α7 integrin is negatively regulated by δEF1 during skeletal myogenesis

Abstract

δ7 integrin levels increase dramatically as myoblasts differentiate to myotubes. A negative regulatory element with putative sites for δEF1 is present in the δ7 proximal promoter region. To define the role of δEF1 in regulating α7 integrin expression, we overexpressed δEF1 in C2C12 myoblasts. This resulted in a major down-regulation of α7 protein expression. Promoter assays revealed that C2C12 myoblasts transfected with δEF1 showed a decrease in activity of the 2.8-kb α7 promoter fragment, indicating regulation of α7 integrin at the transcriptional level. We have identified two E-box-like sites for δEF1 in the negative regulatory region. Mutation of these sites enhanced α7 promoter activity, indicating that these sites function in repression. MYOD, an activator of α7 integrin transcription, can compete with δEF1 for binding at these sites in gel shift assay. By using chromatin immunoprecipitation, we demonstrated a reciprocal binding of δEF1 and MYOD to this regulatory element depending on the stage of differentiation: δEF1 is preferentially bound in myoblasts to this region, whereas MYOD is bound in myotubes. The N-terminal region of δEF1 is necessary for α7 repression, and this region also binds the co-activator p300/CBP. Importantly, we found that the p300/CBP co-activator can overcome repression by δEF1, suggesting that δEF1 can titrate limiting amounts of this co-activator. These findings suggest that δEF1 has a role in suppressing integrin expression in myoblasts by displacing MYOD and competing for p300/CBP co-activator. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.