An E-box within the MHC IIB gene is bound by MyoD and is required for gene expression in fast muscle

The myosin heavy chain (MHC) IIB gene is selectively expressedin skeletal muscles, imparting fast contractile kinetics. Why the MHCIIB gene product is expressed in muscles like the tibialis anterior(TA) and not expressed in muscles like the soleus is currently unclear.It is shown here that the mutation of an E-box within the MHC IIBpromoter decreased reporter gene activity in the fast-twitch TA muscle90-fold as compared with the wild-type promoter. Reporter geneexpression within the TA required this E-box for activation of aheterologous construct containing upstream regulatory regions of theMHC IIB promoter linked to the basal 70-kDa heat shock protein TATApromoter. Electrophoretic mobility shift assays demonstrated thatmutation of the E-box prevented the binding of both MyoD and myogeninto this element. In cotransfected C₂C₁₂myotubes and Hep G2 cells, MyoD preferentially activated the MHC IIBpromoter in an E-box-dependent manner, whereas myogenin activated theMHC IIB promoter to a lesser extent, and in an E-box-independent manner. A time course analysis of hindlimb suspension demonstrated thatthe unweighted soleus muscle activated expression of MyoD mRNA beforethe de novo expression of MHC IIB mRNA. These data suggest a possiblecausative role for MyoD in the observed upregulation of MHC IIB in theunweighted soleus muscle.