Hui Miao

Fluid Shear Stress Activation of IκB Kinase Is Integrin-dependent

Vascular endothelial cells (ECs), forming a boundary between the circulating blood and the vessel wall, are constantly subjected to fluid shear stress due to blood flow. The aim of this study was to determine the role of the recently identified IκB kinases (IKKs) in shear stress activation of NF-κB and to elucidate the upstream signaling mechanism that mediates IKK activation. Our results demonstrate that IKKs in ECs are activated by shear stress in a rapid and transient manner. This IKK activation is followed by IκB degradation and NF-κB translocation into the nucleus. Transfection of plasmids encoding catalytic inactive mutants of IKKs, i.e. hemagglutinin (HA)-IKKα(K44M) and HA-IKKβ(K44A), inhibits shear stress-induced NF-κB translocation. In addition, constructs encoding antisense IKKs, i.e.HA-IKKα(AS) and HA-IKKβ(AS), attenuate shear stress induction of a promoter driven by the κB enhancer element. Preincubation of the EC monolayer with a monoclonal anti-αvβ3integrin antibody (clone LM609) attenuates shear stress induction of IKK. Inhibition of tyrosine kinases by genistein causes a similar down-regulating effect. These results suggest that the integrin-mediated signaling pathway regulates NF-κB through IKKs in ECs in response to shear stress.

Effects of Flow Patterns on the Localization and Expression of VE-Cadherin at Vascular Endothelial Cell Junctions: In vivo and in vitro Investigations

Atherosclerosis occurs preferentially at vascular curvature and branch sites where the vessel walls are exposed to fluctuating shear stress and have high endothelial permeability. Endothelial permeability is modulated by intercellular adhesion molecules such as VE-cadherin. This study was designed to elucidate the effects of different flow patterns on the localization and expression of VE-cadherin in endothelial cells (ECs) both in vivo and in vitro. VE-cadherin staining at EC borders was much stronger in the descending thoracic aorta and abdominal aorta, where the pulsatile flow has a strong net forward component than in the aortic arch and the poststenotic dilatation site beyond an experimental constriction, where the flow near the wall is complex and reciprocating with little net flow. With the use of flow chambers the effects of pulsatile flow (12 ± 4 dyn/cm2 at 1 Hz) and reciprocating flow (0.5 ± 4 dyn/cm2 at 1 Hz) on VE-cadherin organization in endothelial monolayers were studied in vitro. VE-cadherin staining was continuous along cell borders in static controls. Following 6 h of either pulsatile or reciprocating flow, the VE-cadherin staining at cell borders became intermittent. When the pulsatile flow was extended to 24, 48 or 72 h the staining around the cell borders became continuous again, but the staining was still intermittent when the reciprocating flow was similarly extended. Exposure to pulsatile or reciprocating flow for 6 and 24 h neither change the expression level of VE-cadherin nor its distribution between membrane and cytosol fractions as determined by Western blot and compared with static controls. These findings suggest that the cell junction remodeling induced by different flow patterns may result from a redistribution of VE-cadherin within the cell membrane. Both the in vivo and in vitro data indicate that pulsatile and reciprocating flow patterns have different effects on cell junction remodeling. The lack of junction reorganization in regions of reciprocating flow in vivo and in vitro may provide a mechanistic basis for the high permeability and the preferential localization of atherosclerosis in regions of the arterial stress with complex flow patterns and fluctuating shear stress.

Roles of Microtubule Dynamics and Small GTPase Rac in Endothelial Cell Migration and Lamellipodium Formation under Flow

Endothelial cell (EC) migration is required for vascular development and wound healing. We investigated the roles of microtubule (MT) dynamics and the small GTPase Rac in the fluid shear stress-induced protrusion of lamellipodia and enhancement of migration of bovine aortic ECs (BAECs). Shear stress increased lamellipodial protrusion and cell migration. Treating BAECs with paclitaxel (Taxol), an MT-stabilizing agent, inhibited lamellipodial protrusion and reduced migration speed in both the static and sheared groups. After Taxol washout, both lamellipodial protrusion and cell migration increased in the flow direction. Taxol treatment also decreased the shear-induced Rac activation. Transfection of BAECs with a dominant negative mutant of Rac1 inhibited lamellipodial protrusion and cell migration under static and shear conditions. Transfection with an activated mutant of Rac1 induced lamellipodia in all directions and attenuated the shear-induced migration, suggesting that an appropriate level of Rac activity and a polarized lamellipodial protrusion are important for cell migration under static and shear conditions. Our findings suggest that MT dynamics and optimum Rac activation are required for the polarized protrusion of lamellipodia that drives the directional EC migration under flow.

Signal Transduction in Matrix Contraction and the Migration of Vascular Smooth Muscle Cells in Three-Dimensional Matrix

The interaction of vascular smooth muscle cells (SMCs) and extracellular matrix plays important roles in vascular remodeling. We investigated the signaling pathways involved in SMC-induced matrix contraction and SMC migration in three-dimensional (3D) collagen matrix. Matrix contraction is inhibited by the disruption of actin filaments but not microtubules. Therefore, we investigated the roles of signaling pathways related to actin filaments in matrix contraction. SMC-induced matrix contraction was markedly blocked (–80%) by inhibiting the Rho-p160ROCK pathway and myosin light chain kinase, and was decreased to a lesser extent (30–40%) by a negative mutant of Rac and inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase) or p38 mitogen-activated protein kinase (MAPK), but it was not affected by the inhibition of Ras and Cdc42-Wiskott-Aldrich syndrome protein (WASP) pathways. Inhibition of extracellular-signal-regulated kinase (ERK) decreased SMC-induced matrix contraction by only 15%. The migration speed and persistence of SMCs in the 3D matrix were decreased by the inhibition of p160ROCK, PI 3-kinase, p38 MAPK or WASP to different extents, and p160ROCK inhibitor had the strongest inhibitory effect. Our results suggest that the SMC-induced matrix contraction and the migration of SMCs in 3D matrix share some signaling pathways leading to force generation at cell-matrix adhesions and that various signaling pathways have different relative importance in the regulations of these processes in SMCs.