The Antioxidant N-Acetylcysteine Prevents HIF-1 Stabilization under Hypoxia <i>In Vitro</i> but Does Not Affect Tumorigenesis in Multiple Breast Cancer Models <i>In Vivo</i>

<div><p>Intratumoral hypoxia is a poor prognostic factor associated with reduced disease-free survival in many cancer types, including breast cancer. Hypoxia encourages tumor cell proliferation, stimulates angiogenesis and lymphangiogenesis, and promotes epithelial-mesenchymal transition and metastasis. Tumor cells respond to a hypoxic state by stabilizing the Hif-1α subunit of the Hypoxia-Inducible Factor (HIF) transcription factor to promote expression of various tumor- and metastasis-promoting hypoxic response genes. The antioxidant N-acetylcysteine (NAC) was recently shown to prevent Hif-1α stabilization under hypoxia, and has been identified as a potential alternative method to target the hypoxic response in tumors. We utilized three orthotopic syngeneic murine models of breast cancer, the PyMT, EO771 and 4T1.2 models, to investigate the ability of NAC to modulate the hypoxic response <i>in vitro</i> and <i>in vivo</i>. While NAC prevented Hif-1α stabilization under hypoxia <i>in vitro</i> and increased levels of glutathione in the blood of mice <i>in vivo</i>, this did not translate to a difference in tumor growth or the hypoxic state of the tumor compared to untreated control mice. In addition, NAC treatment actually increased metastatic burden in an experimental metastasis model. This work raises questions regarding the validity of NAC as an anti-tumorigenic agent in breast cancer, and highlights the need to further investigate its properties <i>in vivo</i> in different cancer models.</p>