Break the Cycle: The Role of Cell-Cycle Modulation in the Prevention of Vasculoproliferative Diseases

Atherosclerosis is a major health problem in the western world, and the effectiveness of interventional therapeutic modalities for symptomatic atherosclerotic lesions is limited by vessel restenosis. Proliferation and migration of smooth muscle cells (SMCs) play a central role in post-interventional restenosis. Accordingly, many therapeutic approaches attempt to inhibit SMC proliferation. As the cell cycle is a final common pathway in SMC proliferation, proteins of the cell cycle have emerged as logical targets for the treatment and prevention of restenotic lesions. In this review we discuss current approaches that target the cell cycle in smooth muscle cells, and also describe recent and ongoing clinical trials that involve cell-cycle manipulation in the treatment of vasculoproliferative diseases.     

A new concept of development of neointimal hyperplasia

The intima hyperplasia is a major morphological feature of various arterial pathologies such as atherosclerosis, postangioplasty restenosis and transplantation arteriopathy. It is commonly assumed that smooth muscle cells (SMC) comprising loci of the intima hyperplasia originate from arterial media. However, recent studies suggest that the bone marrow could also supply circulating vascular progenitor of SMCs and endothelial cells (EC). Such bone marrow progenitors participate in the formation of a cellular mass of neointima after experimental allotransplantation, mechanical vessel injury or hyperlipidemia induced experimental atherosclerosis. Circulating SMC and EC progenitors are also likely to be involved in the transplantation arteriopathy development in humans but their roles in the atherosclerosis and restenosis remain to be determined. Stages of the mobilization, defferentiation and proliferation of SMC progenitors could provide point of attack for new therapeutic strategies for the treatment of proliferative vascular diseases. The precise understanding of the neointima cells origin could provide a key for development of the optimal therapeutic strategy of treatnent of such disorders. This review is focused on the pathological significance of circulating progenitors of the bone marrow origin, particularly on the SMC progenitors, for development of vascular wall disorders.     

Inhibition of vascular smooth muscle cell proliferation by ribozymes that cleave c-myb mRNA.

Proliferation of injured smooth muscle cells contributes to the reocclusion or restenosis of coronary arteries that often occurs following angioplasty procedures. We have identified and optimized nuclease-resistant ribozymes that efficiently cleave c-myb RNA. Three ribozymes targeting different sites in the c-myb mRNA were synthesized chemically and delivered to rat aortic smooth muscle cells with cationic lipids; all three inhibited serum-stimulated cell proliferation significantly. RNA molecules with two base substitutions in the catalytic core that render the ribozyme catalytically inactive had little effect on smooth muscle cell proliferation. Ribozymes with scrambled binding arm sequences also failed to affect cell cycle progression of vascular smooth muscle cells. Furthermore, inhibition of rat smooth muscle cell proliferation correlated with a reduction in intact c-myb mRNA. Efficacy of the chemically-modified ribozyme was compared directly to phosphorothioate antisense oligodeoxynucleotides targeting the same site in the c-myb RNA; the ribozyme had superior efficacy and showed greater specificity than the antisense molecules. Exogenously delivered ribozymes also inhibited porcine and human smooth muscle cell proliferation effectively. Ribozymes targeting c-myb or other regulators of smooth muscle cell proliferation may represent novel therapeutics for the treatment of restenosis after coronary angioplasty.     

Cytostatic drugs differentially affect phenotypic features of porcine coronary artery smooth muscle cell populations

We studied the effects of cytostatic drugs on porcine coronary artery spindle-shaped (S) and rhomboid (R) smooth muscle cell (SMC) biological activities related to intimal thickening (IT) formation. Imatinib, and to a lesser extent curcumin, decreased proliferation of S- and R-SMCs and migratory and urokinase activities of R-SMCs more efficiently compared with cyclosporine plus rapamycin. Imatinib increased the expression of alpha-smooth muscle actin in both SMC populations and that of smoothelin in S-SMCs. It decreased S100A4 expression in R-SMCs. By promoting SMC quiescence and differentiation imatinib and curcumin may represent valid candidates for restenosis preventive and therapeutic strategies.     

Mechanism by which avenanthramide-c, a polyphenol of oats, blocks cell cycle progression in vascular smooth muscle cells

Previously, we reported that avenanthramide-c (Avn-c), one of the major avenanthramides, polyphenols of oats, inhibited the serum-induced proliferation of vascular smooth muscle cells (SMC), which is an important process in the initiation and development of atherosclerosis. In the present study, we further investigated its cell cycle inhibitory mechanism. Rat embryonic aortic smooth muscle cell line A10 was used in this study. Flow cytometry analysis revealed that treatment of A10 cells with 80 muM Avn-c arrested the cell cycle in G1 phase as indicated by an increase in the number of cells in G1 phase and a decrease in the number of cells in S phase. This cell cycle arrest was associated with a decrease in the phosphorylation of retinoblastoma protein (pRb), whose hyperphosphorylation is a hallmark of the G1 to S transition in the cell cycle. The inhibition of pRb phosphorylation with Avn-c was accompanied by a decrease in cyclin D1 expression and an increase in cyclin-dependent kinase inhibitor p21cip1 expression, without significant changes in p27kip1 expression. Furthermore, Avn-c treatment increased the expression level and stability of p53 protein, which could account for the increase of p21cip1 expression. Our results demonstrate for the first time that Avn-c, which is a unique polyphenol found in oats, arrests SMC proliferation at G1 phase by upregulating the p53-p21cip1 pathway and inhibiting pRB phosphorylation. This inhibitory effect of Avn-c on SMC proliferation is an additional indication for the potential health benefit of oat consumption in the prevention of coronary heart disease beyond its known effect through lowering blood cholesterol.     

Secretion of a new growth factor, smooth muscle cell derived growth factor, distinct from platelet derived growth factor by cultured rabbit aortic smooth muscle cells

In attempts to determine the mechanism of proliferation of arterial smooth muscle cells (SMC) in intimal atheromatous lesions, autocrine secretion of growth factors by SMC has recently received much attention. Here we report a new growth factor named smooth muscle cell derived growth factor (SDGF). Cultured rabbit medial SMC secreted SDGF for 1 week during their incubation in serum-free media only after at least 4 passages. SDGF differed from platelet derived growth factor (PDGF) physicochemically, immunologically, and biologically. The properties of SDGF also seemed different from those of other known growth factors that stimulate the proliferation of mesenchymal cells.     

Heparanase and platelet factor-4 induce smooth muscle cell proliferation and migration via bFGF release from the ECM

Basic fibroblast growth factor (bFGF) has been shown to play an instrumental role in the cascade of events leading to restenosis; however, the mechanisms of bFGF activation following vascular injury have remained elusive. We have demonstrated that heparanase and platelet factor-4 (PF4), released from activated platelets at the site of injury, liberate bFGF from the extracellular matrix (ECM) of vascular smooth muscle cells (SMC), resulting in the induction of SMC proliferation and migration. Increases in proliferation and migration were inhibited by treatment with a bFGF-neutralizing antibody, suggesting that proliferation and migration in response to heparanase or PF4 are mediated by bFGF activation. When platelets were seeded on top of SMCs, degranulation products were found to release bFGF from the ECM, increasing cell proliferation and cell migration. Again, these increases in SMC proliferation and migration were inhibited by treatment with an anti-bFGF antibody. Furthermore, these increases in proliferation were completely inhibited by treatment with an anti-heparanase antibody. Platelet degranulation products, such as heparanase and PF4, may liberate bFGF from extracellular sequestration, activating the growth factor and inducing the SMC proliferation and migration that contribute to the wound healing response following vascular injury.     

Upregulation of intermediate-conductance Ca2+-activated K+ channel (IKCa1) mediates phenotypic modulation of coronary smooth muscle

A hallmark of smooth muscle cell (SMC) phenotypic modulation in atherosclerosis and restenosis is suppression of SMC differentiation marker genes, proliferation, and migration. Blockade of intermediate-conductance Ca(2+)-activated K(+) channels (IKCa1) has been shown to inhibit restenosis after carotid balloon injury in the rat; however, whether IKCa1 plays a role in SMC phenotypic modulation is unknown. Our objective was to determine the role of IKCa1 channels in regulating coronary SMC phenotypic modulation and migration. In cultured porcine coronary SMCs, platelet-derived growth factor-BB (PDGF-BB) increased TRAM-34 (a specific IKCa1 inhibitor)-sensitive K(+) current 20-fold; increased IKCa1 promoter histone acetylation and c-jun binding; increased IKCa1 mRNA approximately 4-fold; and potently decreased expression of the smooth muscle differentiation marker genes smooth muscle myosin heavy chain (SMMHC), smooth muscle alpha-actin (SMalphaA), and smoothelin-B, as well as myocardin. Importantly, TRAM-34 completely blocked PDGF-BB-induced suppression of SMMHC, SMalphaA, smoothelin-B, and myocardin and inhibited PDGF-BB-stimulated migration by approximately 50%. Similar to TRAM-34, knockdown of endogenous IKCa1 with siRNA also prevented the PDGF-BB-induced increase in IKCa1 and decrease in SMMHC mRNA. In coronary arteries from high fat/high cholesterol-fed swine demonstrating signs of early atherosclerosis, IKCa1 expression was 22-fold higher and SMMHC, smoothelin-B, and myocardin expression significantly reduced in proliferating vs. nonproliferating medial cells. Our findings demonstrate that functional upregulation of IKCa1 is required for PDGF-BB-induced coronary SMC phenotypic modulation and migration and support a similar role for IKCa1 in coronary SMC during early coronary atherosclerosis.     

间充质干细胞在动脉粥样硬化治疗中的研究进展

动脉粥样硬化是一种病因复杂的血管壁慢性炎症性疾病。动脉粥样硬化及其相关并发症已成为人类死亡的主要原因,然而,其病因和发病机制尚未完全阐明,治疗效果还不满意。目前已经证实,动脉内皮细胞功能发生障碍是动脉粥样硬化的始动过程,内皮细胞功能失调和内皮细胞丢失是动脉粥样硬化症的主要特点;而血管平滑肌细胞的异常增生在动脉粥样硬化的发生发展中也扮演着重要角色。因此,探索有效措施促进有益的内皮细胞再生并抑制平滑肌细胞增生是血管损伤防治的关键。近年来有研究发现,体外输注的间充质干细胞能够向受损部位募集,并进一步分化为内皮细胞,修复损伤血管。然而,也有研究显示体外输注的间充质干细胞还可以分化为血管平滑肌细胞进而在血管局部增生,参与血管再狭窄的发生。文中综述了间充质干细胞输注对动脉粥样硬化发展的最新研究进展,希望为后续开展的用间充质干细胞治疗动脉粥样硬化的研究提供一定的参考。     

Enhanced proliferation and migration of vascular smooth muscle cells in response to vascular injury under hyperglycemic conditions is controlled by β3 integrin signaling

Atheroma formation and restenosis following percutaneous vascular intervention involve the growth and migration of vascular smooth muscle cells (SMCs) into neointimal lesions, in part due to changes in the extracellular matrix. While some clinical studies have suggested that, in comparison to non-diabetics, β3 integrin inhibition in diabetic patients confers protection from restenosis, little is known regarding the role of β3 integrin inhibition on SMC responses in this context. To understand the molecular mechanisms underlying integrin-mediated regulation of SMC function in diabetes, we examined SMC responses in diabetic mice deficient in integrin β3 and observed that the integrin was required for enhanced proliferation, migration and extracellular regulated kinase (ERK) activation. Hyperglycemia-enhanced membrane recruitment and catalytic activity of PKCβ in an integrin β3-dependent manner. Hyperglycemia also promoted SMC filopodia formation and cell migration, both of which required αVβ3, PKCβ, and ERK activity. Furthermore, the integrin–kinase association was regulated by the αVβ3 integrin ligand thrombospondin and the integrin modulator Rap1 under conditions of hyperglycemia. These results suggest that there are differences in SMC responses to vascular injury depending on the presence or absence of hyperglycemia and that SMC response under hyperglycemic conditions is largely mediated through β3 integrin signaling.     

Differential effect of heme oxygenase-1 in endothelial and smooth muscle cell cycle progression

Arterial remodeling in response to pathological insult is a complex process that depends in part on the balance between vascular cell apoptosis and proliferation. Studies in experimental models suggest that HO-1 mediates neointimal formation while limiting lumen stenosing, indicating a differential effect on vascular endothelial (EC) and smooth muscle cells (SMC). We investigated the effect of HO-1 expression on cell cycle progression in EC and SMC. The addition of SnMP (10 microM), an inhibitor of HO activity, to EC or SMC for 24h, resulted in significant abnormalities in DNA distribution and cell cycle progression compared to cells treated with the HO-1 inducers, heme (10 microM) or SnCl(2) (10 microM). SnMP increased G(1) phase and decreased S and G(2)/M phases in EC while heme or SnCl(2) decreased G(1) phase, but increased S and G(2)/M phases (p<0.05). Opposite effects were obtained in SMC. SnMP decreased G(1) phase and increased S and G(2)/M phases while heme or SnCl(2) increased G(1) phase but decreased S and G(2)/M phases (p<0.05). Our data demonstrate that HO-1 regulates the cell cycle in a cell-specific manner; it increases EC but decreases SMC cycle progression. The mechanisms underlying the HO-1 cell-specific effect on cell cycle progression within the vascular wall are yet to be explored. Nevertheless, these findings suggest that cell-specific targeting of HO-1 expression may provide a novel therapeutic strategy for the treatment of cardiovascular diseases.     

Expression of adhesion molecule T-cadherin is increased during neointima formation in experimental restenosis

Phenotypic modulation, migration and proliferation of vascular smooth muscle cells (SMCs) are major events in restenosis after percutaneous transluminal angioplasty. Surface cell adhesion molecules, essential to morphogenesis and maintenance of adult tissue architecture, are likely to be involved, but little is known about cell adhesion molecules expressed on SMCs. T-cadherin is a glycosyl phosphatidylinositol-anchored member of the cadherin superfamily of adhesion molecules. Although highly expressed in vascular and cardiac tissues, its function in these tissues is unknown. We previously reported increased expression of T-cadherin in intimal SMCs in atherosclerotic lesions and proposed a role for T-cadherin in phenotype control. Here we performed immunohistochemical analysis of spatial and temporal changes in vascular T-cadherin expression following balloon catheterisation of the rat carotid artery. T-cadherin expression in SMCs markedly increases in the media early (1-4 days) after injury, and later (day 7-28) in forming neointima, especially in its preluminal area. Staining for monocyte/macrophage antigen ED-1, proliferating cell nuclear antigen and smooth muscle alpha-actin revealed that spatial and temporal changes in T-cadherin level coincided with the peak in cell migration and proliferation activity during neointima formation. In colchicine-treated cultures of rat aortic SMCs T-cadherin expression is increased in dividing M-phase cells but decreased in non-dividing cells. Together the data support an association between T-cadherin expression and SMC phenotype.     

Stimulation of smooth muscle cell proliferation by ox-LDL- and acetyl LDL-induced macrophage-derived foam cells.

To test the hypothesis that LDL lacking of initial oxidation may also anticipate an essential role in the progression for atherosclerotic lesions, we studied the in vitro effect of foam cells induced by low density lipoprotein (LDL), oxidized (ox)-LDL or acetyl-LDL on smooth muscle cell (SMC) proliferation. Intraperitoneal macrophages collected from ICR mice were incubated with buffered saline LDL, ox-LDL or acetyl-LDL to induce foam cell formation. Porcine aortas with atherosclerotic lesions were collected from 5 pigs fed high cholesterol diets. The results indicate that foam cells induced by ox-LDL and acetyl-LDL, but not by LDL, promoted SMC proliferation. SMC proliferation was also increased by ruptured, ox-LDL- and acetyl-LDL- induced foam cells. Immunohistochemically, epitopes of the LDL, ox-LDL, and malondialdelyde (MDA)-LDL were present in atherosclerotic lesions, but the acetyl epitope was not. We suggest that foam cells, whether induced by the oxidized or acetyl or acetyl (unoxidized) form, play an essential role in the pathogenesis of atherosclerosis by stimulating SMC proliferation.     

IL-10 Accelerates Re-Endothelialization and Inhibits Post-Injury Intimal Hyperplasia following Carotid Artery Denudation

The role of inflammation on atherosclerosis and restenosis is well established. Restenosis is thought to be a complex response to injury, which includes early thrombus formation, acute inflammation and neo-intimal growth. Inflammatory cells are likely contributors in the host response to vascular injury, via cytokines and chemokines secretion, including TNF-alpha (TNF). We have previously shown that IL-10 inhibits TNF and other inflammatory mediators produced in response to cardiovascular injuries. The specific effect of IL-10 on endothelial cell (ECs) biology is not well elucidated. Here we report that in a mouse model of carotid denudation, IL-10 knock-out mice (IL-10KO) displayed significantly delayed Re-endothelialization and enhanced neo-intimal growth compared to their WT counterparts. Exogenous recombinant IL-10 treatment dramatically blunted the neo-intimal thickening while significantly accelerating the recovery of the injured endothelium in WT mice. In vitro, IL-10 inhibited negative effects of TNF on ECs proliferation, ECs cell cycle, ECs-monocyte adhesion and ECs apoptosis. Furthermore, IL-10 treatment attenuated TNF-induced smooth muscle cells proliferation. Our data suggest that IL-10 differentially regulate endothelial and vascular smooth cells proliferation and function and thus inhibits neo-intimal hyperplasia. Thus, these results may provide insights necessary to develop new therapeutic strategies to limit vascular restenosis during percutaneous coronary intervention (PCI) in the clinics.     

Control of smooth muscle cell proliferation in vascular disease

PURPOSE OF REVIEW: Smooth muscle cell proliferation has previously been regarded as a central feature in vascular disease. The role of this process has recently been substantially re-evaluated, and we have reconsidered the functional importance of smooth muscle cell proliferation, the origin of proliferating smooth muscle cells in lesions, and the mechanisms whereby smooth muscle cell proliferation is controlled. In this review, we summarize recent progress in the understanding of smooth muscle cell proliferation, with a particular focus on how interactions between the extracellular matrix, smooth muscle cells, and mitogens control critical steps in this process. RECENT FINDINGS: Irrespective of the origin of smooth muscle cells in vascular lesions, fundamental interactions between the extracellular matrix and cell surface integrins are necessary in order to initiate a proliferative response in a quiescent smooth muscle cell, in a similar manner to any non-malignant cell. These interactions trigger intracellular signaling and cell cycle entry, which facilitate cell cycle progression and proliferation by mitogens. In addition, extracellular matrix interactions may also control the availability and activity of growth factors such as heparin-binding mitogens, which can be sequestered by heparan sulfate containing extracellular matrix components and regulate smooth muscle cell proliferation. SUMMARY: New insights into mechanisms whereby the extracellular matrix takes part in the control of smooth muscle cell proliferation suggest a number of putative targets for future therapies that can be applied to increase plaque stability, prevent the clinical consequences of atherosclerosis and improve outcomes after interventional procedures and organ transplantation.     

The neuropeptide calcitonin gene-related peptide differently modulates proliferation and differentiation of smooth muscle cells in culture depending on the cell type

Calcitonin gene-related peptide (CGRP) is a neuropeptide present around vasculature very early during development, when smooth muscle cells (SMC) are still proliferating and not yet totally differentiated. We investigated the effects of CGRP on proliferation and differentiation of SMC in culture; 10(-7) M CGRP added in the medium of cultured smooth muscle cells every 2 days did not significantly changed cells growth rate in 1% FCS. At the opposite, this treatment modulated proliferation of cells grown in 10% FCS medium. Two distinct populations of SMC with different growth rates were obtained from our primary cultures. SMC which proliferated slowly in the presence of 10% fetal calf serum (FCS) had growth rates positively influenced by CGRP. The quantity of alpha-smooth actin expressed by these cells was not influenced by the peptide. On the contrary, SMC which proliferated more rapidly in 10% FCS medium had growth rate inhibited by CGRP. In these cells, CGRP significantly reduced the amount of expressed alpha-smooth actin, an index of SMC differentiation. In both cases, the peptide significantly increased the level of mRNA for all the actin genes. In the light of this dual role of CGRP, it can be presumed that this peptide controls smooth muscle cells proliferation and differentiation in vivo and could thus regulate the homeostasis of the vessel wall.     

Interactions of arterial cells: III. Stathmokinetic analyses of smooth muscle cells cocultured with endothelial cells

Arterial endothelial cells (EC) or their conditioned medium (ECCM) can alter the proliferation of cocultured arterial smooth muscle cells (SMC). Previously, we have shown, as have others, that EC regulate the growth of cocultured SMC depending on the density of both cell types. To ascertain the rate of cell-cycle traverse in preconfluent arterial SMC cocultured with arterial EC or ECCM (derived from preconfluent EC), we have conducted a series of stathmokinetic experiments using flow cytometry to determine where specific changes may occur in the cell cycle. Results of our experiments indicate for the first time that ECCM stimulates the proliferation of preconfluent SMC by significantly shortening the residence times in the G1 and S phases of the cell cycle. The predominant relative effect occurs within the early G1 (G1A) compartment where pretreatment with ECCM shortens the residence time by approximately 55%. Furthermore, we have observed that preincubation of serum-free ECCM with antiplatelet-derived growth factor (PDGF) antibody abolishes any mitogenic effect on SMC. This suggests that EC secrete PDGF-like molecules which enhance the proliferation rate of preconfluent, cocultured SMC. These findings support the hypothesis that arterial EC may secrete mitogens which stimulate arterial SMC proliferation in the vascular wall.     

Interaction of arterial cells. I. Endothelial cells alter cholesterol metabolism in co-cultured smooth muscle cells

Results of previous in vivo experiments indicated that the presence of arterial endothelium modifies cholesteryl ester (CE) metabolism and the retention of low density lipoproteins (LDL) in injured arteries. We describe herein the effects of bovine arterial endothelial cells (ENDO) on the CE cycle, fluid phase endocytosis, and cell proliferation in co-cultured bovine arterial smooth muscle cells (SMC). Following several days of cultivation on confluent SMC, ENDO were removed from SMC by treatment of the co-cultures with 1.0% collagenase (type II). Removal of only ENDO from the co-culture dishes was confirmed by immunofluorescent staining for Factor VIII antigen, hemotoxylin-eosin staining, and biochemical analyses. We observed that ENDO grown to 75% confluency on confluent SMC induced: 1) a reduction of CE hydrolysis as a result of decreased lysosomal CE hydrolytic activity in SMC as compared to SMC cultured alone; and 2) an increase in the rate of incorporation of labeled oleate into CE as a result of increased acyl CoA:cholesterol O-acyltransferase activity in SMC as compared to SMC cultured alone. Neither endothelial cell-derived culture media (ECDM) nor fibroblasts modulated CE metabolism in co-cultured SMC. Additional experiments showed that the presence of endothelial cells or ECDM decreased the proliferation of co-cultured SMC by 50%, but enhanced the endocytotic rate of labeled sucrose into SMC threefold. Results of experiments described herein demonstrate that, in addition to providing a thrombo-resistant surface and regulating permeability, endothelial cells may also serve to modulate cholesteryl ester metabolism in smooth muscle cells derived from the arterial wall.