Unique Anti-Activator Protein-1 Activity of Retinoic Acid

The anticancer effects of retinoids are mainly mediated by two classes of nuclear receptors, the retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which are encoded by three distinct genes (a, b, and g). Recent studies have demonstrated that RARb plays a critical role in mediating anticancer effects of retinoids. However, how RARb exerts its potent anticancer effects remains largely unknown. In this study, we investigated anti-Activator Protein-1 (AP-1) activity of RARb. In a transient transfection assay, all three RAR subtypes, RARa, RARb, and RARg, could effectively inhibit phorbol ester 12-O-tetradecanoylphorbol13-acetate-induced AP-1 activity and the activity of oncogenes c-Jun and c-Fos on AP-1 containing reporter genes in the presence of retinoic acid (RA). However, RARb showed a strong RA-independent inhibition of AP-1 activity, whereas inhibition of AP-1 activity by RARa and RARg was RA dependent. By using several hybrid receptors that contain either the COOH-terminal portion or the NH2-terminal portion of RARb, we demonstrated that the NH2-terminal portion of RARb, the A/B domain, was mainly responsible for the RA-independent inhibition of AP-1 activity. This activity was not attributable to constitutive AF-1 activity of RARb, because it did not activate several RA response element-containing reporter genes. In addition, inhibition of histone deacetylase activity by trichostatin A did not overcome the inhibitory effect of RARb. In cancer cells, stable transfection of RARb exhibited strong inhibition of AP-1 activity, even in the absence of RA. Moreover, expression of endogenous AP-1-responsive gene collagenase I was strongly repressed in cancer cells stably transfected with RARb. In studying the antitransforming activity of RARb, we observed that the growth of breast cancer MDA-MB231 cells in soft agar was significantly repressed in a RA-independent manner when cells were stably transfected with RARb but not RARa. Together, our results demonstrate that RARb may exert its potent anticancer effect in part through its unique anti-AP-1 activity.