Submitted to: American Journal of Physiology - Heart and Circulatory Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2007
Publication Date: 2/23/2007
Citation: Himburg, H.A., Dowd, S.E., Friedman, M.H. 2007. Frequency-dependent response of the vascular endothelium to pulsatile shear stress. American Journal of Physiology - Heart and Circulatory Physiology. 293:645-653. Interpretive Summary: The pig is exposed to various forms of stress. Each type of stress has a variety of effects on the host that originates from cellular responses. Many of these stress responses involve cell signaling and are uniform among all cell types. Revealing how each type of stress influences the development of cells is important. A unique stress model that induces stress response in endothelial cells was utilized to elucidate changes in cellular genetics. The effects of stress on anti-inflammatory responses and protective responses were revealed under low stress conditions. Under zero stress conditions, beneficial growth factors are expressed. Conversely, stress inducing conditions promote inflammatory responses and other negative cell responses that are associated with disease states. The results of this study provide information on how stress negatively affects the health of endothelial cells. Genes were identified which can be targeted to moderate the negative health impacts of stress.
Technical Abstract: Most cells of the circulatory system are exposed to shear forces that occur at the frequency of the heartbeat. However, as a result of the complicated blood flow patterns that occur at arterial branches, small regions of the arterial wall experience fluctuations in shear stress that are dominated by harmonic frequencies higher than the heart rate. To assess whether variations in the frequency content of the shear stress profile affect endothelial gene expression, a series of in vitro microarray experiments were performed. The gene expression patterns of porcine aortic endothelium exposed to three sinusoidal waveforms (1, 2, and 3 Hz; amplitude = 15 dyn/cm2) and one physiological waveform were compared to the expression profiles exhibited under steady flow. At each frequency, including steady flow, three levels of mean shear stress (0, 7.5, and 15 dyn/cm2) were used. Following 24h shear exposure, phase contrast images of the cells were acquired and RNA was extracted for microarray analysis against 10,665 Sus scrofa oligonucleotides. Microscopic examination showed that cell alignment with the flow was positively correlated with mean shear (p < 0.001) and was independent of frequency. Evaluation of gene expression using a two-way ANOVA identified 232 genes whose transcription was differentially modulated by frequency. The frequency sensitive genes were clustered to identify groups of genes exhibiting similar frequency responses. At 1 Hz, several inflammatory transcripts were repressed relative to steady flow, including VCAM and IL-8, while several atheroprotective transcripts were induced. The antiinflammatory response at 1 Hz was reversed at 2 Hz. Mean shear significantly affected the expression of ~3,000 genes. Purely oscillatory flow (zero mean shear) induced the expression of several growth factors and adhesion molecules (E-selectin, VCAM, MCP-1, IL-8, c-jun) relative to non-reversing flow (15 dyn/cm2 mean shear). The frequency dependent responses of certain atherogenic transcripts, including those encoding VCAM and IL-8, were enhanced as the mean shear was reduced. Thus, the proinflammatory response evoked by frequencies greater than 1 Hz was exacerbated by reversing and oscillatory shear. This study suggests that arterial regions subject to both shear reversal, often seen in areas of low mean shear, and dominant frequencies that exceed the normal heart rate are at greater risk for atherosclerotic lesion development.