 with other protein factors (e.g., MDM2, 53BP1, p300/CBP) and posttranslational modifications of p53 may contribute to the branch point decision.³ We recently observed that protein methylation by CARM1 (coactivator associated arginine methyltransferase 1) is required for activation of genes involved specifically in the cell cycle arrest branch of the DNA damage response pathway.⁴ We therefore suggest that CARM1 is positioned to influence the decision point between arrest/repair versus cell death.

Protein arginine methylation is catalyzed by members of the protein arginine methyltransferase (PRMT) family, which currently has ten members in mammalian cells.^5,6 Protein arginine methylation plays several roles in the DNA damage response pathway. PRMT5 methylates three arginines of tumor suppressor p53 and enhances the activity of p53 in cooperation with Strap protein.⁷ Transcriptional activation of Gadd45 (growth arrest and DNA damage-inducible 45α) by p53 in cell-free assays also requires methylation of histones H3 and H4 by PRMT4/ CARM1 and PRMT1.⁸ CARM1 also methylates coactivator p300 at multiple sites.^4,6 Methylation of p300, specifically at Arg754 in the KIX region, is required for induction of cell cycle regulators like p21^CIP1/WAF1 and Gadd45.⁴ p21^CIP1/WAF1, an inhibitor of cyclin-dependent kinases, binds to the G₁/S-promoting cyclin E/Cdk2 kinase and thereby causes a G₁ to S cell cycle arrest. Gadd45 associates with PCNA and is involved in both cell cycle arrest and nucleotide excision repair. Absence of CARM1 methyltransferase activity led to loss of cell cycle arrest in response to DNA damage.⁴

Although p21^CIP1/WAF1 is a downstream target gene of p53, p21 expression is induced by DNA damage even in p53-deficient cells, CARM1 is involved in p21 induction in p53-dependent and p53-independent pathways.⁴ Furthermore, CARM1 is also involved in the induction of other p53-independent CDK inhibitors like p27 (unpublished data). Thus CARM1 and coactivator methylation have essential roles in cell cycle check point regulation by genotoxic stresses. However, CARM1 is not required for the induction of apoptosis regulators like Bax (BCL2-associated X protein) or PUMA (p53 upregulated modulator of apoptosis), which are also p53 target genes. Instead, expression levels of Bax⁴ and PUMA (unpublished data) are elevated in CARM1 knockout cells. Similarly, expression of PUMA is also upregulated in p300 knockout cells.⁹ This is reminiscent of BRCA1, which is essential for expression of p21 and Gadd45, not for Bax.¹⁰ Thus, CARM1, p300 and BRCA1 are all required for activation of genes involved in the cell cycle arrest branch of the DNA damage response pathway. In contrast, genes in the apoptosis branch of the pathway, such as Bax or PUMA may have different coregulator requirements.

Since CARM1 regulates the interaction between p300 and BRCA1 and thereby the activation of p21 and Gadd45 genes (Fig. 1), CARM1 is in an ideal position to control the branch point decision in the DNA damage response pathway. CARM1 methyltransferase activity may also modulate other tumor suppressors for determination of cell fate. We speculate that post-translational modifications of CARM1, protein-protein interactions or availability of its methyl donor substrate S-adenosylmethionine, could influence CARM1 activity and thereby cell survival.

Control of cell cycle arrest and survival by CARM1 may contribute to DNA damage repair and maintenance of genomic integrity. However, CARM1 may have detrimental effects on cells under consistent and severe DNA insults in vivo. Overexpression or dysregulation of CARM1 might prevent cell death and lead to cell survival in spite of accumulation of chromosomal instability and damage. CARM1 could therefore be like a double-edged sword, both acting as a tumor suppressor and a tumor-promoting factor. Elucidating the roles of CARM1 in DNA damage signaling may provide an effective new strategy for preventing inappropriate cell survival in diseases such as cancer.

Comment on: Lee YH, et al. Genes Dev. 2011;25:176–188. doi: 10.1101/gad.1975811.