Emerging roles of Cxcl12/Cxcr4 signaling axis in breast cancer metastasis

Abstract

Abstract The Cxcl12/Cxcr4 axis promotes breast cancer metastasis in mice and humans. Prior studies have linked Cxcl12/Cxcr4 to cancer cell homing, survival and proliferation at distant sites secreting Cxcl12, such as lungs and liver. However, the mechanism via which Cxcr4+ cancer cells escape primary tumors in the first place (which also highly express Cxcl12), remains elusive. Here, by measuring chemotactic gradients using immunofluorescence, we show that Cxcl12 gradients are concentrically expressed around cancer cell intravasation sites, and spatially associate with Cxcr4+ tumor cells. Without affecting the gradient itself, pharmacological inhibition of Cxcl12 suppresses the translocation of Cxcr4+ cancer cells to intravasation sites. Paradoxically, tumor cell-specific deletion of Cxcr4 results in a suboptimal prometastatic response. We theorized that Cxcr4+ tumor-associated macrophages (TAMs), previously shown to accompany migratory tumor cells, could instead read the Cxcl12 chemotactic gradient, thus partially rescuing the prometastatic phenotype. Indeed, clodronate-mediated TAM depletion abrogates Cxcr4+ cancer cell translocation to intravasation sites and subsequent dissemination to secondary sites. We also found that breast cancer endothelium is lined by Cxcl12+Pdgfrb+ stromal cells; however, Cxcl12 gradients originating from intravasation sites are specifically tied to a subset of Cxcl12High perivascular TAMs. As such, pharmacological inhibition of Pdgfrb depletes Pdgfrb+Cxcl12+ stromal cells, but does not affect Cxcl12+ TAM-dependent translocation of Cxcr4+ tumor cells to intravasation sites. Overall, our data support a new model for the implication of Cxcl12/Cxcr4 in the early stages of metastasis.