Identification of Histidine 303 as the Catalytic Base of Lysyl Oxidase via Site-Directed Mutagenesis

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.

Lysyl oxidase-like 4 exerts an atypical role in breast cancer progression that is dependent on the enzymatic activity that targets the cell-surface annexin A2

Background

LOX family members are reported to play pivotal roles in cancer. Unlike their enzymatic activities in collagen cross-linking, their precise cancer functions are unclear. We revealed that LOXL4 is highly upregulated in breast cancer cells, and we thus sought to define an unidentified role of LOXL4 in breast cancer.

Methods

We established the MDA-MB-231 sublines MDA-MB-231-LOXL4 mutCA and -LOXL4 KO, which stably overexpress mutant LOXL4 that loses its catalytic activity and genetically ablates the intrinsic LOXL4 gene, respectively. In vitro and in vivo evaluations of these cells' activities of cancer outgrowth were conducted by cell-based assays in cultures and an orthotopic xenograft model, respectively. The new target (s) of LOXL4 were explored by the MS/MS analytic approach.

Results

Our in vitro results revealed that both the overexpression of mutCA and the KO of LOXL4 in cells resulted in a marked reduction of cell growth and invasion. Interestingly, the lowered cellular activities observed in the engineered cells were also reflected in the mouse model. We identified a novel binding partner of LOXL4, i.e., annexin A2. LOXL4 catalyzes cell surface annexin A2 to achieve a cross-linked multimerization of annexin A2, which in turn prevents the internalization of integrin β-1, resulting in the locking of integrin β-1 on the cell surface. These events enhance the promotion of cancer cell outgrowth.

Conclusions

LOXL4 has a new role in breast cancer progression that occurs via an interaction with annexin A2 and integrin β-1 on the cell surface.

Reaction mechanism of lysyl oxidase-like 2 (LOXL2) studied by computational methods

Lysyl oxidase-like 2 (LOXL2) is a copper-dependent amine oxidase that catalyzes the oxidative deamination of the ε-amino group of lysines/hydroxylysines on substrate proteins (collagen and elastin) to form aldehyde groups. The generated aldehyde groups are of significance in crosslinking with the adjacent aldehyde or ε-amino group on proteins in extracellular matrix. In this paper, we have studied the reaction mechanism of LOXL2 by means of quantum mechanics (QM) and combined QM and molecular mechanics (QM/MM) methods. This study is divided into two parts, i.e. the biosynthesis of lysine tyrosylquinone (LTQ) cofactor and oxidative deamination of ε-amino group of lysine by LTQ. For the former part, the reaction is driven by a large exothermicity of about 284 kJ/mol. Dopaquinone radical (DPQr) is suggested to be an intermediate state in this reaction. In addition, His652 residue is predicted to serve as proton acceptor. The rate-determining step for the biosynthesis of LTQ is found to be hydrogen-atom abstraction from the benzene ring on substrate by Cu²⁺-hydroxide, which is a proton-coupled electron transfer (PCET) process with an energy barrier of 84 kJ/mol. For the latter part, the reaction is exothermic by about 145 kJ/mol, and the copper ion is proposed to play a role of redox catalyst in the last step to generate the product of aldehyde. However, the copper ion might not be indispensable for the latter part, which is consistent with the previous study.

LOX inhibition downregulates MMP-2 and MMP-9 in gastric cancer tissues and cells

Objective: The objective of this study was to analyze the effects of lysyl oxidase (LOX) on the expression and enzyme activity of the matrix metalloproteinases 2 (MMP-2) and 9 (MMP-9) and to study its preliminary effect mechanisms.

Methods: We collected fresh cancer specimens from 49 gastric cancer patients who underwent surgery. Immunohistochemistry was used to quantitate the protein expression levels of LOX and MMP-9 in gastric cancer tissues and to analyze their correlation. Also, six-week old nude mice were divided into a control group and a LOX inhibition group. SGC-7901 gastric cancer cells were inoculated subcutaneously into the backs of the two groups of these mice to construct a gastric cancer-bearing nude mouse model. In the LOX inhibition group, β-aminopropionitrile (BAPN) was used to inhibit LOX. Western blotting was used to quantitate the relative expression levels of MMP-2 and MMP-9 in mouse tumor tissues, and gelatin zymography was used to quantitate their enzyme activity levels. In addition, BGC-823 gastric cancer cells were cultured, then 0.1 mM, 0.2 mM, and 0.3 mM BAPN and 2.5 nM, 5 nM, and 10 nM LOX were added to treat BGC-823 cells. ELISA and gelatin zymography were used to quantitate the protein concentrations and changes in enzyme activity of MMP-2 and MMP-9 in the culture supernatant. Western blotting was used to quantitate the relative expression levels of platelet derived growth factor receptor (PDGFR) in the BGC-823 gastric cancer cells after LOX inhibition and exogenous LOX addition.

Results: In the tissues from the gastric cancer patients, the relative expression levels of LOX and MMP-9 were positively correlated (r = 0.326, P < 0.05). Compared with the control group, the tumor tissues from mice in the LOX inhibition group had reduced relative expression levels and enzyme activities of MMP-2 and MMP-9 (P < 0.05). After LOX were inhibited with different concentrations of BAPN in BGC-823 gastric cancer cells, the protein concentrations and enzyme activity levels of MMP-2 and MMP-9 in the culture supernatants were decreased (P < 0.05). In addition, the relative expression level of PDGFR in gastric cancer was decreased when BAPN concentrations increased, showing a negative dose-dependent manner (r_PDGFR-α = -0.964, r_PDGFR-β = -0.988, P < 0.05). After exogenous LOX treating BGC-823 cells, the concentrations and enzyme activity levels of MMP-2 and MMP-9 in the cell supernatant were increased (P < 0.05). Further, the relative expression of PDGFR in gastric cancer cells was increased with the increase of exogenous LOX, showing a positive dose-dependent manner (r_PDGFR-α=0.952, r_PDGFR-β=0.953, P<0.05).

Conclusions: LOX inhibition can inhibit the expression and enzyme activity of MMP-2 and MMP-9 in gastric cancer tissues and cells, and the probable mechanism is through its effects on the PDGF-PDGFR signaling pathway.

Lysyl oxidases: from enzyme activity to extracellular matrix cross-links

The lysyl oxidase family comprises five members in mammals, lysyl oxidase (LOX) and four lysyl oxidase like proteins (LOXL1-4). They are copper amine oxidases with a highly conserved catalytic domain, a lysine tyrosylquinone cofactor, and a conserved copper-binding site. They catalyze the first step of the covalent cross-linking of the extracellular matrix (ECM) proteins collagens and elastin, which contribute to ECM stiffness and mechanical properties. The role of LOX and LOXL2 in fibrosis, tumorigenesis, and metastasis, including changes in their expression level and their regulation of cell signaling pathways, have been extensively reviewed, and both enzymes have been identified as therapeutic targets. We review here the molecular features and three-dimensional structure/models of LOX and LOXLs, their role in ECM cross-linking, and the regulation of their cross-linking activity by ECM proteins, proteoglycans, and by inhibitors. We also make an overview of the major ECM cross-links, because they are the ultimate molecular readouts of LOX/LOXL activity in tissues. The recent 3D model of LOX, which recapitulates its known structural and biochemical features, will be useful to decipher the molecular mechanisms of LOX interaction with its various substrates, and to design substrate-specific inhibitors, which are potential antifibrotic and antitumor drugs.

A Three-Dimensional Model of Human Lysyl Oxidase, a Cross-Linking Enzyme

Lysyl oxidase (LOX) is a cross-linking enzyme identified 50 years ago, but its 3D structure is still unknown. We have thus built a 3D model of human LOX by homology modeling using the X-ray structure of human lysyl oxidase-like 2 as a template. This model is the first one to recapitulate all known biochemical features of LOX, namely, the coordination of the copper ion and the formation of the lysine tyrosylquinone cofactor and the disulfide bridges. Furthermore, this model is stable during a 1 μs molecular dynamics simulation. The catalytic site is located in a groove surrounded by two loops. The distance between these loops fluctuated during the simulations, which suggests that the groove forms a hinge with a variable opening, which is able to accommodate the various sizes of LOX substrates. This 3D model is a pre-requisite to perform docking experiments with LOX substrates and other partners to identify binding sites and to design new LOX inhibitors specific for therapeutic purpose.