Lysyl oxidase-variant 2 (LOX-v2) is a novel variant of lysyl oxidase (LOX) that functions as an amine oxidase for the formation of lysine-mediated crosslinks found in collagen and elastin fibrils. In addition to the amine oxidase activity in the extracellular matrix, several novel functions, such as tumor suppression, tumor progression, chemotaxis, cellular senescence, and modification of histones, have been assigned to LOX. In recent years, it has been reported that LOX is also present in nuclear locations, suggesting a novel functional role of LOX in the nucleus. To test the amine oxidase activity of LOX and LOX-v2 to nuclear histone proteins, we expressed and purified LOX and LOX-v2 as recombinant forms and then assessed the amine oxidase activity toward histone H2A in in vitro peroxidase-coupled fluorometric assays. Both LOX and LOX-v2 proteins showed significant levels of amine oxidase activity toward histone H2A in a β -aminopropionitrile-inhibitable manner. In immunofluorescence staining after ectopic expression in cultured cells, LOX was observed in the perinuclear, cytoplasmic, and extracellular areas, whereas LOX-v2 was predominantly detected in the nucleoplasm with a punctuate pattern. These findings suggest that LOX-v2 may play a novel functional role in the nucleus through the amine oxidase activity to the nuclear histone proteins. Elucidation of the specific functional roles of LOX-v2, such as substrate specificity toward different types of nuclear proteins and detailed analysis on subnuclear localization, will provide a significant clue in understanding the diverse functional roles currently assigned to a single enzyme, LOX.

Lysyl oxidase (LOX) family, the copper dependent amine oxidase, oxidizes lysine residues in extracellular collagen and elastin. LOX increases the strength of the extracellular matrix and plays an important role in tumor development and metastasis. It has been reported that increased LOX protein and RNA are found in head and neck squamous cell carcinoma. Moreover some studies regarded LOX as a prognostic marker of oral and oropharyngeal squamous cell carcinoma. However there has not been any report on LOX expression of salivary gland tumors. Here, we investigated LOX expression in mucoepidermoid carcinoma (MEC) and adenoid cystic carcinoma (ACC) of salivary gland and compare it to those of pleomorphic adenoma (PA). We evaluated LOX expression in eighteen MEC, eighteen ACC and twenty PA cases by immunohistochemical examination. Whereas PA showed relatively low density of LOX expression, ACC revealed more cases that showing high staining intensities for LOX. Significantly increased LOX expression was found in the cases of ACC when compared to those of PA (P = 0.010).