Invited Symposium: Signal Transduction in Endothelium: Mechano-Sensing, Ion Channels and Intracellular Calcium
Cardiovascular disease, in particular atherosclerosis, is one of the main causes for death and disability in our society. To date, there are no clear markers for the "predisposition" of the endothelial cell lining to the initial lesions in atherosclerosis-prone regions of the vascular wall. The first observable sign of pre-atherosclerotic alterations of endothelial cells in these "predilection zones" is the focal upregulation of specific adhesion molecules on the endothelial cell surface surface , such as vascular adhesion molecule-1 (Ross, 1993)
In vivo, over time and under certain conditions (genetic predisposition, or exposure to atherosclerotic risk factors), circulating monocytes adhere to these locally activated endothelial cells, extravasate into the subendothelial, where they eventually will take up the "bad" oxidized lipids, and transform into the "foam" cells. Importantly, while in vitro all types of endothelial cells studied so far, express cytokine-stimulable adhesion molecules, it is largely unknown why pre-atherosclerotic lesions, leading to foam cell accumulation, will occur only in large and middle sized muscular arteries and never in veins. One obvious hypothesis is that hemodynamic forces, in particular those found in the areas of perturbed flow occurring at arterial bifurcations, might predispose the cells to become pre-atherosclerotic by enhancing the adhesion of circulating leukocytes, in particular of monocytes.
Quiescent endothelial cells do not constitutively express VCAM-1. The expression of VCAM-1 is down regulated by laminar shear stress (Korenaga, et al., 1997). Recent studies suggest that both oscillatory and pulsatile perturbed flow can enhance endothelial cell VCAM-1 levels (Kettlun et al., 1996; Chappell et al., 1998.) Furthermore, the cellular redox state is differentially affected when EC are exposing to either laminar or oscillatory shear stress (DeKeulenear et al., 1998). While all forms of laminar shear stress induce an oxidative burst, production of reactive oxygen species (ROS) elicited by laminar shear stress is transient, while it is sustained upon exposure of the cells to oscillatory shear stress. These observations support the previous notion that laminar flow acts as an atheroprotective agonist (Traub and Berk, 1998), while non-laminar flow, in particular oscillatory, recirculating flow patterns of low shear might be atherogenic (Gimbrone, et al., 1997).
There is good evidence linking the upregulation of both VCAM-1 to early signs of atherosclerotic lesions (Collins, 1993; Li et al., 1993). The cellular signals and the molecular mechanisms which mediate the altered expression of adhesion molecules are under intense investigation. One of the most conspicuous, common pathways of activation of adhesion molecules by cytokines as well as by oxidative stress involves the transcription factor NF-kappaB (Collins, 1993, Gosh et al., 1998). Activation of NF-kappaB is an ubiquitous, pleiotropic regulational pathway, common to many cell types, which can be activated through multiple cellular pathways. Therefore, although it presumably plays an important regulatory role in the initiation of atherosclerotic lesions, NF-kappaB activation appears to be a necessary, but not a sufficient cellular/molecular mechanism for predisposing endothelial cells to atherosclerotic lesions. Recent evidence suggests, that specific cellular signal transduction pathways and shear responsive promoter elements/enhancers are involved in this process which are directly or indirectly controlled by hemodynamic forces (Rubanyi et al., 1990; Frangos, 1993; Sumpio, 1993; Chien et al., 1998; Lelkes, 1999).
The mechanotransducing machinery in endothelial cells involves the activation of ionic channels, cell-surface receptors, cytoskeletal elements, and, in particular, cell-substrate adhesion molecules (integrins) (Schwartz and Ingber, 1994, Shyy, 1997). Of particular interest is the activation (phosphorylation) of numerous phosphoproteins, such as paxillin, talin and pp125FAK, in the focal adhesion complexes. Activation of any, or all, of these mechanisms induces a particular set of signal transduction pathways (frequently mediated by tyrosine kinase and/or phosphatase activity), which ultimately lead to gene activation (Hanks et al., 1992).
Recently, a short nucleotide consensus sequence (GAGACC) in the 5' promoter region of several genes (e.g. PDGF-B, eNOS and ICAM-1) was identified which seems to mediate the upregulation of these genes by laminar shear stress, but only in a specific range of magnitudes (Resnick et al., 1993). Interestingly, this shear stress responsive element (SSRE) is lacking in the promoter sequences of other adhesion molecules, such as VCAM-1. On the other hand, the gene for endothelin-1 does contain the SSRE, but the ET-1 gene is actually down-regulated by sustained shear stress (Malek and Izumo, 1992). Moreover, mechanical strain seems to regulate gene activation only to a limited extent via the SSRE (Sumpio et al., 1998). These contrasting findings suggest that there must be more than one SSRE-like, mechano-responsive gene elements, which may act distinctly from or in concert with NF-kappaB, to predispose and activate endothelial cells to atherogenic stimuli. Indeed, since the discovery of the prototypic SSRE, several positive and negative mechanosensitive regulatory elements have been described. For recent reviews see (Chien et al., 1998; Lelkes, 1999). It remains to be proven, which of these SSREs might be uniquely sensitive to distinct flow patterns, such as occurring at the sites of atherosclerotic predilection. The aim of this study was to analyze signaling mechanisms involved in the upregulation of VCAM-1 expression in cultured human aortic endothelial cells exposed to pulsatile, perturbed flow.
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|Lelkes, P.I.; Kettlun, C.S.; Wigboldus, J.; Waters, C.R.; Sukovich, D.A.; Rubanyi, G.M.; (1998). Signaling Mechanisms Involved in Endothelial Cell Activation by Perturbed Flow. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Invited Symposium. Available at URL http://www.mcmaster.ca/inabis98/nilius/lelkes0243/index.html|
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