Integrin-mediated expression of bone formation-related genes in osteoblast-like cells in response to fluid shear stress: Roles of extracellular matrix, Shc, and mitogen-activated protein kinase

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 α_vβ₃ 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 α_vβ₃ 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 α_vβ₃ 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.