Skip to main content
NCBI home page
NIHPA Author Manuscripts logoLink to NIHPA Author Manuscripts
. Author manuscript; available in PMC: 2013 Sep 13.
Published in final edited form as: Circ Res. 2009 Jul 23;105(5):471–480. doi: 10.1161/CIRCRESAHA.109.193656

Shear Stress Induces Synthetic-to-Contractile Phenotypic Modulation in Smooth Muscle Cells via PPAR-α/δ Activations by Prostacyclin Released by Sheared Endothelial Cells

Min-Chien Tsai 1,2, Lihong Chen 3, Jing Zhou 2, Zhihui Tang 4, Tzu-Fang Hsu 2, Ying Wang 4, Yu-Tsung Shih 1,2, Hsin-Hsin Peng 2, Nanping Wang 4, Youfei Guan 3, Shu Chien 5, Jeng-Jiann Chiu 1,2,
PMCID: PMC3772782  NIHMSID: NIHMS139819  PMID: 19628794

Abstract

Rationale

Phenotypic modulation of smooth muscle cells (SMCs), which are located in close proximity to endothelial cells (ECs), is critical in regulating vascular function. The role of flow-induced shear stress in the modulation of SMC phenotype has not been well defined.

Objective

The objective was to elucidate the role of shear stress on ECs in modulating SMC phenotype and its underlying mechanism.

Methods and Results

Application of shear stress (12 dynes/cm2) to ECs co-cultured with SMCs modulated SMC phenotype from synthetic to contractile state, with up-regulation of contractile markers, down-regulation of pro-inflammatory genes, and decreased percentage of cells in the synthetic phase. Treating SMCs with media from sheared ECs induced peroxisome proliferator-activated receptor (PPAR)-α, -δ, and -γ ligand binding activities; transfecting SMCs with specific small interfering RNAs (siRNAs) of PPAR-α and -δ, but not -γ, inhibited shear-induction of contractile markers. ECs exposed to shear stress released prostacyclin (PGI2). Transfecting ECs with PGI2 synthase-specific siRNA inhibited shear-induced activation of PPAR-α/δ, up-regulation of contractile markers, down-regulation of pro-inflammatory genes, and decrease in percentage of SMCs in synthetic phase. Mice with PPAR-α deficiency (compared with control littermates) showed altered SMC phenotype toward a synthetic state, with increased arterial contractility in response to angiotensin II.

Conclusions

These results indicate that laminar shear stress induces synthetic-to-contractile phenotypic modulation in SMCs through the activation of PPAR-α/δ by the EC-released PGI2. Our findings provide insights into the mechanisms underlying the EC-SMC interplays and the protective homeostatic function of laminar shear stress in modulating SMC phenotype.

Keywords: Endothelial cell, Peroxisome proliferator-activated receptor, Prostacyclin, Shear stress, Smooth muscle cell

INTRODUCTION

Phenotypic modulation of smooth muscle cells (SMCs) is critical in regulating vascular function in health and disease [1]. During atherosclerotic lesion development, vascular SMCs change from their physiologic contractile phenotype to the pathophysiologic synthetic phenotype and migrate into the intima, where they release pro-inflammatory factors and interact with endothelial cells (ECs) lining the vessel wall and exposed to shear stress [2]. There is increasing evidence that laminar shear stress exerts atheroprotective effects on the vascular wall by regulating EC signaling, gene expression, and function [3]. However, the mechanisms by which shear stress acting on ECs leads to signaling regulation, gene expression, and phenotypic modulation in the neighboring SMCs remains unclear.

Sustained exposure of ECs to laminar shear stress induces their release of several anti-atherogenic factors, including prostacyclin (PGI2) and nitric oxide (NO), which are known to inhibit coagulation, leukocyte migration, and SMC proliferation and inflammation [2, 3]. Recent studies have demonstrated that shear stress applied to ECs induces their release of ligands of peroxisome proliferator-activated receptor (PPAR)-γ [4, 5], which, together with PPAR-α and -δ, constitute a subfamily of nuclear receptors. PPARs have been implicated in metabolic disorders predisposing to atherosclerosis, e.g. dyslipidemia and diabetes [6]. Activation of PPAR-γ in vascular SMCs by the anti-diabetic agent troglitazone inhibits SMC proliferation and migration [7, 8]. The role of PPAR-γ in ECs on the modulation of metabolic phenotype and vasoreactivity has also been reported [9]. Activators of PPAR-α (e.g., Wy14643) inhibit the inflammatory response of aortic SMCs as a result of PPAR-α-repression of nuclear factor-κB signaling [10], thereby suppressing the initiation and progression of atherosclerosis. The role of PPAR-δ in modulating SMC gene expression and function is controversial. It has been shown that PPAR-δ is up-regulated in vascular SMCs during lesion formation and promotes post-confluent proliferation of SMCs [11]. In contrast, activation of PPAR-δ by its ligand L-165041 induces cell cycle arrest in SMCs and inhibits their proliferation and migration [12]. PGI2, a predominant prostanoid that is a product of arachidonic acid metabolism and synthesized by PGI2 synthase (PGIS), has been found to be a putative endogenous ligand for PPAR-α/δ [13]. Overexpression of PGIS by gene transfer inhibits vascular SMC growth and prevents neointimal formation in EC-denuded carotid arteries in rats [14]. These results suggest that PPAR-α/δ may play a role in the pathology of diseases associated with alteration of SMC phenotype and function, e.g. atherosclerosis, hypertension, and restenosis.

In the present study, we investigated the role of shear stress acting on ECs in the modulation of SMC phenotype and the mechanism underlying this shear effect, using our newly developed EC/SMC co-culture flow system in which ECs and SMCs are grown on opposite sides of a porous membrane [15, 16]. We found that laminar shear stress (12 dynes/cm2) acting on ECs induce synthetic-to-contractile phenotypic modulation in SMCs through their PPAR-α/δ activation by EC-released PGI2. Mice with PPAR-α deficiency showed altered SMC phenotype toward a synthetic state in the thoracic aorta, with increased arterial contractility in response to angiotensin II (Ang II), as compared with the wild-type mice. Our findings provide new insights into the mechanisms by which laminar shear stress exerts atheroprotective effects via EC-modulation of SMC phenotype from synthetic toward contractile state.

METHODS

Materials

Mouse monoclonal antibodies against smooth muscle α-actin (SMα-actin), myosin heavy chain (SM-MHC), protein 22-α (SM22α), h-caldesmon, and calponin were purchased from Sigma (St Louis, MO). A detailed list of materials used in this study is shown in online supplemental document.

Cell culture

ECs were isolated from fresh human umbilical cords by collagenase perfusion and grown in medium 199 (Gibco, Grand Island, NY) supplemented with 20% fetal bovine serum (FBS; Gibco). Human umbilical artery SMCs were obtained commercially (Clonetics, Palo Alto, CA) and maintained in F12K medium (Gibco) supplemented with 10% FBS. Human skin fibroblasts were purchased from American Type Culture Collection (Rockville, MD) and grown in minimal essential medium (MEM; Gibco) supplemented with 10% FBS. Cells between passages 4 to 6 were used.

Co-culture modules

ECs (5 × 105 cells/cm2) were seeded onto the outer side of a 10-µm-thick membrane containing 0.4-µm pores (Falcon cell culture inserts; Becton Dickinson, Lincoln Park, NJ), as described [15, 16]; SMCs (2 × 105 cells/cm2) were seeded on the opposite (inner) side to form EC/SMC co-culture. Controls had SMCs as above, but no ECs on the outer side of the membrane (Ø/SMC). Detailed experimental procedures are described in online supplemental document.

Parallel-plate co-culture flow system

The membrane with ECs of EC/SMC was incorporated into a parallel-plate flow chamber, which was connected to a perfusion loop system for the application of shear stress at 12 dynes/cm2 for 24 h, as described [15, 16]. In some experiments, perfusing media collected from 24 h-sheared ECs seeded on 3 glass slides (75 by 38 mm, Corning, NY) were used to elucidate the paracrine effect of factor(s) released from sheared ECs. Detailed experimental procedures are described in online supplemental document.

Animal experiments

Male PPAR-α-knockout (PPARα−/−) and wild-type (WT) mice (9–10 weeks old, 20–25 g weight) on pure 129/SvJ background were used. All experiments were performed in accordance with NIH guidelines and the approval of Animal Research Committees of Peking University Health Science Center. Basal blood pressures in PPARα−/− and WT mice and their responses to Ang II (2 µg/kg) were measured by carotid arterial catheterization as previously reported [17]. The detailed experimental procedures of blood pressure and mesenteric arterial tension measurements and immunohistochemistry were described in online supplemental document.

An expanded Materials and Methods section is provided in the online supplemental document.

RESULTS

Shear stress applied to ECs induces SMC phenotypic change from synthetic to contractile state through paracrine effects of factors released from sheared ECs

The protein expression of contractile markers SMα-actin, SM-MHC, calponin, h-caldesmon, and SM22α in SMCs co-cultured with ECs under static condition was similar to that in the mono-cultured SMCs (Figures 1A and 1B). When the EC side of EC/SMC was exposed to shear stress at 12 dynes/cm2 for 24 h, however, the expression of these contractile markers in SMCs was increased, as compared with the static co-cultured or mono-cultured SMCs. Such shear-induction of SMC contractile markers was not observed when the EC side of EC/SMC was replaced by fibroblasts (Figure 1C), SMCs (Figure 1D), and transcriptional inactive ECs (cycloheximide-treated) (Figure 1E). When SMCs were treated with perfusing media from sheared ECs for 24 h (compared with static media from control ECs), the expression of these contractile markers was also increased (Figures 1A and 1F). These results indicate that shear-induced synthetic-to-contractile phenotypic modulation of SMCs in EC/SMC is specific to ECs and mediated through paracrine effects of mediator(s) released from sheared ECs.

Figure 1. Laminar flow to ECs modulates SMC phenotype.

Figure 1

Figure 1

Open in a new tab

(A) Schematic diagrams of EC/SMC co-culture systems used in this study. (B) ECs and SMCs were seeded on the outer and inner sides of a membrane, respectively (EC/SMC). Controls had SMCs as above, but no ECs on the other side (Ø/SMC). The cells were kept as controls (CL) or subjected to shear stress (12 dynes/cm2) for 24 h (SS). The protein expression of indicated contractile markers in SMCs were examined. The EC side of EC/SMC was replaced by fibroblasts (C), SMCs (D), and cycloheximide (CHX)-treated ECs (E). (F) Ø/SMC was treated with static (CL EC-Med) or perfusing (SS EC-Med) media under static conditions for 24 h. Data in B, C, and F are means ± SEM from 3 to 4 independent experiments. *, P < 0.05 vs. static controls. Results in D and E were representative of 3 independent experiments with similar results.

Perfusing media induce PPAR-mediated transcription and PPAR ligand binding activities in SMCs

SMCs were transfected with a luciferase reporter driven by 3 copies of PPAR-responsive element (PPRE) (i.e., PPRE×3-TK-Luc) and then stimulated with perfusing vs. static media. Perfusing media induced luciferase activity for all three PPAR isoforms approximately 4 times of that obtained using static media (Figure 2A). As positive controls, treating SMCs with agonists of PPAR-α (WY14643; 50 µmol/L), -δ (GW501516; 30 nmol/L), and -γ (troglitazone; 10 µmol/L) under static conditions caused 4~4.5 times inductions. To examine the ligand dependence of the perfusing media-activation of PPARs, SMCs were transfected with a GAL4 reporter construct (i.e., MH100×4-TK-Luc) and a vector of GAL-hPPAR-α-LBD, GAL-hPPAR-δ-LBD, or GAL-hPPAR-γ-LBD fusion protein in which the DNA binding domain of GAL4 is linked to the LBD of the human PPAR-α, -δ, or -γ; the results show that perfusing media caused increases in PPAR-α, -δ, and -γ ligand binding activities in SMCs compared with static controls (Figure 2B). The increases in PPAR-α, -δ, and -γ ligand binding activities induced by perfusing media and their respective agonists were inhibited by pre-treating these transfected SMCs with antagonists of PPAR-α (MK886; 20 µmol/L; Figure 2C), -δ (sulindac sulfide; 100 µmol/L; Figure 2D), and -γ (GW9662; 10 µmol/L; Figure 2E), respectively. Shear stress applied to ECs of EC/SMC for different durations induced the expression of target genes for PPAR-α (carnitine palmitoyltransferase-I [CPT-I]), -δ (adipose differentiation-related protein [ADRP] and angiopoietin-like protein-4 [ANGPTL4]), and -γ (fatty acid translocase [CD36]) in SMCs (Figure 2F). These results suggest that application of shear stress to ECs can induce their releases of PPAR-α, -δ, and -γ ligands, which may subsequently activate the respective PPARs in SMCs.

Figure 2. Laminar flow to ECs induces their release of ligands for SMC PPAR activation.

Figure 2

Figure 2

Open in a new tab

SMCs were transfected with PPRE×3-TK-Luc (1 µg/mL) (A) or co-transfected with MH100×4-TK-Luc and a vector of GAL-hPPAR-α-LBD (B, C), GAL-hPPAR-δ-LBD (B, D), or GAL-hPPAR-γ-LBD (B, E) (1 µg/mL for each) for 48 h, and then stimulated with static vs. perfusing media or an agonist of PPAR-α (WY14643; 50 µmol/L), -δ (GW501516; 30 nmol/L), or -γ (troglitazone; 10 µmol/L) for 24 h. The transfected SMCs were pre-treated with PPAR-α (MK886; 20 µmol/L) (C), -δ (sulindac sulfide; 100 µmol/L) (D), or -γ (GW9662; 10 µmol/L) (E) antagonist for 1 h. (F) The EC side of EC/SMC was exposed to flow and the SMC expression of indicated PPAR target genes was analyzed by real time-PCR. Data are means ± SEM from 3 or 4 independent experiments. *, P < 0.05 vs. static controls. #, P < 0.05 vs. samples without antagonist pre-treatment.

PPAR-α and -δ, but not -γ, are required for shear-induced phenotypic modulation in SMCs of EC/SMC

Transfection of SMCs with specific small interfering RNA (siRNA) of either PPAR-α or PPAR-δ (100 nmol/L), which caused 70–80% reductions in the respective PPAR protein expression in SMCs compared with control siRNA (Figure 3A), inhibited shear-induced expressions of SM-MHC, h-caldesmon, and calponin in SMCs of EC/SMC (Figure 3B). In contrast, PPAR-γ-specific siRNA, which reduced the PPAR-γ protein expression by 80% compared with control siRNA (Figure 3A), had no these inhibitory effects. Thus, while shear stress causes the activations of all three isoforms of PPAR (α, δ, and γ), only PPAR-α/δ are involved in the modulation of SMC phenotype by mediator(s) released from sheared ECs. This involvement of PPAR-α/δ in the modulation of SMC phenotype was further substantiated by the inhibitions of perfusing media-induced contractile marker expression in SMCs by transfections with specific siRNAs of PPAR-α and -δ, but not -γ (Figure 3C). As controls, treating SMCs with agonists of PPAR-α (WY14643; 50 and 100 µmol/L) and -δ (GW501516; 1 and 30 nmol/L) induced SM-MHC, h-caldesmon, and calponin expressions in SMCs (Figure 3D). PPAR-γ agonist troglitazone (5 and 10 µmol/L) had no effect on these contractile marker expressions.

Figure 3. Shear-modulation of SMC phenotype in EC/SMC is mediated by PPAR-α/δ, but not -γ.

Figure 3

Figure 3

Open in a new tab

SMCs were transfected with control siRNA (siCL) or specific siRNA of PPAR-α (siPPARα), -δ (siPPARδ), or -γ (siPPARγ) at the indicated concentrations for 48 h, and the protein expression of these PPARs was examined (A). SMCs were transfected with control or PPARs-specific siRNA (100 nmol/L) for 48 h, and then co-cultured with ECs (EC/SMC) kept under static condition (CL) or exposed to flow (SS) (B), or treated with static vs. perfusing media for 24 h (C). The protein expression of indicated contractile markers in SMCs was examined. SMCs treated with PPAR agonists at the indicated concentrations for 24 h (D). Results were representative of triplicate experiments with similar results.

PGI2 produced by sheared ECs acts as a PPAR-α/δ ligand contributing to shear-induced SMC phenotypic modulation in EC/SMC co-culture

In agreement with previous reports [18], perfusing media from sheared ECs contained much higher levels of 6-keto-prostaglandin F1α (6-keto-PGF1α; ~50 ng/L), the PGI2 stable metabolite, than the static media (~10 ng/L) (Figure 4A). This shear-induction of 6-keto-PGF1α was reduced by transfecting ECs with PGIS-specific siRNA (40 nmol/L), which reduced PGIS protein expression by 60% compared with control siRNA (Figure 4B). Compared with static controls, perfusing media of ECs transfected with control siRNA induced luciferase activities of PPRE (Figure 4C), GAL-hPPAR-α-LBD (Figure 4D), GAL-hPPAR-δ-LBD (Figure 4E), and GAL-hPPAR-γ-LBD (Figure 4F) in SMCs transfected with these respective constructs. Treating SMCs with perfusing media of ECs transfected with PGIS-specific siRNA resulted in reductions in luciferase activities of PPRE, GAL-hPPAR-α-LBD, and GAL-hPPAR-δ-LBD, but the shear-induced increase in GAL-hPPAR-γ-LBD activity was not affected (Figures 4C–4F). As controls, the PGI2 synthetic analog carbaprostacyclin (cPGI2) induced luciferase activities of PPRE, GAL-hPPAR-α-LBD, and GAL-hPPAR-δ-LBD, but not GAL-hPPAR-γ-LBD, in SMCs. Shear stress applied to ECs (transfected with control siRNA) of EC/SMC induced SM-MHC, h-caldesmon, and calponin expressions in SMCs (Figure 4G). These shear-inductions of contractile markers were inhibited by transfecting ECs with PGIS-specific siRNA prior to shearing. The involvement of PGI2 produced by sheared ECs in shear-induction of SMC contractile markers was further substantiated by the reductions of perfusing media-induction of contractile markers in SMCs by treating with perfusing media of ECs transfected with PGIS-specific siRNA (Figure 4H). As positive controls, treating SMCs with cPGI2 (50 and 100 nmol/L) induced SM-MHC, h-caldesmon, and calponin expressions in SMCs (Figure 4I).

Figure 4. PGI2 from sheared ECs acts as a PPAR-α/δ ligand contributing to shear-induced SMC phenotypic modulation.

Figure 4

Figure 4

Open in a new tab

(A) The 6-keto-PGF1α concentrations in static vs. perfusing media of ECs transfected with control or PGIS-specific siRNA (40 nmol/L) were measured. Basal culture medium (BM) was used as negative controls. (B) The efficiency of PGIS-specific siRNA was examined. (C–F) SMCs were transfected with indicated constructs using the protocols same as Figure 2, and then stimulated with cPGI2 (50 nmol/L) or static vs. perfusing media of ECs transfected with control or PGIS-specific siRNA for 24 h. (G) Before co-culture with SMCs and exposing to flow, ECs were transfected with control or PGIS-specific siRNA. SMCs were treated with cPGI2 (I) or static vs. perfusing media of ECs transfected with control or PGIS-specific siRNA (H) for 24 h. Data in A and C–H are means ± SEM from 3 independent experiments. *, P < 0.05 vs. static control cells transfected with siCL. #, P < 0.05 vs. sheared cells transfected with siCL. Results in B and I were representative of triplicate experiments with similar results.

PGI2 produced by sheared ECs induces SM22α gene expression at the transcriptional level and serum response factor (SRF)-DNA binding activity in SMCs

Shear stress applied to ECs of EC/SMC induced SM22α gene expression in SMCs (Figure 5A). This shear-induced SM22α expression in SMCs can be inhibited by transfecting ECs with PGIS-specific siRNA before shearing. For SMCs transfected with SM22α-Luc, which contains 520 bp of 5’-flanking DNA of the human SM22α promoter in front of the luciferase gene [19], perfusing media of ECs transfected with control siRNA caused a 3.7-fold induction of the SM22α-Luc luciferase activity; this response was reduced by transfecting ECs with PGIS-specific siRNA before shearing (Figure 5B). Pre-treating SMCs with antagonists of PPAR-α (MK886; 20 µmol/L) and δ (Sulindac sulfide; 100 µmol/L), but not -γ (GW9662; 10 µmol/L), also caused reductions in this perfusing media-induced increase in SM22α promoter activity (Figure 5C).

Figure 5. SRF is involved in PGI2/shear-induced contractile marker expression in SMCs.

Figure 5

Figure 5

Open in a new tab

(A) The SM22α mRNA expression in SMCs co-cultured with ECs transfected with control or PGIS-specific siRNA was examined by real-time PCR. (B) The SM22α-Luc-transfected SMCs were treated with conditioned media or cPGI2 for 8 h. (C) The transfected SMCs were pre-incubated with PPAR-α (MK886; 20 µmol/L), -δ (sulindac sulfide; 100 µmol/L), or -γ (GW9662; 10 µmol/L) antagonist for 1 h. (D) The efficiency of SRF-specific siRNA (siSRF) was examined. (E) The expression of indicated contractile marker genes in SMCs (transfected with control vs. SRF-specific siRNA) of EC/SMC was examined by real-time PCR. (F) EMSA was perform to determine the SRF binding activity in SMCs treated (24 h) with static vs. perfusing media of ECs transfected with control or PGIS-specific siRNA. The nuclear extracts were pre-incubated with 20-fold excess unlabeled oligonucleotides or an anti-SRF antibody (1 µg). SH: super shift band. Data in A–C and E are means ± SEM from 3 independent experiments. *, P < 0.05 vs. static control cells. #, P < 0.05 vs. sheared cells. Results in D and F are representative of 3 independent experiments with similar results.

Since the promoters of almost all SMC contractile marker genes examined contain the CArG (CC[A/T]6GG) box whose binding with SRF is critical for the expression of these genes [1], we investigated whether SRF is involved in PGI2/shear-induced contractile marker gene expression in SMCs. Transfecting SMCs of EC/SMC with SRF-specific siRNA (10 nmol/L), which reduced the SRF protein expression by 90% compared with control siRNA (Figure 5D), abolished the shear-induced SMα-actin, calponin, and SM22α gene expressions in SMCs (Figure 5E). The electrophoretic mobility shift assay (EMSA) results showed that stimulation with perfusing media of ECs transfected with control siRNA caused an increase in SRF-DNA binding activity in the SMC nucleus (Figure 5F); this response was not observed when SMCs were treated with perfusing media of ECs transfected with PGIS-specific siRNA. As positive controls, treating SMCs with cPGI2 for 24 h induced SRF-DNA binding activity. The specificity of this binding for SRF was shown by its abolition by co-incubation of nuclear proteins with 20-fold unlabeled oligonucleotides and further substantiated by the supershifting in gel mobility of the SRF-oligonucleotide complex after pre-incubation of nuclear proteins with an anti-SRF antibody.

Shear-mediated synthetic-to-contractile phenotypic modulation in SMCs of EC/SMC is accompanied by the attenuation of SMC pro-inflammatory gene expression and proliferation

Compared to static controls, shear stress applied to ECs (transfected with control siRNA) of EC/SMC resulted in reductions in SMC expressions of monocyte chemotatic protein-1 (MCP-1) and interleukin-8 (IL-8), the markers of SMC activation [20] (Figure 6A). These responses were rescued by transfecting ECs with PGIS-specific siRNA before shearing. Flow cytometry showed that application of shear stress to ECs (transfected with control siRNA) of EC/SMC led to increases in the % cell in G0/G1 phases and decreases in S (i.e., synthetic) phase in SMCs, as compared with unsheared control cells (Figure 6B). This shear-induced decrease in % cells in S phase was confirmed by the bromodeoxyuridine (BrdU) incorporation assays (Figure 6C) and rescued by transfecting ECs with PGIS-specific siRNA before shearing.

Figure 6. Shear stress/PGI2 inhibit MCP-1 and IL-8 expressions and proliferation in SMCs of EC/SMC.

Figure 6

Open in a new tab

(A) The MCP-1 and IL-8 mRNA expressions in SMCs co-cultured with ECs transfected with control or PGIS-specific siRNA were determined by real-time RT-PCR. (B) SMC proliferation was analyzed by flow cytometry. (C) The percentage of SMCs incorporating BrdU was determined. Data are mean ± SEM from 3 independent experiments. *, P < 0.05 vs. static control cells. #, P < 0.05 vs. sheared cells.

PPARα−/− mice show altered SMC phenotype, with an increase in arterial contractility in response to Ang II, as compared with the WT mice

To explore whether deficiency in PPAR-α alters SMC phenotype in vivo, we performed studies on PPARα−/− and WT mice. Immunohistochemical staining with anti-SMα-actin and anti-h-caldesmon antibodies on cross sections of the straight part of thoracic aorta, which is known to be exposed to laminar shear stress, showed that the expression of these contractile phenotype markers was decreased in the PPARα−/− mice compared with the WT mice (Figure 7A). We assessed the functional consequence of PPAR-α deficiency by measuring the basal levels of arterial blood pressure and mesenteric arterial tension and their responses to Ang II. The basal levels and the peak response of mean arterial pressure to Ang II in PPARα−/− mice are slightly higher than those in WT mice, but there was no statistically significant difference between these two groups of mice (Figure 7B). However, Ang II induced higher levels of contraction of isolated mesenteric arteries from PPARα−/− mice in comparison to that of the WT littermates (Figure 7C). These results suggest that PPAR-α deficiency may alter SMC phenotype toward a synthetic state and result in higher levels of arterial contraction in response to contractile agonists, e.g., Ang II.

Figure 7. PPARα−/− mice show altered SMC phenotype and arterial contraction in response to Ang II.

Figure 7

Open in a new tab

(A) The cross sections of thoracic aorta from PPAR−/− and WT mice were subjected to immunohistochemical staining with anti-SMα-actin and anti-h-caldesmon antibodies. The results shown are representative of the mice from three separate sets of experiments. The sections were counterstained with hematoxylin. (B) Blood pressure was measured in PPAR−/− and WT mice by the carotid arterial catheterization. (C) Contractions of mesenteric arteriolar rings from PPAR−/− and WT mice in response to Ang II were measured. Contractions are expressed as percent of KCl (100 mmol/L)-induced contraction (control). *, P < 0.05 vs. control mice.

DISCUSSION

In this study, we have characterized a novel mechanism by which laminar shear stress regulates SMC phenotype (from synthetic to contractile state) through PPAR-α/δ activations by PGI2 released from sheared ECs (summarized in Figure 8). Our study has generated the following findings. (i) Shear stress applied to ECs of EC/SMC resulted in increases in contractile marker expression and decreases in pro-inflammatory gene expression and % cells in S phase in SMCs. These responses are EC-specific and mediated through a paracrine effect of EC mediator(s). (ii) Shear stress applied to ECs induced their release of PPAR ligands to activate SMC PPAR-α, -δ, and -γ, but only the former two are involved in shear-modulation of SMC phenotype. (iii) siRNA-knockdown of EC PGIS expression to inhibit the shear-induction of PGI2 inhibited the shear-mediated activation of PPAR-α/δ, induction of contractile markers, and reduction of pro-inflammatory gene expression and % cells in S phase in SMCs. (iv) PPARα−/− mice (compared with WT littermates) showed decreased expression of SMC contractile markers in the thoracic aorta, with increased arterial contraction in response to Ang II. Thus, our findings have helped to elucidate the roles of EC PGI2 and SMC PPAR-α/δ in the shear-modulation of SMC phenotype and the consequent modulation of arterial contractility.

Figure 8.

Figure 8

Open in a new tab

Schematic representation of the signaling pathways regulating the phenotype of SMCs in EC/SMC co-culture as a result of subjecting ECs to laminar shear stress.

PPARs can be activated by a number of natural and synthetic ligands, including thiazolidinedione class of anti-diabetic drugs, polyunsaturated fatty acids, and prostaglandins [6]. Taba et al. [4] demonstrated that laminar shear stress to ECs induces their release of 15-deoxy-Δ12,14-prostaglandin J2 (15d-PGJ2), a natural PPAR-γ ligand. A recent study by Liu et al. [5] further demonstrated that laminar shear stress activates endogenous PPAR-γ in ECs and that the lipid extracts from perfusing media contain PPAR-γ ligands, which exert anti-inflammatory effects in several types of cells. In concert with these previous findings, our study also showed that laminar flow applied to ECs induces their release of PPAR-γ ligands, which can activate PPAR-γ in SMCs. A new finding of this study is that perfusing media of ECs also contain ligands for the activation of SMC PPAR-α/δ. Moreover, the shear/perfusing media-induction of contractile markers in SMCs was inhibited by transfections with specific siRNA of PPAR-α/δ, but not PPAR-γ, suggesting that only PPAR-α/δ, but not PPAR-γ, are involved in shear-induced phenotypic modulation of SMCs in close proximity to ECs.

PGI2 is a potent vasodilator contributing to protection and maintenance of homeostasis in vasculature. In concurrence with the results by previous studies [18], our findings show that shear stress applied to ECs induces their production of PGI2, which may serve as a PPAR-α/δ ligand to activate these PPARs in SMCs, inasmuch as transfecting ECs with PGIS-specific siRNA before shearing inhibited perfusing media-induced PPAR-α/δ ligand binding activities in SMCs. The inhibition of shear-activations of PPAR-α/δ is accompanied by the inhibitions in shear-induction of contractile markers and reduction of MCP-1 and IL-8 expressions and % cells in S phase in SMCs, indicating the importance of PGI2 produced by sheared ECs in regulating SMC phenotype through the ligand-dependent activations of PPAR-α/δ.

PGI2 has a short half-life (< 20 min at physiological pH) and is produced in low amounts by unstimulated cultured ECs [18, 21]. Thus, the effective concentration of PGI2 released from sheared ECs is critical for its paracrine effect on neighboring SMCs to modulate their phenotype. In our current study, the perfusing media were found to contain 6-keto-PGF1α at a concentration of ~50 ng/L and to be able to induce contractile marker expression in SMCs. As controls, cPGI2 of 50 nmol/L also can induce contractile marker expression in SMCs. However, both the perfusing media of ECs transfected with PGIS-specific siRNA, which contain 6-keto-PGF1α at ~10 ng/L, and the cPGI2 at 10 nmol/L did not have this effect. In our EC/SMC co-culture flow system, ECs and SMCs were separated by a porous membrane and hence may have close communications via humoral transmission through the short distance between these cells. It is to be noted, however, that the exposure of the SMCs to the humoral agents released by the ECs is limited to the small pores, which occupy only about 0.2% of the total surface area. In order to achieve a paracrine effect similar to that induced by the perfusing media that contain 50 nmol/L of PGI2 acting on the entire SMC surface available, the local concentration acting on the SMCs in the co-culture system must be much higher, even if one considers the lateral spreading under the pore. This is reasonable because the co-culture does not involve the dilution of the paracrine molecules by the fluid media.

In addition to PGI2, ECs exposed to laminar shear stress release other anti-atherogenic factors, including NO [3]. Using the same flow system, our previous study showed that co-culture with SMCs under static condition results in a down-regulation of eNOS expression in ECs [15]. Application of shear stress to EC/SMC induces EC production of NO [22]. We have found that pre-treating ECs with an eNOS specific inhibitor NG-nitro-L-arginine methyl ester or transfecting ECs of EC/SMC with eNOS-specific siRNA did not inhibit shear-induced PPAR-α/δ ligand binding activities and contractile marker expression in SMCs (online Figure I). These results indicate that the NO released from the sheared ECs probably does not serve as a PPAR-α/δ ligand for the activation of these PPARs in SMCs and the consequent modulation of SMC phenotype.

Our in vitro findings on shear-induction of SMC contractile phenotype through PGI2-activations of PPAR-α/δ suggest that deficiency in PPAR-α, PPAR-δ, or PGI2 may alter SMC phenotype toward a synthetic state, thus affecting the arterial contractility or inflammatory responses in vivo. This notion was supported by a recent study by Yokoyama et al. [23], who showed that PGIS−/− mice exhibit higher levels of blood pressure than the control littermates, with thickening of the thoracic aortic media in aged mice. However, the PPAR-δ agonist GW0742 was found to attenuate Ang II-accelerated atherosclerosis without altering blood pressure [24]. An important finding of the current study is that PPARα−/− mice have decreased expression of contractile phenotype markers in the straight part of thoracic aorta compared with WT mice, supporting our in vitro findings that PPAR-α plays important roles in the maintenance of SMC contractile phenotype in regions subjected to physiological levels of laminar shear stress. Our present study further demonstrated that PPAR-α deficiency leads to higher levels of contraction of mesenteric arteries from PPARα−/− mice in response to Ang II, as compared with the WT littermates. These results suggest that synthetic status of SMC phenotype induced by PPAR-α deficiency is accompanied by an increase in susceptibility to Ang II-induced hypertension or atherosclerosis.

In summary, the present study demonstrated that laminar shear stress applied to ECs modulates the phenotype of underlying SMCs from synthetic to contractile state through activations of SMC PPAR-α/δ by EC-released PGI2. Our previous studies demonstrated that laminar shear stress exerts anti-inflammatory effects by inhibiting SMC-induced pro-inflammatory responses in ECs [15, 16]. The current study advances the new notion that laminar shear stress may serve atheroprotective functions by also modulating SMC phenotype toward a contractile state. Our findings provide insights into the mechanisms underlying the EC-SMC bi-directional interaction and the protective homeostatic function of shear stress in modulating signaling and gene expression in SMCs and the consequent modulation of their phenotype.

Supplementary Material

01

ACKNOWLEDGMENTS

The authors are indebted to Dr. Kenneth K. Wu for helpful discussion.

SOURCES OF FUNDING

This work was supported by National Health Research Institutes Grant ME-097-PP-06 and National Science Council Grants 97-3112-B-400-006/96-2628-B-400-002-MY3 (to J.-J.C.); National Heart, Lung, and Blood Institute Grants HL080518/HL085159 (to S.C.); and National Science Foundation Grants 30725033/30530340 (to Y.G.) and 30890041 (to N.W).

Non-standard Abbreviations and Acronyms

ADRP

adipose differentiation-related protein

ANGPTL4

angiopoietin-like protein-4

Ang II

angiotensin II

BrdU

bromodeoxyuridine

CD36

fatty acid translocase

CHX

cycloheximide

cPGI2

carbaprostacyclin

CPT-I

carnitine palmitoyltransferase-I

EC

endothelial cell

EMSA

electrophoretic mobility shift assay

eNOS

endothelial nitric oxide synthase

IL-8

interleukin-8

6-keto-PGF1α

6-keto-prostaglandin F1α

LBD

ligand binding domain

MAP

mean arterial pressure

MCP-1

monocyte chemotatic protein-1

NO

nitric oxide

PGIS

prostacyclin synthase

PGI2

prostacyclin

PPAR

peroxisome proliferator-activated receptor

PPRE

PPAR-responsive element

siRNA

small interfering RNA

SMα-actin

smooth muscle α-actin

SM22α

smooth muscle protein 22-α

SMC

smooth muscle cell

SM-MHC

smooth muscle myosin heavy chain

SRF

serum response factor

WT

wild type

Footnotes

Subject codes: [95] Endothelium/vascular type/nitric oxide;

[162] Smooth muscle proliferation and differentiation;

[138] Cell signalling/signal transduction;

[134] Pathophysiology;

[97] Other Vascular biology

DISCLOSURES

None

REFERENCE

  • 1.Owens GK, Kumar MS, Wamhoff BR. Molecular regulation of vascular smooth muscle cell differentiation in development and disease. Physiol Rev. 2004;84:767–801. doi: 10.1152/physrev.00041.2003. [DOI] [PubMed] [Google Scholar]
  • 2.Davies PF. Flow-mediated endothelial mechanotransduction. Physiol Rev. 1995;75:519–560. doi: 10.1152/physrev.1995.75.3.519. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 3.Traub O, Berk BC. Laminar shear stress: mechanisms by which endothelial cells transduce an atheroprotective force. Arterioscler Thromb Vasc Biol. 1998;18:677–685. doi: 10.1161/01.atv.18.5.677. [DOI] [PubMed] [Google Scholar]
  • 4.Taba Y, Sasaguri T, Miyagi M, Abumiya T, Miwa Y, Ikeda T, Mitsumata M. Fluid shear stress induces lipocalin-type prostaglandin D2 synthase expression in vascular endothelial cells. Circ Res. 2000;86:967–973. doi: 10.1161/01.res.86.9.967. [DOI] [PubMed] [Google Scholar]
  • 5.Liu Y, Zhu Y, Rannou F, Lee TS, Formentin K, Zeng L, Yuan X, Wang N, Chien S, Forman BM, Shyy JY. Laminar flow activates PPARgamma in vascular endothelial cells. Circulation. 2004;110:1128–1133. doi: 10.1161/01.CIR.0000139850.08365.EC. [DOI] [PubMed] [Google Scholar]
  • 6.Marx N, Duez H, Fruchart JC, Staels B. Peroxisome proliferator-activated receptors and atherogenesis: regulators of gene expression in vascular cells. Circ Res. 2004;94:1168–1178. doi: 10.1161/01.RES.0000127122.22685.0A. [DOI] [PubMed] [Google Scholar]
  • 7.Law RE, Goetze S, Xi XP, Jackson S, Kawano Y, Demer L, Fishbein MC, Meehan WP, Hsueh WA. Expression and function of PPARgamma in rat and human vascular smooth muscle cells. Circulation. 2000;101:1311–1318. doi: 10.1161/01.cir.101.11.1311. [DOI] [PubMed] [Google Scholar]
  • 8.Marx N, Schönbeck U, Lazar MA, Libby P, Plutzky J. Peroxisome proliferator-activated receptor gamma activators inhibit gene expression and migration in human vascular smooth muscle cells. Circ Res. 1998;83:1097–1103. doi: 10.1161/01.res.83.11.1097. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Kanda T, Brown JD, Orasanu G, Vogel S, Gonzalez FJ, Sartoretto J, Michel T, Plutzky J. PPARgamma in the endothelium regulates metabolic responses to high-fat diet in mice. J Clin Invest. 2009;119:110–124. doi: 10.1172/JCI36233. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Staels B, Koenig W, Habib A, Merval R, Lebret M, Torra IP, Delerive P, Fadel A, Chinetti G, Fruchart JC, Najib J, Maclouf J, Tedgui A. Activation of human aortic smooth-muscle cells is inhibited by PPARalpha but not by PPARgamma activators. Nature. 1998;393:790–793. doi: 10.1038/31701. [DOI] [PubMed] [Google Scholar]
  • 11.Zhang J, Fu M, Zhu X, Xiao Y, Mou Y, Zheng H, Akinbami MA, Wang Q, Chen YE. Peroxisome proliferator-activated receptor delta is up-regulated during vascular lesion formation and promotes post-confluent cell proliferation in vascular smooth muscle cells. J Biol Chem. 2002;277:11505–11512. doi: 10.1074/jbc.M110580200. [DOI] [PubMed] [Google Scholar]
  • 12.Lim HJ, Lee S, Park JH, Lee KS, Choi HE, Chung KS, Lee HH, Park HY. PPAR delta agonist L-165041 inhibits rat vascular smooth muscle cell proliferation and migration via inhibition of cell cycle. Atherosclerosis. 2009;202:446–454. doi: 10.1016/j.atherosclerosis.2008.05.023. [DOI] [PubMed] [Google Scholar]
  • 13.Forman BM, Chen J, Evans RM. Hypolipidemic drugs, polyunsaturated fatty acids, and eicosanoids are ligands for peroxisome proliferator-activated receptors alpha and delta. Proc Natl Acad Sci U S A. 1997;94:4312–4317. doi: 10.1073/pnas.94.9.4312. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 14.Numaguchi Y, Naruse K, Harada M, Osanai H, Mokuno S, Murase K, Matsui H, Toki Y, Ito T, Okumura K, Hayakawa T. Prostacyclin synthase gene transfer accelerates reendothelialization and inhibits neointimal formation in rat carotid arteries after balloon injury. Arterioscler Thromb Vasc Biol. 1999;19:727–733. doi: 10.1161/01.atv.19.3.727. [DOI] [PubMed] [Google Scholar]
  • 15.Chiu JJ, Chen LJ, Lee PL, Lee CI, Lo LW, Usami S, Chien S. Shear stress inhibits adhesion molecule expression in vascular endothelial cells induced by coculture with smooth muscle cells. Blood. 2003;101:2667–2674. doi: 10.1182/blood-2002-08-2560. [DOI] [PubMed] [Google Scholar]
  • 16.Chiu JJ, Chen LJ, Lee CI, Lee PL, Lee DY, Tsai MC, Lin CW, Usami S, Chien S. Mechanisms of induction of endothelial cell E-selectin expression by smooth muscle cells and its inhibition by shear stress. Blood. 2007;110:519–528. doi: 10.1182/blood-2006-08-040097. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 17.Zhang Y, Guan Y, Schneider A, Brandon S, Breyer RM, Breyer MD. Characterization of murine vasopressor and vasodepressor prostaglandin E(2) receptors. Hypertension. 2000;35:1129–1134. doi: 10.1161/01.hyp.35.5.1129. [DOI] [PubMed] [Google Scholar]
  • 18.Frangos JA, Eskin SG, McIntire LV, Ives CL. Flow effects on prostacyclin production by cultured human endothelial cells. Science. 1985;227:1477–1479. doi: 10.1126/science.3883488. [DOI] [PubMed] [Google Scholar]
  • 19.Camoretti-Mercado B, Forsythe SM, LeBeau MM, Espinosa R, 3rd, Vieira JE, Halayko AJ, Willadsen S, Kurtz B, Ober C, Evans GA, Thweatt R, Shapiro S, Niu Q, Qin Y, Padrid PA, Solway J. Expression and cytogenetic localization of the human SM22 gene (TAGLN) Genomics. 1998;49:452–457. doi: 10.1006/geno.1998.5267. [DOI] [PubMed] [Google Scholar]
  • 20.Rainger GE, Nash GB. Cellular pathology of atherosclerosis: smooth muscle cells prime co-cultured endothelial cells for enhanced leukocyte adhesion. Circ Res. 2001;88:615–622. doi: 10.1161/01.res.88.6.615. [DOI] [PubMed] [Google Scholar]
  • 21.Morigi M, Zoja C, Figliuzzi M, Remuzzi G, Remuzzi A. Supernatant of endothelial cells exposed to laminar flow inhibits mesangial cell proliferation. Am J Physiol. 1993;264:C1080–C1083. doi: 10.1152/ajpcell.1993.264.4.C1080. [DOI] [PubMed] [Google Scholar]
  • 22.Sakamoto N, Ohashi T, Sato M. Effect of fluid shear stress on migration of vascular smooth muscle cells in cocultured model. Ann Biomed Eng. 2006;34:408–415. doi: 10.1007/s10439-005-9043-y. [DOI] [PubMed] [Google Scholar]
  • 23.Yokoyama C, Yabuki T, Shimonishi M, Wada M, Hatae T, Ohkawara S, Takeda J, Kinoshita T, Okabe M, Tanabe T. Prostacyclin-deficient mice develop ischemic renal disorders, including nephrosclerosis and renal infarction. Circulation. 2002;106:2397–2403. doi: 10.1161/01.cir.0000034733.93020.bc. [DOI] [PubMed] [Google Scholar]
  • 24.Takata Y, Liu J, Yin F, Collins AR, Lyon CJ, Lee CH, Atkins AR, Downes M, Barish GD, Evans RM, Hsueh WA, Tangirala RK. PPARdelta-mediated antiinflammatory mechanisms inhibit angiotensin II-accelerated atherosclerosis. Proc Natl Acad Sci U S A. 2008;105:4277–4282. doi: 10.1073/pnas.0708647105. [DOI] [PMC free article] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

01
NIHMS139819-supplement-01.pdf (95.8KB, pdf)

RESOURCES