Neutrophil gelatinase-associated lipocalin (Ngal), also known as siderocalin, forms a complex with ironbinding siderophores (Ngal:siderophore:Fe). This complex converts renal progenitors into epithelial tubules.In this study, we tested the hypothesis that Ngal:siderophore:Fe protects adult kidney epithelial cells or accelerates their recovery from damage. Using a mouse model of severe renal failure, ischemia-reperfusion injury, we show that a single dose of Ngal (10 μg), introduced during the initial phase of the disease, dramatically protects the kidney and mitigates azotemia. Ngal activity depends on delivery of the protein and its siderophore to the proximal tubule. Iron must also be delivered, since blockade of the siderophore with gallium inhibits the rescue from ischemia. The Ngal:siderophore:Fe complex upregulates heme oxygenase-1, a protective enzyme, preserves proximal tubule N-cadherin, and inhibits cell death. Because mouse urine contains an Ngaldependent siderophore-like activity, endogenous Ngal might also play a protective role. Indeed, Ngal is highly accumulated in the human kidney cortical tubules and in the blood and urine after nephrotoxic and ischemic injury. We reveal what we believe to be a novel pathway of iron traffic that is activated in human and mouse renal diseases, and it provides a unique method for their treatment.

Background and objectives: Serum creatinine (Scr) does not allow for early diagnosis of acute kidney injury (AKI).

Neutrophil gelatinase-associated lipocalin (Ngal), also known as siderocalin, forms a complex with ironbinding siderophores (Ngal:siderophore:Fe). This complex converts renal progenitors into epithelial tubules.In this study, we tested the hypothesis that Ngal:siderophore:Fe protects adult kidney epithelial cells or accelerates their recovery from damage. Using a mouse model of severe renal failure, ischemia-reperfusion injury, we show that a single dose of Ngal (10 μg), introduced during the initial phase of the disease, dramatically protects the kidney and mitigates azotemia. Ngal activity depends on delivery of the protein and its siderophore to the proximal tubule. Iron must also be delivered, since blockade of the siderophore with gallium inhibits the rescue from ischemia. The Ngal:siderophore:Fe complex upregulates heme oxygenase-1, a protective enzyme, preserves proximal tubule N-cadherin, and inhibits cell death. Because mouse urine contains an Ngaldependent siderophore-like activity, endogenous Ngal might also play a protective role. Indeed, Ngal is highly accumulated in the human kidney cortical tubules and in the blood and urine after nephrotoxic and ischemic injury. We reveal what we believe to be a novel pathway of iron traffic that is activated in human and mouse renal diseases, and it provides a unique method for their treatment.

AKI is characterized by increased catecholamine levels and hypertension. Renalase, a secretory flavoprotein that oxidizes catecholamines, attenuates ischemic injury and the associated increase in catecholamine levels in mice. However, whether the amine oxidase activity of renalase is involved in preventing ischemic injury is debated. In this study, recombinant renalase protected human proximal tubular (HK-2) cells against cisplatinand hydrogen peroxide-induced necrosis. Similarly, genetic depletion of renalase in mice (renalase knockout) exacerbated kidney injury in animals subjected to cisplatin-induced AKI. Interestingly, compared with the intact renalase protein, a 20-amino acid peptide (RP-220), which is conserved in all known renalase isoforms, but lacks detectable oxidase activity, was equally effective at protecting HK-2 cells against toxic injury and preventing ischemic injury in wild-type mice. Furthermore, in vitro treatment with RP-220 or recombinant renalase rapidly activated Akt, extracellular signal-regulated kinase, and p38 mitogen-activated protein kinases and downregulated c-Jun N-terminal kinase. In summary, renalase promotes cell survival and protects against renal injury in mice through the activation of intracellular signaling cascades, independent of its ability to metabolize catecholamines, and we have identified the region of renalase required for these effects. Renalase and related peptides show potential as therapeutic agents for the prevention and treatment of AKI. AKI is a common clinical condition affecting up to 20% of hospitalized patients and is frequently associated with sepsis, surgery, and certain drugs. Epidemiologic data indicate a positive association between the severity of AKI and in-hospital and long-term mortality. 1,2 Unfortunately, the development of effective therapy for AKI has been hampered by (1) an inherent delay in diagnosis, a consequence of relying on serum creatinine, which only increases 48-72 hours after the original insult, and (2) the paucity of validated targets of therapy. There is an urgent need to identify novel therapeutic modalities.Renalase is a novel secretory flavoprotein with amine oxidase activity. [3][4][5] In vitro, renalase metabolizes epinephrine, norepinephrine, and dopamine and also possesses significant intrinsic nicotinamide adenine dinucleotide (NADH) oxidase activity. 6,7 Other investigators have questioned the amine oxidase activity of renalase. 8,9 We had proposed that, in contrast to the classic amine oxidases, renalase reacts with oxygen to generate superoxide anions and hydrogen peroxide, with subsequent oxidation of catecholamines to their respective aminochromes. Recent results indicate that renalase functions as an oxidase/anomerase, using molecular oxygen to convert a-NAD(P)H to b-NAD + , with hydrogen peroxide as reaction byproduct. 10 Because

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