Overexpression of Protein Kinase C - C Stimulates

A function for protein kinase C(PKC), a member of the phorbol ester nonresponsive atypical protein kinase C subfamily, in modulating differentiation was examined in the leukemic U937 cell. Transfected U937 cells stably overexpressing PKCdisplayed a longer doubling time, lower saturation density at confluency, and an increase in adherence to plastic as compared to control cells. PKCcells expressed a more differentiated phenotype as assessed by changes in morphology, surface antigen expression, and lysosomal enzyme activities and were distinct from parental U937 cells stimulated to differentiate by exposure to phorbol esters. In contrast to parental U937 cells, PKC-C cells constitutively expressed mRNA transcripts for c-jun and a low mobility AP-1 binding activity. Thus, PKC-C overexpression stimulates a type of phenotypic differentiation that differs significantly from maturation occurring upon activation of other PKC subfamilies induced by phorbol ester treatment. Increased expression of the c-jun protooncogene and an increase in AP-1 binding activity in PKC-C cells provides a potential mechanism for explaining the altered differentiation status of this cell.