N 6 -methyladenosine (m 6 A) modification of mRNA plays a role in regulating embryonic stem cell pluripotency. However, the physiological signals that determine the balance between methylation and demethylation have not been described, nor have studies addressed the role of m 6 A in cancer stem cells. We report that exposure of breast cancer cells to hypoxia stimulated hypoxiainducible factor (HIF)-1α-and HIF-2α-dependent expression of AlkB homolog 5 (ALKBH5), an m 6 A demethylase, which demethylated NANOG mRNA, which encodes a pluripotency factor, at an m 6 A residue in the 3′-UTR. Increased NANOG mRNA and protein expression, and the breast cancer stem cell (BCSC) phenotype, were induced by hypoxia in an HIF-and ALKBH5-dependent manner. Insertion of the NANOG 3′-UTR into a luciferase reporter gene led to regulation of luciferase activity by O 2 , HIFs, and ALKBH5, which was lost upon mutation of the methylated residue. ALKBH5 overexpression decreased NANOG mRNA methylation, increased NANOG levels, and increased the percentage of BCSCs, phenocopying the effect of hypoxia. Knockdown of ALKBH5 expression in MDA-MB-231 human breast cancer cells significantly reduced their capacity for tumor initiation as a result of reduced numbers of BCSCs. Thus, HIF-dependent ALKBH5 expression mediates enrichment of BCSCs in the hypoxic tumor microenvironment.hypoxia-inducible factors | metastasis | pluripotency factors | self-renewal | tumorigenesis B reast cancer has the highest incidence of all cancers affecting women (1). It is also the leading cause of cancer-related death for women worldwide (2). Although progress has been made in treating early-stage breast cancer, there is no effective strategy to prevent or treat metastasis, which is the major cause of breast cancer-related mortality (3). Within breast cancers, a small subpopulation of cells, which are designated as tumor-initiating cells or breast cancer stem cells (BCSCs), have the unique property of generating daughter BCSCs, which are capable of infinite proliferation through self-renewal, and transient amplifying cells, which are capable of a limited number of cell divisions and give rise to differentiated breast cancer cells (4, 5). Only BCSCs are capable of forming a secondary (recurrent or metastatic) tumor (5, 6). As in the case of ES cells (ESCs) (7), the BCSC phenotype is specified by the expression of core pluripotency factors, including Kruppel-like factor 4 (KLF4), Octamer-binding transcription factor 4 (OCT4), SRY-box 2 (SOX2), and NANOG (8-12). Delineation of the molecular mechanisms that regulate the BCSC phenotype is needed to better understand metastasis and design more effective therapies.Intratumoral hypoxia is a common finding in advanced cancers. The median partial pressure of oxygen (pO 2 ) within primary breast cancers is 10 mm Hg [1.4% (vol/vol) O 2 ], compared with 65 mm Hg [9.3% (vol/vol) O 2 ] in normal breast tissue (13). Hypoxia-inducible factors (HIFs) HIF-1 and HIF-2 are heterodimeric transcriptional activators, consisting of an O 2 -regu...

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Cucurbitacins are triterpenoids that confer a bitter taste in cucurbits such as cucumber, melon, watermelon, squash, and pumpkin. These compounds discourage most pests on the plant and have also been shown to have antitumor properties. With genomics and biochemistry, we identified nine cucumber genes in the pathway for biosynthesis of cucurbitacin C and elucidated four catalytic steps. We discovered transcription factors Bl (Bitter leaf) and Bt (Bitter fruit) that regulate this pathway in leaves and fruits, respectively. Traces in genomic signatures indicated that selection imposed on Bt during domestication led to derivation of nonbitter cucurbits from their bitter ancestors.