MMP9 and osteosarcoma: Acidosis has also been demonstrated to increase the secretion of proteases such as cathepsin B in melanoma cells and matrix metalloproteinase-9 (MMP-9) in human osteosarcoma cells (Rozhin et al., 1994; Matsubara et al., 2013), which may facilitate the degradation of extracellular matrix proteins and subsequently accelerate tumor cell invasion.