Mitogenic factors released from smooth muscle cells are responsible for neointimal cell proliferation after balloon catheter deendothelialization

Mechanobiological model of arterial growth and remodeling

A coupled agent-based model (ABM) and finite element analysis (FEA) computational framework is developed to study the interplay of bio-chemo-mechanical factors in blood vessels and their role in maintaining homeostasis. The agent-based model implements the power of REPAST Simphony libraries and adapts its environment for biological simulations. Coupling a continuum-level model (FEA) to a cellular-level model (ABM) has enabled this computational framework to capture the response of blood vessels to increased or decreased levels of growth factors, proteases and other signaling molecules (on the micro scale) as well as altered blood pressure. Performance of the model is assessed by simulating porcine left anterior descending artery under normotensive conditions and transient increases in blood pressure and by analyzing sensitivity of the model to variations in the rule parameters of the ABM. These simulations proved that the model is stable under normotensive conditions and can recover from transient increases in blood pressure. Sensitivity studies revealed that the model is most sensitive to variations in the concentration of growth factors that affect cellular proliferation and regulate extracellular matrix composition (mainly collagen).

Ensuring congruency in multiscale modeling: towards linking agent based and continuum biomechanical models of arterial adaptation

There is a need to develop multiscale models of vascular adaptations to understand tissue-level manifestations of cellular level mechanisms. Continuum-based biomechanical models are well suited for relating blood pressures and flows to stress-mediated changes in geometry and properties, but less so for describing underlying mechanobiological processes. Discrete stochastic agent-based models are well suited for representing biological processes at a cellular level, but not for describing tissue-level mechanical changes. We present here a conceptually new approach to facilitate the coupling of continuum and agent-based models. Because of ubiquitous limitations in both the tissue- and cell-level data from which one derives constitutive relations for continuum models and rule-sets for agent-based models, we suggest that model verification should enforce congruency across scales. That is, multiscale model parameters initially determined from data sets representing different scales should be refined, when possible, to ensure that common outputs are consistent. Potential advantages of this approach are illustrated by comparing simulated aortic responses to a sustained increase in blood pressure predicted by continuum and agent-based models both before and after instituting a genetic algorithm to refine 16 objectively bounded model parameters. We show that congruency-based parameter refinement not only yielded increased consistency across scales, it also yielded predictions that are closer to in vivo observations.

Toward a multi-scale computational model of arterial adaptation in hypertension: verification of a multi-cell agent based model

Agent-based models (ABMs) represent a novel approach to study and simulate complex mechano chemo-biological responses at the cellular level. Such models have been used to simulate a variety of emergent responses in the vasculature, including angiogenesis and vasculogenesis. Although not used previously to study large vessel adaptations, we submit that ABMs will prove equally useful in such studies when combined with well-established continuum models to form multi-scale models of tissue-level phenomena. In order to couple agent-based and continuum models, however, there is a need to ensure that each model faithfully represents the best data available at the relevant scale and that there is consistency between models under baseline conditions. Toward this end, we describe the development and verification of an ABM of endothelial and smooth muscle cell responses to mechanical stimuli in a large artery. A refined rule-set is proposed based on a broad literature search, a new scoring system for assigning confidence in the rules, and a parameter sensitivity study. To illustrate the utility of these new methods for rule selection, as well as the consistency achieved with continuum-level models, we simulate the behavior of a mouse aorta during homeostasis and in response to both transient and sustained increases in pressure. The simulated responses depend on the altered cellular production of seven key mitogenic, synthetic, and proteolytic biomolecules, which in turn control the turnover of intramural cells and extracellular matrix. These events are responsible for gross changes in vessel wall morphology. This new ABM is shown to be appropriately stable under homeostatic conditions, insensitive to transient elevations in blood pressure, and responsive to increased intramural wall stress in hypertension.

Transcriptional activation of p21 by Tranilast is mediated via transforming growth factor beta signal pathway

Tranilast, an antiallergic medication, is a very promising inhibitor of restenosis after balloon angioplasty. Tranilast can prevent the proliferation and migration of smooth muscle cells by activating the gene expression of p21, a strong cyclin/cyclin-dependent kinase (CDK) inhibitor, and by arresting cell growth at the G0/G1 phase. The signaling pathway of Tranilast in regulating p21 is to our best interest and is elucidated in the present study. The major emphasis was weighted on exploring the regulatory effects of Tranilast on promoter activity of p21. By serial deletion analysis, the sequence between -74 and -83 bp of the p21 promoter, previously identified as the transforming growth factor-beta (TGF-beta)-response element, was found sufficient, where as most of the promoter region 5' to -111 bp was found unnecessary for the transcriptional activation of p21 by both TGF-beta1 and Tranilast. Tranilast was also found to induce phosphorylation of Smad2 (a cytoplasmic signaling molecule essential for mediating TGF-beta signal transduction). Transfection of DeltakTbetaRII, a truncated form of TGF-beta type II receptor known to exert a dominant-negative effect on TGF-beta signaling, was found to suppress the signaling of both Tranilast and TGF-beta1 to a similar extent. These results suggested that induction of p21 by Tranilast might be closely related to TGF-beta signal transduction pathway.

Early activation of internal medial smooth muscle cells in the rabbit aorta after mechanical injury: relationship with intimal thickening and pharmacological applications

1. Smooth muscle cells (SMC) participate in both inflammatory and dedifferentiation processes during atherosclerosis, as well as during mechanical injury following angioplasty. In the latter, we studied medial SMC differentiation and inflammation processes implicated early after de-endothelialization in relation to mechanical stresses. We hypothesized that activation of a subpopulation of SMC within the media plays a crucial role in the early phase of neointimal formation. 2. For this purpose, we used a rabbit model of balloon injury to study activation and differentiation of medial SMC in the early time after denudation and just before neointima thickening. Inflammation was evaluated by the expression of vascular cell adhesion molecule (VCAM)-1, integrin alpha4beta1 and nuclear factor (NF)-kB. Myosin isoforms and 2P1A2 antigen, a membrane protein expressed by rabbit dedifferentiated SMC, were used as markers of differentiation. 3. On day 2 after de-endothelialization, VCAM-1, alpha4beta1 and NF-kB were coexpressed by a well-defined subpopulation of SMC of the internal part of the media, in the vicinity of the blood stream. At the same time, the majority of SMC throughout the media expressed non-muscle myosin heavy chain-B (nm-MHC-B) and 2P1A2 antigen. On day 7, when intimal thickening appeared, SMC of the media were no longer activated, whereas some intimal SMC expressed the activation markers. Thus, after de-endothelialization, early dedifferentiation occurs in most of the medial SMC, whereas activation concerned only a subpopulation of SMC located in the internal media. Using the T-type voltage-operated calcium channel blocker mibefradil (0.1-1 micromol/L) in SMC culture, we showed that this agent exhibited an antiproliferative effect in a dose-dependent manner only on undifferentiated cells. 4. In conclusion, the results suggest that the activated SMC represent cells that are potentially able to migrate and participate in the intimal thickening process. Thus, the medial SMC inflammatory process, without any contribution of inflammatory cells, may represent a major mechanism underlying the development of intimal thickening following mechanical stress. In humans, inhibition of T-type calcium channels may be a tool to prevent the early proliferation step leading to neointimal formation.

Stem cell factor and c-kit are expressed by and may affect vascular SMCs through an autocrine pathway

INTRODUCTION

Stem cell factor (SCF) is a membrane-bound and soluble growth factor that activates the c-kit tyrosine kinase receptor. Given the similarities between c-kit and platelet-derived growth factor (PDGF) receptors, we hypothesized that similar to PDGF, SCF/c-kit signaling may play a role in smooth muscle cell (SMC) function and thus the development of intimal hyperplasia.

MATERIALS AND METHODS

Human saphenous vein SMCs were harvested from veins procured at the time of bypass grafting. Carotid arteries from rats that were balloon injured (n = 12) at variable time points were compared to sham-operated controls (n = 3). Expression of SCF and c-kit was measured by immunohistochemistry (IHC) and Western blotting.

RESULTS

Western blotting revealed that human SMCs express membrane-bound SCF. In separate experiments, we found that this growth factor undergoes proteolytic cleavage to its soluble form following exposure to matrix metalloproteinase-9 (MMP-9), a ubiquitous MMP released at the time of arterial injury. We next evaluated in human SMCs, expression of the SCF receptor, c-kit. Western blotting of human SMC lysates revealed minor but consistent expression of c-kit. IHC demonstrated c-kit expression to be localized to the media. To determine if c-kit is up-regulated during the development of intimal hyperplasia, we evaluated expression of this receptor in a rat carotid balloon injury model. Quantification of IHC staining on injured vessels revealed that c-kit expression within the media was significantly increased at 3, 7, 14, and 28 days following injury (28.1, 30.8, 16, and 10.4% increase over sham controls, respectively, P < 0.05). Furthermore, c-kit expression was prominent within the neointima and maximal at 7 days (53.4 +/- 7.8% of area c-kit positive).

CONCLUSION

Human vascular SMCs express the growth factor SCF and its receptor, c-kit. SCF is released from its membrane-bound form via MMP-9. This finding and the dramatic increase in c-kit expression observed in the rat carotid artery after balloon injury suggests SCF/c-kit signaling may affect SMC function via an autocrine pathway.

Factor Xa induces mitogenesis of vascular smooth muscle cells via autocrine production of epiregulin

Factor Xa has been reported to elicit smooth muscle cell proliferation via autocrine release of platelet-derived growth factor. However, this study has shown that factor Xa-induced mitogenesis of rat aortic smooth muscle cell is independent of platelet-derived growth factor. We also could not observe any platelet-derived growth factor isoforms in the cultured medium of factor Xa-stimulated cells. Our finding that the cultured medium of factor Xa-stimulated cells strongly induces rat aortic smooth muscle cell mitogenesis in the absence of factor Xa activity led us to explore the existence of a novel autocrine pathway. The autocrine growth factor was purified from the cultured medium and was identified to be epiregulin. Recombinant epiregulin was also able to induce the mitogenesis. The secretion of epiregulin from factor Xa-stimulated rat aortic smooth muscle cell required mRNA expression and protein synthesis of the growth factor. The mitogenic effect of factor Xa on rat aortic smooth muscle cell was significantly reduced by anti-epiregulin antibody or by antisense oligodeoxynucleotide to epiregulin. Several lines of experimental evidence clearly indicate that the autocrine production of epiregulin, an epidermal growth factor-related ligand, is induced in the factor Xa-stimulated mitogenic process of rat aortic smooth muscle cell.

Shear stress-induced release of basic fibroblast growth factor from endothelial cells is mediated by matrix interaction via integrin alpha(v)beta3

Considering that chronic elevation of shear stress results in remodeling of the vasculature, we analyzed whether mechanical load could mediate basic fibroblast growth factor (bFGF) release and whether bFGF would act as mediator of shear stress-induced endothelial proliferation and differentiation. Supernatant media of shear stress-exposed endothelial cells (EC) contained significantly higher amounts of bFGF than medium from static cells. Released bFGF was fully intact with regard to its function as an inductor of proliferation and differentiation. Shear stress-conditioned media induced capillary-like structure formation, whereas static control medium did not. Likewise, only shear stress-conditioned medium induced proliferation of serum starved EC. Both capillary-like structure formation and proliferation could be inhibited by neutralization of bFGF or its receptor. The release of bFGF was subject to specific, integrin-mediated control, since inhibition of alpha(v)beta(3) integrin prevented it, whereas inhibition of alpha(5)beta(1) integrin had no effect. We conclude that shear stress induces the release of bFGF from EC in a tightly controlled manner. The release is dependent on specific cell-matrix interactions via alpha(v)beta(3) integrins. The effects on cell proliferation and differentiation suggest that release of bFGF is functionally significant and may represent a necessary initial step in adaptive remodeling processes induced by shear stress.

Collagen biosynthesis by neointimal smooth muscle cells cultured from rabbit aortic explants 15 weeks after de-endothelialization

Extracellular matrix (ECM) accumulation in arterial neointima, developed in response to de-endothelialization, is a prolonged process. In this study, we examined the relationship between increased collagen accumulation and synthetic activity of neointimal smooth muscle cells (SMCs) derived from aortic explants fifteen weeks after balloon catheter injury. Freshly confluent SMCs, derived either from normal aorta or from aortic neointima, were used in this study. The newly synthesized collagen was analysed by measuring [3H]-proline incorporation; and the mRNA expression for two major types of collagen, collagen type I and type III, was studied by Northern blot analysis. Our results indicated a three fold increase in protein (collagen) synthesis by neointimal SMCs. At the same time, the steady-state mRNA for procollagen I and procollagen III was elevated five and three times, respectively. These data indicate that persistent synthesis contributes to collagen accumulation in the arterial neointima and both transcriptional and post-transcriptional regulation take part in this process.

Expression of syndecan-1 in rabbit neointima following de-endothelialization by a balloon catheter

Enrichment of proteoglycans is prominent in early atherogenesis, contributing not only to SMC migration and proliferation, but also to low density lipoprotein retention. A family of integral cell membrane proteoglycans termed syndecans has recently been recognized. Among syndecans, syndecan-1, the first isolated member, has received most research attention. In this study, we examined the expression of syndecan-1 in rabbit aorta and aortic neointima, developed in response to a balloon catheter-induced de-endothelialization. The tissues were processed for Northern blot analysis, in situ hybridization, immunohistochemical staining and immunoblotting. Our results indicate that in normal aorta, the signal for syndecan-1 is weak. However, arterial injury induces syndecan-1 expression at both mRNA and protein levels. The presence of syndecan-1 in the neointimal tissue is persistent, prominent even at the 12th week after injury. Syndecan positive cells are distributed in the whole layer of the neointima, but are not visible in the underlying media. The presence of syndecan-1 in arterial neointima suggests a novel means of mediating interactions between neointimal cells and various agents, including extracellular matrix components, growth factors and lipoproteins.