Shedding of kidney injury molecule-1 by kidney proximal tubular epithelial cells: the role of matrixmetalloproteinase-3

Abstract

Regardless of the original cause and etiology, the progression of kidney disease follows a final common pathway associated with tubulointerstitial injury, in which proximal tubular epithelial cells (PTEC) are instrumental. Kidney injury molecule-1 (KIM-1) is an emerging biomarker of kidney tubular damage. It is markedly expressed and released into urine in various animal models and human kidney diseases. This study aimed to explore the underlying mechanism regulating the release of KIM-1 by PTEC.

First, expression and release of KIM-1 by primary cultured human PTEC were examined. In quiescent PTEC, KIM-1 was detected at the plasma membrane and in the cytoplasm. A transwell system, in which PTEC were grown as monolayer on permeable membrane, was used to examine the polarized release of KIM-1. PTEC constitutively released KIM-1 from their apical surface, and the release was independent of gene expression or protein synthesis. The KIM-1 release process by PTEC was enhanced dose- and time-dependently by two important kidney injury mediators, human serum albumin (HSA) and tumor necrosis factor (TNF)-α, and was inhibited by the presence of broad-spectrum inhibitors of matrix metalloproteinases (MMP).

Second, the potential sheddases responsible for KIM-1 shedding were identified by quantitative polymerase chain reaction (PCR) array system, in which the gene expression of a panel of MMP members was screened. The gene expression of MMP-3, MMP-7 and MMP-9 was up-regulated by PTEC under HSA or TNF-α activation. Blockade experiments with synthetic MMP inhibitors or MMP gene knockdown by small interfering RNA transfection, revealed that the constitutive or accelerated KIM-1 shedding was mediated by MMP-3, but not MMP-7 or MMP-9. The role of MMP-3 in KIM-1 shedding was further defined by additional data showing the enhanced MMP-3 synthesis by HSA- or TNF-α-stimulated PTEC, and the up-regulated KIM-1 shedding by PTEC following exogenous MMP-3 treatment.

Third, the regulatory mechanism of MMP-3-mediated KIM-1 shedding was investigated. Treatment of PTEC with HSA or TNF-α up-regulated the reactive oxygen species (ROS) generation, and its kinetics ran parallel to the increase of KIM-1 shedding and MMP-3 synthesis. In addition, exogenous hydrogen peroxide dose-dependently induced KIM-1 shedding and MMP-3 synthesis, which were abolished by the presence of an oxidation inhibitor. These evidence suggest that ROS play an essential role in regulating the MMP-3-mediated KIM-1 shedding by PTEC.

Finally, a mouse model of acute kidney injury induced by renal ischemia and reperfusion (I/R) was established to translate the in vitro findings. Reduced kidney function and increased urinary KIM-1 level were observed in mice after renal I/R treatment. Strikingly, the expression of MMP-3 and KIM-1 in the I/R treated mice was most profound in the S3 segments of the proximal tubules, where is the most susceptible area to oxidative stress. Taken together, these in vivo data have further strengthened the distinct roles of ROS and MMP-3 in KIM-1 shedding during PTEC injury.

In conclusion, ROS generated by the injured PTEC activate MMP-3, which release the soluble KIM-1 through the ectodomain shedding process.