Lysyl oxidase oxidizes cell membrane proteins and enhances the chemotactic response of vascular smooth muscle cells

Lysyl oxidase in colorectal cancer

Colorectal cancer is the third most prevalent form of cancer worldwide and fourth-leading cause of cancer-related mortality, leading to ~600,000 deaths annually, predominantly affecting the developed world. Lysyl oxidase is a secreted, extracellular matrix-modifying enzyme previously suggested to act as a tumor suppressor in colorectal cancer. However, emerging evidence has rapidly implicated lysyl oxidase in promoting metastasis of solid tumors and in particular colorectal cancer at multiple stages, affecting tumor cell proliferation, invasion, and angiogenesis. This emerging research has stimulated significant interest in lysyl oxidase as a strong candidate for developing and deploying inhibitors as functional efficacious cancer therapeutics. In this review, we discuss the rapidly expanding body of knowledge concerning lysyl oxidase in solid tumor progression, highlighting recent advancements in the field of colorectal cancer.

The expression of lysyl-oxidase gene family members in myeloproliferative neoplasms

Myeloproliferative neoplasms (MPNs) are malignant disorders originating from clonal expansion of a single neoplastic stem cell and characteristically show an increase in bone marrow reticulin fibers. Lysyl oxidases (LOXs) are copper-dependent amine oxidases that play a critical role in the biogenesis of connective tissue by crosslinking extracellular matrix proteins, collagen and elastin. Expression of LOX gene family members is increased in disorders associated with increased fibrosis. To evaluate involvement of LOX gene family in various MPNs. In-situ hybridization was used to detect Lysyl-Oxidase family members in bone marrow biopsies from patients with different MPNs. We compared normal bone marrows and those from patients with polycythemia vera, essential thrombocythemia, chronic myeloid leukemia, and primary myelofibrosis (PMF). Serum levels of lysyl-oxidase from patients with PMF and healthy controls were also examined. LOX gene family was not detected in normal bone marrows. All members of the LOX gene family were over expressed in PMF. In other MPNs a differential pattern of expression was observed. Differences in gene expression were statistically significant (P < 0.010). The medianserum LOX levels in normal controls was 28.4 ± 2.5 ng\ml and 44.6 ± 9.44 ng\ml in PMF (P = 0.02). The varying pattern of expression of LOX genes may reflect differences in the pathophysiology of bone marrow fibrosis in these MPNs. These observations could be used as the basis for future targeted therapy directed against bone marrow fibrosis.

Hepatic stellate cells and portal fibroblasts are the major cellular sources of collagens and lysyl oxidases in normal liver and early after injury

Liver fibrosis is characterized by excessive deposition of extracellular matrix proteins by myofibroblasts derived from hepatic stellate cells and portal fibroblasts. Activation of these precursors to myofibroblasts requires matrix stiffness, which results in part from increased collagen cross-linking mediated by lysyl oxidase (LOX) family proteins. The aims of this study were to characterize the mechanical changes of early fibrosis, to identify the cells responsible for LOX production in early injury, and to determine which cells in normal liver produce collagens and elastins, which serve as substrates for LOXs early after injury. Hepatocytes and liver nonparenchymal cells were isolated from normal and early-injured liver and examined immediately for expression of LOXs and matrix proteins. We found that stellate cells and portal fibroblasts were the major cellular sources of fibrillar collagens and LOXs in normal liver and early after injury (1 day after bile duct ligation and 2 and 7 days after CCl(4) injury). Activity assays using stellate cells and portal fibroblasts in culture demonstrated significant increases in LOX family enzymatic activity as cells became myofibroblastic. LOX family-mediated deoxypyridinoline and pyridinoline cross-links increased after CCl(4)-mediated injury. There was a significant association between liver stiffness (as quantified by the shear storage modulus G') and deoxypyridinoline levels; increased deoxypyridinoline levels were also coincident with significantly increased elastic resistance to large strain deformations, consistent with increased cross-linking of the extracellular matrix. These data suggest a model in which the liver is primed to respond quickly to injury, activating potential mechanical feed-forward mechanisms.

Novel role of copper transport protein antioxidant-1 in neointimal formation after vascular injury

OBJECTIVE

Vascular smooth muscle cell (VSMC) migration is critically important for neointimal formation after vascular injury and atherosclerosis lesion formation. Copper (Cu) chelator inhibits neointimal formation, and we previously demonstrated that Cu transport protein antioxidant-1 (Atox1) is involved in Cu-induced cell growth. However, role of Atox1 in VSMC migration and neointimal formation after vascular injury is unknown.

APPROACH AND RESULTS

Here, we show that Atox1 expression is upregulated in injured vessel, and it is colocalized with the Cu transporter ATP7A, one of the downstream targets of Atox1, mainly in neointimal VSMCs at day 14 after wire injury. Atox1(-/-) mice show inhibition of neointimal formation and extracellular matrix expansion, which is associated with a decreased VSMCs accumulation within neointima and lysyl oxidase activity. Mechanistically, in cultured VSMC, Atox1 depletion with siRNA inhibits platelet-derived growth factor-induced Cu-dependent VSMC migration by preventing translocation of ATP7A and small G protein Rac1 to the leading edge, as well as Cu- and Rac1-dependent lamellipodia formation. Furthermore, Atox1(-/-) mice show decreased perivascular macrophage infiltration in wire-injured vessels, as well as thioglycollate-induced peritoneal macrophage recruitment.

CONCLUSIONS

Atox1 is involved in neointimal formation after vascular injury through promoting VSMC migration and inflammatory cell recruitment in injured vessels. Thus, Atox1 is a potential therapeutic target for VSMC migration and inflammation-related vascular diseases.

Lysyl oxidase plays a critical role in endothelial cell stimulation to drive tumor angiogenesis

Identification of key molecules that drive angiogenesis is critical for the development of new modalities for the prevention of solid tumor progression. Using multiple models of colorectal cancer, we show that activity of the extracellular matrix-modifying enzyme lysyl oxidase (LOX) is essential for stimulating endothelial cells in vitro and angiogenesis in vivo. We show that LOX activates Akt through platelet-derived growth factor receptor β (PDGFRβ) stimulation, resulting in increased VEGF expression. LOX-driven angiogenesis can be abrogated through targeting LOX directly or using inhibitors of PDGFRβ, Akt, and VEGF signaling. Furthermore, we show that LOX is clinically correlated with VEGF expression and blood vessel formation in 515 colorectal cancer patient samples. Finally, we validate our findings in a breast cancer model, showing the universality of these observations. Taken together, our findings have broad clinical and therapeutic implications for a wide variety of solid tumor types.

Lysyl oxidase plays a critical role in endothelial cell stimulation to drive tumor angiogenesis

Identification of key molecules that drive angiogenesis is critical for the development of new modalities for the prevention of solid tumor progression. Using multiple models of colorectal cancer, we show that activity of the extracellular matrix-modifying enzyme lysyl oxidase (LOX) is essential for stimulating endothelial cells in vitro and angiogenesis in vivo. We show that LOX activates Akt through platelet-derived growth factor receptor β (PDGFRβ) stimulation, resulting in increased VEGF expression. LOX-driven angiogenesis can be abrogated through targeting LOX directly or using inhibitors of PDGFRβ, Akt, and VEGF signaling. Furthermore, we show that LOX is clinically correlated with VEGF expression and blood vessel formation in 515 colorectal cancer patient samples. Finally, we validate our findings in a breast cancer model, showing the universality of these observations. Taken together, our findings have broad clinical and therapeutic implications for a wide variety of solid tumor types.

Oxidases and reactive oxygen species during hematopoiesis: a focus on megakaryocytes

Reactive oxygen species (ROS), generated as a result of various reactions, control an array of cellular processes. The role of ROS during megakaryocyte (MK) development has been a subject of interest and research. The bone marrow niche is a site of MK differentiation and maturation. In this environment, a gradient of oxygen tension, from normoxia to hypoxia results in different levels of ROS, impacting cellular physiology. This article provides an overview of major sources of ROS, their implication in different signaling pathways, and their effect on cellular physiology, with a focus on megakaryopoiesis. The importance of ROS-generating oxidases in MK biology and pathology, including myelofibrosis, is also described.

Megakaryocyte pathology and bone marrow fibrosis: the lysyl oxidase connection

Megakaryocytes (MKs), the platelet precursors, are capable of accumulating DNA greater than a diploid content as part of their cell cycle. MKs have been recognized as mediating fibrosis in a subset of hematologic malignancies, including acute megakaryoblastic leukemia and a subset of myeloproliferative neoplasms. The mechanisms responsible for fibrosis remain only partially understood. Past studies highlighted the role of growth factors in such pathologies, and recently, the protein lysyl oxidase (LOX) has been implicated in proliferation of MKs, ploidy and deposition of fibers. LOX was initially characterized as a protein responsible for the intermolecular cross-linking of elastin and collagen, and in recent years it has been identified as regulator of various pathologies, such as cancer and inflammation. Here, we review recent advances in the understanding of the contribution of MKs to the progression of myelofibrosis, highlighting the newly identified role of LOX.

Lysyl oxidase, extracellular matrix remodeling and cancer metastasis

Lysyl oxidase (LOX) family oxidases, LOX and LOXL1-4, oxidize lysine residues in collagens and elastin, resulting in the covalent crosslinking and stabilization of these extracellular matrix (ECM) structural components, thus provide collagen and elastic fibers much of their tensile strength and structural integrity. Abnormality in LOX expression and/or activity results in connective tissue disorders and fibrotic diseases. Despite LOX family oxidases have been reported to function as tumor suppressors, recent studies have highlighted the roles of LOX family oxidases in promoting cancer metastasis. LOX family oxidases are highly expressed in invasive tumors, and are closely associated with metastasis and poor patient outcome. Consistent to their roles in connective tissue homeostasis, LOX family oxidases expedite tumorigenesis and metastasis through active remodeling of tumor microenvironment. LOX family oxidases are also actively involved in the process of epithelial-mesenchymal transition (EMT), an event critical in cancer cell invasion and metastasis. In this review, we will summarize the recent progress on LOX family oxidases, with much of the focus on the roles and mechanism of LOX in tumor progression and metastasis.

Lysyl oxidase-like protein-2 regulates sprouting angiogenesis and type IV collagen assembly in the endothelial basement membrane

Sprouting angiogenesis is associated with extensive extracellular matrix (ECM) remodeling. The molecular mechanisms involved in building the vascular microenvironment and its impact on capillary formation remain elusive. We therefore performed a proteomic analysis of ECM from endothelial cells maintained in hypoxia, a major stimulator of angiogenesis. Here, we report the characterization of lysyl oxidase-like protein-2 (LOXL2) as a hypoxia-target expressed in neovessels and accumulated in the endothelial ECM. LOXL2 belongs to the lysyl oxidase family of secreted enzymes involved in ECM crosslinking. Knockdown experiments in Tg(fli1:egfp)y1 zebrafish embryos resulted in lack of intersegmental vessel circulation and demonstrated LOXL2 involvement in proper capillary formation. Further investigation in vitro by loss and gain of function experiments confirmed that LOXL2 was required for tubulogenesis in 3D fibrin gels and demonstrated that this enzyme was required for collagen IV assembly in the ECM. In addition, LOXL2 depletion down-regulated cell migration and proliferation. These data suggest a major role for LOXL2 in the organization of endothelial basal lamina and in the downstream mechanotransductive signaling. Altogether, our study provides the first evidence for the role of LOXL2 in regulating angiogenesis through collagen IV scaffolding.

Control of megakaryocyte expansion and bone marrow fibrosis by lysyl oxidase

Lysyl oxidase (LOX), a matrix cross-linking protein, is known to be selectively expressed and to enhance a fibrotic phenotype. A recent study of ours showed that LOX oxidizes the PDGF receptor-β (PDGFR-β), leading to amplified downstream signaling. Here, we examined the expression and functions of LOX in megakaryocytes (MKs), the platelet precursors. Cells committed to the MK lineage undergo mitotic proliferation to yield diploid cells, followed by endomitosis and acquisition of polyploidy. Intriguingly, LOX expression is detected in diploid-tetraploid MKs, but scarce in polyploid MKs. PDGFR-BB is an inducer of mitotic proliferation in MKs. LOX inhibition with β-aminopropionitrile reduces PDGFR-BB binding to cells and downstream signaling, as well as its proliferative effect on the MK lineage. Inhibition of LOX activity has no influence on MK polyploidy. We next rationalized that, in a system with an abundance of low ploidy MKs, LOX could be highly expressed and with functional significance. Thus, we resorted to GATA-1(low) mice, where there is an increase in low ploidy MKs, augmented levels of PDGF-BB, and an extensive matrix of fibers. MKs from these mice display high expression of LOX, compared with control mice. Importantly, treatment of GATA-1(low) mice with β-aminopropionitrile significantly improves the bone marrow fibrotic phenotype, and MK number in the spleen. Thus, our in vitro and in vivo data support a novel role for LOX in regulating MK expansion by PDGF-BB and suggest LOX as a new potential therapeutic target for myelofibrosis.

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.

Role of lysyl oxidase propeptide in secretion and enzyme activity

Lysyl oxidase (LOX) is secreted as a proenzyme (proLOX) that is proteolytically processed in the extracellular milieu to release the propeptide and mature, active LOX. LOX oxidizes lysyl residues of a number of protein substrates in the extracellular matrix and on the cell surface, which impacts several physiological and disease states. Although the LOX propeptide (LOX-PP) is glycosylated, little is known about the role of this modification in LOX secretion and activity. To gain insight into this issue, cells were transfected with native, full-length LOX cDNA (pre-pro-LOX), the N-glycosylation null pre-[N/Q]pro-LOX cDNA and the deletion mutant pre-LOX cDNA, referred to as secretory LOX, in which mature LOX is targeted to the secretory pathway without its N-terminal propeptide sequence. The results show that glycosylation of the LOX-PP is not required for secretion and extracellular processing of pro-LOX but it is required for optimal enzyme activity of the resulting mature LOX. Complete deletion of the propeptide sequence prevents mature LOX from exiting the endoplasmic reticulum (ER). Taken together, our study points out the requirement of the LOX-PP for pro-LOX exit from the ER and is the first to highlight the influence of LOX-PP glycosylation on LOX enzyme activity.

Lysyl oxidase, a critical intra- and extra-cellular target in the lung for cigarette smoke pathogenesis

Cigarette smoke (CS), a complex chemical mixture, contains more than 4,800 different compounds, including oxidants, heavy metals, and carcinogens, that individually or in combination initiate or promote pathogenesis in the lung accounting for 82% of chronic obstructive pulmonary disease (COPD) deaths and 87% of lung cancer deaths. Lysyl oxidase (LO), a Cu-dependent enzyme, oxidizes peptidyl lysine residues in collagen, elastin and histone H1, essential for stabilization of the extracellular matrix and cell nucleus. Considerable evidences have shown that LO is a tumor suppressor as exemplified by inhibiting transforming activity of ras, a proto oncogene. CS condensate (CSC), 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and cadmium (Cd), major components of CS, down-regulate LO expression at such multiple levels as mRNA, protein and catalytic activity in lung cells in vitro and in vivo indicating LO as a critical intra- and extracellular target for CS pathogenesis in the lung. In view of multiple biological functions and regulation characteristics of the LO gene, molecular mechanisms for CS damage to lung LO and its role in emphysema and cancer pathogenesis are discussed in this review.

Lysyl oxidase-like-2 (LOXL2) is a major isoform in chondrocytes and is critically required for differentiation

The lysyl oxidase family is made up of five members: lysyl oxidase (LOX) and lysyl oxidase-like 1-4 (LOXL1-LOXL4). All members share conserved C-terminal catalytic domains that provide for lysyl oxidase or lysyl oxidase-like enzyme activity; and more divergent propeptide regions. LOX family enzyme activities catalyze the final enzymatic conversion required for the formation of normal biosynthetic collagen and elastin cross-links. The importance of lysyl oxidase enzyme activity to normal bone development has long been appreciated, but regulation and roles for specific LOX isoforms in bone formation in vivo is largely unexplored. Fracture healing recapitulates aspects of endochondral bone development. The present study first investigated the expression of all LOX isoforms in fracture healing. A remarkable coincidence of LOXL2 expression with the chondrogenic phase of fracture healing was found, prompting more detailed analyses of LOXL2 expression in normal growth plates, and LOXL2 expression and function in developing ATDC5 chondrogenic cells. Data show that LOXL2 is expressed by pre-hypertrophic and hypertrophic chondrocytes in vivo, and that LOXL2 expression is regulated in vitro as a function of chondrocyte differentiation. Moreover, LOXL2 knockdown studies in vitro show that LOXL2 expression is required for ATDC5 chondrocyte cell line differentiation through regulation of SNAIL and SOX9, important transcription factors that control chondrocyte differentiation. Taken together, data provide evidence that LOXL2, like LOX, is a multifunctional protein. LOXL2 promotes chondrocyte differentiation by mechanisms that are likely to include roles as both a regulator and an effector of chondrocyte differentiation.

Unexpected role of the copper transporter ATP7A in PDGF-induced vascular smooth muscle cell migration

RATIONALE

Copper, an essential nutrient, has been implicated in vascular remodeling and atherosclerosis with unknown mechanism. Bioavailability of intracellular copper is regulated not only by the copper importer CTR1 (copper transporter 1) but also by the copper exporter ATP7A (Menkes ATPase), whose function is achieved through copper-dependent translocation from trans-Golgi network (TGN). Platelet-derived growth factor (PDGF) promotes vascular smooth muscle cell (VSMC) migration, a key component of neointimal formation.

OBJECTIVE

To determine the role of copper transporter ATP7A in PDGF-induced VSMC migration.

METHODS AND RESULTS

Depletion of ATP7A inhibited VSMC migration in response to PDGF or wound scratch in a CTR1/copper-dependent manner. PDGF stimulation promoted ATP7A translocation from the TGN to lipid rafts, which localized at the leading edge, where it colocalized with PDGF receptor and Rac1, in migrating VSMCs. Mechanistically, ATP7A small interfering RNA or CTR small interfering RNA prevented PDGF-induced Rac1 translocation to the leading edge, thereby inhibiting lamellipodia formation. In addition, ATP7A depletion prevented a PDGF-induced decrease in copper level and secretory copper enzyme precursor prolysyl oxidase (Pro-LOX) in lipid raft fraction, as well as PDGF-induced increase in LOX activity. In vivo, ATP7A expression was markedly increased and copper accumulation was observed by synchrotron-based x-ray fluorescence microscopy at neointimal VSMCs in wire injury model.

CONCLUSIONS

These findings suggest that ATP7A plays an important role in copper-dependent PDGF-stimulated VSMC migration via recruiting Rac1 to lipid rafts at the leading edge, as well as regulating LOX activity. This may contribute to neointimal formation after vascular injury. Our findings provide insight into ATP7A as a novel therapeutic target for vascular remodeling and atherosclerosis.

Toxicity of Non-protein Amino Acids to Humans and Domestic Animals

Non-protein amino acids are common in plants and are present in widely consumed animal feeds and human foods such as alfalfa ( Medicago sativa), which contains canavanine, and lentil ( Lens culinaris), which contains homoarginine. Some occur in wild species that are inadvertently harvested with crop species. Some nonprotein amino acids and metabolites can be toxic to humans, e.g. Lathyrus species contain a neurotoxic oxalyl-amino acid. Some potential toxins may be passed along a food chain via animal intermediates. The increased interest in herbal medicines in the Western countries will increase exposure to such compounds.

Cell signaling, internalization, and nuclear localization of the angiotensin converting enzyme in smooth muscle and endothelial cells

The angiotensin converting enzyme (ACE) catalyzes the extracellular formation of angiotensin II, and degradation of bradykinin, thus regulating blood pressure and renal handling of electrolytes. We have previously shown that exogenously added ACE elicited transcriptional regulation independent of its enzymatic activity. Because transcriptional regulation generates from protein-DNA interactions within the cell nucleus we have investigated the initial cellular response to exogenous ACE and the putative internalization of the enzyme in smooth muscle cells (SMC) and endothelial cells (EC). The following phenomena were observed when ACE was added to cells in culture: 1) it bound to SMC and EC with high affinity (K(d) = 361.5 +/- 60.5 pM) and with a low binding occupancy (B(max) = 335.0 +/- 14.0 molecules/cell); 2) it triggered cellular signaling resulting in late activation of focal adhesion kinase and SHP2; 3) it modulated platelet-derived growth factor receptor-beta signaling; 4) it was endocytosed by SMC and EC; and 5) it transited through the early endosome, partially occupied the late endosome and the lysosome, and was localized to the nuclei. The incorporation of ACE or a fragment of it into the nuclei reached saturation at 120 min, and was preceded by a lag time of 40 min. Internalized ACE was partially cleaved into small fragments. These results revealed that extracellular ACE modulated cell signaling properties, and that SMC and EC have a pathway for delivery of extracellular ACE to the nucleus, most likely involving cell surface receptor(s) and requiring transit through late endosome/lysosome compartments.

Potential anticancer application of polyamine oxidation products formed by amine oxidase: a new therapeutic approach

The polyamines spermine, spermidine and putrescine are ubiquitous cell components. These molecules are substrates of a class of enzymes that includes monoamine oxidases, diamine oxidases, polyamine oxidases and copper-containing amine oxidases. Amine oxidases are important because they contribute to regulate levels of mono- and polyamines. In tumors, polyamines and amine oxidases are increased as compared to normal tissues. Cytotoxicity induced by bovine serum amine oxidase (BSAO) and spermine is attributed to H(2)O(2) and aldehydes produced by the reaction. This study demonstrated that multidrug-resistant (MDR) cancer cells (colon adenocarcinoma and melanoma) are significantly more sensitive than the corresponding wild-type (WT) ones to H(2)O(2) and aldehydes, the products of BSAO-catalyzed oxidation of spermine. Transmission electron microscopy (TEM) observations showed major ultrastructural alterations of the mitochondria. These were more pronounced in MDR than in WT cells. Increasing the incubation temperature from 37 to 42 degrees Celsius enhances cytotoxicity in cells exposed to spermine metabolites. The combination BSAO/spermine prevents tumor growth, particularly well if the enzyme has been conjugated to a biocompatible hydrogel polymers. Since both wild-type and MDR cancer cells after pre-treatment with MDL 72527, a lysosomotropic compound, are sensitized to subsequent exposure to BSAO/spermine, it is conceivable that combined treatment with a lysosomotropic compound and BSAO/spermine would be effective against tumor cells. It is of interest to search for such novel compounds, which might be promising for application in a therapeutic setting.

Lysyl oxidase resolves inflammation by reducing monocyte chemoattractant protein-1 in abdominal aortic aneurysm

Lysyl oxidase (LOX) is an enzyme critical for the stability of extracellular matrix and also known to have diverse biological functions. Little is known, however, about the role of LOX in regulating inflammation. Here we demonstrate that LOX suppresses secretion of monocyte chemoattractant protein-1 (MCP-1) in cultured vascular smooth muscle cells. Furthermore, enhancement of LOX activity reduces MCP-1 in a mouse model of abdominal aortic aneurysm (AAA), thereby preventing macrophage infiltration and AAA progression. These findings suggest that LOX has a novel function in resolving inflammation by reducing MCP-1 in AAA.

Differential regulation of elastic fiber formation by fibulin-4 and -5

Fibulin-4 and -5 are extracellular glycoproteins with essential non-compensatory roles in elastic fiber assembly. We have determined how they interact with tropoelastin, lysyl oxidase, and fibrillin-1, thereby revealing how they differentially regulate assembly. Strong binding between fibulin-4 and lysyl oxidase enhanced the interaction of fibulin-4 with tropoelastin, forming ternary complexes that may direct elastin cross-linking. In contrast, fibulin-5 did not bind lysyl oxidase strongly but bound tropoelastin in terminal and central regions and could concurrently bind fibulin-4. Both fibulins differentially bound N-terminal fibrillin-1, which strongly inhibited their binding to lysyl oxidase and tropoelastin. Knockdown experiments revealed that fibulin-5 controlled elastin deposition on microfibrils, although fibulin-4 can also bind fibrillin-1. These experiments provide a molecular account of the distinct roles of fibulin-4 and -5 in elastic fiber assembly and how they act in concert to chaperone cross-linked elastin onto microfibrils.