Localization of human placenta lysyl oxidase on human placenta, skin and aorta by immunoelectronmicroscopy

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.

Development of collagen fibres and lysyl oxidase expression in the presumptive dermis of chick limb bud

Lysyl oxidase (LOX) plays a critical role in the formation of cross-linkages in extracellular matrix molecules. Thus, it is essential for the biogenesis and homeostasis of the connective tissue matrix. During development, collagen fibres and elastic system fibres emerge and accumulate in a temporospatial manner in the presumptive dermis of chicks. In this study, we investigated LOX mRNA expression by laser capture microdissection and RT-qPCR and LOX protein localization by immunohistochemistry. The picrosirius polarization method was used to investigate a relation between collagen accumulation and LOX expression. PCR analysis showed that the expression of LOX mRNA in the presumptive dermis became apparent at embryonic day 13 and increased considerably by ED17. Immunohistochemical staining for LOX in the dermis was very low at all stages of development. Accumulation of collagen fibres was seen in the dermis on ED10, and higher wavelengths of birefringence became evident by ED13. Our findings suggest that the temporal pattern of LOX mRNA expression correlates with collagen fibre accumulation in the dermis of the developing chick limb bud, whereas LOX expression was relatively constant at the protein level.

Feto-maternal interaction: a mathematical model simulating placental response in hypertensive disorders of pregnancy

Elevated maternal blood pressure (BP) is common in pregnancies complicated by hypertensive disorders. In response, increased production and accumulation of elastin occurs in the feto-placental blood vessels. This results in increased vascular wall stiffness that increases the resistance to flow. To study the interaction between the stiffness of the fetoplacental blood vessels, fetoplacental blood flow and BP, a mathematical model of the fetoplacental vascular tree was developed. The model describes an elastic structure exposed to external pressure. Model results indicate that increased vascular stiffness in the fetal blood vessels may contribute to optimizing fetoplacental blood flow in hypertensive pregnancies. According to model predictions, uncontrolled lowering of BP following vascular adaptation may adversely affect fetoplacental blood flow.

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.

Lysyl oxidase in development, aging and pathologies of the skin

Lysyl oxidase (LOX) is a copper- and lysyl-tyrosyl cofactor containing amine oxidase that has been known to play a critical role in the catalysis of lysine-derived crosslinks in extracellular matrix (ECM) proteins in the dermis. Changes in the composition and crosslinked state of the ECM and alterations in LOX synthesis and activity are known to be associated with aging and a range of acquired and heritable skin disorders. It has been assumed until recently that the LOX-related changes in the skin are mediated through the catalytic activity of LOX. However, work by several laboratories over the last few years has shown that LOX is a multifunctional protein. In this review we discuss the regulation of expression, localization and activation of LOX in the normal developing and adult skin, and alterations in LOX expression and activity associated with skin aging and senescence, and in pathological conditions, including wound healing, fibrosis, hypertrophic scarring, keloids, scleroderma, and diabetic skin. We further evaluate the role of LOX in skin ECM changes associated with the normal aging process and with these pathological states. In addition to collagen and elastin cross-linkages, regulatory and activation mechanisms and cell type specific LOX interactions may contribute to a range of novel intra- and extracellular LOX functions that appear critical determinants of the cellular microenvironment in the normal skin and in these skin disorders.

Comparative immunocytochemical localization of lysyl oxidase (LOX) and the lysyl oxidase-like (LOXL) proteins: changes in the expression of LOXL during development and growth of mouse tissues

Lysyl oxidase (LOX) and lysyl oxidase-like (LOXL) are extracellular enzymes that deaminate peptidyl lysyl residues involved in the cross-linking of fibrillar collagens and elastin. While LOX is required for the survival of newborn mice, the role of LOXL during development remains unclear. Studies have shown that the same cell types express LOX and LOXL in the same tissues, but no functional differences have been established. We have compared the immunohistochemical localization of LOX and LOXL in various tissues from normal, young adult mice. LOX and LOXL were co-localized in the skin, aorta, heart, lung, liver and cartilage, but were localized to different areas in the kidney, stomach, small intestine, colon, retina, ovary, testis and brain. LOXL expression was further examined in tissues from different developmental stages. In embryonic mice (10.5-14.5 dpc), LOXL immunostaining was abundant in the heart, liver, intestine, and neural tube. LOXL was present in most major organs in late fetal (16.5 dpc) and newborn mice, but generally diminished as animals aged. Immunoreactivity was significantly reduced in the heart, lung, kidney and liver of 2 year-old mice, but remained prevalent in the skin and tongue. LOX and LOXL were also found in the nuclei of cells in a number of tissues. These results indicate that LOXL has a role during mouse development and in the maintenance of adult tissues.

Lysyl oxidase-like and lysyl oxidase are present in the dermis and epidermis of a skin equivalent and in human skin and are associated to elastic fibers

Elastic fiber formation involves the secretion of tropoelastin which is converted to insoluble elastin by cross-linking, initiated by the oxidative deamination of lysine residues by lysyl oxidase. Five lysyl oxidase genes have been discovered. This study deals with the expression of two isoforms, LOX and LOX-like (LOXL), in human foreskin and in a human skin-equivalent (SE) model that allows the formation of elastic fibers. In this model, keratinocytes are added to a dermal equivalent made of fibroblasts grown on a chitosan-cross-linked collagen-GAG matrix. LOX and LOXL were detected by immunohistochemistry in the dermis and the epidermis of both normal skin and in a SE. This expression was confirmed by in situ hybridization on the SE. LOX and LOXL expression patterns were confirmed in human skin. The ultrastructural localization of LOXL was indicative of its association with elastin-positive materials within the SE and human skin, though interaction with collagen could not be discarded. LOX was found on collagen fibers and could be associated with elastin-positive materials in the SE and human skin. LOXL and LOX were detected in keratinocytes where LOX was mainly expressed by differentiating keratinocytes, in contrast to LOXL that can be found in both proliferating and differentiating fibroblasts. These data favor a role for LOXL in elastic fiber formation, together with LOX, and within the epidermis where both enzymes should play a role in post-translational modification of yet unknown substrates.

Codistribution analysis of elastin and related fibrillar proteins in early vertebrate development

Elastin is an extracellular matrix protein found in adult and neonatal vasculature, lung, skin and connective tissue. It is secreted as tropoelastin, a soluble protein that is cross-linked in the tissue space to form an insoluble elastin matrix. Cross-linked elastin can be found in association with several microfibril-associated proteins including fibrillin-1, fibrillin-2 and fibulin-1 suggesting that these proteins contribute to elastic fiber assembly, structure or function. To date, the earliest reported elastin expression was in the conotruncal region of the developing avian heart at 3.5 days of gestation. Here we report that elastin expression begins at significantly earlier developmental stages. Using a novel immunolabeling method, the deposition of elastin, fibrillin-1 and -2 and fibulin-1 was analyzed in avian embryos at several time points during the first 2 days of development. Elastin was found at the midline associated with axial structures such as the notochord and somites at 23 h of development. Fibrillin-1 and -2 and fibulin-1 were also expressed at the embryonic midline at this stage with fibrillin-1 and fibulin-1 showing a high degree of colocalization with elastin in fibers surrounding midline structures. The expression of these genes was confirmed by conventional immunoblotting and mRNA detection methods. Our results demonstrate that elastin polypeptide deposition occurs much earlier than was previously appreciated. Furthermore, the results suggest that elastin deposition at the early embryonic midline is accompanied by the deposition and organization of a number of extracellular matrix polypeptides. These filamentous extracellular matrix structures may act to transduce or otherwise stabilize dynamic forces generated during embryogenesis.

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.

The LOXL2 gene encodes a new lysyl oxidase-like protein and is expressed at high levels in reproductive tissues

We have reported in this paper the complete cDNA sequence, gene structure, and tissue-specific expression of LOXL2, a new amine oxidase and a member of an emerging family of human lysyl oxidases. The predicted amino acid sequence, from several overlapping cDNA clones isolated from placenta and spleen cDNA libraries, shared extensive sequence homology with the conserved copper-binding and catalytic domains of both lysyl oxidase (LOX) and the lysyl oxidase-like (LOXL) protein. These conserved domains are encoded by five consecutive exons within the LOX, LOXL, and LOXL2 genes that also maintained exon-intron structure conservation. In contrast, six exons encoding the amino-terminal domains diverged both in sequence and structure. Exon 1 of the LOXL2 gene does not encode a signal sequence that is present in LOX and LOXL, suggesting a different processing and intracellular localization for this new protein. Expression of the LOXL2 gene was detected in almost all tissues with the highest steady state mRNA levels in the reproductive tissues, placenta, uterus and prostate. In situ hybridization identified placental syncytial and cytotrophoblasts responsible for the synthesis of LOXL2 mRNA and demonstrated a spatial and temporal expression pattern unique to the LOXL2 gene.

Elastic fiber during development and aging

Elastin molecules aggregate in the extracellular space where they are crosslinked by stable desmosine bridges. The resulting polymer is structurally organized as branched fibers and lamellae, which, in skin, are wider (a few microns) in the deep dermis and become progressively thinner (fraction of a micron) towards the papillary dermis. Several general and local factors seem to regulate elastin gene expression, deposition and degradation. In skin, the volume density of the elastin network increases from birth up to maturity, when it accounts for about 3-4% of the tissue. However, its amount and distribution depend on dermis areas, which are different among subjects and change with age. Several matrix molecules (glycosaminoglycans, decorin, biglycan, osteopontin) have been found to be associated with elastin into the normal fiber, and several others have been recognized within pathologic elastic fiber (osteonectin, vitronectin, alkaline phosphatase in PXE). With age, and in some pathologic conditions, skin elastin may undergo irreversible structural and compositional changes, which seem to progress from localized deposition of osmiophilic materials to the substitution of the great majority of the amorphous elastin with interwoven filaments negative for elastin specific antibodies.

Detection of lysyl oxidase gene expression in rat skin during wound healing

Lysyl oxidase (LOX) initiates the crosslinking of the lysine-derived aldehyde and plays an essential role in maturation of collagen, for example in wound healing. Although the activity of this enzyme has been examined in various disorders, and a further intriguing aspect of the relationship between LOX and tumorigenesis has recently emerged, its gene expression pattern in tissues is still unknown. We examined LOX gene expression during wound healing in rat skin. In addition, type III collagen gene expression was studied to determine the formation of fibrils. The LOX mRNA level reached a peak by day 3 after injury, which was earlier than that of type III collagen, and continued at a high level until day 22. The type III collagen mRNA level began to rise from day 3 and had increased intensely by day 22. In situ hybridization revealed grains corresponding to LOX mRNA in the fibroblasts of the granulomatous tissue. These results suggest that LOX is produced before collagen synthesis in preparation for crosslinking in the early phase of wound healing.

Increased expression of lysyl oxidase in skin with scleroderma

Lysyl oxidase initiates cross-linkage of collagen and elastin by catalysing the formation of a lysine-derived aldehyde. In order to study cross-linking in scleroderma, we used monoclonal antibodies to lysyl oxidase to determine the localization of this enzyme in systemic and localized scleroderma, and compared the distributions obtained with that in normal skin. Using an indirect immunofluorescent antibody method and an avidin-biotinylated enzyme complex method, 11 cases of diffuse type of systemic scleroderma and seven cases of localized scleroderma were studied. In the oedematous stage of systemic scleroderma, intracellular and extracellular lysyl oxidase were remarkably increased in the dermis, particularly in groups around blood vessels. In the sclerotic stage of systemic scleroderma, lysyl oxidase was detected intracellularly in fibroblasts and extracellularly among collagen bundles between the lower dermis and the subcutaneous fat tissue. In localized scleroderma, a marked increase in lysyl oxidase was observed in mononuclear cells and fibroblasts near blood vessels in the lower dermis and in the subcutaneous fat tissue, in addition to the extracellular deposits between collagen bundles. The increase in lysyl oxidase in localized scleroderma was much more common than in the oedematous stage of systemic scleroderma. These findings indicated that intracellular and extracellular expression of lysyl oxidase expression was greater in sclerodermatous skin than in normal skin.

Primary structure and developmental expression of Fbn-1, the mouse fibrillin gene

Previous studies have reported > 10 kilobases of human fibrillin-1 cDNA sequence, but a consensus regarding the 5' end of the transcript remains to be worked out. One approach to developing a clear consensus would be to search for regions of evolutionary conservation in transcripts from a related species such as mouse. As reported here, the mouse fibrillin-1 transcript encodes a highly conserved polypeptide of 2,871 amino acids. The upstream sequence that flanks the ATG is considerably less well conserved, however. Indeed, the ATG codon (which occurs in the context of a Kozak consensus sequence and is located just upstream of a consensus signal peptide) signals the point where human and mouse fibrillin-1 sequences cease to be nearly identical. Together, these results are consistent with previous efforts by Pereira et al. (Pereira, L., D'Alessio, M., Ramirez, F., Lynch, J. R., Sykes, B., Pangilinan, T., and Bonadio, J. (1993) Human Mol. Genet. 2, 961-968) to identify the human fibrillin-1 translational start site. Sequences immediately upstream of the ATG are GC-rich and devoid of TATA and CCAAT boxes, which suggests that the mouse fibrillin-1 gene will be broadly expressed. A survey of expression in mouse embryo tissues is consistent with this hypothesis and suggests two novel functions for fibrillin-associated microfibrils in non-elastic connective tissues.

An immunocytochemical study of the routes of secretion of collagen and phosphophoryn from odontoblasts into dentin

Polyclonal antibodies to rat incisor phosphophoryns and to the amino-telopeptide of the alpha1 (I)-chain of type I collagen were used to follow the pathways of movement of collagen I (COL1) and phosphophoryns (PP) from synthesis in the odontoblast to secretion into the mineralized dentin. The antibodies were detected at the transmission electron microscopic level by their reaction with Protein A-colloidal gold conjugates. Special care was given in specimen preparation to retention of maximal antigenicity during fixation while maintaining cellular and extracellular ultrastructure at the mineralization front (MF) in nondemineralized sections. Intracellularly, COL1 and PP were detected within the endoplasmic reticulum (ER), the Golgi (G) and secretory granules (SG). However, as determined by double-immunolabeling with different size gold particles the COL1 and PP were not found together within the same ER, G or SG compartments. PP was localized within the tubular ER, round-shaped transitional vesicles, the Golgi and in narrow asymmetric SG. These asymmetric SG were found in abundance in the odontoblastic process. PP secretion from these vesicles was near the MF at the predentin-dentin boundary. COL1 was localized within rosette form ER compartments, the Golgi and in large, distinctive SG. COL1 was deposited at the cell-predentin boundary. No COL1 SG were seen within the odontoblastic process near the MF. In the region of the MF, prior to mineralization, the PP was localized along the surfaces of the COL1 fibrils of the predentin. The mineral phase etched surfaces revealed both COL1- and abundant mineral-associated PP. These data support the hypotheses that, in dentin, the interaction between COL1 and PP may initiate crystal nucleation and that additional interactions between PP and the growing crystals may modulate the crystal growth pattern and crystal size.

Immunohistochemical localization of lysyl oxidase in normal human skin

Lysyl oxidase (EC 1.4.3.13), a copper-dependent enzyme which catalyses the formation of aldehyde cross-links, and acts primarily on collagen and elastin, is known to be increased during wound healing and in fibrotic disorders including liver cirrhosis and atherosclerosis, and to be decreased in some hereditary connective tissue diseases and in malignant cell lines. A recent study showed that lysyl oxidase might possess tumour suppressor activity as an antioncogene for ras. Little is known about the localization of this enzyme in human skin. In this study, we determined immunohistochemically the localization of lysyl oxidase in normal skin of young and elderly subjects obtained from sun-exposed and unexposed regions of the body. All skin samples tested had similar distributions of lysyl oxidase. The enzyme was present both extracellularly and intracellularly. Extracellularly, a few granular aggregates of immunoreactants were observed along collagen and elastic fibres. These granules were more common in the adventitial portion of the dermis than in the reticular portion. Of all sun-exposed and unexposed regions studied, the skin of the face displayed the greatest amount of extracellular immunoreactants. Immunopositive granules were observed intracellularly in fibroblasts, vascular endothelial cells, sweat glands, sebaceous glands, arrector pili muscles and some keratinocytes. These findings provide evidence that, as suggested in recent reports, lysyl oxidase may have a variety of intracellular functions.

Properties and function of lysyl oxidase

Lysyl oxidase catalyzes the oxidation of peptidyl lysine to alpha-aminoadipic-delta-semialdehyde, the precursor to the covalent crosslinkages that stabilize fibers of elastin and collagen. This enzyme contains both copper and a carbonyl cofactor consistent with an o-quinone. The proposed mechanism of action is derived from available kinetic and chemical data and also can account for mechanism-based inhibition of the enzyme by specific monoamines and diamines. Recent evidence for biosynthetic precursors and for the regulation of lysyl oxidase in fibrotic and malignant diseases is discussed.