Histone deacetylase inhibitors regulate retinoic acid receptor β expression in neuroblastoma cells by both transcriptional and posttranscriptional mechanisms

The retinoic acid receptor beta ( RAR beta) is a retinoic acid ( RA)- inducible tumor suppressor, which plays an important role in the arrest of neuroblastoma cell growth. Using human neuroblastoma SH- SY5Y cells, we have examined the regulation of RAR beta expression by histone deacetylase inhibitors ( HDACi), considered to be promising agents in anticancer therapy. Our results show that HDACi cooperated with RA to increase RAR beta mRNA levels and to activate the RAR beta 2 promoter in transient transfection assays. Chromatin immunoprecipitation assays showed that the basal RAR beta 2 promoter that contains the RA response element was refractory to acetylation by both HDACi and RA. In addition, HDACi caused a transient increase in acetylation of a downstream RAR beta 2 region, even though global histones remain hyperacetylated after prolonged treatment with the inhibitors. RA potentiated this response and maintained acetylation for a longer period. Despite the cooperation of RA with HDACi to increase transcription of the RAR beta gene, these inhibitors caused a paradoxical reduction of the cellular levels of the RAR beta protein in cells treated with the retinoid. This reduction is secondary to a change in the protein half- life that is decreased by the HDACi due to increased ubiquitinin-dependent proteasomal degradation. These results show that HDACi regulate expression of the tumor suppressor gene RAR beta by both transcriptional and posttranscriptional mechanisms and might then modulate sensitivity to the retinoid in neuroblastoma cells.