DIVERSE SIGNALING PATHWAYS INVOLVED IN PDGF STIMULATED SECRETION OF BASEMENT MEMBRANE PROTEINS BY SKELETAL MUSCLE 455

Abstract

The basement membrane of skeletal muscle provides a mechanical link between muscle cells and the extracellular matrix, that can be remodeled during modified muscle use. In this study, we have examined how growth factors play a role in basement membrane remodeling. We have shown that platelet-derived growth factor (PDGF) stimulates secretion of three basement membrane components of skeletal muscle; laminin (70% increase), fibronectin (30%), and type IV collagen (70%). Furthermore, we have found using the signal transduction inhibitors, genistein (tyrosine kinase inhibitor; 122nM), PMA(protein kinase C (PKC) inhibitor; 600nM), thapsigargan (depletes intracellular Ca2+ stores; 50nM), and H89 (protein kinase A (PKA) inhibitor; 30uM), that PDGF stimulated secretion of these proteins occurs through distinct signaling pathways. Densitometry of western blots of L6 myoblast supernatant indicates that the PDGF induced increase in secretion of laminin and type IV collagen is tyrosine kinase dependent. The increase in type IV collagen secretion also shows dependence on PKC as well as the release of intracellular Ca2+. Inhibition of either of these pathways reduces the increase in type IV collagen secretion to 20%. In contrast, the PDGF induced increase in laminin secretion is unaffected by inhibition of either PKC or intracellular Ca2+ release. PDGF induced fibronectin secretion is not dependent on tyrosine kinase activity but is dependent on PKA as well as the release of intracellular Ca2+. These divergent signaling pathways provide for independent regulation of basement membrane protein secretion, allowing a muscle cell to modify both the quantity and composition of its basement membrane in response to its environment.