A cytoplasmic tyrosine kinase‐based signaling pathway regulating cytoskeleton dynamics and podosome formation in macrophages

Podosomes are protrusive structures implicated in macrophage extracellular matrix degradation and three-dimensional migration through cell barriers and the interstitium. Podosome formation and assembly are regulated by cytoskeleton remodeling requiring cytoplasmic tyrosine kinases of the Src and the Abl families. Considering that Abl has been reported to phosphorylate the guanine nucleotide exchange factor (GEF) Sos1, eliciting its Rac-GEF activity, and Rac regulates podosome formation in myeloid cells and invadopodia formation in cancer cells, we addressed whether Sos1 is implicated in podosome formation and function in macrophages. We found that ectopically expressed Abl or the Src kinase Fgr phosphorylate Sos1, and the Src kinases Hck and Fgr are required for Abl and Sos1 phosphorylation and Abl/Sos1 interaction in macrophages. Sos1 localizes to podosomes in both murine and human macrophages and its silencing by siRNA results in disassembly of murine macrophage podosomes and a marked reduction of GTP- loading on Rac. Matrix degradative capacity and trans-migration through an endothelial cell monolayer of Sos1-silenced macrophages were either suppressed or markedly inhibited, respectively. Additionally, Sos1- or Abl-silenced macrophages, or macrophages treated with the selective Abl inhibitor imatinib mesylate had a reduced capability to migrate into breast tumor spheroids, the majority of cells remaining at the margin and the outer layers of the spheroid itself. To note, neoplastic cells form podosome-like structures, called invadopodia, that have been implicated in cancer cell migratory ability and metastatic potential. Considering that both Src and Abl are implicated in cancer cell invadopodia formation, our studies suggest that targeting the SFKs/Abl/Sos1/Rac pathway may result in reduction of cancer cell invasive capacity but also of myeloid leukocyte recruitment into the tumor. Notably, macrophage recruitment into tumors has emerged as one of the hallmarks of cancer, thus pointing to the exciting possibility that targeting this pathway may represent a double-edged sword to control both cancer invasive capacities and cancer-related inflammation.