MOLECULAR-BASIS OF TRIIODOTHYRONINE MYOGE NIC INFLUENCE

The importance of thyroid hormone in the regulation of in vivo myogeni c processes is well documented. This hormone stimulates muscle growth by increasing the fibers number and diameter. In addition, T3 affects the expression of different myosin isoforms, by a direct transcription al pathway in cardiac muscle, indirectly in skeletal muscle. Last, acc ording to the strong effect of this hormone upon mitochondriogenesis a nd mitochondrial activity, it could be also involved in the processes of metabolic differentiation. However, the molecular basis of these in fluences remained largely unknown. Cell culture experiments provide st rong evidence that T3 stimulates myoblast differentiation, by increasi ng the rate of myoblast withdrawal from the cell cycle. Recent data su ggest that in murine myoblasts, MyoD and myo-genin genes transcription is directly stimulated by T3; however, as a similar phenomenon does n ot occur in avian myoblasts in which T3 displays a stronger myogenic i nfluence, such a mechanism is probably not essential. Studies performe d in avian cells (secondary cultures and QM7 line) indicate that inhib ition of AP-1 activity is probably a crucial target involved in the T3 myogenic influence.