RETINOID-DEPENDENT TRANSCRIPTIONAL SUPPRESSION OF CYTOKERATIN GENE-EXPRESSION IN HUMAN EPIDERMAL SQUAMOUS-CELL CARCINOMA-CELLS

We have previously demonstrated that cytokeratin levels are coordinately regulated in normal cultured human keratinocytes. In the present study we examine the mechanism of this regulation using human squamous cell carcinoma (SCC) cells. Treatment of SCC-13 cells with 20 or 200 nM trans-retinoic acid results in nearly complete suppression of cytokeratin K5 and K6 expression. This change is accompanied by a simultaneous reduction (> 20-fold) in the level of the mRNAs encoding K5 and K6. Transcriptional analysis indicates that the transcription rate of the K5 and K6 genes drops by approximately four to fivefold in retinoid treated nuclei. Retinol (2000 nM) also promotes this change. In contrast, cytokeratin K19 does not increase in the presence of retinoic acid, thus the normal coordinate regulation of keratin gene expression by retinoids appears to be uncoupled in SCC-13 cells. However, this does not represent a general defect in positive regulation of gene expression by retinoids, since in a transient transfection assay trans-retinoic acid positively regulates a reporter plasmid containing the retinoid response element from the retinoic acid receptor-beta gene. The synthetic retinoids Ro 13-6298 (ethyl ester) and its metabolic derivative Ro 13-7410 (free acid) are both active in modulating the differentiation of normal keratinocytes. In contrast, only Ro 13-7410 is active in SCC-13 cells. As Ro 13-6298 binds poorly to the retinoic acid receptors, this suggests that SCC-13 cells, unlike normal keratinocytes, lack the ability to convert Ro 13-6298 to the active Ro 13-7410. These results further suggest that an inability to metabolize retinoid ethyl esters may be a property of transformed epidermal keratinocytes.