We reported previously that the expression of the gene encoding MUC5AC mucin in human airway epithelial cells is controlled by retinoic acid via the retinoic acid receptor (RAR)-α and that 3,3′,5-tri-iodothyronine (T3) inhibits the expression of MUC5AC. The purpose of the present study was to identify mechanisms mediating the effect of T3. T3 has been shown to inhibit gene expression via several mechanisms, either by enhancing or repressing the transcription of target genes or by the regulation of post-transcriptional events. Results showed that T3 strongly inhibited MUC5AC-driven luciferase activity in normal human tracheobronchial epithelial cells that had been transiently transfected with a MUC5AC–luciferase reporter construct; however, it did not affect MUC5AC mRNA stability. These results indicate that T3 suppresses MUC5AC expression at the transcriptional level. An analysis of deletion constructs showed that deletion of the region downstream of 3kb resulted in markedly decreased levels of MUC5AC transcription in the absence of T3 (i.e. under control conditions) as well as a loss of responsiveness to the inhibitory effects of T3. This suggests that this region might contain elements important for the activation as well as the repression of MUC5AC transcription. To determine whether T3 modulates retinoic-acid-dependent MUC5AC transcription via an alteration in the abundance of retinoid receptor proteins, we examined the type and abundance of these receptors in nuclear extracts of airway epithelial cells grown in the presence or absence of T3. Western blots showed that T3 markedly decreased several types of retinoid receptor while not affecting T3 receptor proteins. Consistent with this finding were gel-shift assays revealing a decrease in RAR–retinoic acid response element complexes obtained from T3-treated cells. We propose that T3 might inhibit retinoid-dependent MUC5AC expression by decreasing retinoid receptor levels and thereby decreasing the transcriptional activation of this gene for mucins.

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