Expression of retinoic acid receptor gamma correlates with retinoic acid sensitivity and metabolism in head and neck squamous cell carcinoma cell lines

Retinoids, analogues of vitamin A, can reverse premalignant lesions and prevent second primary tumors in patients with head and neck squamous cell carcinoma (HNSCC). The effects of retinoids are mediated by retinoic acid receptors (RARs) and retinoid X receptors (RXRs), which act as ligand-activated transcription factors. The regulation of cell growth, differentiation and retinoid metabolism in normal, premalignant and malignant cells by retinoids is thought to be a result of their effects on gene expression. We investigated mRNA expression of RARs (α, β, and γ) and RXR-β by means of RNase protection and related this to retinoic acid (RA)-induced growth inhibition and RA turnover in four HNSCC cell lines (UM-SCC-14C, UM-SCC-22A, UM-SCC-35 and VU-SCC-OE). An RA-resistant subline of UM-SCC-35 was generated by exposure to increasing concentrations of RA for 8 months (designated UM-SCC-35R). RA turnover was determined on the basis of decreasing RA levels in the cells and culture medium after exposure to I μM RA. We found that RAR-γ mRNA expression was strongly correlated with RA-induced growth inhibition (p = 0.016, R = 0.92) and RA turnover (p = 0.041, R = 0.86). RAR-β transcript levels were reduced in three of five cell lines compared with normal mucosa, and these did not correlate with RA-induced growth inhibition and RA turnover. Expression of RAR-α and RXR-β was not substantially altered in any of the cell lines. These findings suggest that in HNSCC cell lines RAR-γ is the most important retinoid receptor for regulation of RA turnover rate and RA-induced growth inhibition.