Retinoids inhibit the growth and enhance the differentiation of murine S91-C2 melanoma cells. Specific alterations in gene expression are a plausible mechanism for these effects. Since nuclear retinoic acid receptors (RAR) are likely mediators of retinoid-induced changes in gene expression, we used Northern blotting to analyze the expression of RARα, RARβ, and RARγ in S91-C2 cells. mRNA for both RARα and RARγ was detected in these cells, but no RARβ mRNA could be found. Treatment with 10-7 and 10-6 m β-all-trans-retinoic acid (RA) for 24 h caused a 1.5- to 2-fold increase in RARα and RARγ mRNA, whereas lower concentrations of RA were ineffective. RARβ mRNA, which was undetectable in untreated cells, was detected after 24 h of treatment with a RA concentration as low as 10-9 m, and its level increased with up to 10-6 m RA. At the latter dose, RARβ mRNA induction occurred by 4 h and increased progressively, reaching a plateau after 24 h of treatment. RARβ mRNA induction at 4 h was not inhibited by cycloheximide at a concentration that suppressed protein synthesis by more than 90%. Several retinoids and related synthetic compounds, including 13-cis RA, TTNPB, Ch55, Am80, and the trifluoromethyl nonyloxyphenyl analog of RA, also induced RARβ mRNA, whereas a 24-h treatment with 10-6 m retinol, TTNP (a decarboxylated analog of TTNPB), or the phenyl analog of RA failed to induce RARβ mRNA. With the exception of retinol and the trifluoromethyl nonyloxyphenyl analog of RA, the ability of the retinoids to induce RARβ mRNA and their growth inhibitory effect were correlated. However, S91-C154, a RA-resistant mutant subclone de rived from S91-C2 cells, showed mRNA levels of RARα and RARγ and induction of RARβ by RA similar to those detected in the sensitive S91-C2 cells. Like the S91 melanoma cells, two other mouse melanoma cell lines, K-1735P and B16-F1, constitutively expressed RARαand RARγ mRNAs. The level of RARβ mRNA was increased by RA only in B16-F1 cells, although the growth of both was inhibited by RA. These results demonstrate that RA can, directly and rapidly, induce the expression of mRNA for a high affinity nuclear receptor in some murine melanoma cells and that this induction is not sufficient to inhibit growth. © 1990 by The Endocrine Society.
