Modulation by retinoic acid (RA) of squamous cell differentiation, cellular RA-binding proteins, and nuclear RA receptors in human head and neck squamous cell carcinoma cell lines (original) (raw)

carcinomas in patients with head and neck or lung cancer, and skin cancers in patients with xeroderma pigmentosum (7â€"12). To evaluate the potential application ofretinoids for the prevention and treatment of head and neck premalignancies and SCC,3 we investigated the effects of retinoids on the growth and differentiation of Cultured HNSCC cells (13â€"18). We found that BA inhibited the proliferation of the majority of the HNSCC cell lines we examined and suppressed their transformed phenotype (13, 15, 18). These findings contrasted with previous reports that BA enhanced the growth of normal buccal mucosa cells (19), suggesting a selective inhibition oftumor cells. In addition, BA suppressed squamous differentiation of HNSCC cells (14, 15, 17, 20) and modulated two differentiation pathways in normal laryngeal keratino cytes and papilloma cells (21, 22). The mechanism(s) by which retinoids suppress carcinogenesis and regulate differentiation and the expression of the transformed pheno type in malignant cells has not been elucidated. It is thought that nuclear retinoid receptors, members of the steroid/thyroid hormone! vitamin D receptor family that act as ligand-activated transacting transcription factors, mediate the effects of retinoids on gene expres sion and thereby alter the growth and differentiation of normal and tumor cells. There are two types of retinoid nuclear receptors, RARs and RXRs. Each of the receptor types has at least three subtypes,-a,-@, and-‘y (3, 23â€"26). The RARs bind BA and 9-cis RA, whereas the RXRs bind 9-cis BA but not BA. The RXRs and RARs form het erodimers before binding to specific DNA sequences characterized by direct repeats of (A/G)GCITCA separated by two or five nucleotides, although complex elements have also been identified (24â€"26).Other factors affecting the retinoid-signaling pathway are the CRABP-I and CRABP-II, which have been implicated in controlling the level of BA available for interaction with nuclear receptors by sequestering the retinoid or increasing its catabolism (27, 28). The study presented here was designed to examine the expression of BA-binding proteins and receptors in HNSCC cell lines and their relationship to cell differentiation and responses to BA. MATERIALS AND METHODS Cell CUltUreand BA Treatment Procedures. The fourHNSCCcell lines (Table 1) were grown in monolayer cultures in a 1:1 (v/v) mixture of Dulbec co's modified Eagle's medium and Ham's F12 medium containing 5% FBS at 37°C. BA was dissolved in DMSO at a concentration of 102 Mand was stored in the dark at â€"20°C in N2. Stock solutions were diluted to the appropriate concentration with growth medium. Control cultures received the same amount of DMSO as treated cultures. For growth inhibition studies, cells were grown for 7 days in the absence (control) or presence of 1 @M RA. Medium was 3 The abbreviations used are: SCC, squamous cell carcinoma; HNSCC, head and neck