MITOGENIC VERSUS APOPTOTIC ESTRADIOL ACTION: ROLE OF ERb ACTIVATED RAPID SIGNAL TRANSDUCTION PATHWAYS

Estrogens exert multiple effects on target cells probably related to various action mechanisms which start after hormone binding to both estrogen receptor isoforms (ERa and ERb). 17b-estradiol (E2)-induced rapid/non genomic signal transduction pathways (i.e., AKT and ERK phosphorylation) are both necessary and sufficient to mediate G1-S phase transition in several target cells. Here, the ability of the two ER isoforms to activate different signal transduction pathways and to drive cells into or out of the cell cycle has been analyzed. HeLa cells, devoid of any endogenous ER, have been used as experimental model after the transient transfection with expression vectors encoding for human ERa or ERb. The results show that E2 induces target gene promoter activity in HeLa cells only after transfection with ERa or ERb. Furthermore, E2 increases the number of ERa-transfected HeLa cells in the G1 phase of the cell cycle. On the contrary, in the presence of ERb, E2 decreases the cellular population in the G1 phase of the cell cycle and increases the number of cells in sub-G1 phase, a marker of DNA fragmentation. These data are consistent with the ability of E2 to trigger both caspase-3 activation and the cleavage of PARP in ERb-expressing HeLa cells. The role played by signal transduction pathways shows that the E2-ERb complex does not activate any of the E2-ERa-activated signal molecules involved in cell growth (i.e., AKT and ERK). However, in the presence of ERb, E2 induces a persistent phosphorylation of MAPK/p38 which is involved in both the regulation of caspase-3 activation and the cleavage of PARP. On the other hand, in the presence of ERa, the E2-dependent MAPK/p38 activation is transient. Accordingly, in a human adenocarcinoma cell line (DLD-1), which expresses only ERb, E2 induces a persistent activation of the MAPK/p38 which is involved in the induction of caspase-3 whereas in a human hepatoma cell line (HepG2), which expresses only ERa, only the transient E2-induced activation of MAPK/p38 can be detected. Taken together, these results demonstrate the ability of ERb isoform to activate specific signal transduction pathways starting from plasma membrane that may justify the effect of E2 in inducing cell proliferation or apoptosis in cancer cells. In particular this hormone promotes cell survival through ERa non-genomic signalling and cell death through ERb non-genomic signalling.