RARβ involvement in enhancement of lung tumor cell immunogenicity revealed by array analysis

The retinoid receptors (RARs and RXRs) are mediators of the multiple effects of retinoic acid. Of these, the retinoic acid receptor β2 (RARβ2) has frequently been shown to be the principal mediator of the growth and tumor suppressive effects of retinoic acid; this gene is inactivated in many epithelial tumors and their derived cell lines. We have searched for genes that are regulated by this isoform and are potentially involved in tumor suppression. Using the Atlas human cDNA array I, we identified 27 genes (not counting RARβ itself) that are regulated, directly or indirectly, by RARβ2 when it is transfected into Calu-1, a lung tumor-derived line that does not normally express RARβ. Several of the affected genes code for proteins whose functions would augment the process of apoptosis and/or the host's immune response. The latter group included ICAM-1 and MHC class I heavy chain, whose protein products play particularly important roles in the mounting of an effective antitumor response. We then confirmed by flow cytometry that the observed increases in message levels were reflected in increased cell surface protein levels for ICAM-1 and MHC class I in RARβ2 transfectants of two RARβ-deficient lines, Calu-1 and the epidermoid lung cancer-derived line SK- MES. Finally, we showed that RARβ2 transfection of Calu-1 cells enhanced the heterologous CTL response in both the induction and the effector phases by up to threefold. These results support the hypothesis that down-regulation of these genes (and possibly others) in RARβ-deficient tumor cells contributes to immune system evasion, and suggest a novel therapeutic approach for this disease.