The estrogen-responsive B box protein (EBBP) restores retinoid sensitivity in retinoid-resistant cancer cells via effects on histone acetylation

Retinoids have significant clinical activity in several human cancers, yet the factors determining retinoid sensitivity in cancer cells are still unclear. Retinoid-induced expression of retinoic acid receptor (RAR) β2 is a necessary component of the retinoid anticancer signal in cancer cells. We have previously identified the Estrogen-responsive B Box Protein (EBBP), a member of the Tripartite Motif (TRIM) protein family, as a novel RARβ2 transcriptional regulator in the retinoid signal. Here we examined the mechanism of the EBBP effect on the retinoid anticancer signal. We assessed retinoid-responsive RARβ2 transcription in retinoid-resistant breast and lung cancer cells in the presence of chromatin modifying agents. A histone deacetylase (HDAC) inhibitor alone, or in combination with retinoid, was more effective than a demethylating agent in restoring RARβ2 transcription in resistant cells. Overexpression of EBBP alone markedly increased histone acetylation. The effect of EBBP on retinoid-responsive transcription appeared to be limited to genes with the retinoic acid response element (βRARE) regulatory sequence, such as CYP26A1. EBBP inhibited cell growth by effects on cyclin D1 and Phospho-Rb, and, reduced cell viability in retinoid-resistant cancer cells. The viability of non-cancer cells was unaffected by EBBP overexpression. Taken together our data suggests that EBBP acts to de-repress transcription of RARβ2 and CYP26A1, by modifying histone acetylation in retinoid-resistant cancer cells, and, is an important target for drug discovery in retinoid-resistant cancers.