Inhibition of phorbol ester−induced monocytic differentiation by dexamethasone is associated with down−regulation of c−fos and c−jun (AP−1)

Previous studies have shown that treatment of human myeloid leukemia cells with 12−O−tetradecanoylphorbol−13−acetate (TPA) is associated with induction of monocytic differentiation and expression of the c−jun and c−fos early response genes. The present work demonstrates that the glucocorticoid dexamethasone inhibits TPA−induced increases in c−jun and c−fos mRNA levels in U−937 leukemia cells. These findings were associated with a block in appearance of the monocytic phenotype, including inhibition of TPA−induced increases in lamin A, lamin C, and vimentin transcripts. Other studies have demonstrated that TPA−induced monocytic differentiation and expression of the c−jun and c−fos genes in myeloid leukemia cells are regulated by protein kinase C (PKC). The finding that dexamethasone has no effect on TPA−induced activation of PKC suggests that this glucocorticoid inhibits signals downstream or parallel to this enzyme. Nuclear run−on assays demonstrate that: (1) induction of c−jun and c−fos expression by TPA is regulated by transcriptional mechanisms, (2) TPA−induced expression of c−jun and c−fos does not require protein synthesis, and (3) TPA−induced expression of both genes is inhibited at the transcriptional level by dexamethasone. To further define the effects of dexamethasone at the molecular level, we prepared a series of deleted c−jun promoter fragments linked to the chloramphenicol acetyltransferase (CAT) gene. Increases in CAT activity during transient expression of these constructs in TPA−treated U−937 cells could be assigned to the region (−97 to −20) of the promoter that contains the AP−1 binding site. This induction of CAT activity was sensitive to dexamethasone. These findings suggest that dexamethasone down−regulates TPA−induced transcription of the c−jun gene during monocytic differentiation by inhibiting activation of the AP−1 sit