The leucine-rich repeat domain of human peroxidasin 1 promotes binding to laminin in basement membranes

Modification of extracellular matrix proteins by oxidants and electrophiles

The extracellular matrix (ECM) is critical to biological architecture and determines cellular properties, function and activity. In many situations it is highly abundant, with collagens and elastin being some of the most abundant proteins in mammals. The ECM comprises of multiple different protein species and sugar polymers, with both different isoforms and post-translational modifications (PTMs) providing a large variety of microenvironments that play a key role in determining tissue structure and health. A number of the PTMs (e.g. cross-links) present in the ECM are critical to integrity and function, whereas others are deleterious to both ECM structure and associated cells. Modifications induced by reactive oxidants and electrophiles have been reported to accumulate in some ECM with increasing age. This accumulation can be exacerbated by disease, and in particular those associated with acute or chronic inflammation, obesity and diabetes. This is likely to be due to higher fluxes of modifying agents in these conditions. In this focused review, the role and effects of oxidants and other electrophiles on ECM are discussed, with a particular focus on the artery wall and atherosclerotic cardiovascular disease. Modifications generated on ECM components are reviewed, together with the effects of these species on cellular properties including adhesion, proliferation, migration, viability, metabolic activity, gene expression and phenotype. Increasing data indicates that ECM modifications are both prevalent in human and mammalian tissues and play an important role in disease development and progression.

Identification of tyrosine brominated extracellular matrix proteins in normal and fibrotic lung tissues

Peroxidasin (PXDN) is a secreted heme peroxidase that catalyzes the oxidative crosslinking of collagen IV within the extracellular matrix (ECM) via intermediate hypobromous acid (HOBr) synthesis from hydrogen peroxide and bromide, but recent findings have also suggested alternative ECM protein modifications by PXDN, including incorporation of bromide into tyrosine residues. In this work, we sought to identify the major target proteins for tyrosine bromination by HOBr or by PXDN-mediated oxidation in ECM from mouse teratocarcinoma PFHR9 cells. We detected 61 bromotyrosine (BrY)-containing peptides representing 23 proteins in HOBr-modified ECM from PFHR9 cells, among which laminins displayed the most prominent bromotyrosine incorporation. Moreover, we also found that laminin α1, laminin β1, and tubulointerstitial nephritis antigen-like (TINAGL1) contained BrY in untreated PFHR9 cells, which depended on PXDN. We extended these analyses to lung tissues from both healthy mice and mice with experimental lung fibrosis, and in lung tissues obtained from human subjects. Analysis of ECM-enriched mouse lung tissue extracts showed that 83 ECM proteins were elevated in bleomycin-induced fibrosis, which included various collagens and laminins, and PXDN. Similarly, mRNA and protein expression of PXDN and laminin α/β1 were enhanced in fibrotic mouse lung tissues, and also in mouse bone-marrow-derived macrophages or human fibroblasts stimulated with transforming growth factor β1, a profibrotic growth factor. We identified 11 BrY-containing ECM proteins, including collagen IV α2, collagen VI α1, TINAGL1, and various laminins, in both healthy and mouse fibrotic lung tissues, although the relative extent of tyrosine bromination of laminins was not significantly increased during fibrosis. Finally, we also identified 7 BrY-containing ECM proteins in human lung tissues, again including collagen IV α2, collagen VI α1, and TINAGL1. Altogether, this work demonstrates the presence of several bromotyrosine-modified ECM proteins, likely involving PXDN, even in normal lung tissues, suggesting a potential biological function for these modifications.

Peroxidasin Inhibition by Phloroglucinol and Other Peroxidase Inhibitors

Human peroxidasin (PXDN) is a ubiquitous peroxidase enzyme expressed in most tissues in the body. PXDN represents an interesting therapeutic target for inhibition, as it plays a role in numerous pathologies, including cardiovascular disease, cancer and fibrosis. Like other peroxidases, PXDN generates hypohalous acids and free radical species, thereby facilitating oxidative modifications of numerous biomolecules. We have studied the inhibition of PXDN halogenation and peroxidase activity by phloroglucinol and 14 other peroxidase inhibitors. Although a number of compounds on their own potently inhibited PXDN halogenation activity, only five were effective in the presence of a peroxidase substrate with IC50 values in the low μM range. Using sequential stopped-flow spectrophotometry, we examined the mechanisms of inhibition for several compounds. Phloroglucinol was the most potent inhibitor with a nanomolar IC50 for purified PXDN and IC50 values of 0.95 μM and 1.6 μM for the inhibition of hypobromous acid (HOBr)-mediated collagen IV cross-linking in a decellularized extracellular matrix and a cell culture model. Other compounds were less effective in these models. Most interestingly, phloroglucinol was identified to irreversibly inhibit PXDN, either by mechanism-based inhibition or tight binding. Our work has highlighted phloroglucinol as a promising lead compound for the design of highly specific PXDN inhibitors and the assays used in this study provide a suitable approach for high-throughput screening of PXDN inhibitors.

The role of peroxidasin in solid cancer progression

Peroxidasin is a heme-containing peroxidase enzyme that plays a vital role in the cross-linking of collagen IV molecules in basement membranes. Collagen IV cross-links are essential for providing structure and mechanical stability throughout tissue development, homeostasis, and wound healing. During cancer progression, the basement membrane is degraded, and proteins typically found in the basement membrane, including peroxidasin and collagen IV, can be found spread throughout the tumour microenvironment where they interact with cancer cells and alter cell behaviour. Whilst peroxidasin is reported to be up-regulated in a number of different cancers, the role that it plays in disease progression and metastasis has only recently begun to be studied. This review highlights the current literature exploring the known roles of peroxidasin in normal tissues and cancer progression, regulators of peroxidasin expression, and the reported relationships between peroxidasin expression and patient outcome in cancer.

Spatial composition and turnover of the main molecules in the adult glomerular basement membrane

The glomerular basement membrane (GBM) is an important tissue structure in kidney function. It is the membrane through which filtrate and solutes must pass to reach the nephron tubules. This review focuses on the spatial location of the main extracellular matrix components of the GBM. It also attempts to explain this organization in terms of their synthesis, transport, and loss. The picture that emerges is that the collagen IV and laminin content of GBM are in a very slow dynamic disequilibrium, leading to GBM thickening with age, and in contrast, some heparan sulfate proteoglycans are in a dynamic equilibrium with a very rapid turnover (i.e. half-life measured in ~hours) and flow direction against the flow of filtrate. The highly rapid heparan sulfate turnover may serve several roles, including an unclogging mechanism for the GBM, compressive stiffness of the GBM fiber network, and/or enabling podocycte-endothelial crosstalk against the flow of filtrate.

Uric Acid Reacts with Peroxidasin, Decreases Collagen IV Crosslink, Impairs Human Endothelial Cell Migration and Adhesion

Uric acid is considered the main substrate for peroxidases in plasma. The oxidation of uric acid by human peroxidases generates urate free radical and urate hydroperoxide, which might affect endothelial function and explain, at least in part, the harmful effects of uric acid on the vascular system. Peroxidasin (PXDN), the most recent heme-peroxidase described in humans, catalyzes the formation of hypobromous acid, which mediates collagen IV crosslinks in the extracellular matrix. This enzyme has gained increasing scientific interest since it is associated with cardiovascular disease, cancer, and renal fibrosis. The main objective here was to investigate whether uric acid would react with PXDN and compromise the function of the enzyme in human endothelial cells. Urate decreased Amplex Red oxidation and brominating activity in the extracellular matrix (ECM) from HEK293/PXDN overexpressing cells and in the secretome of HUVECs. Parallelly, urate was oxidized to 5-hydroxyisourate. It also decreased collagen IV crosslink in isolated ECM from PFHR9 cells. Urate, the PXDN inhibitor phloroglucinol, and the PXDN knockdown impaired migration and adhesion of HUVECs. These results demonstrated that uric acid can affect extracellular matrix formation by competing for PXDN. The oxidation of uric acid by PXDN is likely a relevant mechanism in the endothelial dysfunction related to this metabolite.

Mammalian peroxidasin (PXDN): From physiology to pathology

Heme-containing peroxidases catalyze the oxidation of a variety of substrates by consuming hydrogen peroxide (H2O2), and play diversified roles in physiology and pathology including innate immunity, the synthesis of thyroid hormone and the extracellular matrix, as well as the pathogenesis of several inflammatory diseases. Peroxidasin (PXDN), also known as Vascular Peroxidase-1 (VPO1), is a newly identified peroxidase and expresses in multiple cells and tissues including cardiovascular system and the lung. Recent studies imply its roles in the innate immunity, cardiovascular physiology and diseases, and extracellular matrix formation. Studies on the role of PXDN in human diseases are entering a new and exciting stage, and this review provides the insights into this emerging field of PXDN.

Peroxidasin protein expression and enzymatic activity in metastatic melanoma cell lines are associated with invasive potential

Peroxidasin, a heme peroxidase, has been shown to play a role in cancer progression. mRNA expression has been reported to be upregulated in metastatic melanoma cell lines and connected to the invasive phenotype, but little is known about how peroxidasin acts in cancer cells. We have analyzed peroxidasin protein expression and activity in eight metastatic melanoma cell lines using an ELISA developed with an in-house peroxidasin binding protein. RNAseq data analysis confirmed high peroxidasin mRNA expression in the five cell lines classified as invasive and low expression in the three non-invasive cell lines. Protein levels of peroxidasin were higher in the cell lines with an invasive phenotype. Active peroxidasin was secreted to the cell culture medium, where it accumulated over time, and peroxidasin protein levels in the medium were also much higher in invasive than non-invasive cell lines. The only well-established physiological role of peroxidasin is in the formation of a sulfilimine bond, which cross-links collagen IV in basement membranes via catalyzed oxidation of bromide to hypobromous acid. We found that peroxidasin secreted from melanoma cells formed sulfilimine bonds in uncross-linked collagen IV, confirming peroxidasin activity and hypobromous acid formation. Moreover, 3-bromotyrosine, a stable product of hypobromous acid reacting with tyrosine residues, was detected in invasive melanoma cells, substantiating that their expression of peroxidasin generates hypobromous acid, and showing that it does not exclusively react with collagen IV, but also with other biomolecules.

Characterization of the Proprotein Convertase-Mediated Processing of Peroxidasin and Peroxidasin-like Protein

Peroxidasin (PXDN) and peroxidasin-like protein (PXDNL) are members of the peroxidase-cyclooxygenase superfamily. PXDN functions in basement membrane synthesis by forming collagen IV crosslinks, while the function of PXDNL remains practically unknown. In this work, we characterized the post-translational proteolytic processing of PXDN and PXDNL. Using a novel knock-in mouse model, we demonstrate that the proteolytic cleavage of PXDN occurs in vivo. With the help of furin-specific siRNA we also demonstrate that the proprotein-convertase, furin participates in the proteolytic processing of PXDN. Furthermore, we demonstrate that only the proteolytically processed PXDN integrates into the extracellular matrix, highlighting the importance of the proteolysis step in PXDN’s collagen IV-crosslinking activity. We also provide multiple lines of evidence for the importance of peroxidase activity in the proteolytic processing of PXDN. Finally, we show that PXDNL does not undergo proteolytic processing, despite containing sequence elements efficiently recognized by proprotein convertases. Collectively, our observations suggest a previously unknown protein quality control during PXDN synthesis and the importance of the peroxidase activity of PXDN in this process.

Enzymatic cross-linking of collagens in organ fibrosis - resolution and assessment

Introduction: Enzymatic cross-linking of the collagens within the extracellular matrix (ECM) catalyzed by enzymes such as lysyl oxidase (LOX) and lysyl oxidase like-enzymes 1-4 (LOXL), transglutaminase 2 (TG2), and peroxidasin (PXDN) contribute to fibrosis progression through extensive collagen cross-linking. Studies in recent years have begun elucidating the important role of collagen cross-linking in perpetuating progression of organ fibrosis independently of inflammation through an increasingly stiff and noncompliant ECM. Therefore, collagen cross-linking and the cross-linking enzymes have become new targets in anti-fibrotic therapy as well as targets of novel biomarkers to properly assess resolution of the fibrotic ECM.Areas covered: The enzymatic actions of enzymes catalyzing collagen cross-linking and their relevance in organ fibrosis. Potential biomarkers specifically quantifying proteolytic fragments of collagen cross-linking is discussed based on Pubmed search done in November 2020 as well as the authors knowledge.Expert opinion: Current methods for the assessment of fibrosis involve the use of invasive and/or cumbersome and expensive methods such as tissue biopsies. Thus, an unmet need exists for the development and validation of minimally invasive biomarkers of proteolytic fragments of cross-linked collagens. These biomarkers may aid in the development and proper assessment of fibrosis resolution in coming years.