Integrins play significant roles in mechanical responses of cells on extracellular matrix (ECM). We studied the roles of integrins and ECM proteins (fibronectin [FN], type I collagen [COL1], and laminin [LM]) in shear-mediated signaling and the expression of bone formation-related genes (early growth response-1 [Egr-1], c-fos, cyclooxygenase-2 [Cox-2], and osteopontin [OPN]) in human osteosarcoma MG63 cells. MG63 cells on FN, COL1, and LM were kept as controls or subjected to shear stress (12 dynes/cm2), and the association of αvβ3 and β1 integrins with Shc, phosphorylation of mitogen-activated protein kinases (MAPKs, i.e., extracellular signal-regulated kinase [ERK], c-jun-NH2- terminal kinase [JNK], and p38), and expressions of Egr-1, c-fos, Cox-2, and OPN were determined. In MG63 cells, shear stress induces sustained associations of αvβ3 and β1 with Shc when seeded on FN, but sustained associations of only β1 with Shc when seeded on COL1/LM. Shear inductions of MAPKs and bone formation-related genes were sustained (24 h) in cells on FN, but some of these responses were transient in cells on COL1/LM. The shear activations of ERK, JNK, and p38 were mediated by integrins and Shc, and these pathways differentially modulated the downstream bone formation-related gene expression. Our findings showed that β1 integrin plays predominant roles for shear-induced signaling and gene expression in osteoblast-like MG63 cells on FN, COL1, and LM and that αvβ3 also plays significant roles for such responses in cells on FN. The β1/Shc association leads to the activation of ERK, which is critical for shear induction of bone formation-related genes in osteoblast-like cells.
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