Osteoarthritis (OA) is the most common degenerative joint disease, and it is characterized by articular cartilage loss. In part, OA is caused by aberrant anabolic and catabolic activities of the chondrocyte, the only cell type present in cartilage. These chondrocyte activities depend on the intra- and extracellular signals that the cell receives and integrates into gene expression. The key proteins for this integration are transcription factors. A large number of transcription factors exist, and a better understanding of the transcription factors activated by the various signaling pathways active during OA can help us to better understand the complex etiology of OA. In addition, establishing such a profile can help to stratify patients in different subtypes, which can be a very useful approach towards personalized therapy. In this review, we discuss crucial transcription factors for extracellular matrix metabolism, chondrocyte hypertrophy, chondrocyte senescence, and autophagy in chondrocytes. In addition, we discuss how insight into these factors can be used for treatment purposes.

Osteoarthritis (OA) is a degenerative joint disease characterized by irreversible cartilage damage, inflammation and altered chondrocyte phenotype. Transforming growth factor-β (TGF-β) signaling via SMAD2/3 is crucial for blocking hypertrophy. The post-translational modifications of these SMAD proteins in the linker domain regulate their function and these can be triggered by inflammation through the activation of kinases or phosphatases. Therefore, we investigated if OA-related inflammation affects TGF-β signaling via SMAD2/3 linker-modifications in chondrocytes. We found that both Interleukin (IL)-1β and OA-synovium conditioned medium negated SMAD2/3 transcriptional activity in chondrocytes. This inhibition of TGF-β signaling was enhanced if SMAD3 could not be phosphorylated on Ser213 in the linker region and the inhibition by IL-1β was less if the SMAD3 linker could not be phosphorylated at Ser204. Our study shows evidence that inflammation inhibits SMAD2/3 signaling in chondrocytes via SMAD linker (de)-phosphorylation. The involvement of linker region modifications may represent a new therapeutic target for OA.

During osteoarthritis (OA), hypertrophy-like chondrocytes contribute to the disease process. TGF-β’s signaling pathways can contribute to a hypertrophy(-like) phenotype in chondrocytes, especially at high doses of TGF-β. In this study, we examine which transcription factors (TFs) are activated and involved in TGF-β-dependent induction of a hypertrophy-like phenotype in human OA chondrocytes. We found that TGF-β, at levels found in synovial fluid in OA patients, induces hypertrophic differentiation, as characterized by increased expression of RUNX2, COL10A1, COL1A1, VEGFA and IHH. Using luciferase-based TF activity assays, we observed that the expression of these hypertrophy genes positively correlated to SMAD3:4, STAT3 and AP1 activity. Blocking these TFs using specific inhibitors for ALK-5-induced SMAD signaling (5 µM SB-505124), JAK-STAT signaling (1 µM Tofacitinib) and JNK signaling (10 µM SP-600125) led to the striking observation that only SB-505124 repressed the expression of hypertrophy factors in TGF-β-stimulated chondrocytes. Therefore, we conclude that ALK5 kinase activity is essential for TGF-β-induced expression of crucial hypertrophy factors in chondrocytes.

Osteoarthritis (OA) is the most prevalent joint disease, and it is characterized by cartilage degeneration, synovitis, and bone sclerosis, resulting in swelling, stiffness, and joint pain. TAM receptors (Tyro3, Axl, and Mer) play an important role in regulating immune responses, clearing apoptotic cells, and promoting tissue repair. Here, we investigated the anti-inflammatory effects of a TAM receptor ligand, i.e., growth arrest-specific gene 6 (Gas6), in synovial fibroblasts from OA patients. TAM receptor expression was determined in synovial tissue. Soluble Axl (sAxl), a decoy receptor for the ligand Gas6, showed concentrations 4.6 times higher than Gas6 in synovial fluid of OA patients. In OA fibroblast-like synoviocytes (OAFLS) exposed to inflammatory stimuli, the levels of sAxl in the supernatants were increased, while the expression of Gas6 was downregulated. In OAFLS under TLR4 stimulation by LPS (Escherichia coli lipopolysaccharide), the addition of exogenous Gas6 by Gas6-conditioned medium (Gas6-CM) reduced pro-inflammatory markers including IL-6, TNF-α, IL-1β, CCL2, and CXCL8. Moreover, Gas6-CM downregulated IL-6, CCL2, and IL-1β in LPS-stimulated OA synovial explants. Pharmacological inhibition of TAM receptors by a pan inhibitor (RU301) or by a selective Axl inhibitor (RU428) similarly abrogated Gas6-CM anti-inflammatory effects. Mechanistically, Gas6 effects were dependent on Axl activation, determined by Axl, STAT1, and STAT3 phosphorylation, and by the downstream induction of the suppressors of the cytokine signaling family (SOCS1 and SOCS3). Taken together, our results showed that Gas6 treatment dampens inflammatory markers of OAFLS and synovial explants derived from OA patients associated with SOCS1/3 production.

Osteoarthritis (OA) is characterized by progressive articular cartilage loss. Human mesenchymal stromal cells (MSCs) can be used for cartilage repair therapies based on their potential to differentiate into chondrocytes. However, the joint microenvironment is a major determinant of the success of MSC-based cartilage formation. Currently, there is no tool that is able to predict the effect of a patient's OA joint microenvironment on MSC-based cartilage formation. Our goal was to develop a molecular tool that can predict this effect before the start of cartilage repair therapies. Six different promoter reporters (hIL6, hIL8, hADAMTS5, hWISP1, hMMP13, and hADAM28) were generated and evaluated in an immortalized human articular chondrocyte for their responsiveness to an osteoarthritic microenvironment by stimulation with OA synovium-conditioned medium (OAs-cm) obtained from 32 different knee OA patients. To study the effect of this OA microenvironment on MSC-based cartilage formation, MSCs were cultured in a three-dimensional pellet culture model, while stimulated with OAs-cm. Cartilage formation was assessed histologically and by quantifying sulfated glycosaminoglycan (sGAG) production. We confirmed that OAs-cm of different patients had significantly different effects on sGAG production. In addition, significant correlations were obtained between the effect of the OAs-cm on cartilage formation and promoter reporter outcome. Furthermore, we validated the predictive value of measuring two promoter reporters with an independent cohort of OAs-cm and the effect of 87.5% of the OAs-cm on MSC-based cartilage formation could be predicted. Together, we developed a novel tool to predict the effect of the OA joint microenvironment on MSC-based cartilage formation. This is an important first step toward personalized cartilage repair strategies for OA patients. Impact statement: We describe the development of a novel molecular tool to predict if an osteoarthritis joint microenvironment is permissive for cartilage repair or not. Such a tool is of great importance in determining the success of mesenchymal stromal cell-based cartilage repair strategies.