Estrogen activates raf-1 kinase and induces expression of Egr-1 in MCF-7 breast cancer cells.

We have investigated whether the raf-1 kinase, a downstream mediator of both receptor tyrosine kinase and protein kinase C signalling, is activated by estrogen (E2) in an estrogen receptor positive human breast cancer cell line. Autophosphorylation of raf-1 kinase was studied after treatment of MCF-7 cells with E2. E2-deprived cells contained low levels of raf-1 kinase activity. Treatment of cells for 1 min with E2 resulted in raf-1 autophosphorylation which was maximal within 5 min. Western blot analysis showed that raf-1 undergoes an electrophoretic mobility shift following E2 treatment. Egr-1 is a zinc finger-containing transcription factor which is expressed in association with raf-1 activation. Untreated MCF-7 cells expressed low levels of Egr-1 while E2 treatment resulted in an induction of egr-1 mRNA expression. These kinetics followed closely behind the E2 induction of c-myc mRNA. Egr-1 protein was similarly low in E2-deprived MCF-7 cells and was transiently increased following E2 treatment. Several studies have suggested that kinase activity may play a role in estrogen receptor (ER) activation. While activated v-raf failed to augment ER activation of transcription in transient transfection assays, a dominant negative mutant of raf-1 inhibited E2-induced transcription by 50% primarily as a result of increased baseline levels of E2 independent transcription. The results show that E2 can induce raf-1 kinase activity in MCF-7 breast cancer cells associated with the expression of an early growth response gene and modulation of ER signalling.