Cisplatin is one of the most widely used and most potent chemotherapy drugs. However, side effects in normal tissues and organs, notably nephrotoxicity in the kidneys, limit the use of cisplatin and related platinum-based therapeutics. Recent research has shed significant new lights on the mechanism of cisplatin nephrotoxicity, especially on the signaling pathways leading to tubular cell death and inflammation. Renoprotective approaches are being discovered, but the protective effects are mostly partial, suggesting the need for combinatorial strategies. Importantly, it is unclear whether these approaches would limit the anticancer effects of cisplatin in tumors. Examination of tumor-bearing animals and identification of novel renoprotective strategies that do not diminish the anticancer efficacy of cisplatin are essential to the development of clinically applicable interventions.

The mechanism of mitochondrial damage, a key contributor to renal tubular cell death during acute kidney injury, remains largely unknown. Here, we have demonstrated a striking morphological change of mitochondria in experimental models of renal ischemia/reperfusion and cisplatin-induced nephrotoxicity. This change contributed to mitochondrial outer membrane permeabilization, release of apoptogenic factors, and consequent apoptosis. Following either ATP depletion or cisplatin treatment of rat renal tubular cells, mitochondrial fragmentation was observed prior to cytochrome c release and apoptosis. This mitochondrial fragmentation was inhibited by Bcl2 but not by caspase inhibitors. Dynamin-related protein 1 (Drp1), a critical mitochondrial fission protein, translocated to mitochondria early during tubular cell injury, and both siRNA knockdown of Drp1 and expression of a dominant-negative Drp1 attenuated mitochondrial fragmentation, cytochrome c release, caspase activation, and apoptosis. Further in vivo analysis revealed that mitochondrial fragmentation also occurred in proximal tubular cells in mice during renal ischemia/ reperfusion and cisplatin-induced nephrotoxicity. Notably, both tubular cell apoptosis and acute kidney injury were attenuated by mdivi-1, a newly identified pharmacological inhibitor of Drp1. This study demonstrates a rapid regulation of mitochondrial dynamics during acute kidney injury and identifies mitochondrial fragmentation as what we believe to be a novel mechanism contributing to mitochondrial damage and apoptosis in vivo in mouse models of disease.

Autophagy is induced in renal tubular cells during acute kidney injury, however, whether this is protective or injurious remains controversial. We address this question by pharmacologic and genetic blockade of autophagy using mouse models of cisplatin- and ischemia-reperfusion induced acute kidney injury. Chloroquine, a pharmacological inhibitor of autophagy, blocked autophagic flux and enhanced acute kidney injury in both models. Rapamycin, however, activated autophagy and protected against cisplatin-induced acute kidney injury. We also established a renal proximal tubule-specific autophagy-related gene 7 knockout mouse model shown to be defective in both basal and cisplatin induced autophagy in kidneys. Compared with wild-type littermates, these knockout mice were markedly more sensitive to cisplatin-induced acute kidney injury as indicated by renal functional loss, tissue damage, and apoptosis. Mechanistically, these knockout mice had heightened activation of p53 and c-Jun N terminal kinase, signaling pathways contributing to cisplatin acute kidney injury. Proximal tubular cells isolated from the knockout mice were more sensitive to cisplatin-induced apoptosis than cells from wild-type mice. In addition, the knockout mice were more sensitive to renal ischemia-reperfusion injury than their wild-type littermates. Thus, our results establish a renoprotective role of tubular cell autophagy in acute kidney injury where it may interfere with cell killing mechanisms.