Retinoic acid induces neuroblastoma cell death by inhibiting proteasomal degradation of retinoic acid receptor alpha.

To seek a novel therapeutic approach to neuroblastoma (NBL), we used three NBL cell lines (SK-N-DZ, NH12, and SK-N-SH) to examine the underlining molecular mechanisms of cellular reactions and sensitivity to all-trans-retinoic acid (ATRA). SK-N-DZ cells expressed relatively high levels of retinoic acid receptor alpha (RAR-alpha) and underwent ATRA-induced cell death that was blocked by an RAR-alpha antagonist. By contrast, RAR-alpha expression gradually decreased in NH12 and SK-N-SH cells, which did not experience increased cell death in response to ATRA. We report here the ubiquitin-dependent down-regulation of RAR-alpha expression during ATRA treatment. Our data suggest that SK-N-DZ cells have a defect in RAR-alpha down-regulation, resulting in sustained high expression of RAR-alpha that confers high sensitivity to ATRA. Accordingly, treatment with a proteasome inhibitor dramatically increased ATRA-induced cell death in NH12 and SK-N-SH cell lines. Our results reveal the crucial involvement of the RAR-alpha signaling pathway in NBL cell death and show that three NBL cell lines are differentially sensitive to ATRA. These data suggest a potential novel therapy for NBL involving retinoic acid treatment combined with the inhibition of RAR-alpha degradation.