Ultrasound-induced angiogenic response in endothelial cells

Mechanical forces are known to affect endothelial cell (EC) function and can promote the formation of mature, muscular arterioles and arteries (arteriogenesis). The present study explores the possible angiogenic role of ultrasonic irradiation on EC phenotype using an in-vitro approach. Therapeutic ultrasound (TUS) stimulation at 1-MHz frequency was applied to bovine aortic endothelial cells (BAECs) in 2-D monolayer cultures and 3-D spheroid cultures. An angiogenic EC phenotype was characterized by the proliferation rate, migration, sprouting and Flk-1 expression in response to ultrasound stimulation. Irradiation lasting as long as 30 min caused a down-regulation and redistribution of Flk-1, increased EC proliferation rates and enhanced migration and sprouting in the 3-D spheroid cultures. The ultrasound-mediated EC stimulation in monolayers may be attributed to stable cavitation and micro-streaming, which are induced by pulsating microbubbles near the EC surface. Three-dimensional EC spheroid cultures surrounded by a highly viscous gel phase also exhibited ultrasound-induced angiogenic characteristics, although microbubbles may not participate in this response because of the impeding medium. The described in-vitro influence of low-intensity ultrasound on angiogenic EC phenotype has implications for TUS as a safe and controlled noninvasive stimulus for vascular regeneration.