Posttranscriptional regulation of myosin heavy chain expression in the heart by triiodothyronine

Abstract

Triiodothyronine (T3) regulates cardiac contractility in part by regulating the expression of several important cardiac myocyte genes. In the rat, the T3-mediated induction of α-myosin heavy chain (MHC) transcription in hypothyroid hearts is rapid, exhibiting zero-order kinetics, whereas the repression of β-MHC in these same hearts is much slower. To elucidate the mechanism for T3transcriptional as well as posttranscriptional regulation of both MHC gene isoforms, we used an RT-PCR-based transcription assay and the RNA polymerase II inhibitor actinomycin D in an in vivo model to simultaneously measure specific α- and β-MHC heterogeneous nuclear RNA (hnRNA), mRNA kinetics, and MHC antisense RNA. In vivo actinomycin D treatment blocked α-MHC transcription in euthyroid rats by >80% at 2 h and suggested a half-life of α-MHC hnRNA of ∼1 h, whereas actinomycin D inhibited β-MHC transcription in hypothyroid rats by >75% at 6 h, suggesting a significantly longer hnRNA half-life of ∼4 h. The effect of actinomycin D on β-MHC transcription was independent of T3. T3treatment in hypothyroid animals caused β-MHC mRNA to decline more rapidly than β-MHC hnRNA, demonstrating, for the first time, a posttranscriptional mechanism(s). The measured change in β-MHC mRNA half-life indicates a T3-mediated destabilization of β-MHC mRNA. To understand the mechanism by which T3destabilizes β-MHC mRNA, we measured β-MHC antisense RNA. β-MHC antisense RNA is present in euthyroid myocytes, but levels are not significant in hypothyroid myocytes. This differential expression may explain some of the effects of T3on MHC posttranscriptional regulation.