Abstract A208: Comparison of expression and function of two functional RARβ isoforms in human cancer: Clinical implications for the differential splicing of the RARβ gene

Abstract

Abstract Purpose: Retinoids are derivatives of vitamin A that are used for treating and preventing certain cancers. Retinoid sensitivity in the lung is linked to expression of a specific isoform of the retinoic acid receptor β (RARβ), termed RARβ1′. This study compared the function of RARβ1′ and RARβ2 and assessed the expression of these isoforms in a panel of cancers. Experimental Design: RARβ1′ and RARβ2 were independently co-transfected into BEAS-2B human bronchial epithelial cells along with a retinoid responsive reporter plasmid. Transfectants were then treated with 4 µM all-trans retinoic acid or control and reporter activation was measured. An isoform-specific antibody for detection of RARβ1′ was also developed and validated. Using this antibody, RARβ1′ expression was evaluated by immunoblot analyses of normal lung versus lung cancer tissue specimens as well as in a panel of cancer cell lines including those of breast, colon, prostate, glioblastoma (GBM) and lung cancer origins. RARβ1′ expression was then examined in an extended panel of GBM cell lines. GBM cell lines were independently treated with 13-cis-retinoic acid, and growth assays and microarray analyses were each performed. Results: In the absence of retinoid treatment, retinoid reporter activation was increased by co-transfection of RARβ2 (P = .02) but not RARβ1′ (P = .21) in BEAS-2B cells. When treated with the retinoid, transfection of either isoform enhanced reporter activation. RARβ1′ was noted to be expressed in normal lung tissue but absent in tumor tissue from patients with each of the common types of non-small cell lung cancer. Analysis of multiple cancer cell lines revealed RARβ1′ expression only in the GBM cell lines. No appreciable expression was detected in the breast, colon, prostate, or lung cancer cell lines. Analysis of an extended panel of GBM cell lines revealed RARβ1′ expression in all examined GBM cell lines. The examined GBM cell lines were found to be sensitive to treatment with pharmacologically achievable dosages of 13-cis retinoic acid. Microarray analyses revealed several novel candidate retinoid target genes activated in SNB-19 GBM cells. Conclusions: RARβ1′ and RARβ2 have differing effects on ligand-independent signaling. RARβ1′ was repressed in all examined lung cancer cases and in most of the cancer cell lines studied. RARβ1′ protein was abundantly expressed in GBM cell lines which may help to explain the activity of 13-cis retinoic acid for the treatment of GBM despite the modest single agent activity of retinoids for treating many epithelial cancers. Microarray data of GBM cells have uncovered several candidate retinoid-target genes that may represent novel therapeutic targets. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A208.