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 that Src and Ras are required for oestradiol and progestin‐dependent progression of cells through the cell cycle. The antitumoral ansamycin antibiotic, geldanamycin, disrupts the steroid‐induced Ras–Raf‐1 association and prevents Raf‐1 activation and steroid‐induced DNA synthesis. Furthermore, the selective MEK 1 inhibitor, PD 98059, inhibits oestradiol and progestin stimulation 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 in 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 wild‐type or the transcriptionally inactive mutant oestradiol 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 transcriptionally inactive mutant. The data presented support the view that non‐transcriptional action of the two steroids plays a major role in cell cycle progression.