Effects of chronic phorbol ester treatment on protein kinase C activity, content, and gene expression in the human monoblastoid U937 cell.

Immediate and sustained signal transduction is involved in mediating phorbol ester-induced changes in growth and differentiation. Activation of protein kinase C (PKC) is the initial step in phorbol ester-induced signal transduction. By virtue of preferential down-regulation of individual isoforms and generation of proteolytically derived kinase activities, the signal transduced by sustained activation of this pathway may differ substantially from that generated initially upon application of the phorbol ester. To examine the effect of chronic phorbol ester-induced activation of this pathway, the relationship between PKC activity/content and AP-1 binding activity and gene expression was studied in the U937 cell. Phorbol ester-induced differentiation of the U937 cell into a monocyte/macrophage-like cell requires sustained activation of the PKC pathway. AP-1 binding activity was enhanced by 12-O-tetradecanoylphorbol-13-acetate (TPA) and in a temporally dependent manner, with conversion of a high to low mobility band shift occurring after a 12-h exposure to TPA. After a 72-h exposure, AP-1 binding activity was maximally increased by 1 nM TPA and remained elevated to a similar degree even after treatment with 600 nM TPA. Enhanced AP-1 binding activity was dependent upon continuous exposure to TPA and was not secondary to differentiation. A 72-h treatment with one nM TPA maximally increased expression of c-jun, krox-24, and jun-B mRNA transcripts. Exposure to higher TPA concentrations decreased the content of these transcripts. Maximal expression of collagenase and plasminogen activator receptor transcripts required exposure to much higher TPA concentrations (100 nM).(ABSTRACT TRUNCATED AT 250 WORDS)