Differential responsiveness to phorbol esters correlates with differential expression of protein kinase C in KG-1 and KG-1a human myeloid leukemia cells

The KG-1 myeloid leukemia cell line differentiates into macrophages in response to 12-O-tetradecanoylphorbol 13-acetate (TPA), while its spontaneous variant, the KG-1a cell line, is resistant to the differentiative effects of TPA. To determine the mechanism underlying these differential responses to TPA, experiments were performed to determine the relative numbers of TPA binding sites, protein kinase C (PKC) enzyme activity levels, PKC translocation responses and PKC gene expression in these cell lines. KG-1a cells exhibited 40% fewer high affinity binding sites for TPA than KG-1 cells. Although PKC translocation from cytosol to the membrane fraction was observed in both cell types, total PKC activity, measured in vitro using type III-S histone as substrate, was reduced by 70% in KG-1a cells. These biochemical differences between the parental line and the phorbol ester non-responsive variant were correlated with the depressed level of PKC-β RNA abundance in KG-1a cells. Both lines expressed PKC-α RNA at comparable levels. Chronic exposure to TPA resulted in down-regulation of PKC enzyme activity in both cell lines, and a selective decrease in PKC-β RNA transcripts in both cell types. In contrast, chronic TPA treatment had no effect on the levels of PKC-α RNA in KG-1 and KG-1a cells. Our results indicate a correlation between the level of PKC-β expression and the responsiveness of myeloid lineage precursor cells to the differentiative effects of TPA.