Pre-metastatic neutrophils directly support highly tumourigenic breast cancer cells during lung metastasis via a leukotriene-ERK1/2 axis

Evidence is mounting that cancer development and progression depends on a supporting microenvironment or stroma. Pro-tumourigenic and pro-metastatic properties of inflammation are long recognised and systemic or intra-tumoural presence of the cellular inflammatory mediators neutrophils shows strong associations with poor prognosis in the clinic. Thus, we were prompted to investigate the contribution of neutrophils to tumourigenesis and metastasis. Taking advantage of mouse models for lung metastatic breast cancer, we find neutrophils to be the predominant inflammatory component to strongly accumulate in the pre-metastatic organ of tumour-bearing mice prior to metastatic colonisation by cancer cells. Preventing increased neutrophil presence in the lung during metastasis initiation resulted in a pronounced decrease of metastatic burden. In fact, we unravelled a novel function of mammary tumour-induced neutrophils at the distant site to directly aid proliferation and initiation of metastatic outgrowth of cancer cells. Neutrophils specifically promote the intrinsically highly potent metastasis-initiating subpopulation of mammary cancer cells during initiation of metastatic lung colonisation via secretion of Alox5 metabolites, the lipid mediators leukotrienes. Engagement of leukotriene receptors with their ligands induced cell proliferation by activation of ERK1/2 kinases in mammary cancer cells. Leukotriene receptor expression is strongly enriched on metastasis-initiating cells and makes them susceptible to the neutrophil-derived proliferation-inducing signals leading to their expansion at the metastatic site. In fact, leukotriene receptor expression itself might represent a novel marker to identify highly tumourigenic cancer stem cells. Interference with neutrophil-derived Alox5 metabolites/leukotrienes holds potential to weaken the highly potent cancer stem cell-like subpool, the main cellular cause of metastasis initiation and relapse. Importantly, genetic or pharmacologic block of the Alox5 enzyme prevents the proliferation and expansion of metastasis-initiating cells and subsequently the metastasis-promoting activity of neutrophils. The Alox5 inhibitor Zileuton, which is routinely used in the clinic to treat asthmatic patients, significantly reduced lung metastasis in three mouse models of breast cancer. This observation, together with expression of leukotriene receptors in the majority of examined human breast cancers and lymph node metastases, suggests a promising novel therapeutic approach targeting the tumour stroma to limit metastatic progression. In summary, we found neutrophils, an inflammatory component of the metastatic microenvironment, to act pro-metastatic. Neutrophils specifically promote early events of metastasis initiation at the distant site by providing a direct, proliferation-inducing signal to intrinsically highly potent metastasis-initiating cells. Interference with the neutrophil-leukotriene-ERK1/2 axis-dependent support might hold great potential to be exploited in the clinic. Please note that part of the data contained in this PhD thesis was first published by the Nature Publishing Group (Wculek and Malanchi, 2015).