Mechanosignal transduction coupling between endothelial and smooth muscle cells: role of hemodynamic forces.

HEMODYNAMICS play an important role in the focal nature of atherosclerosis. Mounting evidence has demonstrated that fluid shear stress is intimately involved in the biological activities of vascular endothelial cells (ECs). However, the role of neighboring smooth muscle cells (SMCs) in modulating the endothelial phenotype in the presence of shear stress remains undefined. Emerging EC and SMC coculture systems have provided the basis to elucidate the mechanosignal transduction coupling between ECs and SMCs. Hasting et al. (9) as well as Chiu et al. (2–5) have demonstrated novel transcriptional regulation to support the notion that endothelial functional phenotypes are not only influenced by hemodynamic forces but also by neighboring SMCs. Vascular ECs in resistant arteries are constantly exposed to the dynamic changes of blood flow, namely, hemodynamic forces. These homodynamic forces can be resolved into three components: 1) shear stress, the tangential frictional force acting at the EC surface; 2) hydrostatic pressure, the perpendicular force acting on the vascular wall; and 3) cyclic strain, the circumferential stretch of the vessel wall (6, 7, 11, 13, 17). Emerging lines of evidence have supported the role of shear stress in mechanosignal transduction between vascular ECs and SMCs (3, 9). ECs are subject to fluid shear stress in the presence of neighboring SMCs (Fig. 1). The bidirectional communication between ECs and SMCs influences the homeostasis of the function and structure of the blood vessel wall. As an interface between blood and the vessel wall, ECs sense and respond to hemodynamic forces. EC and SMC coculture systems have elucidated novel transcriptional regulation between ECs and SMCs (4, 9), providing evidence that endothelial functional phenotypes are not only influenced by hemodynamic forces but also by neighboring SMCs (3, 10) (Fig. 2) . The coculture system represents a significant advance over homogeneous culture to assess the molecular mechanisms whereby shear stresses regulate EC function in the presence of SMCs (1, 14, 15). Using the perfused transcapillary coculture model, Remond et al. (15) reported that ECs protect against flow-induced SMC migration and flow-induced EC plasminogen activator inhibitor type 1. Using the parallel plate EC/SMC coculture system, Chiu et al. (4) reported that laminar shear stress significantly inhibits SMC-induced adhesion molecule gene expression. Furthermore, SMCs induce an upregulation of proinflammatory gene expression in ECs that are located in close proximity to SMCs. However, laminar shear stress acts as a negative regulator by regulating NFB binding sites in the promoters of these inflammatory genes expressed in the presence of SMCs (2). Chiu et al. (3) further elucidated the molecular mechanism whereby JNK and p38 in MAPK pathways are activated to induce E-selectin expression in cocultures. Gel shifting and chromatin immunoprecipitation assays showed that SMC coculture increased the NFB-promoter binding activity in ECs, whereas preshearing of ECs at 12 dyn/cm inhibited coculture-induced EC signaling and E-selectin expression (3). Thus, mechanosignal transduction in the EC/SMC coculture system provides molecular insights into the atheroprotective role of unidirectional laminar shear stress (12 dyn/ cm) to downregulate proinflammatory gene expressions in ECs induced by coculture with SMCs. The coculture in vitro model further allowed for testing the hypothesis that differential human-derived hemodynamic flow patterns applied to ECs influence SMC phenotypic modulation (1). Using a modified cone and plate flow device, Hastings et al. (9) developed a dynamic system to accommodate a 75-mm Transwell culture dish (polycarbonate, 10 m thickness and 0.4 m pore diameter, Corning) in which ECs were exposed two distinct flow profiles, namely, pulsatile shear stress in the common carotid artery and oscillatory shear stress in the lateral wall or point of flow separation in the internal carotid artery (Fig. 2, A and C). Similar to the parallel plate coculture flow system (5), ECs and SMCs were separated by a porous membrane with only the EC side subjected to the flow condition (Fig. 2, B and D). The former enabled the testing of atherogenic hemodynamics in the presence of SMCs (1). The latter elucidated new insights into the molecular mechanisms