Lysyl oxidase (LOX), also known as protein-lysine 6-oxidase, is an enzyme that, in humans, is encoded by the LOX gene. [5] [6] It catalyzes the conversion of lysine residues into its aldehyde derivative allysine. [7] Allysine form cross-links in extracellular matrix proteins. Inhibition of lysyl oxidase can cause osteolathyrism, but, at the same time, its upregulation by tumor cells may promote metastasis of the existing tumor, causing it to become malignant and cancerous.
In the yeast species Pichia pastoris , lysyl oxidase constitutes a homodimeric structure. Each monomer consists of an active site that includes a Cu(II) atom, coordinated by three histidine residues, as well as 2,4,5-trihydroxyphenylalanine quinone (TPQ), a crucial cofactor. [8]
In humans, the LOX gene is located on chromosome 5 q23.3-31.2. The DNA sequence encodes a polypeptide of 417 amino acids, the first 21 residues of which constitute a signal peptide, [6] with a weight of approximately 32 kDa. [9] The carboxyterminus contains the active copper (II) ion, lysine, tyrosine, and cysteine residues that comprise the catalytically active site. [10] The three-dimensional structure of human lysyl oxidase has not yet been resolved. [11]
Lysyl oxidase the terminal carbon of the side chain of lysyl residue side chain. [9] The enzyme belongsthe category of quinone-containing copper amine oxidases. The reaction requires the cofactor lysyl tyrosylquinone (LTQ). The LTQ cofactor is unique among quinones because it contains an 1,2-benzoquinone substituent. Furthermore, it is neutral charge at physiological pH. [12] [13] The ε-amine is condenses with LTQ to give the Schiff base via reaction with LTQ. The rate-limiting removal of a ε-proton yields an imine. Subsequent hydrolysis of the imine leads to release of the allysine residue. Molecular oxygen and the copper ion are utilized to reoxidize the cofactor, producing hydrogen peroxide as a side product. [14]
Lysyl oxidase is an extracellular copper-dependent enzyme that catalyzes formation of aldehydes from lysine residues in collagen and elastin precursors. [15] [16] These aldehydes react with unmodified lysine residues, resulting in cross-linking collagen and elastin, which is essential for stabilization of collagen fibrils and for the integrity and elasticity of mature elastin. [5]
Complex cross-links are formed in collagen (pyridinolines derived from three lysine residues) and in elastin (desmosines derived from four lysine residues) that differ in structure. [17]
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 β-aminopropionitrile (BAPN), an inhibitor of lysyl oxidase. [18] This resulted in lathyrism, characterized by poor bone formation and strength, hyperextensible skin, weak ligaments, and increased occurrence of aortic aneurysms. These abnormalities correlated well with decreased cross-linking of collagen and elastin. [19]
Developmentally, reduced lysyl oxidase activity have been implicated in Menkes disease and occipital horn syndrome, two X-linked recessive disorders characterized by a mutation in a gene coding for a protein involved in copper transport. Thus, not only is LOX crucial to cardiovascular development, it plays a major role in connective tissue development and may also be important in neurological function. [20]
Lysyl oxidase has also proven crucial to the development of the 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 homozygous null models (Lox -/-), the activity of LOX was reduced by up to 80%, and the phenotype of the lungs resembles those of patients with emphysema and dilated distal airways. [20]
Lysyl oxidase plays a crucial role in the commitment step of adipocyte, or fat cell, formation from pluripotent stem cells during development. Its absence may lead to defects in the transforming growth factor beta superfamily of proteins, which control cell growth and differentiation. [21]
LOX expression is regulated by 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. [22] In fact, recent research has shown overexpression of LOX as crucial to promoting tumor growth and metastasis in several cancers, including breast cancer, [23] non-small cell lung cancer, [24] and colorectal cancer. [25]
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. [26] LOX secreted by hypoxic breast tumor cells crosslinks collagen in the 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. [27]
In cells lacking TGF-β receptors, a deficiency that is characteristic of 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 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. [28]
Lysyl oxidase has been newly implicated in tumor angiogenesis, or blood vessel formation, both in vivo and in vitro. Subcutaneous tumor-derived LOX was shown to increase vascular endothelial growth factor (VEGF) expression and secretion, which then promotes angiogenesis by phosphorylation of protein kinase B, or Akt, through platelet-derived growth factor receptor β (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. [29]
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 extracellular matrix, including neurodegenerative and cardiovascular diseases. [30]
Collagen is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole-body protein content. Collagen consists of amino acids bound together to form a triple helix of elongated fibril known as a collagen helix. It is mostly found in connective tissue such as cartilage, bones, tendons, ligaments, and skin. Vitamin C is vital for collagen synthesis, and Vitamin E improves the production of collagen.
Ubiquitin is a small regulatory protein found in most tissues of eukaryotic organisms, i.e., it is found ubiquitously. It was discovered in 1975 by Gideon Goldstein and further characterized throughout the late 1970s and 1980s. Four genes in the human genome code for ubiquitin: UBB, UBC, UBA52 and RPS27A.
Elastin is a protein that in humans is encoded by the ELN gene. Elastin is a key component of the extracellular matrix in gnathostomes. It is highly elastic and present in connective tissue allowing many tissues in the body to resume their shape after stretching or contracting. Elastin helps skin to return to its original position when it is poked or pinched. Elastin is also an important load-bearing tissue in the bodies of vertebrates and used in places where mechanical energy is required to be stored.
Elastic fibers are an essential component of the extracellular matrix composed of bundles of proteins (elastin) which are produced by a number of different cell types including fibroblasts, endothelial, smooth muscle, and airway epithelial cells. These fibers are able to stretch many times their length, and snap back to their original length when relaxed without loss of energy. Elastic fibers include elastin, elaunin and oxytalan.
Lysyl hydroxylases are alpha-ketoglutarate-dependent hydroxylases enzymes that catalyze the hydroxylation of lysine to hydroxylysine. Lysyl hydroxylases require iron and vitamin C as cofactors for their oxidation activity. It takes place following collagen synthesis in the cisternae (lumen) of the rough endoplasmic reticulum (ER). There are three lysyl hydroxylases (LH1-3) encoded in the human genome, namely: PLOD1, PLOD2 and PLOD3. From PLOD2 two splice variant can be expressed, where LH2b differs from LH2a by incorporating the small exon 13A. LH1 and LH3 hydroxylate lysyl residues in the collagen triple helix, whereas LH2b hydroxylates lysyl residues in the telopeptides of collagen. In addition to its hydroxylation activity, LH3 has glycosylation activity that produces either monosaccharide (Gal) or disaccharide (Glc-Gal) attached to collagen hydroxylysines.
Desmosine is an amino acid found uniquely in elastin, a protein found in connective tissue such as skin, lungs, and elastic arteries.
Allysine is a derivative of lysine that features a formyl group in place of the terminal amine. The free amino acid does not exist, but the allysine residue does. It is produced by aerobic oxidation of lysine residues by the enzyme lysyl oxidase. The transformation is an example of a post-translational modification. The semialdehyde form exists in equilibrium with a cyclic derivative.
Isodesmosine is a lysine derivative found in elastin. Isodesmosine is an isomeric pyridinium-based amino acid resulting from the condensation of four lysine residues between elastin proteins by lysyl-oxidase. These represent ideal biomarkers for monitoring elastin turnover because these special cross-links are only found in mature elastin in mammals.
Lysine-specific histone demethylase 1A (LSD1) also known as lysine (K)-specific demethylase 1A (KDM1A) is a protein that in humans is encoded by the KDM1A gene. LSD1 is a flavin-dependent monoamine oxidase, which can demethylate mono- and di-methylated lysines, specifically histone 3, lysine 4 (H3K4). Other reported methylated lysine substrates such as histone H3K9 and TP53 have not been biochemically validated. This enzyme plays a critical role in oocyte growth, embryogenesis, hematopoiesis and tissue-specific differentiation. LSD1 was the first histone demethylase to be discovered though more than 30 have since been described.
Procollagen-proline dioxygenase, commonly known as prolyl hydroxylase, is a member of the class of enzymes known as alpha-ketoglutarate-dependent hydroxylases. These enzymes catalyze the incorporation of oxygen into organic substrates through a mechanism that requires alpha-Ketoglutaric acid, Fe2+, and ascorbate. This particular enzyme catalyzes the formation of (2S, 4R)-4-hydroxyproline, a compound that represents the most prevalent post-translational modification in the human proteome.
Matrilysin also known as matrix metalloproteinase-7 (MMP-7), pump-1 protease (PUMP-1), or uterine metalloproteinase is an enzyme in humans that is encoded by the MMP7 gene. The enzyme has also been known as matrin, putative metalloproteinase-1, matrix metalloproteinase pump 1, PUMP-1 proteinase, PUMP, metalloproteinase pump-1, putative metalloproteinase, MMP). Human MMP-7 has a molecular weight around 30 kDa.
Enhancer of zeste homolog 2 (EZH2) is a histone-lysine N-methyltransferase enzyme encoded by EZH2 gene, that participates in histone methylation and, ultimately, transcriptional repression. EZH2 catalyzes the addition of methyl groups to histone H3 at lysine 27, by using the cofactor S-adenosyl-L-methionine. Methylation activity of EZH2 facilitates heterochromatin formation thereby silences gene function. Remodeling of chromosomal heterochromatin by EZH2 is also required during cell mitosis.
Lysyl oxidase homolog 2 is an enzyme that in humans is encoded by the LOXL2 gene.
Collagen alpha-1(XV) chain is a protein that in humans is encoded by the COL15A1 gene.
Lysyl oxidase homolog 1, also known as LOXL1, is an enzyme which in humans is encoded by the LOXL1 gene.
Procollagen-lysine,2-oxoglutarate 5-dioxygenase 3 is an enzyme that in humans is encoded by the PLOD3 gene.
Lysyl oxidase homolog 4 is an enzyme that in humans is encoded by the LOXL4 gene.
Lysyl oxidase homolog 3 is an enzyme that in humans is encoded by the LOXL3 gene.
Pyridinoline, also known as Hydroxylysylpyridinoline, is a fluorescent cross-linking compound of collagen fibers. Crosslinks in collagen and elastin are derived from lysyl and hydroxylysyl residues, a process catalyzed by lysyl oxidase. Fujimoto and colleagues first described the isolation and characterization of a fluorescent material in bovine Achilles tendon collagen and termed it pyridinoline. It is reported to be present in collagen of bone and cartilage, but is absent in collagen of skin. It is not present in newly synthesized collagen and is formed from aldimine cross-links during maturation of collagen fibers.
Copper peptide GHK-Cu is a naturally occurring copper complex of the tripeptide glycyl-L-histidyl-L-lysine. The tripeptide has strong affinity for copper(II) and was first isolated from human plasma. It can be found also in saliva and urine.
