Transcriptional responses to hypoxia are primarily mediated by hypoxia-inducible factor (HIF), a heterodimer of HIF-␣ and the aryl hydrocarbon receptor nuclear translocator subunits. The HIF-1␣ and HIF-2␣ subunits are structurally similar in their DNA binding and dimerization domains but differ in their transactivation domains, implying they may have unique target genes. Previous studies using Hif-1␣ ؊/؊ embryonic stem and mouse embryonic fibroblast cells show that loss of HIF-1␣ eliminates all oxygen-regulated transcriptional responses analyzed, suggesting that HIF-2␣ is dispensable for hypoxic gene regulation. In contrast, HIF-2␣ has been shown to regulate some hypoxia-inducible genes in transient transfection assays and during embryonic development in the lung and other tissues. To address this discrepancy, and to identify specific HIF-2␣ target genes, we used DNA microarray analysis to evaluate hypoxic gene induction in cells expressing HIF-2␣ but not HIF-1␣. In addition, we engineered HEK293 cells to express stabilized forms of HIF-1␣ or HIF-2␣ via a tetracycline-regulated promoter. In this first comparative study of HIF-1␣ and HIF-2␣ target genes, we demonstrate that HIF-2␣ does regulate a variety of broadly expressed hypoxia-inducible genes, suggesting that its function is not restricted, as initially thought, to endothelial cell-specific gene expression. Importantly, HIF-1␣ (and not HIF-2␣) stimulates glycolytic gene expression in both types of cells, clearly showing for the first time that HIF-1␣ and HIF-2␣ have unique targets.Oxygen (O 2 ), the final electron acceptor during oxidative phosphorylation, is absolutely required for invertebrate and vertebrate life. The immediate response to O 2 deprivation (hypoxia) is a defense phase, which suppresses ATP consumption by arresting protein translation and ion channel activity, two major ATP sinks during normoxia. During a rescue phase, in spite of a general reduction in RNA synthesis, transcription of some genes increases dramatically under low O 2 (21, 34). These hypoxia-responsive genes are involved in glucose transport, glycolysis, erythropoiesis, angiogenesis, vasodilation, and respiratory rate, and together they function to minimize the effects caused by low O 2 at cellular, tissue and systemic levels (93, 106).The activation of many O 2 -regulated genes is mediated by hypoxia-inducible factor (HIF), a heterodimer consisting of HIF-1␣ and HIF-1␤ (also called the aryl hydrocarbon receptor nuclear translocator [ARNT]) in most cells (52,104,105). Both HIF-1␣ and ARNT belong to the basic helix-loop-helix (bHLH)-Per-Arnt-Sim (PAS) family of transcription factors, which share several conserved structural domains, including a bHLH region for DNA binding and two PAS domains for target gene specificity and dimerization (102). Although ARNT is absolutely required for HIF activity (63, 110), HIF function is primarily regulated by HIF-1␣ protein stability (37,46,84). Under normoxia, HIF-1␣ is ubiquitinated through interaction with the von Hippel-Lindau tumor suppr...

Epithelial-mesenchymal transition (EMT) is a tightly regulated process that is critical for embryogenesis but is abnormally activated during cancer metastasis and recurrence. Here we show that a switch in CD44 alternative splicing is required for EMT. Using both in vitro and in vivo systems, we have demonstrated a shift in CD44 expression from variant isoforms (CD44v) to the standard isoform (CD44s) during EMT. This isoform switch to CD44s was essential for cells to undergo EMT and was required for the formation of breast tumors that display EMT characteristics in mice. Mechanistically, the splicing factor epithelial splicing regulatory protein 1 (ESRP1) controlled the CD44 isoform switch and was critical for regulating the EMT phenotype. Additionally, the CD44s isoform activated Akt signaling, providing a mechanistic link to a key pathway that drives EMT. Finally, CD44s expression was upregulated in high-grade human breast tumors and was correlated with the level of the mesenchymal marker N-cadherin in these tumors. Together, our data suggest that regulation of CD44 alternative splicing causally contributes to EMT and breast cancer progression.

Breast cancer recurrence is a fundamental clinical manifestation of tumor progression and represents the principal cause of death from this disease. Using a conditional transgenic mouse model for the recurrence of HER2/neu-induced mammary tumors, we demonstrate that the transcriptional repressor Snail is spontaneously upregulated in recurrent tumors in vivo and that recurrence is accompanied by epithelial-to-mesenchymal transition (EMT). Consistent with a causal role for Snail in these processes, we show that Snail is sufficient to induce EMT in primary tumor cells, that Snail is sufficient to promote mammary tumor recurrence in vivo, and that high levels of Snail predict decreased relapse-free survival in women with breast cancer. In aggregate, our observations strongly implicate Snail in the process of breast cancer recurrence.

We have identi®ed a novel protein, BAP1, which binds to the RING ®nger domain of the Breast/Ovarian Cancer Susceptibility Gene product, BRCA1. BAP1 is a nuclearlocalized, ubiquitin carboxy-terminal hydrolase, suggesting that deubiquitinating enzymes may play a role in BRCA1 function. BAP1 binds to the wild-type BRCA1-RING ®nger, but not to germline mutants of the BRCA1-RING ®nger found in breast cancer kindreds. BAP1 and BRCA1 are temporally and spatially coexpressed during murine breast development and remodeling, and show overlapping patterns of subnuclear distribution. BAP1 resides on human chromosome 3p21.3; intragenic homozgyous rearrangements and deletions of BAP1 have been found in lung carcinoma cell lines. BAP1 enhances BRCA1-mediated inhibition of breast cancer cell growth and is the ®rst nuclearlocalized ubiquitin carboxy-terminal hydrolase to be identi®ed. BAP1 may be a new tumor suppressor gene which functions in the BRCA1 growth control pathway.