Retinoic acid receptor beta induces terminal differentiation of squamous cell carcinoma lines in the absence of cyclin-dependent kinase inhibitor expression.

The chemotherapeutic agent and vitamin A metabolite retinoic acid (RA) has been used to treat many tumor types. The effects of RA are mediated by a family of ligand-dependent transcription factors, the retinoic acid receptors (RAR) and the retinoid X receptors. Alterations in RAR expression have been implicated in tumorigenesis. We showed previously that loss of RARbeta expression and target gene responsiveness to RA were consistent features of head and neck squamous cell carcinoma lines. To begin to elucidate the role of RARbeta in the malignant transformation of stratified squamous epithelia, we reexpressed RARbeta in squamous cell carcinoma (SCC) lines by stable transfection. SCC lines expressing RARbeta produced large numbers of flat cells with abundant cytoplasm that died and detached from the culture dish. These cells morphologically resembled the differentiated cells of normal stratified squamous epithelium in culture. These cells did not exhibit the characteristic DNA fragmentation pattern of apoptotic cells, nor did they label in a fluorescent apoptosis assay. Northern and Western blotting revealed induction of terminal differentiation markers in these cells. RARbeta produced alterations in levels of RA-responsive gene expression in these cells, and terminal differentiation occurred in the absence of detectable cyclin-dependent kinase inhibitor expression. We concluded that RARbeta expression induced terminal differentiation in SCC lines, suggesting a role for this transcription factor in the normal maturation of stratified squamous epithelium.