Retinoids and their receptors in modulation of differentiation, development, and prevention of head and neck cancers.

Retinoids suppress oral premalignant lesions (OPLs) and decrease the incidence of second primary tumors in head and neck (HN) cancer patients. The optimal clinical use of retinoids depends on the understanding of the mechanism by which they suppress carcinogenesis. It is thought that retinoids restore normal cell growth and differentation by means of nuclear retinoic acid (RA) receptors (RARα, β and γ) and retinoid X receptors (RXRα, β and γ). We used cultured HNSCC cell lines and surgical specimens from normal oral mucosa and from premalignant oral lesions and HN squamous cell carcinomas (HNSCCs) to understand fundamental aspects of retinoids mode of action. RA inhibited the growth and suppressed the aberrant squamous cell differentiation in the majority of HNSCC cell lines examined. Four cell lines expressed mRNAs for RARα, RARβ, RARγ, and RXRα. RA treatment increased the levels of the three RAR mRNAs in most of the cell lines but had no effect on the RXR mRNAs. An inverse relationship has been established between the level of RARβ and squamous differentiation markers. The expression of retrovirally transduced RARβ in a HNSCC cell line, which did not express RARβ constitutively, showed a 1,000-fold increase in sensitivity to the suppression of squamous cell differentiation by RA. The expression of retrovirally transduced RARγ sense rendered the cells more sensitive, and expression of retrovirally transduced RARγ anti-sense made them less sensitize to growth inhibition by RA than parental cells. These results indicate that RARβ may mediate suppression of squamous differentiation, whereas RARγ may mediate the growth inhibition. RARα, RARβ, RARγ, RXRα, and RXRβ mRNAs were detected by in situ hybridization in specimens from normal oral mucosa, and all but RAR-β were also expressed in most of the adjacent normal, hyperplastic, dysplastic, and malignant HN tissues. In contrast, RARβ mRNA was detected in only 70% of adjacent normal and hyperplastic HN lesions, and its expression was decreased further to 56% of dysplastic lesions and to 35% of SCCs. Likewise, RARβ was expressed in only 40% of OPLs. Treatment of OPLs with 13-cis-RA increased RARβ expression from 40% to 90% of the cases. The increase in RARβ level was associated with clinical response. These results show that selective loss of RARβ mRNA expression occurs in the early stages of carcinogenesis and may be involved in HN cancer development ; and demonstrate that RARβ is induced by 13-cis-RA in humans.