Endothelial responses to environmental stimuli: The role of mitochondrial reactive oxygen species in mechanotransduction and hypoxia

Environmental stresses elicit a variety of responses in cells including transcriptional activation of specific genes, but the underlying process by which environmental forces trigger these responses is not established. The response to mechanical forces is especially important in the vascular endothelium, where hemodynamic forces play an integral role in normal physiological function and in the pathogenesis of diseases such as atherosclerosis. The present studies demonstrate that endothelial cells (HUVEC) utilize mitochondria in the mechanotransduction process by stimulating their release of reactive oxygen species (ROS) in response to mechanical strain. The biological significance of this ROS signaling is manifested by an activation of NF-κB, an increase VCAM-1 mRNA expression, and an upregulation of VCAM-1 at the cell surface. Strain-induced responses were inhibited by specific mitochondrial inhibitors that attenuated ROS signaling or by antioxidants that blocked redox signaling, and were selectively abolished in mutant HUVEC (ρ0 cells) lacking a functional electron transport chain. The data reveal a relationship between the actin cytoskeleton and mitochondria that is critical for specific endothelial responses to strain. These signaling pathways also mediate strain-induced activation of focal adhesion kinase and endothelial nitric oxide synthase, suggesting the mitochondria functions as the primary endothelial mechanotransducer in response to cyclic stretch. Mitochondria also serve as the cellular oxygen sensor; our studies indicate that endothelial cells generate mitochondrial ROS to respond to hypoxia by increasing cytokine secretion and paracellular permeability. Hypoxia elicited increases in DCF fluorescence and DHE fluorescence that were abrogated by the mitochondrial electron transport (ET) inhibitors that prevent ROS generation. The same ET inhibitors also attenuated hypoxia-induced increases in NF-κB activation, although they did not abrogate NF-κB activation in response to endotoxin (LPS). ET inhibition also abolished the hypoxia-induced increases in IL-6 mRNA expression, hypoxia-stimulated IL-6 secretion into the media, and the hypoxia-induced increases in HUVEC monolayer permeability. Collectively, these findings identify a signal transduction mechanism by which environmental stimuli may be converted into a biological signal capable of triggering both physiological and pathological responses. Mitochondria thus serve as environmental sensory organelles, generating reactive oxygen species in response to mechanical strain and hypoxia to alter endothelial cell signaling.