Hypothyroidism Effects on Wild Type and Mutant Rats with Altered Cardiac Titin Expression

Abstract

Thyroid hormone levels play an important role in cardiac regulation, and a state of hypothyroidism leads to dilated forms of cardiomypathy. Inappropriate thyroid levels can also impact the sarcomeric protein titin, which is responsible for maintaining passive tension and structural integrity. The effects of hypothyroidism were studied after administration of propylthiouracil (PTU) to wild type (Wt) and homozygous mutant (Hm) rats (Greaser et al J Mol Cellul Cardiol 44:983, 2008) (the latter of which express a giant titin isoform). Enzyme-linked immunosorbent assay results demonstrated that PTU lowered thyroxine levels in the treated rats. Electrophoretic cardiac myosin heavy chain analysis indicated that hypothyroidism induced a transition to predominantly beta myosin heavy chain expression, while Wt control samples continued to express an age-appropriate range of alpha and beta isoforms. PTU-treated Wt rats had a larger N2BA to N2B titin ratio, and the slower migrating N2BA was in higher proportion than the faster N2BA. PTU did not affect titin isoform expression in Hm. Because it is believed that thyroid hormone regulates titin isoform expression through the PI3K/Akt pathway, we investigated several participants by immunoblotting. The Akt PH domain was not altered by PTU administration, although expression was higher in Hm rats. Akt phospho-Ser473 did not vary between treatments, but mTOR phospho-Ser2448 demonstrated a higher phosphorylation state in Wt. Echocardiography data showed significantly decreased fractional shortening in Hm control, Hm PTU, and Wt PTU relative to Wt control. Wt control also exhibited a higher ejection fraction and a lower isovolumic relaxation time compared to the other treatments. It can be concluded that hypothyroidism has significant effects on titin isoform expression and other cardiac functional characteristics. Supported by NIH HL77196.