Fig. 3.

IGF-I responsiveness in type IIb MyHC promoter activity is lost with 5′-deletions. C₂C₁₂ myocytes were harvested after 3 days of differentiation in DM (control) or DM with 250 ng/ml IGF-I (IGF-I), and type IIb MyHC promoter activity was assessed. A: successive 5′-deletions were made to the 3-kb type IIb MyHC promoter. Values are expressed as a percentage of control 3-kb type IIb MyHC promoter activity and are reported as the average ± SE of n = 4 wells/group. *P < 0.05 from control within same promoter length, $P < 0.05 from IGF-I 3-kb activity. B: an additional 50-bp 5′-deletion between 1.3 kb and 1.2 kb revealed a potential IGF-I-responsive region in the type IIb MyHC promoter. Values are expressed as a percentage of control 3-kb type IIb MyHC promoter activity and are reported as the average ± SE of n = 4 wells/group. *P < 0.05 from control within same promoter length, $P < 0.05 from IGF-I 1.3-kb and IGF-I 1.25-kb activity.