Retinoids have shown significant activities in
cancer prevention and
therapy. Many of their effects are mediated by nuclear
retinoid receptors including
retinoic acid receptors (RARs alpha, beta and gamma) and
retinoid X receptors (RXRs alpha, beta and gamma). Human
retinoic acid receptor beta (RARbeta) has three different
isoforms: beta1, beta2 and beta4. The
tumor suppressive characteristics of RARbeta2, its silencing by promoter hypermethylation, and its reexpression following demethylation have been reported. In contrast, RARbeta1, an embryonic
isoform with restricted expression in adult tissues has been linked to
carcinogenesis. However, factors regulating RARbeta1 expression have not yet been clarified. During studies on the
head and neck squamous cell carcinoma cells, we found that the expression of RARbeta increased in cells grown to high density. Real-time
reverse-transcriptase polymerase chain reaction revealed that the
isoform increased in these cells was RARbeta1. Epigenetic modifications of this
isoform were tested using combined
bisulfite restriction analysis and
chromatin immunoprecipitation assays. The UMSCC38 cell line showed significant RARbeta1 expression (p < 0.001), which was dependent on cell density and culture duration. The increased expression of RARbeta1 was not due to demethylation of its promoter. However, higher cell densities were associated with increased acetylation of
histone 3 at
lysine 9 in RARbeta1 but not in RARbeta2. These findings reveal that the expression of RARbeta1 is regulated by cell density through changes in
histone acetylation.