Synergism between protein‐kinase C and cAMP‐dependent pathways in the expression of the interleukin‐1β gene is mediated via the activator‐protein‐1 (AP‐1) enhancer activity

In many different cell types treatment with phorbol esters (e.g. 4β‐phorbol 12‐myristate 13‐acetate, PMA) leads to the activation of protein‐kinase C (PKC) and subsequently to the activation of the activator‐protein‐1(AP‐1)‐responsive gene expression. We have previously reported that a structural analog of cAMP (dibutyryl cAMP, Bt 2 cAMP) or agents elevating the endogenous cAMP levels strongly enhanced the PMA‐induced interleukin‐1β(IL‐1β)‐gene expression in human myeloid leukemia cells (THP‐1, HL‐60). We have now examined the role of AP‐1 in the regulation of the IL‐1β gene expression by PKC and cAMP in THP‐1 cells. AP‐1 is a complex composed of products of the jun and fos gene families. Our studies show that Bt 2 cAMP enhances the PMA‐induced c‐ fos and jun ‐B expression, but inhibits c‐ jun expression. Electrophoretic mobility‐shift assay revealed that Bt 2 cAMP also increased the PMA‐induced AP‐1 DNA‐binding activity. The functional role of the increased AP‐1 DNA‐binding activity was studied by transfecting THP‐1 cells with reporter constructs containing AP‐1 sites [Col‐TREx5/TK‐CAT and IL‐1β‐X‐CAT, which contains the putative 12‐ O ‐tetradecanoyl‐phorbol‐13‐acetate(TPA)‐responsive element of the IL‐1β gene]. Transient transfection assay demonstrated that Bt 2 cAMP similarily increased the PMA‐induced transcription from both of these reporter constructs. Taken together, these results suggest that cAMP increases the PMA‐induced AP‐1 activity which then leads to increased IL‐1β expression.