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

Inactivation of renal mitochondrial respiratory complexes and manganese superoxide dismutase during sepsis: mitochondria-targeted antioxidant mitigates injury

Acute kidney injury (AKI) is a complication of sepsis and leads to a high mortality rate. Human and animal studies suggest that mitochondrial dysfunction plays an important role in sepsis-induced multi-organ failure; however, the specific mitochondrial targets damaged during sepsis remain elusive. We used a clinically relevant cecal ligation and puncture (CLP) murine model of sepsis and assessed renal mitochondrial function using high-resolution respirometry, renal microcirculation using intravital microscopy, and renal function. CLP caused a time-dependent decrease in mitochondrial complex I and II/III respiration and reduced ATP. By 4 h after CLP, activity of manganese superoxide dismutase (MnSOD) was decreased by 50% and inhibition was sustained through 36 h. These events were associated with increased mitochondrial superoxide generation. We then evaluated whether the mitochondria-targeted antioxidant Mito-TEMPO could reverse renal mitochondrial dysfunction and attenuate sepsis-induced AKI. Mito-TEMPO (10 mg/kg) given at 6 h post-CLP decreased mitochondrial superoxide levels, protected complex I and II/III respiration, and restored MnSOD activity by 18 h. Mito-TEMPO also improved renal microcirculation and glomerular filtration rate. Importantly, even delayed therapy with a single dose of Mito-TEMPO significantly increased 96-h survival rate from 40% in untreated septic mice to 80%. Thus, sepsis causes sustained inactivation of three mitochondrial targets that can lead to increased mitochondrial superoxide. Importantly, even delayed therapy with Mito-TEMPO alleviated kidney injury, suggesting that it may be a promising approach to treat septic AKI.

Rolipram improves renal perfusion and function during sepsis in the mouse

Microcirculatory dysfunction is correlated with increased mortality among septic patients and is believed to be a major contributor to the development of acute kidney injury (AKI). Rolipram, a selective phosphodiesterase 4 (PDE4) inhibitor, has been shown to reduce microvascular permeability and in the kidney, increase renal blood flow (RBF). This led us to investigate its potential to improve the renal microcirculation and preserve renal function during sepsis using a murine cecal ligation and puncture (CLP) model to induce sepsis. Rolipram, tested at doses of 0.3-10 mg/kg i.p., acutely restored capillary perfusion in a bell-shaped dose-response effect with 1 mg/kg being the lowest most efficacious dose. This dose also acutely increased RBF despite transiently decreasing mean arterial pressure. Rolipram also reduced renal microvascular permeability. It is noteworthy that delayed treatment with rolipram at 6 hours after CLP restored the renal microcirculation, reduced blood urea nitrogen and serum creatinine, and increased glomerular filtration rate at 18 hours. However, delayed treatment with rolipram did not reduce serum nitrate/nitrite levels, a marker of nitric oxide production, nor reactive nitrogen species generation in renal tubules. These data show that restoring the microcirculation with rolipram, even with delayed treatment, is enough to improve renal function during sepsis despite the generation of oxidants and suggest that PDE4 inhibitors should be evaluated further for their ability to treat septic-induced AKI.

Acute kidney injury in acute liver failure: a review

Acute liver failure is a rare and often devastating condition consequent on massive liver cell necrosis that frequently affects young, previously healthy individuals resulting in altered cognitive function, coagulopathy and peripheral vasodilation. These patients frequently develop concurrent acute kidney injury (AKI). This abrupt and sustained decline in renal function, through a number of pathogenic mechanisms such as renal hypoperfusion, direct drug-induced nephrotoxicity or sepsis/systemic inflammatory response contributes to increased morbidity and is strongly associated with a worse prognosis. Improved understanding of the pathophysiology AKI in the context of acute liver failure may be beneficial in a number of areas; the development of new and sensitive biomarkers of renal dysfunction, refining prognosis and organ allocation, and ultimately leading to the development of novel treatment strategies, these issues are discussed in more detail in this expert review.

Endothelial peroxisomal dysfunction and impaired pexophagy promotes oxidative damage in lipopolysaccharide-induced acute kidney injury

AIMS

We examined that (a) how the endotoxic stress affects peroxisomal function and autophagic degradation of peroxisomes-pexophagy, (b) how a superimposed dysfunction of lysosomes and pexophagy modifies responses to lipopolysaccharide (LPS), and (c) the mechanisms of peroxisomal contribution to renal injury. To accomplish this, we used lysosome-defective Lyst-mice in vivo and primary endothelial cells in vitro, and compared the responses with wild-type (WT) littermates.

RESULTS

LPS induced pexophagic degradation, followed by proliferation of peroxisomes in WT mice, which was abolished in Lyst-mice. Lyst-mice exhibited impaired activation of catalase, which together with preserved hydrogen peroxide-generating β-oxidation resulted in redox disequilibrium. LPS treatment induced a heightened inflammatory response, increased oxidative damage, and aggravated renal injury in Lyst-mice. Similarly, as in vivo, LPS-activated lysosomal (LYS) pexophagy and transiently repressed peroxisomes in vitro, supported by reduced peroxisomal density in the vicinity of lysosomes. Peroxisomal dynamics was also abolished in lysosome-defective cells, which accumulated peroxisomes with compromised functions and intraorganellar redox imbalance.

INNOVATION

We demonstrated that pexophagy is a default response to endotoxic injury. However, when LYS dysfunction (a frequent companion of chronic diseases) is superimposed, recycling and functioning of peroxisomes are impaired, and an imbalance between hydrogen peroxide-generating β-oxidation and hydrogen peroxide-detoxifying catalase ensues, which ultimately results in peroxisomal burnout.

CONCLUSION

Our data strongly suggest that pexophagy, a cellular mechanism per se, is essential in functional maintenance of peroxisomes during LPS exposure. Inhibition of pexophagy results in accumulation of impaired peroxisomes, redox disequilibrium, and aggravated renal damage.

Peptides modeled on the RGG domain of AUF1/hnRNP-D regulate 3' UTR-dependent gene expression

Messenger RNA binding proteins control post-transcriptional gene expression of targeted mRNAs. The RGG (arginine-glycine-glycine) domain of the AUF1/hnRNP-D mRNA binding protein is a regulatory region that is essential for protein function. The AUF1-RGG peptide, modeled on the RGG domain of AUF1, represses expression of the macrophage cytokine, VEGF. This report expands studies on the AUF1-RGG peptide and evaluates the role of post-translational modifications of the AUF1 protein. Results show that a minimal 31-amino acid AUF1-RGG peptide that lacks poly-glutamine and nuclear localization motifs retains suppressive activity on a VEGF-3'UTR reporter. Arginine residues in RGG motifs may be methylated with resulting changes in protein function. Mass spectroscopy analysis was performed on AUF1 expressed in RAW-264.7 cells. In resting cells, arginines in the first and second RGG motifs are monomethylated. Following activation with lipopolysaccharide, the arginines are dimethylated. To evaluate if the arginine residues are essential for AUF1-RGG activity, the methylatable arginines in the AUF1-3RGG peptide were mutated to lysine or alanine. The R→K and R→A mutants lack activity. We also demonstrate that PI3K/AKT inhibitors reduce VEGF gene expression. Although immunoscreening of AUF1 suggests that LPS and PI3K inhibitors alter the phosphorylation status of AUF1-p37, mass spectroscopy results show that the p37 AUF1 isoform is not phosphorylated with or without lipopolysaccharide stimulation. In summary, arginines in the RGG domain of AUF1 are methylated, and AUF1-RGG peptides may be novel reagents that reduce macrophage activation in inflammation.

Paclitaxel ameliorates lipopolysaccharide-induced kidney injury by binding myeloid differentiation protein-2 to block Toll-like receptor 4-mediated nuclear factor-κB activation and cytokine production

Recent studies suggest that paclitaxel, an anticancer agent, may modulate the injury and inflammatory responses in normal tissues. However, the underlying mechanism is not fully understood. Here we have examined the effect of paclitaxel on lipopolysaccharide (LPS)-induced acute kidney injury (AKI) in mice and further studied the mechanism. At relatively low doses, paclitaxel protected against LPS-induced AKI and improved animal survival. The beneficial effects of paclitaxel were accompanied by the downregulation of tumor necrosis factor-α, interleukin-1, and interleukin-6 production. In cultured renal tubular HK-2 cells, paclitaxel decreased the DNA-binding activity of nuclear factor-κB (NF-κB) during LPS treatment, inhibited the degradation of the inhibitor of κB-α, and blocked the expression and activation of NF-κB p65. At the upstream level, paclitaxel reduced LPS-induced association of myeloid differentiation protein-2 (MD-2) with Toll-like receptor 4 (TLR4). In an in vitro assay, paclitaxel was shown to directly bind recombinant MD-2. The inhibitory effect of paclitaxel on NF-κB activation and cytokine expression disappeared in MD-2 knockdown cells, indicating that paclitaxel acts through MD-2. Collectively, these results suggest that paclitaxel may bind MD-2 to block MD-2/TLR4 association during LPS treatment, resulting in the suppression of NF-κB activation and inhibition of proinflammatory cytokine production.