| Abstract: | The human promyelocytic leukemia cell line HL-60 can be induced to differentiate into macrophage-like cells by nanomolar concentrations of phorbol esters. A phorbol ester-resistant variant R1B6 obtained by culturing HL-60 cells with increasing concentrations of 12-O-tetradecanoylphorbol-13-acetate, is reversibly resistant. These cells have been growing continuously in the presence of phorbol esters for more than 1 yr, but when the phorbol ester is removed, the cells gradually regain their sensitivity and express characteristics of macrophage-like cells upon readdition of phorbol ester. The concentration of phorbol ester receptors in R1B6 is about one-third that in the parental HL-60 cells. The reversion of the variants to sensitivity to phorbol esters is associated with the up regulation of the cytosol and membrane phorbol ester receptors. When partially purified, these receptor populations contain protein kinase C activity, in support of the identity of protein kinase C and the receptor. This study demonstrates that a phenotypic change in a clonal cell population correlates with the up regulation of the phorbol ester receptor-calcium-activated phospholipid-dependent protein kinase. This variant cell line is a useful model for analyzing the relationship between phorbol ester binding and protein kinase C during differentiation of HL-60 cells. |
