Authors: Thi MM, Suadicani SO, Spray DC.
While load-induced mechanical signals play a key role in bone formation and maintenance of bone mass and structure, the cellular mechanisms involved in the translation of these signals are still not well understood. Recent identification of a novel flow-induced mechanosignaling pathway involving vascular endothelial growth factor (VEGF) in osteoblasts and the known VEGF regulation of actin reorganization in various cell types has led us to hypothesize that fluid shear stress induced Vegf upregulation underlies the actin cytoskeleton adaptation observed in osteoblasts during mechanotransduction. Our results show that MC3T3-E1 cells secret significant VEGF in response to 5h pulsatile fluid shear stress (PFSS - 5 dyne/cm2 at 1 Hz) while expression of VEGF receptors (VEGFR-1, VEGFR-2 or NRP1) is unaffected. These receptors, in particular VEGFR-2, participate in PFSS-induced VEGF release. Exposure to flow-conditioned media or exogenous VEGF significantly induces stress fiber formation in osteoblasts that is comparable to PFSS-induced stress fiber formation, whereas VEGF knock-down abrogates this response to PFSS, thereby providing evidence that flow-induced VEGF release plays a role in actin polymerization. Using neutralizing antibodies against the receptors and VEGF isoforms, we found that soluble VEGFs, in particular VEGF164, play a crucial role in transient stress fiber formation during osteoblast mechanotransduction, most likely through VEGFR-2 and NRP1. Based on these data we conclude that flow-induced VEGF release from osteoblasts regulates osteoblast actin adaptation during mechanotransduction and that VEGF paracrine signaling may provide a potent crosstalk among bone cells and endothelial cells that is essential for fracture healing, bone remodeling and osteogenesis.