Enhanced interstitial flow as a contributing factor in neointima formation: (shear) stressing vascular wall cell types other than the endothelium.

AS BLOOD FLOWS THROUGH the vascular network, it creates fluid mechanical forces that can be predicted based on established physical principles of fluid dynamics. The major components of flow-generated force in the vasculature include pressure and shear stress, both of which are imparted directly onto the vessel wall. The realization that different patterns (laminar vs. oscillatory), magnitudes (high vs. low), and spatial gradients of shear stress can influence the overall vessel homeostasis has led to an intense investigation into how shear stress regulates the structural and functional phenotype of the blood vessel. Through the work of many dedicated laboratories, we are now certain that shear stress exerts a profound effect on endothelial cells which line all vessels. The endothelium, in turn, can communicate the condition of the hemodynamic environment to which it is exposed to underlying vascular components such as smooth muscle and adventitial fibroblasts, resulting in a propagation of shear stress effects to the entire vessel wall. To illustrate this point, changes in shear stress are perceived by the endothelium and converted in biochemical signals, which are then relayed to enzyme systems that generate vasoactive substances such as nitric oxide (NO). Through a paracrine effect, NO activates the molecular machinery that governs smooth muscle contractility, thereby translating changes in shear stress into an alteration of vessel diameter. In the normal, intact vasculature, the endothelial layer forms an interface between the blood and the remaining components of the vascular wall. Thus vascular smooth muscle and adventitial fibroblasts are remote from the actual hemodynamic shear forces. As a result, the direct influence of shear stress on these mural cell types has received little attention. Rather, the stretch generated via the rhythmic expansion and contraction of the vessel wall with each successive pulse of pressure and blood flow appears to be the dominant hemodynamic stimuli for smooth muscle and, potentially, adventitial cells. Indeed, cyclic stretch has been shown to activate mechanotransduction pathways that lead to functional responses in these cell, and readers are directed to an excellent review in this area (4). So, if smooth muscle and adventitial cells are shielded, in a relative sense, from shear stress and are most responsive to hemodynamic-imposed stretch, the influences of shear stress would seem to be of little concern in these cell types. However, if one considers the forces produced by the flow of interstitial fluid through the vessel wall, then shear stress forces may in fact be an important factor in regulating smooth muscle cells and vessel wall integrity. This concept may have clinical ramifications given that factors which enhance interstitial flow (i.e., chemical or mechanical injury to the endothelium and inflammation and hypertension induced enhancement of vascular permeability) are associated with vessel remodeling and neointima formation. The basic notion that vascular smooth muscle and fibroblasts are responsive to shear stress has been tested by several investigative teams in the past. Laminar shear stress ranging from 10 to 25 dyn/cm decreases smooth muscle proliferation (10, 12) and migration (3, 7) and induces a phenotypic shift from a synthetic to a more differentiated and contractile morphology (6). Initial mechanotransduction responses to shear stress appear to involve calcium influx (8), production of prostaglandins (1) and NO (3), and regulation of matrix metalloproteinases (MMPs) (7). The few studies that have been performed with vascular fibroblasts show that these cells are also sensitive to shear stress and that cell confluency, hence phenotype, modulates the degree to which fibroblasts respond to shear stress (2). While these findings confirm that vascular cells other than the endothelium have the ability to sense and respond to shear stress, a major limitation develops in extrapolating these results to more physiologically relevant settings since the magnitudes of shear stress used in most of these studies were relatively high and unlikely to be encountered by these cell types in vivo, even under conditions where interstitial flow is substantially raised. The study reported in the American Journal of PhysiologyHeart and Circulatory Physiology by Tarbell’s laboratory (9) continues a line of inquiry aimed at understanding the effects of interstitial flow on smooth muscle and adventitial cell function. Their earlier study aptly modeled transmural flow through the arterial wall and calculated values of shear stress around smooth muscle cells residing in various locations (i.e., near or away from the fenestral pore of the internal elastic lamina) in the medial layer of the vessel (11). To address more basic questions regarding shear stress effects on smooth muscle and adventitial cells, this group created a system where vascular cell types are suspended in a three-dimensional (3-D) collagen-I ECM to more faithfully represent the in vivo environment (13). This methodology, which is used in the current study, represents a significant advance over past approaches performed with parallel plate, cone and plate, and similar devices where cells are grown two dimensionally and then subjected to laminar shear stress. While the 3-D model demonstrated higher Darcy permeability (Kp) and interstitial flow velocities compared with the normal aorta, the estimated shear stress on the embedded cells was in good agreement with expected values from an intact vessel. Interestingly, the magnitudes of shear stress are in the range of 0.05 to 0.36 dyn/cm, which is 100 lower than those used in the two-dimensional (2-D) culture studies described above, illustrating again that this system can recapitulate key aspects of the in vivo environment. Initial studies to demonstrate that smooth muscle cells are responsive to changes in interstitial flow in the 3-D collagen gel model were reported by Tarbell’s group (13) in 2000. They Address for reprint requests and other correspondence: V. Rizzo, Cardiovascular Research Center and Dept. of Anatomy and Cell Biology, Temple Univ. School of Medicine, 3420 N. Broad St., Philadelphia, PA 19140 (e-mail: [email protected]). Am J Physiol Heart Circ Physiol 297: H1196–H1197, 2009; doi:10.1152/ajpheart.00499.2009.