Rheumatoid arthritis (RA) is an autoimmune disease of unknown etiology, with persistent synovial inflammation, massive release of inflammatory mediators, and progressive cartilage and bone erosion as its main pathological features (Mcinnes and Schett, 2011, Nygaard and Firestein, 2020). This feature leads to progressive and large-scale structural destruction of joints, which in turn leads to chronic pain, dysfunction and other clinical symptoms and ultimately has a serious impact on the physical and mental health and quality of life of patients (Schett, 2017). Therefore, early diagnosis and treatment are crucial for preventing joint injury and dysfunction in RA (Li et al., 2024). Fibroblast-like synovial cells (FLSs) are the main contributors to synovial hyperplasia and inflammation in RA (Jay et al., 2000). Their activated type has tumour-like biological characteristics, manifested as uncontrolled proliferation, abnormal secretion of pro-inflammatory cytokines/chemokines, and enhanced pathological invasion ability, thereby driving typical pathological processes such as synovial inflammatory response, synovial hyperplasia, and pannus formation (Ma et al., 2019). During the course of RA, FLSs mediate the destruction of articular cartilage and subchondral bone by forming an inflammatory cascade reaction through the production of pro-inflammatory cytokines (such as IL-6, TNF-α and IL-1β), granulocyte-macrophage colony-stimulating factor (Reparon-Schuijt et al., 2000), and chemokines, thereby accelerating the progression of RA (Fang et al., 2020, Damerau and Gaber, 2020). Therefore, inhibiting the functional activation of RA-FLSs can effectively control synovial hyperplasia and inflammation, prevent cartilage erosion, and ultimately alleviate or delay the progression of joint injury in RA.

Glycolysis is a metabolic pathway that converts glucose to pyruvate, which can be transferred from oxidative phosphorylation of the tricarboxylic acid (TCA) cycle and converted to lactic acid by lactate dehydrogenase (LDH) in the cytoplasm (Audzeyenka et al., 2024). Studies have shown that a high glycolysis rate can regulate immune function through multiple mechanisms (Nathan and Ding, 2010). For example, increased glycolysis may promote the polarization and/or activation of immune cells (O'neill et al., 2016). Recently, the intricate interplay between metabolic pathways and cell signaling has also become an important focus in the field of inflammation research. Various glycolytic enzymes participate in aerobic glycolysis, among which the metabolic switch with the dynamic regulation of key glycolytic enzymes as the core has attracted much attention. During this process, pyruvate kinase, as a key rate-limiting enzyme catalyzing the final reaction and rate-limiting reaction in the glycolytic pathway, can significantly alter metabolic flux. There are four pyruvate kinase subtypes (M1, M2, L and R) in mammals, but immune cells preferentially express the PKM1 and PKM2 subtypes (Alquraishi et al., 2019). PKM1 is expressed in highly differentiated tissues, while PKM2 is expressed in all cells with a high rate of nucleic acid synthesis (Jurica et al., 1998). Among these subtypes, PKM2 plays a crucial role in metabolic reprogramming (Wang et al., 2020, Zhang et al., 2019). It can promote glycolysis by redirecting carbon from glucose to macromolecular biosynthesis, ultimately leading to metabolic reconfiguration of increased glucose uptake and excessive lactic acid accumulation (Zheng et al., 2021, Shirai et al., 2016). The expression profile of PKM2 has significant pathological correlation. It is not only widely present in various malignant tumours, but also shows abnormally high expression in specific physiological states (such as the cell proliferation phase) and activated immune cells (Gui et al., 2013). In recent years, research has gradually revealed the core regulatory position of PKM2 in the metabolic immune axis. As a key molecule in immune cell metabolic reprogramming, PKM2 is deeply involved in the molecular regulatory network of inflammatory initiation and progression by driving the remodeling of aerobic glycolytic metabolic flow (Dhanesha et al., 2022). It is particularly noteworthy that in classical inflammatory models (such as lipopolysaccharide (LPS)-activated macrophages), PKM2-dependent glycolysis can significantly up-regulate the release of injury-related molecular patterns such as IL-1β and HMGB1, resulting in an inflammatory cascade (Palsson-Mcdermott et al., 2015). Shen's team further found that PKM2-mediated metabolic remodeling can exacerbate RA-related autoimmune responses by promoting Th17 cell lineage-directed differentiation (Shen et al., 2022). In terms of clinical relevance, metabolomics analysis shows that the levels of lactate, pyruvate, and other glycolytic metabolites—such as glucose-6-phosphate and fructose-1,6-bisphosphate—are significantly elevated in the synovial fluid and serum of RA patients, and are positively correlated with the disease activity score (DAS28). This suggests that glycolytic abnormalities are not only part of the pathologic mechanism of RA but may also serve as potential biomarkers for monitoring disease progression and treatment response (Chang and Wei, 2011). Meanwhile, existing animal model studies have shown that small molecule inhibitors targeting PKM2 or specific metabolic interventions can effectively reduce synovial inflammatory responses and inhibit the destruction of articular cartilage and bone tissues (Wu et al., 2023). These findings suggest that PKM2 is not only a key regulator in the pathogenic mechanism of RA but also a potential translational target for the development of new therapeutic strategies. However, there is still a research gap regarding the specific mechanisms by which PKM2 regulates the metabolic-inflammatory interactions of RA-FLSs, the core effector cells. These interactions subsequently affect the pathological microenvironment of the synovial membrane. Therefore, regulating the function of PKM2 in RA-FLSs and inhibiting its abnormal activation of glycolysis and inflammatory signal transduction may become effective strategies for treating RA.

Protein kinase B/mammalian rapamycin target protein (Akt/mTOR) is an important intracellular signal transduction hub, and its abnormal activation has been found to participate in the homeostasis of various pathophysiological processes by regulating downstream effector molecules. Research has confirmed that this pathway can mediate key biological events such as inflammation, cell apoptosis, and autophagy through phosphorylation cascade reactions (Fruman et al., 2017). It is worth noting that PKM2, as a key enzyme in glycolysis, has been found to have a bidirectional regulatory relationship with the Akt/mTOR signaling network. During the process of energy metabolism reprogramming, PKM2 can not only enhance the phosphorylation of serine at position 473 of Akt through allosteric effect (Sun et al., 2011). Recent studies have further revealed the pathological significance of the PKM2-Akt/mTOR axis in neurological diseases. In the status epilepticus model, PKM2 induces the activation of microglia TLR4/MyD88 signaling, driving Akt/mTOR-dependent NLRP3 inflammasome assembly, leading to the excessive release of pro- inflammatory cytokines such as IL-1β and IL-18 (Cui et al., 2024). Meanwhile, the role of PKM2 in immune regulation is gradually being clarified. For example, PKM2 can drive the differentiation of Th17 cells by phosphorylating STAT3, thereby promoting autoimmune inflammation (Damasceno et al., 2020). Clinical evidence also indicates that the overactivation of the Akt/mTOR pathway in RA patients is closely related to the degree of joint destruction and poor response to traditional DMARDs such as methotrexate (Xu et al., 2015). Therefore, targeting the PKM2-Akt/mTOR axis, which plays a key role in the underlying pathogenic mechanisms, may offer a potential strategy to improve the efficacy of treatment for refractory RA. Although existing studies have revealed the important role of PKM2 in inflammatory diseases, the specific mechanisms of its involvement in the pathogenesis of RA remain unclear. In particular, how the metabolic regulatory network mediated by PKM2 shapes the inflammatory phenotype in RA-FLSs, and how its synergistic effect with the Akt/mTOR pathway drives bone and cartilage destruction, still requires further systematic elucidation.

Our study aims to explore the role and related mechanisms of PKM2 in synovial inflammation and joint destruction in the pathogenesis of RA. We have demonstrated that PKM2 activates the Akt/mTOR signaling pathway, which subsequently enhances glycolytic metabolism and promotes transcription and release of inflammatory cytokines, thereby promoting synovial inflammation and joint destruction in RA. These findings not only elucidate the central role of PKM2 as a metabolic checkpoint of RA, but also construct a pathological regulatory framework of the trinity of metabolic enzymes-signaling pathways-phenotypic remodeling, providing an innovative target system for the development of precision treatment strategies based on metabolic reprogramming interventions.