Increased Flow and Shear Stress Enhance In Vivo Transforming Growth Factor-β1 After Experimental Arterial Injury

Abstract

Abstract-We have previously demonstrated that high-flow (HF) conditions inhibit experimental intimal hyperplasia.We hypothesized that such flow conditions may alter transforming growth factor-␤1 (TGF-␤1) after mural injury. The right common carotid artery (CCA) was balloon-injured in 54 New Zealand White male rabbits. Flow was thereafter preserved (normal flow [NF]), reduced by partial outflow occlusion (low flow [LF]), or increased by ligation of the left CCA (HF). Four sham-operated animals served as uninjured controls. Mean blood flow and pressure in the right CCA were measured before and after flow modulation and before euthanasia (3, 7, and 14 days). TGF-␤1 mRNA and protein levels in the right CCA were determined by Northern and ELISA analyses at each time point. At 7 and 14 days, intimal hyperplasia was quantified, and the transmural localization of TGF-␤1 was determined by immunohistochemical analysis. Mean flow was reduced from 22Ϯ1 to 10Ϯ3 mL/min in the LF group and increased to 34Ϯ2 mL/min in the HF group (PϽ0.001). Blood pressure was not different among the flow groups for all time points. Wall shear stress was markedly decreased in the LF group to 14Ϯ4 dyne/cm 2 and increased in the HF group to 63Ϯ6 dyne/cm 2 at 7 days compared with values in uninjured controls (39Ϯ2 dyne/cm 2 , PϽ0.001) and the NF group (44Ϯ7 dyne/cm 2 , PϽ0.001). At 14 days, wall shear stress was similar among the flow groups. The intima-to-media ratio was 5-and 2-fold greater in the LF group than in the HF and NF groups at 14 days. mRNA levels for TGF-␤1 and its active ligand were increased in the HF group by at least 2-and 3-fold, respectively, at 3 and 7 days compared with levels in uninjured controls and the LF group (PϽ0.05) but were not different among the flow groups at 14 days. TGF-␤1 preferentially localized in the abluminal vascular smooth muscle cells of the HF arterial segments. Flow-and shear-mediated release of TGF-␤1 may therefore play a role in abrogating the proliferative and migratory response of vascular smooth muscle cells in the early stages after mural injury.

Wall Tissue Remodeling Regulates Longitudinal Tension in Arteries

“…To confirm that shear-induced remodeling did not indirectly affect axial strain, we assessed axial strain in left carotid arteries 1 week after ligation of the right carotid artery. We 7 and others 8 have shown that this procedure causes a doubling of shear stress in the left carotid artery.…”

“…To further assess possible effects of shear on axial strain, we performed ligations of the contralateral carotid artery, which causes a doubling of shear stress. 7,8 Seven days later, axial strain (60.0Ϯ7%) was not significantly different from that of control arteries (PϾ0.05). These findings indicate that changes in neither circumferential tensile stress nor shear stress can account for the rapid remodeling that normalized increases in axial strain.…”

Fibroblast Growth Factor-2 Expression Is Altered in Lambs With Increased Pulmonary Blood Flow and Pulmonary Hypertension

“…TGF-␤1 expression was increased in 4-wk-old shunt animals relative to controls (8), which may be due in part to increased TGF-␤1 expression and release by PASMC exposed to stretch (30). Furthermore, TGF-␤1 expression was enhanced by increased flow and shear stress in vivo (31), and TGF-␤1 has been shown to up-regulate FGF-2 in vascular SMC (32). Together, these data suggest that induction of TGF-␤1 expression and release by biomechanical forces may stimulate FGF-2 expression in PASMC.…”

Interstitial fluid flow induces myofibroblast differentiation and collagen alignment in vitro

“…As for the fundamental mechanotransduction mechanism triggering these responses, it is still unclear exactly how the cell might sense such low levels of interstitial flow. It has been established that mechanical forces like stretch can drive fibroblast differentiation toward a myofibroblast phenotype (Arora and McCulloch, 1994;Hinz et al, 2001a;Hinz and Gabbiani, 2003b), although the mechanism remains unclear; furthermore, 2D shear stress can induce autocrine TGF-␤ 1 expression in vascular smooth muscle cells both in vitro on confluent cell monolayers (Ueba et al, 1997) or in vivo after experimental artery injury (Song et al, 2000). Here, the levels of interstitial flow imposed are extremely small: based on a measured average hydraulic conductivity of 1ϫ10 -9 cm 2 at the beginning of the experiment that decreased to 2ϫ10 -10 cm 2 after 5 days of interstitial flow owing to matrix remodeling (Ng and Swartz, 2003), we estimated the average fluid shear stress on the cells to vary between 0.15 and 0.33 dyn/cm 2 (Wang and Tarbell, 1995).…”