Angiopoietin-Like Protein 2 Induces Synovial Inflammation in the Facet Joint Leading to Degenerative Changes via Interleukin-6 Secretion

Transcriptomic analyses and machine-learning methods reveal dysregulated key genes and potential pathogenesis in human osteoarthritic cartilage

Aims

This study aimed to explore the biological and clinical importance of dysregulated key genes in osteoarthritis (OA) patients at the cartilage level to find potential biomarkers and targets for diagnosing and treating OA.

Methods

Six sets of gene expression profiles were obtained from the Gene Expression Omnibus database. Differential expression analysis, weighted gene coexpression network analysis (WGCNA), and multiple machine-learning algorithms were used to screen crucial genes in osteoarthritic cartilage, and genome enrichment and functional annotation analyses were used to decipher the related categories of gene function. Single-sample gene set enrichment analysis was performed to analyze immune cell infiltration. Correlation analysis was used to explore the relationship among the hub genes and immune cells, as well as markers related to articular cartilage degradation and bone mineralization.

Results

A total of 46 genes were obtained from the intersection of significantly upregulated genes in osteoarthritic cartilage and the key module genes screened by WGCNA. Functional annotation analysis revealed that these genes were closely related to pathological responses associated with OA, such as inflammation and immunity. Four key dysregulated genes (cartilage acidic protein 1 (CRTAC1), iodothyronine deiodinase 2 (DIO2), angiopoietin-related protein 2 (ANGPTL2), and MAGE family member D1 (MAGED1)) were identified after using machine-learning algorithms. These genes had high diagnostic value in both the training cohort and external validation cohort (receiver operating characteristic > 0.8). The upregulated expression of these hub genes in osteoarthritic cartilage signified higher levels of immune infiltration as well as the expression of metalloproteinases and mineralization markers, suggesting harmful biological alterations and indicating that these hub genes play an important role in the pathogenesis of OA. A competing endogenous RNA network was constructed to reveal the underlying post-transcriptional regulatory mechanisms.

Conclusion

The current study explores and validates a dysregulated key gene set in osteoarthritic cartilage that is capable of accurately diagnosing OA and characterizing the biological alterations in osteoarthritic cartilage; this may become a promising indicator in clinical decision-making. This study indicates that dysregulated key genes play an important role in the development and progression of OA, and may be potential therapeutic targets.

Intervertebral disc degeneration and osteoarthritis: a common molecular disease spectrum

Intervertebral disc degeneration (IDD) and osteoarthritis (OA) affecting the facet joint of the spine are biomechanically interdependent, typically occur in tandem, and have considerable epidemiological and pathophysiological overlap. Historically, the distinctions between these degenerative diseases have been emphasized. Therefore, research in the two fields often occurs independently without adequate consideration of the co-dependence of the two sites, which reside within the same functional spinal unit. Emerging evidence from animal models of spine degeneration highlight the interdependence of IDD and facet joint OA, warranting a review of the parallels between these two degenerative phenomena for the benefit of both clinicians and research scientists. This Review discusses the pathophysiological aspects of IDD and OA, with an emphasis on tissue, cellular and molecular pathways of degeneration. Although the intervertebral disc and synovial facet joint are biologically distinct structures that are amenable to reductive scientific consideration, substantial overlap exists between the molecular pathways and processes of degeneration (including cartilage destruction, extracellular matrix degeneration and osteophyte formation) that occur at these sites. Thus, researchers, clinicians, advocates and policy-makers should consider viewing the burden and management of spinal degeneration holistically as part of the OA disease continuum.

Facet overhang: A novel parameter in the pathophysiology of multifidus muscle atrophy

The relationship between degenerative zygapophysial joint (facet) arthropathy and multifidus muscle atrophy has not been rigorously evaluated. The purpose of this study was to determine if specific morphological features of degenerative facet arthropathy are correlated with multifidus muscle atrophy. We retrospectively reviewed medical records and imaging studies of patients with lumbar spinal stenosis. Facet overhang, bridging osteophyte formation, facet effusion, and facet angles were evaluated by univariable and multivariable regression to identify independent associations with deep and superficial parts of the multifidus total cross-sectional area (tCSA), functional cross-sectional area (fnCSA), and fatty infiltration (FI). Facet overhang was classified as severe in 50 females (53.2%) versus 56 males (36.9%) (p = 0.030). Severity of facet overhang and female sex were independently associated with smaller deep part of the multifidus tCSA and fnCSA as well as higher FI, reflecting greater atrophy of the deep region compared to total muscle mass. In comparison, severe facet overhang (p < 0.001; OR = 3.47, 95% CI = 2.13-5.66) and female sex (p < 0.001; OR = 4.19, 95% CI = 2.58-6.79) were independently associated only with higher superficial part of the multifidus FI, reflecting muscle steatosis without significant lean muscle atrophy. In patients with degenerative lumbar spinal stenosis, facet overhang is an independent risk factor for deep part of the multifidus atrophy. Bridging osteophyte formation, facet effusion, and facet angles were not independently associated with deep part of the multifidus atrophy.

Emerging roles of angiopoietin-like proteins in inflammation: Mechanisms and potential as pharmacological targets

Angiopoietin-like proteins (ANGPTLs), a family of eight secreted glycoproteins termed ANGTPL1-8, are involved in angiogenesis, lipid metabolism, cancer progression, and inflammation. Their roles in regulating lipid metabolism have been intensively studied, as some ANGPTLs are promising pharmacological targets for hypertriglyceridemia and associated cardiovascular disease. Recently, the emerging roles of ANGPTLs in inflammation have attracted great attention. First, elevated levels of multiple circulating ANGPTLs in inflammatory diseases make them potential disease biomarkers. Second, multiple ANGPTLs regulate acute or chronic inflammation via various mechanisms, including triggering inflammatory signaling through their action as ligands for integrin or forming homo- /hetero-oligomers to regulate signal transduction via extra- or intracellular mechanisms. As dysregulation of the inflammatory response is a critical trigger in many diseases, understanding the roles of ANGPTLs in inflammation will aid in drug/therapy development. Here, we summarize the roles, mechanisms, and potential therapeutic values for ANGPTLs in inflammation and inflammatory diseases.

ANGPTL2 Induces Synovial Inflammation via LILRB2

Angiopoietin-like proteins (ANGPTLs) are circulating proteins that are expressed in various cells and tissues and are thought to be involved in the repair and remodeling of damaged tissues; however, ANGPTL2 hyperfunction has been shown to cause chronic inflammation, leading to the progression of various diseases. ANGPTL2 is known to exert cellular effects via receptors such as integrin α5β1 and leukocyte immunoglobulin-like receptor subfamily B member 2 (LILRB2); however, their roles in ANGPTL2-induced inflammation remain unclear. In this study, we investigated the mechanisms underlying ANGPTL2-induced inflammation involving LILRB2 and various signaling pathways in human fibroblast-like synoviocytes (HFLS). The effects of ANGPTL2 and an anti-LILRB2 antibody on the gene expression of various inflammation-related factors were examined using real-time RT-PCR, while their effects on MAPK, NF-κB, and Akt phosphorylation were analyzed by western blotting. We found that the addition of ANGPTL2 enhanced the gene expression of inflammatory factors, whereas pretreatment with the anti-LILRB2 antibody for 12 h decreased the expression of these factors. Similarly, ANGPTL2 addition activated the phosphorylation of ERK, p38, JNK, NF-κB, and Akt in HFLS; however, this effect was significantly inhibited by pretreatment with the anti-LILRB2 antibody. Together, the findings of this study demonstrate that ANGPTL2 induces the expression of inflammatory factors via LILRB2 in synovial cells. Therefore, LILRB2 could be a potential therapeutic agent for treating matrix degradation in osteoarthritis.

Protective Effect of Irbesartan by Inhibiting ANGPTL2 Expression in Diabetic Kidney Disease

Angiopoietin-like protein 2 (ANGPTL2) stimulates inflammation and is important in the pathogenesis of diabetic kidney disease (DKD). Irbesartan is helpful in reducing diabetes-induced renal damage. In this study, the effects of irbesartan on DKD and its renal protective role involving ANGPTL2 in DKD rats were examined. Wistar rats were divided into normal, DKD, and DKD + irbesartan groups. The DKD + irbesartan group was treated once daily for 8 weeks with 50 mg/kg irbesartan via intragastric gavage. The 24-h urinary albumin was determined each week, renal pathological changes were observed, and expression of ANGPTL2 and nuclear factor-kappa B (NF-κB) in rat renal tissue was assessed by immunohistochemistry. Mouse podocytes cultured in a high concentration of glucose were classified into four groups based on the irbesartan concentrations (0, 25, 50, and 75 ºg/mL). Expression of ANGPTL2 and phosphorylated IκB-α was assessed by Western blotting. The mRNA levels of ANGPTL2 and monocyte chemotactic protein 1 (MCP-1) were assessed by real-time polymerase chain reaction. The DKD rats displayed proteinuria, podocyte injury, and increased ANGPTL2 and NF-κB expression. All were relieved by irbesartan treatment. In podocytes cultured in elevated glucose, ANGPTL2 and phosphorylated IκB-α were overexpressed at the protein level, and ANGPTL2 and MCP-1 were highly expressed at the mRNA level. Irbesartan down-regulated ANGPTL2 and phosphorylated IκB-αexpression at the protein level and inhibited ANGPTL2 and MCP-1 expression at the mRNA level. The ameliorative effects of irbesartan against DKD involves podocyte protection and suppression of ANGPTL2.