BTB-Kelch protein Krp1 regulates proliferation and differentiation of myoblasts

The BTB-Kelch protein Krp1 is highly and specifically expressed in skeletal muscle, where it is proposed to have a role in myofibril formation. We observed significant upregulation of Krp1 in C2 cells early in myoblast differentiation, well before myofibrillogenesis. Krp1 has a role in cytoskeletal organization and cell motility; since myoblast migration and elongation/alignment are important events in early myogenesis, we hypothesized that Krp1 is involved with earlier regulation of differentiation. Krp1 protein levels were detectable by 24 h after induction of differentiation in C2 cells and were significantly upregulated by 48 h, i.e., following the onset myogenin expression and preceding myosin heavy chain (MHC) upregulation. Upregulation of Krp1 required a myogenic stimulus as signaling derived from increased myoblast cell density was insufficient to activate Krp1 expression. Examination of putative Krp1 proximal promoter regions revealed consensus E box elements associated with myogenic basic helix-loop-helix binding. The activity of a luciferase promoter-reporter construct encompassing this 2,000-bp region increased in differentiating C2 myoblasts and in C2 cells transfected with myogenin and/or MyoD. Knockdown of Krp1 via short hairpin RNA resulted in increased C2 cell number and proliferation rate as assessed by bromodeoxyuridine incorporation, whereas overexpression of Krp1-myc had the opposite effect; apoptosis was unchanged. No effects of changed Krp1 protein levels on cell migration were observed, either by scratch wound assay or live cell imaging. Paradoxically, both knockdown and overexpression of Krp1 inhibited myoblast differentiation assessed by expression of myogenin, MEF2C, MHC, and cell fusion.