DEFECTIVE TRANSLOCATION OF PROTEIN-KINASE-C IN MULTIDRUG-RESISTANT HL-60 CELLS CONFERS A REVERSIBLE LOSS OF PHORBOL ESTER-INDUCED MONOCYTICDIFFERENTIATION

Previous studies have demonstrated that human HL-60 myeloid leukemia c ells differentiate in response to phorbol esters. This event is associ ated with induction of the c-jun early response gene and appearance of a monocytic phenotype. The present studies have examined the effects of vincristine-selected, multidrug resistance on 12-O-tetradecanoylpho rbol-13-acetate (TPA)-induced HL-60 cell differentiation. The results demonstrate that multidrug-resistant HL-60 cells, designated HL-60/vin c, fail to respond to TPA with an increase in c-jun transcripts or oth er phenotypic characteristics of monocytic differentiation. By contras t, treatment of HL-60/vinc cells with okadaic acid, an inhibitor of se rine/threonine protein phosphatases, induces c-jun transcription, grow th arrest, and expression of the c-fms gene. Studies were also perform ed with an HL-60/vinc revertant (HL-60/vinc/R) line that has regained partial sensitivity to vincristine. The finding that HL-60/vinc/R cell s respond to TPA with induction of a monocytic phenotype, but not c-ju n expression, suggests that c-jun induction is not obligatory for mono cytic differentiation. Other studies further demonstrate that the jun- B and fra-1 genes are induced by TPA in both HL-60/vinc and HL-60/vinc /R cells, whereas c-fos expression is attenuated in the HL-60/vinc lin e. Since TPA activates protein kinase C (PKC), we examined translocati on of PKC from the cytosol to the membrane fraction. Although HL-60 an d HL-60/vinc/R cells demonstrated translocation of PKC activity, this subcellular redistribution was undetectable in HL-60/vinc cells. Activ ity of the mitogen-activated protein kinase family with associated pho sphorylation of c-Jun Y-peptide was markedly diminished in TPA-treated HL-60/vinc cells, but not in response to okadaic acid. Taken together , these findings suggest that vincristine resistance confers insensiti vity to TPA-induced differentiation and can include defects in PKC-med iated signaling events and induction of jun/fos early response gene ex pression.