This is the first study to profile the expression of all MMP
genes in synovium and compare tissue from OA patients with those who have a fracture to the NOF. Konttinen et al
. had previously used conventional reverse-transcriptase PCR to profile the expression of 16 MMPs
in synovium (MMP1–3
), with induction of MMP1
in synovium from rheumatoid arthritis compared with trauma controls [31
Recent studies show that inflammatory changes in the OA synovium are variable throughout the tissue [23
]. This might be expected to confound measurement of regulated gene expression in this tissue because variation across the tissue within a single joint would lead to greater variation in gene expression within each patient group. This is borne out by our study, in which more genes showed significant regulation between OA and fractured NOF in cartilage than synovium.
Several studies have measured MMPs in synovium or synovial fluid and demonstrated that levels of MMP-1 and MMP-3 are elevated in rheumatoid arthritis (compared with OA or other control groups, such as joint trauma; e.g. [32
]). High expression of a number of MMPs has been demonstrated in rheumatoid pannus, suggesting a role mediating cartilage destruction in RA (e.g. [31
]). Indeed, levels of synovial fluid MMP-1 and MMP-3 correlate to synovial hyperplasia [38
]. Although levels of synovial fluid MMPs are generally raised in RA compared with OA, the levels of at least MMP-1–3, MMP-8, MMP-9 and TIMP-1 are reported to be raised in OA compared with control, nonarthritic, synovial fluid (e.g. [39
In this study, the four most significantly upregulated genes (P
< 0.0001) in OA synovium compared with fracture of the NOF are MMP28
. Interestingly, we recently published the first report of MMP28
gene expression in cartilage and their upregulation in OA [26
]. Although these enzymes have no known function, the fact that their expression is switched on in at least two tissues of the OA joint underlines their probable role in this disease. ADAMTS17
is another recently cloned ADAMTS family member with unknown function and this is the first report of its expression in synovium. TIMP-2 can inhibit the activity of all the MMPs and the increase in expression in OA synovium might represent an attempt to control proteolysis. The expression of MMP13
, the probable key collagenase in OA, is also upregulated in OA synovium, in addition to cartilage.
MMP-1, MMP-2, MMP-9–11, MMP-13 and TIMP-2 are all expressed in human growth plate hypertrophic chondrocytes [41
]. Interestingly, a significant subset of these (MMP2
) is upregulated in OA cartilage.
MMP-1 and MMP-3 are the most frequently measured enzymes in RA, where correlations of synovial fluid or serum levels to disease activity have been reported [36
]. Our data show that in synovium there is no difference in expression between OA and fracture to the NOF, whereas in cartilage, as we and others have previously reported, there is a significant downregulation of expression in OA [26
]. This might reflect the late stage of the disease tissue in our study (see below), or, for MMP-3, a maintenance role in cartilage metabolism. Our data support the fact that MMP3
is a highly expressed gene in synovium, as previously reported [37
Other genes expressed at significantly higher levels in OA synovium compared with fracture to the NOF are MMP9
, although the absolute expression of ADAMTS8
is low. MMP-9 has previously been reported as elevated in RA synovium (e.g. [31
]) and in synovium and synovial fluid in a dog model of OA [43
]. Elevated MMP11
expression has also been previously reported in late-stage OA [42
expression has been reported in synovium, but was not associated with disease [31
], whereas ADAMTS8
have not previously been described in synovium. ADAMTS-8 is capable of degrading aggrecan [44
], although with lesser catalytic efficiency compared with, for example, ADAMTS-4 or ADAMTS-5.
The expression of MMP10
was the most significantly downregulated of any MMP
in OA synovium compared with fractured NOF. MMP10
expression has been previously described in synovium, but its regulation has not been described in disease [31
expression has also been associated with the invasive potential of synovial fibroblasts in both OA and RA [45
We previously reported that the expression of ADAMTS4
, all aggrecanases, was downregulated in OA cartilage compared with fractured NOF [26
]. These genes, particularly ADAMTS4
, are also downregulated in OA synovium. This could reflect a role for aggrecanases in the earlier phases of OA, but similar expression patterns in cartilage and synovium could also suggest a role for the synovium in aggrecan turnover and breakdown. Synovium has been shown to express soluble aggrecanase activity [46
], and ADAMTS-5 has been localized to human synovium [47
A comparison of the expression patterns of MMP
genes in cartilage in this study with those in our previous report shows predominantly similar data. Overall, a greater number of genes were regulated between OA and fractured NOF samples in the current study. This might reflect differences in the actual patient groups in each study or the rapidity of tissue dissection and processing. Genes induced in OA with weak significance in the current study, MMP17
, all show a trend towards induction in our previous cohort but did not reach significance. The same is true for MMP12
showed upregulation in OA in the current study at P
< 0.05 and similarly significant downregulation in our previous study. This might represent variation because of multiple testing. More difficult to explain are the significantly upregulated genes in OA in the current study, MMP21
< 0.001), MMP19
< 0.0001) and MMP23
< 0.0001); these genes showed no significant difference in our earlier cohort. Where genes are annotated as 'not detected' (Figures , , and [26
], these might not be expressed at all in the tissue or the median CT
is 40, with a few samples showing a low level of expression.
For MMP9, MMP10, MMP12, MMP17, MMP23, MMP28, ADAMTS4, and ADAMTS9, there is a significant correlation between expression levels in the synovium and cartilage. This presumably reflects the fact that these tissues are exposed to a similar environment in terms of growth factors and cytokines. For genes where expression between the two tissues does not correlate, there must therefore be other factors driving expression, such as mechanical load, oxidative stress and cell-matrix interaction.
To resolve previously raised questions, we used plasmid cDNA for MMP13, MMP28 and ADAMTS16 to construct standard curves for these genes. This demonstrated that MMP13 was the most abundant mRNA of the three genes. It remains impossible to speculate about their relative importance because the substrates, and therefore the specific activities of MMP-28 and ADAMTS-16, are unknown.