Macrophage subtypes inhibit breast cancer proliferation in culture.

Macrophages are a highly plastic cell type that adopt distinct subtypes and functional states depending on environmental cues. These functional states can vary wildly, with distinct macrophages capable of displaying opposing functions. We sought to understand how macrophage subtypes that exist on two ends of a spectrum influence the function of other cells. We used a co-culture system with primary human macrophages to probe the effects of macrophage subtypes on breast cancer cell proliferation. Our studies revealed a surprising phenotype in which both macrophage subtypes inhibited cancer cell proliferation compared to cancer cells alone. Of particular interest, using two different proliferation assays with two different breast cancer cell lines, we showed that differentiating macrophages into a "pro-tumor" subtype inhibited breast cancer cell proliferation. These findings are inconsistent with the prevailing interpretation that "pro-tumor" macrophages promote cancer cell proliferation and suggest a re-evaluation of how these interpretations are made.

Lack of Paxillin phosphorylation promotes single-cell migration in vivo.

Focal adhesions are structures that physically link the cell to the extracellular matrix for cell migration. Although cell culture studies have provided a wealth of information regarding focal adhesion biology, it is critical to understand how focal adhesions are dynamically regulated in their native environment. We developed a zebrafish system to visualize focal adhesion structures during single-cell migration in vivo. We find that a key site of phosphoregulation (Y118) on Paxillin exhibits reduced phosphorylation in migrating cells in vivo compared to in vitro. Furthermore, expression of a non-phosphorylatable version of Y118-Paxillin increases focal adhesion disassembly and promotes cell migration in vivo, despite inhibiting cell migration in vitro. Using a mouse model, we further find that the upstream kinase, focal adhesion kinase, is downregulated in cells in vivo, and cells expressing non-phosphorylatable Y118-Paxillin exhibit increased activation of the CRKII-DOCK180/RacGEF pathway. Our findings provide significant new insight into the intrinsic regulation of focal adhesions in cells migrating in their native environment.