SRF protein is upregulated during stretch-induced hypertrophy of rooster ALD muscle

Serum response element 1 has previously been reported to be necessary and s ufficient for activation of the skeletal oc-actin promoter during hypertrop hy of the anterior latissimus dorsi (ALD) muscle of roosters [J. A. Carson, R. J. Schwartz, and F. W Booth. Am. J. Physiol. 270 (Cell Physiol. 39): C1 624-C1633, 1996]. Serum response factor (SRF) protein is the transcription factor that binds as a homodimer to serum response element 1 and activates the skeletal alpha-actin promoter. An increased expression of exogenous SRF protein in replicating C2C12 myoblasts induced a three- to fourfold activa tion of the skeletal alpha-actin promoter (L. Wei, W. Zhou, J. D. Croissant , F.-E. Johansen, R. Prywes, A. Balasubramamyan, and R. J. Schwartz. J. Bio l. Chem. 273: 30287-30294, 1998). Thus we hypothesized that SRF protein con centration would be increased during hypertrophy of skeletal muscle. In the present study, 10% of the rooster's body weight was attached to the left w ing to induce enlargement of the ALD muscle compared with the contralateral muscle. With Western analysis, a significant increase in SRF protein per g ram of wet weight of the ALD muscle was noted at 7 and 13 days of hypertrop hy. Furthermore, the increase in SRF protein occurred in both crude nuclear protein and cytoplasmic fractions in 7-day stretched ALD muscles. This is the first report showing increased protein concentration for a transcriptio n factor whose regulatory element in the Skeletal ol-actin promoter has pre viously been shown to be required for the transduction of a hypertrophy sig nal in overloaded skeletal muscle of an animal.