Phorbol ester-mediated downregulation of tropoelastin expression is controlled by a posttranscriptional mechanism

Expression of tropoelastin, the principal precursor of elastic fibers, is tissue-specific and is limited to a brief developmental period. Little is known, however, about the mechanisms that regulate the tissue- and temporal-specific expression of elastogenesis. The tropoelastin promoter contains putative phorbol ester responsive elements, or AP-1 binding sites, but the functional significance of these sequences is unknown. To test if tropoelastin expression is influenced by phorbol esters, we exposed elastogenic fetal bovine chondrocytes to 10(-7) M 12-O-tetradecanoylphorbol 13-acetate (TPA). Tropoelastin mRNA levels decreased greater than 10-fold in response to TPA, and this downregulation was paralleled by a decline in the secretion of tropoelastin protein into the culture medium. As determined by nuclear-runoff assay and transient transfection with a human gene promoter-CAT construct, tropoelastin transcription was unaffected after exposure to TPA. As indicated by actinomycin D experiments, the half-life of tropoelastin mRNA in control cells was about 20 h, but exposure to TPA resulted in an accelerated decay of the tropoelastin transcript (t1/2 = 2.2 h). These data indicate that downregulation of tropoelastin expression was controlled by a posttranscriptional mechanism and that the AP-1 elements in the bovine tropoelastin promoter may not be involved in regulation of production.