Cancer Prevention Research Antitumor Effect of Retinoic Acid Receptor-β2 Associated with Suppression of Cyclooxygenase-2

Retinoic acid receptor-β2 (RAR-β2) is a putative tumor suppressor gene in various cancers. To determine the underlying molecular mechanisms, we transfected RAR-β2 cDNA into esophageal cancer TE-1 and TE-8 cells and found that RAR-β2 suppressed tumor cell growth in vitro and tumor formation in nude mice in TE-8 cells, whereas the stable transfection of RAR-β2 did not restore retinoid sensitivity or inhibit tumor formation in nude mouse in TE-1 cells. Molecularly, we revealed that RAR-β2 antitumor activity was associated with expression and suppression of cyclooxygenase-2 (COX-2) in these tumor cell lines. Moreover, antisense RAR-β2 cDNA induced COX-2 expression in TE-3 cells. Furthermore, when COX-2 expression is first blocked by using antisense COX-2 expression vector, the effect of RAR-β2 is diminished in these tumor cells. In addition, we analyzed expression of RAR-β2 and COX-2 mRNA in tissue specimens and found that RAR-β2 expression is associated with low levels of COX-2 expression in esophageal cancer tissues. Induction of RAR-β2 expression in oral leukoplakia tissues after the patients treated with 13-cis RA correlated with a reduction in COX-2 expression and clinical response. Our findings indicate that some of RAR-β2 antitumor activities are mediated by suppression of COX-2 expression in some of these esophageal cancer cells. After correlating antitumor effect of RAR-β2 with COX-2 expression in the published studies, we also found the association. Thus, further studies will determine whether manipulation of COX-2 expression in different cancers can antagonize RAR-β2 activity.