Proliferation suppression by thyroid hormone requires thyroid hormone receptor in fetal cardiomyocytes (850.1)

Abstract

Objectives: Ovine fetal cardiomyocyte (CM) maturation begins after 100 days gestation (dGA). At term (145dGA) ~60% of CMs are binucleated and cannot divide. Thyroid hormone (T3) rises near term and suppresses proliferation by insulin‐like growth factor‐1 (IGF‐1) in vitro. Despite T3 activation of extracellular regulatory kinase (ERK) and AKT we hypothesize that T3‐induced inhibition of proliferation is thyroid receptor (TR) mediated and reversed by the TR antagonist, NH3. Methods: Fetal CMs were isolated from 102±3 and 135±1dGA sheep (n=11/group). Cells were treated with T3 (0.37, 1.5nM), IGF‐1 (1ug/ml), NH3 (100nM) or a combination. Proliferation was quantified by BrdU uptake (10uM). Western blots measured ERK, AKT, TRa, and p21 expression. Results: IGF‐1 stimulated proliferation at both ages. T3 reduced this mitotic activity by ~50% in 100dGA and ~75% in 135dGA. At both ages T3 increased p21 (p<0.001), and NH3 blocked the T3+IGF‐1 reduction of BrdU uptake without altering ERK or AKT. In 100dGA CMs NH3 did not reduce T3‐induced p21 expression but upregulated TRa (p<0.05). In 135dGA CMs, NH3 alone reduced BrdU uptake as well as T3‐induced p21 (p<0.05). Conclusion: Fetal ovine CMs are sensitive to the effects of NH3 differently depending on maturation state. NH3 inhibition of proliferation suggests that TRs are needed for normal proliferation in the developing myocardium and that p21 may be the main effector. Grant Funding Source: Supported by NIH NHLBI and NICHD