Selective iNOS inhibition for the treatment of sepsis-induced acute kidney injury

Expression and cellular localization of inducible nitric oxide synthase in lipopolysaccharide-treated rat kidneys

Although inducible nitric oxide synthase (iNOS) is known to play significant roles in the kidney, its renal localization has long been controversial. To resolve this issue, the authors identified iNOS-positive cell types in rat kidneys using double immunohistochemistry and confirmed iNOS positivity using enzyme histochemistry with NADPH-diaphorase (NADPH-d) and in situ RT-PCR. Adult male Sprague-Dawley rats were injected intraperitoneally with lipopolysaccharide (LPS) or saline as a control and sacrificed at various time intervals after injection. Quantitative real-time reverse transcriptase polymerase chain reaction showed that iNOS was not expressed in control kidneys but was induced in LPS-treated kidneys. iNOS immunostaining was strongest 6 to 18 hr after injection and decreased gradually to control levels by day 7. Double immunohistochemistry and NADPH-d revealed that iNOS expression was induced in the interstitial cells, glomerular parietal epithelial cells, the proximal part of the short-looped descending thin limb, the upper and middle papillary parts of the long-looped descending thin limb, some inner medullary collecting duct cells, and almost all calyceal and papillary epithelial cells. The present study determines the precise localization of iNOS in LPS-treated rat kidneys and provides an important morphological basis for examining the roles of iNOS in sepsis-induced acute kidney injury.

Pharmacological targets in the renal peritubular microenvironment: implications for therapy for sepsis-induced acute kidney injury

One of the most frequent and serious complications to develop in septic patients is acute kidney injury (AKI), a disorder characterized by a rapid failure of the kidneys to adequately filter the blood, regulate ion and water balance, and generate urine. AKI greatly worsens the already poor prognosis of sepsis and increases cost of care. To date, therapies have been mostly supportive; consequently there has been little change in the mortality rates over the last decade. This is due, at least in part, to the delay in establishing clinical evidence of an infection and the associated presence of the systemic inflammatory response syndrome and thus, a delay in initiating therapy. A second reason is a lack of understanding regarding the mechanisms leading to renal injury, which has hindered the development of more targeted therapies. In this review, we summarize recent studies, which have examined the development of renal injury during sepsis and propose how changes in the peritubular capillary microenvironment lead to and then perpetuate microcirculatory failure and tubular epithelial cell injury. We also discuss a number of potential therapeutic targets in the renal peritubular microenvironment, which may prevent or lessen injury and/or promote recovery.

Aldosterone increases kidney tubule cell oxidants through calcium-mediated activation of NADPH oxidase and nitric oxide synthase

Chronic hyperaldosteronism has been associated with an increased cancer risk. We recently showed that aldosterone causes an increase in cell oxidants, DNA damage, and NF-κB activation. This study investigated the mechanisms underlying aldosterone-induced increase in cell oxidants in kidney tubule cells. Aldosterone caused an increase in both reactive oxygen and reactive nitrogen (RNS) species. The involvement of the activation of NADPH oxidase in the increase in cellular oxidants was demonstrated by the inhibitory action of the NADPH oxidase inhibitors DPI, apocynin, and VAS2870 and by the migration of the p47 subunit to the membrane. NADPH oxidase activation occurred as a consequence of an increase in cellular calcium levels and was mediated by protein kinase C. The prevention of RNS increase by BAPTA-AM, W-7, and L-NAME indicates a calcium-calmodulin activation of NOS. A similar pattern of effects of the NADPH oxidase and NOS inhibitors was observed for aldosterone-induced DNA damage and NF-κB activation, both central to the pathogenesis of chronic aldosteronism. In summary, this paper demonstrates that aldosterone, via the mineralocorticoid receptor, causes an increase in kidney cell oxidants, DNA damage, and NF-κB activation through a calcium-mediated activation of NADPH oxidase and NOS. Therapies targeting calcium, NOS, and NADPH oxidase could prevent the adverse effects of hyperaldosteronism on kidney function as well as its potential oncogenic action.

Development of oxidative stress in the peritubular capillary microenvironment mediates sepsis-induced renal microcirculatory failure and acute kidney injury

Acute kidney injury is a frequent and serious complication of sepsis. To better understand the development of sepsis-induced acute kidney injury, we performed the first time-dependent studies to document changes in renal hemodynamics and oxidant generation in the peritubular microenvironment using the murine cecal ligation and puncture (CLP) model of sepsis. CLP caused an increase in renal capillary permeability at 2 hours, followed by decreases in mean arterial pressure, renal blood flow (RBF), and renal capillary perfusion at 4 hours, which were sustained through 18 hours. The decline in hemodynamic parameters was associated with hypoxia and oxidant generation in the peritubular microenvironment and a decrease in glomerular filtration rate. The role of oxidants was assessed using the superoxide dismutase mimetic/peroxynitrite scavenger MnTMPyP [Mn(III)tetrakis(1-methyl-4-pyridyl)porphyrin]. At 10 mg/kg administered 6 hours after CLP, MnTMPyP did not alter blood pressure, but blocked superoxide and peroxynitrite generation, reversed the decline in RBF, capillary perfusion, and glomerular filtration rate, preserved tubular architecture, and increased 48-hour survival. However, MnTMPyP administered at CLP did not prevent capillary permeability or the decrease in RBF and capillary perfusion, which suggests that these early events are not mediated by oxidants. These data demonstrate that renal hemodynamic changes occur early after sepsis and that targeting the later oxidant generation can break the cycle of injury and enable the microcirculation and renal function to recover.

PPARα mediates the anti-inflammatory effect of simvastatin in an experimental model of zymosan-induced multiple organ failure

BACKGROUND AND PURPOSE

Zymosan-induced non-septic shock is a multi-factorial pathology that involves several organs including the kidneys, liver and lungs. Its complexity and diversity presents a continuing therapeutic challenge. Given their pleiotropic effect, statins could be beneficial in non-septic shock. One of the molecular mechanisms underlying the anti-inflammatory effect of statins involves the peroxisome proliferator-activated receptor (PPAR) α. We used a zymosan-induced non-septic shock experimental model to investigate the role of PPARα in the anti-inflammatory effects of simvastatin.

EXPERIMENTAL APPROACH

Effects of simvastatin (5 or 10 mg·kg(-1) i.p.) were analysed in PPARα knock-out (KO) and PPARα wild type (WT) mice after zymosan or vehicle administration. Organ injury in lung, liver, kidney and intestine was evaluated by immunohistology. PPARα mRNA expression and nuclear factor-κB activation were evaluated in all experimental groups, 18 h after study onset. Cytokine levels were measured in plasma, and nitrite/nitrate in plasma and peritoneal exudate. Nitric oxide synthase, nitrotyrosine and poly ADP-ribose were localized by immunohistochemical methods.

KEY RESULTS

Simvastatin significantly and dose-dependently increased the zymosan-induced expression of PPARα levels in all tissues analysed. It also dose-dependently reduced systemic inflammation and the organ injury induced by zymosan in lung, liver, intestine and kidney. These effects were observed in PPARαWT mice and in PPARαKO mice.

CONCLUSIONS AND IMPLICATIONS

Simvastatin protected against the molecular and cellular damage caused by systemic inflammation in our experimental model. Our results also provide new information regarding the role of PPARα in the anti-inflammatory effects of statins.

Role of mitochondrial oxidants in an in vitro model of sepsis-induced renal injury

Oxidative stress has been implicated to play a major role in multiorgan dysfunction during sepsis. To study the mechanism of oxidant generation in acute kidney injury (AKI) during sepsis, we developed an in vitro model of sepsis using primary cultures of mouse cortical tubular epithelial cells exposed to serum (2.5-10%) collected from mice at 4 h after induction of sepsis by cecal ligation and puncture (CLP) or Sham (no sepsis). CLP serum produced a concentration-dependent increase in nitric oxide (NO) (nitrate + nitrite) release at 6 h and cytotoxicity (lactate dehydrogenase release) at 18 h compared with Sham serum treatment. Before cytotoxicity there was a decrease in mitochondrial membrane potential, which was followed by increased superoxide and peroxynitrite levels compared with Sham serum. The role of oxidants was evaluated by using the superoxide dismutase mimetic and peroxynitrite scavenger manganese(III)tetrakis(1-methyl-4-pyridyl)porphyrin tetratosylate hydroxide (MnTmPyP). MnTmPyP (10-100 μM) produced a concentration-dependent preservation of ATP and protection against cytotoxicity. MnTmPyP blocked mitochondrial superoxide and peroxynitrite generation produced by CLP serum but had no effect on NO levels. Although MnTmPyP did not block the initial CLP serum-induced fall in mitochondrial membrane potential, it allowed mitochondrial membrane potential to recover. Data from this in vitro model suggest a time-dependent generation of mitochondrial oxidants, mitochondrial dysfunction, and renal tubular epithelial cell injury and support the therapeutic potential of manganese porphyrin compounds in preventing sepsis-induced AKI.

Resveratrol improves renal microcirculation, protects the tubular epithelium, and prolongs survival in a mouse model of sepsis-induced acute kidney injury

The mortality rate of patients who develop acute kidney injury during sepsis nearly doubles. The effectiveness of therapy is hampered because it is usually initiated only after the onset of symptoms. As renal microvascular failure during sepsis is correlated with the generation of reactive nitrogen species, the therapeutic potential of resveratrol, a polyphenol vasodilator that is also capable of scavenging reactive nitrogen species, was investigated using the cecal ligation and puncture (CLP) murine model of sepsis-induced acute kidney injury. Resveratrol when given at 5.5 h following CLP reversed the decline in cortical capillary perfusion, assessed by intravital microscopy, at 6 h in a dose-dependent manner. Resveratrol produced the greatest improvement in capillary perfusion and increased renal blood flow and the glomerular filtration rate without raising systemic pressure. A single dose at 6 h after CLP was unable to improve renal microcirculation assessed at 18 h; however, a second dose at 12 h significantly improved microcirculation and decreased the levels of reactive nitrogen species in tubules, while improving renal function. Moreover, resveratrol given at 6, 12, and 18 h significantly improved survival. Hence, resveratrol may have a dual mechanism of action to restore the renal microcirculation and scavenge reactive nitrogen species, thus protecting the tubular epithelium even when administered after the onset of sepsis.

Sepsis-induced acute kidney injury

Acute kidney injury (AKI) is a common sequel of sepsis in the intensive care unit. It is being suggested that sepsis-induced AKI may have a distinct pathophysiology and identity. Availability of biomarkers now enable us to detect AKI as early as four hours after it's inception and may even help us to delineate sepsis-induced AKI. Protective strategies such as preferential use of vasopressin or prevention of intra-abdominal hypertension may help, in addition to the other global management strategies of sepsis. Pharmacologic interventions have had limited success, may be due to their delayed usage. Newer developments in extracorporeal blood purification techniques may proffer effects beyond simple replacement of renal function, such as metabolic functions of the kidney or modulation of the sepsis cascade.

Clearance and beyond: the complementary roles of GFR measurement and injury biomarkers in acute kidney injury (AKI)

Acute kidney injury (AKI) is a common and frequently fatal illness in critically ill patients. The reliance on daily measurements of serum creatinine as a surrogate of glomerular filtration rate (GFR) not only delays diagnosis and development of successful therapies but also hinders insight into the pathophysiology of human AKI. Measurement of GFR under non-steady-state conditions remains an elusive gold standard against which biomarkers of renal injury need to be judged. Approaches to the rapid (near real-time) measurement of GFR are explored. Even if real-time GFR was available, absent baseline information will always limit diagnosis of AKI based on GFR or serum creatinine to a detection of change. Biomarkers of renal cellular injury have provided new strategies to facilitate detection and early intervention in AKI. However, the diagnostic and predictive performance of urinary biomarkers of injury vary, depending on both the time after renal injury and on the preinjury GFR. Progress in understanding the role of each novel biomarker in the causal pathways of AKI promises to enhance their diagnostic potential. We predict that combining rapid measures of GFR with biomarkers of renal injury will yield substantive progress in the treatment of AKI.

Regulation of lymphocytes by nitric oxide

Shortly after the identification of nitric oxide (NO) as a product of macrophages, it was discovered that NO generated by inducible NO synthase (iNOS) inhibits the proliferation of T lymphocytes. Since then, it has become clear that iNOS activity also regulates the development, differentiation, and/or function of various types of T cells and B cells and also affects NK cells. The three key mechanisms underlying the iNOS-dependent immunoregulation are (a) the modulation of signaling processes by NO, (b) the depletion of arginine, and (c) the alteration of accessory cell functions. This chapter highlights important principles of iNOS-dependent immunoregulation of lymphocytes and also reviews more recent evidence for an effect of endothelial or neuronal NO synthase in lymphocytes.

Pulmonary and renal protection: targeting PARP to ventilator-induced lung and kidney injury?

Both acute lung injury and acute kidney injury (AKI) are frequent and serious problems in intensive care medicine. Therefore, the avoiding of any iatrogenic insult to these organs is of great importance. While an increasing body of evidence suggests that mechanical ventilation is capable of inducing lung and distant organ injury, the complex underlying molecular mechanisms remain insufficiently understood. In the previous issue of Critical Care, Vaschetto and colleagues reported the results of an experimental study designed to further explore pathways linking injurious ventilation with AKI. The authors demonstrated that scavenging of peroxynitrite or inhibiting poly(ADP-ribose) polymerase (PARP) afforded protection against AKI induced by double-hit lung injury. Although PARP inhibition or peroxynitrite detoxification or both may become viable candidates for a protective strategy in this setting, the implementation of a lung-protective ventilatory strategy remains the only clinical tool to mitigate the lung biotrauma and its systemic consequences.

In vitro model of sepsis-induced renal epithelial reactive nitrogen species generation

Sepsis-induced acute kidney injury (AKI) is a complex disease characterized by generation of inducible nitric oxide synthase (iNOS)-derived reactive nitrogen species (RNS) by the renal tubular epithelium. While most in vitro models of sepsis use combinations of lipopolysaccharide and cytokines to simulate exposure to inflammatory mediators thought to play a role in sepsis, the relevance of these models is limited. To address the need for a model that more closely mimics the tubular microenvironment during sepsis, we developed an in vitro model where mIMCD-3 (murine tubular epithelial) cells are treated with media containing 5% serum collected from mice at 4 h after cecal ligation and puncture (CLP) or sham surgery (no sepsis). After exposure to CLP serum, induction of iNOS messenger RNA occurred and NO generation was significantly increased compared to sham. This increase was accompanied by increased RNS as measured by oxidation of 5-(and-6)-carboxy-2,7'-dichlorodihydrofluorescein diacetate (carboxy-H(2)DCF-DA) and 2-(3,6-diamino-9H-xanthen-9-yl)-benzoic acid, methyl ester (dihydrorhodamine 123) and moderate cytotoxicity in cells treated with CLP serum, similar to what is observed in mice subjected to CLP. Since iNOS has been shown to play an important role in sepsis-induced AKI, the iNOS inhibitor L-N(6)-(1-iminoethyl)-lysine (L-NIL) was tested in this in vitro model. L-NIL completely blocked NO generation, RNS generation, and cytotoxicity, similar to its effects in vivo. Therefore, this new in vitro model exhibits many of the characteristics observed in vivo, suggesting that it is a relevant model for studying the mechanism of sepsis-induced renal epithelial RNS generation and injury.