Extracellular Matrix Fibronectin Attenuates PDGF‐Induced Intracellular Calcium Release

Abstract

The extracellular matrix (ECM) has structural and signaling roles within tissues. Fibronectin is an ECM protein that is secreted by fibroblasts in a protomeric form and assembled into a fibrillar matrix by a cell‐mediated process termed fibronectin matrix assembly. ECM proteins affect growth factor signaling. Platelet derived growth factor (PDGF) is a major mitogen for fibroblasts that promotes proliferation and migration. The objective of this study was to determine the effect of ECM fibronectin on the response of fibroblasts to PDGF. The response of cells to PDGF was quantified by measuring intracellular calcium release in Fluo‐4 loaded cells. Calcium release was measured in fibronectin‐null mouse embryonic fibroblasts, allowing for control over the amount of fibronectin to which cells were exposed and the initiation of fibronectin matrix assembly. Results showed that in the absence of fibronectin, addition of 50 ng/mL PDGF to cells transiently increased intracellular calcium concentration. In contrast, incubation of cells with 25 nM fibronectin for 20 hr significantly attenuated the PDGF‐induced increase in calcium release. Fibronectin‐mediated attenuation of PDGF‐induced calcium release was blocked by an inhibitor of fibronectin matrix assembly, indicating that the ECM form of fibronectin is required for fibronectin‐mediated attenuation of PDGF signaling. A fibronectin fragment engineered to mimic the ECM form of fibronectin also attenuated PDGF‐induced calcium release. Attenuation of PDGF‐induced calcium release was not due to downregulation of PDGF receptor β expression. This study demonstrates that fibronectin matrix assembly attenuates PDGF‐induced intracellular calcium release by an as‐yet unidentified mechanism.Supported by: NIH/NHLBI: T32 HL066988