One of the mechanisms involved includes the secretion of matrix metalloproteinase-9 (MMP-9) by M2 macrophages, which promotes the establishment of the pre-metastatic niche, as well as the extravasation of cancer cells to the metastatic site since it induces EMT of cancer cells and supports the invasion phenotype of cancer cells [81,82,83]. Here, MMP9 is linked to cancer.