Effects of thyroid hormones on myofibrillar proteolysis and activities of calpain, proteasome, and cathepsin in primary cultured chick muscle cells.

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 (60 ng/mL) or T3 (12 ng/mL) for 6 d. Calpain, cathepsins, and proteasome activities and N tau-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 tau-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.