Skeletal muscle differentiation, maturation, and regeneration are regulated by interactions between intrinsic genetic programs controlled by myogenic transcription factors, including members of the MyoD and MEF2 families, and environmental cues mediated by hormones and growth factors. Insulin-like growth factors (IGFs) also play key roles in muscle development, and in the maintenance and repair of mature muscle, but their mechanisms of interaction with other muscle regulatory networks remain undefined. To evaluate the potential interplay between MyoD and IGF signaling pathways, we have studied muscle differentiation in C3H 10T1/2 fibroblasts acutely converted to myoblasts by quantitative infection with a recombinant adenovirus encoding mouse MyoD. In these cells, IGF-II gene and protein expression are induced as early events in differentiation, and the IGF-I receptor and downstream signaling molecules, including Akt, are rapidly activated. Interference with IGF-II production by a tetracycline-inhibited adenovirus expressing an IGF-II cDNA in the antisense orientation reversibly inhibited both production of muscle-specific structural proteins and myocyte fusion to form multinucleated myotubes. Similar results were achieved with a tetracycline-inhibited adenovirus expressing dominant-negative Akt. Our observations identify a robust autocrine amplification network in which MyoD enhances the later steps in muscle differentiation by induction of a locally acting growth factor.
This work was supported by National Institutes of Health Research Grant 5RO1-DK42748 (to P. R.) and by the Muscular Dystrophy Association. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
