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

Abstract

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 site.