Interdependence of pulsed ultrasound and shear stress effects on cell morphology and gene expression

Fluid shear stress is a key biomechanical regulatory factor in a several biological systems including bone tissue. Bone cells are also regulated by exogenous acoustic vibration, which has therapeutic benefits. In this study, we determined the effects of shear stress and pulsed ultrasound (US), alone and in series on osteoblast morphology and gene expression. We observed that shear stress (19 dyne/cm(2)) elongated SaOS-2 cells at 3, 6, 24, and 48 h decreasing their shape index from control values of 0.51 +/- 0.01, 0.60 +/- 0.05, 0.59 +/- 0.04, and 0.45 +/- 0.01 to 0.45 +/- 0.04, 0.47 +/- 0.03, 0.39 +/- 0.02, and 0.33 +/- 0.01, respectively. This morphological effect was inhibited at 24 and 48 h but not at 3 and 6 h by a 20 min pre-exposure to pulsed US (1.5 MHz, 30 mW/cm(2)). Shear stress significantly decreased Bone Morphogenetic Protein-4 (BMP-4) mRNA levels at 1, 2, 3, 6, and 24 h by 32.5 +/- 1.8%, 30.8 +/- 3.5%, 49.6 +/- 2.8%, 23.5 +/- 5.0%, 24.4 +/- 2.3%, respectively. A 20 min pulsed US exposure had no significant effect. However, a 20 min pre-exposure to pulsed US caused significant 39.6 +/- 3.0% and 25.6 +/- 2.7% decreases in BMP-4 levels in shear stress treated cells at 3 and 24 h, respectively. These results show for the first time that pulsed US alters the mechanotransductive effects of shear stress indicating a more comprehensive understanding of therapeutic US will be obtained when it is studied in conjunction with in vivo, regulatory biomechanical forces.