In the present study, we demonstrated that the highly expressed SLC7A5 in the synoviocytes is induced by proinflammatory IL-1β via NF-κB signaling activation. Overexpressed SLC7A5 promotes mTOR-P70S6K signals and enhances the expression of MMP3 and MMP13 at the protein level in RA FLS.
Studies regarding the extent of metabolic changes and the types of metabolites involved may provide us suitable biomarkers for RA diagnosis. Mounting evidence supports the notion that the metabolic changes occurring in the pathogenesis of RA are different from those found in other arthritis diseases [
17]. Multiple amino acids such as glycine, leucine, serine, tyrosine, isoleucine, and proline have been reported in the synovial fluid of patients with RA [
18]. However, only a few studies are available focusing on the involvement of amino acid transporter in RA pathogenesis. The metabolic changes in joint inflammation are complicated, and many interactions may take place, leading to a complex communication network between different cell types. Therefore, more knowledge is needed to unveil the critical interactions between amino acid transporter and FLS function in the arthritis process. In this study, we established that an amino acid transporter, SLC7A5, is overexpressed in FLS of RA patients, resulting in the upregulation of the MMPs at the protein level, which plays a critical role in maintaining FLS invasive phenotype and degradation of the extracellular matrix. Blocking SLC7A5 activity may slow down the FLS infiltration hence reducing MMP3 and MMP13 expression in RA development.
To understand the mechanism of SLC7A5 upregulation in RA FLS, the FLS inflammatory microenvironment was studied in this study. In RA synovial fluid, a lot of inflammatory mediators are secreted by immune cells. IFN-γ and TNF-α are secreted by activated T cells [
19]. IL-17 is produced by Th17 and mast cells [
20]. Activated macrophages have been reported to secrete other cytokines like IL-1β, IL-6, and TNF-α [
21]. These inflammatory cytokines are well known to accelerate the process of matrix degradation in RA. Hence, we used a series of cytokines to stimulate FLS and observed the behavior of SLC7A5 in vitro. We found that IL-1β-treated cells exhibited upregulated
SLC7A5 expression via NF-κB activation. Yoon et al. have recently demonstrated that SLC7A5 expression was upregulated by LPS in RA monocytes [
22]. More interestingly, HIF-2α was found binding to the
Slc7a5 promoter and increased the Slc7a5 expression in normal liver and lung tissues [
12]. Hypoxia is an important micro-environmental characteristic of RA, and hypoxia-inducible factors (HIFs) are key transcriptional factors that are highly expressed in RA synovium and are reported to modulate the expression of mediators that are involved in cellular infiltration of the synovial tissue, cartilage destruction, and bone erosion [
23]. These findings suggested the potential role of SLC7A5 in RA pathogenesis.
To investigate the function of overexpressed SLC7A5 in RA, we focused on the amino acid transportability of this molecule. We hypothesized that amino acid transport may modulate the FLS infiltration in RA. Our data shows that siRNA knockdown0 or antibody blocking of SLC7A5 suppressed the protein levels of MMP3 and MMP13 significantly. And these two proteinases are responsible to mediate the cleavage of aggrecan and collagen in damaged cartilage [
24]. Raposo et al. found that using the amino acid transport inhibitor 2-(methylamino) isobutyric acid could attenuate the severity of arthritis in diseased animals [
25], suggesting that the amino acids and their transporters might be the key factor in RA pathogenesis. As we mentioned before, some studies suggest the synovial infiltration and tumor cell-like behavior of FLS in RA. Likewise, many SLC7A5-related tumor studies have demonstrated a critical role of SLC7A5 in tumor migration and invasion. For example, Janpipatkul et al. showed that the downregulation of the SLC7A5 expression suppressed cholangiocarcinoma cell migration and invasion [
26]. Further, SLC7A5 regulated by miR-126-3p exhibited a strong association with cellular migration and metastasis in thyroid cancer cells [
27]. All these data support the participation of SLC7A5 in regulating cellular infiltration and invasion, and MMP expression in RA FLS. We observed that antibody blocking of SLC7A5 suppressed the MMP3 and MMP13 expression only at the protein level, and not affected their mRNA levels, suggesting that blocked SLC7A5 might contribute to modulate the amino acid-sensing mechanism. mTOR is a central nutrient sensor that signals a cell to grow and proliferate. One of the important functions of the mTOR complex (mTORC) is to maintain the available amino acid pool by regulating protein translation [
28]. Dysregulation of the mTOR pathway leads to aberrant protein translation which manifests into various pathological states [
29]. We showed that knocked down by siRNA, SLC7A5 could decrease the protein level of MMP3 and MMP13 via suppressing the phosphorylation of mTOR and P70S6K significantly. MMP3 and MMP13 expression was also decreased significantly when we used rapamycin to inhibit mTORC1 signaling. Ito et al. have shown that knockdown of Raptor (a component of mTORC1), reduced the MMP3 and MMP13 expression in nucleus pulposus cells of the human intervertebral disc treated with IL-1β [
30]. Cejka et al. showed that inhibited mTORC1 via sirolimus or everolimus could reduce synovial osteoclast formation and protect against local bone erosions and cartilage loss [
31]. Curcumin, another inhibitor of mTOR signaling, was also reported to alleviate rheumatoid arthritis-induced inflammation and synovial hyperplasia by reducing inflammatory mediators like IL-1β, TNF-α, MMP-1, and MMP-3 [
32]. SLC7A5 mediates amino acid flux and activates mTORC1 signaling in tumors as well as immune cells [
12,
22]. SLC7A5 knockout cancer cell lines showed decreased P70S6K phosphorylation and compromised cell proliferation [
33]. Intracellular amino acids induce mTOR phosphorylation which activates its downstream target P70S6K [
34]. P70S6K plays important roles in cell growth, proliferation, and differentiation by regulating cell cycle progression and ribosome biogenesis [
35,
36]. It phosphorylates multiple components of the translational machinery and related regulators and increases translation by stimulating rRNA and tRNA synthesis [
37]. It is reported that SLC7A5 together with SLC3A2 participates in transporting large neutral amino acids such as Phe, tyrosine, or Trp into the cell [
38]. As shown in the present study, FLS cultured in a Trp-deficient medium exhibited a decreased expression of MMP13. In contrast, when FLS were cultured in the Trp supplemented medium, the expression of MMP13 was increased. At the same time, the supplemented Trp could activate the mTOR complex 1 signaling. Likewise, Kyn (a metabolite of Trp) was also able to activate mTOR signaling in T cells of SLE patients [
39], suggesting that amino acids and their metabolites may also play important roles in regulating autoimmune response. Moreover, the upstream of mTOR signal, Akt activation, could also induce MMP3 and MMP13 expression in the microglia [
40]. In the present study, we also found that SLC7A5 siRNA could cause increased IL-10 and TIMP1 and decreased PDGF-BB protein production in RA FLS supernatant. Previously, it was reported that IL-10 works as an anti-inflammatory cytokine, inhibits VEGF [
41], and suppresses inflammatory response [
42]. The increased Timp1 has been reported to ameliorate cartilage destruction in collagen-induced arthritis in rats [
43]. All these findings are in accordance with our hypothesis that the amino acid transporter SLC7A5 takes part in cellular invasion and regulates protein levels of MMP3 and MMP13 via the mTOR signaling pathway in RA FLS. Downregulated or blocked SLC7A5 in FLS could serve as an anti-inflammatory molecule and a potential therapeutic target in arthritis.