OA is the one of most common arthritis leading to pain, joint destruction and disability characterized by cartilage degradation and synovial inflammation[1]. Metabolism is important for cartilage and synovial joint function[13]. In the past few years, several studies have demonstrated that metabolism has a key role in inflammatory joint diseases[15]. Evidence suggests that patients with OA fall into multiple phenotypic subgroups defined on the basis of the main driver of disease, included a metabolic phenotype and a synovitis-driven inflammatory phenotype, although all OA phenotypes probably involve metabolic alterations[15]. Glucose is an important metabolic fuel being vital for extracellular matrix synthesis and degradation, and it is metabolized via glycolysis[15][33]. Warburg effect, exhibiting enhanced glycolysis even under aerobic conditions, with the purpose of providing sufficient energy to support rapid biosynthesis, also exists in the pathogenesis of OA[13, 34]. Systematic analysis of the interaction between glycolysis and OA patient prognosis and its potential associations with immune infiltration is lacking but urgently needed. In this study, we aimed to distinguish distinct metabolic phenotypes related to glycolysis and figured out the correlation between metabolic patterns with inflammatory microenvironment in synovial tissues of OA.
Firstly, we observed a significant difference in the expression of 56 glycolysis regulator factors between OA patients and normal controls. Next, we established a glycolysis nomogram for predicting the risk of OA from the perspective of metabolism. Different scores were assigned to factors such as STC1, VEGFA, KDELR3, DDIT4 and PGAM1. Consistent results were obtained via RT-qPCR, which validated our findings. Moreover, we investigated the association between glycolysis regulatory factors and the immune properties of OA, including the gene set for immune cell infiltration and inflammatory response. Unsupervised clustering of OA samples using glycolysis regulator expression profiles led to two subtypes with distinctive glycolysis patterns. Furthermore, 56 glycolysis DEGs was used to generate glycolysis scores for every patient by PCA. Most patients from glycolysis cluster A were further classified into high glycolysis score group and an inflamed phenotype; whereas patients from glycolysis cluster B were classified into low glycolysis score group and a non-inflamed phenotype. Our results also in the two distinct glycolysis patterns there is a significant difference of gene expression of MAPK, TLR, necroptosis and TGF-β pathways which all significantly involved in OA[35][36].
Among the five genes, STC1 (Stanniocalcin 1) is a paracrine factor associated with inflammation and carcinogenesis that can help mesenchymal cells to protect cancer cells from apoptosis and enhance the Warburg effect[37]. DDIT4 is a mammalian target of rapamycin (mTOR) inhibitor[38]. Eddie et al showed that IL-10 inhibits lipopolysaccharide-induced glucose uptake and glycolysis and promotes oxidative phosphorylation through the induction of an mTOR inhibitor, DDIT4[38]. PGAM1, phosphoglycerate mutase 1, plays a key role in glycolysis. Shen et al. demonstrated that S1P/S1PR3 axis promotes aerobic glycolysis by YAP/c-MYC/PGAM1 axis in osteosarcoma[38]. These suggested that these glycolysis regulators may also involve in OA and served as biomarkers of diagnosis.
OA patients often exhibit inflammatory infiltration of synovial membranes by macrophages, T cells, mast cells, B cells, plasma cells, natural killer cells, dendritic cells, granulocytes, etc[39]. Glycolysis also have a neglectable role in immune infiltration as well. For example, glycolysis plays an important role in T cells, which are assumed to be associated with the pathogenesis of OA, as significant abnormalities in the T-cell profile have been found in the synovial membranes of OA patients[39]. Both CD8 and CD4 T cell-dependent immune responses and the helper T cell-dependent inflammation were attenuated by Pgam1 deficiency[40]. Macrophages are among the most abundant cell type present in the cellular infiltrates found in the inflamed synovium in OA[41]. Moreover, Cai et al. showed that the glycolysis inhibitor 2-deoxyglucose ameliorates adjuvant-induced arthritis by regulating macrophage polarization in an AMPK-dependent manner[42]. All these studies had implications in our results; however, further research is urgently acquired to further demonstrate the correlation between glycolysis and inflammatory environment in OA.
Taken together, glycolysis pathway plays a significant role in OA. Firstly, we constructed a glycolysis OA nomogram to assess the risk of OA, thus providing a reference for the clinical diagnosis of OA. Then two distinct glycolysis patterns and a strong correlation between these patterns and inflammatory environment were identified. Meanwhile, a novel scoring system to quantify glycolysis pattern in individual patients was built. Our findings provided novel ideas for promoting personalized immunotherapy, and demonstrating a new horizon for the investigations of the pathogenesis of OA in the future.