Articular cartilage consists of a relatively small number of cells and an abundant ECM. The major components of the ECM are collagen fibrils and aggregating proteoglycan aggrecan. Collagen fibrils, mainly type II collagen together with minor types IX and XI, form a meshwork that provides the tensile strength of the tissue. Aggrecan forms a large aggregated complex interacting with hyaluronan via link proteins and fills the interstitium of the collagen meshwork. Aggrecan provides a hydrated gel that gives cartilage its ability to withstand compression.
In normal cartilage, the turnover and synthesis of ECM macromolecules is at equilibrium, but in rheumatoid arthritis (RA) and osteoarthritis (OA) the loss of ECM components exceeds new synthesis. The primary cause of this imbalance is elevated activity of the proteinase that degrades aggrecan and collagen. Aggrecan loss initially occurs most markedly just beneath the joint surface, which is followed by mechanical failure of the tissue and collagen degradation [1
MMPs are a family of extracellular zinc metalloendopeptidases that function in the turnover of components of the ECM [3
]. They are produced by many types of cells, but their synthesis is regulated by many factors such as inflammatory cytokines, growth factors, cellular transformation and physical stimuli [3
Certain members of the MMP family have been considered to be the major enzymes that participate in the degradation of aggrecan and collagen in cartilage. Collagenases (MMP-1, MMP-8 and MMP-13), gelatinase A (MMP-2) and gelatinase B (MMP-9), stromelysin 1 (MMP-3), matrilysin 1 (MMP-7) and membrane-type MT1-MMP (MMP-14) are found in cartilage, and most are elevated in the synovium and in the cartilage from patients with RA and OA [5
All of these MMPs cleave the aggrecan core protein at various sites, but the critical site is the Asn341–Phe342 bond located in the interglobular domain located between the two N-terminal globular domains G1 and G2, as this cleavage can release aggrecan molecules from the cartilage [7
]. The N-terminal fragments with the C-terminal sequence Val-Asp-Ile-Pro-Glu-Asn341 are found in both OA and RA cartilage as well as in normal cartilage [9
]. On the contrary, Sandy et al
] found that the core protein was cleaved at the Glu373–Ala374 bond, but not at the Asn341–Phe342 bond, when bovine cartilage in culture was stimulated by IL-1. This activity was called 'aggrecanase'. The products resulting from this cleavage accumulate in the synovial fluids of patients with OA or inflammatory joints [11
Two enzymes responsible for this cleavage have been purified and cloned. They are referred to as aggrecanase 1 and aggrecanase 2 (also ADAMTS-4 and ADAMTS-5, members of the ADAM protein family, respectively) [13
]. Later, it was also found that ADAMTS-1 has aggrecanase activity [15
]. The degradation of type II collagen occurs slower than aggrecan degradations in arthritis. This is all due to the action of MMPs, and potential collagenolytic enzymes are MMP-1, MMP-2, MMP-8, MMP-13 and MMP-14.
MMP activities in the tissue are regulated by endogenous inhibitor TIMPs [16
]. Four TIMPs (TIMP-1, TIMP-2, TIMP-3, TIMP-4) are found in humans. They are homologous with each other and consist of two domains, an N-terminal inhibitory domain of about 125 amino acids and a C-terminal domain of about 65 amino acids. Each domain is stabilized by three conserved disulfide bonds. While the N-terminal domains of TIMPs (N-TIMPs) are primarily responsible for the inhibition of MMPs [17
], the C-terminal domains can also influence their binding affinity. The balance between the metalloproteinases and their endogenous inhibitors is critical for the appropriate maintenance of tissues.
Early work by Dean et al
] showed that both MMP levels and TIMP levels were elevated in OA cartilage compared with unaffected cartilage, but that the total amount of MMP was slightly higher than that of TIMP, whereas this balance was reverse in the unaffected cartilage. This subtle difference in the ratio of MMPs and TIMPs is considered to be a cause of the gradual degradation of the cartilage matrix.
TIMP-1, TIMP-2 and TIMP-3 are present in the joint tissue. Some elevated levels of TIMP-1 were reported in synovial fluids [19
] and in serum [20
] of RA patients, but not in the serum of OA patients [22
]. However, the changes of TIMP-1 levels are not very large compared with the over-expression of MMPs. Overexpression of TIMP-1 using systemic adenovirus-based gene delivery reduced destruction of the joints of TNF-α transgenic mice [23
]. On the contrary, the overexpression of TIMP-1 did not prevent osteochondral injury in the mouse model of collagen-induced arthritis [24
]. Since there are differences in specificity among TIMPs, further investigation is clearly needed to elucidate the biological and pathological significance of TIMPs.