Effects of Thyroid Hormones on Myofibrillar Proteolysis and Activities of Calpain, Proteasome, and Cathepsin in Primary Cultured Chick Muscle Cells

Abstract

The effects of thyroxine (T₄) and triiodothyronine (T₃) on growth, muscle protein degradation, and proteases activities in cultured chick muscle cells were studied. The cells were treated with a physiological level of T₄ (60ng/mL) or T₃ (12ng/mL) for 6d. Calpain, cathepsins, and proteasome activities and N^τ-methylhistidine release were measured as indexes of myofibrillar protein breakdown. Creatine kinase activity was also measured as an index of myotube formation. Calpain activity was increased by T₄ and T₃. Cathepsin D and proteasome activities and N^τ-methylhistidine release were increased by T₃, but not by T₄. Neither were cathepsin B and B+L activities affected by T₃ or T₄. Creatine kinase activity was increased by T₄ and T₃. The results suggest that myotube formation is accelerated by T₄ and T₃, whereas myofibrillar protein degradation is accelerated by T₃, but not by T₄.