Estradiol modulates breast cancer cell apoptosis: a novel nongenomic steroid action relevant to carcinogenesis

It is known that steroids can induce cell surface receptor aggregation foll owed by activation of receptor and nonreceptor tyrosine kinases. It has bee n shown recently that 17 beta-estradiol (E-2) can stimulate the Src/p21(ras )/mitogen-activated protein kinase pathway in breast cancer cells, and this effect is supposed to mediate the E-2-induced stimulation of breast cancer cell proliferation, possibly via activation of the c-fos and c-jun early g enes or of genes involved in cell cycle control. Here we demonstrate the ex istence of an alternative mechanism of the cancer-promoting effect of E-2. Human breast cancer cells (MCF-7) were exposed to the known proapoptotic ag ent vitamin E succinate (VES), added alone or together with different conce ntrations of E-2. E-2 conjugated with bovine serum albumin (E-2-BSA), which cannot cross the plasma membrane of living cells, was also used in some ex periments to assess whether E-2 acted on the cell surface or at intracellul ar receptors. Apoptosis was analyzed by fluorescence-activated cell sorting after cell staining with propidium iodide and FITC-labeled annexin V. E-2 showed a concentration-dependent stimulatory effect on spontaneous apoptosi s but inhibited the VES-induced apoptosis. However, effects produced by the same molar concentrations of E-2 were different when the hormone was free and when it was used in the form of the E-2-BSA conjugate. The effects of E -2 and E-2-BSA were sensitive to genistein, a tyrosine kinase inhibitor. Th ese data show that E-2 modulates apoptosis of breast cancer cells, probably acting both at the cell surface and inside the cells. Tyrosine phosphoryla tion is involved in the signaling pathways mediating this E-2 effect. (C) 1 999 Elsevier Science Inc. All rights reserved.