The mobilization of bone marrow-derived cells (BMDCs) to distant tissues prior to the arrival of disseminated tumor cells has been shown preclinically to facilitate metastasis through the establishment of metastatic niches. Primary tumor hypoxia has been demonstrated to play a pivotal role in the production of chemokines and cytokines responsible for the mobilization of these BMDCs, especially in breast cancer. Carbonic Anhydrase IX (CAIX) expression is highly upregulated in hypoxic breast cancer cells through the action of Hypoxia-Inducible Factor-1 (HIF-1). Preclinical evidence has demonstrated that CAIX is required for breast tumor growth and metastasis, however, the mechanism by which CAIX exerts its prometastatic function is not well understood. Here, we show that CAIX is indispensable for the production of G-CSF by hypoxic breast cancer cells and tumors. Furthermore, we demonstrate that CAIX expression in the tumor is required for the G-CSF-driven mobilization of granulocytic myeloid-derived suppressor cells (MDSCs) to the breast cancer lung metastatic niche. We also determined that CAIX expression is required for the activation of NF-κB in hypoxic breast cancer cells and constitutive activation of the NF-κB pathway in CAIX-depleted cells restored G-CSF secretion. Together, these findings identify a novel hypoxia-induced CAIX-NF-κB-G-CSF cellular signalling axis culminating in the mobilization of granulocytic MDSCs to the breast cancer lung metastatic niche.


