Evidence for Separate Control by Phorbol Esters of CD18-dependent Adhesion and Translocation of Protein Kinase C in U-937 Cells¹

Treatment of U-937 GTB cells with tumor-promoting phorbol esters induced adherence of the cells to plastic, with a t_½ of 20 min. The ED₅₀ was determined to 3.3 nm for phorbol-12,13-dibutyrate and 0.3 nm for 12-O-tetradecanoyl-4β-phorbol-13-acetate, whereas the non-tumor-promoting analogue 12-O-tetradecanoyl-4α-phorbol-13-acetate was ineffective at concentrations up to 100 nm. The adherence process showed characteristics typical of leucocyte adhesion and was inhibited by a monoclonal antibody to the leucocyte adhesion molecule CD18. The sublines of U-937, RES and RESREV made resistant to the action of low doses of phorbol ester regarding inhibition of DNA synthesis and containing lower levels of protein kinase C compared to U-937 GTB, were desensitized with respect to the adhesion response. Translocation of protein kinase C from cytosol to the particulate fraction occurred at about 10-fold higher concentrations of phorbol ester than the adhesion response in U-937 GTB cells, under otherwise similar conditions, whereas no difference in sensitivity was observed between the sublines. Also phorbol ester stimulation of choline incorporation into lipids exhibited lower sensitivity compared to the adhesion response with no difference observed between the various cell lines. The results indicate that CD18-dependent adhesion, like DNA synthesis, is controlled by phorbol esters in a manner unrelated to the translocation of protein kinase C and that the control mechanism might involve forms of protein kinase C which are subject to stable down-modulation following TPA adaption of the cells.