An investigation of vascular cell responses to physiologically relevant mechanical and biochemical stimuli
In vivo, vascular cells experience a combination of mechanical forces and biochemical factors which dictate cellular functions and ultimately contribute to vessel homeostasis. Therefore, the purpose of the present in vitro study was to examine the individual and combined effects of laminar and stenotic flow and physiological and pathological pressure stimuli on endothelial and smooth muscle cell functions. Previous results from our lab have confirmed up regulation of endothelial cell mRNA expression of COX-2, ecNOS, and PDGF-B under laminar fluid flow. The current study further showed that mRNA expression of COX-2, ecNOS, and PDGF-B by endothelial cells exposed to physiological and pathological pressures was decreased when compared to laminar flow values. This demonstrates force-dependent influences on cell functions and suggests unique signaling pathways in response to these forces. To better model arterial geometry, this study utilized a custom-built, simulated vessel, flow model with a 75% area reduction in the flow orifice to create pathological (stenotic) flow. Cell exposure to such flows resulted in decreased mRNA expression of COX-2, ecNOS, and PDGF-B in the flow region downstream of the stenosis. A novel aspect of the current study was to investigate the simultaneous exposure of vascular cells to mechanical stimuli and conditioned medium (or medium containing soluble and transferable factors released by endothelial cells in response to mechanical stimuli). When endothelial cells were exposed to laminar flow and flow-conditioned medium, mRNA expression of COX-2, ecNOS, and PDGF-B was significantly increased when compared to either cells under static conditions or laminar flow only. In addition, endothelial and smooth muscle cell growth responses to each type of media under static conditions were opposite of one another. Specifically, when exposed to flow-, pressure-, or stenotic-conditioned medium for up to five days, endothelial cell growth was inhibited, while smooth muscle cell growth was enhanced. In combination, results of the present studies suggest that the biochemical communication pathways between vascular cells are dependent on the specific and combinational effects of mechanical forces and biochemicals acting on the cells. Understanding the nature of these variables will be critical in designing pharmacological agents for treating diseases like atherosclerosis. ^
Karen M. Haberstroh, Purdue University, Thomas J. Webster, Purdue University.