Dexamethasone and retinoic acid regulate the expression of epidermal growth factor receptor mRNA by distinct mechanisms

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Abstract
Retinoic acid and dexamethasone have antagonistic effects on epidermal growth factor (EGF) receptor expression in fetal rat lung (FRL) cells: Receptor synthesis is enhanced by retinoic acid and reduced by dexamethasone. In the presence of actinomycin D, neither agent has the capacity to modify receptor synthesis or 125I-EGF binding capacity. Northern blot analysis demonstrates a tenfold increase in EGF mRNA following retinoic acid treatment and a 60% decrease in receptor message levels after dexamethasone treatment. To dissect the mechanisms of these effects, the expression of mRNA was separated from effects requiring protein synthesis by the use of cycloheximide and actinomycin D. Ligand binding, EGF receptor protein synthesis, and mRNA levels were measured in cultures of FRL cells that were incubated with retinoic acid or dexamethasone in the presence of cycloheximide, then washed and reincubated with fresh media containing actinomycin D, but not retinoic acid, dexamethasone, or cycloheximide. The results demonstrate that dexamethasone reduces the expression of EGF receptor mRNA in the absence of protein synthesis. In contrast, the mechanism by which retinoic acid increases the expression of EGF receptor mRNA requires protein synthesis. These data indicate that, in FRL cells, dexamethasone negatively regulates EGF receptor mRNA in a direct manner, while retinoic acid controls transcription of an intermediate protein, possibly a transcription factor, that subsequently increases transcription of receptor message.