Modulation by Retinoids of mRNA Levels for Nuclear Retinoic Acid Receptors in Murine Melanoma Cells

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Abstract
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 RAR.alpha., RAR.beta., and RAR.gamma. in S91-C2 cells. mRNA for both RAR.alpha. and RAR.gamma. was detected in these cells, but no RAR.beta. mRNA could be found. Treatment with 10-7 and 10-6 M .beta.-all-trans-retinoic acid (RA) for 24 h caused a 1.5- to 2-fold increase in RAR.alpha. and RAR.gamma. mRNA, whereas lower concentrations of RA were ineffective. RAR.beta. 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, RAR.beta. mRNA induction occurred by 4 h and increased progressively, reaching a plateau after 24 h of treatment. RAR.beta. 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 RAR.beta. 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 RAR.beta. mRNA. With the exception of retinol and the trifluoromethyl nonyloxyphenyl analog of RA, the ability of the retinoids to induce RAR.beta. mRNA and their growth inhibitory effect were correlated. However, S91-C154, a RA-resistant mutant subclone derived from S91-C2 cells, showed mRNA levels of RAR.alpha. and RAR.gamma. and induction of RAR.beta. 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 RAR.alpha. and RAR.gamma. mRNAs. The level of RAR.beta. 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.