Induction of apoptosis of mononucleated cells is a physiological process for regulating the intensity of the immune response. The female steroid hormones estrogen (E2) and progesterone(Prog) are known to modulate the reactivity of the immune system; recently it has been demonstrated that they can regulate induction of apoptosis of endothelial cells and osteoblasts. TNF-α-mediated induction of apoptosis has been well characterized in myeloid cells. We investigated whether E2 and Prog could interfere with TNF-α-induced apoptosis of the monoblastoid U937 cell line. Treatment with E2 or Prog increased survival and prevented apoptosis induced by TNF-α in both undifferentiated and macrophage-like PMA-differentiated U937 cells, as assessed by trypan blue exclusion cell counting, thymidine incorporation, AnnexinV labeling, followed by flow cytometry and DNA fragmentation studies. This effect can be associated with the activation of specific hormone receptors, since we observed the expression of the estrogen receptor α (ER-α), ER-β, and progesterone receptor (PR) mRNAs; the ER-α protein expression was confirmed by immunocytochemical analysis. In addition, hormone-mediated survival against apoptosis was concentration dependent, reaching the half-maximal effect at 10 nM and blocked by the ER antagonist ICI 182,780 in undifferentiated cells, further supporting a receptor-mediated mechanism of cell survival. Other steroid receptor drugs such as Raloxifene, RU486, or the ICI 182,780 in PMA-differentiated cells displayed agonist activity by preventing TNF-α-induced apoptosis as efficiently as the hormones alone, providing further evidence to the notion that steroid receptor drugs may manifest agonist or antagonist activities depending on the cellular context in which they are studied. Treatment with E2 was also associated with a time-dependent decrease in the mRNA level of the proapoptotic Nip-2 protein, supporting the hypothesis that hormone responsiveness of U937 cells is mediated by target gene transcription. Together, these results demonstrate that ER and PR can be activated by endogenous or exogenous ligands to induce a genetic response that impairs TNF-α-induced apoptosis in U937 cells. The data presented here suggest that the female steroid receptors play a role in regulation of the immune response by preventing apoptosis of monoblastoid cells; this effect might have important consequences in the clinical use of steroid receptor drugs.-Vegeto, E., Pollio, G., Pellicciari, C., Maggi, A. Estrogen and progesterone induction of survival of monoblastoid cells undergoing TNF-α-inuced apoptosis.