Role of Tumor Oxygen Tension in Signaling and Response to Targeted Therapies

Abstract

Most tumor cells in solid tumors are exposed to oxygen levels ranging from 0.5% to 5%, but never to ambient air oxygen levels of about 21%. We developed an approach that allows collection, processing and evaluation of cancer and non-cancer cells under physioxia (3%-5% oxygen), ensuring little to no exposure to ambient air. This approach allowed for comparison of baseline and targeted therapy-induced changes in signaling pathways in cells under physioxia and ambient air and to identify potentially efficacious therapeutic combinations based on signaling pathways uniquely active under physioxia. Using tumor cells from two transgenic models of breast cancer and cells from breast tissues of clinically breast cancer-free women, we demonstrate oxygen level-dependent differences in cell preference for EGFR or PDGFRβ signaling. Physioxia caused PDGFRβ-mediated activation of AKT and ERK that reduced tumor cell sensitivity to EGFR and PIK3CA inhibition and maintained PDGFRβ+ epithelial-mesenchymal hybrid cells with potential cancer stem cell properties. Cells in ambient air displayed differential EGFR activation and were sensitive to EGFR and PIK3CA inhibition. Tumor cells grown under physioxia were sensitive to high affinity PDGFRβ inhibitor sunitinib. Furthermore, significantly higher synergistic growth inhibition and apoptosis was observed with lapatinib (a clinically used dual EGFR and ErbB2/HER2 inhibitor) and sunitinib combination only in tumor cells under physioxia both in vitro and in vivo. Our data emphasize the importance of oxygen considerations in preclinical cancer research to evaluate clinically relevant signaling pathways and identify novel drug targets or combination therapy approaches. We suggest that evaluation of candidate drugs for their efficacy under physiologic oxygen levels in preclinical models, prior to transitioning into clinical trials, would not only accelerate the development of effective drugs but also reduce failure at the clinical trial stage.