Non-transcriptional action of oestradiol and progestin triggers DNA synthesis

The recent findings that oestradiol and progestins activate the Src/Ras/Erk (s) signalling pathway raise the question of the role of this stimulation. Microinjection experiments of human mammary cancer-derived cells (MCF-7 and T47D) with cDNA of catalytically inactive Src or anti-Ras antibody prove t hat Src and Ras are required for oestradiol and progestin-dependent progres sion of cells through the cell cycle, The antitumoral ansamycin antibiotic, geldanamycin, disrupts the steroid-induced Ras-Raf-1 association and preve nts Raf-1 activation and steroid-induced DNA synthesis. Furthermore, the se lective MEK 1 inhibitor, PD 98059, inhibits oestradiol and progestin stimul ation of Erk-2 and the steroid-dependent S-phase entry, The MDA-MB231 cells , which do not express oestradiol receptor, fail to respond to oestradiol i n terms of Erk-2 activation and S-phase entry. Fibroblasts are made equally oestradiol-responsive in terms of DNA synthesis by transient transfection with either the wildtype or the transcriptionally inactive mutant oestradio l receptor (HE241G). Co-transfection of catalytically inactive Src as well as treatment with PD98059 inhibit the oestradiol-dependent S-phase entry of fibroblasts expressing either the wild-type oestrogen receptor or its tran scriptionally inactive mutant, The data presented support the view that non -transcriptional action of the two steroids plays a major role in cell cycl e progression.