Autocrine regulation of TPA-induced apoptosis in monoblastic cell-line U-937: Role for TNF-alpha, MnSOD and IL-6

The present studies were undertaken to analyse the factors regulating TPA-i nduced apoptosis. Treatment of the monoblastic U-937 cells with the phorbol ester, TPA, was found to induce apoptosis in two distinct phases. In phase I (from 0 to 72 hours following TPA induction), apoptotic cells appeared, despite the expression of high levels of the anti-apoptotic Bcl-2 protein. After 96 h. of TPA treatment (phase II), the percentage of apoptotic cells increased as did the cell differentiation stage. The first phase apoptotic response could be significantly reduced (70%) by treatment with anti-tumor necrosis factor-alpha (TNF-alpha) antibody. TNF-alpha protein required de n ovo RNA and protein synthesis and was found to be mediated by protein kinas e and protein tyrosine kinases. Manganese superoxide dismutase (MnSOD) inhi bited, whereas IL-6 increased TPA-induced apoptosis. These findings suggest that both TPA, via TNF-alpha synthesis, exerts its protective function int racellularly by inducing MnSOD production and IL-6 may be an effective adju nct to TNF-alpha in the clinic, increasing the antitumor potency of this cy tokine.