OKADAIC ACID, A PHOSPHATASE INHIBITOR, ENHANCES THE PHORBOL ESTER-INDUCED INTERLEUKIN-1-BETA EXPRESSION VIA AN AP-1-MEDIATED MECHANISM

Protein kinase C (PKC)-activating phorbol esters are known to induce the expression of several genes in monocytic cells. As the effect of serine-threonine kinases, such as PKC, is often counteracted by specific protein phosphatases, we have now examined the role of phosphatases in the regulation of the phorbol ester (PMA) -induced interleukin-1beta (IL-1beta) gene expression in the THP-1 monocytic leukaemia cell line. Okadaic acid (OA) is a potent tumour promoter, the function of which is based on its activity to inhibit the serine/threonine specific phosphatases 1 and 2A (PP1 and PP2A, respectively). Thus, it mimicks or potentiates the action of PKC activators in several cell types. Our data demonstrate that alone OA induced a very weak expression of IL-1beta mRNA, but it strongly enhanced the PMA-induced IL-1beta expression. To analyse the site of action of OA, the cells were transiently transfected with a chloramphenicol acetyl transferase (CAT) -reporter plasmid containing the AP-1 binding site as the enhancer. Alone, OA was a weak inducer of CAT-activity in these cells, but again it strongly enhanced the PMA-induced response. Similar data were obtained with cells transfected with a reporter plasmid containing the PMA-responsive element (containing a putative AP-1 binding site) of the IL-1beta gene. Thus, these data indicate that the PMA-induced AP-1 enhancer activity, which is required for the expression of the IL-1beta gene, is controlled in these cells by PPI and/or PP2A. As OA did not synergize with PMA in the induction of expression of genes encoding the AP-1 proteins (c-fos, c-jun, junB), it is likely that OA potentiates the AP-1 enhancer activity by its effect on protein phosphorylation.