MMP9 and esophageal squamous cell carcinoma: After observing that a CCL3–CCR5 axis promoted the migration and invasion of the three ESCC cell lines via PI3K/Akt and MEK/ERK pathways as described above, we investigated whether CCL3 upregulated the expressions of matrix metalloprotease-2 (MMP-2) and matrix metalloprotease-9 (MMP-9), which are well known to contribute to cell invasion by extracellular matrix remodeling in the tumor microenvironment [6].