The ability to respond to differential levels of oxygen is important to all respiring cells. The response to oxygen deficiency, or hypoxia, takes many forms and ranges from systemic adaptations to those that are cell autonomous. Perhaps the most ancient of the cell-autonomous adaptations to hypoxia is a metabolic one: the Pasteur effect, which includes decreased oxidative phosphorylation and an increase in anaerobic fermentation. Because anaerobic fermentation produces far less ATP than oxidative phosphorylation per molecule of glucose, increased activity of the glycolytic pathway is necessary to maintain free ATP levels in the hypoxic cell. Here, we present genetic and biochemical evidence that, in mammalian cells, this metabolic switch is regulated by the transcription factor HIF-1. As a result, cells lacking HIF-1α exhibit decreased growth rates during hypoxia, as well as decreased levels of lactic acid production and decreased acidosis. We show that this decrease in glycolytic capacity results in dramatically lowered free ATP levels in HIF-1α-deficient hypoxic cells. Thus, HIF-1 activation is an essential control element of the metabolic state during hypoxia; this requirement has important implications for the regulation of cell growth during development, angiogenesis, and vascular injury.

Adaptation to hypoxia is a critical step in tumor progression and is, in part, regulated by the transcription factor hypoxia-inducible factor-1α (HIF-1α). Xenograft models have been extensively used to characterize the role of HIF-1α in experimental cancers. Although these models provide an understanding of tumor growth at terminal stages of malignancy, they do not address tumor initiation or metastatic progression. To elucidate these roles, HIF-1α was conditionally deleted in the mammary epithelium of a transgenic mouse model for metastatic breast cancer. Conditional deletion of HIF-1α in the mammary epithelium resulted in delayed tumor onset and retarded tumor growth; this was correlated with decreased tumor cell proliferation. Tumors with conditional deletion of HIF-1α were also less vascular during early tumor progression. Perhaps most surprisingly, deletion of HIF-1α in the mammary epithelium resulted in decreased pulmonary metastasis. These results show that whereas HIF-1α is not required for the initiation of breast tumor growth or tumor cell metastasis, the transcriptional activity of HIF-1α is a significant positive regulator of tumor progression and metastatic potential. [Cancer Res 2007;67(2):563–72]

Introduction: Overexpression of the oxygen-responsive transcription factor hypoxia-inducible factor 1α (HIF-1α) correlates with poor prognosis in breast cancer patients. The mouse mammary tumor virus polyoma virus middle T (MMTV-PyMT) mouse is a widely utilized preclinical mouse model that resembles human luminal breast cancer and is highly metastatic. Prior studies in which the PyMT model was used demonstrated that HIF-1α is essential to promoting carcinoma onset and lung metastasis, although no differences in primary tumor end point size were observed. Using a refined model system, we investigated whether HIF-1α is directly implicated in the regulation of tumor-initiating cells (TICs) in breast cancer. Methods: Mammary tumor epithelial cells were created from MMTV-PyMT mice harboring conditional alleles of Hif1a, followed by transduction ex vivo with either adenovirus β-galactosidase or adenovirus Cre to generate wild-type (WT) and HIF-1α-null (KO) cells, respectively. The impact of HIF-1α deletion on tumor-initiating potential was investigated using tumorsphere assays, limiting dilution transplantation and gene expression analysis. Results: Efficient deletion of HIF-1α reduced primary tumor growth and suppressed lung metastases, prolonging survival. Loss of HIF-1α led to reduced expression of markers of the basal lineage (K5/K14) in cells and tumors and of multiple genes involved in the epithelial-to-mesenchymal transition. HIF-1α also enhanced tumorsphere formation at normoxia and hypoxia. Decreased expression of several genes in the Notch pathway as well as Vegf and Prominin-1 (CD133)was observed in response to Hif1a deletion. Immunohistochemistry confirmed that CD133 expression was reduced in KO cells and in tumorspheres. Tumorsphere formation was enhanced in CD133^hi versus CD133^neg cells sorted from PyMT tumors. Limiting dilution transplantation of WT and KO tumor cells into immunocompetent recipients revealed > 30-fold enrichment of TICs in WT cells. Conclusion: These results demonstrate that HIF-1α plays a key role in promoting primary mammary tumor growth and metastasis, in part through regulation of TICs. HIF-1α regulates expression of several members of the Notch pathway, CD133 and markers of the basal lineage in mammary tumors. Our results suggest that CD133, which has not been profiled extensively in breast cancer, may be a useful marker of TICs in the PyMT mouse model. These data reveal for the first time that HIF-1α directly regulates breast TIC activity in vivo.