integrin review 整合素受体信号通路综述

Rheumatol IntDOI 10.1007/s00296-014-3137-5Role of integrins and their ligands in osteoarthritic cartilageJian Tian · Fang‑Jie Zhang · Guang‑Hua LeiReceived: 25 May 2014 / Accepted: 17 September 2014 © Springer-Verlag Berlin Heidelberg 2014[1]. Radiographic evidence of OA occurs in the majority of people by 65 years of age, and among them about 80 % in people who aged over 75 years [2]. However, the pathogen-esis of this disease is not fully elucidated.Cartilage damage is one of the major pathological changes in OA. Articular cartilage is an avascular, a neu-ral, alymphatic, and viscoelastic connective tissue that functions autonomously to bear loads and provide almost friction-free movement of diarthrodial joints [3]. Chondro-cytes, the only cell population of adult articular cartilage, are strongly involved in maintaining the dynamic equi-librium between synthesis and degradation of the extra-cellular matrix (ECM) [4]. Collagens represent the major structural components of the articular cartilage. Cartilage is made up of two main ECM macromolecules: type II collagen and aggrecan, a large aggregating proteoglycan [5, 6]. Cartilage destruction is thought to be mediated by two main enzyme families: the matrix metalloproteinases (MMPs) are responsible for the cartilage collagen break-down, whereas enzymes from disintegrin and metallopro-teinase domain with thrombospondin motifs (ADAMTS) family mediate cartilage aggrecan loss [7]. Activation of biochemical pathways involves the production of proin-flammatory cytokines, inflammation, degradation of the ECM by MMPs and ADAMTS, and cessation of ECM syn-thesis via dedifferentiation and apoptosis of chondrocytes [8, 9]. Therefore, the ECM is a vital cellular environment, and interactions between the cell and ECM are important in regulating many biological processes, which include cell growth, differentiation, and survival [10, 11].Cell–matrix interactions control cell function and behav-ior by signal transduction through a variety of cell sur-face receptors. The integrins are the major family of ECM receptors, which can transmit information from the matrix to the cell. Integrin binding of ECM ligands results in theAbstract Osteoarthritis (OA) is a degenerative disease, which is characterized by articular cartilage destruction, and mainly affects the older people. The extracellular matrix (ECM) provides a vital cellular environment, and interactions between the cell and ECM are important in reg-ulating many biological processes, including cell growth, differentiation, and survival. However, the pathogenesis of this disease is not fully elucidated, and it cannot be cured totally. Integrins are one of the major receptors in chondro-cytes. A number of studies confirmed that the chondrocytes express several integrins including α5β1, αV β3, αV β5, α6β1, α1β1, α2β1, α10β1, and α3β1, and some integrins ligands might act as the OA progression biomarkers. This review focuses on the functional role of integrins and their extracellular ligands in OA progression, especially OA car-tilage. Clear understanding of the role of integrins and their ligands in OA cartilage may have impact on future develop-ment of successful therapeutic approaches to OA.Keywords Chondrocyte · Integrin · Fibronectin · Tenascin C · Osteopontin · Osteoarthritis · CartilageIntroductionOsteoarthritis (OA) is a degenerative disease and is char-acterized by articular cartilage destruction along with changes occurring in other joint components including bone, menisci, synovium, ligaments, capsule, and musclesRheumatologyINTERNATIONALJ. Tian · F.-J. Zhang · G.-H. Lei (*)Department of Orthopaedics, Xiangya Hospital, Central South University, No. 87 Xiangya Road, Changsha 410008, Hunan, Chinae-mail: gh.lei9640@; lgh9640@Rheumatol Intformation of signaling complexes, which play a key role in the regulation of cell survival, adhesion, proliferation, dif-ferentiation, and matrix remodeling [11, 12]. To develop new and successful approaches for the treatment for OA, it is essential to elucidate the role of integrins and their ligands in the pathogenesis of OA. In this study, we have reviewed the role of integrins and their ligands on the OA cartilage, consequently which contributes to OA progression. Integrins structure and functionThe first integrin was identified almost 30 years ago; “integrin” was named for this protein complex because of its role as an integral membrane complex involved in the transmembrane association between the ECM and the cytoskeleton [13]. The first integrin of which cDNA was sequenced encodes a polypeptide of 89 kD, with the pres-ence of a large N-terminal extracellular domain, a single transmembrane segment, and a small C-terminal cytoplas-mic domain. The extracellular domain contains a threefold repeat of a novel 40 residue cysteine-rich segment, and the cytoplasmic domain contains a tyrosine residue that is a potential site for phosphorylation by tyrosine kinases [13]. So far, it is well known as a family of heterodimeric trans-membrane receptors consisting of an α and a β subunit, which each have a large ectodomain, a single transmem-brane domain, and a generally short cytoplasmic tail. All of the different 18 α and 8 β subunits are known in humans, which can be combined to 24 different integrin receptors [14, 15]. Multiple α subunits can combine with single βsubunits (and vice versa), giving rise to “combinatorial” ligand specificity, as shown in Fig. 1.The 24 known integrin heterodimers can be classified as arginine–glycine–aspartate (RGD)-binding, the α4 family, leukocyte adhesion integrins, laminin-binding, and I-domain collagen-binding, as shown in the Table 1. All of these integ-rins can be further segregated into two groups, either contain-ing or the other lacking an extra von Willebrand factor type A domain (known as αA or αI in integrins) in their α subunits. The I-domain subunits contain α1, α2, α10, α11, αL, αM, αX, αD, and αE, and non-I-domain subunits are α3, α4, α5, α6, α7, α8, α9, αV, and α IIb, as shown in Fig. 1. In I-domain integ-rins, the I-domains play a central role in ligand binding and intercellular adhesion, whereas in integrins, which lack the αI domain, the binding site in the integrin “head” is formed by structural contributions of both the α and β chains [16].Although the 24 heterodimers can be defined into different groups, different heterodimers can also be expressed on a sin-gle cell and each can interact with multiple intracellular sign-aling cascades. Depending on the cellular microenvironment, the biological effect of ligating or activating an integrin can vary dramatically [15, 17]. The regulation of integrin activ-ity is complex. Integrin affinities for their cognate extracellu-lar ligands, such as fibronectin, fibrinogen, and collagen, are regulated by cellular signaling, resulting in integrin activation through “inside–out” signaling [15, 18] leading to conforma-tional changes that result in increased affinity for extracellu-lar ligands [18]. Inside–out signaling controls the adhesion strength and enables sufficiently strong interactions between integrins and ECM proteins to allow integrins to transmit the forces required for cell migration and ECM remodeling and assembly [18]. Integrins have no intrinsic enzymatic activity but, following binding to extracellular ligands, they become activated, can cluster on the cell surface, and undergo con-formational changes that propagate across the membraneFig. 1 Integrins superfamily. All 18 different α and 8 dif-ferent β subunits are known in humans, which can combine to24 different integrin receptorsRheumatol Int(“outside–in”) to activate cytoplasmic kinase- and cytoskele-tal-signaling cascades. These in turn control cell attachment, movement, growth and differentiation, and survival [15, 17]. Therefore, integrin activation can increase ligand binding, resulting in outside–in signaling. Converse ligand binding can generate signals that cause inside–out signaling [18].Expression of integrins in chondrocytesPrevious studies confirmed that the chondrocytes express several integrins including α5β1, αV β3, αV β5, α6β1, α1β1,α2β1, and α10β1 [18–23], while α3β1 was expressed by occasional cells only [24]. The expression level of above-mentioned integrins was in different percentages and in dif-ferent zones. Fetal chondrocytes strongly expressed β1 and β5 chains [24, 25]. Chondrocytes from osteoarthritic car-tilage expressed high levels of β1 integrin and all of the α chains. The α1 was the most frequently expressed α chain, followed by α3, α5, α2, αv. Integrin expression decreased from the least to the most damaged zone of articular car-tilage, and cell cycle analysis showed that proliferating chondrocytes (S phase) were prevalent in the latter zone. The expression of β2, β3, β2, and β5 is usually very lowTable 1 24 human integrin heterodimers and their ligands ADAMs a disintegrin and metalloproteinases, ICAM intercellular adhesion molecules, VCAM vascular adhesion molecules, TGF β LAP trans-forming growth factor β latency-associated peptide, MadCaM mucosal address in cell adhesion molecule, VEGF vascular endothelial growth factorHuman integrins Ligands Cellular and tissue distributionRGD -binding α5β1FibronectinChondrocytes , endothelial cellsα8β1Fibronectin, vitronectin, tenascin C, osteopontin, nefronectin Smooth muscle cells αV β1Fibronectin, vitronectin Smooth muscle cells, fibroblasts, osteoclasts, tumor cells αV β3Fibrinogen, fibronectin, vitronectin, tenascin C, osteopontin, bone sialoprotein, MMP-2Smooth muscle cells, fibroblasts, osteoclasts, tumor cells, Chondrocytes, endothelial cells, platelets, epithelial cells,leukocytesαV β5Vitronectin Smooth muscle cells, fibroblasts, osteoclasts, Chondrocytes,platelets, leukocytes, epithelial cellsαV β6Fibronectin, TGF-β LAP Epithelial cells, carcinoma cells αV β8Vitronectin Melanoma, kidney, brian, ovary, uterus, placenta αIIb β3Fibrinogen Fibronectin, vitronectinPlatelets The α4 family α4β1Fibronectin, VCAMLeukocytes, endothelial cells,α4β7Fibronectin, VCAM, MadCaMLeukocytes,α9β1Tenascin C, osteopontin, ADAMs, factor XIII, VCAM, VEGF-C, VEGF-DEndothelial cells, keratinocytesLeukocyte adhesion integrins αD β2ICAM, VCAM Leukocytes αM β2ICAM, VCAM, iC3b, factor X, fibrinogen Leukocytes αL β2ICAM Leukocytes αX β2Fibrinogen, plasminogen, heparin, iC3b Leukocytes αE β7E-cadherin Leukocytes,Laminin -binding α3β1Laminins (collagens)Keratinocytesα6β1Laminins, ADAMs Endothelial cells, Chondrocytes α6β4Laminins Endothelial cellsα7β1LamininsDifferentiated muscle cells I -domain collagen -binding α1β1Collagens, semaphorin7A, (laminins)Endothelial cells, Chondrocytesα2β1Collagens, tenascin C, (laminins)Keratinocytes, endothelial cells, Chondrocytes, platelets α10β1Collagens Chondrocytesα11β1CollagensMesenchymal non-muscle cellsRheumatol Int[25]. With immunohistochemical methods using monoclo-nal and polyclonal antibodies, the integrin pattern in joint cartilage from rats corresponded largely to integrin expres-sion described for human cartilage tissue: β1, α1, α3, and αv subunits and the α5β1 and αvβ3 heterodimers were con-sistently expressed [26]. Moreover, an inverse correlation was demonstrated between the severity of the anatomical changes found in the zones and the phenotypic/metabolic changes in the cells. These results, together with the well-known inside–out signaling function of the adhesion mol-ecules, highlight the key role of matrix interactions in the pathogenesis of the anatomic changes in OA cartilages [22, 27]. Expression of integrins on chondrocytes is correlated with the degree of cartilage damage in human OA [22].All of the α5β1, αVβ3, and αVβ5 contain the RGD-binding domain; α6β1 and α3β1 contain the laminin-binding domain, while α1β1, α2β1, and α10β1 contain the I-domain collagen-binding motif. The α5β1 integrin serves as the primary chondrocyte fibronectin (FN) receptor [28], while αV-containing integrins bind to vitronectin [29] and osteopontin (OPN) [30], and may serve as alternative FN receptors. αVβ3 integrin binds to tenascin C [31]. All of the α1β1, α2β1, and α10β1 integrins can serve as receptors for collagens [32–34], and α6β1 and α3β1 integrins could bind to certain cartilage extracellular matrix proteins such as laminin [35–37]. The aforementioned integrins and their corresponding ligands all played the important roles in OA pathologic changes.Important roles of integrins in OA cartilageIntegrins mediate cells adhesionThe cartilage surface defect is a common change in OA. The initial adhesion of transplanted chondrocytes to sur-rounding host cartilage may be important in the repair of articular defects [38]. Adhesion may set position for cells to secrete molecules that fill the defect and integrate repair tissue with host tissue, while chondrocytes are known to become increasingly adherent to cartilage with time. It is well known that Annexins (mainly A5), CD44, and integ-rins are the important molecules involved in chondrocyte adhesion with ECM.In vitro experiments, under the conditions in which chondrocytes were cultured in high-density monolayer, released with trypsin, and allowed to recover in suspen-sion for 2 h at 37 °C, β1-integrins appear to mediate chon-drocyte adhesion to a cut cartilage surface. Delineation of the mechanisms of adhesion may have clinical impli-cations by allowing cell manipulations or matrix treat-ments to enhance chondrocyte adhesion and retention at a defect site [39]. Under the culture and seeding conditions in high-density or low-density monolayer, β1, α5β1, and αVβ5 integrins mediate human chondrocyte adhesion to cartilage [19]. These chondrocyte integrins have a potential role in the initial adhesion and retention of chondrocytes at a cartilage defect site following clinical procedures of chondrocyte transplantation [19]. In an experiment about the chondrocytes attached to hyaline or calcified carti-lage and bone, freshly isolated (primary) or passaged (P1) chondrocytes were seeded on the top of bone plugs having either a surface composed of mid-deep zone hyaline carti-lage or calcified cartilage or bone only. Both primary and passaged chondrocytes attached efficiently to all of the three surfaces (over 88 % of seeded cells). The chondro-cytes showed a punctate distribution of β1-integrin and vin-culin, which are colocalized with actin, suggesting that the cells formed focal adhesions. Blocking either β1-integrin or αVβ5 integrin partially inhibited (between 27–48 and 26–37 %, respectively) attachment of both primary and pas-saged chondrocytes to all surfaces. Blocking αVβ3 had no effect on adhesion [38].Besides cell adhesion, integrins also mediate chondro-cytes adhesion to their extracellular ligands. Cell adhesion assays revealed that both α1β1 and α2β1 can serve as chon-drocyte adhesion receptors for types II and VI collagen. In cell lines expressing both integrins, α1β1 was the preferen-tial receptor for type VI collagen, while α2β1 was the pref-erential receptor for type II collagen [23]. Thus, α1β1 and α2β1 integrins play the roles to mediate chondrocyte adhe-sion to types II and VI collagen, respectively [23]. α1β1 also mediates chondrocyte adhesion to type VI collagen [40]. Integrins also mediate attachment of chondrocytes to fibronectin and matrix Gla protein (MGP) [41].Integrins in chondrocytes mechanotransductionIn OA, mechanical forces play an important role in tissue homeostasis and remodeling [42]. Chondrocytes are poten-tially exposed to a variety of different mechanical forces including stretch, shear, or compressive forces in vivo [42]. Matrix synthesis and chondrocyte proliferation are up-regulated by the physiological levels of mechanical forces [43]. It is well know that integrins as mechanoreceptors regulate the cellular response to both changes in the ECM and mechanical stresses that chondrocytes are subjected to [44–46]. Integrin activity is important in the early cel-lular responses to mechanical stimulation, regulating the activation of a number of intracellular cascades that induce changes in gene expression and tissue remodeling. In nor-mal human articular chondrocytes, integrin activation, con-sequent to mechanical stimulation in vitro, results in tyros-ine phosphorylation of regulatory proteins and subsequent secretion of autocrine and paracrine acting soluble media-tors including substance P and interleukin 4 [47]. NormalRheumatol Intchondrocytes in monolayer exposed to 0.33-Hz mechanical stimulation for 20 min resulted in increased GAG synthesis that was blocked by the presence of antibodies to α5 and αVβ5 integrins and CD47. These studies suggested that αVβ5 integrin plays a role in the regulation of chondrocyte responses to biomechanical stimulation [48]. In vitro stud-ies showed that the primary monolayer cultures of human chondrocytes have an electrophysiological response after intermittent pressure-induced strain characterized by a membrane hyperpolarization of approximately 40 %. The cultured chondrocyte’s hyperpolarization was found to be inhibited by RGD peptides and antibodies to the α5 and β1 integrin subunits [49], and the hyperpolarization response was associated with opening of small conductance (SK) calcium-dependent K+ channels via α5β1 integrin stretch activated ion channels and a number of integrin-associated signaling molecules including the involvement of the actin cytoskeleton and tyrosine phosphorylation [50]. Thus, α5β1 is an important chondrocyte mechanoreceptor and a potential regulator of chondrocyte function [49]. Integrin α1β1 is a key participant in chondrocyte transduction of a hypo-osmotic stress. Furthermore, integrin α1β1 influences osmotransduction is independent of matrix binding, but likely dependent on the chondrocyte osmosensor transient receptor potential vanilloid-4 [51].Treatment of chondrocytes with interleukin-1 (IL-1) resulted in diminished synthesis and enhanced catabolism of matrix proteoglycans [52]. Within chondrocytes, expo-sure of interleukin-1β (IL-1β) induces the release of nitric oxide (NO) and prostaglandin E2 (PGE2) via activation of inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX-2) enzymes, independently of integrins [53, 54]. This effect can be reversed by integrin with the applica-tion of dynamic compression three-dimension agarose con-structs. Mechanical loading and IL-1β influence the release of NO and PGE2 from articular chondrocytes. The integ-rin-binding peptide, GRGDSP, abolishes or reverses the compression-induced alterations in the presence or absence of IL-1β. Thus, integrins act abrogating the NO and PGE2 release by directly influencing the expression levels of iNOS and COX-2 in the presence and absence of IL-1β in three-dimension agarose constructs [55, 56].In the mechanical stress stimuli situation, integrins regu-late responses of human articular chondrocytes to mechani-cal stimulation via several pathway or downstream com-ponents. For example, mechanical signals control SOX-9, VEGF, and c-Myc expression and cell proliferation dur-ing inflammation via integrin-linked kinase, B-Raf, and ERK1/2-dependent signaling in articular chondrocytes [43]. Integrin-associated protein (CD47/IAP) is necessary for chondrocyte mechanotransduction. Through interac-tions with α5β1 integrin and thrombospondin, CD47/IAP may modulate chondrocyte responses to mechanical signals [57]. Furthermore, ankle joint chondrocytes appeared to show significant differences in levels of the integrin-asso-ciated proteins CD98, CD147, and galectin 3, PKC gamma, and differences in responses to glutamate were seen. This might be related to modified integrin-dependent mecha-notransduction as a result of changes in the expression of integrin regulatory molecules such as CD98 or differen-tial expression and function of downstream components of the mechanotransduction pathway such as PKC or NMDA receptors [58]. RACK1-mediated translocation of activated PKCα to the cell membrane and modulation of integrin-associated signaling are likely to be important in regula-tion of downstream signaling cascades controlling chon-drocyte responses to mechanical stimuli [59]. Recently, Whitney et al. [60] found that ultrasound (US) has emerged as a technique to deliver mechanical stress, and their find-ings suggested US signals through integrin receptors to the MAPK/Erk pathway via a mechanotransduction pathway involving FAK, Src, p130Cas, and CrkII.Integrins regulate cells proliferation and differentiationCell–cell interactions play an important role in the develop-ment of cartilage. Heterologous and homologous cell–cell interactions are critical for chondrogenic differentiation during development. Chondrocyte survival and in situ dif-ferentiation are integrin-mediated [61]. Integrin β1, β5, and α5 might be involved in signal transmission for the chon-drocyte survival and dedifferentiation [62, 63]. The lack of β1 integrins on chondrocytes leads to severe chondrodys-plasia associated with high mortality rate around birth [64]. Deletion of β1 integrins in the limb bud results in multi-ple abnormalities of the knee joints; however, it neither accelerate articular cartilage destruction, perturb cartilage metabolism, nor influence intracellular mitogen-activated proteins kinase (MAPK) signaling pathways [64]. When β1 integrin gene is inactivated in the mutant mice chondro-cytes, chondrodysplasia of various severity is developed in mice. β1-deficient chondrocytes have an abnormal shape, and they are failed to arrange into columns in the growth plate [65]. This is caused by the lack of motility, which is in turn caused by a loss of adhesion to collagen type II, reduced binding to and impaired spreading on fibronectin, and an abnormal F-actin organization. In addition, mutant chondrocytes show decreased proliferation caused by a defect in G1/S transition and cytokinesis. Altogether, these findings establish that β1-integrin-dependent motility and proliferation of chondrocytes are mandatory events for endochondral bone formation to occur [65].Cell–cell interactions between articular chondrocytes and synovial fibroblasts have enhanced binding between these two cell types compared to background binding of the labeled cells to the tissue culture plastic surface andRheumatol Intchondrocytes, specifically bound to synovial fibroblasts through RGD-dependent receptors. Therefore, β1 integrins are involved in this adhesion process, and these heterolo-gous cell interactions appear to have a negative influence on chondrogenic differentiation [66]. Articular chondrocytes undergo an obvious phenotypic change when cultured in monolayers. During this change, or dedifferentiation, α5β1 integrin was found to be involved in the induction of type I and type III pro-collagen expression. Elated RAS viral (r-ras) oncogene homolog (RRAS) was considered to regu-late the progression of dedifferentiation by modulating the affinity and avidity of α5β1 integrin to ligands. Echistatin (a potent disintegrin) inhibits dedifferentiation of monolayer-cultured chondrocytes [67]. In chondrocytes, during expan-sion for tissue engineering, a candidate for signal transmis-sion during dedifferentiation is integrin α5β1 in conjunction with its ligand fibronectin [68]. Other receptors, like vitron-ectin and OPN (αVβ3) or laminin (α6β1) or their ligands, do not seem to be involved in signal transmission for dedi-fferentiation. In addition, the GPIIb/IIIa receptor seems to assist the process of dedifferentiation. Intracellularly, ILK, ICAP1, and CD47 might assist the transduction of the inte-grin-dependent signals [68]. In tissue engineering research, it was confirmed that the mesenchymal stem cells (MSCs) with high chondrogenic differentiation potential are highly α10 positive and propose α10 as a potential marker to pre-dict the differentiation state of MSCs [69].The signaling cascades involved in these processes of integrin regulating cells proliferation and differentiation mainly were MAPK, and GTPases as Ras and Raf, and subsequent apoptosis in human articular chondrocytes. Ras activation stimulates the extracellular signal-regulated kinase (ERK) MAPK cascade [70]. Loss of chondrogenic potential is accompanied by reduced expression in key signaling proteins of the MAPK pathway and apoptosis [71]. Activation of the chondrogenic transcription factor Sox-9 seems to be mediated by the MAPK pathway [72]. Ras-activated Raf–MEK–ERK signaling pathway can specifically control the expression of individual integrin subunits in a variety of human and mouse cell lines [73]. In articular chondrocytes, the affinity of αVβ5 integrin for ligands was regulated by the small GTPase R-Ras. R-Ras was gradually activated in monolayer-cultured chondro-cytes after plating, which caused a gradual decline in the cartilage matrix gene expression through enhanced Vβ5 integrin activation and the subsequent increase in ERK signaling [74].Integrins in cartilage homeostasisOsteoarthritis-affected cartilage exhibits enhanced expres-sion of FN and OPN mRNA. Ligation of α5β1 using acti-vating mAb JBS5 (which acts as agonist similar to FN N-terminal fragment) up-regulates the inflammatory medi-ators such as NO and PGE2, as well as the cytokines, IL-6, and IL-8. In contrast, αVβ3 complex-specific function-blocking mAb (LM609), which acts as an agonist similar to OPN, attenuates the production of IL-1β, NO, and PGE2 in a dominant negative fashion by osteoarthritis-affected carti-lage and activated bovine chondrocytes. These demonstrate a cross talk in signaling mechanisms among integrins and show that integrin-mediated “outside–in” and “inside–out” signaling very likely influences cartilage homeostasis, and its deregulation may play a role in the pathogenesis of oste-oarthritis [75]. In the α1-KO mice, more severe cartilage degradation, glycosaminoglycan depletion, and synovial hyperplasia were found as compared with the wild-type (WT) mice [76]. MMP-2 and MMP-3 expressions were increased in the OA-affected areas. In cartilage from α1-KO mice, the cellularity was reduced and the frequency of apoptotic cells was increased. Therefore, deficiency in the α1 integrin subunit is associated with an earlier deregula-tion of cartilage homeostasis and an accelerated, aging-dependent development of OA [76].Integrin α1β1 plays a vital role in mediating chondrocyte responses to two contrasting factors that are critical play-ers in the onset and progression of OA—inflammatory IL-1 and anabolic TGF-β [77]. In a rat OA experimental model, an increased expression of α5 and α2 integrins was found at OA late stages, which was correlated with the changes in the ECM content, as a consequence of the increased MMPs activity. In addition, in the rat OA experimental model, the presence of α4 integrin since OA early stages was corre-lated with the loss of proteoglycans and clusters formation [78]. However, at late OA stages, the increased expression of α4 integrin in the middle and deep zones of the cartilage was also correlated with the abnormal endochondral ossi-fication of the cartilage through its interaction with OPN. Finally, these findings concluded that ECM–chondrocytes interaction through specific cell receptors is essential to maintain the cartilage homeostasis. However, as the integ-rins cell signaling is ligand-dependent, changes in the ECM contents may induce the activation of either anabolic or catabolic processes, which limits the reparative capacity of chondrocytes, favoring OA severity [78].Fibroblast growth factor (FGF) and insulin-like growth factor (IGF) have been implicated as contributing factors in cartilage homeostasis [79, 80]. FGF-18 most likely exerts anabolic effects in human articular chondrocytes by induc-ing ECM formation, chondrogenic cell differentiation, and inhibiting cell proliferation [79, 81]. The role of FGF-8 has been identified as a catabolic mediator in rat and rab-bit articular cartilage [82]. IGF-1 is a major growth factor involved in cartilage matrix synthesis and repair. IGF-1 promotes synthesis of collagen type II, proteoglycans, and other matrix components [83]. Chondrocytes from。