Repair of articular cartilage and growth factors in the pathogenesis of osteoarthritis

Thanks to the progress of biotechnology, in particular cloning technology, recently a growing list of growth factors, which, being anabolic factors, play an important but not completely understandable role in the pathogenesis of osteoarthritis.

The first group of growth factors, which will be discussed below, are the IGF. They are in large quantities found in serum, have a number of common properties with insulin. IGF-2 is more characteristic for the embryonic stage of development, whereas IGF-1 is the dominant group representative in an adult. Both representatives of this group act by binding to the I type IGF receptors. If the function of IGF-2 remains unknown, the value of IGF-1 has already been determined - it is able to stimulate the synthesis of proteoglycans by chondrocytes and significantly inhibit catabolic processes in the articular cartilage. IGF-1 is the main anabolic stimulus for the synthesis of proteoglycans by chondrocytes, present in serum and synovial fluid. IGF-1 is an important factor in the cultivation of chondrocytes in experimental models of in vitro models of choleraemia . It is suggested that IGF-1 enters the synovial fluid from the blood plasma. In addition, normal chondrocytes produce both factors - the expression of IGF-1 and IGF-2 is found in the synovial membrane and cartilage of patients with osteoarthritis. In normal cartilage, IGF-1 does not have mitogenic properties, but it can stimulate the proliferation of cells in the damaged matrix, which indicates participation in reparative processes.

Biologically active substances that stimulate reparation and depress the degradation of articular cartilage

• Insulin
• Gamma-interferon
• Growth hormone, androgens
• Somatomedins (IPF-1 and -2)
• TGF-beta (tissue growth factor)
• The growth factor derived from platelets
• The main growth factor of fibroblasts
• EFR
• Il-1 receptor antagonist
• TNF-a-binding proteins
• Tissue inhibitors of metalloproteases
• and _2- macroglobulin
• ai-antitrypsin
• Pr-macroglobulin
• Pr-antichymotrypsin

The actions of IGF-1 and IGF-2 are controlled by various IGF-binding proteins (IGF-SB), which are also produced by chondrocytes. IGF-SB can act as a carrier, and also have blocking IGF activity Isolated from the articular cartilage of patients with sostoarthrosis, cells produce an excessive amount of IGF-SB, indicating that they block the effects of IGF. J. Martel-Pelletier and co-authors (1998) have shown that although the synthesis of IGF-1 in the cartilage with osteoarthritis increases, the chondrocytes respond poorly to the stimulation of IGF-1. It turned out that this phenomenon is associated (at least in part) with an increase in the level of IGF-SB. IGF-SB has a high affinity for IGF and is an important biomodulator of its activity. To date, seven types of IGF-SB have been studied, disruption of IGF-SB-3 and IGF-SB-4 regulation plays an important role in osteoarthritis.

Another category of growth factors exhibiting different effects on chondrocytes includes platelet derived growth factor (PDGF), FGF and TGF-beta. These factors are produced not only by chondrocytes, but also by activated synovitis. FGF has both anabolic and catabolic properties depending on the concentration and condition of the articular cartilage. The PDGF takes part in maintaining the VKM homeostasis of the articular cartilage, not having obvious mitogenic properties. For this growth factor, the ability to enhance the synthesis of proteoglycans and reduce their degradation is known.

TGF-beta is of particular interest in terms of studying its role in the pathogenesis of osteoarthritis. He is a member of a large TGF superfamily, has common functional and signaling properties with newly discovered growth factors of BMP (bone morphogenetic proteins).

TGF-beta-pleiotropic factor: on the one hand, it has immunosuppressive properties, on the other - it is a chemotactic factor and a potent stimulator of proliferation of fibroblasts. The unique properties of TGF-beta are the ability to inhibit the release of enzymes from different cells and significantly increase the production of enzyme inhibitors (eg, TIMP). TGF-beta is considered an important regulator of tissue damage due to inflammation. So, in the tissue of articular cartilage, TGF-beta significantly stimulates the production of the matrix by chondrocytes, especially after pre-exposure with this factor. Normal cartilage is insensitive to TGF-beta. In patients with OA, TGF-P stimulates the production of aggrecan and small proteoglycans in the articular cartilage.

TGF-beta is produced by many cells, in particular chondrocytes. It is released in a latent form associated with a special protein called the "protein associated with latency" (BAL). Dissociation with this protein is carried out by proteases, which are produced in large quantities in inflamed tissues. Apart from TGF-beta, which is produced by activated cells, the latent form of this factor is an important element of the reactivity of TGF-beta in tissue after local damage. TGF-beta in a significant amount is contained in the synovial fluid, synovial membrane and cartilage of the joint affected by osteoarthritis. In areas of damaged tissue where there are inflammatory infiltrates, coexpression of TNF and IL-1 is detected, whereas in areas with fibrosis phenomena only the expression of TGF-beta is detected.

Incubation of the culture of chondrocytes obtained from patients with osteoarthritis with TGF-beta causes a significant increase in the synthesis of proteoglycans by these cells. Stimulation of TGF-beta of normal chondrocytes causes an increase in the synthesis of proteoglycans only after many days of incubation. Perhaps this time is necessary to change the phenotype of cells under the influence of TGF-beta (for example, to change the so-called compartmentalization of proteoglycans: the newly created proteoglycans are localized only around the chondrocytes).

It is known that the activation of the synthesis of growth factors, in particular, TGF-beta, is an important link in the pathogenesis of kidney and liver fibrosis, scar formation during wound healing. An increase in the load on chondrocytes in vitro leads to hyperproduction of TGF-beta, whereas a decrease in the synthesis of proteoglycans after limb immobilization can be leveled by TGF-beta. TGF-beta induces the formation of osteophytes in the marginal zone of the joints as a mechanism to adapt to changes in load. IL-1, causing a moderate inflammatory process in synovia in response to joint damage, promotes the formation of chondrocytes with a modified phenotype, which produce an excessive amount.

Repeated local injections of recombinant TGF-beta in high concentrations led to the development of osteoarthritis in C57B1 mice - the formation of osteophytes, which is characteristic of human osteoarthrosis, and a significant loss of proteoglycans in the "wavy border" zone.

In order to understand how the excess of TGF-beta causes known changes in cartilage, it should be noted that exposure of TGF-P induces a characteristic chondrocyte phenotype with a change in the subclass of synthesized proteoglycans and a violation of the normal integration of VCR elements. Both IGF-1 and TGF-beta stimulate the synthesis of proteoglycans by chondrocytes cultured in the alginate, but the latter also induces the so-called compartmentalization of proteoglycans. Moreover, it has been found that TGF-beta increases the level of collagenase-3 (MMP-13) in activated chondrocytes, which is at odds with the general view of TGF-beta as a factor that, on the contrary, reduces the release of destructive proteases. Although it is not known whether TGF-beta-induced synthesis of MMP-13 is involved in the pathogenesis of OA. TGF-beta not only stimulates the synthesis of proteoglycans, but also promotes their deposition in ligaments and tendons, increasing stiffness and reducing the amount of movement in the joints.

CIP are members of the TGF-beta superfamily. Some of them (CML-2, CML-7 and CMS-9) have the property of stimulating the synthesis of proteoglycans by chondrocytes. CMPs perform their effects by binding to specific receptors on the cell surface; the signaling pathways of TGF-beta and CMS are somewhat different. Like TGF-beta, the signal from the CMP is transmitted through a serine / threonine kinase type I and II receptor complex. In this complex, the type II receptor istrans-phosphorylated and activates the Type I receptor, which transmits the signal to signal molecules called Smad. After receiving the Smad signal, they are rapidly phosphorylated. It is now known that Smad-1, -5 and -8 are phosphorylated in the signal pathway of CMP, and Smd-2 and Smad-3 in the TGF-beta signaling pathway. Then named Smad is associated with Smad-4, which is common to the signaling pathways of all representatives of the TGF-beta superfamily. This fact explains the presence of cross-functions in the members of the TGF-beta superfamily, as well as the phenomenon of mutual inhibition of the TGF-beta and CMS signaling pathways by competition for common components. Not so long ago, another class of Smad proteins was identified, which is represented by Smad-6 and -7. These molecules act as regulators of the signaling pathways of TGF-beta and CML.

Despite the fact that the stimulating effect of CMP on the synthesis of proteoglycans has been known for a long time, their role in regulating the function of articular cartilage remains controversial due to the known ability of CML to induce dedifferentiation of cells, stimulate calcification and the formation of bone tissue. M. Enomoto-Iwamoto and co-authors (1998) showed that the interaction of the CML with the type II CML-receptor is necessary to maintain a differentiated phenotype of chondrocytes, as well as control their proliferation and hypertrophy. According to LZ Sailor and co-authors (1996), CmP-2 supports the phenotype of chondrocytes in culture for 4 weeks without causing their hypertrophy. CMP-7 (identical to osteogenic protein-1) has long maintained the phenotype of mature chondrocytes of articular cartilage cultivated in the alginate.

The introduction of KMP-2 and -9 into the knee joints of mice increased the synthesis of proteoglycans by 300%, and significantly more than TGF-beta. However, the stimulating effect turned out to be temporary, and after a few days the level of synthesis returned to the original one. TGF-beta caused a longer stimulation of proteoglycan synthesis, which is probably due to autoinduction of TGF-beta and sensitization of chondrocytes to this factor.

TGF-beta is responsible for the formation of chondrophytes, which can be considered an undesirable effect of its action; CmP-2 also promotes the formation of chondrophytes, but in the other part of the joint margin (mainly in the area of the growth plate).

[1], [2], [3], [4], [5], [6], [7]

Cartilage morphogenetic proteins

Cartilage morphogenetic proteins (XMP-1 and -2) are yet another representative of the superfamily TGF-beta, necessary for the formation of cartilaginous tissue during the development of the limbs. The mutation of the gene of HMP-1 causes chondrodysplasia. Perhaps, the KMP has a more selective, cartilage-oriented profile. Despite the fact that TGF-beta and CMP are able to stimulate chondrocytes, they can act on many other cells, so their use for cartilage repair can be accompanied by side effects. Both types of CMP are found in the cartilage of healthy joints affected by osteoarthrosis, they contribute to the repair of ECM of articular cartilage after enzymatic degradation, supporting a normal phenotype.

[8], [9], [10], [11], [12], [13], [14], [15], [16], [17]

Synergism of growth factors

One growth factor is capable of inducing itself, as well as other growth factors, this interaction is finely regulated. For example, FGF together with other growth factors provides more effective repair of articular cartilage after a traumatic defect. IGF-1 together with TGF-beta significantly induce a normal phenotype of chondrocytes when they are cultivated in vitro. It was demonstrated that TGF-beta interferes with the production of IGF-1 and IGF-SB, and also dephosphorylates the IGF-1 receptor, stimulates the binding of IGF-1. In the intact cartilage of mice, a synergism of IGF-1 with many growth factors was observed. However, the mild reaction of chondrocytes in IGF-1 can not be leveled using in combination with other growth factors.

Interaction of anabolic and destructive cytokines

Growth factors demonstrate a complex interaction with IL-1. For example, pre-exposure of chondrocytes in FRF increases the release of proteases after exposure to IL-1; perhaps, this is due to an increase in the expression of IL-1 receptors. PDGF also stimulates IL-1-dependent release of proteases, but it reduces IL-1-mediated inhibition of proteoglycan synthesis. This may mean that some growth factors can simultaneously stimulate the process of cartilage repair and contribute to its destruction. Other growth factors, such as IGF-1 and TGF-P, stimulate the synthesis of the articular matrix and inhibit IL-1-mediated destruction of articular cartilage, i.e. Their activity is associated only with tissue repair. Such interaction does not depend on pre-exposure of chondrocytes IL-1. Interestingly, the kinetics of the effects of IL-1 and TGF-beta can be different: the ability of TGF-beta to inhibit articular cartilage degradation is weakened by its slow action on TIMP mRNA. On the other hand, there is an increase in the level of hNOC and N0 in the absence of TGF-beta. Considering the NO-dependence of the suppressor effect of IL-1 on the synthesis of proteoglycans by chondrocytes, one can explain why we observe much stronger counteraction of TGF-beta to IL-1-dependent oppression of proteoglycan synthesis in comparison with the destruction of proteoglycans in vivo.

In a study in mice injected with IL-1 and growth factors, TGF-beta was shown to significantly counteract IL-1-mediated inhibition of the synthesis of proteoglycans of articular cartilage, whereas CMP-2 is not capable of such counteraction: its stimulatory potential is completely inhibited IL-1 even under the condition of a high concentration of CMP-2. It is noteworthy that in the absence of IL-1, CML-2 stimulated the synthesis of proteoglycans much more than TGF-beta).

In addition to influencing the synthesis of proteoglycans, TGF-beta also significantly influences the IL-1-induced decrease in proteoglycan content in the cartilage. Perhaps, depending on the relative concentration of IL-1 and TGF-beta, the content of proteoglycans decreases or increases. Interestingly, the above-described counteraction of IL-1 and TGF-beta was observed in the thickness of the cartilage, but there was no such phenomenon near the chondrophytes at the edges of the joint surfaces. The formation of chondrophytes is induced by TGF- (3, which affects the chondrogenic cells in the perioth, causing the development of chondroblasts and the deposition of proteoglycans.) It seems that the ethochondroblasts are not sensitive to IL-1.

HL Glansbeek and co-authors (1998) studied the ability of TGF-beta and CML-2 to oppress the synthesis of proteoglycans in the joints of mice with zymozaneducated arthritis (ie, in the model of "pure" IL-1-induced inflammation). The intra-articular injection of TGF-beta significantly counteracted the inhibition of proteoglycan synthesis caused by inflammation, while CMP-2 was virtually incapable of counteracting this IL-1-dependent process. Repeated injections of TGF-P into the knee of the animals under study significantly stimulated the synthesis of proteoglycans by chondrocytes, promoted the preservation of existing proteoglycans of inflammatory cartilage, but did not inhibit the inflammatory process.

When studying the proteoglycansynthesizing function of chondrocytes using experimental models of osteoarthrosis in animals, an increase in the content and stimulation of the synthesis of proteoglycans in the early stages of OAc was noted, in contrast to inflammatory models in which there is considerable inhibition of synthesis (IL-1-dependent process). An increase in the activity of anabolic factors, in particular growth factors, which is observed in osteoarthritis, neutralizes the action of such suppressor cytokines as IL-1. Among the growth factors, the most important is TGF-beta; CIC-2 is unlikely to play a significant role in this process. Although IGF-1 is able to stimulate proteoglycan synthesis in vitro, in conditions in vivo is a property not observed with local application of IGF-1. Perhaps this is due to the fact that the endogenous level of this growth factor is optimal. At later stages of osteoarthritis, there are signs of inhibition of proteoglycan synthesis, which is probably due to the dominant effect of IL-1 and the inability of growth factors to counteract it due to decreased activity.

Growth factor analysis in STR / ORT mice with spontaneous osteoarthrosis demonstrated an increase in the level of TGF-R and IL-1 mRNA in the damaged cartilage. It should be noted that the activation of TGF-beta from the latent form is an important element of tissue repair. Understanding the role of TGF-beta complicates the results of the study of the expression of TGF-beta type II receptors in rabbits of the ACL line. Immediately after the induction of osteoarthritis, a reduced level of these receptors was detected, which indicates an insufficient signal function of TGF-beta. Interestingly, in the receptor-deficient TGF-beta 11 type of mice, signs of spontaneous osteoarthrosis are found, which also indicates the important role of the signal function of TGF-beta in the deterioration of cartilage repair and the development of osteoarthrosis.

The absolute content of growth factors in the joints of patients with rheumatoid arthritis or osteoarthrosis may indicate their possible role in the pathogenesis of these diseases. However, despite the fact that joints with osteoarthritis and rheumatoid arthritis show high concentrations of growth factors, the nature of the processes of degradation and repair in both diseases is completely different. Probably, there are other factors not yet identified, which play a major role in the pathogenesis of these diseases, or other aspects of the phenomena studied that determine the course of the processes of degradation and repair in joint tissues (for example, the expression of certain receptors on the surface of chondrocytes, soluble receptors binding proteins, or imbalance anabolic and destructive factors).