Regulation of mucin gene expression in human tracheobronchial epithelial cells by thyroid hormone

We reported previously that the expression of the gene encoding MUC5AC muci n in human airway epithelial cells is controlled by retinoic acid via the r etinoic acid receptor (RAR)-alpha and that 3,3',5-tri-iodothyronine (T-3) i nhibits the expression of MUC5AC. The purpose of the present study was to i dentify mechanisms mediating the effect of T-3. T-3 has been shown to inhib it gene expression via several mechanisms, either by enhancing or repressin g the transcription of target genes or by the regulation of post-transcript ional events. Results showed that T-3 strongly inhibited MUC5AC-driven luci ferase activity in normal human tracheobronchial epithelial cells that had been transiently transfected with a MUC5AC-luciferase reporter construct; h owever, it did not affect MUC5AC mRNA stability. These results indicate tha t T-3 suppresses MUC5AC expression at the transcriptional level. An analysi s of deletion constructs showed that deletion of the region downstream of 3 kb resulted in markedly decreased levels of MUC5AC transcription in the ab sence of T-3 (i.e. under control conditions) as well as a loss of responsiv eness to the inhibitory effects of T-3. This suggests that this region migh t contain elements important for the activation as well as the repression o f MUC5AC transcription. To determine whether T-3 modulates retinoic-acid-de pendent MUC5AC transcription via an alteration in the abundance of retinoid receptor proteins, we examined the type and abundance of these receptors i n nuclear extracts of airway epithelial cells grown in the presence or abse nce of T-3. Western blots showed that T-3 markedly decreased several types of retinoid receptor while not affecting T-3 receptor proteins. Consistent with this finding were gel-shift assays revealing a decrease in RAR-retinoi c acid response element complexes obtained from T-3-treated cells. We propo se that T-3 might inhibit retinoid-dependent NUC5AC expression by decreasin g retinoid receptor levels and thereby decreasing the transcriptional activ ation of this gene for mucins.