Methylation, a key step for nongenomic estrogen signaling in breast tumors

Estrogen receptor alpha (ERalpha) is a member of a large conserved superfamily of steroid hormone nuclear receptors which regulates many physiological pathways by acting as a ligand-dependent transcription factor. Evidence is emerging that estrogens also induce rapid signaling to the downstream kinase cascades; however the mechanisms underlying this nongenomic function remain poorly understood. We have recently shown that ERalpha is methylated specifically by the arginine methyltransferase PRMT1 at arginine 260 in the DNA-binding domain of the receptor. This methylation event is required for mediating the extra-nuclear function of the receptor which would thereby interact with Src/FAK and p85 and propagate the signal to downstream transduction cascades that orchestrate cell proliferation and survival. Of particular interest, a possible role of methylated ERalpha in mammary tumorigenesis is also evident by the fact that, as demonstrated by immunohistochemical studies on a cohort of breast cancer patients, ERalpha is methylated in normal epithelial breast cells and is hypermethylated in a subset of breast cancers. Hypermethylation of ERalpha in breast cancer might cause hyperactivation of cellular kinase signaling, notably of Akt, described as a selective survival advantage for primary tumor cells even in the presence of anti-estrogens. A detailed understanding of the molecular mechanisms that control estrogen signaling in breast cancer is a crucial step in identifying new effective therapies.