PRMT5 triggers glucocorticoid-induced cell migration in triple-negative breast cancer.

Triple-negative breast cancers (TNBCs) are the most aggressive breast cancers, and therapeutic options mainly rely on chemotherapy and immunotherapy. Although synthetic glucocorticoids (GCs) are given to alleviate the side effects of these treatments, GCs and their receptor, the glucocorticoid receptor (GR), were recently associated with detrimental effects, albeit the mechanisms involved remain elusive. Here, we identified the arginine methyltransferase PRMT5 as a master coregulator of GR, serving as a scaffold protein to recruit phospho-HP1γ and subsequently RNA polymerase II, independently of its methyltransferase activity. Moreover, the GR/PRMT5/HP1γ complex regulated the transcription of GC-target genes involved in cell motility and triggering cell migration of human TNBC cells in vitro and in a zebrafish model. Of note, we observed that GR/PRMT5 interaction was low in primary tumors but significantly increased in residual tumors treated with chemotherapy and GCs in neoadjuvant setting. These data suggest that the routine premedication prescription of GCs for early TNBC patients should be further assessed and that this complex could potentially be modulated to specifically target deleterious GR effects.

Using proximity ligation assay to detect protein arginine methylation.

Arginine methylation is now recognized as a major contributor to proteome diversity and is, as such, involved in a large range of cellular processes. There is a growing need for assessing endogenous protein arginine methylation in cells. Besides the classical immunoprecipitation, in situ proximity ligation assay (PLA) is a useful technique allowing at the same time the detection, localization and quantification of arginine methylation of a given protein within a cellular context. Here, we described in depth a standard PLA protocol applied to the detection of arginine methylation in combination with RNA interference and specific methyltransferase inhibitors. We demonstrated that the glucocorticoid receptor is methylated by the arginine methyltransferase PRMT5 inside the nucleus of MCF-7 cells. In addition, the automated quantification of protein arginine methylation performed using Image J is reported. Hence, we demonstrated that PLA offers a novel approach to study protein arginine methylation and could be extended to other post-translational modifications when specific antibodies are available.

The arginine methyltransferase PRMT1 regulates IGF-1 signaling in breast cancer.

Aside from its well-known nuclear routes of signaling, estrogen also mediates its effects through cytoplasmic signaling. Estrogen signaling involves numerous posttranslational modifications of its receptor ERα, the best known being phosphorylation. Our research group previously showed that upon estrogen stimulation, ERα is methylated on residue R260 and forms the mERα/Src/PI3K complex, central to the rapid transduction of nongenomic estrogen signals. Regulation of ERα signaling via its phosphorylation by growth factors is well recognized, and we wondered whether they could also trigger ERα methylation (mERα). Here, we found that IGF-1 treatment of MCF-7 cells induced rapid ERα methylation by the arginine methyltransferase PRMT1 and triggered the binding of mERα to IGF-1R. Mechanistically, we showed that PRMT1 bound constitutively to IGF-1R and that PRMT1 became activated upon IGF-1 stimulation. Moreover, we found that expression or pharmacological inhibition of PRMT1 impaired mERα and IGF-1 signaling. Our findings were substantiated in a cohort of breast tumors in which IGF-1R expression was positively correlated with ERα/Src and ERα/PI3K expression, hallmarks of nongenomic estrogen signaling, reinforcing the link between IGF-1R and mERα. Altogether, these results provide a new insight into ERα and IGF-1R interference, and open novel perspectives for combining endocrine therapies with PRMT1 inhibitors in ERα-positive tumors.