Silencing of hypoxia-inducible tumor suppressor lysyl oxidase gene by promoter methylation activates carbonic anhydrase IX in nasopharyngeal carcinoma

Lysyl oxidase (LOX) is an oxidative enzyme known to initiate the cross-linking of collagens and elastin, and suggested recently as a tumor suppressor for several tumor types including lung, pancreatic and gastric cancers. Previously we showed that LOX is strongly induced upon hypoxia in nasopharyngeal carcinoma (NPC) cell lines CNE2 and HONE1 but only slightly in HK1 and not in C666-1. Here, we further studied the regulatory mechanism and functions of LOX in NPC. LOX is widely expressed in human normal tissues with variations in expression levels. LOX was expressed in most NPC cell lines except for C666-1, while HK1 and FaDu (laryngeal cancer) only expressed low level of LOX. Methylation analysis showed that the LOX promoter was methylated in C666-1 and partially methylated in HK1. After demethylation with 5-aza-2’-deoxycytidine, LOX expression was reactivated along with increased unmethylated alleles. LOX promoter methylation was detected in 42/49 (85.7%) of NPC primary tumors but only 3/16 (18.75%) of nose swab samples from NPC patients. LOX overexpression reduced the clonogenicity and cell growth of NPC cells, and also inhibited the migration and invasion of the NPC cells. Carbonic anhydrase IX (CA9) mRNA level was obviously decreased in HK1 cells after transfection with LOX. The elevation of CA9 protein upon hypoxia was inhibited in LOX-transfected HK1 cells. The protein levels of an apoptosis marker cPARP were increased in LOX-transfected HK1 cells upon hypoxia treatment. Our data showed that silencing or down-regulation of LOX in NPC was due to its promoter methylation and LOX acts as a tumor suppressor in NPC. LOX silencing would facilitate NPC cells to escape from hypoxia-induced apoptosis and maintains a malignant and metastatic phenotype.