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.