Bioeffects of low-intensity ultrasound in vitro: Apoptosis, protein profile alteration, and potential molecular mechanism

Objective: The purpose of this study was to evaluate the potential molecular mechanism of low-intensity ultrasound-induced apoptosis by analyzing protein profile alteration in response to ultrasound exposure. Methods: Human hepatocarcinoma SMMC-7721 cells were used in this study. Cell viability was measured by a trypan blue dye exclusion test. Morphologic changes were examined by light microscopy. Apoptosis was assessed by phosphatidylserine externalization and DNA fragmentation. The pattern of the mitochondrial membrane potential decrease was determined by flow cytometry. Protein profile alteration was analyzed by comparative proteomics based on 2-dimensional polyacrylamide gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Results: Low-intensity ultrasound (3.0 W/cm2, 1 minute, cells incubated for 6 hours after ultrasound exposure) induced early apoptosis (mean ± SD, 26.5% ± 6.2%) significantly (P < .05) with minimal lysis in human hepatocarcinoma cells in vitro. On a molecular level, several proteins, eg, cellular tumor antigen protein 53, BH3-interacting domain death agonist, apoptosis regulator Bcl-2, and heme oxygenase 1 were identified as responding to ultrasound irradiation, suggesting that mitochondrial dysfunction and oxidative stresses were involved in ultrasoundinduced apoptosis. It was also assumed that mitofilin-regulated crista remodeling may be a potential channel of mitochondrial membrane permeabilization pore formation involved in low-intensity ultrasound-induced apoptosis. Conclusions: This study suggests that 2 potential molecular signaling pathways are involved in ultrasound- induced apoptosis. It is a first step toward low-intensity ultrasound-induced apoptotic cancer therapy via understanding its relevant molecular signaling and key proteins.