transcriptional targets include TRAIL receptors, Noxa, Bax, p53-upregulated modulator of apoptosis (PUMA), and p53-induced protein with a death domain (PIDD). The expression of the latter two is critical for p53 nephrotoxicity. PUMA antagonizes Bcl-xL. PIDD promotes the formation of a multiprotein complex, the PIDDosome, leading to caspase-2 activation, which causes the release of AIF from mitochondria. Inhibition of p53, caspase-2, or apoptosis-inducing factor (AIF) markedly protected against cisplatin-induced apoptosis in cultured tubular cells. p53 nontranscriptional actions include inactivating Bcl2/BclxL and activating Bax. In addition, cisplatin activates mitogen-activated protein kinases. In the context of cisplatin nephrotoxicity, extracellular signalregulated kinase (ERK) promotes apoptosis, contrary to its usual role in cell death regulation. Cdk2 and E2F1 also participate in cisplatin-induced tubular cell death.

Puromycin aminonucleoside (PAN), a drug commonly used to induce experimental nephrotic syndrome in rats, also induces podocyte apoptosis. PAN-induced podocyte apoptosis is mediated by ROS, Bax, p53, and AIF. However, PAN does not induce proteinuria in mice and is not in clinical use.

4.2.5. Ischemia-reperfusion and sepsis

Ischemia-reperfusion is a frequent cause of AKI, especially in renal transplantation and intensive care units. Mitochondria, death receptors, p53, caspases, and ER stress have all been implicated by interventional studies in tubular cell death after ischemia-reperfusion. In this model, Bid connects the death receptor and mitochondrial pathways. In the intensive care setting, renal ischemia-reperfusion usually coexists with other causes of AKI, such as sepsis and nephrotoxins. Multiple cytokines contribute to renal injury in sepsis. Bacterial LPS itself increases Bak and downregulates BclxL, inducing apoptosis in glomerular endothelial cells and upregulating Fas in tubular and mesangial cells.

5. THERAPEUTIC APPROACHES

Some of the drugs currently in use for CKD or glomerular injury have been recently shown to target renal cell apoptosis, besides having other beneficial effects (Table 22-3). In addition, new drugs targeting apoptosis are under development. The characterization of the molecular pathways activated at each stage of renal injury, the cell targets, and the time frames will be crucial to develop sensible therapeutic strategies. Although lethal factors result in tissue injury, it is commonly thought that competition for survival factors is a key determinant of survival during the second compensatory wave

Table 22-3. Therapy targeting apoptosis in kidney injury

Note: Recent research has identified inhibition of renal cell apoptosis as a mechanism of action of some drugs currently used in nephrology. In addition, some novel approaches have been successful in animal or cell models. ACEI, angiotensin-converting enzyme inhibitors; ARB, angiotensin receptor blockers.

of apoptosis, leading to disappearance of hyperplasia and recovery from AKI. Given the potential of apoptosis modulation to interfere with physiologic apoptosis, the most likely initial clinical translation of antiapoptotic drugs will be processes in which there is limited systemic exposure to the drug or the drug is administered during a short time period. Prevention of AKI will be the most likely objective for novel antiapoptotic drugs. Inclusion of antiapoptotic drugs in solutions used to preserve organs for transplantation with the aim of reducing ischemia-reperfusion injury will limit drug exposure in time and space. In addition, short-term prophylactic administration in situations in which AKI is highly likely may be explored. Such situations include extracorporeal circulation cardiac surgery or administration of nephrotoxic drugs such as cidofovir. Nephrotoxicity is the doselimiting effect of this antiviral drug, which is administered iv every 2 weeks, thus facilitating prophylactic intervention. In the following discussion, we focus on approaches that directly target apoptosis, skipping alternative therapeutic approaches such as decreasing access of nephrotoxins to tubular cells and other maneuvers. Small molecules may be bound to carriers that lead to specific proximal tubular uptake and organ protection.

Among currently used drugs that also target apoptosis, we find angiotensin-converting enzyme inhibitors (ACEIs), angiotensin type 1 receptor blockers (ARBs), steroids, and erythropoietins.

ACEIs and ARBs are antihypertensive drugs used to treat CKD when proteinuria is present. In addition to reducing systemic and intraglomerular pressures, angiotensin II blockade decreases podocyte apoptosis induced by either angiotensin II or mechanical stretch. Corticosteroids are immunosuppressive drugs that have long been used to treat proteinuric kidney disease of immune origin and are the mainstay of therapy of minimal-change nephrotic syndrome. Dexamethasone markedly reduces apoptosis in cultured podocytes by decreasing p53, increasing Bcl-xL, and inhibiting AIF translocation. Erythropoietin and darbepoetin are used in CKD patients for the treatment of renal anemia. They also have antiapoptotic and tissue-protective actions. Darbepoetin protects podocytes from sublethal injury and apoptotic cell death. Ongoing clinical trials are exploring the role of erythropoietin in prevention of AKI after kidney transplantation. Statins are frequently used in proteinuric patients to lower LDL cholesterol levels. Experimental animal and cell culture studies suggest that statins inhibit cultured podocyte apoptosis by stimulating Akt activity. It is interesting to note that lovastatin induces apoptosis in actively proliferating mesangial cells and spares quiescent cells grown in serum-free conditions. This property could be used therapeutically to target proliferating mesangial cells in vivo. However, statins also induce apoptosis in proliferating tubular cells.

Among potential novel targets we find growth factors, cytokines, Bcl2-like proteins, caspases, and p53. Survival factors and anti-cytokine strategies have been used to prevent apoptosis in animal models, but clinical trials have not been performed, or, in the case of IGF-1 for AKI, have failed to demonstrate benefit. A decrease in BclxL levels is a common event in tubular cell death induced by different mechanisms. BclxL over-expression protected tubular cells from apoptosis induced by acetaminophen, CsA, and death receptors. More recently, the cell-permeable BclxL-like molecule TAT-BH4 containing the BH4 domain of BclxL fused to the protein transduction domain of HIV TAT has efficiently prevented apoptosis in cultured cells and in vivo. A KU-70–derived Bax-targeting peptide afforded protection in tubular cell culture studies.

In vivo caspase inhibitors protect against ischemic injury in kidney. The pan caspase inhibitor zVAD prevented renal function impairment at an early time point (24 hours) when administered at the time of reperfusion. It was much less effective when administered 2 hours later. Longer follow-up studies are needed to exclude the possibility that zVAD is only retarding cell death and favoring more injurious necrotic cell death. In

this regard, zVAD exacerbated TNFα toxicity by enhancing oxidative stress and mitochondrial damage, resulting in hyperacute hemodynamic collapse, kidney failure, and death. In tubular cells exposed to TWEAK, TNFα, and IFNγ, inhibition of caspase-8 or multiple caspases transformed a weak apoptotic response into massive reactive oxygen species–dependent necrosis. In addition to their role in apoptosis, caspases have also nonapoptotic roles in inflammation, cell proliferation, and differentiation that may complicate their therapeutic targeting. Thus interference with inflammation via IL-18 was instrumental in protection against ischemiareperfusion injury afforded by caspase-1 deficiency or inhibition.

Basp1 was recently shown to be required for high glucose-induced apoptosis in tubular cells and Basp1 targeting by siRNA was protective.

The small-molecule p53 inhibitor pifithrin-α prevented apoptosis and protected renal function in experimental ischemia-reperfusion and cisplatin nephrotoxicity.

SUGGESTED READINGS

Docherty NG, O’Sullivan OE, Healy DA, Fitzpatrick JM, Watson RW. Evidence that inhibition of tubular cell apoptosis protects against renal damage and development of fibrosis following ureteric obstruction. Am J Physiol Renal Physiol. 2006;290:F4–13

Hamar P, Song E, Kokeny G, Chen A, Ouyang N, Lieberman J. Small interfering RNA targeting Fas protects mice against renal ischemia-reperfusion injury. Proc Natl Acad Sci U S A. 2004;101:14883–8

Hughes J, Savill JS. Apoptosis in glomerulonephritis. Curr Opin Nephrol Hypertens. 2005;14:389–95

Lorz C, Benito-Mart´ın A, Boucherot A, Ucero AC, Rastaldi MP, Henger A, Armelloni S, Santamar´ıa B, Kretzler M, Egido J, Ortiz A. The death ligand TRAIL in diabetic nephropathy. J Am Soc Nephrol. 2008;19;904–14

Moreno JA, Sanchez-Nino˜ MD, Sanz AB, Lassila M, Holthofer H, Blanco-Colio LM, Egido J, Ruiz-Ortega M, Ortiz A. A slit in podocyte death. Curr Med Chem 2008;15:1645–54

Padanilam BJ. Cell death induced by acute renal injury: a perspective on the contributions of apoptosis and necrosis. Am J Physiol Renal Physiol 2003;284:F608–27

Sanchez-Nino,˜ M.D, Benito-Martin, A, Ortiz, A New paradigms in cell death in human diabetic nephropathy. Kidney Int

2010;78:737–44

Sanz AB, Santamaria B, Ruiz Ortega M, Egido J, Ortiz A. Mechanisms of renal apoptosis in health and disease. J Am Soc Nephrol. 2008;19:1634–42

Schiffer M, Bitzer,M, Roberts,IS et al. Apoptosis in podocytes induced by TGF-beta and Smad7. J Clin Invest 2001;108: 807–16