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Unique anti-activator protein-1 activity of retinoic acid receptor beta
1The Burnham Institute Cancer Center, La Jolla, California 92037, USA.
Abstract:
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 (alpha, beta, and gamma). Recent studies have demonstrated that RARbeta plays a critical role in mediating anticancer effects of retinoids. However, how RARbeta exerts its potent anticancer effects remains largely unknown. In this study, we investigated anti-Activator Protein-1 (AP-1) activity of RARbeta. In a transient transfection assay, all three RAR subtypes, RARalpha, RARbeta, and RARgamma, could effectively inhibit phorbol ester 12-O-tetradecanoylphorbol-13-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, RARbeta showed a strong RA-independent inhibition of AP-1 activity, whereas inhibition of AP-1 activity by RARalpha and RARgamma was RA dependent. By using several hybrid receptors that contain either the COOH-terminal portion or the NH2-terminal portion of RARbeta, we demonstrated that the NH2-terminal portion of RARbeta, 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 RARbeta, 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 RARbeta. In cancer cells, stable transfection of RARbeta 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 RARbeta. In studying the antitransforming activity of RARbeta, 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 RARbeta but not RARalpha. Together, our results demonstrate that RARbeta may exert its potent anticancer effect in part through its unique anti-AP-1 activity.
Insights
Retinoic acid receptor beta (RARbeta) uniquely inhibits Activator Protein-1 (AP-1) activity independently of retinoic acid (RA). This RARbeta anti-AP-1 activity, mediated by its A/B domain, contributes to its potent anticancer effects, including inhibiting cancer cell growth.
Area of Science:
- Molecular Biology
- Oncology
- Cell Biology
Background:
- Retinoids mediate anticancer effects via retinoic acid receptors (RARs) and retinoid X receptors (RXRs).
- RARbeta is critical for retinoid-mediated anticancer effects, but its mechanisms are unclear.
- Activator Protein-1 (AP-1) is a key transcription factor involved in cancer development.
Purpose of the Study:
- To investigate the anti-AP-1 activity of RARbeta.
- To determine the role of RARbeta in mediating anticancer effects through AP-1 inhibition.
- To identify the domains of RARbeta responsible for AP-1 inhibition.
Main Methods:
- Transient and stable transfection assays in cancer cells.
- Reporter gene assays to measure AP-1, c-Jun, and c-Fos activity.
- Analysis of RARbeta domains and their effect on AP-1 activity.
- Soft agar colony formation assays to assess antitransforming activity.
Main Results:
- RARbeta, RARalpha, and RARgamma inhibited AP-1 activity in a retinoic acid (RA)-dependent manner.
- RARbeta uniquely inhibited AP-1 activity independently of RA.
- The NH2-terminal A/B domain of RARbeta mediated this RA-independent inhibition.
- Stable RARbeta expression repressed endogenous AP-1-responsive genes and inhibited cancer cell growth in soft agar.
- RARbeta's anti-AP-1 activity was not linked to constitutive activation of RA response elements or histone deacetylase inhibition.
Conclusions:
- RARbeta possesses a unique RA-independent anti-AP-1 activity.
- The A/B domain of RARbeta is crucial for its RA-independent anti-AP-1 function.
- RARbeta's anti-AP-1 activity contributes to its anticancer effects, including inhibiting cell transformation and proliferation.