Published online before print October 9, 2003, doi: 10.1161/01.ATV.0000099882.52647.E4
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© 2003 American Heart Association, Inc.
Brief Reviews |
Anoïkis in the Cardiovascular System
Known and Unknown Extracellular Mediators
From INSERM Unit 460, CHU Xavier Bichat, Paris, France.
Correspondence to Jean-Baptiste Michel, INSERM Unit 460, CHU Xavier Bichat, 46, rue Henri Huchard, 75877 Paris Cedex 18, France. E-mail jbmichel{at}bichat.inserm.fr
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Abstract |
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 Top Abstract IntroductionCell Adhesion and Tensional...References |
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Anoïkis is defined as programmed cell death induced by the loss of cell/matrix interactions. Adhesion to structural glycoproteins of the extracellular matrix is necessary for survival of the differentiated adherent cells in the cardiovascular system, including endothelial cells, smooth muscle cells, fibroblasts, and cardiac myocytes. Adhesion is also a key factor for the differentiation of mesenchymal stem cells. In particular, fibronectin is considered a factor of survival and differentiation for many adherent cells. Adhesion generates cell tensional integrity (tensegrity) and repression of apoptotic signals, whereas detachment has the opposite effect. Anoïkis plays a physiological role by regulating cell homeostasis in tissues. However, anoïkis can also be involved in pathological processes, as illustrated by the resistance to anoïkis in cancer and its enhancement in degenerative tissue remodeling. Extracellular mediators of anoïkis include matrix retraction, leading to loss of tensegrity in fibroblasts, pharmacological disengagement of integrins by RGD-like peptides and fragments of fibronectin, and focal adhesion disassembly by fragments of thrombospondin, plasminogen activator-1, and high-molecular-weight kininogen. In addition to binding of the RGD peptide by integrins, the engagement of the heparin binding sites of adhesive glycoproteins with glycosaminoglycans on the cell surface is also involved in the prevention of cell detachment–induced apoptosis. Proteases able to degrade adhesive glycoproteins, such as fibronectin, induce anoïkis of vascular adherent cells. Active proteases can either be secreted directly by inflammatory cells, as elastase and cathepsin G by polymorphonuclear leukocytes, chymase and tryptase by mast cells, and granzymes by lymphocytes, or generated from circulating zymogens by activation in close contact with the cells. This is the case for the pericellular conversion of plasminogen to plasmin, which degrades fibronectin and induces anoïkis of smooth muscle cells. Involvement of proteases has also been proposed in the apoptotic response of cultured adherent cells to serum starvation. Anoïkis is probably involved in pathological remodeling of cardiovascular tissues, including cardiac myocyte detachment in heart failure, deendothelialization and plaque rupture in atherosclerosis, and smooth muscle cell disappearance in aneurysms and varicose veins. The absence of cell adhesion and growth resulting from cleavage of adhesive proteins also represents a major impediment to cellular healing, including the absence of cell recolonization of proteolytically injured tissue and the low efficacy of cell transplantation. However, the exact role of anoïkis in cardiovascular pathologies remains to be further defined.
Key Words: cell adhesion • smooth muscle cell • endothelium • apoptosis • aneurysm
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Introduction |
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TopAbstractIntroductionCell Adhesion and Tensional...References |
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The term anoïkis (from the ancient Greek word for homelessness) was first proposed by Frish and Francis in 1994.1 Anoïkis is defined as a process of programmed cell death induced by the loss of cell/matrix interactions. The role of matrix integrity as a survival factor for adherent cells was first described by Meredith et al.2 In these initial studies, anoïkis was described in endothelial2 and epithelial1 cells. Anoïkis could be a physiological mechanism, allowing the maintenance of tissue cell homeostasis. The concept of anoïkis and of resistance to anoïkis has been therefore extended to cancer, in which cell survival becomes independent of anchorage to matrix. Resistance to anoïkis has been demonstrated to be involved in the loss of cell homeostasis, cancer growth, and metastasis.3,4
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Cell Adhesion and Tensional Integrity |
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TopAbstractIntroductionCell Adhesion and Tensional...References |
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The matrix to which cells adhere is a solid network of proteins, secreted by the cells themselves, and insolubilized in the extracellular space by cross-linking. It can thus be solubilized only by proteolytic enzymes. Adhesive proteins such as fibronectin, laminin, and vitronectin are associated with this solid matrix phase and play the role of an intermediary between cells and insoluble matrix. The rigidity of the matrix may also play a role by strengthening the integrin-cytoskeleton linkage.5 Whereas integrins on the cells and RGD-like peptides on the adhesive proteins of the extracellular matrix play a predominant role in adhesion, other molecules, such as proteoglycans on the cell surface, and other functional domains of adhesive proteins, such as sulfated polysaccharide binding domains, are also involved and may modulate the cell response to adhesion.6 For adherent cells, the anchorage to matrix is a necessary condition for survival.2 In contrast, nonadherent cells, such as blood leukocytes, including circulating progenitors, can normally survive without anchorage to extracellular matrix. These specific properties are probably attributable to differences in the organization of the cytoskeleton between adherent and nonadherent cells. Adhesion to matrix is the main determinant of differentiation of stromal stem cells of bone marrow origin into somatic mesenchymal cells.7
Adhesion of cells to matrix, predominantly via the integrin molecules, generates an endogenous tensile stress within the cell named tensegrity (for tensional integrity).8 Tensional integrity involves matrix, focal adhesion, integrin/cytoskeleton interactions, folding of intracellular proteins associated with cytoskeleton,9 and interactions of cytoskeletal microtubules with calcium and nuclear functions.10 In the cardiovascular system, tensegrity mediates the control of cell behavior by mechanical forces, such as shear stress for endothelial cells and tensile stress for vascular smooth muscle cells. All of the molecular components mentioned above are involved in the mechanotransmission of hemodynamic forces within the vascular system. For example, in hypertension, the extracellular matrix of the arterial wall is submitted to greater stretch than in normotension, leading to an increase in smooth muscle cell tensegrity. In response, global and specific gene transcription increases, leading to hypertrophy and phenotype changes. Similarly, shear stress induces strain of the endothelial cell/matrix interactions, leading to an enhancement of endothelial tensegrity.11 In addition, adhesion-induced cell tensegrity is a necessary condition for growth factor–induced cell proliferation.12 Conversely, the loss of cell tensegrity, mainly attributable to loss of anchorage to matrix, induces a program of death in adherent cells (Figure 1). It seems that whereas circulating nonadherent progenitors are able to adhere and differentiate, adherent cells are not capable of reversing to a less-differentiated nonadherent state and thus cannot survive in the nonanchored state.
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The intracellular signaling leading to apoptosis attributable to the loss of matrix anchorage has been explored mainly in the context of resistance to anoïkis in malignancy.13 In this context, intracellular mechanisms of anoïkis involve the effect of PI3K/Akt on focal adhesion and integrin-linked kinases (FAK, Shc, and ILK), impediment of Raf-ERK signaling and jun N-Terminal kinases, bcl-2 repression,14 FAS ligand,15 and caspase activation.16 Contrasting with numerous studies focused on the intracellular signaling triggered by rupture of appropriate cell/matrix interactions, the extracellular mediators able to induce cell anoïkis have been less explored.
Physiological Anoïkis
In several situations, anoïkis is a physiological process participating in tissue and cell homeostasis during development, renewal, and regression. Phylogenically, in cells of invertebrate origin, adhesion to extracellular matrix is a necessary condition for tissue organization and survival.17 In mammals, physiological anoïkis has been described in the involution and renewal of tissue18,19 (Figure 2).
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Extracellular Mediators of Anoïkis
Besides the involvement of anoïkis in physiological tissue processing, anoïkis also participates in pathological processes. As cited above, resistance to anoïkis is involved in malignancy. However, enhancement of anoïkis, not compensated by cell healing or overcompensated by a dysfunctional healing process, such as in fibrosclerosis, could also participate in abnormal tissue remodeling. This is probably the case in cardiovascular degenerative pathologies. Vascular cell apoptosis is one of the main phenomena leading to cell disappearance in the evolution of atheromatous lesions such as plaque complications and aneurysm formation.20,21 Cardiomyocyte disappearance occurs early in left ventricular overload and contributes to the progression toward heart failure.22 Nevertheless, there are multiple mechanisms leading to apoptosis of adherent cells in the cardiovascular system. The loss of cell tensional integrity, mainly attributable to the loss of cell anchorage to matrix, is one of the extracellular mediators able to induce apoptosis of cardiovascular cells.
Several different mechanisms can lead to the loss of cell tensegrity and anchorage to matrix, among which are matrix retraction, pharmacological or endogenous integrin disengagement, and pericellular proteolysis.
Matrix Retraction
The contraction of the extracellular matrix as a triggering factor of cell apoptosis has been mainly explored in dermal fibroblasts in the context of the homeostasis of wound healing. Fibroblasts embedded in a noncontractile collagen gel do not undergo apoptosis. By contrast, a decrease in mechanical tension in collagen matrices during gel contraction triggers fibroblast apoptosis.23–25 The magnitude of fibroblast apoptosis is proportional to the degree of gel contraction.26 This phenomenon is dependent on integrins and on the decrease in tensional forces within the cell.27 Similarly, pharmacological adjunction of cytochalasin-D to adherent cells, which disrupts the cytoskeleton and abolishes tensegrity, also induces apoptosis.28 These studies suggest that a reduction in the mechanical tension in the extracellular matrix serves as a trigger for the initiation of fibroblast apoptosis, thus participating in cell homeostasis during physiological wound healing.29 Conversely, the loss of such a cell/matrix homeostasis can lead to the formation of an abnormal, fibroproliferative, keloid scar.30
Integrin Disengagement
Competition of cell/matrix interactions by synthetic arginine-glycine-aspartate (RGD)-like peptides is the usual pharmacological means of inducing anoïkis of adherent cells in culture. In molluscs, RGD-containing peptides impair the spreading of oyster hemocytes and induce cell death.31 The pharmacological observation that RGD-like peptides induce inhibition of cell spreading and trigger apoptosis has been reported for many cell types, including endocrine32 and mesenchymal cells. RGD peptides have been used extensively to demonstrate the necessity of adhesion and spreading for the survival of endothelial cells,33 mesangial cells,34 and fibroblasts35 and to assess the role of integrins in adhesion, migration, and intimal proliferation of smooth muscle cells.36–38 Physiologically, it has been recently reported that soluble RGD-like peptides, released by the endogenous carboxy-terminal processing of ßig-h3 (an adhesive protein) in response to transforming growth factor (TGF)-ß, induce cell apoptosis and participate in the proapoptotic effect of TGF-ß.39 Similarly, the disintegrins, a family of low-molecular-weight RGD-containing peptides, are able to inhibit angiogenesis by provoking endothelial cell detachment and apoptosis.40
RGD-like peptides also have been extensively used for exploring the interaction of adherent cells with fibronectin. Fibronectin, one of the predominant adhesive glycoproteins, possesses 1 RGD motif in its central type III domain. This domain is enriched by fibrin-, collagen-, and heparin-binding domains. Fibronectin exists in 2 forms, a soluble form, synthesized by the liver and secreted into the plasma, and a cellular form, directly synthesized and secreted by adherent cells, such as smooth muscle cells and fibroblasts. This latter form is bound to the insoluble extracellular matrix and used by these cells as a substrate for adhesion. The soluble form is entrapped by fibrin during thrombus formation, producing a copolymer substrate involved in cellular healing and regrowth in response to injury. Fibronectin is considered a survival factor41 for different cell types, including endocrine cells32 and smooth muscle cells.42 For example, fibronectin deposits support neuronal survival and reduce brain injury after transient focal cerebral ischemia.43 Similarly, fibronectin acts as a rescue factor for a liver cell line.28 Numerous proteases, eg, plasmin, thrombin, elastase, cathepsin G, tryptase, chymase, and granzymes, cleave the fibronectin molecule and generate different fragments. These soluble peptide fragments, containing the RGD domain, could interfere with cell adhesion to the intact fibronectin molecule and induce cell apoptosis.44,45 Fibronectin is also the most commonly used substrate to obtain adhesion and spreading of circulating or bone marrow progenitors46 and their differentiation into endothelial cells47 or smooth muscle cells.48
It has recently been demonstrated that both the RGD domain and the heparin-binding domain of fibronectin play an important role in cell survival. Mutations in the heparin-binding domain induced fibroblast apoptosis despite the conservation of the RGD peptide,49 and fibroblast survival is dependent on peptides containing both integrin and glycosaminoglycan-binding domains.50 The specific association of tissue transglutaminase with the heparin-binding domain of fibronectin also induces RGD-independent, distinct adhesive and prosurvival properties for cells.51 Thus, these 2 signals, 1 from the integrins and 1 from membrane-bound proteoglycans, linked to the expression of P53, C-myc,52 and protein kinase C,51 cooperatively regulate fibronectin-mediated cell survival.
Similarly to fibronectin fragments, other matricellular proteins can induce detachment when presented as soluble proteins to cells strongly anchored to their surrounding matrix. Soluble thrombospondin, tenascin-L, and SPARC stimulate the disassembly of focal adhesion complexes.53 Soluble thrombospondin-1 stimulates the loss of focal adhesion in endothelial and smooth muscle cells54 and in fibroblasts adherent to fibronectin. This effect is independent of the substrate used to support adherence. The NH2-terminal glycosaminoglycan-binding domain of thrombospondin contains the focal adhesion reorganizing activity.55 This effect of focal adhesion disassembly is induced by the interaction of the thrombospondin domain with the N-terminal domain of calreticulin at the cell surface.56 The heparin-binding domain of thrombospondin can be proteolytically released by thrombin, plasmin,57 leukocyte cathepsin G, and elastase.58
Plasminogen activator inhibitor 1 (PAI-1) and 2-chain high-molecular-weight kininogen each exert antiadhesive properties on vitronectin-dependent adhesion of endothelial and smooth muscle cells. The effect of PAI-1 is similar to that observed with the cyclic RGD peptide.59,60 PAI-1 can bind to vitronectin and to urokinase-type plasminogen activator (uPA) and its receptor (uPAR), and the new complexes also interact with different integrins. There is competition between PAI-1 and uPAR to bind the NH2-terminal domain of vitronectin, which is in close proximity to the single RGD sequence of the molecule. Therefore, binding of PAI-1 to pericellular vitronectin inhibits integrin-mediated adhesion to vitronectin.61,62 Another explanation has recently been proposed. The process involves a protein complex formation including integrins, uPA, uPAR, PAI-1, and the endocytic clearance of the complexes,59 leading to disengagement of integrins from the extracellular matrix and cell detachment. In this last study, this effect seems not to be limited to vitronectin as the substrate for adhesion but can also occur with fibronectin and type 1 collagen and also depends on the cell type.
The detachment effect of high-molecular-weight kininogen (HMK) activated by tissue kallikrein was first identified by Asakura et al63 in 1992. This team isolated a plasma globulin that induced detachment of osteosarcoma, melanoma, and endothelial and monocytic cells from vitronectin and fibrinogen substrates but not from fibronectin. This proanoïkis effect is reproduced in part by the domain 5 of HMK, is dependent on the integrin 
vß3 and on uPAR-mediated cell attachment and spreading, and is prevented by heparin and PAI-1 but not by cyclic RGD peptides.64,65 Low-molecular-weight kininogen, which does not possess the histidine-rich D5 domain, does not share similar detachment properties.66 The authors suggest that HMK inhibits adhesion via competition with urokinase-protease-activated receptor (u-PAR) and the vitronectin NH2-terminal domain.
Pericellular Proteolysis
The third mechanism able to induce anoïkis of vascular cells is the pericellular proteolysis of molecules involved in integrin-matrix interactions. Pericellular proteolysis can shed membrane-bound proteins, leading to their solubilization, such as vascular cell adhesion molecule and ACE; can release latent proteins bound to extracellular matrix, such as PAI-1 and TGF-ß, leading to their activation; and can act on large adhesive glycoproteins, such as fibronectin and laminin, leading to their fragmentation and to the subsequent solubilization of these fragments. The proteases capable of inducing pericellular proteolysis can be directly released from inflammatory cells, such as elastase, cathepsin G, chymase, and granzyme, or produced by activation of a zymogen at the cell surface, such as thrombin and plasmin.
It has been known for more than 20 years that activated polymorphonuclear leukocytes are able to detach endothelial cells.67 This activity is dependent on the release of proteases by granules, inhibited by neutral protease inhibitors, reproduced by conditioned medium of activated polymorphonuclear leukocytes (PMNs), and associated with the proteolytic degradation of fibronectin. This PMN-induced endothelial cell anoïkis is dependent both on the degree of PMN activation and also on the degree of endothelial cell activation, which determines the extent of PMN adhesion.68 This endothelial cell anoïkis effect mainly involves elastase but also cathepsin G and proteolysis of cell-cell connections via 
E cadherin69 and loss of cell/matrix interactions. Pure leukocyte elastase is able to reproduce endothelial cell anoïkis because of its proteolytic activity on fibronectin and other adhesive glycoproteins, which include laminin, thrombospondin, and von Willebrand factor.58 Cathepsin G also reproduces a similar effect of cell retraction and detachment. This effect is dose- and time-dependent and inhibited by eglin, an inhibitor of cathepsin-G.70
Recently, these data have been extended to vascular smooth muscle cells. Conditioned medium of fMLP-activated PMNs induced human and rat smooth muscle cell detachment and apoptosis.71 This effect is inhibited by elastase inhibitors, elafin, leukocyte proteinase inhibitor (LPI), and 1-antitrypsin and depends on the degree of PMN activation. Mediators or synthetic compounds capable of limiting PMN activation in response to fMLP decrease the efficiency of activated PMNs to induce smooth muscle cell anoïkis. Moreover, it has been recently reported that not only PMNs but also macrophages could express and secrete elastase in atherosclerosis.72 The data concerning cathepsin G have been also recently extended to cardiomyocyte detachment and apoptosis.73 This effect of cardiomyocyte detachment is independent of the ability of cathepsin G to activate PAR on the cell surface. Similar results have been reported with PMN released proteases on airway epithelial cells.74,75
Other enzymes, able to degrade fibronectin, originating from other inflammatory cells, reproduce similar effects. This is the case for chymase produced by mast cells. Chymase is able to activate MMPs,76 degrade fibronectin,77 and induce smooth muscle cell detachment and apoptosis.78 Mast cell chymase is also capable of retracting a confluent endothelial cell monolayer79 and of inducing endothelial cell apoptosis.80 Similarly, chymase causes apoptosis of cardiomyocytes.81 This alteration of cell adhesion by mast cell chymase is not restricted to cardiovascular cells and is linked to the degradation of the extracellular glycoproteins, fibronectin and vitronectin.82 Chymase binds to pericellular glycosaminoglycans, focusing its action in the pericellular space. Tryptase, another protease of mast cell granules, is also capable of degrading fibronectin.83 Like plasmin, these serine proteases can also activate metalloproteinases.83–85
Granzymes, serine-proteases expressed exclusively by cytotoxic T lymphocytes and natural killers and stored in lysosome-like secretory granules, are also able to cleave extracellular matrix proteins, such as proteoglycans, type IV collagens, laminin, and fibronectin.86 This potential extracellular effect of adhesive protein cleavage could participate in the apoptotic effect of killer cells in association with their well-documented direct intracellular effect mediated by perforin.87 Proteases can also be of bacterial origin.88
It has been recently shown that smooth muscle cells can convert plasminogen to plasmin, leading to pericellular fibronectin proteolysis,89 cell detachment, and apoptosis.90 These results were obtained in rat and human smooth muscle cells in culture and were also reproduced in tissue culture of aortic rings. Conditioned medium of plasminogen-incubated smooth muscle cells contained degraded fragments of fibronectin. Under these experimental conditions, plasminogen activation is dependent on the binding of plasminogen and tissue plasminogen activator (tPA) to the cell. Annexin 2 has been identified as a receptor for tPA and plasminogen on the endothelial cell surface, and the tPA binding site has been mapped to the amino-terminal domain of annexin 2.91 Concomitant binding of tPA and plasminogen on endothelial annexin 2 results in a 60-fold increase in catalytic efficiency of plasmin generation. Similarly, tPA binds to human smooth muscle cells92 via the type II transmembrane protein p63.93 This cell-bound pericellular activation of plasminogen is not inhibited by 2-antiplasmin, a physiological circulating inhibitor of plasmin, whereas soluble unbound plasmin, present in the conditioned medium, is totally inhibited. However, a direct proteolytic action of plasmin on cell-cell and cell-matrix adhesion molecules cannot be excluded. In contrast, despite the activation of MMPs by generated plasmin, MMP inhibition does not prevent anoïkis. These data have been recently extended to CHO cells, which also express tPA, and where the pericellular overexpression after transfection of a tissue-bound antiprotease, such as protease nexin-1 (bound to the pericellular proteoglycans) prevented the anoïkis induced by the pericellular activation of plasminogen.94 Analogous results have been already published showing that plasmin directly prevented the adhesion of keratinocytes to vitronectin.95 Similarly, it has been recently reported that reticulated endostatin (a fragment of type XVIII collagen that is a powerful inhibitor of tumor angiogenesis) induces endothelial cell detachment by binding of t-PA and potentiation of plasminogen activation.96 The authors suggest that overstimulation of t-PA by binding to a macromolecular phase (reticulated endostatin) may result in excessive matrix degradation, thereby preventing angiogenesis. The role of plasminogen activation in cell detachment has also been recently extended to neuronal cells in response to ischemia.97 In this study, experimentally induced ischemia of the retina induced both u-PA overexpression and activity and plasma plasminogen leak within the tissue, leading to in situ plasmin formation, laminin degradation, and cell loss.
A similar mechanism of anoïkis has been also proposed in the apoptotic response of cultured cells to serum withdrawal. Ikari et al98 have shown that antiproteases present in serum are required for vascular smooth muscle cell spreading in a fibrin gel. These antiproteases prevented smooth muscle cell apoptosis associated with proteolytic fibronectin degradation. The proteases involved in serum starvation–induced cell anoïkis remain undefined. Similar results have been reported in other cell types, including thyroid cells99 and endothelial cells, in which serum withdrawal induced loss of fibronectin-integrin interaction, attributable to fibronectin degradation, and apoptosis.
However, pericellular proteolysis does not necessarily lead to cell detachment and apoptosis. Pericellular degradation of the extracellular matrix is also a necessary condition for cell migration. It has been shown that elastase activity participates in smooth muscle cell migration100 and overexpression of an elastase inhibitor prevents intimal proliferation.101,102 Similarly, plasminogen activation is involved in migration of many cell types.103 Therefore, the outcome of the effects of pericellular proteolysis, ie, permitting cell migration or inducing cell detachment, probably depends on the intensity of the proteolysis, its clustering at the cell surface or its diffusion around the cell, the endogenous or exogenous source of proteases, and the switch of the smooth muscle cell from a contractile to a synthetic phenotype.
Involvement of Anoïkis in Vascular Disease
How is anoïkis involved in vascular pathology and therapeutics? Disappearance of endothelial cells could play a role in the initiation of atherosclerosis, whereas smooth muscle cell apoptosis is claimed to play a predominant role in atherothrombotic complications. Anoïkis could be one of the mechanisms leading to vascular cell disappearance. For example, it has been shown that neutrophils, which can release elastase and cathepsin G, are the predominant circulating leukocytes recruited by activated endothelium in experimental atherosclerosis in vivo.104 In addition, neutrophil infiltration has been observed in human culprit coronary lesions in acute syndromes105 and leukocyte elastase has been detected in human carotid complicated plaques removed during surgery.72 Similarly, chymase and tryptase release106 by mast cells107 have been detected in human atherosclerotic lesions, particularly associated with plaque rupture.108,109
Smooth muscle cell disappearance as well as matrix degradation are necessary conditions for the development of aneurysms.110 Aneurysms are characterized by chronic proteolytic injury of the arterial wall, not compensated by a cellular healing process. Smooth muscle apoptosis has been observed in aneurysms in different localizations, including atheromatous aneurysms of the abdominal aorta,111,112 popliteal aneurysms,113 degenerative aneurysms of the ascending aorta,114 and aneurysms of intracerebral arteries.115 We have recently shown that plasminogen adsorbed in the mural thrombus of abdominal aortic aneurysms is activated by plasminogen activators from the aneurysmal wall at the interface between the thrombus and wall, generating plasmin in close contact with the smooth muscle cells, releasing proteolytically degraded fibronectin and active metalloproteinases.116 Such a proteolytic cascade is probably involved in the disappearance of smooth muscle cells. Similarly, the dilated segments of varicose veins are characterized by the disappearance of smooth muscle cells, and varicose vein development is attributable to PMN margination and activation in response to blood stasis and relative hypoxia.117,118 Release of proteases by PMNs in hypoxia-reperfusion injury is also probably involved in endothelial cell detachment and disappearance. It has also been shown that plasminogen119,120 and t-PA,121 by degrading laminin, potentiate the neuronal damage provoked by excitotoxic and ischemic injury in the brain. In all of these situations, the exact role of the pericellular activation of plasminogen, proteolysis of adhesive glycoproteins, and cell retraction and detachment remain to be clarified.
Anoïkis and inhibition of cell adhesion, spreading, and growth on the extracellular matrix are probably one of the main impediments to the cellular healing process and represent a possible therapeutic target. Adhesion to intact matrix, and particularly to fibronectin,48 is a necessary condition for progenitor cell differentiation, growth, and tissue regeneration.46 Indeed, intact fibronectin promotes the survival and efficacy of stem cell transplantation in injured brain.122 Similarly, adhesion of somatic cells to extracellular matrix and tensegrity are necessary for the action of growth factors leading to cellular healing.123,124 Conversely, the presence of proteases, by inhibition of cell adhesion on matrix, prevents cellular healing. This point is well illustrated by the absence of wound healing in varicose leg ulcers. Chronic leg ulcers are characterized by the presence of proteases, ie, elastase125 and plasmin,126 and of degraded antiproteases127 as well as adhesive glycoproteins, such as fibronectin128 and tenascin.129 The release of proteinases, particularly elastase, by the chronic leg ulcer represents a poor prognosis for the cellular healing process.130 Therefore, the absence of adhesion and spreading is probably the main cause of poor cell survival in cell transplantation.131,132 In the myocardium, attempts at postinfarct cell therapy have also shown a poor rate of cell adhesion and survival, probably mainly attributable to the pericellular action of proteases.133 Therefore, prevention of anoïkis and enhancement of cell adhesion and spreading are one of the major aims in the development of cell transplantation techniques, including the therapeutic use of progenitor cells.
In conclusion, cell adhesion to extracellular matrix generates tensional integrity within the different cell types of the cardiovascular system. This physiological cellular process is necessary for mesenchymal cell differentiation, survival, and growth. In contrast, loss of cell/matrix interactions leads to the programmed death of adherent cells named anoïkis. Anoïkis plays a role in many physiological processes, and resistance to anoïkis characterizes neoplastic transformation. In the cardiovascular system, anoïkis is probably one of the processes involved in cell disappearance. In various pathological contexts, molecules capable of inducing integrin disengagement and proteinases able to degrade adhesive glycoproteins could play a predominant role. Such proteinases could be directly released by leukocytes or generated from zymogens by a cell-dependent activation. Anoïkis is implicated in both the pathophysiology of cell disappearance in cardiovascular tissues and the absence of cellular healing of proteolytically injured tissues.
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Acknowledgments |
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Acknowledgments
The studies performed in the author’s laboratory were supported by OPAL and the Leducq Foundation. The author wishes to acknowledge the essential contributions of Olivier Meilhac, Xavier Houard, Vincent Fontaine, Patrick Rossignol, Magalie Ancelin, Marie-Paule Jacob, Roger Vranckx, and Eduardo Angles-Cano. The author also thanks Mary Osborne-Pellegrin for editing this review.
Received August 22, 2003; accepted September 16, 2003.
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