The synovium of RA exhibits endothelial hyperplasia and transformation into pannus tissue that destroys the articular cartilage and bone, with occasional lymphatic-like aggregates. A large number of inflammatory cytokines (IL-1β, TNF-ɑ, etc.) stimulate FLS to proliferate abnormally and exhibit anti-apoptosis. The imbalance between FLS anti-apoptotic and pro-apoptotic factors increases the number of FLS considerably, which directly leads to synovial hyperplasia. The FLS in the synovial lining layer is increased from the normal 1–3 to 10–15 cell layers (Neumann et al., 2010). The proliferated FLS develops into lymphoid-like structures, interacting with immune cells to form lymphoid organs and releasing pro-inflammatory factors and inflammatory mediators. Growth factors, such as platelet-derived growth factor (PDGF), transforming growth factor-β (TGF-β), and stimulatory cytokines in the synovial tissue, induce FLS proliferation through the activation of the signaling pathway. Along with the in situ proliferative capacity of FLS, the expression of anti-apoptotic molecules is also increased. The anti-apoptotic molecule FLICE inhibitory protein (FLIP) suppresses intracellular apoptosis-triggering cystatase-8, decreasing apoptosis and causing synovial proliferation (Bartok and Firestein, 2010).

Cartilage and bone destruction are hallmarks of RA. MMPs expressed by FLS degrade the chondral matrix, leading to impaired nutrient supply to the articular cartilage and tissue joint destruction.

FLS are known to contribute significantly to RA by secreting inflammatory chemokines that interact with synovial infiltrating cells. The chemokines secreted by FLS, including, CXC motif chemokine 8 (CXCL-8), CXCL-10, and CC motif chemokine ligand 2 (CCL2), can recruit a range of immune cells into the synovial tissue. Then, the inflammatory mediators, for example, IL, TNF-α, and TGF-β1, from these immune cells in turn stimulate FLS activation, resulting in a vicious circle. Macrophages are constantly affected by inflammatory stimuli and participate in the development of chronic synovitis, bone erosion, and cartilage erosion. Macrophages express a lot of molecules on their surface, such as Fc-gamma receptors (FcγRs), TLR, and the major histocompatibility complex class II (MHCII), which in turn, regulate their own activities, activate other cells in the local microenvironment, or attract immune cells outside the joint. TNF-α, IL-6, IL-1β, IL-23, and a wide range of CXCL and CCL chemokines promote and maintain inflammation by recruiting and activating polymorphonuclear leukocytes, T cells, B cells, or monocytes.

The MAPK signaling pathway is associated with various kinases, such as P38, c-Jun N-terminal kinase (JNK), and extracellular regulated protein kinases (ERKs), which are involved in the proliferation, apoptosis, and migration of FLS, with the addition of cytokine secretion (Harigai et al., 2004; Tang et al., 2019). ERK is involved in the secretion of certain cytokines and cell proliferation and differentiation through the regulation of B-cell lymphoma 2 (Bcl-2). JNK decreases proteoglycan synthesis and enhances MMP-13 synthesis, which are necessary for bone deterioration and joint inflammation. p38 is associated with the cytokine secretion of MMP. Through inhibiting p38, MMP reduces cartilage degradation and inhibits osteoclast formation. Additionally, the MAPK pathway contributes to the FLS’s increase in TNF-α expression, amplifying inflammatory signals, inducing FLS proliferation, aggravating inflammation, and damaging joints (Zuo et al., 2015; Kadkhoda et al., 2016). An increasing number of studies have shown that the MAPK pathway is activated in immune and autoimmune response conditions, regulating the cell responses of division, differentiation, apoptosis, inflammation, and stress and also participating in the activation of FLS (Müller-Ladner et al., 2007; Bustamante et al., 2017). In addition, MAPK activates downstream transcription factors that promote synovial cell proliferation and chondrocyte apoptosis. It also leads to high expression of multiple MMPs in synovial cells and chondrocytes and overhydrolysis of the extracellular matrix, resulting in joint damage. Therefore, MAPK is one of the most studied targets to inhibit RA-FLS (Wang et al., 2010).

Here, we review the synthetic small molecule compounds and natural products in the recent 3 years targeted to MAPK for FLS in Table 1, and the natural products regulating MAPK are shown in Figure 3. It is important to note that the majority of drugs affected numerous signaling pathways and multiple targets. As an MAPK downstream effector, p38 is considered a possible target for RA, but only few p38 inhibitors have been tested in humans. Tacrolimus as a macrolide calcineurin inhibitor immunosuppressant drug decreased the production of angiopoietin-1 (Ang1), tyrosine-protein kinase receptor (Tie-2), and VEGF in human FLS by preventing the activation of the IL-1β-mediated JNK and p38 MAPK pathways. Sugiura et al.’s (2020) study was very interesting. They found that glycogen synthase kinase 3 (GSK-3) inhibitors significantly reduced synovial fibroblast migration after 72 h and decreased Akt phosphorylation [Ser (473)] after 48 h in vitro, which might have therapeutic efficacy targeting the invasion and migration of synovial fibroblasts. Also, 3′3-diindolylmethane exhibited the possibility of anti-RA-FLS activitiy in vivo and in vitro (Du et al., 2019). The small molecule compounds reported in recent years that could alter FLS in vivo and in vitro were elutriated extirpate, dasatinib, 4-phenylbutyric acid, and 3-(4-hydroxy-3-methoxy-phenyl)-1-3-[1]-phenyl-propenone. Unfortunately, these medications are still in the laboratory stage. Because of their poor performance, p38 inhibitors have limited efficacy in RA treatment. Also, blocking p38’s downstream had a compensatory effect on other kinases, so alternative options for p38 have been progressively explored (Guma et al., 2012). Regulation of MAPK kinases upstream of p38, the human mitogen-activated protein kinase kinase (MKK), such as MKK6 and MKK1, could selectively block the production of MMPs and pro-inflammatory cytokines in FLS (Hammaker et al., 2012). In addition, ubiquitin D might be considered a possible therapeutic target for RA-FLS (Chen et al., 2023).

In natural products in Table 1 and Figure 3, alkaloids and flavonoids were more frequently reported and studied for their effects on the MAPK signaling pathway of FLS. Other categories, such as iridoids and saponins, were also found to have an impact. It is well known that flavonoids possess anti-oxidant and anti-inflammatory properties. Flavonoids can inhibit the inflammatory response and reduce the symptoms of inflammation while scavenging free radicals, reducing oxidative stress, and protecting cells from oxidative damage. Flavonoids usually inhibit FLS proliferation, migration, and invasion by inhibiting p38 and JNK. To our surprise, alkaloids also showed up significantly in the treatment of FLS. Preparations of berberine and paclitaxel were available for clinical use, but they have no indication for the treatment of RA.

Triptolide and tetrandrine from Tripterygium wilfordii Hook F. and Stephania tetrandra root, respectively, have anti-rheumatic effects in the classic sense. Tripterygium glycoside preparations have been clinically used for the treatment of RA. As the representative, we concentrate on triptolide, which has been studied more and has been proven to have multiple signaling pathways. The treatment with triptolide decreased the expression of phosphorylated JNK that TNF-α-produced, but it had no effect on the expression of phosphorylated p38 or ERK (Yang et al., 2016) and reduced FLS migration and invasion by targeting the JNK/MAPK signaling pathway (Tang et al., 2020). Triptolide dramatically increased the p-Akt/Akt ratio, and inhibiting the PI3K/Akt signaling pathway in MH7A cells caused autophagy to be triggered, indicating that triptolide repressed autophagy via activating p-Akt/Akt (Xie et al., 2019). Other natural products, such as Paris saponin VII/Chonglou, geniposide, and gentiopicroside, shown in Table 1, also have the potential to regulate FLS against RA. However, it is currently in the experimental stage.

As a major signaling transcription factor, NF-κB contributes to synovial inflammation, proliferation, and decay in bones in RA and regulates inflammatory gene expression and cell proliferation. Both innate and adaptive immune cells include NF-B, which is a key mediator of the stimulation of pro-inflammatory genes (Liu et al., 2017). In a normal situation, NF-κB is bound to its repressor protein IκB and not activated. The nuclear-localization sequence (NLS) that belongs to NF-κB is covered by the IκB unable to undergo nuclear translocation. However, in RA due to the activators (TNF-α, IL-17, etc.), IκB is phosphorylated, ubiquitinated by IκB kinase, and eventually degraded by the enzyme, releasing NF-κB. Following that, NF-κB p65 enters the nucleus and combines with target genes (Aupperle et al., 1999). The production of inflammatory mediators such as TNF-α, COX-2, and IL-1β increases as a result of this nuclear translocation in the synovium. Those activated sustaining states lead to massive abnormal activation of FLS (Saravanan et al., 2014). NF-κB p65 regulates apoptosis and inhibits protein expression, which has an antagonistic effect on apoptosis in FLS (Kadkhoda et al., 2016), leading to synovial hyperplasia and aggravating joint destruction (Yin et al., 2015). In addition, p38 mediates IκB phosphorylation, which is involved in regulating NF-κB activation (Carter et al., 1999; Kaminska, 2005).

The small molecule drugs and natural products targeted at NF-κB in recent 3 years are summarized in Table 2, and the classification of the natural products is in Figure 3. There have been many studies on small molecule compounds that modulate FLS in the NF-κB signaling pathway, such as TAK-242 (Samarpita et al., 2020), CKD-506 (Park et al., 2020), and synthetic derivatives from natural products that also showed the activity of inhibiting proliferation. For example, oxymatrine hydrazone synthesized from oxidized bitter ginseng induced apoptosis and prevented TNF-α-mediated enhanced viability of RA-FLS (Zhang et al., 2021). Paeoniflorin-6′-O-benzene sulfonate (CP-25), a paeoniflorin derivative, had the ability to decrease membrane expression and the combination of these proteins (Wang et al., 2020; Wang et al., 2023). Edaravone, roflumilast, sorafenib, dexmedetomidine, and alogliptin have been used clinically, without the indication for the treatment of RA. The existing experiments showed that they have the anti-proliferation ability of FLS and were worthy of inclusion in the secondary development of drugs. In the natural products in Figure 4, flavonoids still predominated, such as diosmetin, icariin, isoginkgetin, and tectoridin. In a similar situation with the MAPK inhibitions for RA-FLS, these natural products were in the experimental stage. In addition, some inhibitors modulated both NF-κB and MAPK pathways to regulate FLS activity, such as tectoridin and corilagin.

JAK/STAT signaling has been instrumental in regulating immune and inflammatory responses. The JAK/STAT pathway can be segmented into three components: receptor-associated tyrosine kinase, JAK tyrosine kinase, and STAT transcription factor. The JAK kinase activates JAK upon receptor binding, leading to JAK-mediated phosphorylation of STAT. Among the STAT family, STAT1 and STAT3 serve as the primary activators (Kim et al., 2011). The expression and activity of STAT1 are elevated in the initial synovial tissue of RA, while STAT3 facilitates the survival of synovial fibroblasts. Elevated STAT3 expression contributes to the inhibition of programmed cell death-induced anti-apoptotic molecule expression, blocks apoptosis in RA-FLS, and promotes RA synovial thickening (Yang et al., 2017). The JAK/STAT pathway is also involved in regulating the response of RA-FLS to pro-inflammatory cytokines and plays an essential role in the pro-inflammatory response and invasive behavior of FLS (Diller et al., 2019).

Inhibitors of JAKs could block the activation of STATs in RA-LS in the synthesis of various drugs and in the study of natural products. We included the synthetic small molecule compounds and natural products in the last 3 years in Table 3. Tofacitinib is a Food and Drug Administration (FDA)- and European Medicines Agency (EMA)-approved JAK inhibitor that effectively treats RA (Vomero et al., 2022). The synthetic small molecule compounds of peficitinib, fingolitinib, takinib, tolvamycin, baricitinib, and abatinib all demonstrated monotherapy effectiveness in clinical trials in RA. The synthetic JAK inhibitors appeared to be an important treatment choice for difficult-to-treat RA patients and researchers (Kubo et al., 2023). Momelotinib had no indication for the treatment of RA in the clinic, but could inhibit the proliferation and migration of FLS (Srivastava et al., 2022). On the contrary, there are few research reports on the natural products in the JAK/STAT signal pathway.

The PI3K/Akt signaling pathway is involved in regulating cell growth, proliferation, differentiation, and survival and is associated with the production of pro-inflammatory cytokines, degrading enzymes of the extracellular matrix, and other factors in FLS. The activation of PI3K induces the phosphorylation of Akt and p-Akt. As a downstream effector, it can be involved in FLS invasion by regulating the transcriptional levels of MMPs. The Akt phosphorylation also activates downstream mTOR complex 1 (mTORC1). mTORC1 translates mRNA into proteins to regulate the cell activities of metabolism, growth, and differentiation and is involved in RA-FLS proliferation and survival (Wendel et al., 2004; Malemud, 2013).

Table 4 is a summary of the synthetic small molecules and natural drugs that have been developed recently that target PI3k/Akt. Metformin, a drug used to treat type 2 diabetes, has been shown to have a protective effect against the development of RA (Liang et al., 2023), and RA-FLS proliferation is inhibited by metformin in a dose- and time-dependent manner (Chen et al., 2019). The natural products targeted at PI3k/Akt regulating FLS came from a variety of sources. Against the development of inflammatory arthritis, ginger is a preventive substance. There was evidence that ginger helped reduce RA-related joint pain (Al-Nahain et al., 2014). The active ingredients of ginger, 6-shogaol, and 8-shogaol reduced the production of TNF-α, IL-1β, IL-6, etc., prevented migration, invasion, and population growth, and ameliorated joint destruction in mice (N. Li et al., 2023; Jo et al., 2022).

The Wnt signaling cascade participates in regulating the growth, differentiation, production, and apoptosis of osteoblasts. The conventional Wnt/β-catenin cascade, Wnt/Ca₂ ⁺ signaling cascade, and Wnt/JNK signaling cascade coordinate with each other to regulate the dynamic balance between osteoclasts and osteoblasts. Once the balance is disturbed, it might lead to bone erosion and bone destruction (Walsh et al., 2009; De, 2011; Deal, 2012). Studies had shown that the growth Wnt3a/5a proteins could activate the Wnt signaling cascade as well as downstream genes, thus increasing fibronectin expression and promoting FLS function. The aforementioned processes also promoted the proliferation of RA synovial tissue without pro-inflammatory factors (Kim et al., 2010; Rabelo Fde et al., 2010; Maeda et al., 2013). Researchers (Cici et al., 2019) suggested that the inflammatory activation of the Wnt pathway might inhibit T-cell function and exacerbate the immune response [181]. In the recent 3 years, we inquired natural products, including paeoniflorin (Yang et al., 2022), 7-hydroxycoumarin (Umbelliferone) (Cai et al., 2022; Cai et al., 2022), and penta-acetyl geniposide (Cai et al., 2021).