Three-dimensional invasion of macrophages is mediated by cysteine cathepsins in protrusive podosomes

Podosomes, specialized actin-rich structures in macrophages (Mfs), degrade the extra-cellular matrix (ECM) and are involved in cell migration. On two-dimensional (2D) surfaces Mfs form spot-like podosomes at the ventral cell surface that develop into protrusive structures in a three-dimensional (3D) environment resembling the ECM. We have shown that the tips of these protrusive podosomes are characterized by increased accumulation of cysteine cathepsins (Cts) B, X, S, H, and L, both in human blood Mfs and in human monocytic cell line U-937. Monocyte-to-Mf differentiation induces an increase in cysteine cathepsin expression and activity, promoting their translocation to the cell surface, where they interact with ECM. This group of proteases is crucial for the extracellular as well as intracellular degradation of ECM, as demonstrated by quantitative monitoring of collagen IV degradation. Furthermore, inhibiting CtsB, X, and S significantly impairs Mf invasion through the 3D matrix. Time-lapse live-cell imaging of CtsB activity revealed that the extracellular and the intracellular ECM degradation are associated with extensive endocytosis at the tip of protrusive podosomes. The targeting of cysteine cathepsins, as the major mediators of human Mf 3D invasion, could be an approach to the treatment of inflammatory and cancerous diseases.