Cisplatin nephrotoxicity: Mechanisms and renoprotective strategies

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. 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. Cisplatin is one of the most remarkable successes in ‘the war on cancer.’ Since the accidental discovery over four decades ago, cisplatin has been widely used for chemotherapy.1.Rosenberg B. Vancamp L. Krigas T. Inhibition of cell division in Escherichia coli by electrolysis products from a platinum electrode.Nature. 1965; 205: 698-699Crossref PubMed Scopus (1320) Google Scholar, 2.Wang D. Lippard S.J. Cellular processing of platinum anticancer drugs.Nat Rev Drug Discov. 2005; 4: 307-320Crossref PubMed Scopus (1473) Google Scholar, 3.Cohen S.M. Lippard S.J. Cisplatin: from DNA damage to cancer chemotherapy.Prog Nucleic Acid Res Mol Biol. 2001; 67: 93-130Crossref PubMed Google Scholar, 4.Arany I. Safirstein R.L. Cisplatin nephrotoxicity.Semin Nephrol. 2003; 23: 460-464Abstract Full Text Full Text PDF PubMed Scopus (384) Google Scholar, 5.Siddik Z.H. Cisplatin: mode of cytotoxic action and molecular basis of resistance.Oncogene. 2003; 22: 7265-7279Crossref PubMed Scopus (1340) Google Scholar It is potent, demonstrating one of the highest cure rates, for example, over 90% in testicular cancers.2.Wang D. Lippard S.J. Cellular processing of platinum anticancer drugs.Nat Rev Drug Discov. 2005; 4: 307-320Crossref PubMed Scopus (1473) Google Scholar, 3.Cohen S.M. Lippard S.J. Cisplatin: from DNA damage to cancer chemotherapy.Prog Nucleic Acid Res Mol Biol. 2001; 67: 93-130Crossref PubMed Google Scholar, 4.Arany I. Safirstein R.L. Cisplatin nephrotoxicity.Semin Nephrol. 2003; 23: 460-464Abstract Full Text Full Text PDF PubMed Scopus (384) Google Scholar, 5.Siddik Z.H. Cisplatin: mode of cytotoxic action and molecular basis of resistance.Oncogene. 2003; 22: 7265-7279Crossref PubMed Scopus (1340) Google Scholar Cisplatin and related platinum-based therapeutics are now being used for the treatment of testicular, head and neck, ovarian, cervical, nonsmall cell lung carcinoma, and many other types of cancer.2.Wang D. Lippard S.J. Cellular processing of platinum anticancer drugs.Nat Rev Drug Discov. 2005; 4: 307-320Crossref PubMed Scopus (1473) Google Scholar, 3.Cohen S.M. Lippard S.J. Cisplatin: from DNA damage to cancer chemotherapy.Prog Nucleic Acid Res Mol Biol. 2001; 67: 93-130Crossref PubMed Google Scholar, 4.Arany I. Safirstein R.L. Cisplatin nephrotoxicity.Semin Nephrol. 2003; 23: 460-464Abstract Full Text Full Text PDF PubMed Scopus (384) Google Scholar, 5.Siddik Z.H. Cisplatin: mode of cytotoxic action and molecular basis of resistance.Oncogene. 2003; 22: 7265-7279Crossref PubMed Scopus (1340) Google Scholar Unlike most cancer therapy drugs, which are usually complex organic compounds, cisplatin is a simple inorganic molecule. The mechanism of the anticancer activity of cisplatin is not completely understood, but a widely held view is that cisplatin binds to DNA, leading to the formation of inter- and intrastrand cross-links.2.Wang D. Lippard S.J. Cellular processing of platinum anticancer drugs.Nat Rev Drug Discov. 2005; 4: 307-320Crossref PubMed Scopus (1473) Google Scholar, 6.Ciccarelli R.B. Solomon M.J. Varshavsky A. et al.In vivo effects of cis- and trans-diamminedichloroplatinum(II) on SV40 chromosomes: differential repair, DNA-protein cross-linking, and inhibition of replication.Biochemistry. 1985; 24: 7533-7540Crossref PubMed Google Scholar, 7.Heiger-Bernays W.J. Essigmann J.M. Lippard S.J. 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Mildly damaged DNA can be repaired, whereas extensive DNA damage leads to irreversible injury and cell death. Although cisplatin has been a mainstay for cancer therapy, its use is mainly limited by two factors: acquired resistance to cisplatin and severe side effects in normal tissues.2.Wang D. Lippard S.J. Cellular processing of platinum anticancer drugs.Nat Rev Drug Discov. 2005; 4: 307-320Crossref PubMed Scopus (1473) Google Scholar,5.Siddik Z.H. Cisplatin: mode of cytotoxic action and molecular basis of resistance.Oncogene. 2003; 22: 7265-7279Crossref PubMed Scopus (1340) Google Scholar The molecular mechanism of cisplatin resistance has been studied extensively, which may involve decreased uptake or increased efflux of cisplatin, neutralization of cisplatin by glutathione and other sulfur-containing molecules, increased DNA repair, and defective apoptotic signaling in response to DNA damage.5.Siddik Z.H. Cisplatin: mode of cytotoxic action and molecular basis of resistance.Oncogene. 2003; 22: 7265-7279Crossref PubMed Scopus (1340) Google Scholar, 11.Kartalou M. Essigmann J.M. Mechanisms of resistance to cisplatin.Mutat Res. 2001; 478: 23-43Crossref PubMed Scopus (508) Google Scholar, 12.Wernyj R.P. Morin P.J. Molecular mechanisms of platinum resistance: still searching for the Achilles' heel.Drug Resist Updat. 2004; 7: 227-232Abstract Full Text Full Text PDF PubMed Scopus (57) Google Scholar, 13.Siddik Z.H. Biochemical and molecular mechanisms of cisplatin resistance.Cancer Treat Res. 2002; 112: 263-284Crossref PubMed Google Scholar The other major limiting factor in the use of cisplatin is the side effects in normal tissues, which include neurotoxicity, ototoxicity, nausea and vomiting, and nephrotoxicity. For years, various approaches have been attempted to curtail these side effects. One strategy is to synthesize and screen for novel cisplatin analogues that have lower toxicity in normal tissues. In this direction, several cisplatin analogues, such as carboplatin, have been identified with less severe side effects.14.Pasetto L.M. D'Andrea M.R. Brandes A.A. et al.The development of platinum compounds and their possible combination.Crit Rev Oncol Hematol. 2006; 60: 59-75Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar Another approach that has been used with some success is to hydrate the patients during cisplatin treatment.15.Cornelison T.L. Reed E. Nephrotoxicity and hydration management for cisplatin, carboplatin, and ormaplatin.Gynecol Oncol. 1993; 50: 147-158Abstract Full Text PDF PubMed Scopus (135) Google Scholar,16.Bajorin D.F. Bosl G.J. Alcock N.W. et al.Pharmacokinetics of cis-diamminedichloroplatinum(II) after administration in hypertonic saline.Cancer Res. 1986; 46: 5969-5972PubMed Google Scholar Despite these efforts, the side effects of cisplatin, particularly nephrotoxicity, remain a major factor that limits the use and efficacy of cisplatin in cancer therapy. Understanding the mechanism of cisplatin nephrotoxicity could lead to novel renoprotective interventions. Nephrotoxicity was reported in the initial clinical trials of cisplatin chemotherapy.17.Hill J.M. Speer R.J. Organo-platinum complexes as antitumor agents (review).Anticancer Res. 1982; 2: 173-186PubMed Google Scholar Now, it is recognized that the prevalence of cisplatin nephrotoxicity is high, occurring in about one-third of patient undergoing cisplatin treatment.4.Arany I. Safirstein R.L. Cisplatin nephrotoxicity.Semin Nephrol. 2003; 23: 460-464Abstract Full Text Full Text PDF PubMed Scopus (384) Google Scholar,18.Beyer J. Rick O. Weinknecht S. et al.Nephrotoxicity after high-dose carboplatin, etoposide and ifosfamide in germ-cell tumors: incidence and implications for hematologic recovery and clinical outcome.Bone Marrow Transplant. 1997; 20: 813-819Crossref PubMed Google Scholar Clinically, cisplatin nephrotoxicity is often seen after 10 days of cisplatin administration and is manifested as lower glomerular filtration rate, higher serum creatinine, and reduced serum magnesium and potassium levels.4.Arany I. Safirstein R.L. Cisplatin nephrotoxicity.Semin Nephrol. 2003; 23: 460-464Abstract Full Text Full Text PDF PubMed Scopus (384) Google Scholar, 19.Gonzales-Vitale J.C. Hayes D.M. Cvitkovic E. et al.The renal pathology in clinical trials of cis-platinum (II) diamminedichloride.Cancer. 1977; 39: 1362-1371Crossref PubMed Google Scholar, 20.Gomez Campdera F.J. Gonzalez P. Carrillo A. et al.Cisplatin nephrotoxicity: symptomatic hypomagnesemia and renal failure.Int J Pediatr Nephrol. 1986; 7: 151-152PubMed Google Scholar On the other hand, the long-term effects of cisplatin on renal function are not completely understood, but it is believed that cisplatin treatment may lead to subclinical but permanent reduction in glomerular filtration rate.21.Brillet G. Deray G. Jacquiaud C. et al.Long-term renal effect of cisplatin in man.Am J Nephrol. 1994; 14: 81-84Crossref PubMed Google Scholar The pathophysiological basis of cisplatin nephrotoxicity has been studied for the last three decades. However, only recently has the research been directed toward the understanding of the cellular and molecular mechanism. The emerging picture (Figure 1) is that the exposure of tubular cells to cisplatin activates complex signaling pathways that lead to tubular cell injury and death. Meanwhile, a robust inflammatory response is stimulated, further exacerbating renal tissue damage. Cisplatin may also induce injury in renal vasculature and result in decreased blood flow and ischemic injury of the kidneys, contributing to a decline in glomerular filtration rate. These events, together, culminate in the loss of renal function during cisplatin nephrotoxicity, triggering acute renal failure. Apparently, this is a simplistic view and as discussed in the following sections, cisplatin nephrotoxicity is now recognized as a very complex multifactorial process. The mechanism of intracellular transport of cisplatin is not clear and may vary from one cell type to another. Earlier work using cancer cell lines concluded that about half of the cisplatin uptake is due to passive diffusion through the plasma membrane, and the remaining half is mediated by an unknown transporter.22.Gately D.P. Howell S.B. Cellular accumulation of the anticancer agent cisplatin: a review.Br J Cancer. 1993; 67: 1171-1176Crossref PubMed Google Scholar In 2002, Ishida et al.23.Ishida S. Lee J. Thiele D.J. et al.Uptake of the anticancer drug cisplatin mediated by the copper transporter Ctr1 in yeast and mammals.Proc Natl Acad Sci USA. 2002; 99: 14298-14302Crossref PubMed Scopus (394) Google Scholar showed that deletion of the Ctr1, a high-affinity copper transporter, results in reduced intracellular accumulation of cisplatin in yeast, which is associated by increased resistance to cisplatin toxicity. Similar observations were shown in mouse cell lines lacking one or both mouse Ctr1 (mCtr1) alleles, suggesting that copper transporters may mediate cisplatin uptake in both yeast and mammals. In addition, ATP7B, a copper efflux transporter, has been shown to be overexpressed in cancer cell lines that are resistant to cisplatin.24.Kuo M.T. Chen H.H. Song I.S. et al.The roles of copper transporters in cisplatin resistance.Cancer Metastasis Rev. 2007; 26: 71-83Crossref PubMed Scopus (132) Google Scholar,25.Safaei R. Howell S.B. Copper transporters regulate the cellular pharmacology and sensitivity to Pt drugs.Crit Rev Oncol Hematol. 2005; 53: 13-23Abstract Full Text Full Text PDF PubMed Scopus (166) Google Scholar Together, these studies, mainly from cancer cells, have suggested a critical role for the copper transport system in cisplatin uptake by cells. It is interesting to note that Ctr1 is also highly expressed in proximal tubular cells,26.Kuo Y.M. Gybina A.A. Pyatskowit J.W. et al.Copper transport protein (Ctr1) levels in mice are tissue specific and dependent on copper status.J Nutr. 2006; 136: 21-26Crossref PubMed Google Scholar although the role of these transporters in cisplatin uptake has not been studied in renal models. On the other hand, in the renal system, the organic cation transporters (OCTs) have been implicated in cisplatin uptake.27.Ludwig T. Riethmuller C. Gekle M. et al.Nephrotoxicity of platinum complexes is related to basolateral organic cation transport.Kidney Int. 2004; 66: 196-202Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar,28.Yonezawa A. Masuda S. Nishihara K. et al.Association between tubular toxicity of cisplatin and expression of organic cation transporter rOCT2 (Slc22a2) in the rat.Biochem Pharmacol. 2005; 70: 1823-1831Crossref PubMed Scopus (77) Google Scholar OCTs mediate the basolateral-to-apical transport of several cationic compounds in renal tubular cells. In 2004, Ludwig et al.27.Ludwig T. Riethmuller C. Gekle M. et al.Nephrotoxicity of platinum complexes is related to basolateral organic cation transport.Kidney Int. 2004; 66: 196-202Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar showed that cisplatin, when applied at the basolateral side, induced higher toxicity in MDCK cells than apical side application. The results suggest that cisplatin-induced tubular cell injury may be related to basolateral organic cation transport. Notably, cimetidine, an inhibitor of OCTs, could partially prevent cisplatin-induced cytotoxicity shown by the decrease of transepithelial electrical resistance.27.Ludwig T. Riethmuller C. Gekle M. et al.Nephrotoxicity of platinum complexes is related to basolateral organic cation transport.Kidney Int. 2004; 66: 196-202Abstract Full Text Full Text PDF PubMed Scopus (67) Google Scholar Three isoforms of OCTs are expressed in renal proximal tubules, mainly at the basolateral side.29.Motohashi H. Sakurai Y. Saito H. et al.Gene expression levels and immunolocalization of organic ion transporters in the human kidney.J Am Soc Nephrol. 2002; 13: 866-874Crossref PubMed Google Scholar Recent work by Ciarimboli et al.30.Ciarimboli G. Ludwig T. Lang D. et al.Cisplatin nephrotoxicity is critically mediated via the human organic cation transporter 2.Am J Pathol. 2005; 167: 1477-1484Abstract Full Text Full Text PDF PubMed Google Scholar provided evidence to suggest that OCT2 is the critical OCT responsible for cisplatin uptake in the kidney. They showed that cisplatin uptake was increased by OCT2 overexpression in HEK293 cells, which was associated with increased cellular sensitivity to cisplatin toxicity. Consistently, cimetidine decreased cisplatin uptake in freshly isolated human proximal tubular cells. In addition, proximal tubular cells isolated from a human diabetic kidney showed reduced cisplatin uptake, which was attributed to the well-documented lower expression of OCT2 in diabetes. Interestingly, cisplatin did not interact with OCT1. It was speculated that since OCT1 is mainly expressed in the liver and OCT2 in the kidneys, this differential expression pattern of OCTs in different tissues might account for organ-specific toxicity of cisplatin. Of note, the less nephrotoxic analogues of cisplatin such as carboplatin and oxaliplatin did not interact with OCT2.30.Ciarimboli G. Ludwig T. Lang D. et al.Cisplatin nephrotoxicity is critically mediated via the human organic cation transporter 2.Am J Pathol. 2005; 167: 1477-1484Abstract Full Text Full Text PDF PubMed Google Scholar After entry into the cell, cisplatin may interact with various kinds of reactive groups. In the kidney, it has been suggested that the nephrotoxicity of cisplatin may depend on metabolic activation, which involves a pathway including γ-glutamyl transpeptidase and cysteine-S-conjugate β-lyase.31.Townsend D.M. Deng M. Zhang L. et al.Metabolism of cisplatin to a nephrotoxin in proximal tubule cells.J Am Soc Nephrol. 2003; 14: 1-10Crossref PubMed Scopus (128) Google Scholar Inhibition of either of the enzymes led to amelioration of cisplatin nephrotoxicity in mice.32.Hanigan M.H. Gallagher B.C. Taylor Jr, P.T. et al.Inhibition of gamma-glutamyl transpeptidase activity by acivicin in vivo protects the kidney from cisplatin-induced toxicity.Cancer Res. 1994; 54: 5925-5929PubMed Google Scholar,33.Hanigan M.H. Lykissa E.D. Townsend D.M. et al.Gamma-glutamyl transpeptidase-deficient mice are resistant to the nephrotoxic effects of cisplatin.Am J Pathol. 2001; 159: 1889-1894Abstract Full Text Full Text PDF PubMed Google Scholar Notably, prostate cancer xenografts overexpressing γ-glutamyl transpeptidase were more resistant to cisplatin therapy, suggesting that inhibition of the cisplatin activation pathway may reduce the nephrotoxicity of cisplatin, yet enhance its antitumor effect.34.Hanigan M.H. Gallagher B.C. Townsend D.M. et al.Gamma-glutamyl transpeptidase accelerates tumor growth and increases the resistance of tumors to cisplatin in vivo.Carcinogenesis. 1999; 20: 553-559Crossref PubMed Google Scholar Further investigation is needed to directly test this possibility and determine whether the bioactivation is indeed critical to cisplatin nephrotoxicity. Renal tissue damage, characterized by tubular cell death, is a common histopathological feature of cisplatin nephrotoxicity. Under this condition, cell death in the form of both necrosis and apoptosis is identified. Recent studies have provided new insights into the possible connections between these two forms of cell death. In addition, the cell types that undergo apoptosis during cisplatin nephrotoxicity have been further investigated. Using cultured renal tubular cells, earlier observations by Lieberthal et al.35.Lieberthal W. Triaca V. Levine J. Mechanisms of death induced by cisplatin in proximal tubular epithelial cells: apoptosis vs. necrosis.Am J Physiol. 1996; 270: F700-F708PubMed Google Scholar suggested that the dosage of cisplatin might determine whether the cells die by necrosis or apoptosis. Necrotic cell death was observed when a high concentration of cisplatin (millimolar) was used, while lower concentrations of cisplatin (micromolar) led to apoptosis. Nevertheless, in vivo in animals, both necrosis and apoptosis were induced in renal tubules following cisplatin administration.36.Megyesi J. Safirstein R.L. Price P.M. Induction of p21WAF1/CIP1/SDI1 in kidney tubule cells affects the course of cisplatin-induced acute renal failure.J Clin Invest. 1998; 101: 777-782Crossref PubMed Google Scholar, 37.Ramesh G. Reeves W.B. TNFR2-mediated apoptosis and necrosis in cisplatin-induced acute renal failure.Am J Physiol Renal Physiol. 2003; 285: F610-F618Crossref PubMed Google Scholar, 38.Liu H. Baliga R. Cytochrome P450 2E1 null mice provide novel protection against cisplatin-induced nephrotoxicity and apoptosis.Kidney Int. 2003; 63: 1687-1696Abstract Full Text Full Text PDF PubMed Scopus (71) Google Scholar Is there a relationship between necrosis and apoptosis in the injured tissues? Necrosis can be induced directly by severe toxic injury. However, it can also be a postmortem result of apoptosis, termed secondary necrosis. In addition, despite their distinct morphologies, necrosis and apoptosis may share some important signaling events. For example, mitochondrial damage by proapoptotic proteins such as Bid and Bax could destine the cells to death. Under this condition, if downstream apoptotic events are defective or aborted, the cells will become passive and finally end up being lysed in the form of necrosis. Although in vivo evidence to support this scenario is lacking, renoprotective agents frequently prevent both necrosis and apoptosis in renal tissues during cisplatin nephrotoxicity.39.Faubel S. Ljubanovic D. Reznikov L. et al.Caspase-1-deficient mice are protected against cisplatin-induced apoptosis and acute tubular necrosis.Kidney Int. 2004; 66: 2202-2213Abstract Full Text Full Text PDF PubMed Scopus (84) Google Scholar, 40.Ramesh G. Reeves W.B. p38 MAP kinase inhibition ameliorates cisplatin nephrotoxicity in mice.Am J Physiol Renal Physiol. 2005; 289: F166-F174Crossref PubMed Scopus (125) Google Scholar, 41.Li S. Basnakian A. Bhatt R. et al.PPAR-alpha ligand ameliorates acute renal failure by reducing cisplatin-induced increased expression of renal endonuclease G.Am J Physiol Renal Physiol. 2004; 287: F990-F998Crossref PubMed Scopus (59) Google Scholar Moreover, knockout of known apoptotic genes such as Bax diminishes tubular cell apoptosis as well as necrosis under this condition.42.Wei Q. Dong G. Franklin J. et al.The pathological role of Bax in cisplatin nephrotoxicity.Kidney Int. 2007; 72: 53-62Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar It is well recognized that renal tubules are the major sites of cell injury and death during cisplatin nephrotoxicity. Earlier work suggested that the distal tubules were the primary site of apoptosis.36.Megyesi J. Safirstein R.L. Price P.M. Induction of p21WAF1/CIP1/SDI1 in kidney tubule cells affects the course of cisplatin-induced acute renal failure.J Clin Invest. 1998; 101: 777-782Crossref PubMed Google Scholar However, recent studies indicated that proximal tubular cells also undergo apoptosis during cisplatin nephrotoxicity.41.Li S. Basnakian A. Bhatt R. et al.PPAR-alpha ligand ameliorates acute renal failure by reducing cisplatin-induced increased expression of renal endonuclease G.Am J Physiol Renal Physiol. 2004; 287: F990-F998Crossref PubMed Scopus (59) Google Scholar,43.Tsuruya K. Ninomiya T. Tokumoto M. et al.Direct involvement of the receptor-mediated apoptotic pathways in cisplatin-induced renal tubular cell death.Kidney Int. 2003; 63: 72-82Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar Nevertheless, these studies did not clearly identify the cell type(s) of apoptosis using specific markers. A more recent study has addressed this question by using proximal and distal tubule-specific lectins. It was shown that many apoptotic cells were stained by phytohemagglutinin, a proximal tubule-binding lectin, whereas significantly fewer apoptotic cells were stained by peanut lectin agglutinin, a distal tubule-binding lectin.42.Wei Q. Dong G. Franklin J. et al.The pathological role of Bax in cisplatin nephrotoxicity.Kidney Int. 2007; 72: 53-62Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar Thus, apoptosis occurs in both tubular segments, but the majority is in proximal tubules. These observations justify the extensive use of primary as well as immortalized proximal tubular cells for mechanistic studies of cisplatin nephrotoxicity. Interestingly, cisplatin can also induce necrosis and apoptosis in cultured endothelial cells, although evidence is yet to be demonstrated for endothelial cell apoptosis during cisplatin nephrotoxicity in vivo.44.Dursun B. He Z. Somerset H. et al.Caspases and calpain are independent mediators of cisplatin-induced endothelial cell necrosis.Am J Physiol Renal Physiol. 2006; 291: F578-F587Crossref PubMed Scopus (46) Google Scholar During the last few years, apoptosis of renal tubular cells has been a focus of mechanistic investigation of cisplatin nephrotoxicity. Several pathways of apoptosis have been implicated, including the extrinsic pathway mediated by death receptors, the intrinsic pathway centered on mitochondria, and the endoplasmic reticulum (ER)-stress pathway (Figure 2). In the extrinsic pathway, binding of the death receptors by ligands at the plasma membrane leads to the recruitment and activation of caspase-8, which further activate downstream caspases to induce apoptosis.45.Strasser A. O'Connor L. Dixit V.M. Apoptosis signaling.Annu Rev Biochem. 2000; 69: 217-245Crossref PubMed Scopus (1104) Google Scholar Major death receptors include Fas, tumor-necrosis factor-α (TNF-α) receptor (TNFR) 1 and 2. In cultured human proximal tubular cells, Razzaque et al.46.Razzaque M.S. Koji T. Kumatori A. et al.Cisplatin-induced apoptosis in human proximal tubular epithelial cells is associated with the activation of the Fas/Fas ligand system.Histochem Cell Biol. 1999; 111: 359-365Crossref PubMed Scopus (39) Google Scholar showed an upregulation of Fas and Fas ligand by cisplatin, which was associated with apoptosis. However, definitive evidence for a role of Fas in cisplatin-induced tubular cell apoptosis was not established. On the other hand, Tsuruya et al.43.Tsuruya K. Ninomiya T. Tokumoto M. et al.Direct involvement of the receptor-mediated apoptotic pathways in cisplatin-induced renal tubular cell death.Kidney Int. 2003; 63: 72-82Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar demonstrated the amelioration of cisplatin-induced tubular cell apoptosis and renal failure in TNFR1-deficient cells and mice, suggesting the involvement of TNFR1 signaling in cisplatin nephrotoxicity. In 2002, Ramesh and Reeves47.Ramesh G. Reeves W.B. TNF-alpha mediates chemokine and cytokine expression and renal injury in cisplatin nephrotoxicity.J Clin Invest. 2002; 110: 835-842Crossref PubMed Scopus (377) Google Scholar demonstrated an impressive induction of TNF-α by cisplatin in mice. Importantly, pharmacological and genetic inhibition of TNF-α attenuated the production of various cytokines and chemokines, which was accompanied by the amelioration of cisplatin nephrotoxicity. A follow-up study by these investigators further showed that TNFR2-deficient, but not TNFR1-deficient, mice were resistant to cisplatin-induced renal injury.37.Ramesh G. Reeves W.B. TNFR2-mediated apoptosis and necrosis in cisplatin-induced acute renal failure.Am J Physiol Renal Physiol. 2003; 285: F610-F618Crossref PubMed Google Scholar While these studies support a role for TNF-α in the inflammatory response and cisplatin nephrotoxicity, it is suggested that TNFR2, and not TNFR1, is mainly responsible for the pathogenic signaling of TNF-α.37.Ramesh G. Reeves W.B. TNFR2-mediated apoptosis and necrosis in cisplatin-induced acute renal failure.Am J Physiol Renal Physiol. 2003; 285: F610-F618Crossref PubMed Google Scholar Of note, TNF-α and its receptors may induce renal injury primarily by mounting a disastrous inflammatory response rather than by directly activating the extrinsic pathway of apoptosis.48.Ramesh G. Reeves W.B. Inflammatory cytokines in acute renal failure.Kidney Int Suppl. 2004; 66: S56-S61Abstract Full Text Full Text PDF Google Scholar On the other hand, the intrinsic or mitochondrial pathway has emerged as the major apoptotic pathway in cisplatin nephrotoxicity. In the intrinsic pathway, cellular stress leads to the activation of the proapoptotic Bcl-2 family proteins Bax and Bak, which form porous defects on the outer membrane of mitochondria, resulting in the release of apoptogenic factors from the organelles.45.Strasser A. O'Connor L. Dixit V.M. Apoptosis signaling.Annu Rev Biochem. 2000; 69: 217-245Crossref PubMed Scopus (1104) Google Scholar, 49.Danial N.N. Korsmeyer S.J. Cell death: critical control points.Cell. 2004; 116: 205-219Abstract Full Text Full Text PDF PubMed Scopus (2860) Google Scholar, 50.Green D.R. Reed J.C. Mitochondria and apoptosis.Science. 1998; 281: 1309-1312Crossref PubMed Google Scholar The apoptogenic factors released from mitochondria include cytochrome c, AIF (apoptosis-inducing factor), Smac/DIABLO, endonuclease G, and others. Cytochrome c, after being released into the cytosol, binds to and induces conformational changes in the adaptor protein Apaf-1, leading to the recruitment and activation of caspase-9, which in turn after proteolytic processing activates downstream caspases for caspase-dependent apoptosis. Smac, after being released into cytosol, can bind and antagonize the caspase inhibitor proteins, IAPs (inhibitor of apoptosis proteins) to further augment caspase activation. In contrast, AIF, after being released from mitochondria, accumulates in the nucleus to induce apoptosis in a caspase-independent manner. The involvement of the intrinsic pathway of apoptosis in