The tumor suppressor NOR1 suppresses cell growth, invasiveness, and tumorigenicity in glioma.

Oxidored-nitro domain-containing protein 1 (NOR1) is a tumor suppressor downregulated in various human cancers, including nasopharyngeal carcinoma (NPC), lung cancer, and testicular cancer. NOR1 protein is highly expressed in the normal brain, however, its role in brain tumors remains unknown. In this study, we demonstrated that the NOR1 protein level was decreased in glioma tissue samples as compared to its normal counterpart. Exogenous expressed NOR1 protein in glioma U251 cells inhibits tumor cell proliferation, migration, and invasion. Re-expression of NOR1 induced cell cycle S to G2 phase arrest and suppressed its tumorigenicity in nude mice. Overexpression of NOR1 in U251 cells also led to a decrease of Ki67 expression in xenografts. Transcriptomic analysis revealed that NOR1 expression altered the expression of genes favored cell proliferation. Among the differentially expressed genes, FOXR2, a member of the FOX gene family, which promotes glioma progression, was decreased in NOR1 expressing cells. The downregulation of FOXR2 by NOR1 was validated in vitro and in vivo. Our findings suggest for the first time that NOR1 suppresses glioma progression via modulating the FOXR2 expression.