Shear Stress Induces Apoptosis via Cytochrome C Release from Dynamic Mitochondria in Endothelial Cells

Mechano‐sensors on endothelial cells (ECs) detect shear stress and transduce it into biochemical signals to trigger vascular adaptive responses. The variations in shear stress patterns can result in different consequences of mechanotransduction and propensity for atherosclerosis. In the regions of disturbed, oscillatory flow with low shear stress, excess cell turnover generates spatial differences in apoptosis in the endothelium. The aims of the present study were to determine whether oscillatory shear stress (OS) and laminar shear stress (LS) could induce EC apoptosis and modulate the cytochrome c (Cyt c) release from mitochondria. ECs were subjected to oscillatory and laminar flows, with shear stresses of 0.4±4 and 12 dynes/cm 2 , respectively. A significantly increased apoptosis was seen following OS over 24 h, compared with LS.Apoptosis (%) 4 h 24 hOscillatory Flow / Static 1.85±0.06 2.89±0.05 * Laminar Flow / Static 1.73±0.03 1.24±0.04Values are mean±SEM. * P<0.05 for Oscillatory vs. Laminar Flow.Endothelial mitochondria are essential to the functional integrity of ECs as they integrate a wide range of cellular processes. The mitochondrial morphology changes dynamically due to their frequent fusion and fission in response to various cellular conditions. Mitochondrial fission is an early event during apoptosis and it occurs very close in time to Cyt c release, and the fragmented mitochondria can readily be seen to retain Cyt c. Cyt c staining of the mitochondria was detected in ECs under both types of flow. In comparison to static control, Cyt c was significantly reduced in the mitochondria of ECs exposed to both flow patterns for 4 h and 24 h. The reduction of Cyt c in endothelial mitochondria was greater under oscillatory flow for 24 h.Mitochondrial Cyt c 4 h 24 hOscillatory Flow / Static 0.16±0.05 0.59±0.05 * Laminar Flow / Static 0.19±0.03 0.90±0.03Values are mean±SEM. * P<0.05 for Oscillatory vs. Laminar Flow.We used time‐lapse imaging to monitor the spatial‐temporal dynamics of mitochondria in live ECs transfected with RFP‐mito. During the shearing of live ECs, mitochondria networks are dramatically reorganized from long filamentous, interconnected tubules into small punctate spheres under both flow patterns. Under oscillatory flow, more small fragmentations of endothelial mitochondria were observed than under laminar flow. The results show that mechanical forces affect the mitochondria dynamics in ECs to cause Cyt c release, which was greater under oscillatory than laminar flow. The findings suggest that shear stress induces apoptosis via mitochondrial pathway. Understanding of the endothelial mitochondria responses to different flow patterns will help to elucidate the mechanism of the region‐specific localization of atherosclerosis in the arterial system. Support or Funding Information This work was supported by NHLBI Research Grants HL‐104402 and HL‐106579 (S.C.)