Lysyl oxidase

Lysyl oxidase (LOX), also known as protein-lysine 6-oxidase, is a is_associated_with::protein that, in humans, is encoded by the LOX is_associated_with::gene. Its inhibition can cause is_associated_with::osteolathyrism, but, at the same time, its is_associated_with::upregulation by is_associated_with::tumor cells may promote is_associated_with::metastasis of the existing tumor, causing it to become malignant and cancerous.

Structure
In the yeast strain is_associated_with::Pichia pastoris, lysyl oxidase constitutes a is_associated_with::homodimeric structure. Each is_associated_with::monomer consists of an active site that includes a Cu(II) atom coordinated with three is_associated_with::histidine residues as well as 2,4,5-trihydroxyphenalanine quinone (TPQ), a crucial cofactor.

In humans, the LOX gene is located on chromosome 5q23.3-31.2. The DNA sequence encodes a polypeptide of 417 amino acids, the first 21 residues of which constitute a signal peptide, with a weight of approximately 32 kDa. The carboxyterminus contains the active copper (II) ion, lysine, tyrosine, and cysteine residues that comprise the catalytically active site. The three-dimensional structure of human lysyl oxidase has not yet been resolved.

Mechanism
The mechanism of lysyl oxidase occurs via modification of the ε-amino group of a is_associated_with::lysine side chain. The enzyme falls into the category of is_associated_with::quinone-containing copper amine oxidases, and the reaction is highly dependent on the cofactor lysyl tyrosylquinone (LTQ). The LTQ cofactor is unique among quinones due to its ortho/benzoquinone structure and neutral charge under physiological pH. This can be contrasted with the similar ubiquitous quinocofactor TPQ, which exists as a negatively charged structure under physiological conditions and includes ortho/para-carbonyl resonance functionality.

LTQ is crucial in LOX-catalyzed conversion of lysine residues to α-aminoadipidic-δ-semialdehydes, generally referred to as is_associated_with::allysines. In the oxidation of lysine, the ε-amine is first converted to a Schiff base via reaction with LTQ. While LTQ is still bound to the substrate, rate-limiting removal of the ε-proton yields an imine intermediate. Subsequent hydrolysis of the imine leads to release of the aldehyde product, allysine. Molecular oxygen and the copper ion are utilized to reoxidize the cofactor and yield another imine, producing hydrogen peroxide as a side product. Additional hydrolysis releases ammonia and the original cofactor, completing the catalytic cycle.

Biological function
Lysyl is_associated_with::oxidase is an extracellular copper is_associated_with::enzyme that catalyzes formation of aldehydes from is_associated_with::lysine residues in is_associated_with::collagen and is_associated_with::elastin precursors. These aldehydes are highly reactive, and undergo spontaneous chemical reactions with other lysyl oxidase-derived aldehyde residues, or with unmodified lysine residues. This results in cross-linking collagen and elastin, which is essential for stabilization of collagen fibrils and for the integrity and elasticity of mature elastin.

Complex cross-links are formed in collagen (is_associated_with::pyridinolines derived from three lysine residues) and in elastin (is_associated_with::desmosines derived from four lysine residues) that differ in structure.

The importance of lysyl oxidase-derived cross-linking was established from animal studies in which lysyl oxidase was inhibited either by nutritional copper-deficiency or by supplementation of diets with is_associated_with::β-aminopropionitrile (BAPN), an inhibitor of lysyl oxidase. This resulted in is_associated_with::lathyrism, characterized by poor bone formation and strength, hyperextensible skin, weak ligaments, and increased occurrence of aortic is_associated_with::aneurysms. These abnormalities correlated well with decreased cross-linking of collagen and elastin.

Developmentally, reduced lysyl oxidase levels have been implicated in is_associated_with::Menkes disease and is_associated_with::Occipital horn syndrome, two X-linked recessive disorders characterized by a mutation in a gene for copper transportation. Thus, not only is LOX crucial to cardiovascular development, it is thought to play a major role in connective tissue development and may also be important in neurological function.

Lysyl oxidase has also proven crucial to the development of the is_associated_with::respiratory system and the skin, as collagen and elastin represent 50-60% of the composition of the lung, and 75% of the skin. In LOX double knockout models (Lox -/-), function of LOX was reduced by up to 80%, and the phenotype of the lungs resembles those of human patients with is_associated_with::emphysema and dilated distal airways.

Finally, lysyl oxidase plays a crucial role in the commitment step of is_associated_with::adipocyte, or fat cell, formation from pluripotent is_associated_with::stem cells during development. Its absence may lead to defects in the is_associated_with::transforming growth factor beta superfamily of proteins, which control is_associated_with::cell growth and differentiation.

Clinical significance
LOX expression is regulated by is_associated_with::hypoxia-inducible factors (HIFs), and, hence, LOX expression is often upregulated in hypoxic breast and head and neck tumors. Patients with high LOX-expressing tumors have poor overall survival. Furthermore, inhibition of LOX has been demonstrated to eliminate metastases in mice. Secreted LOX is responsible for the invasive properties of hypoxic cancer cells through focal adhesion kinase activity and cell-to-matrix adhesion. LOX may be required to create a niche permissive for metastatic growth and, thus, may be required for hypoxia-induced metastasis. In fact, recent research has shown overexpression of LOX as crucial to promoting tumor growth and metastasis in several cancers, including breast cancer, melanoma, non-small cell lung cancer, and colorectal cancer.

LOX expression was also detected in megakaryocytes, or bone marrow cells responsible for the production of platelets. Data derived from a mouse model of myelofibrosis implicated LOX in bone marrow fibrosis.

In a rodent model of breast cancer, a small-molecule or antibody inhibitors of LOX abolished metastasis. LOX secreted by hypoxic breast tumor cells crosslinks is_associated_with::collagen in the is_associated_with::basement membrane and is essential for CD11b+ myeloid cell recruitment. CD11b+ cells in turn adhere to crosslinked collagen and produce matrix metalloproteinase-2, which cleaves collagen, enhancing the invasion of metastasizing tumor cells. In contrast, LOX inhibition prevents CD11b+ cell recruitment and metastatic growth.

In cells lacking TGF-β receptors, a deficiency that is characteristic of is_associated_with::lung cancer, lysyl oxidase is found in high concentrations. LOX immunostaining has revealed that high LOX expression is associated with high extent of carcinoma invasion in samples obtained from surgically removed lung is_associated_with::adenocarcinomas. Additionally, LOX expression is an indicator of 5-year survival in patients, with a 71% chance of survival for patients with low LOX levels, compared to 43% for patients with high LOX levels. Thus, upregulation of lysyl oxidase is a predictor of poor prognosis in early-stage adenocarcinoma patients.

Lysyl oxidase has been newly implicated in tumor is_associated_with::angiogenesis, or is_associated_with::blood vessel formation, both in vivo and in vitro. Subcutaneous tumor-derived LOX was shown to increase is_associated_with::vascular endothelial growth factor (VEGF) expression and secretion, which then promotes angiogenesis by phosphorylation of is_associated_with::protein kinase B, or Akt, through platelet-derived growth factor receptor β (is_associated_with::PDGFRB). High levels of LOX were associated with high blood vessel density in patient samples. Clinically relevant LOX inhibitors may help slow cancer progression by downregulating crucial growth factors that promote solid tumor progression.

Hence, inhibitors of the LOX enzyme may be useful in preventing angiogenesis, tumor progression, and metastasis as well as treating other fibrotic disease involving remodeling of the is_associated_with::extracellular matrix, including neurodegenerative and cardiovascular diseases.