Lysyl oxidase exacerbates rheumatoid arthritis through promoting angiogenesis and the proliferation of fibroblast-like synoviocytes

Abstract

Rheumatoid arthritis (RA) is an autoimmune disorder characterized by synovial hyperplasia and pannus formation, which serves as its primary pathological feature and may ultimately result in joint deformities. Lysyl oxidase (LOX) is involved in the formation and remodeling of the extracellular matrix, but its role in RA is not yet clear. This study aims to investigate the mechanism of lysyl oxidase (LOX) in synovial hyperplasia and pannus formation associated with rheumatoid arthritis (RA). Synovial, serum, and synovial fluid samples are collected from RA, osteoarthritis (OA), and knee injury patients and subsequently analyzed via HE staining, immunohistochemistry, and ELISA. Compared with those of the OA and injury groups, the RA synovium presents increased thickness, disorganized cell layers, increased microvascular density (MVD), and elevated LOX expression. Moreover, LOX levels are positively correlated with the MVD. Both synovial fluid and fibroblast-like synoviocytes (FLSs) derived from RA patients present significantly elevated concentrations of LOX. In vitro experiments reveal that LOX dose-dependently promotes the proliferation of FLSs derived from both RA patients and healthy individuals (MH7A/HFLS) by accelerating S/M-phase cell cycle progression while simultaneously stimulating angiogenesis in human umbilical vein endothelial cells (HUVECs). In contrast, the LOX inhibitor BAPN suppresses these effects. Mechanistic analysis further reveals that LOX increases the phosphorylation of the PI3K-AKT signaling pathway, an effect that is reversible by BAPN. In conclusion, LOX may induce abnormal fibroblast proliferation and endothelial neovascularization via activation of the PI3K/AKT pathway, thus aggravating synovial hyperplasia and pathological membrane formation in RA. These findings provide a theoretical foundation for the development of targeted LOX treatments for RA.

1. Introduction

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by a complex onset and pathological process that remains incompletely understood [1]. The clinical manifestations of RA commonly involve joint lesions, primarily characterized by persistent synovitis, which leads to synovial hyperplasia and pannus formation in the joints [2]. Synovial hyperplasia arises from the excessive proliferation of synovial fibroblasts, while the pannus consists of hyperplastic synovial cells, newly formed microvessels, inflammatory cells, and localized fibrosis. This pannus formation serves as the pathological basis for joint lesions and the destruction of bone and cartilage [3,4].

Lysyl oxidase (LOX) mediates the covalent cross-linking of collagen and elastin fibers, playing a crucial role in the formation and remodeling of the extracellular matrix, thereby maintaining its appropriate strength and elasticity to the extracellular matrix [5]. In recent years, LOX has been shown to be involved in the occurrence and development of various diseases, including glioblastoma growth, diabetic neovascularization, tumor progression, and metastasis [6-8]. Multiple cell types in the human body are capable of synthesizing LOX, with fibroblasts identified as the primary source of LOX in tissues [9]. However, whether synovial fibroblasts can produce LOX and whether LOX affects synovial proliferation and pannus formation in the pathological process of RA remain largely unexplored.

In this study, we examined the expression of LOX in joint synovial tissue from RA patients and evaluated its correlation with microvascular density. The LOX content in the synovial fluid and blood of patients was also quantified. Furthermore, primary synovial fibroblasts were isolated from RA patients, and established synovial fibroblast lines were utilized to assess LOX production. The cells were treated with exogenous LOX and its inhibitor BAPN to examine the effects on synovial fibroblast proliferation, the cell cycle, and HUVEC angiogenesis. These findings aim to enhance the understanding of RA pathogenesis and provide a theoretical foundation for new insights into clinical treatment.