MMP9 and cancer: CO2 also significantly inhibits tumor growth, tumor new blood vessel formation, and cancer cell invasion by affecting the expression of hypoxia-inducible factor 1-alpha (HIF-1a), vascular endothelial growth factor (VEGF), matrix metalloproteinase-2 (MMP-2), and matrix metalloproteinase-9 (MMP-9) [37,38].