Retinoic acid‐induced AP‐1 transcriptional activity regulates B16 mouse melanoma growth inhibition and differentiation

Publisher Website
Google Scholar
Abstract
Retinoic acid (RA) inhibits growth and induces differentiation of B16 mouse melanoma cells. These effects are accompanied by a large increase in PKCα mRNA and protein levels and surprisingly an increase in activating protein‐1 (AP‐1) transcriptional activity. To further investigate the RA‐induced AP‐1 activity we established clones of B16 cells stably expressing an AP‐1‐luciferase reporter gene. Treatment of these clones with phorbol dibutyrate increased AP‐1 activity which peaked at 2–4 h and returned to baseline level by 24 h. In contrast, RA treatment resulted in a slow increase in AP‐1 activity that reached a maximum level at 48 h and was maintained for the duration of the treatment. We tested the importance of the RA‐induced AP‐1 activity by establishing clones which stably express a dominant negative fos gene (A‐fos) and have greatly diminished AP‐1 activity. Growth rates of untreated A‐fos expressing cells were similar to wt B16 and clones not expressing A‐fos. However, clones expressing the dominant‐negative fos had a markedly decreased sensitivity to RA‐induced inhibition of anchorage‐dependent and ‐independent growth. Treatment of wt B16 cells for 48 h with RA increased melanin production by two to fourfold, but this effect was completely lost in the A‐fos clones. The ability of RA to induce RARβ and PKCα expression was retained in A‐fos clones, suggesting that A‐fos was not interfering with RAR transcription activation functions. We tested whether the RA‐induced AP‐1 activity might be mediated by the ERK1/2 MAPK pathway. Inhibition of ERK1/2 phosphorylation stimulated AP‐1 activity, which was not additive to that induced by RA. This finding raises the possibility that this MAPK pathway may be a target of retinoid action. Our observations suggest that AP‐1 transcriptional activity induced by RA likely plays an important role in the biological changes mediated by this retinoid in B16 melanoma cells.