Caspase-3-Independent Internucleosomal DNA Fragmentation in Ischemic Acute Kidney Injury

Abstract

Background/Aims: Renal tubular cell death in ischemia-reperfusion does not follow the classical apoptosis or necrosis phenotype. We characterized the morphological and biochemical features of injured tubular epithelial cells in ischemic acute kidney injury (AKI). Methods: Ischemic AKI was induced in rats by 60 min of ischemia followed by 24 h of reperfusion. Light and electron microscopic TUNEL (LM-TUNEL and EM-TUNEL), gel electrophoresis of extracted DNA, and caspase-3 involvement were examined during the development of death. Results: Damaged tubular epithelial cells with condensed and LM-TUNEL-positive (+) nuclei were prominent at 12 and 18 h after reperfusion with DNA ‘ladder’ pattern on gel electrophoresis. EM-TUNEL+ cells were characterized by nuclei with condensed and clumping chromatin, whereas the cytoplasm showed irreversible necrosis. The protein levels and activity of caspase-3 did not increase in kidneys after reperfusion. In addition, caspase inhibitor (ZVAD-fmk) failed to inhibit DNA fragmentation and prevent tubular epithelial cell death in ischemic AKI. Conclusion: Caspase-3-independent internucleosomal DNA fragmentation occurs in injured tubular epithelial cells undergoing irreversible necrosis in ischemic AKI. The manner of this cell death may be identical to the cell death termed apoptotic necrosis, aponecrosis, or necrapoptosis. Ischemia-reperfusion injury activates caspase-3-independent endonuclease, which in turn induces irreversible damage of tubular epithelial cells, and may contribute to the initiation and development of AKI.