Hypoxia inducible factor-1alpha promotes primary tumor growth and tumor-initiating cell activity in breast cancer
Menée à l'aide d'un modèle murin, cette étude met en évidence le rôle clé joué par le facteur HIF-1alpha dans la croissance d'une tumeur mammaire et dans le processus métastatique, notamment par la régulation de la population des cellules initiatrices de tumeurs
INTRODUCTION:Over-expression of the oxygen-responsive transcription factor hypoxia inducible factor (HIF)-1alpha correlates with poor prognosis in breast cancer patients. The MMTV-PyMT (polyoma virus middle T) mouse is a widely utilized pre-clinical mouse model that resembles luminal breast cancer and is highly metastatic. Prior studies in the PyMT model demonstrated that HIF-1 is essential to promote carcinoma onset and lung metastasis, although no differences in primary tumor endpoint size were observed. Using a refined model system, we investigated whether HIF-1 is directly implicated in regulation of tumor-initiating cells (TICs) in breast cancer.METHODS:Mammary tumor epithelial cells (MTECs) were created from MMTV-PyMT mice harboring conditional alleles of Hif1a, followed by transduction ex vivo with either denovirusbeta-galactosidase or Cre to generate wild type (WT) and HIF-1 null (KO) cells, respectively. The impact of HIF-1 deletion on tumor-initiating potential was investigated using tumorsphere assays, limiting dilution transplantation and gene expression analysis.RESULTS:Efficient deletion of HIF-1alpha reduced primary tumor growth and suppressed lung metastases, prolonging survival. Loss of HIF-1 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-1alpha also enhanced tumorsphere formation at normoxia and hypoxia. Decreased expression of several genes in the Notch pathway, Vegf and Prominin-1 (CD133) were observed in response to HIF deletion. Immunohistochemistry confirmed that CD133 expression was reduced in KO cells and in tumorspheres. Tumorsphere formation was enhanced in CD133hi versus CD133neg 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-1alpha plays a key role in promoting primary mammary tumor growth and metastasis, in part through regulation of TICs. HIF-1 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 model. These data reveal for the first time that HIF-1 directly regulates breast TIC activity in vivo.