Proconvertase proteolytic processing of an enzymatically active myeloperoxidase precursor

Granulocyte pro-myeloperoxidase is redundantly processed by proprotein convertase furin and PC7 in HL-60 cells

Neutrophil myeloperoxidase/H2O2/chloride system is a key mechanism to control pathogen infection. This enzyme, myeloperoxidase, plays a pivotal role in the arsenal of azurophilic granules that are released through degranulation upon neutrophil activation, which trigger local hypochlorous acid production. Myeloperoxidase gene encodes a protein precursor named promyeloperoxidase that arbors a propeptide that gets cleaved later during secretory routing in post-endoplasmic reticulum compartments. Although evidence suggested that this processing event was performed by one or different enzymes from the proprotein convertases family, the identity of this enzyme was never investigated. In this work, the naturally producing myeloperoxidase promyelocytic cell line HL-60 was used to investigate promyeloperoxidase cleavage during granulocytic differentiation in response to proprotein convertase inhibitors decanoyl-RVKR-chloromethylketone and hexa-d-arginine. Stable PC knockdown of endogenously expressed proprotein convertases, furin and PC7, was achieved using lentiviral delivery of shRNAs. None of the knockdown cell line could reproduce the effect of the pan-proprotein convertases inhibitor decanoyl-RVKR-chloromethylketone that accumulated intracellular promyeloperoxidase stores in HL-60 cells, therefore illustrating that both furin and PC7 redundantly process this proprotein.

Posttranslational modification and heme cavity architecture of human eosinophil peroxidase-insights from first crystal structure and biochemical characterization

Eosinophil peroxidase (EPO) is the most abundant granule protein exocytosed by eosinophils, specialized human phagocytes. Released EPO catalyzes the formation of reactive oxidants from bromide, thiocyanate, and nitrite that kill tissue-invading parasites. However, EPO also plays a deleterious role in inflammatory diseases, making it a potential pharmacological target. A major hurdle is the high similarity to the homologous myeloperoxidase (MPO), which requires a detailed understanding of the small structural differences that can be used to increase the specificity of the inhibitors. Here, we present the first crystal structure of mature leukocyte EPO at 1.6 Å resolution together with analyses of its posttranslational modifications and biochemical properties. EPO has an exceptionally high number of positively charged surface patches but only two occupied glycosylation sites. The crystal structure further revealed the existence of a light (L) and heavy (H) chain as a result of proteolytic cleavage. Detailed comparison with the structure of human MPO allows us to identify differences that may contribute to the known divergent enzymatic properties. The crystal structure revealed fully established ester links between the prosthetic group and the protein, the comparably weak imidazolate character of the proximal histidine, and the conserved structure of the catalytic amino acids and Ca2+-binding site. Prediction of the structure of unprocessed proeosinophil peroxidase allows further structural analysis of the three protease cleavage sites and the potential pro-convertase recognition site in the propeptide. Finally, EPO biosynthesis and its biochemical and biophysical properties are discussed with respect to the available data from the well-studied MPO.

Integrating network pharmacology and transcriptomic validation to investigate the efficacy and mechanism of Mufangji decoction preventing lung cancer

ETHNOPHARMACOLOGICAL RELEVANCE

Mufangji decoction (MFJD), a famous traditional Chinese medicine formula in Synopsis of Golden Chamber (Jingui yaolue), has been utilized to treat cough and asthma and release chest pain over 2000 years in China. Chinese old herbalist doctor use MFJD to treat lung cancer and cancerous pleural fluid, but the preventive effect of MFJD on lung cancer and the underlying mechanism are indefinite.

AIM OF THE STUDY

The goal of this study is to explore the efficacy and mechanism of Mufangji decoction preventing lung cancer referring to the traditional use.

MATERIALS AND METHODS

Tumor allograft experiment and host versus tumor experiment were used to observe the direct anti-tumor effect and indirect anti-tumor immune effect, the mouse lung carcinogenic model was used to evaluate the dose-response and the preventive effect of MFJD on lung cancer. The active ingredients of MFJD were obtained by UPLC-MS/MS. The potential targets of MFJD were screened by network pharmacology and transcriptomics. The therapeutic targets and pathways of MFJD on lung cancer were obtained by protein-protein interaction, molecular docking and David database. The predicted results were verified in vitro and in vivo.

RESULTS

MFJD could significantly prevent tumor growth in host versus tumor experiment but could not in tumor allograft experiment, indicating an anti-tumor immune effect against lung cancer. MFJD could reduce lung nodules with a dose-response in mouse lung carcinogenic model. Myeloperoxidase (MPO) was selected as the core target due to the highest degree value in Protein-Protein interaction network and had potently binding activity to sinomenine and dehydrocostus lactone in molecular docking. In vivo, MPO-expressed neutrophils are negatively correlated with lung cancer progression and MFJD could promote the neutrophil-related immune surveillance. In vitro, sinomenine and dehydrocostus lactone could promote neutrophil phagocytosis, MPO and ROS production in a dose dependent manner. The major compounds from MFJD were identified to regulate 36 targets for lung cancer prevention by UPLC-MS/MS, network pharmacology and transcriptomics. David database exhibited that MFJD plays an important role in immunoregulation by modulating 4 immune-related biological processes and 3 immune-related pathways.

CONCLUSIONS

MFJD prevents lung cancer by mainly promoting MPO expression to maintain neutrophil immune surveillance, its key compounds are sinomenine and dehydrocostus lactone.

Recombinant Myeloperoxidase as a New Class of Antimicrobial Agents

Heme-containing peroxidases are widely distributed in the animal and plant kingdoms and play an important role in host defense by generating potent oxidants. Myeloperoxidase (MPO), the prototype of heme-containing peroxidases, exists in neutrophils and monocytes. MPO has a broad spectrum of microbial killing. The difficulty of producing MPO at a large scale hinders its study and utilization. This study aimed to overexpress recombinant human MPO and characterize its microbicidal activities in vitro and in vivo. A human HEK293 cell line stably expressing recombinant MPO (rMPO) was established as a component of this study. rMPO was overexpressed and purified for studies on its biochemical and enzymatic properties, as well as its microbicidal activities. In this study, rMPO was secreted into culture medium as a monomer. rMPO revealed enzymatic activity similar to that of native MPO. rMPO, like native MPO, was capable of killing a broad spectrum of microorganisms, including Gram-negative and -positive bacteria and fungi, at low nM levels. Interestingly, rMPO could kill antibiotic-resistant bacteria, making it very useful for treatment of nosocomial infections and mixed infections. The administration of rMPO significantly reduced the morbidity and mortality of murine lung infections induced by Pseudomonas aeruginosa or methicillin-resistant Staphylococcus aureus. In animal safety tests, the administration of 100 nM rMPO via tail vein did not result in any sign of toxic effects. Taken together, the data suggest that rMPO purified from a stably expressing human cell line is a new class of antimicrobial agents with the ability to kill a broad spectrum of pathogens, including bacteria and fungi with or without drug resistance.

IMPORTANCE Over the past 2 decades, more than 20 new infectious diseases have emerged. Unfortunately, novel antimicrobial therapeutics are discovered at much lower rates. Infections caused by resistant microorganisms often fail to respond to conventional treatment, resulting in prolonged illness, greater risk of death, and high health care costs. Currently, this is best seen with the lack of a cure for coronavirus disease 2019 (COVID-19). To combat such untreatable microorganisms, there is an urgent need to discover new classes of antimicrobial agents. Myeloperoxidase (MPO) plays an important role in host defense. The difficulty of producing MPO on a large scale hinders its study and utilization. We have produced recombinant MPO at a large scale and have characterized its antimicrobial activities. Most importantly, recombinant MPO significantly reduced the morbidity and mortality of murine pneumonia induced by Pseudomonas aeruginosa or methicillin-resistant Staphylococcus aureus. Our data suggest that recombinant MPO from human cells is a new class of antimicrobials with a broad spectrum of activity.

Characterization of the turbot Scophthalmus maximus (L.) myeloperoxidase. An insight into the evolution of vertebrate peroxidases

We have completed the characterization of the turbot (Scophthalmus maximus) myeloperoxidase (mpx) gene and protein, which we partially described in a previous study. The turbot mpx gene has 15 exons that encode a protein of 767 aa, with a signal peptide, propeptide and light and heavy chains, and also with haem cavities, a Ca⁺²-binding motif and several N- and O-glycosylation sites. The mature protein forms homodimers of about 150 kDa and is very abundant in turbot neutrophils. In addition to the mpx (epx2a) gene, another three peroxidase genes, named epx1, epx2b1 and epx2b2, were identified in the turbot genome. Epx1, Epx2b1 and Epx2b2 proteins also have signal peptides and many structural characteristics of mammalian MPO and eosinophil peroxidase (EPX). Mpx was strongly expressed in head kidney, while epx2b1 and epx2b2 were strongly expressed in the gills, and epx1 was not expressed in any of the tissues or organs analysed. In vitro stimulation of head kidney leucocytes with the parasite Philasterides dicentrarchi caused a decrease in mpx expression and an increase in epx2b1 expression over time. In turbot infected experimentally with P. dicentrarchi a significant increase in mpx expression in the head kidney was observed on day 7 postinfection, while the other genes were not regulated. However, mpx, epx2b1 and epx2b2 were downregulated in the gills of infected fish, and epx1 expression was not affected. These results suggest that the four genes responded differently to the same stimuli. Interestingly, BLAST analysis revealed that Epx1 and Mpx showed greater similarity to mammalian EPX than to MPO. Considering the phylogenetic and synteny data obtained, we concluded that the epx/mpx genes of Gnathostomes can be divided into three main clades: EPX1, which contains turbot epx1, EPX2, which contains turbot mpx (epx2a) and epx2b1 and epx2b2 genes, and a clade containing mammalian EPX and MPO (EPX/MPO). EPX/MPO and EPX2 clades share a common ancestor with the chondrichthyan elephant shark (Callorhinchus milii) and the coelacanth (Latimeria chalumnae) peroxidases. EPX2 was only found in fish and includes two sister groups. One of the groups includes turbot mpx and was only found in teleosts. Finally, the other group contains epx2b1 and epx2b2 genes, and epx2b1-2b2 loci share orthologous genes with other teleosts and also with holosteans, suggesting that these genes appeared earlier on than the mpx gene.

The Enzymatic and Non-Enzymatic Function of Myeloperoxidase (MPO) in Inflammatory Communication

Myeloperoxidase is a signature enzyme of polymorphonuclear neutrophils in mice and humans. Being a component of circulating white blood cells, myeloperoxidase plays multiple roles in various organs and tissues and facilitates their crosstalk. Here, we describe the current knowledge on the tissue- and lineage-specific expression of myeloperoxidase, its well-studied enzymatic activity and incoherently understood non-enzymatic role in various cell types and tissues. Further, we elaborate on Myeloperoxidase (MPO) in the complex context of cardiovascular disease, innate and autoimmune response, development and progression of cancer and neurodegenerative diseases.

Biosynthesis of human myeloperoxidase

Members of Chordata peroxidase subfamily [1] expressed in mammals, including myeloperoxidase (MPO), eosinophil peroxidase (EPO), lactoperoxidase (LPO), and thyroid peroxidase (TPO), express conserved motifs around the heme prosthetic group essential for their activity, a calcium-binding site, and at least two covalent bonds linking the heme group to the protein backbone. Although most studies of the biosynthesis of these peroxidases have focused on MPO, many of the features described occur during biosynthesis of other members of the protein subfamily. Whereas MPO biosynthesis includes events typical for proteins generated in the secretory pathway, the importance and consequences of heme insertion are events uniquely associated with peroxidases. This Review summarizes decades of work elucidating specific steps in the biosynthetic pathway of human MPO. Discussion includes cotranslational glycosylation and subsequent modifications of the N-linked carbohydrate sidechains, contributions by molecular chaperones in the endoplasmic reticulum, cleavage of the propeptide from proMPO, and proteolytic processing of protomers and dimerization to yield mature MPO. Parallels between the biosynthesis of MPO and TPO as well as the impact of inherited mutations in the MPO gene on normal biosynthesis will be summarized. Lastly, specific gaps in our knowledge revealed by this review of our current understanding will be highlighted.

Structure of human promyeloperoxidase (proMPO) and the role of the propeptide in processing and maturation

Myeloperoxidase (MPO) is synthesized by neutrophil and monocyte precursor cells and contributes to host defense by mediating microbial killing. Although several steps in MPO biosynthesis and processing have been elucidated, many questions remained, such as the structure-function relationship of monomeric unprocessed proMPO versus the mature dimeric MPO and the functional role of the propeptide. Here we have presented the first and high resolution (at 1.25 Å) crystal structure of proMPO and its solution structure obtained by small-angle X-ray scattering. Promyeloperoxidase hosts five occupied glycosylation sites and six intrachain cystine bridges with Cys-158 of the very flexible N-terminal propeptide being covalently linked to Cys-319 and thereby hindering homodimerization. Furthermore, the structure revealed (i) the binding site of proMPO-processing proconvertase, (ii) the structural motif for subsequent cleavage to the heavy and light chains of mature MPO protomers, and (iii) three covalent bonds between heme and the protein. Studies of the mutants C158A, C319A, and C158A/C319A demonstrated significant differences from the wild-type protein, including diminished enzymatic activity and prevention of export to the Golgi due to prolonged association with the chaperone calnexin. These structural and functional findings provide novel insights into MPO biosynthesis and processing.

Cellular Uptake and Delivery of Myeloperoxidase to Lysosomes Promote Lipofuscin Degradation and Lysosomal Stress in Retinal Cells

Neutrophil myeloperoxidase (MPO) catalyzes the H₂O₂-dependent oxidation of chloride anion to generate hypochlorous acid, a potent antimicrobial agent. Besides its well defined role in innate immunity, aberrant degranulation of neutrophils in several inflammatory diseases leads to redistribution of MPO to the extracellular space, where it can mediate tissue damage by promoting the oxidation of several additional substrates. Here, we demonstrate that mannose 6-phosphate receptor-mediated cellular uptake and delivery of MPO to lysosomes of retinal pigmented epithelial (RPE) cells acts to clear this harmful enzyme from the extracellular space, with lysosomal-delivered MPO exhibiting a half-life of 10 h. Lysosomal-targeted MPO exerts both cell-protective and cytotoxic functions. From a therapeutic standpoint, MPO catalyzes the in vitro degradation of N-retinylidene-N-retinylethanolamine, a toxic form of retinal lipofuscin that accumulates in RPE lysosomes and drives the pathogenesis of Stargardt macular degeneration. Furthermore, chronic cellular uptake and accumulation of MPO in lysosomes coincides with N-retinylidene-N-retinylethanolamine elimination in a cell-based model of macular degeneration. However, lysosomal-delivered MPO also disrupts lysosomal acidification in RPE cells, which coincides with nuclear translocation of the lysosomal stress-sensing transcription factor EB and, eventually, cell death. Based on these findings we predict that under periods of acute exposure, cellular uptake and lysosomal degradation of MPO mediates elimination of this harmful enzyme, whereas chronic exposure results in progressive accumulation of MPO in lysosomes. Lysosomal-accumulated MPO can be both cell-protective, by promoting the degradation of toxic retinal lipofuscin deposits, and cytotoxic, by triggering lysosomal stress and cell death.

Proprotein Convertase Processing Enhances Peroxidasin Activity to Reinforce Collagen IV

The basement membrane (BM) is a form of extracellular matrix that underlies cell layers in nearly all animal tissues. Type IV collagen, a major constituent of BMs, is critical for tissue development and architecture. The enzyme peroxidasin (Pxdn), an extracellular matrix-associated protein, catalyzes the formation of structurally reinforcing sulfilimine cross-links within the collagen IV network, an event essential to basement membrane integrity. Although the catalytic function of Pxdn is known, the regulation of its activity remains unclear. In this work we show through N-terminal sequencing, pharmacologic studies, and mutational analysis that proprotein convertases (PCs) proteolytically process human Pxdn at Arg-1336, a location relatively close to its C terminus. PC processing enhances the enzymatic activity of Pxdn and facilitates the formation of sulfilimine cross-links in collagen IV. Thus, PC processing of Pxdn is a key regulatory step that contributes to its function and, therefore, supports BM integrity and homeostasis.

T47D Cells Expressing Myeloperoxidase Are Able to Process, Traffic and Store the Mature Protein in Lysosomes: Studies in T47D Cells Reveal a Role for Cys319 in MPO Biosynthesis that Precedes Its Known Role in Inter-Molecular Disulfide Bond Formation

Among the human heme-peroxidase family, myeloperoxidase (MPO) has a unique disulfide-linked oligomeric structure resulting from multi-step processing of the pro-protein monomer (proMPO) after it exits the endoplasmic reticulum (ER). Related family members undergo some, but not all, of the processing steps involved with formation of mature MPO. Lactoperoxidase has its pro-domain proteolytically removed and is a monomer in its mature form. Eosinophil peroxidase undergoes proteolytic removal of its pro-domain followed by proteolytic separation into heavy and light chains and is a heterodimer. However, only MPO undergoes both these proteolytic modifications and then is further oligomerized into a heterotetramer by a single inter-molecular disulfide bond. The details of how and where the post-ER processing steps of MPO occur are incompletely understood. We report here that T47D breast cancer cells stably transfected with an MPO expression plasmid are able to efficiently replicate all of the processing steps that lead to formation of the mature MPO heterotetramer. MPO also traffics to the lysosome granules of T47D cells where it accumulates, allowing in-depth immunofluorescent microscopy studies of MPO trafficking and storage for the first time. Using this novel cell model we show that formation of MPO’s single inter-molecular disulfide bond can occur normally in the absence of the proteolytic events that lead to separation of the MPO heavy and light chains. We further demonstrate that Cys³¹⁹, which forms MPO’s unique inter-molecular disulfide bond, is important for events that precede this step. Mutation of this residue alters the glycosylation and catalytic activity of MPO and blocks its entry into the endocytic pathway where proteolytic processing and disulfide bonding occur. Finally, using the endocytic trafficking of lysosomal hydrolases as a guide, we investigate the role of candidate receptors in the endocytic trafficking of MPO.