Cloning of rat aorta lysyl oxidase cDNA: complete codons and predicted amino acid sequence

Lysyl Oxidases as Targets for Cancer Therapy and Diagnostic Imaging

The understanding of the contribution of the tumour microenvironment to cancer progression and metastasis, in particular the interplay between tumour cells, fibroblasts and the extracellular matrix has grown tremendously over the last years. Lysyl oxidases are increasingly recognised as key players in this context, in addition to their function as drivers of fibrotic diseases. These insights have considerably stimulated drug discovery efforts towards lysyl oxidases as targets over the last decade. This review article summarises the biochemical and structural properties of theses enzymes. Their involvement in tumour progression and metastasis is highlighted from a biochemical point of view, taking into consideration both the extracellular and intracellular action of lysyl oxidases. More recently reported inhibitor compounds are discussed with an emphasis on their discovery, structure-activity relationships and the results of their biological characterisation. Molecular probes developed for imaging of lysyl oxidase activity are reviewed from the perspective of their detection principles, performance and biomedical applications.

Analysis of lysyl oxidase as a marker for diagnosis of canine mammary tumors

Lysyl oxidase (LOX) is an extracellular metalloenzyme which mediates crosslinking of collagen and elastin. It has been reported to play a pivotal role in cancer metastasis especially in women suffering from breast cancer. The present study is the first to evaluate the gene expression levels of LOX by Real time-polymerase chain reaction (Real time-PCR) in dogs with mammary tumor besides molecular cloning and expression of canine lysyl oxidase gene (lox). Real time-PCR studies showed a significant upregulation (threefold higher) of lox in mammary tumor cases as compared to healthy dogs indicating its possible diagnostic and prognostic role in canine mammary tumors (CMTs). Cloning and sequencing of lox gene revealed 1230 bp CDS which is mostly conserved in C-terminal region. Sequence analysis of canine lox showed that it shares 99% homology with the predicted sequence available on NCBI and had greatest identity with the lox gene from cat. Protein structure predicted with homology modelling was validated by Ramachandran plot analysis which revealed most (approximately 95%) of the amino acids in favoured region. Additionally, recombinant lysyl oxidase expressed as His-tagged fusion protein in prokaryotic expression vector (pPROExHTa) was used in an ELISA for detection of circulating protein LOX in serum of CMT subjects. Receiver operating characteristics analysis of the ELISA revealed high sensitivity (90%) and specificity (85%) with histopathology as reference standard. Taken together, we propose LOX as a diagnostic biomarker and a putative prognostic candidate in CMT cases.

LOX/LOXL in pulmonary fibrosis: potential therapeutic targets

Lysyl oxidase (LOX) and lysyl oxidase-like proteins (LOXL), a family of extracellular matrix (ECM) crosslinking enzymes that have been recognised as playing an important role in fibrogenesis for more than 40 years, are logical targets for antifibrotic treatments. Pulmonary fibrosis, especially idiopathic pulmonary fibrosis (IPF), is a progressive and lethal disease characterised by excessive deposition of ECM in the lung parenchyma. In this review, we discuss the current clinical approaches for IPF and review members of LOX family-LOX, LOXL1, LOXL2, LOXL3 and LOXL4 in IPF patients and in animal models of bleomycin-induced pulmonary fibrosis. Although these findings are controversial and require further validation, LOX/LOXL1/LOXL2 as potential therapeutic targets for IPF deserve continued attention. So far to our knowledge, LOXL3 or LOXL4 has not clearly shown specific therapeutic potential.

Evaluation of Fluorine-18-Labeled α1(I)-N-Telopeptide Analogs as Substrate-Based Radiotracers for PET Imaging of Melanoma-Associated Lysyl Oxidase

Accumulating evidence suggests an unequivocal role of lysyl oxidases as key players of tumor progression and metastasis, which renders this enzyme family highly attractive for targeted non-invasive functional imaging of tumors. Considering their function in matrix remodeling, malignant melanoma appears as particularly interesting neoplasia in this respect. For the development of radiotracers that enable PET imaging of the melanoma-associated lysyl oxidase activity, substrates derived from the type I collagen α1 N-telopeptide were labeled with fluorine-18 using N-succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) as prosthetic reagent. With regards to potential crosslinking to tumor-associated collagen in vivo, their interaction with triple-helical type I collagen was studied by SPR. A mouse model of human melanoma was established on the basis of the A375 cell line, for which the expression of the oncologically relevant lysyl oxidase isoforms LOX and LOXL2 was demonstrated in Western blot and immunohistochemical experiments. The radiopharmacological profiles of the peptidic radiotracers were evaluated in normal rats and A375 melanoma-bearing mice by ex vivo metabolite analysis, whole-body biodistribution studies and dynamic PET imaging. Out of three ¹⁸F-labeled telopeptide analogs, the one with the most favorable substrate properties has shown favorable tumor uptake and tumor-to-muscle ratio. Lysyl oxidase-mediated tumor uptake was proven by pharmacological inhibition using β-aminopropionitrile and by employing negative-control analogs of impeded or abolished targeting capability. The latter were obtained by substituting the lysine residue by ornithine and norleucine, respectively. Comparing the tumor uptake of the lysine-containing peptide with that of the non-functional analogs indicate the feasibility of lysyl oxidase imaging in melanoma using substrate-based radiotracers.

Decreased copper concentrations but increased lysyl oxidase activity in ischemic hearts of rhesus monkeys

Myocardial ischemia leads to a decrease in copper (Cu) concentrations, along with collagen deposition in which Cu-dependent lysyl oxidase (LOX) catalyzes the cross-linking of collagens leading to tissue stiffness. The present study was undertaken to determine the relationship between decreased Cu concentrations and LOX activities in ischemic hearts of monkeys. Rhesus monkeys were subjected to coronary artery ligation, leading to ischemic infarction. At 8 weeks after the surgery, Cu concentrations and Cu-dependent cytochrome c oxidase (CCO) activities in the infarct area were significantly decreased. Unexpectedly, the Cu-dependent LOX activities in the same area were significantly increased. LOX proteins were accumulated in the cytosol of myofibroblasts, endothelial cells, and residual cardiomyocytes in the infarct area. In contrast, LOX was only found in fibroblasts and myocardial intercalated discs between cardiomyocytes in sham-operated controls. The LOX mRNA level was also increased in the infarct area compared to the sham operated control. This upregulation of LOX was associated with significant increases in collagen deposition; protein levels of type I and III collagens were significantly increased along with increases in their mRNA levels in the infarct area. This finding indicates that under myocardial infarction, Cu-dependent CCO activities were depressed but LOX activities were increased most likely through Cu redistribution although Cu concentrations were significantly depressed.

Lysyl Oxidase Isoforms and Potential Therapeutic Opportunities for Fibrosis and Cancer

INTRODUCTION

The lysyl oxidase family of enzymes is classically known as being required for connective tissue maturation by oxidizing lysine residues in elastin and lysine and hydroxylysine residues in collagen precursors. The resulting aldehydes then participate in cross-link formation, which is required for normal connective tissue integrity. These enzymes have biological functions that extend beyond this fundamental biosynthetic role, with contributions to angiogenesis, cell proliferation, and cell differentiation. Dysregulation of lysyl oxidases occurs in multiple pathologies including fibrosis, primary and metastatic cancers, and complications of diabetes in a variety of tissues.

AREAS COVERED

This review summarizes the major findings of novel roles for lysyl oxidases in pathologies, and highlights some of the potential therapeutic approaches that are in development and which stem from these new findings.

EXPERT OPINION

Fundamental questions remain regarding the mechanisms of novel biological functions of this family of proteins, and regarding functions that are independent of their catalytic enzyme activity. However, progress is underway in the development of isoform-specific pharmacologic inhibitors, potential therapeutic antibodies and gaining an increased understanding of both tumor suppressor and metastasis promotion activities. Ultimately, this is likely to lead to novel therapeutic agents.

Activation of cellular chemotactic responses to chemokines coupled with oxidation of plasma membrane proteins by lysyl oxidase

Lysyl oxidase (LOX) is a potent chemokine inducing the migration of varied cell types. Here we demonstrate that inhibition of cellular LOX activity by preincubation of vascular smooth muscle cells (VSMC) with β-aminopropionitrile (BAPN), the irreversible inhibitor of LOX activity, resulted in the marked suppression of the chemotactic response and sensitivity of these cells toward LOX and toward PDGF-BB. Plasma membranes purified from VSMC not previously exposed to BAPN contained a group of oxidized plasma membrane proteins, including the PDGF receptor, PDGFR-β. The oxidation of this receptor and other membrane proteins was largely prevented in cells preincubated with BAPN. Addition of purified LOX to BAPN-free cells, which had been previously exposed to BAPN, restored the profile of oxidized proteins towards that of control cells. The high affinity and capacity for the binding of PDGF-BB by cells was significantly diminished when compared with cells in which oxidation by LOX was prevented by BAPN. The chemotactic responses of LOX knock-out mouse embryonic fibroblasts mirrored those obtained with VSMC treated with BAPN. These novel findings suggest that LOX activity is essential to generate optimal chemotactic sensitivity of cells to chemoattractants by oxidizing specific cell surface proteins, such as PDGFR-β.

Vascular extracellular matrix and arterial mechanics

An important factor in the transition from an open to a closed circulatory system was a change in vessel wall structure and composition that enabled the large arteries to store and release energy during the cardiac cycle. The component of the arterial wall in vertebrates that accounts for these properties is the elastic fiber network organized by medial smooth muscle. Beginning with the onset of pulsatile blood flow in the developing aorta, smooth muscle cells in the vessel wall produce a complex extracellular matrix (ECM) that will ultimately define the mechanical properties that are critical for proper function of the adult vascular system. This review discusses the structural ECM proteins in the vertebrate aortic wall and will explore how the choice of ECM components has changed through evolution as the cardiovascular system became more advanced and pulse pressure increased. By correlating vessel mechanics with physiological blood pressure across animal species and in mice with altered vessel compliance, we show that cardiac and vascular development are physiologically coupled, and we provide evidence for a universal elastic modulus that controls the parameters of ECM deposition in vessel wall development. We also discuss mechanical models that can be used to design better tissue-engineered vessels and to test the efficacy of clinical treatments.

mRNA expression of lysyl oxidase and matrix metalloproteinase-12 in mouse skin

Elastic fibers in the dermis play an important role in skin elasticity. The desmosine crosslinking structure constructed of lysyl oxidase (LOX) in elastic fibers contributes to elasticity, while elastic fibers are primarily degraded by one of the matrix metalloproteinases (MMPs), MMP-12. We investigated the gender differences and diurnal variation of these enzymes. Gender-based differences in LOX mRNA expression were detected, and were significantly lower in females. In contrast, higher MMP-12 mRNA expression was observed in the light period, suggesting that elastic fibers might be degraded in the light rather than the dark period.

Catalytic properties and structural components of lysyl oxidase

Key aspects of the biosynthesis and catalytic specificity of lysyl oxidase (LO) have been explored. Oxidation of peptidyl lysine in synthetic oligopeptides is markedly sensitive to the presence of vicinal dicarboxylic ami/no acid residues. Optimal activity is obtained with the -Glu-Lys- sequence within a polyglycine 11-mer, whereas the -Lys-Glu- sequence is much less efficiently oxidized. The -Asp-Glu-Lys- sequence is a very poor substrate, although this sequence is oxidized in type I collagen fibrils. These results are considered in the light of a model requiring collagen to be assembled as fibrils prior to oxidation by LO. An in vitro system for the expression of catalytically active LO has been devised. Deletion or inclusion of the cDNA coding for the propeptide region in the expressed construct results in apparently identical, catalytically active enzyme products, indicating the lack of essentiality of this region for active enzyme production. These effects are considered with respect to the conservation of the amino acid sequence of LO produced by different species.

Cloning and characterization of the rat lysyl oxidase gene promoter: identification of core promoter elements and functional nuclear factor I-binding sites

Lysyl oxidase (LO) stabilizes the extracellular matrix by cross-linking collagen and elastin. To assess the transcriptional regulation of LO, we cloned the 5'-flanking region with 3,979 bp of the rat LO gene. LO transcription started at multiple sites clustered at the region from -78 to -51 upstream of ATG. The downstream core promoter element functionally independent of the initiator predominantly activated the TATA-less LO gene. 5' Deletion assays illustrated a sequence of 804 bp upstream of ATG sufficient for eliciting the maximal promoter activity and the region -709/-598 exhibiting strongly enhancing effects on the reporter gene expression in transiently transfected RFL6 cells. DNase I footprinting assays showed a protected pattern existing in the fragment -612/-580, which contains a nuclear factor I (NFI)-binding site at the region -594/-580 confirmed by electrophoretic mobility supershift assays. Mutations on this acting site decreased both NFI binding affinity in gel shift assays and stimulation of SV40 promoter activities in cells transfected with the NFI-binding site-SV40 promoter chimeric construct. Furthermore, at least two functional NFI-binding sites, including another one located at -147/-133, were identified in the LO promoter region -804/-1. Only NFI-A and NFI-B were expressed in rat lung fibroblasts, and their interaction with the LO gene was sensitively modulated by exogenous stimuli such as cigarette smoke condensate. In conclusion, the isolated rat LO gene promoter contains functionally independent initiator and downstream core promoter elements, and the conserved NFI-binding sites play a critical role in the LO gene activation.

The Tumor Suppressor Activity of the Lysyl Oxidase Propeptide Reverses the Invasive Phenotype of Her-2/neu–Driven Breast Cancer

Expression of the lysyl oxidase gene (LOX) was found to inhibit the transforming activity of the ras oncogene in NIH 3T3 fibroblasts and was hence named the ras recision gene (rrg). Lysyl oxidase (LOX) is synthesized and secreted as a 50-kDa inactive proenzyme (Pro-LOX), which is processed by proteolytic cleavage to a functional 32-kDa enzyme and an 18-kDa propeptide (LOX-PP). Recently, the ras recision activity of the LOX gene in NIH 3T3 cells was mapped to its propeptide region. Here, we show for the first time that LOX-PP inhibits transformation of breast cancer cells driven by Her-2/neu, an upstream activator of Ras. LOX-PP expression in Her-2/neu-driven breast cancer cells in culture suppressed Akt, extracellular signal-regulated kinase, and nuclear factor-kappaB activation. Her-2/neu-induced epithelial to mesenchymal transition was reverted by LOX-PP, as judged by reduced levels of Snail and vimentin; up-regulation of E-cadherin, gamma-catenin, and estrogen receptor alpha; and decreased ability to migrate or to form branching colonies in Matrigel. Furthermore, LOX-PP inhibited Her-2/neu tumor formation in a nude mouse xenograft model. Thus, LOX-PP inhibits signaling cascades induced by Her-2/neu that promote a more invasive phenotype and may provide a novel avenue for treatment of Her-2/neu-driven breast carcinomas.

LOXL as a target to increase the elastin content in adult skin: a dill extract induces the LOXL gene expression

The lysyl oxidases lysyl oxidase (LOX) and lysyl oxidase-like (LOXL) are responsible for elastin cross-linking. It was shown recently that LOXL is essential for the elastic fibres homeostasis and for their maintenance at adult age. We first determined whether or not elastin, LOX and LOXL are less expressed during adulthood. The LOX and LOXL mRNA level, quantified by real-time reverse transcriptase-polymerase chain reaction decreased in adult skin fibroblasts compared with fibroblasts from children. In contrast, the elastin mRNA level remains stable at all ages. The goal of this study was to induce elastogenesis at the adult age. Therefore, both enzymes, and in particular LOXL, of which expression is the most affected by age, could be targeted to induce elastogenesis in adult skin. We screened a library of about 1000 active ingredients to find activators capable to stimulate specifically the LOXL gene expression in adult dermal fibroblasts. The positive effect of selected active ingredients was confirmed on fibroblasts grown on monolayers and on dermal and skin equivalent cultures. One extract, obtained from dill (LYS'LASTINE V, Engelhard, Lyon, France), stimulates the LOXL gene expression in dermal equivalents (+64% increase in the LOXL mRNA level when compared with control). At the same time, the elastin detection is increased in dermal equivalents and under the dermal-epidermal junction of skin equivalents, without increase of the elastin mRNA. In conclusion, LOXL can be considered as a new target to reinduce elastogenesis. Its stimulation by a dill extract is correlated with increased elastin detection, suggesting an increase in elastogenesis efficiency.

Inhibition of the expression of lysyl oxidase and its substrates in cadmium-resistant rat fetal lung fibroblasts

Copper (Cu)-dependent lysyl oxidase (LO) catalyzes crosslinking of collagen and elastin stabilizing the extracellular matrix (ECM). Chronic inhalation of cadmium (Cd), a toxic metal, induces emphysema. To probe mechanisms of Cd injury to the lung, we developed Cd-resistant (CdR) cells from rat fetal lung fibroblasts (RFL6) by chronic exposure to CdCl(2) from 1 to 40 microM and further examined their expressions of LO, LO substrates, and Cu-scavenging thiols. Levels of cellular thiols, metallothionein, and glutathione in CdR cells were elevated to 13.0- and 3.2-fold of parental controls, respectively, whereas LO mRNA and protein levels were markedly reduced in these cells, with catalytic activity declining to only 16% of the parental control. A conspicuous 52 kDa species rather then the normal 50 kDa proenzyme appeared in the CdR cell extract but not in the conditioned medium, which was codistributed with the endoplasmic reticulum marker [DiOC5(3)] within the cell, implying the Cd-induced 52 kDa species as a product of an abnormal LO-processing defect in secretion. Addition of Cu into CdR cell cultures enhanced the expression of LO mRNA, protein and catalytic activities reflecting limitation of Cu bioavailability for LO in these cells. With inhibition of LO, CdR cells also displayed downregulation of collagen and elastin, substrates of LO. Restoration of collagen synthesis by exposure of CdR cells to purified LO or Cu suggests that inhibition of LO and limitation of Cu cofactor by Cd, as key phenotype changes, accelerated collagen and elastin damage, a critical event pertinent to emphysema pathogenesis.

Lysyl oxidase is essential for normal development and function of the respiratory system and for the integrity of elastic and collagen fibers in various tissues

Lysyl oxidases, a family comprising LOX and four LOX-like enzymes, catalyze crosslinking of elastin and collagens. Mouse Lox was recently shown to be crucial for development of the cardiovascular system because null mice died perinatally of aortic aneurysms and cardiovascular dysfunction. We show here that Lox is also essential for development of the respiratory system and the integrity of elastic and collagen fibers in the lungs and skin. The lungs of E18.5 Lox(-/-) embryos showed impaired development of the distal and proximal airways. Elastic fibers in E18.5 Lox(-/-) lungs were markedly less intensely stained and more disperse than in the wild type, especially in the mesenchyme surrounding the distal airways, bronchioles, bronchi, and trachea, and were fragmented in pulmonary arterial walls. The organization of individual collagen fibers into tight bundles was likewise abnormal. Similar elastic and collagen fiber abnormalities were seen in the skin. Lysyl oxidase activity in cultured Lox(-/-) skin fibroblasts and aortic smooth muscle cells was reduced by approximately 80%, indicating that Lox is the main isoenzyme in these cells. LOX abnormalities may thus be critical for the pathogenesis of several common diseases, including pulmonary, skin, and cardiovascular disorders.

Abnormal deposition of collagen around hepatocytes in Wilson's disease is associated with hepatocyte specific expression of lysyl oxidase and lysyl oxidase like protein-2

BACKGROUND/AIMS

Lysyl-oxidases catalyze the oxidation of lysine residues in collagen and elastin thereby promoting their polymerization. We have studied here the expression of four lysyl-oxidases in normal and diseased human liver.

METHODS

The expression of the different lysyl-oxidases in paraffin embedded liver sections was studied using in-situ hybridization and immunohistochemistry. The enzymatic activity of lysyl-oxidase like protein-2 (Loxl2 or LOR-1) using a previously described lysyl-oxidase assay.

RESULTS

We have found that the four lysyl-oxidases which we examined are not significantly expressed in the normal liver. By contrast, Wilson's disease and primary biliary cirrhosis (PBC) patients express lysyl-oxidase (Lox) and lysyl-oxidase like protein-2 (Loxl2 or LOR-1) in hepatocytes, and the expression is accompanied by collagen deposition around the hepatocytes. Lysyl-oxidases are also expressed in additional fibrotic liver diseases such as hepatitis B and C but in these diseases the expression is confined to the fibrotic lesions and collagen does not accumulate around hepatocytes. We have found that Loxl2 is able to oxidize lysine residues of collagen, and behaves in that respect similarly to Lox. The copper chelator D-penicillamine inhibits Loxl2 induced oxidation of collagen but the Lox inhibitor beta-aminopropionitrile did not inhibit the oxidation using a BAPN concentration at which Lox activity was completely inhibited. Loxl2 also catalyzed the oxidation of cell surface proteins on HepG2 hepatoblastoma cells and inhibited their proliferation.

CONCLUSIONS

Upregulation of Lox and Loxl2 in hepatocytes of Wilson's disease and PBC patients may contribute to liver damage by various mechanisms. The upregulation of Lox and Loxl2 in Wilson's disease could perhaps be utilized for diagnostic purposes since their expression is up-regulated in hepatocytes even before the onset of fibrosis.

A procollagen C-proteinase inhibitor diminishes collagen and lysyl oxidase processing but not collagen cross-linking in osteoblastic cultures

The deposition of insoluble functional collagen occurs following extracellular proteolytic processing of procollagens by procollagen N- and C-proteinases, fibril formation, and lysyl oxidase dependent cross-linking. Procollagen C-proteinases in addition process and activate lysyl oxidase. The present study evaluates a possible role for procollagen C-proteinases in controlling different aspects of collagen deposition in vitro. Studies determine whether inhibition of procollagen C-proteinase activity with a specific BMP-1 inhibitor results in perturbations in lysyl oxidase activation, and in collagen processing, deposition, and cross-linking in phenotypically normal cultured murine MC3T3-E1 cells. Data show that BMP-1 Inhibitor dose dependently inhibits lysyl oxidase activation by up to 50% in undifferentiated proliferating cells. In differentiating cultures, BMP-1 inhibitor decreased collagen processing but did not inhibit the accumulation of mature collagen cross-links. Finally, electron microscopy studies show that collagen fibril diameter increased. Thus, inhibition of procollagen C-proteinases results in perturbed collagen deposition primarily via decreased collagen processing.

The Propeptide Domain of Lysyl Oxidase Induces Phenotypic Reversion of Ras-transformed Cells

Lysyl oxidase is an extracellular enzyme critical for the normal biosynthesis of collagens and elastin. In addition, lysyl oxidase reverts ras-mediated transformation, and lysyl oxidase expression is down-regulated in human cancers. Since suramin inhibits growth factor signaling pathways and induces lysyl oxidase in ras-transformed NIH3T3 cells (RS485 cells), we sought to investigate the effects of suramin on the phenotype of transformed cells and the role of lysyl oxidase in mediating these effects. Suramin treatment resulted in a more normal phenotype as judged by growth rate, cell cycle parameters, and morphology. beta-aminopropionitrile, the selective inhibitor of lysyl oxidase enzyme activity, was remarkably unable to block suramin-induced reversion. By contrast, ectopic antisense lysyl oxidase demonstrated that lysyl oxidase gene expression mediated phenotypic reversion. Since lysyl oxidase is synthesized as a 50 kDa precursor and processed to a 30 kDa active enzyme and 18 kDa propeptide, the effects of these two products on the transformed phenotype of RS485 cells were then directly assessed in the absence of suramin. Here we report, for the first time, that the lysyl oxidase propeptide, and not the lysyl oxidase enzyme, inhibits ras-dependent transformation as determined by effects on cell proliferation assays, growth in soft agar, and Akt-dependent induction of NF-kappaB activity. Thus, the lysyl oxidase propeptide, which is released during extracellular proteolytic processing of pro-lysyl oxidase, functions to inhibit ras-dependent cell transformation.

Regulation of collagen deposition and lysyl oxidase by tumor necrosis factor-alpha in osteoblasts

Tumor necrosis factor-alpha (TNF-alpha) inhibits osteoblast function in vitro by inhibiting collagen deposition. Studies generally support that TNF-alpha does not inhibit collagen biosynthesis by osteoblasts but that collagen deposition is in some way diminished. The study investigated TNF-alpha regulation of biosynthetic enzymes and proteins crucial for posttranslational extracellular collagen maturation in osteoblasts including procollagen C-proteinases, procollagen C-proteinase enhancer, and lysyl oxidase. The working hypothesis is that such regulation could inhibit collagen deposition by osteoblasts. We report that in phenotypically normal MC3T3-E1 osteoblasts, TNF-alpha decreases collagen deposition without decreasing collagen mRNA levels or procollagen protein synthesis. Analyses of the cell layers revealed that TNF-alpha diminished the levels of mature collagen cross-links, pyridinoline and deoxypyridinoline. Further analyses revealed that the mRNA expression for lysyl oxidase, the determining enzyme required for collagen cross-linking, is down-regulated by TNF-alpha in a concentration- and time-dependent manner by up to 50%. The decrease was accompanied by a significant reduction of lysyl oxidase protein levels and enzyme activity. By contrast, Northern and Western blotting studies revealed that procollagen C-proteinases bone morphogenic protein-1 and mammalians Tolloid and procollagen C-proteinase enhancer were expressed in MC3T3-E1 cells and not down-regulated. The data together demonstrate that TNF-alpha does not inhibit collagen synthesis but does inhibit the expression and activity of lysyl oxidase in osteoblasts, thereby contributing to perturbed collagen cross-linking and accumulation. These studies identify a novel mechanism in which proinflammatory cytokine modulation of an extracellular biosynthetic enzyme plays a determining role in the control of collagen accumulation by osteoblasts.

Characterization of AOC2 gene encoding a copper-binding amine oxidase expressed specifically in retina

We have previously cloned a human, retina-specific, amine oxidase gene (RAO, gene symbol: AOC2), a member of the copper-binding amine oxidase super family. AOC2 shares sequence identity with the human kidney amine oxidase gene (KAO, gene symbol: AOC1) and the vascular adhesion protein-1 gene (VAP-1, gene symbol: AOC3). For further analysis of AOC2, the sequences surrounding the human AOC2 and the complete mouse and partial rat homologue of AOC2 were cloned for characterization. Real-time quantitative PCR, in situ hybridization, and immunohistochemistry were performed to determine the specific expression of AOC2 in the mouse retina and especially in the retinal ganglion cells. Our results demonstrated that the copper-binding motif and the enzyme active site of AOC1 and AOC3 were both conserved in mouse AOC2. The human and mouse AOC2 was flanked by two genes, the Psme3 gene for PA-28 gamma subunit and, surprisingly, the AOC3 gene. Rat AOC2 contained a stop codon that terminated the peptide length to 127 amino acids. The presence of human and rat AOC pseudogene in this region, in addition to the tandemly positioned two AOC genes, indicates the possibility of successful AOC3 replication to retina-specific AOC2 for human and mouse but unsuccessful for rat.

Trace elements in human physiology and pathology. Copper

Copper is a trace element, important for the function of many cellular enzymes. Copper ions can adopt distinct redox states oxidized Cu(II) or reduced (I), allowing the metal to play a pivotal role in cell physiology as a catalytic cofactor in the redox chemistry of enzymes, mitochondrial respiration, iron absorption, free radical scavenging and elastin cross-linking. If present in excess, free copper ions can cause damage to cellular components and a delicate balance between the uptake and efflux of copper ions determines the amount of cellular copper. In biological systems, copper homeostasis has been characterized at the molecular level. It is coordinated by several proteins such as glutathione, metallothionein, Cu-transporting P-type ATPases, Menkes and Wilson proteins and by cytoplasmic transport proteins called copper chaperones to ensure that it is delivered to specific subcellular compartments and thereby to copper-requiring proteins.

Accelerated calcification represses the expression of elastic fiber components and lysyl oxidase in cultured bovine aortic smooth muscle cells

Vascular calcification is a common feature of advanced atherosclerosis resulting in reduced elasticity of elastic arteries. However, the relationship between elastic fibers and vascular calcification at the molecular and cellular levels remains unknown. We investigated the expression of major elastic fiber components such as tropoelastin (TE) and fibrillin-1 (FBN1) and elastin-related enzyme, lysyl oxidase (LO), in a calcification model using beta-glycerophosphate (beta-GP) in cultured bovine aortic smooth muscle cells (BASMCs). Ten mM of beta-GP stimulated calcium deposition in a time-dependent manner. As determined by Western blot analysis, 10 mM of beta-GP time-dependently decreased TE and FBN1 protein levels. TE, FBN1, and LO mRNA levels, assessed by reverse transcription-polymerase chain reaction, were also decreased by exposure to 10 mM beta-GP. Furthermore, we investigated whether the processes of calcification in BASMCs directly control these regulations. In experiments using levamisole, an alkaline phosphatase inhibitor, and DMDP, a bisphosphonate, both inhibitors inhibited down-regulation during beta-GP-induced calcification, suggesting that the down-regulation of TE, FBN1, and LO directly relates to calcium deposition. In cases of vascular calcification, the decreased expression of TE, FBN1, and LO may be partially responsible for decreased vascular elasticity and also for the decreased formation of new elastic fibers.

Enhancing mechanical properties of tissue-engineered constructs via lysyl oxidase crosslinking activity

A number of strategies have been investigated to enhance the mechanical stability of engineered tissues. In this study, we utilized lysyl oxidase (LO) to enzymatically crosslink extracellular matrix (ECM) proteins, particularly collagen and elastin, to enhance the mechanical integrity of the ECM and thereby impart mechanical strength to the engineered tissue. Vascular smooth muscle cells (VSMCs) were liposomally transfected with the LO gene. Both Northern and Western analyses confirmed increased LO expression. Increased LO activity was demonstrated using a fluorescent enzyme substrate assay and by observation of the presence of increased levels of desmosine, a product of LO crosslinking, in the ECM. The mechanical effects of altered crosslink densities within tissue-engineered constructs were demonstrated in a VSMC-populated collagen gel model. When smooth muscle cells transfected with lysyl oxidase were seeded in collagen gels, the tensile strength and elastic modulus in these constructs increased by approximately two-fold compared to constructs seeded with mock-transfected VSMCs. Also, desmosine levels in the LO-populated collagen gels were higher than they were in mock-seeded gels, as demonstrated via immunohistochemical staining. Compositional analysis of the ECM deposited by the transformed cells showed similar collagen and elastin levels, and cell proliferation rates were similar as well, thus attributing increased mechanical properties to ECM crosslinking.

Autocrine growth factor regulation of lysyl oxidase expression in transformed fibroblasts

Lysyl oxidase catalyzes oxidative deamination of peptidyl-lysine and hydroxylysine residues in collagens and lysine residues in elastin to form peptidyl aldehydes that are required for the formation of covalent cross-links in normal extracellular matrix biosynthesis. Lysyl oxidase in addition has tumor suppressor activity, and phenotypic reversion of transformed cell lines is accompanied by increased lysyl oxidase expression. The mechanism of low expression of lysyl oxidase in tumor cells is unknown. The present study investigates the hypothesis that autocrine growth factor pathways maintain low lysyl oxidase expression levels in c-H-ras-transformed fibroblasts (RS485 cell line). Autocrine pathways were blocked with suramin, a general inhibitor of growth factor receptor binding, and resulted in more than a 10-fold increase in lysyl oxidase expression and proenzyme production. This regulation was found to be reversible and occurred at the transcriptional level determined using lysyl oxidase promoter/reporter gene assays. Function blocking anti-fibroblast growth factor-2 (FGF-2) antibody enhanced lysyl oxidase expression in the absence of suramin. Finally, the addition of FGF-2 to suramin-treated cells completely reversed suramin stimulation of lysyl oxidase mRNA levels. Data support that an FGF-2 autocrine pathway inhibits lysyl oxidase transcription in the tumorigenic-transformed RS485 cell line. This finding may be of therapeutic significance and, in addition, provides a new experimental approach to investigate the mechanism of the tumor suppressor activity of lysyl oxidase.

Characterization of an in vitro model of calcification in retinal pigmented epithelial cells

Little is known about the relationship at the molecular and cellular levels between vascular calcification and elastic fibers essential for elasticity. To gain a better understanding of the physiological function of elastin in vascular calcification, we developed a calcification model on cultured bovine retinal-pigmented-epithelial cells (RPEs) that do not express endogenous tropoelastin. The addition of inorganic phosphate (NaH2PO4; Pi) induced calcium deposition in RPEs. The Pi-induced calcification, as assessed by the o-cresolphthalein complexone method, Goldenbergs method, and von Kossa staining, was completely inhibited by treatment with clodronate (DMDP) and phosphonoformic acid (PFA) and was weakly suppressed by treatment with levamisole. Moreover, the osteopontin mRNA expression was upregulated in the Pi-induced calcification of RPEs. These reactions in RPEs were characteristically consistent with those already established in cultured bovine aortic smooth muscle cells (BASMCs). Furthermore, bacterially expressed tropoelastin inhibited calcium deposition in RPEs as well as in BASMCs. Finally, Pi-induced calcification was partially suppressed after the addition of tropoelastin due to elastic fiber formation. In conclusion, we suggest that this calcification model in RPEs is useful for analyzing the relation between elastic fibers and vascular calcification, and that tropoelastin and elastic fibers may contribute to the inhibition of vascular calcification.

Lysyl oxidase: properties, specificity, and biological roles inside and outside of the cell

Lysyl oxidase (LO) plays a critical role in the formation and repair of the extracellular matrix (ECM) by oxidizing lysine residues in elastin and collagen, thereby initiating the formation of covalent crosslinkages which stabilize these fibrous proteins. Its catalytic activity depends upon both its copper cofactor and a unique carbonyl cofactor and has been shown to extend to a variety of basic globular proteins, including histone H1. Although the three-dimensional structure of LO has yet to be determined, the present treatise offers hypotheses based upon its primary sequence, which may underlie the prominent electrostatic component of its unusual substrate specificity as well as the catalysis-suppressing function of the propeptide domain of prolysyl oxidase. Recent studies have demonstrated that LO appears to function within the cell in a manner, which strongly modifies cellular activity. Newly discovered LO-like proteins also likely play unique roles in biology.

Evidence for the involvement of TGF-β and PDGF in the regulation of prolyl 4-hydroxylase and lysyloxidase in cultured rat lung fibroblasts

Lung fibrosis is the end-point of numerous lung disorders induced by a pneumonia or by a variety of different noxes, one of which is the cytostatic drug bleomycin (BLM). Fibrosis is characterized by excessive extracellular matrix accumulation. Macrophage-fibroblast interactions are suggested to play an important role in the development of this disease. The present study was addressed to investigate one possible pathway of this interaction, the influence of soluble mediators produced by BLM-stimulated macrophages on lung fibroblast collagen synthesis and modification. Conditioned media (CM) of BLM-exposed macrophages of the cell line NR8383 submitted to rat lung fibroblast cultures increased the activity of prolyl 4-hydroxylase (P4H) in fibroblasts in a dose dependent manner. CM of stimulated macrophages increased the collagen concentration in fibroblast culture supernatant. The level of mRNAs specific for the alpha-subunit of P4H and that for alpha1(I) collagen were found to be increased by about two-fold, that for lysyloxidase (LO) by about 2.5-fold in fibroblasts cultured in CM of stimulated macrophages. Pre-incubation of CM of BLM-exposed macrophages with neutralizing antibodies against TGF-beta or against PDGF resulted in a partial reversal of the increasing effect of the CM on P4H- and LO-activities in fibroblasts. Both growth factors, TGF-beta and PDGF, added to fibroblast cultures led to significant increases of P4H activity in the treated cells. We conclude that TGF-beta and PDGF produced by stimulated macrophages are involved in the regulation of the expression of alpha1(I) collagen, of P4H-alpha-subunit and LO in lung fibroblasts. The results indicate that this is not a direct effect but involves the action of a so far unidentified mediator responsible for autocrine stimulation of collagen production.

PKC-MEK-MAPK-dependent signal transduction pathway mediates the stimulation of lysyl oxidase expression by serum and PDGF in rat aortic smooth muscle cells

Lysyl oxidase (LO) plays a critical role in the stabilization and insolubilization of fibrous structural proteins of the extracellular matrix and has been implicated in the suppression of Ras-induced tumorigenesis. Several prior reports demonstrate that the expression of this catalyst is strongly influenced by a variety of effectors of cell function and is responsive to the growth state of fibrogenic cells. Using specific inhibitors of components of signal transduction pathways, the present study reveals that a PKC-MEK-MAPK-dependent pathway is critical to the enhanced expression of the LO gene in response to variations in the levels of the serum component of the growth medium and in response to platelet-derived growth factor (PDGF). PDGF is shown to be the major component of fetal bovine serum, which stimulates the activity of a LO promoter construct.

Regulation of the expression of lysyl oxidase mRNA in cultured rabbit retinal pigment epithelium cells

Lysyl oxidase, an extracellular amine oxidase, controls the maturation of collagen and elastin. We examined the regulation of lysyl oxidase mRNA in cultured rabbit retinal pigment epithelium (RPE) cells in relation to the changes in subretinal fluid transport and phenotype of RPE cells. The level of the mRNA in cells grown on microporous membranes was markedly increased by application of hyperosmotic mannitol solution on the apical side (191% of control), implying that RPE cells express more lysyl oxidase in the condition which may cause the accumulation of subretinal fluid. Platelet-derived growth factor increased the mRNA level in subconfluent cells in culture (137% of control) and basic fibroblast growth factor decreased it (79% of control). In addition, exposure of cells to retinoic acid alone or in combination with dibutyryl cAMP for 22 days markedly decreased the level of lysyl oxidase mRNA (52 or 35% of control) while increasing the level of mRNA of N-acetylglucosaminidase (NAG), a marker enzyme for lysosomes (162 or 142% of control). Moreover, the level of lysyl oxidase mRNA in cells grown on microporous membranes was lower than that in cells grown on plastic dishes, while the level of NAG mRNA in the former cells was higher than that in the latter. Taken together, the expression of lysyl oxidase seemed to increase during proliferation of RPE cells and decrease toward differentiation. beta-Aminopropionitrile, an inhibitor of lysyl oxidase, significantly inhibited the contraction of collagen gels by fetal calf serum, suggesting that lysyl oxidase may be involved in pathogenesis caused by RPE cells.

Cytokine regulation of gingival fibroblast lysyl oxidase, collagen, and elastin

BACKGROUND

Systemic therapy with cyclosporin A, phenytoin, and nifedipine modulates cytokine levels in human gingival tissues. Functional relationships between altered cytokine levels and gingival extracellular matrix production are partially characterized. The present study investigates in cultured human gingival fibroblasts the regulation of lysyl oxidase, alpha-1 type I collagen, and elastin by selected cytokines that are elevated in drug-induced gingival overgrowth tissues.

METHODS

Normal human gingival fibroblasts were cultured and then treated with selected cytokines: interleukin (IL)-1beta, IL-6, platelet-derived growth factor (PDGF)-BB, and basic fibroblast growth factor (bFGF or FGF-2). Cells were harvested at intervals, and changes in lysyl oxidase enzyme activity, and in mRNA levels of lysyl oxidase, alpha-1 type I collagen, and elastin were determined.

RESULTS

bFGF reproducibly and significantly decreased human gingival fibroblast lysyl oxidase and alpha-1 type I collagen mRNA levels in a dose- and time-dependent manner; 1 nM bFGF reduced lysyl oxidase and collagen mRNA levels to 53% and to less than 10% of control after 48 hours of treatment. Interestingly, bFGF downregulated lysyl oxidase enzyme activity by 10% to 20%. IL-1, IL-6, and PDGF-BB did not significantly regulate lysyl oxidase enzyme activity, or alpha-1 type I collagen, elastin, and lysyl oxidase mRNA levels under the conditions tested.

CONCLUSIONS

Previous studies have shown that modulated levels of bFGF occur in gingiva as a result of certain pharmacologic therapies. The present study suggests that modulated levels of bFGF likely influence gingival connective tissue metabolism.

Lysyl oxidase-catalyzed cross-linking and insolubilization reactions of Lys-containing polypeptides and synthetic adhesive proteins

The lysyl oxidase- (LO-) mediated insolubilization reactions of the Lys-containing polypeptides have been examined using poly(L-Lys) with degrees of polymerization (Dps) ranging 1 from 2300, copoly(LysxAlay) (x:y = 1:4, 1:3; 1:1, 2:1, and 3:1), copoly(LysxGlyy) (x:y = 1:1 and 2:1), and synthetic adhesive proteins with sequential repetitive units enriched in the Lys residues, poly(Ala-Lys-Pro-Ser-Tyr-Pro-Pro-Thr-Tyr-Lys), poly(Ala-Gly-Tyr-Gly-Gly-Ala-Lys), and poly(Gly-Gly-Gly-Tyr-Gly-Gly-Tyr-Gly-Lys). All of the substrates were insolubilized by the LO-catalyzed oxidation of the epsilon-amino group in the Lys residues. The Dps of the polypeptide substrates did not affect the kinetic constants, the Km and Vmax values. The Km and Vmax values and the insolubilization rates varied depending on the Lys contents in the substrate polypeptides, which were enriched in Gly and Ala residues. As the Lys content increased, the Km and Vmax values became lower and higher, respectively. The insolubilization rates decreased with increase of the Lys content. The time-dependent changes in the LO-catalyzed aldehyde production, the insolubilization, and remaining LO activity demonstrated that the cross-linking and the insolubilization steps occurred along with LO deactivation, indicating that the enzymatic and chemical processes in the LO-mediated insolubilization occur in order.

A fluorometric assay for detection of lysyl oxidase enzyme activity in biological samples

Lysyl oxidase catalyzes the final known enzymatic step required for collagen and elastin cross-linking in the biosynthesis of normal mature functional insoluble extracellular matrices. In addition, lysyl oxidase has been identified as a possible tumor suppressor. Lysyl oxidase activity in biological samples is traditionally and most reliably assessed by tritium release end-point assays using radiolabeled collagen or elastin substrates involving laborious vacuum distillation of the released tritiated water. In addition, a less sensitive fluorometric method exists that employs nonpeptidyl amine lysyl oxidase substrates and measures hydrogen peroxide production with horseradish peroxidase coupled to homovanillate oxidation. The present study describes a more sensitive fluorescent assay for lysyl oxidase activity that utilizes 1,5-diaminopentane as substrate, and released hydrogen peroxide is detected using Amplex red in horseradish peroxidase-coupled reactions. This method allows the detection of 40 ng of enzyme per 2 ml assay at 37 degrees C and is 7.5 times more sensitive than the currently available fluorometric assay for enzyme activity. This method eliminates the interference that occurs in some biological samples and can be successfully used to detect lysyl oxidase activity in cell culture experiments.

Lysyl oxidases: a novel multifunctional amine oxidase family

Lysyl oxidase (LOX), a copper-containing amine oxidase, belongs to a heterogeneous family of enzymes that oxidize primary amine substrates to reactive aldehydes. LOX has been traditionally known for one function, the extracellular catalysis of lysine-derived cross-links in fibrillar collagens and elastin. More recently, diverse roles have been attributed to lysyl oxidase and these novel activities cover a spectrum of diverse biological functions such as developmental regulation, tumor suppression, cell motility, and cellular senescence. Lysyl oxidase has also been shown to have both intracellular and intranuclear locations. The multifunctional properties of lysyl oxidase (LOX) and our recent discovery of three novel members of this amine oxidase family, LOX-like (LOXL), LOXL2, and LOXL3, indicate the possibility that these varied functions are performed in both intracellular and extracellular environments by individual novel members of the LOX amine-oxidase family. Structural similarities of the highly conserved copper-binding and lysyl-tyrosylquinone cofactor sites among the LOX and LOX-like proteins may result in similar amine oxidase activities. However, specific novel functions, such as a potential role in cell adhesion and cell growth control, will be determined by other, conserved domains such as the cytokine receptor-like domain that is shared by all LOXs and by multiple scavenger receptor cysteine-rich (SRCR) domains present in LOXL2 and LOXL3. Furthermore, these functions may be carried out in a temporally and spatially regulated fashion.

Cloning and characterization of a fifth human lysyl oxidase isoenzyme: the third member of the lysyl oxidase-related subfamily with four scavenger receptor cysteine-rich domains

We report the complete cDNA sequence of the human lysyl oxidase-like 4 (LOXL4) gene, a new member of the lysyl oxidase (LO) gene family. The predicted polypeptide is 756 amino acids long, including a 24-residue signal peptide. The C-terminal region contains a LO domain similar to those of LOX, LOXL, LOXL2 and LOXL3. The N-terminal region has four subregions similar to scavenger receptor cysteine-rich domains that are highly conserved with LOXL2 and LOXL3. The LOXL4 mRNA is approximately 4 kb in size and is expressed in many tissues, the highest levels among the tissues studied being in the skeletal muscle, testis and pancreas. Recombinant LOXL4 expressed in HT-1080 cells was secreted into the culture medium with no evident proteolytic processing.

Multiple bone morphogenetic protein 1-related mammalian metalloproteinases process pro-lysyl oxidase at the correct physiological site and control lysyl oxidase activation in mouse embryo fibroblast cultures

Lysyl oxidase catalyzes the final enzymatic step required for collagen and elastin cross-linking in extracellular matrix biosynthesis. Pro-lysyl oxidase is processed by procollagen C-proteinase activity, which also removes the C-propeptides of procollagens I-III. The Bmp1 gene encodes two procollagen C-proteinases: bone morphogenetic protein 1 (BMP-1) and mammalian Tolloid (mTLD). Mammalian Tolloid-like (mTLL)-1 and -2 are two genetically distinct BMP-1-related proteinases, and mTLL-1 has been shown to have procollagen C-proteinase activity. The present study is the first to directly compare pro-lysyl oxidase processing by these four related proteinases. In vitro assays with purified recombinant enzymes show that all four proteinases productively cleave pro-lysyl oxidase at the correct physiological site but that BMP-1 is 3-, 15-, and 20-fold more efficient than mTLL-1, mTLL-2, and mTLD, respectively. To more directly assess the roles of BMP-1 and mTLL-1 in lysyl oxidase activation by connective tissue cells, fibroblasts cultured from Bmp1-null, Tll1-null, and Bmp1/Tll1 double null mouse embryos, thus lacking BMP-1/mTLD, mTLL-1, or all three enzymes, respectively, were assayed for lysyl oxidase enzyme activity and for accumulation of pro-lysyl oxidase and mature approximately 30-kDa lysyl oxidase. Wild type cells or cells singly null for Bmp1 or Tll1 all produced both pro-lysyl oxidase and processed lysyl oxidase at similar levels, indicating apparently normal levels of processing, consistent with enzyme activity data. In contrast, double null Bmp1/Tll1 cells produced predominantly unprocessed 50-kDa pro-lysyl oxidase and had lysyl oxidase enzyme activity diminished by 70% compared with wild type, Bmp1-null, and Tll1-null cells. Thus, the combination of BMP-1/mTLD and mTLL-1 is shown to be responsible for the majority of processing leading to activation of lysyl oxidase by murine embryonic fibroblasts, whereas in vitro studies identify pro-lysyl oxidase as the first known substrate for mTLL-2.

Building Elastin. Incorporation of recombinant human tropoelastin into extracellular matrices using nonelastogenic rat-1 fibroblasts as a source for lysyl oxidase

The purpose of this study was to assess the feasibility of crosslinking exogenously produced tropoelastin, the precursor of insoluble elastin, into existing elastin. Tritiated recombinant human tropoelastin (rhTE) was added to neonatal rat aorta smooth-muscle cell (NNRSMC) cultures. As much as 12% of the added rhTE was incorporated into the NNRSMC-derived insoluble elastin with the formation of the elastin crosslinks desmosine (DES) and isodesmosine (IDES) in a time-dependent fashion. The ratio of radioactivity found in DES and IDES crosslinks to that found in lysyl residues increased from 0.18 immediately after incubation with rhTE to 0.76 after 14 d. Crosslinking of rhTE into elastin and the accompanying formation of tritiated water was inhibited by beta-aminoproprionitrile, a potent inhibitor of lysyl oxidase, an enzyme critical for the post-translational processing of elastin and collagen. Acellular NNRSMC matrices were produced and replated with Rat-1 fibroblasts, cells that were found to express lysyl oxidase but not tropoelastin. At 14 d after incubation with rhTE, the ratio of DES and IDES radioactivity to that of lysine in the insoluble elastin was 0.38. We show for the first time that cells expressing lysyl oxidase, but not elastin, as well as elastogenic cells can incorporate rhTE into insoluble elastin with the formation of elastin crosslinks. This novel approach might be used to augment elastin repair in certain pathologic states.

Molecular events that contribute to lysyl oxidase enzyme activity and insoluble collagen accumulation in osteosarcoma cell clones

Maximum collagen synthesis and maximum accumulation of insoluble collagen occur at different phenotypic stages in developing osteoblastic cell cultures. Insoluble collagen accumulation depends in part on the activity of extracellular enzymes including procollagen N-proteinases, procollagen C-proteinase (derived from the BMP1 gene), and lysyl oxidase. In addition to its action on procollagen, procollagen C-proteinase processes prolysyl oxidase to mature 32-kDa lysyl oxidase. The regulation of extracellular activities that control insoluble collagen accumulation has not been studied extensively. The present study compares molecular events that control production of a collagenous mineralized extracellular matrix in vitro among five different murine osteosarcoma cell clones derived from the same tumor, but which differ in their ability to produce an insoluble mineralized matrix. Levels of insoluble type I collagen, insoluble calcium, bone morphogenetic protein 1 (BMP-1), and lysyl oxidase expression, lysyl oxidase biosynthesis, lysyl oxidase activity, and prolysyl oxidase processing activity were determined. Results surprisingly indicate that lysyl oxidase activity is not related closely to lysyl oxidase messenger RNA (mRNA) levels among the different cell clones. However, it appears that BMP-1-dependent prolysyl oxidase processing could contribute to the observed lysyl oxidase activity. Highest collagen and BMP-1 mRNA levels, prolysyl oxidase processing activity, and lysyl oxidase activity occurred in a cell clone (K8) that showed the highest levels of insoluble collagen accumulation. Culture media from a cell clone (K37) that accumulates little insoluble collagen or calcium but expresses high levels of lysyl oxidase mRNA contained low molecular weight fragments of lysyl oxidase protein and showed low lysyl oxidase activity. By contrast the K14 cell line exhibits relatively high lysyl oxidase activity and collagen accumulation, but low levels of mature lysyl oxidase protein. Together, these studies indicate that catabolic as well as anabolic activities are important in regulating insoluble collagen accumulation in osteoblastic cells. In addition, results suggest that products of genes homologous to lysyl oxidase may contribute to observed lysyl oxidase activity.

Physiological importance of quinoenzymes and the O-quinone family of cofactors

O-quinone cofactors derived from tyrosine and tryptophan are involved in novel biological reactions that range from oxidative deaminations to free-radical redox reactions. The formation of each of these cofactors appears to involve post-translational modifications of either tyrosine or tryptophan residues. The modifications result in cofactors, such as topaquinone (TPQ), tryptophan tryptophylquinone (TTQ), lysine tyrosylquinone (LTQ) or the copper-complexed cysteinyl-tyrosyl radical from metal-catalyzed reactions. Pyrroloquinoline quinone (PQQ) appears to be formed from the annulation of peptidyl glutamic acid and tyrosine residues stemming from their modification as components of a precursor peptide substrate. PQQ, a primary focus of this review, has invoked considerable interest because of its presence in foods, antioxidant properties and role as a growth-promoting factor. Although no enzymes in animals have been identified that exclusively utilize PQQ, oral supplementation of PQQ in nanomolar amounts increases the responsiveness of B- and T-cells to mitogens and improves neurologic function and reproductive outcome in rodents. Regarding TPQ and LTQ, a case may be made that the formation of TPQ and LTQ is also influenced by nutritional status, specifically dietary copper. For at least one of the amine oxidases, lysyl oxidase, enzymatic activity correlates directly with copper intake. TPQ and LTQ are generated following the incorporation of copper by a process that involves the two-step oxidation of a specified tyrosyl residue to first peptidyl dopa and then peptidyl topaquinone to generate active enzymes, generally classed as "quinoenzymes." Limited attention is also paid to TTQ and the copper-complexed cysteinyl-tyrosyl radical, cofactors important to fungal and bacterial redox processes.

Regulation of lysyl oxidase by interferon-gamma in rat aortic smooth muscle cells

Lysyl oxidase is an essential catalyst for the cross-linking of extracellular collagen and elastin. Abnormalities in lysyl oxidase activity may contribute to the pathogenesis of arterial diseases characterized by abnormal matrix remodeling. This study tested the hypothesis that interferon (IFN)-gamma, a proinflammatory cytokine present in aortic aneurysm and arteriosclerotic plaque rupture, downregulates lysyl oxidase gene expression in rat aortic smooth muscle cells. Steady-state lysyl oxidase mRNA levels decreased in a concentration- and time-dependent manner to 30% of control levels after 24 hours of treatment with IFN-gamma. Cell layer lysyl oxidase activity decreased in parallel with the observed changes in steady-state mRNA. Nuclear runoff studies suggested that transcriptional regulation was responsible for at least 40% of the observed downregulation. mRNA decay studies suggested that IFN-gamma also decreased lysyl oxidase mRNA half-life from 9 to 6 hours. Downregulation of lysyl oxidase by IFN-gamma did not appear to require new protein synthesis. This study documents that IFN-gamma downregulates lysyl oxidase gene expression in rat aortic smooth muscle cells by transcriptional and posttranscriptional mechanisms. If similar regulation occurs in vivo, it is possible that IFN-gamma-mediated changes in lysyl oxidase may contribute to arterial diseases characterized by abnormal extracellular matrix.

Lysyl oxidase: properties, regulation and multiple functions in biology

Lysyl oxidase (LO) is a copper-dependent amine oxidase that plays a critical role in the biogenesis of connective tissue matrices by crosslinking the extracellular matrix proteins, collagen and elastin. Levels of LO increase in many fibrotic diseases, while expression of the enzyme is decreased in certain diseases involving impaired copper metabolism. While the three-dimensional structure of the enzyme is not yet available, many of its physical-chemical properties, its novel carbonyl cofactor, and its catalytic mechanism have been described. Lysyl oxidase is synthesized as a preproprotein, secreted as a 50 kDa, N-glycosylated proenzyme and then proteolytically cleaved to the 32 kDa, catalytically active, mature enzyme. Within the past decade, the gene encoding LO has been cloned, facilitating investigations of the regulation of expression of the enzyme in response to diverse stimuli and in numerous disease states. Transforming growth factor-beta, platelet-derived growth factor, angiotensin II, retinoic acid, fibroblast growth factor, altered serum conditions, and shear stress are among the effectors or conditions that regulate LO expression. New, LO-like genes have also been identified and cloned, suggesting the existence of a multigene family. It has also become increasingly evident that LO may have other important biological functions in addition to its role in the crosslinking of elastin and collagen in the extracellular matrix.

Micro-injection of recombinant lysyl oxidase blocks oncogenic p21-Ha-Ras and progesterone effects on Xenopus laevis oocyte maturation

Previous evidence suggested an anti-oncogenic role for lysyl oxidase, mainly in ras-transformed cells. Here we prove that recombinant lysyl oxidase is actually able to antagonize p21-Ha-Ras-induced Xenopus laevis oocyte maturation. Lysyl oxidase was also effective on progesterone-dependent maturation, indicating a block lying downstream of Ras. Maturation induced by activated 'maturation promoting factor', normally triggered by progesterone, was also inhibited by lysyl oxidase. Finally, lysyl oxidase did not abolish p42Erk2 phosphorylation upon maturation triggering, suggesting a block downstream of Erk2. Further investigation showed that lysyl oxidase action depends on protein synthesis and is therefore probably mediated by a newly synthesized protein.

Incorporation of copper into lysyl oxidase

Lysyl oxidase is a copper-dependent enzyme involved in extracellular processing of collagens and elastin. Although it is known that copper is essential for the functional activity of the enzyme, there is little information on the incorporation of copper. In the present study we examined the insertion of copper into lysyl oxidase using 67Cu in cell-free transcription/translation assays and in normal skin fibroblast culture systems. When a full-length lysyl oxidase cDNA was used as a template for transcription/translation reactions in vitro, unprocessed prolysyl oxidase appeared to bind copper. To examine further the post-translational incorporation of copper into lysyl oxidase, confluent skin fibroblasts were incubated with inhibitors of protein synthesis (cycloheximide, 10 microg/ml), glycosylation (tunicamycin, 10 microg/ml), protein secretion (brefeldin A, 10 microg/ml) and prolysyl oxidase processing (procollagen C-peptidase inhibitor, 2.5 microg/ml) together with 300 microCi of carrier-free 67Cu. It was observed that protein synthesis was a prerequisite for copper incorporation, but inhibition of glycosylation by tunicamycin did not affect the secretion of 67Cu as lysyl oxidase. Brefeldin A inhibited the secretion of 67Ci-labelled lysyl oxidase by 46%, but the intracellular incorporation of copper into lysyl oxidase was not affected. In addition, the inhibition of the extracellular proteolytic processing of prolysyl oxidase to lysyl oxidase had minimal effects on the secretion of protein-bound 67Cu. Our results indicate that, similar to caeruloplasmin processing [Sato and Gitlin (1991) J. Biol. Chem. 266, 5128-5134], copper is inserted into prolysyl oxidase independently of glycosylation.

Repair of elastase-digested elastic fibers in acellular matrices by replating with neonatal rat-lung lipid interstitial fibroblasts or other elastogenic cell types

Disruption of elastic fibers is a major factor in the pathogenesis of pulmonary emphysema. Elastic fibers in culture, injured by exposure to elastase, undergo repair in the presence of elastogenic cells that restores the fibers toward normal as determined by biochemical and ultrastructural methods. The repair appears to be the result of both salvage and de novo repair mechanisms. The evidence for salvage repair is that hot-alkali resistance, lost as a result of elastase treatment, is restored to previously radiolabeled elastic fibers. This repair mechanism has been shown in aortic smooth muscle cell cultures. In order to determine the potential relevance of this mechanism for elastic fiber repair in the lungs, experiments were carried out using neonatal rat lung lipid interstitial fibroblasts (LIF). Treatment of the LIF cultures with elastase, in the absence of serum, caused solubilization of 12% of elastin; however, 81% of the elastin protein and 80% of the elastin-associated radioactivity (EAR) were solubilized by subsequent hot-alkali treatment, indicating that most of the elastin was retained in the matrix but was damaged. Ultrastructurally, the elastic fibers were frayed. After 6 additional wk in culture, hot-alkali resistant elastin protein and EAR were restored to 88 and 62% of control values, respectively, and the ultrastructural appearance of elastic fibers was restored to normal. We calculate that about 42% of the restored elastin represented salvage repair; the remainder was new elastin. No repair occurred in matrices rendered acellular by azide treatment; however, when acellular matrices were replated with LIF, repair was complete at 6 wk. No repair was seen when acellular matrices were replated with a transformed mouse macrophage cell line. We conclude that lung LIF are capable of mounting a robust repair process after elastolytic injury of elastin and that the repair is the result of both salvage and de novo repair mechanisms.

Transforming growth factor-beta 1 increases lysyl oxidase enzyme activity and mRNA in rat aortic smooth muscle cells

PURPOSE

This investigation was designed to test the hypothesis that transforming growth factor-beta 1 (TGF-beta 1) regulates lysyl oxidase secretion from vascular smooth muscle cells. Lysyl oxidase is an enzyme that catalyzes an essential step in collagen and elastin cross-linking in the extracellular matrix, and TGF-beta 1 has been implicated in the pathogenesis of restenosis after vascular injury. The effect of TGF-beta 1 on lysyl oxidase in vascular smooth muscle cells has not been previously defined.

METHODS

Rat aortic smooth muscle cells were grown in culture to confluence. Cells in passage 2 to 6 were incubated for 24 hours in media containing 0.1, 0.5, 1.0, or 10.0 ng/ml of TGF-beta 1. Lysyl oxidase activity in the media was quantitated with a tritium-release bioassay against an insoluble 3H-labeled aortic clastin substrate. Northern blot analyses were performed to determine steady-state levels of lysyl oxidase mRNA in the smooth muscle cells.

RESULTS

Lysyl oxidase activity in the media increased 1.5-fold above control levels after exposure to 10 ng/ml of TGF-beta 1 (p < 0.01). This increase in lysyl oxidase activity was associated with a concentration-dependent increase in steady-state levels of lysyl oxidase mRNA, being 4.3- and 6.2-fold above control levels after exposure to 1 and 10 ng/ml TGF-beta 1, respectively (p < 0.01). The observed increase in steady-state lysyl oxidase mRNA after exposure to TGF-beta 1 was also time-dependent over the 24-hour experimental period.

CONCLUSIONS

TGF-beta 1 appears to regulate lysyl oxidase in cultured rat aortic smooth muscle cells. Increases in lysyl oxidase activity may be one of the mechanisms by which TGF-beta 1 contributes to arterial restenosis after vascular injury.