Myofibroblasts protect myoblasts from intrinsic apoptosis associated with differentiation via β1 integrin-PI3K/Akt pathway

Skeletal myoblasts withdrawing from cell cycle is a prerequisite for myodifferentiation, while upon proliferation/differentiation transformation, a large portion of myoblasts will undergo apoptosis. Skeletal fibroblasts, residing in muscle tissue both during and post myogenesis, have been proofed to play pivotal roles in muscle development, while their effect on myoblast apoptosis being coincident with differentiation has not been reported. Using a membrane insert co-culture system, we studied it and found that the mitochondrial pathway played a crucial role in myoblast apoptosis during differentiation, and fibroblasts promoted not only cell cycle withdrawal but also myoblast survival in a paracrine fashion, which was coupled with upregulations of beta 1 integrin, phosphorylated Akt and anti-apoptotic protein Bcl2. To determine the effect of beta 1 integrin in the process, we transfected myoblasts with siRNA specific for beta 1 integrin before co-culture and found that beta 1 integrin knockdown abolished anti-apoptotic ability of myoblasts and inhibited Akt activation and Bcl2 expression. Blockage of PI3K/Akt pathway with wortmannin also seriously impaired the protective effect of fibroblasts on myoblasts and fibroblast-induced Bcl2 expression. The data demonstrated that fibroblasts protected myoblasts from intrinsic apoptosis associated with differentiation, and beta 1 integrin-PI3K/Akt pathway activation was required for the process.