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Retinol and retinoic acid dose-response curves were obtained for promotion of the differentiation of F9 murine embryonal carcinoma cells with an enzyme-linked immunoadsorbent assay for laminin, a product of differentiated F9 cells. Retinoic acid produced a half-maximum response at 1.3 nM and a maximum response at about 30 nM; retinol was 1/175th as potent. Maximum differentiation required 48 hr of continuous exposure to retinoic acid, whereas retinol required 72 hr of exposure. The half-time of retinoic acid conversion into polar metabolites was 3.5 hr; metabolism was accelerated by pretreating F9 cells with retinoic acid. An inhibitor of oxidative metabolism, ketoconazole, decreased the rate of retinoic acid metabolism and decreased the concentration of retinoic acid required to produce a half-maximum response. Unchanged retinoic acid was the sole compound isolated from nuclei of F9 cells incubated with retinoic acid. Retinol had a half-life approximately 5-fold longer than retinoic acid, attained greater cell concentrations, and was converted into retinoic acid by F9 cells. These data indicate that retinoic acid itself directs the differentiation of F9 cells and may mediate differentiation induced by retinol.
Williams et al. (Mon,) studied this question.