MMP9 and acute kidney injury: Under physiological conditions, kidney tubular epithelial cells produce low levels of the matrix metalloproteinases MMP-2 and MMP-9; however, following lead intoxication and renal fibrosis, the mRNA transcription levels of MMP-9 and MMP-2 are significantly increased and serve as sensitive biomarkers of acute kidney injury (Cheng et al. 2017; Wiercinska et al. 2011).