Synergistic effect of co-treatment with all-trans retinoic acid and 9-cis retinoic acid on human lung cancer cell line at molecular level.

The major challenge in treating cancers with ATRA is the limited availability inside the cell and resistance developed in prolonged treatment. We made an attempt for co-treatment of human NSCLC cell lines (A549) with ATRA and its isomeric precursor (9cisRA). In this study, the growth inhibitory effect of ATRA, 9cisRA and combination of both were tested in A549 cells by MTT and Trypan blue assays. As the effects of retinoid are mediated through their receptors, their gene expression levels were analyzed by RT-PCR. The target gene receptor, RAR-β protein expression, was analyzed by immunocytochemistry. The cancer cell (A549) growth inhibitory effect was significantly (p ≤ 0.001) enhanced in combination treatment when compared with the result of individual treatments. The mRNA expression levels of both RAR-β and RXR-β were found to be increased in co-treatment (band density of 0.75 and 0.806, respectively) when compared with 9cisRA treatment (0.25 and 0.112) and ATRA treatment (0.01 and 0.081). A concomitant enhancement in the target RAR-β protein expression was observed in co-treated cells when compared with individual treatments. We thus conclude that the co-treatment had increased the availability of ATRA, by isomerization of the 9cisRA which then resulted in an increased expression of both RAR-β and RXR-β receptors and the target protein RAR-β which in turn inhibited lung cancer cell growth. Our study results have explored the mechanism of synergistic effect of co-treatment with ATRA and 9cisRA and further preclinical studies are necessary to validate the application of co-treatment of retinoid in clinical use.