Canonical and noncanonical contribution of thyroid hormone receptor isoforms alpha and beta to cardiac hypertrophy and heart rate in male mice

Abstract Background Stimulation of ventricular hypertrophy and heart rate are two major cardiac effects of thyroid hormone (TH). Aim of this study was to determine in vivo which TH receptor (TR), α or β, and which mode of TR action, canonical gene expression or DNA-binding independent noncanonical action, mediate these effects. Material and methods We compared global TRα and TRβ knockout mice (TRα KO ; TRβ KO ) with WT mice to determine the TR isoform responsible for T3 effects. The relevance of TR DNA- binding was studied in mice with a mutation in the DNA-binding domain that selectively abrogates DNA binding and canonical TR action (TRα GS ; TRβ GS ). Hearts were studied with echocardiography at baseline and after seven weeks T3-treatment. Gene expression was measured with real-time PCR. Heart rate was recorded with radiotelemetry transmitters for seven weeks in untreated, hypothyroid and T3-treated mice. Results T3 induced ventricular hypertrophy in WT and TRβ KO mice, but not in TRα KO mice. Hypertrophy was also induced in TRα GS mice. Thus, hypertrophy is mostly mediated by noncanonical TRα action. Similarly, repression of Mhy7 occurred in WT and TRα GS mice. Basal heart rate was largely dependent on canonical TRα action. But responsiveness to hypothyroidism and T3-treatment as well as expression of pacemaker gene Hcn2 were still preserved in TRα KO mice, demonstrating that TRβ could compensate for absence of TRα. Conclusion T3-induced cardiac hypertrophy could be attributed to noncanonical TRα action, whereas heart rate regulation was mediated by canonical TRα action. TRβ could substitute for canonical, but not noncanonical TRα action.