Osteocytes exposed to far field of therapeutic ultrasound promotes osteogenic cellular activities in pre-osteoblasts through soluble factors

• Published in: Ultrasonics 2014-07, Vol.54 (5), p.1358-1365
• Main Authors: Fung, Chak-Hei, Cheung, Wing-Hoi, Pounder, Neill M., Harrison, Andrew, Leung, Kwok-Sui
• Format: Article
• Language: English
• Subjects: Alkaline Phosphatase - metabolism; Animals; beta Catenin - metabolism; Cell Differentiation; Cell Line; Cell Movement; Cell Proliferation; Enzyme-Linked Immunosorbent Assay; Far field; Fracture Healing - physiology; Low-intensity pulsed ultrasound; Mechanotransduction; Mechanotransduction, Cellular; Mice; Nitric Oxide - metabolism; Osteoblasts - diagnostic imaging; Osteoblasts - physiology; Osteocyte; Osteocytes - diagnostic imaging; Osteocytes - physiology; Staining and Labeling; Ultrasonic Therapy; Ultrasonography
• ISSN: 0041-624X; 1874-9968
• DOI: 10.1016/j.ultras.2014.02.003

•The mechanisms of different axial distances of LIPUS influence the mechanotransduction of bone cells are not understood.•This is the first study investigating the effect of axial distances of LIPUS on mechanotransduction in bone cells.•Osteocytes sense the acoustic differences of axial distances, and react by regulating the activities of pre-osteoblasts.•Ultrasound beyond near field transmits energy to osteocytes more efficiently.•Factors secreted by far field LIPUS exposed osteocytes led to promotion in osteogenic activities in pre-osteoblasts. Low intensity pulsed ultrasound (LIPUS) was reported to accelerate the rate of fracture healing. When LIPUS is applied to fractures transcutaneously, bone tissues at different depths are exposed to different ultrasound fields. Measurement of LIPUS shows pressure variations in near field (nearby transducer); uniform profile was found beyond it (far field). Moreover, we have reported that the therapeutic effect of LIPUS is dependent on the axial distance of ultrasound beam in rat fracture model. However, the mechanisms of how different axial distances of LIPUS influence the mechanotransduction of bone cells are not understood. To understand the cellular mechanisms underlying far field LIPUS on enhanced fracture healing in rat model, the present study investigated the effect of ultrasound axial distances on (1) osteocyte, the mechanosensor, and (2) mechanotransduction between osteocyte and pre-osteoblast (bone-forming cell) through paracrine signaling. We hypothesized that far field LIPUS could enhance the osteogenic activities of osteoblasts via paracrine factors secreted from osteocytes. The objective of this study was to investigate the effect of axial distances of LIPUS on osteocytes and osteocyte–osteoblast mechanotransduction. In this study, LIPUS (plane; 2.2cm in diameter, 1.5MHz sine wave, ISATA=30mW/cm2) was applied to osteocytes (mechanosensor) at three axial distances: 0mm (near field), 60mm (mid-near field) and 130mm (far field). The conditioned medium of osteocytes (OCM) collected from these three groups were used to culture pre-osteoblasts (effector cell). In this study, (1) the direct effect of ultrasound fields on the mechanosensitivity of osteocytes; and (2) the osteogenic effect of different OCM treatments on pre-osteoblasts were assessed. The immunostaining results indicated the ultrasound beam at far field resulted in more β-catenin nuclear translocation in osteocytes than all other groups. This i