12-O-tetradecanoylphorbol-13-acetate-induced apoptosis is mediated by tumor necrosis factor α in human monocytic U937 cells

12-O-tetradecanoylphorbo-13-acetate (TPA), a phorbol ester that is known as a tumor promoter, induces differentiation of myeloid cells and suppresses their proliferation. We studied the regulation of apoptosis by TPA in human monocytic cell line U937 cells that lack p53. Untreated U937 cells constitutively underwent apoptosis, and TPA enhanced apoptosis in these cells. Further studies showed that TPA increased production of tumor necrosis factor-alpha (TNF alpha) in U937 cells, and exogenously added TNF alpha induced apoptosis, Moreover, the induction of apoptosis by TPA was blocked by anti-TNF alpha antibody. Similar results were obtained in the myeloblastic cell line KY821 cells, We also found that the induction of apoptosis by TPA was increased in cells overexpressed with TNF receptor 1 but not in control cells. Furthermore, TPA failed to induce the production of TNF alpha and apoptosis in cells with either their protein kinase C or mitogen-activated protein kinase pathway blocked. Our results indicate that TPA induces apoptosis, at least in part, through a pathway that requires endogenous production of TNF alpha in U937 cells. Our data also suggest that the induction of apoptosis by TPA occurs through activation of protein kinase C and mitogen-activated protein kinase and TNF alpha is an autocrine-stimulating factor for the induction of apoptosis in these cells.