Recent studies of tissue culture cells have defined a widespread system of oxygen-regulated gene expression based on the activation of a heterodimeric transcription factor termed hypoxia-inducible factor-1 (HIF-1). To determine whether the HIF-1 transcriptional response is activated within solid tumors and to define the consequences, we have studied tumor xenografts of a set of hepatoma (Hepa-1) cells that are wild type (wt), deficient (c4), and revertant (Rc4) for an obligatory component of the HIF-1 heterodimer, HIF-1␤. Because HIF-1␤ is also essential for the xenobiotic response (in which it is termed the aryl hydrocarbon receptor nuclear translocator), we also studied c31 cells, which have a different defect in the xenobiotic response and form the HIF-1 complex normally. Two genes that show different degrees of HIF-1-dependent hypoxia-inducible expression in cell culture were selected for analysis-the glucose transporter, GLUT3, and vascular endothelial growth factor (VEGF). In situ hybridization showed intense focal induction of gene expression in tumors derived from wt, Rc4, and c31 cells, which was reduced (VEGF) or not seen (GLUT3) in those derived from c4 cells. In association with these changes, tumors of c4 cells had reduced vascularity and grew more slowly. These findings show that HIF-1 activation occurs in hypoxic regions of tumors and demonstrate a major inf luence on gene expression, tumor angiogenesis, and growth.

PR-104, currently in phase II clinical trials, is a phosphate ester pre-prodrug which is converted in vivo to its cognate alcohol, PR-104A, a prodrug designed to exploit tumor hypoxia. Bioactivation occurs via one-electron reduction to DNA crosslinking metabolites in the absence of oxygen. However, certain tumor cell lines activate PR-104A in the presence of oxygen, suggesting the existence of an aerobic nitroreductase. Microarray analysis identified a cluster of five aldo-keto reductase (AKR) family members whose expressions correlated with aerobic metabolism of PR-104A. Plasmid-based expression of candidate genes identified aldo-keto reductase 1C3 as a novel nitroreductase. AKR1C3 protein was detected by Western blot in 7 of 23 cell lines and correlated with oxic PR-104A metabolism, an activity which could be partially suppressed by Nrf2 RNAi knockdown (or induced by Keap1 RNAi), indicating regulation by the ARE pathway. AKR1C3 was unable to sensitize cells to 10 other bioreductive prodrugs and was associated with single-agent PR-104 activity across a panel of 9 human tumor xenograft models. Overexpression in two AKR1C3-negative tumor xenograft models strongly enhanced PR-104 antitumor activity. A population level survey of AKR1C3 expression in 2,490 individual cases across 19 cancer types using tissue microarrays revealed marked upregulation of AKR1C3 in a subset including hepatocellular, bladder, renal, gastric, and non-small cell lung carcinoma. A survey of normal tissue AKR1C3 expression suggests the potential for tumor-selective PR-104A activation by this mechanism. These findings have significant implications for the clinical development of PR-104. Cancer Res; 70(4); 1573-84.