Activation of the skeletal alpha-actin promoter during muscle regeneration

Little is known concerning promoter regulation of genes in regenerating skeletal muscles. In young rats, recovery of muscle mass and protein content is complete within 21 days. During the initial 5-10 days of regeneration, mRNA abundance for IGF-I, myogenin and MyoD have been shown to be dramatically increased. The skeletal alpha-actin promoter contains E box and serum response element (SRE) regulatory regions which are directly or indirectly activated by myogenin (or MyoD) and IGF-I proteins, respectively. We hypothesized that the skeletal alpha-actin promoter activity would increase during muscle regeneration, and that this induction would occur before muscle protein content returned to normal. Total protein content and the percentage content of skeletal alpha-actin protein was diminished at 4 and 8 days and re-accumulation had largely occurred by 16 days post-bupivacaine injection. Skeletal alpha-actin mRNA per whole muscle was decreased at day 8, and thereafter returned to control values. During regeneration at day 8, luciferase activity (a reporter of promoter activity) directed by -424 skeletal alpha-actin and -99 skeletal alpha-actin promoter constructs was increased by 700% and 250% respectively; however, at day 16, skeletal alpha-actin promoter activities were similar to control values. Thus, initial activation of the skeletal alpha-actin promoter is associated with regeneration of skeletal muscle, despite not being sustained during the later stages of regrowth. The proximal SRE of the skeletal alpha-actin promoter was not sufficient to confer a regeneration-induced promoter activation, despite increased serum response factor protein binding to this regulatory element in electrophoretic mobility shift assays. Skeletal alpha-actin promoter induction during regeneration is due to a combination of regulatory elements, at least including the SRE and E box.