title: 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 alpha-myosin heavy chain (MHC) transcription in hypothyroid hearts is rapid, exhibiting zero-order kinetics, whereas the repression of beta-MHC in these same hearts is much slower. To elucidate the mechanism for T3 transcriptional 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 alpha- and beta-MHC heterogeneous nuclear RNA (hnRNA), mRNA kinetics, and MHC antisense RNA. In vivo actinomycin D treatment blocked alpha-MHC transcription in euthyroid rats by >80% at 2 h and suggested a half-life of alpha-MHC hnRNA of approximately 1 h, whereas actinomycin D inhibited beta-MHC transcription in hypothyroid rats by >75% at 6 h, suggesting a significantly longer hnRNA half-life of approximately 4 h. The effect of actinomycin D on beta-MHC transcription was independent of T3. T3 treatment in hypothyroid animals caused beta-MHC mRNA to decline more rapidly than beta-MHC hnRNA, demonstrating, for the first time, a posttranscriptional mechanism(s). The measured change in beta-MHC mRNA half-life indicates a T3-mediated destabilization of beta-MHC mRNA. To understand the mechanism by which T3 destabilizes beta-MHC mRNA, we measured beta-MHC antisense RNA. beta-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 T3 on MHC posttranscriptional regulation.
authors.affiliation: Division of Endocrinology and Department of Medicine, North Shore University Hospital/New York University School of Medicine and North Shore-LIJ Research Institute, Manhasset, New York, USA.
authors.forename: Sara
authors.initials: S
authors.lastname: Danzi
authors.forename: Irwin
authors.initials: I
authors.lastname: Klein
mesh_terms: D000818:Animals; D003609:Dactinomycin / Q000494:pharmacology; D015870:Gene Expression / Q000187:drug effects / Q000502:physiology; D006321:Heart / Q000502:physiology*; D007037:Hypothyroidism / Q000235:genetics / Q000503:physiopathology*; D008297:Male; D018995:Myosin Heavy Chains / Q000235:genetics*; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D012323:RNA Processing, Post-Transcriptional / Q000187:drug effects / Q000502:physiology*; D016372:RNA, Antisense; D012332:RNA, Heterogeneous Nuclear / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D014158:Transcription, Genetic / Q000187:drug effects / Q000502:physiology; D014284:Triiodothyronine / Q000494:pharmacology*
chemical_list: C000594638:MYH7 protein, rat; D011500:Protein Synthesis Inhibitors; D016372:RNA, Antisense; D012332:RNA, Heterogeneous Nuclear; D012333:RNA, Messenger; D014284:Triiodothyronine; D003609:Dactinomycin; D018995:Myosin Heavy Chains