Lin F, Xiao D, Kolluri S K, Zhang X
The Burnham Institute Cancer Center, La Jolla, California 92037, USA.
Cancer Res. 2000 Jun 15;60(12):3271-80.
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.
类视黄醇的抗癌作用主要由两类核受体介导，即维甲酸受体（RARs）和类视黄醇X受体（RXRs），它们由三个不同的基因（α、β和γ）编码。最近的研究表明，RARβ在介导类视黄醇的抗癌作用中起关键作用。然而，RARβ如何发挥其强大的抗癌作用在很大程度上仍然未知。在本研究中，我们研究了RARβ的抗激活蛋白-1（AP-1）活性。在瞬时转染实验中，在维甲酸（RA）存在的情况下，所有三种RAR亚型，即RARα、RARβ和RARγ，都能有效抑制佛波酯12-O-十四酰佛波醇-13-乙酸酯诱导的AP-1活性以及癌基因c-Jun和c-Fos对含AP-1报告基因的活性。然而，RARβ对AP-1活性表现出强烈的不依赖RA的抑制作用，而RARα和RARγ对AP-1活性的抑制则依赖于RA。通过使用几种包含RARβ羧基末端部分或氨基末端部分的杂合受体，我们证明RARβ的氨基末端部分，即A/B结构域，主要负责对AP-1活性的不依赖RA的抑制作用。这种活性并非归因于RARβ的组成型AF-1活性，因为它没有激活几个含RA反应元件的报告基因。此外，曲古抑菌素A对组蛋白脱乙酰酶活性的抑制并不能克服RARβ的抑制作用。在癌细胞中，即使在没有RA的情况下，RARβ的稳定转染也表现出对AP-1活性的强烈抑制。此外，在稳定转染RARβ的癌细胞中，内源性AP-1反应基因胶原酶I的表达受到强烈抑制。在研究RARβ的抗转化活性时，我们观察到，当乳腺癌MDA-MB231细胞稳定转染RARβ而非RARα时，其在软琼脂中的生长以不依赖RA的方式受到显著抑制。总之，我们的结果表明，RARβ可能部分通过其独特的抗AP-1活性发挥其强大的抗癌作用。
