Treatment with retinoic acid (RA) or carnosol, two structurally unrelated compounds with anticancer properties, inhibited phorbol ester (PMA)-mediated induction of activator protein-1 (AP-1) activity and cyclooxygenase-2 (COX-2) expression in human mammary epithelial cells. The induction of COX-2 transcription by PMA was mediated by increased binding of AP-1 to the cyclic AMP response element (CRE) of the COX-2 promoter. Inhibition of the histone acetyltransferase activity of CREB- binding protein (CBP)/p300 blocked the induction of COX-2 by PMA. Treatment with carnosol but not RA blocked increased binding of AP-1 to the COX-2 promoter. Because AP-1 binding was unaffected by RA, we investigated whether RA inhibited COX-2 transcription via effects on the coactivator CBP/p300. Treatment with RA stimulated an interaction between RA receptor-alpha and CBP/p300; a corresponding decrease in the interaction between CBP/p300 and c-Jun was observed. Importantly, overexpressing CBP/p300 or dominant-negative RA receptor-alpha relieved the suppressive effect of RA on PMA-mediated stimulation of the COX-2 promoter. To elucidate the mechanism by which carnosol inhibited COX-2 transcription, its effects on protein kinase C (PKC) signaling were determined. Carnosol but not RA inhibited the activation of PKC, ERK1/2, p38, and c-Jun NH2-terminal kinase mitogen-activated protein kinase. Overexpressing c-Jun but not CBP/p300 reversed the suppressive effect of carnosol on PMA-mediated stimulation of COX-2 promoter activity. Thus, RA acted by a receptor-dependent mechanism to limit the amount of CBP/p300 that was available for AP-1-mediated induction of COX-2. By contrast, carnosol inhibited the induction of COX-2 by blocking PKC signaling and thereby the binding of AP-1 to the CRE of the COX-2 promoter. Taken together, these results show that small molecules can block the activation of COX-2 transcription by distinct mechanisms.