Identification of Novel HIF1A Target Genes That Regulate Tumor Progression and Metastasis

Abstract

DEDICATION ABSTRACTHypoxia is a hallmark of most solid tumors. In response to hypoxic stress tumor cells adapt by regulating survival, metabolism and angiogenesis. The heterodimeric Hypoxia-Inducible Factor (HIF) transcription factors are the master regulators of this response. HIFs play key roles in many critical aspects of cancer biology including angiogenesis, stem cell maintenance, metabolic reprogramming, invasion, metastasis and resistance to radiation therapy and chemotherapy. Overexpression of HIF-1α and HIF-2α has been documented in multiple human cancers and HIF-1α protein is over-expressed in ~30% of primary breast tumors and ~70% of metastases, which independently correlates with poor prognosis and decreased survival in patients. A precise role for HIF-2α in breast cancer is still being elucidated.Our lab has established primary mammary tumor epithelial cells (MTECs) from late stage carcinomas originating in PyMT+; Hif1a floxed mice. These MTECs were exposed to either Adenovirus-beta-gal or -Cre to create wild-type (WT) and knockout (KO) cells, respectively. Deletion of HIF-1 activity reduced primary tumor growth by ~60% and the formation of lung macrometastases originating from mammary fat pad tumors by >90%. In addition, deletion of Hif1a reduced mammary tumorsphere formation efficiency (TSE) in vitro and tumor initiating cell (TIC) frequency in vivo. In contrast, in triple negative models of breast cancer, knockdown of HIF1A had the opposite phenotype, increasing TSE in vitro and increased primary mammary tumor growth in vivo. ITGA6 (CD49f) was identified as one HIF-1α-dependent cancer stem cell marker that enriches for both primary mammary tumor growth and metastasis to the lung. Microarray profiling conducted to identify genes differentially expressed between PyMT WT and KO cells and end-stage WT and KO tumors revealed several genes that were down-regulated in both data sets in response to deletion of Hif1a. One mRNA of particular interest, creatine kinase brain isoform (Ckb) was down-regulated in KO cells >100 fold and >2 fold in end-stage tumors. Transplantation of Ckb knockdown (KD) PyMT cells to the mammary fat pad delayed initiation of palpable tumors by >30 days. When cells were introduced into the circulation via tail vein injection, 20% of mice injected with Ckb KD cells developed lung metastases whereas 100% of mice injected with WT cells developed metastases. Likewise, when mice injected with WT PyMT cells were treated daily with a CKB chemical inhibitor, cyclocreatine (cCr), lungs of vehicle treated (saline) mice were almost completely covered with surface metastases, while lungs of cCr treated mice contained very few metastases.Overall, our data show that HIF-1α strongly promotes mammary tumor initiation, progression and metastasis, in part through regulation of TIC activity. Metastasis is the major cause of mortality in breast cancer patients. Further characterization of the distinct roles of HIF-1α versus HIF-2α in breast cancer and metastasis, and genes downstream of the HIFs, suc...