Hypoxia is known to favor tumor survival and progression. Numerous studies have shown that hypoxiainducible factor 1a (HIF-1a), an oxygen-sensitive transcription factor, is overexpressed in various types of human cancers and upregulates a battery of hypoxia-responsive genes for the growth and survival of cancer cells. Although tumor progression involves the acquisition of genetic and/or epigenetic changes that confer additional malignant traits, the underlying mechanisms of these changes remain obscure. We recently identified an alternative mechanism of HIF-1a function by which HIF-1a suppresses DNA repair by counteracting c-Myc transcriptional activity that maintains gene expression. Here, we show that this HIF-a-c-Myc pathway plays an essential role in mediating hypoxic effects on malignant progression via genetic alterations, resulting in the formation of malignant tumors with aggressive local invasion and epithelial-mesenchymal transition. We show an absolute requirement of the HIF-a-c-Myc pathway for malignant progression, whereas the canonical transcription function of HIF-1a alone is insufficient and seemingly dispensable. This study indicates that HIF1a induction of genetic alteration is the underlying cause of tumor progression, especially by the hypoxic microenvironment.

Hypoxia, a key microenvironmental factor for tumor development, not only stimulates angiogenesis and glycolysis for tumor expansion, but also induces cell cycle arrest and genetic instability for tumor progression. Several independent studies have shown hypoxic blockade of cell cycle progression at the G 1 /S transition, arising from the inactivation of S-phase-promoting cyclin E-CDK2 kinase complex. Despite these findings, the biochemical pathways leading to the cell cycle arrest remain poorly defined. We recently showed that hypoxic activates the expression of CDNK1A encoding the CDK2 inhibitor p21 Cip1 , through a novel HIF-1a-Myc pathway that involves Myc displacement from the CDNK1A promoter by the hypoxia-inducible transcription factor HIF-1a. In pursuit of further understanding of the hypoxic effects on cell cycle in tumor cells, here we report that hypoxia inhibits the expression of CDC25A, another cell cycle gene encoding a tyrosine phosphatase that maintains CDK2 activity. In accordance with the HIF-1a-Myc pathway, hypoxia requires HIF-1a for CDC25A repression, resulting in a selective displacement of an activating Myc from the CDC25A promoter without affecting a canonical Myc binding in the intron. Intriguingly, HIF-1a alone fails to recapitulate the hypoxic effect, indicating that HIF-1a is necessary but insufficient for the hypoxic repression. Taken together, our studies indicate that hypoxia inhibits cell cycle progression by controlling the expression of various cell cycle genes.

Although tumor progression involves genetic and epigenetic alterations to normal cellular biology, the underlying mechanisms of these changes remain obscure. Numerous studies have shown that hypoxia-inducible factor 1α (HIF-1α) is overexpressed in many human cancers and up-regulates a host of hypoxia-responsive genes for cancer growth and survival. We recently identified an alternative mechanism of HIF-1α function that induces genetic alterations by suppressing DNA repair. Here, we show that long-term hypoxia, which mimics the tumor microenvironment, drives a perpetual epithelial-mesenchymal transition (EMT) through up-regulation of the zinc finger E-box binding homeobox protein ZEB2, whereas short-term hypoxia induces a reversible EMT that requires the transcription factor Twist1. Moreover, we show that the perpetual EMT driven by chronic hypoxia depends on HIF-1α induction of genetic alterations rather than its canonical transcriptional activator function. These mesenchymal tumor cells not only acquire tumorigenicity but also display characteristics of advanced cancers, including necrosis, aggressive invasion, and metastasis. Hence, these results reveal a mechanism by which HIF-1α promotes a perpetual mesenchymal phenotype, thereby advancing tumor progression.

Cancer is a disease of genomic aberration. The hypoxic microenvironment is believed to promote tumor progression via the induction of genetic instability. To understand how hypoxia drives tumor progression, we have shown recently that the hypoxia-inducible transcription factor, HIF-1alpha, is critical for transcriptional repression of DNA repair genes by a noncanonical mode of action referred to as the "HIF-1alpha-c-Myc axis." HIF-1alpha action via the HIF-1alpha-c-Myc axis is independent of its DNA-binding and transactivation domains; instead it requires the PAS-B domain to displace the transcription activator c-Myc from the target gene promoter for gene repression. Owing to the functional compromise on DNA repair, tumor cells with activated HIF-1alpha-c-Myc axis display persistent DNA damage, genetic alterations, and malignant progression. However, apoptosis-proficient cells are resistant to such changes. These findings argue that the hypoxic microenvironment plays a critical role in driving genetic alterations especially in apoptosis-deficient cells for malignant progression.

For those with allergy, vaccination with a specific allergen has often been used as a major therapeutic measure. However, the universal application of this technique in clinics have been restricted due to its low success rates and the risk of active systemic anaphylactic shock (ASAS). In this regard, we constructed a fusion protein (OVA-DT), ovalbumin (OVA) fused with diphtheria toxin protein (DT), which may exert a specific cytotoxicity to cells bearing OVA-specific IgE. Its therapeutic effect was evaluated in mice (BALB/c) sensitized with OVA (Os-mice). OVA challenges to the OVA-sensitized mice (Os-mice) caused ASAS to death within 30 min, but OVA-DT treatment afforded mice complete protection. When OVA-DT was treated to the Os-mice, none showed the signs of ASAS when re-challenged 48 h after the treatment. OVA-DT itself was not found to be toxic or allergenic in normal mice. The effect of OVA-DT on the biological functions of mast cells was also studied. Binding of OVA-DT to OVA-specific IgE bearing mast cells and the inhibition of histamine release from these cells were observed. In addition, OVA-DT treatment inhibited the proliferation of OVA-specific B cells in mice. In Os-mice treated with OVA-DT, levels of anti-OVA IgG2a in serum and the production of IFN-gamma by splenic lymphocytes were found to increase, but the production of IL-4 by these cells decreased. Re-direction of cytokine profiles from OVA-specific Th2 to OVA-specific Thl is suggested. These results indicate that OVA-DT can protect Os-mice from ASAS due to OVA challenge, because it inactivates OVA-specific IgE-expressing cells, including mast cells and B cells.