CXCR2 Mediates Distinct Neutrophil Behavior in Brain Metastatic Breast Tumor

SIMPLE SUMMARY: Brain metastasis is one of the common complications associated with breast cancers. Neutrophils form the predominant type of circulating white blood cells and play an important role in tumor pathogenesis. However, the role of neutrophils in the evolution of brain metastasis of breast cancers has not been clearly understood. Using simple neutrophil-tumor cell-culture models, we studied the possible biomolecular mechanisms by which the brain-metastatic breast cancer cells could potentially re-program the neutrophils as a tumor-promoting strategy, and how drug-induced inhibition of certain key neutrophil functionalities could help reverse this behavior. ABSTRACT: Brain metastasis is one of the main causes of mortality among breast cancer patients, but the origins and the mechanisms that drive this process remain poorly understood. Here, we report that the upregulation of certain CXCR2-associated ligands in the brain metastatic variants of the breast cancer cells (BrM) dynamically activate the corresponding CXCR2 receptors on the neutrophils, thereby resulting in the modulation of certain key functional neutrophil responses towards the BrM. Using established neutrophil-tumor biomimetic co-culture models, we show that the upregulation of CXCR2 increases the recruitment of Tumor-Associated Neutrophils (TANs) towards the BrM, to enable location-favored formation of Neutrophil Extracellular Traps (NETs). Inhibition of CXCR2 using small molecule antagonist AZD5069 reversed this behavior, limiting the neutrophil responses to the BrM and retarding the reciprocal tumor development. We further demonstrate that abrogation of NETs formation using Neutrophil Elastase Inhibitor (NEI) significantly decreases the influx of neutrophils towards BrM but not to their parental tumor, suggesting that CXCR2 activation could be used by the brain metastatic tumors as a mechanism to program the tumor-infiltrating TANs into a pro-NETotic state, so as to assume a unique spatial distribution that assists in the subsequent migration and invasion of the metastatic tumor cells. This new perspective indicates that CXCR2 is a critical target for suppressing neutrophilic inflammation in brain metastasis.