Down-regulation of Glucocorticoid Binding Sites by Retinoic Acid in Squamous Carcinoma Cells Resistant to the Induction of Keratinization by Hydrocortisone

Physiologic concentrations of retinoic acid strongly inhibit the in vitro maturation of human squamous carcinoma cells in serum-free medium. Differentiation, as measured by the capacity to synthesize cornified cell envelopes, could be induced by hydrocortisone in retinoic acid-treated SqCC/Y1 and CE-81T cells. However, two other cell lines (C4-1 and A431) were less competent to spontaneously form cornified cell envelopes and resistant to the induction of envelope competence by hydrocortisone in the presence of retinoic acid. To investigate the mechanism underlying the resistance of these two lines to hydrocortisone, the characteristics of glucocorticoid receptors were analyzed. Whole cell dexamethasone binding sites ranged from 1300 to 9000 sites per cell for the four cell lines. The binding affinity for dexamethasone was similar in all four squamous carcinoma cell lines (1.32 to 4.75 nM). During retinoic acid-treatment, the binding of dexamethasone by intact SqCC/Y1 and CE-81T cells increased 1.5- to 3.0-fold over 48 h. In contrast, the number of dexamethasone binding sites were decreased by 80% in retinoic acid-treated A431 and C4-1 cells. In each case, the regulation of dexamethasone binding was dependent on the concentration of retinoic acid, with maximal effects being observed at 10-6 M. Thus, the manner in which retinoic acid regulates the availability of dexamethasone binding sites might explain, in part, the effects of glucocorticoids on differentiation of retinoic acid-treated squamous carcinoma cell lines