FGFR3 expression was reduced in RA synovial tissues. This was confirmed in RASF in vitro. Thus, the reduction of FGFR3 is conserved in vitro over at least 4–6 passages. This raised the question whether decreased expression reflects reduced transport to the plasma membrane or more generally decreased synthesis or increased degradation.
The ratio of FGFR3 on the cell membrane versus total in permeabilized cells was not reduced, suggesting a normal transport to the cell surface. In addition, quantifications of FGFR3 transcripts showed no difference between RASF and OASF. FGFR3 transcripts can be targeted by microRNAs, e.g. miR-24 and miR-100 [12, 16] but this is probably not the major factor here, since FGFR3 transcript half-life was not different between OASF and RASF. The most prominent differences in vitro are observed in the permeabilized cells, because 1) the transport mechanism to the cell membrane is not affected and 2) the test becomes more sensitive due to the higher proportion of positive cells.
The ROC analysis confirmed the highly significant diagnostic predictive value of FGFR3 expression in permeabilized SF. The benefits or potential use of this finding appear limited by the invasive nature of biopsy or surgery. An alternative could be to check whether fibroblasts in RA synovial fluids [17] obtained by arthrocentesis also have low FGFR3 expression.
During the wound healing process, FGFR3 protein expression on fibroblasts and myofibroblasts is reduced [18]. Low FGFR3 also been described in urothelial bladder cancer associated with poor outcome [19]. In the context of arthritis, low FGFR3 could also have a pathological relevance. This is suggested by FGFR3 knockout mice; thus, the absence of signaling through FGFR3 in the joints of FGFR3(-/-) mice leads to premature cartilage degeneration and early arthritis [20].
Our first hypothesis was that reduced FGFR3 in RA could be due to chronic stimulation by pro-inflammatory mediators, such as TNFα, or to the action of its ligand, particularly FGF2. However, this was not the case. Together with the fact that RASF conserved the low FGFR3 phenotype after more than 4 passages in vitro, this indicates that pro-inflammatory or growth-promoting environments alone are not sufficient to explain the reduced FGFR3 expression. Regarding FGFR2, short-time exposure to TNFa increases it expression, as reported by others [21], while chronic exposure increased it. Our results indicated that the expression of FGFR2, but not of FGFR1 or FGFR3, can be modulated by the pro-inflammatory milieu. FGFR2 appears to be differently regulated than FGFR1 or FGFR3, e.g. particularly in the context of RA and inflammatory bowel diseases FGFR2 expression could be affected by micro-RNAs [22].
Since FGFR3 promoter can be hypermethylated under certain circumstances, we tested the effect of 5-azacytdine on its expression. However, treatment with 5-azacytidine resulted in a decreased FGFR3 expression in both OASF and RASF. This suggested an indirect effect of a methylation-sensitive factor. Low FGFR3 in RASF could be linked to the global DNA hypomethylation that has been described previously [15]. This possibility has to be investigated in more detail.
The catalytic subunit of PKA (PKACA) is a gene product that is downregulated upon treatment of OASF with 5-azacytidine [15]. Since decreased PKA activity could be associated with reduced FGFR3 expression [11], we tested the effects of PKA-mimics on RASF and PKA-inhibiting agents on OASF. However, the effects were modest and transient only.
In the hyperplastic RA synovial tissue, hypoxia is an important factor influencing cell activity. Since hypoxia can modify FGFR3 expression [12], with tested two chemical hypoxia-mimics. Particularly DFO can reduce the expression of FGFR3 in permeabilized OASF. Again, however, the changes were transient only. In addition, hypoxia would not explain low FGFR3 in RASF kept over 4 passages in normal conditions, i.e. 20% O2.
Most importantly, we showed that proteasome inhibitors (MG-132 or bortezomib) restored the expression of FGFR3 in RASF to the levels measured in OASF. This increase of FGFR3 remained at least for 24–48 hours and the FGFR1/FGFR3 ratio was normalized for at least 48 hours. An increased proteasome activity might also affect the expression of other proteins in RASF, such as DNMT1, as previously suggested [15]. The inhibition of proteasomes does not affect the expression of FGFR3 in OASF because it is already maximally expressed.
We hypothesized that lower expression of FGFR3 could have an influence on the response to FGF2. To test this, we performed a proliferation assay with or without pre-treatment with MG-132. As expected, MG-132 abolished the enhanced proliferative response of RASF to FGF2. The situation is different in OASF, in which an equilibrated FGFR1-2/FGFR3 balance is able to limit the effect of FGF2. FGFR3, in contrast to other FGFRs, is a strong client of the chaperone HSP90 and is subject of ubiquitination by E3-ubiquitin ligases [13]. This can render it susceptible to lysozymal and proteasomal degradation.
Congruently, in RASF, increased 20S proteasome activity correlated with decreased expression of FGFR3. The measurement of FGFR3 in permeabilized cells allowed to estimate the total expression of the protein, including the cytoplasmic and cell membrane compartments. This total expression correlates better with the 20S proteasome activity than the cell surface expression, probably because the latter is regulated by additional independent mechanisms influencing the transport on the cell surface.
Interestingly, the expression levels of FGFR1 and FGFR3 declined in synovial fibroblasts after blocking the sonic hedgehog-Gli signaling pathway with a Gli specific inhibitor (Gli-antagonist 61, GANT61) [23]. GANT61 inhibits RASF cell proliferation and increases cell apoptosis but did not the affect the FGFR1/FGFR3 ratio. Unfortunately, the effect of FGF2 in this system was not investigated.
An argument against our hypothesis could be that Kaempferol which inhibit FGF2-FGFR3–ribosomal S6 kinase 2 (RSK2) signalling suppresses the in vitro proliferation and migration of RASF [7]. However, in the hyperplastic synovial tissue, CD68 + macrophages, but not fibroblasts, showed phosphorylated RSK2. This might be due to the downregulation of FGFR3 on fibroblasts. Again, the effect of FGF2 in this system have to be analysed. It is also important to note that Kaempferol has multiple pharmacological properties that at end results in less inflammation, less oxidative stress and more apoptosis [24, 25]. Recently, it has been reported that kaempferol dramatically suppressed TNFα-induced MAPK activation [26], which is certainly a FGFR3-independent process.