Cross-talk between transcriptional regulation by thyroid hormone and myogenin: new aspects of the Ca2+-dependent expression of the fast-type sarcoplasmic reticulum Ca2+-ATPase.

We have previously demonstrated an interaction between the major determinants of skeletal muscle phenotype by showing that continuous contractile activity represses the thyroid hormone (3,3', 5-tri-iodothyronine; T3)-dependent transcriptional activity of fast-type sarcoplasmic/endoplasmic-reticulum Ca2+-ATPase (SERCA1), a characteristic of the fast phenotype. Both the free cytosolic Ca2+ concentration ([Ca2+]i) and the myogenic determination factors MyoD and myogenin have been implicated as mediators of the effect of contractile activity on skeletal muscle phenotype. Using L6 cells we have shown that an increase in the steady-state [Ca2+]i above the resting level of 120 nM indeed can mimic the effect of contractile activity on T3-dependent SERCA1 expression. We now show that the repressing effect of increased [Ca2+]i on T3-dependent SERCA1 expression in L6 cells is exerted at a pre-translational level and is accompanied by increased myogenin mRNA expression. Myogenin overexpression in these cells revealed that increased expression of myogenin alone strongly decreases the T3-dependent stimulation of SERCA1 promoter activity. These results suggest a pathway for the regulation of skeletal muscle phenotype in which [Ca2+]i mediates the effect of contractile activity by regulating the expression of myogenin, which in turn interferes with transcriptional regulation by T3.