In chondrocytes, the hypoxic environment of cartilage leads to the generation of over 75% of total cellular ATP through glycolysis and the remaining energy through oxidative phosphorylation (OXPHOS). Glucose is considered the main metabolic fuel and structural precursor in this process (2). During glycolysis, a single molecule of glucose can generate two molecules of pyruvate while producing two molecules of ATP (34). Pyruvate can then reach the mitochondria in aerobic circumstances, where pyruvate dehydrogenase complexes convert it into acetyl-CoA, thus integrating it into the tricarboxylic acid cycle (TCA). This cycle produces GTP, which is the energy equivalent of ATP, as well as NADH and FADH₂, crucial electron carriers in the electron transport chain for oxidative phosphorylation. This process ultimately leads to the generation of ATP (35). In the cytosol, pyruvate is converted to lactate to restore the NAD⁺ levels essential for the continuous synthesis of ATP by phosphorylation of the substrate via anaerobic glycolysis (31). OA chondrocytes exhibit an elevated anaerobic glycolysis rate (36). In Table 1 , the key targets in the pathophysiology of glycolysis-related OA and their subtypes are summarized. The metabolic function of each process and its role in OA pathogenesis are outlined.

In OA chondrocytes, the glycolytic process undergoes a shift, and the GLUTs and multiple enzymes are considered to be involved in the pathogenesis. Various types of the GLUTs family, including GLUTs-1, -3, -6, -8, -9, and -10, have been identified in human chondrocytes through protein analysis (21, 22). GLUT-1 is essentially responsible for basal glucose transfer in chondrocytes, glycolysis’s first rate-limiting phase (41), which is indispensable for chondrogenesis, embryonic development, and skeletal system development (42). However, the expression of GLUT-1 is increased during hypoxia and glucose deficiency but downregulated in high-glucose circumstances (40). When glucose levels increase, chondrocytes that cannot adapt may exhibit heightened glucose absorption and subsequently generate elevated levels of reactive oxygen species (40). The disruption of cellular growth and matrix synthesis in the growth plate and articular cartilage occurs due to GLUT1 gene deletion, leading to long-term bone dysplasia and cartilage fibrosis (43, 44). Conversely, prolonged elevation of GLUT1 expression can detrimentally impact cartilage by promoting excessive glucose absorption and the accumulation of advanced glycation end-products (37–39). Thus, in the presence of pro-inflammatory cytokines, chondrocyte GLUT1 expression is remarkably elevated (41), suggesting that GLUT-1 may serve as a potential therapeutic target to treat OA.

Glycolysis is a highly regulated process wherein a multitude of enzymes play relevant roles (45). These enzymes include hexokinase (HK), pyruvate kinase (PK), phosphofructokinase (PFK), and lactate dehydrogenase A (LDHA). The subtype HK2 is an important regulator that facilitates the passage of glucose metabolism from oxidative phosphorylation to aerobic glycolysis (46). The glycolysis initial rate-limiting enzyme, HK2, is capable of catalyzing the conversion of glucose to glucose-6-phosphate (G-6-P) and is implicated in the primary glycolysis pathways (47). In OA chondrocytes, transforming growth factor beta 1 (TGF-β1) promotes HK2 expression (48). Compared to the group with healthy synovial tissue, OA synovial tissue (FLS) had a greater HK2 level of expression. In OA FLS, overexpression of HK2 increases RNA expression levels of pro-inflammatory cytokines such as IL-6, IL-8, and MMPs (49). PK catalyzes the final and rate-limiting glycolytic process to transform phosphoenolpyruvate to pyruvate and produce ATP. The isoenzyme PKM2 is increased while ATP generation is reduced in human OA chondrocytes. PKM2 inhibition can limit OA chondrocyte growth, induce cell apoptosis, and diminish COL21 and SOX9 expression levels. PKM2 overexpression causes lactate accumulation and creates an acidic microenvironment in OA chondrocytes. The acidic microenvironment impairs the production of chondrocyte matrix and may increase cartilage deterioration in OA (50), indicating that PK might play a key function in OA progression and could be an essential target to reverse OA pathogenesis.

The enzyme PFK, notably Phosphofructokinase-2/Fructose-2, 6-Bisphosphatase 3 (PFKFB3), is important for glycolysis stimulation. When chondrocytes are activated with tumor necrosis factor (TNF) or IL-1, PFKFB3 is decreased. In addition, PFKFB3 can increase chondrocyte vitality, inhibit caspase-3 activation, and stimulate the expression of aggrecan and type II collagen, which could serve as a target for treating and preventing OA (51).

LDHA is required for lactate synthesis (52, 53). Some synovial fluid studies revealed that OA patients have higher levels of lactic acid in the absence of sepsis, which suggests that LDHA is likely to play a pathogenic role in human OA (54). LDH activity and expression are greatly increased in IL-1-treated primary chondrocytes. In an inflammatory condition, LDHA can enhance the production of ROS in chondrocytes (40). Therefore, LDHA could be another therapeutic vision for OA treatment.

Additionally, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), which is primarily responsible for glucose breakdown in glycolysis, participates in a variety of cell functions, such as phosphotransferase activity, RNA export, DNA replication, gene transcription activation, and gene translocation regulation. The study revealed that the expression of GAPDH is influenced by hypoxic conditions in 3-dimensional (3D) culture (55). This association observed in 3D culture could be related to hypoxia-induced extracellular matrix formation (55). This indicates that hypoxia might contribute to the development of OA.

Chondrocyte GLUTs exhibit sensitivity to mechanical pressure or loading. Pressure loading reduces glucose transport via GLUTs. Glucose transport in chondrocytes may also be influenced by several growth factors and cytokines. Interleukin-1, transforming growth factor-1 (TGF-1), insulin-like growth factor-1, TNF, and others can enhance glucose absorption by chondrocytes via several pathways (56).

Lipids, characterized by their intricate structures such as fatty acids, glycerol, and numerous functional groups, are among the most important molecules in biology. They are essential for maintaining biological activities by performing functions including energy storage, cell membrane structure, and signaling (57). Within the human body, there are four major types of lipids: cholesterol, fatty acids (FAs), triglycerides (TGs), and phospholipids. Despite constituting less than 1% of the wet weight of adult articular cartilage, lipids are present in both the chondrocytes and matrix (58, 59).

Several studies have found that dysregulated lipid metabolism contributes to the susceptibility to OA by promoting inflammation, cartilage deterioration, and imbalances in joint tissue homeostasis (60). Specifically, elevated blood cholesterol levels have been related to generalized OA, implying that cholesterol may contribute as a risk factor for OA (61). Moreover, abnormal HDL and higher levels of total cholesterol and TG in the bloodstream have been associated with the progression of bone marrow lesions (62, 63). Bone marrow lesions can cause discomfort and may accelerate cartilage loss in the knees of OA patients (64–67).

Dyslipidemia, particularly reduced HDL levels, can impair cartilage homeostasis and contribute to OA progression (63). Decreased HDL levels result in cholesterol accumulation within the cartilage tissue, compromising the regular metabolic functions of chondrocytes and the structural integrity of the extracellular matrix. This may lead to low-grade inflammation, oxidative stress, and cartilage deterioration (68, 69). Therefore, controlling dyslipidemia and maintaining normal HDL levels may prevent or reduce the progression of OA.

However, in osteoarthritic chondrocytes, important lipid deposit reserves have been noted (58). A positive correlation exists between the gravity of OA and the quantity of intracellular lipid deposit reserves (58). The downregulation of cholesterol efflux genes such as APOA1 and ABCA1 in OA cartilage contributes to lipid buildup in chondrocytes. This disruption of cholesterol metabolism may disturb the normal lipid balance within the joint, resulting in increased inflammation and oxidative stress, both of which are known to contribute to the progression of OA. Thus, the impaired expression of lipid-regulating genes appears to have a significant impact on the etiology of OA by contributing to lipid buildup and its negative effects on cartilage health (70). According to the research of Lippiello (1991), the distribution profile of individual fatty acids in both healthy and osteoarthritic cartilage was kept at a specific level, with 85% of the total fatty acids being composed of palmitic, oleic, and linoleic acids. The study also revealed that there were no changes in cholesterol content. However, OA samples had significantly greater levels of total fatty acids and arachidonic acid, and these raised levels were linked to increasing histological severity (58). Baker (2012) discovered in the Multicenter OA Study that there is a positive correlation between synovitis and omega-6 PUFA, suggesting that higher omega-6 PUFA intake may worsen inflammation in OA. In contrast, they found an adverse association between total omega-3 PUFA levels and patellofemoral cartilage loss, indicating that a higher intake of omega-3 PUFA may have a preventive effect on cartilage health in OA (71). These findings suggest that maintaining a balance of omega-3 and omega-6 PUFA in the diet may influence the progression of OA.

The synovium consists of macrophages, fibroblasts, and endothelial cells (72). In OA, these cells can trigger the release of cartilage substances such as IL-1 and TNF-α, which generate MMPs and inhibit the production of collagen and proteoglycan, leading to low-grade synovitis (73). Inflammatory conditions may result in LDL oxidation, which is subsequently absorbed by synovial cells via scavenger receptors (72). An elevated level of LDL could trigger synovial cell activation, potentially resulting in increased synovial thickness (72). Thus, lipid metabolism is related to synovitis during OA ( Figure 1 ).

Also, cholesterol levels in OA chondrocytes are higher due to the increased synthesis of oxysterol metabolites and the activation of cholesterol hydroxylase (27).

To carry out their normal activity, chondrocytes need to be supplied with energy (99). The mitochondria produce ATP via the TCA, also known as the Krebs cycle or citric acid cycle, and OXPHOS (99). OXPHOS is essential for ATP generation in chondrocytes. The electron transport chain (ETC) establishes an imbalance of protons across the inner mitochondrial membrane (IMM) and generates mitochondrial membrane potential, which leads to complex V (also known as ATP synthase) producing ATP (100). Additionally, several protein complexes located within the inner mitochondrial membrane promote the movement of electrons and the pumping of protons along the mitochondrial respiratory chain to produce ATP (80). These complexes include NADH dehydrogenase (complex I), succinate dehydrogenase (complex II), Cyt-C reductase (complex III), and Cyt-C oxidase (complex IV). Mitochondrial dysfunction in OA can result in decreased activity of respiratory chain complexes I, II, III, and V, loss of MMP, and decreases in OXPHOS (101), leading to inflammation and IL-1β production (102).

Given the importance of glycolysis in chondrocyte metabolism, multiple researchers have concentrated on its exposure in high-glucose environments. Therefore, the ability of normal chondrocytes to respond to normal glucose levels is lost during OA, which is the absolute cause of excessive glucose absorption and possible glucose poisoning (25). Exposure to high glucose levels can lead to localized toxicity in joint tissue, causing an increase in oxidative stress, cytokines, proteolytic enzyme synthesis, and the accumulation of advanced glycation end products (AGEs) (127–129). Unfortunately, there is a scarcity of qualified research studies in animal models of diabetes that are related to OA. In a recent study, cartilage injury was seen after eight weeks of hyperglycemia in mice with streptozotocin (STZ)-induced type 1 diabetes, and there were higher levels of circulating AGEs (130). Pioglitazone, a diabetes medication, improved both anomalies. The authors concluded that the medication’s response indicated a downregulation in PPARγ expression; however, it was not so clear whether this was connected to hyperglycemia improvement or PPARγ suppression (131). A noteworthy investigation employed the diet-induced-obesity (DIO) model on the C57Bl/6 strain, meticulously assessing physiological parameters and histological effects related to OA (132). This study related a high-fat diet (60% calories) with the meniscal ligament damage paradigm to produce OA. The mice group that obtained both the high-fat diet and ligament injury had higher OA scores (increased joint degradation). However, because hyperglycemia was not developed until the final month of the experiment, it is uncertain whether hyperadiposity (133) or increasing hyperglycemia was the main factor behind increased OA development. This essential study describes that acceleration of joint degradation is related to metabolic abnormalities commonly observed in diabetes mellitus (DM) patients (131). Increased proteoglycan degradation has been found in the non-articular connective tissues of diabetic animals (134). The composition of cartilage’s ECM significantly affects its biomechanical qualities. Furthermore, there is some evidence that metabolic problems related to diabetes affect the cartilage ECM. Previous experiments in animal models of diabetes demonstrated decreased collagen formation (135) and enhanced proteoglycan degradation. Thus, these works show glycolytic metabolism alterations in chondrocytes in diabetes-related OA.

In an in vitro study, Laiguillon (2015) used diverse methods such as (14C)-2-deoxyglucose to assess glucose uptake, quantitative RT-PCR, and ELISA/EIA to observe the expression and release of pro-inflammatory mediators, and ROS and nitric oxide (NO) production were measured. Under high-glucose conditions, they showed enhanced articular chondrocyte glucose absorption, specifically in response to IL-1β stimulation (127). IL-1β has been shown to promote GLUT-1 and GLUT-9 production in chondrocytes (21). They obtained comparable results for GLUT-1. Under ordinary glucose circumstances, IL-1β only moderately promoted glucose absorption despite increased GLUT-1 expression. This absorption was abundantly boosted when cells were generated under high glucose conditions (127). Also, in another study, an author observed that OA-affected chondrocytes exposed to high glucose levels were unable to downregulate GLUT-1, resulting in higher glucose accumulation inside the chondrocytes and the production of more ROS, which is deleterious to the ECM (136), which leads to mitochondrial dysfunction and cartilage degradation. Excessive oxidative stress may also result from the crucial cellular transfer of glucose (131). Therefore, these studies proved that chondrocytes cannot downregulate glucose concentration under high glucose conditions, but they absorb it, which may deteriorate cartilage, thus enhancing OA progression.

It has been noted that OA and lipid metabolism are closely related (137, 138). Disrupted lipid metabolism is linked to obesity (139). Obesity is distinguished by high joint loading and aberrant lipid profiles, such as dyslipidemia (60). Obesity-related dyslipidemia is defined by elevated plasma levels of TGs, low levels of HDL cholesterol (HDL-c), often modestly raised levels of LDL cholesterol (LDL-c), and higher levels of FFAs (139). Previous studies have shown a potential role for HDL malfunction in the pathophysiology of OA, based on the finding that OA patients have lower serum HDL-c levels (140). To further study this, Eva Thijssen (2015) compared the development of OA in LCAT−/− and ApoA-I−/− mice and that of C57BL/6 control mice. These knockout (KO) mice had significantly lower levels of functional HDL. On the other hand, cartilage fibrillation, vertical clefts, chondrocyte clustering, and reduced levels of proteoglycan were observed in both LCAT−/− and ApoA-I−/− mice fed a Western-type diet (WTD, 42% calories from fat), while control mice did not exhibit any of these characteristics. Furthermore, KO mice on WTD showed elevated cartilage protein levels of MMP-2, MMP-9, and MMP-13. A decrease followed this enhanced MMP expression in collagen type II protein levels (60). So, all these studies support the idea that reduced HDL levels are involved in OA progression by affecting cartilage homeostasis.

Excessive levels of serum low-density lipoprotein (LDL) and oxidized LDL have been linked to pro-inflammatory effects (141). According to research by De Munter (2016), mice given a diet high in cholesterol developed synovitis, accumulated LDL in synovial cells, and produced more ectopic bone formation. They suggest that this process is caused by elevated levels of oxidized LDL activating endothelial, fibroblast, and synovial macrophages, which in turn cause ectopic bone formation, local inflammation, and cartilage loss (142). Oxidized LDL promotes OA by activating inflammatory pathways such as NF-kB and oxidative stress, leading to the generation of ROS and the expression of inflammatory mediators (MMPs), contributing to cartilage degradation in OA (143).

As was previously highlighted, increased systemic FFA levels are also an important factor in obesity-related dyslipidemia. FFAs can activate macrophages by engaging Toll-like receptor 2/4 (TLR2/4), which results in downstream c-Jun N-terminal kinase signaling and macrophage activation (93). Consequently, pro-inflammatory mediators such as TNF-α can be secreted by macrophages (60). Furthermore, TNF-α exacerbates inflammation by stimulating the synthesis of pro-inflammatory cytokines and chemokines such as IL-6, IL-8, monocyte chemoattractant protein 1, and CC-chemokine ligand 5 (144–147). In OA, high levels of FFAs in cartilage tissues are related to significant tissue damage (58). In vitro, palmitate (SFA) stimulates the synthesis of pro-inflammatory cytokines by chondrocytes and synoviocytes, activates TLR-4, and has pro-apoptotic properties (148).

In vivo studies showed that a high-fat diet exacerbated OA, but n-3 PUFAs reduced disease severity by lowering inflammation and cartilage breakdown, and n-6 PUFAs had no deleterious effects on the condition (149). A diet high in eicosapentaenoic acids and docosahexaenoic acids may reduce joint stiffness and pain in individuals with arthritis (150, 151). Thus, the COX enzyme oxidizes n-6 PUFAs, producing prostaglandins (PGE2 and PGF2α), as well as leukotrienes such as LTB4. Whereas, n-3 PUFA oxidation produces less inflammatory compounds such as PGE3 and LTB5, following COX oxidation and lipooxygenase activity (150). Additionally, the study utilizing the destabilization of the medial meniscus (DMM) model demonstrates that n-3 PUFA-derived 17-hydroxy docosahexaenoic acid (17-HDHA) is linked to reduced pain during loading, confirming the murine DMM model’s utility in investigating the 17-HDHA pathway as a potential therapeutic targeting for alleviating OA pain (152).

Mitochondrial dysfunction and damage, which can lead to anomalies in chondrocyte function and viability, exacerbate cartilage degradation in OA (112). These abnormalities involve the following aspects: increased inflammatory responses, such as matrix catabolism induced by interleukin-1ß (IL-1ß) and TNF, as well as impairment of chondrocyte growth and anabolic responses, excessive oxidative stress, apoptosis of chondrocytes, and calcification of the cartilage matrix (112, 119). In their study, Yun Wang (2015) revealed a dysfunction in mitochondrial biogenesis capacity, which may lead to deficits in the physiologic mitochondrial activities of chondrocytes in human knee OA (101). Thus, they connected decreased mitochondrial biogenesis capability in well-established human knee OA chondrocytes to mtDNA content and reduction in mass, as well as decreased mitochondrial function, as demonstrated through decreased baseline oxygen consumption and intracellular ATP levels (101). Multiple studies have demonstrated that mitochondrial dysfunction has an important influence on catabolic gene expression in chondrocytes. In their study, Mohammad Y (2020) demonstrated that mitochondrial roles are compromised in OA cartilage in vivo compared with healthy cartilage (153). Furthermore, their research demonstrated a substantial enhancement in the level of mitochondrial superoxide in human OA cartilage in vivo, which was associated with the expression of catabolic genes in OA cartilage. They used carbonyl cyanide 3-chlorophenylhydrazone (CCCP) to induce mitochondrial malfunction and discovered the associated signaling route. According to their findings, when CCCP-induced mitochondrial malfunction was observed in vitro, increases in type II collagen and proteoglycan degradation were discovered, which is consistent with the enhancement in their gene expression in vivo. In human and mouse cartilage explants, they also observed an enhancement in the protein expression of matrix-degrading proteases MMP-3, -9, -13, and ADAMTS5 (153).

In vivo and in vitro models, by using different assessment methods such as mitochondrial DNA analysis, respiration studies, and mitochondrial membrane potential, researchers have received vital insights regarding the role of mitochondrial dysfunction in the expansion and advancement of OA. These discoveries have opened the door to more research into the underlying processes of mitochondrial dysfunction in OA, in addition to developing specialized therapeutic options for the effective treatment of this debilitating ailment.