Resistance to phorbol ester-induced differentiation of a U-937 myeloid leukemia cell variant with a signaling defect upstream to Raf-1 kinase.

Previous studies have demonstrated that treatment of human U-937 myeloid leukemia cells with 12-O-tetradecanoylphorbol-13-acetate (TPA) is associated with growth arrest and induction of monocytic differentiation. The present work describes the isolation of a U-937 cell variant, designated TUR, which is unresponsive to the growth-inhibitory effects of this agent. The results demonstrate that, in contrast to U-937 cells, the TUR line fails to respond to TPA with induction of the c-jun, junB, c-fos, and EGR-1 early response genes. The finding that these cells also fail to exhibit adherence or induction of the tumor necrosis factor and c-fms genes further supports their resistance to TPA-induced differentiation. In contrast, TUR cells responded to 1,25-dihydroxyvitamin D3, another inducer of monocytic differentiation, with growth arrest and induction of early response gene and c-fms transcripts. TUR cells also responded to okadaic acid, an inhibitor of type 1 and 2A protein phosphatases, with similar changes in gene expression. Further characterization of TUR cells has demonstrated decreased expression of protein kinase C as compared to wild-type U-937 cells. We also demonstrate that although treatment of U-937 cells with TPA is associated with activation of the Raf-1 serine/threonine kinase, there was no detectable decrease in electrophoretic mobility of this protein in TPA-treated TUR cells. Taken together, these findings indicate that the TUR variant is defective in TPA-induced signaling events upstream to activation of Raf-1 kinase.