text: Regulation of c-jun expression during induction of monocytic differentiation by okadaic acid. 
  The present work has examined the effects of okadaic acid, an inhibitor of type 1 and 2A protein phosphatases, on the regulation of c-jun expression during monocytic differentiation of U-937 leukemia cells. The results demonstrate that okadaic acid treatment is associated with induction of a differentiated monocyte phenotype characterized by: (a) growth arrest; (b) increases in Mac-1 cell surface antigen expression; (c) down-regulation of c-myc transcripts; and (d) induction of tumor necrosis factor gene expression. This induction of monocytic differentiation was associated with transient increases in c-jun mRNA levels, which were maximal at 6 h. Similar effects were obtained for the c-fos gene. Run-on analysis demonstrated detectable levels of c-jun transcription in U-937 cells and that this rate is increased approximately 40-fold following okadaic acid exposure. c-jun mRNA levels were superinduced in cells treated with both okadaic acid and cycloheximide, whereas inhibition of protein synthesis had little, if any, effect on okadaic acid-induced c-jun transcription. The half-life of c-jun mRNA was similar (45-50 min) in both untreated and okadaic acid-induced cells. In contrast, treatment with both okadaic acid and cycloheximide was associated with stabilization (t 1/2 = 90 min) of c-jun transcripts. Taken together, these findings indicate that the induction of c-jun transcription by okadaic acid is controlled primarily by a transcriptional mechanism. Since previous studies have demonstrated that the c-jun gene is autoinduced by Jun/AP-1, we also studied transcription of c-jun promoter (positions -132/+170)-reporter gene constructs with and without a mutated AP-1 element. (ABSTRACT TRUNCATED AT 250 WORDS) 