Lipoprotein modulation of the intracellular localization of protein kinase C and alteration of phorbol ester-stimulated differentiation in the human monoblastic U937 cell line

The subcellular localization of protein kinase C and the ability of phorbol esters to alter cell phenotype were examined in the U937 monoblastic cell line. Protein kinase C activity was evaluated using an in vitro assay measuring histone phosphorylation in the cytosolic and detergent extracted particulate fractions obtained after disrupting cells that had been cultured previously under varying conditions. Depriving cells of serum for 2–3 days resulted in a time-dependent decrease in protein kinase C activity of the particulate fraction. The addition of as little as 0.5–1% fetal bovine serum to serum-deprived cells increased protein kinase C in the particulate fraction by up to 2- to 3-fold. In contrast lipoprotein-deficient serum did not mimic the effect of whole serum. However addition of high or low density lipoproteins to cells grown in lipoprotein-deficient serum or serum-free medium produced a concentration-dependent 2- to 3-fold increase in particulate protein C kinase activity. The maximal lipoprotein effect was similar to that observed with 5% fetal bovine serum and the concentrations of lipoproteins needed to increase protein kinase C activity were in the physiological range. Adherence to plastic was used as a marker of the differentiated phenotype. Cells cultured in lipoprotein-deficient serum did not differentiate in response to phorbol ester stimulation as well as cells cultured in 5% fetal bovine serum. These results suggest that serum lipoprotelns modulate protein kinase C localization and the response to phorbol ester stimulation in the U937 cell.