Comparison of retinoic acid and phorbol myristate acetate as inducers of monocytic differentiation. 
Several human myeloid leukemia cell lines growing in vitro can be induced to differentiate to more mature monocyte/macrophage-like cells by treatment with protein kinase C-activating phorbol esters, such as PMA. In addition to PMA, cells of the THP-1 myeloid leukemia cell line acquire macrophage-like characteristics after treatment with all-trans retinoic acid (RA). To analyze the signal transduction mechanisms induced by RA, we first compared the effects of PMA and RA on the expression of genes which are known to be regulated during monocytic differentiation. Both RA and PMA effectively down-regulated c-myc expression, while c-myb expression decreased only after PMA treatment. Expression of the beta 2-integrin genes, CD11a and CD11b, was clearly increased after both of these treatments. Their effects on the src-family tyrosine kinase genes were different: hck expression was similarly induced by these agents but lyn expression was stronger and more rapid after RA treatment. RA also enhanced lyn mRNA production rapidly in HL-60, indicating that the activation of lyn gene expression is common in monocytic and granulocytic maturation of myeloid leukemia cells. To examine whether the AP-1 enhancer activity is involved in RA-induced monocytic differentiation, THP-1 cells were transiently transfected with a chloramphenicol acetyl transferase (CAT)-reporter gene containing 5 copies of the AP-1 binding sites. In contrast to PMA, RA did not induce any CAT activity in these cells, thus suggesting that the RA-induced changes in the expression of those genes described above were not dependent on the AP-1 enhancer activity.