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Rapamycin Prevents Thyroid Hormone-Induced Cardiac Hypertrophy

Sanford Research/University of South Dakota, Sanford School of Medicine, Cardiovascular Research Institute, Sioux Falls, South Dakota 57105

Address all correspondence and requests for reprints to: A. Martin Gerdes, Ph.D., Cardiovascular Research Institute, 1100 East 21st Street, Suite 700, Sioux Falls, South Dakota 57105. E-mail: mgerdes{at}usd.edu.

Thyroid hormones (THs) have many effects on the cardiovascular system including cardiac hypertrophy. Although THs induce cardiac hypertrophy, the mechanism through which they exert this effect is unknown. We previously found that THs activate signaling related to increased protein synthesis [mammalian target of rapamycin (mTOR) and p70 S6 kinase] in the heart. It is unknown whether this activation contributes to TH-induced hypertrophy or whether it is merely incidental. In this study, we used rapamycin to inhibit mTOR function in mice and neonatal cardiomyocyte cultures treated with THs to test whether mTOR/S6 kinase signaling is involved in TH-mediated cardiac hypertrophy. C57 mice were treated with T₄ for 3 d, 1 wk, 2 wk, or 1 month with either placebo, T₄ (50 µg/100 g body weight·d), rapamycin (200 µg/100 g body weight·d) or T₄/rapamycin by sc slow-release pellets. At the end of the treatment period, hemodynamics and physical data were collected and hearts were frozen for Western blot analysis or myocytes were isolated. The effects of T₃ and rapamycin were also investigated using neonatal cardiomyocytes. THs activated specific components of the AKT signaling pathway in vivo and in vitro. THs induced cardiac hypertrophy, which was completely inhibited by rapamycin. Our results suggest that TH-induced hypertrophy is mediated by AKT/mTOR/S6 kinase signaling, which is important in the regulation of protein synthesis, a hallmark of cardiac hypertrophy.

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