In this study, we have shown that the tumour-associated proinvasive glycoprotein podoplanin is highly expressed in synovial lining layer cells in RA but is rarely found in OA synovial specimens. The expression of podoplanin was most pronounced in areas with signs of inflammation (that is, the presence of leukocyte infiltrates and ectopic lymphoid structures) and synovial transformation (indicated by lining layer hyperplasia, MMP-9 expression and upregulation of cadherin-11 and α-sma). Furthermore, the podoplanin-expressing lining layer cells expressed cadherin-11 but not the macrophage marker CD68, suggesting that these synoviocytes were FLSs rather than synovial macrophages.
All included RA patients had progressed to erosive disease (Larsen index score >1), and all except one had a high podoplanin expression score (IHC score >1) of the synovial tissue from the replaced joint (Table ). However, without the rarely available tissue specimens from nonerosive and early RA joints to compare these tissues with, we could not analyze whether there is a correlation between erosive disease and podoplanin expression.
The function of podoplanin is far from elucidated. On one hand, this small glycoprotein is constitutively expressed on the apical surface of lymph endothelia as well as on specialised epithelia (for example, podocytes) facing fluid compartments [28
]. On the other hand, podoplanin is crucial for processes involving cell migration, such as the specific embryologic development of deep lymphatics [28
] and the invasion and metastasis of certain tumour cells or tissues [32
]. Podoplanin has been shown to bind ezrin, an actin filament membrane linker protein, on the inside of the cell in vitro
]. It has therefore been suggested that podoplanin is involved in directing actin polymerisation, thereby forming the cellular protrusions needed for migration.
In our study, the marked and widespread expression of podoplanin in lining layer cells in RA was not restricted to the apical cell surface. Instead, it resembled the strong whole cell surface-staining pattern of podoplanin in tumour tissues [30
]. It has been shown that RA-FLSs of highly inflammatory synovial tissue show a gene expression profile characteristic of myofibroblasts [23
]. We detected coexpression of podoplanin and α-sma of FLSs in areas of synovial transformation and found that the expression of E-cadherin was low or absent in the podoplanin-expressing lining layer cells. We know from earlier studies that podoplanin can promote EMT of epithelial Madin-Darby canine kidney cells in vitro
]. EMT is a biologic process in which polarised epithelial cells undergo sequential changes into a mesenchymal cell phenotype with increased migratory potential and the production of ECM components [24
]. Loss of E-cadherin and gain of α-sma expression constitute examples of such changes. We therefore hypothesise that podoplanin is involved in an EMT-like transdifferentiation of RA-FLSs into myofibroblasts.
Podoplanin has been observed in interstitial fibroblasts in different inflammatory environments in vivo
and in vitro
]. In agreement with this observation, we found a locally increased expression of podoplanin in interstitial cells of the sublining connective tissue in specimens from patients with RA. However, it is difficult to determine whether upregulated podoplanin expression in the sublining in some RA specimens was a result of general inflammation or whether this phenomenon was part of a specific activation and transdifferentiation of FLSs in RA.
To confirm the specificity of the D2-40 antibody and the expression of podoplanin in RA synovial tissue, we performed SDS-PAGE and Western blot analysis of protein extracts showing a distinct band of about 45 kDa. The mature glycosylated form of podoplanin has been estimated to be about 38 to 40 kDa [27
]. The difference in approximated molecular weight could be explained by the reported heterogeneity of podoplanin in SDS-PAGE, which arises as a result of heavily O
-linked glycosylation of the core protein [27
] as well as a slightly unspecific migration of the used molecular weight markers.
Characteristics of RA are the phenotypic changes and hyperplasia of FLSs of the lining layer. Conventional isolation of FLSs from synovial tissue yields homogeneous fibroblast cultures [45
], but the interindividual morphological variation is large, and cultures presumably arise from both the synovial lining and sublining layers.
We established primary cultures of FLSs from human synovial tissues by enzyme digestion and found that the cells had typical fibroblast morphology. Nearly all of the primary FLSs stained positive for the fibroblast marker CD90/Thy-1 and most expressed β1 integrins. However, the IHC staining of human synovia using the anti-CD90 antibody revealed positive expression in the sublining, but not in the lining layer cells (Figures and ). Fibroblasts possess a remarkable phenotypic plasticity [41
] as well as a positional identity [46
]. The synovium (lining layer versus sublining layer) of both healthy and RA patients harbour phenotypically different (by morphology and expression of surface markers) populations of fibroblasts. CD90 might therefore be a good marker for interstitial tissue fibroblasts, but not for the FLSs forming the epithelium-like lining of the synovium. In addition, fibroblasts change the expression of several surface molecules in vitro
and acquire an "active" phenotype with prominent stress fibres and focal adhesions [47
] when cultured on plastic. We therefore concluded that most of the established primary FLS cultures in this study originated from the sublining connective tissue or acquired a sublining fibroblast phenotype (with respect to CD90 expression) in culture. Using IHC, we found that cadherin-11 was expressed both in the lining layer and in cells of the sublining tissue in reactive areas, but when using flow cytometry, we found it on average in only 10% of the isolated primary FLSs. These data support the assumption that the isolated primary FLSs in these experiments originated from the sublining rather than from the lining layer.
Fibroblasts have been shown to upregulate podoplanin in culture [34
]. In this study, we did not observe any significant difference in mean podoplanin expression between the RA-FLS and OA-FLS cultures. Only one culture, derived from an RA patient, was growing without contact inhibition, a characteristic of activated FLSs in RA. All cells of this culture were expressing podoplanin. Taken together, our results suggest that cultures of the lining layer FLS phenotype are hard to establish by using this technique and that primary FLSs, like other fibroblasts, probably upregulate podoplanin in culture. The observed upregulated expression of α-sma of the primary FLSs constitutes another example of an acquired feature of cultured fibroblasts.
Finally, we found a more than twofold increase in podoplanin expression in primary FLSs after stimulation with IL-1β and TNFα compared with controls. Interestingly, we also detected an increase in podoplanin expression in response to TGF-β1 stimulation. TGF-β1 is a key mediator of EMT and promotes the differentiation of fibroblasts into myofibroblasts in wound healing and fibrosis. Furthermore, TGF-β1-induced podoplanin in human fibrosarcomas [48
] was found to be increased in arthritic joints in RA [49
] and promoted EMT of FLS in vitro
]. The fact that proinflammatory cytokines and growth factors, known to be present in high concentrations in the RA joint, stimulate podoplanin expression in primary FLSs in vitro
supports our finding that podoplanin is upregulated in the synovium of RA patients and might be involved in the transdifferentiation of FLSs in RA.