Figure 4.

The N-Box motif in the utrophin promoter mediates transcriptional activation by heregulin in cultured muscle cells. The mutant utrophin promoter–luciferase reporter construct ΔNBox (deleted in the N-box) was cotransfected into L6 muscle cell lines along with transfection control plasmid pRL and assayed after 24 h of incubation with either heregulin-containing medium or controls. (A) The ΔNBox-derived firefly luciferase activity is normalized to pRL-derived renilla luciferase activity (internal control) as 100% in the untreated state and expressed as luciferase activity (normalized). The stippled bars represent mutant utrophin promoter activity in untreated cells, and cross-hatched bars indicate the levels in heregulin (HRG)-treated cultures. (B) Schematic of the ΔNBox–luciferase construct; the cross depicts the site of deletion mutation removing the N-box. Heregulin does not activate the mutant utrophin promoter bearing a deletion of the N-Box. Error bars indicate SEM; n = 36. The differences of expression are not statistically significant.