Overexpression of retinoic acid receptor β induces growth arrest and apoptosis in oral cancer cell lines

Expression of retinoic acid receptor beta (RAR beta) is reported to be absent or down-regulated in oral squamous cell carcinomas. Recently, we found that the growth-inhibitory effect of 9-cis-retinoic acid (9CRA) on oral squamous cell carcinoma may depend on the expression levels of endogenous RAR beta, In order to clarify the role of RAR beta in growth and differentiation, we transfected RAR beta expression vector into oral squamous carcinoma cell lines, HSC-4 and Ho-1-N-1. Both RAR beta -transfected cell lines displayed growth inhibition. Moreover, RAR beta -transfected clones underwent morphological changes, and RAR beta -transfected HSC-4 clones underwent apoptosis even in the absence of 9CRA treatment. In contrast, RAR beta -transfected Ho-1-N-1 clones exhibited cell cycle arrest without undergoing apoptosis initially; however apoptosis was induced in these cells after 6 days of 9CRA treatment. RAR alpha and RAR gamma expression was reduced at both the protein and mRNA levels in RAR beta transfectants, whereas the expression of retinoid X receptor alpha (RXR alpha) was not altered. RAR beta transfectants exhibited alterations in the levels of cell cycle-associated proteins, histone acetyltransferase (HAT) and apoptosis-associated proteins. After 6 days of 9CRA treatment, RAR beta transfectants overexpressed Waf1/Cip1/Sdi1/p21, Kip1/p27, chk1, p300/CBP, BAX, Bah, Apaf 1, caspase 3 and caspase 9, Conversely, E2F1, cdc25B and HDAC1 were down-regulated in these transfectants. In addition, histone H4 acetylation was induced in RAR beta transfectants, These findings suggest that histone acetylation mediated by histone acetyltransferase and p300/CBP may play a role in the growth arrest and apoptosis induced by RAR beta transfection in oral squamous cell carcinoma.