Cloning and characterization of phorbol ester differentiation-resistant U937 cell variants.

Differentiation-resistant U937 cells were derived from parental U937 human promonocytic leukemia cells by selecting for a nonadherent phenotype in cell cultures continuously exposed to 12-O-tetradecanoylphorbol-13-acetate (TPA). Subsequent analysis indicated no differences between wildtype (wt) and resistant U937 cells with respect to protein kinase C (PKC) isozyme expression, activation, or down-modulation. The subcellular localization of PKCs is identical in wt and resistant cells with the exception of PKC beta 2, which no longer colocalizes with microtubules in the TPA-resistant cell lines. In contrast to wt-U937 cells, the resistant cells do not express beta 2-integrin adhesion molecules, cd11b and cd11c, on the cell surface following TPA treatment but do express cd11b and cd11c in intracellular vesicles. TPA stimulates the translocation of these vesicles to the cell surface in wt U937 cells but not in the resistant cells. These results suggest that events downstream of PKC activation may mediate cytoskeletal reorganization and beta 2-integrin transport to the cell surface in wt-U937 cells but not in the differentiation-resistant cells.