CTSL and osteosarcoma: Recent studies implicated OSM in macrophage M2 polarization, which might promote tumour progression.36, 37, 38 OSM reportedly increases the level of metastasis‐related proteins, including matrix metalloproteinase‐1 (MMP‐1) in osteogenic differentiation of mouse MC3T3 osteoblasts,39 MMP‐2 in human trophoblast cell line40 and cathepsin L in osteosarcoma cells.41