Human AKI and heme oxygenase-1

Heme oxygenase-1 as a predictor of sepsis-induced acute kidney injury: a cross-sectional study

BACKGROUND

Sepsis patients suffer from severe inflammation and poor prognosis. Oxidative stress and local inflammation that results from sepsis can trigger organ injury, including acute kidney injury (AKI). Previous studies have shown that heme oxygenase-1 (HO-1) is overexpressed in proximal tubular cells under oxidative stress and has significant cytoprotective and anti-inflammatory effects. Heme-induced inflammation in sepsis is antagonized by increased tissue expression of heme oxygenase-1 (HO-1), which impacts on AKI development. The investigators observed intrarenal HO-1 expression and corresponding potential increases in plasma and urinary HO-1 protein concentrations in four different AKI models. Since serum levels of HO-1 reflect HO-1 expression, we aimed to investigate whether serum HO-1 could predict the development of AKI in sepsis patient.

METHODS

A total of 83 sepsis patients were enrolled in this study including septic patients with AKI and sepsis patients without AKI. According to the definition of septic shock and the global kidney diagnostic criteria described in the Kidney Disease: Improving Global Outcomes (KDIGO), patients were allocated to the sepsis and septic shock groups with and without AKI, respectively. The serum levels of HO-1 were measured by enzyme-linked immunosorbent assays (ELISA). Statistical analyses were performed using SPSS software.

RESULTS

There were statistically significant differences between septic patients with AKI and sepsis patients without AKI in terms of Sequential Organ Failure Assessment (SOFA) score, hospitalization time, and laboratory indicators including serum HO-1, creatine kinase MB (CK-MB), troponin I (TnI), urea, myoglobin (MYO), serum creatinine (Scr), procalcitonin, and activated partial thromboplastin time. Serum levels of alkaline phosphatase (ALP), urea, MYO, Scr, procalcitonin, activated partial thromboplastin time, and prothrombin time exhibited significant differences among the four groups. The concentration of serum HO-1 was higher in sepsis-induced AKI compared with sepsis patients without AKI. Serum HO-1 levels were increased in patients with sepsis shock-induced AKI. The area under the receiver operating characteristic (ROC) curve for serum HO-1 combined with Scr was 0.885 [95% confidence interval (CI): 0.761-1.000].

CONCLUSIONS

Serum HO-1 is positively correlated with sepsis-induced AKI. These findings suggest that measurement of serum HO-1 may play a diagnostic and prediction role in sepsis-induced AKI.

Mitochondria bridge HIF signaling and ferroptosis blockage in acute kidney injury

Ferroptosis, a form of regulated cell death, plays an important role in acute kidney injury (AKI). Previous studies have shown that prolyl hydroxylase domain protein (PHD) inhibitors that activate HIF signaling provide strong protection against AKI, which is characterized by marked cell death. However, the relationship between PHD inhibition/HIF signaling and ferroptosis in AKI has not been elucidated. Here, we review recent studies to explore the issue. First, we will review the literature concerning the functions of HIF in promoting mitophagy, suppressing mitochondrial respiration and modulating redox homeostasis. Second, we will describe the current understanding of ferroptosis and its role in AKI, particularly from the perspective of mitochondrial dysfunction. Finally, we will discuss the possibility that mitochondria link PHD inhibition/HIF signaling and ferroptosis in AKI. In conclusion, we propose that HIF may protect renal cells against ferroptosis in AKI by reducing mitochondrial oxidative stress and damage.

Tissue Culture Models of AKI: From Tubule Cells to Human Kidney Organoids

AKI affects approximately 13.3 million people around the world each year, causing CKD and/or mortality. The mammalian kidney cannot generate new nephrons after postnatal renal damage and regenerative therapies for AKI are not available. Human kidney tissue culture systems can complement animal models of AKI and/or address some of their limitations. Donor-derived somatic cells, such as renal tubule epithelial cells or cell lines (RPTEC/hTERT, ciPTEC, HK-2, Nki-2, and CIHP-1), have been used for decades to permit drug toxicity screening and studies into potential AKI mechanisms. However, tubule cell lines do not fully recapitulate tubular epithelial cell properties in situ when grown under classic tissue culture conditions. Improving tissue culture models of AKI would increase our understanding of the mechanisms, leading to new therapeutics. Human pluripotent stem cells (hPSCs) can be differentiated into kidney organoids and various renal cell types. Injury to human kidney organoids results in renal cell-type crosstalk and upregulation of kidney injury biomarkers that are difficult to induce in primary tubule cell cultures. However, current protocols produce kidney organoids that are not mature and contain off-target cell types. Promising bioengineering techniques, such as bioprinting and "kidney-on-a-chip" methods, as applied to kidney nephrotoxicity modeling advantages and limitations are discussed. This review explores the mechanisms and detection of AKI in tissue culture, with an emphasis on bioengineered approaches such as human kidney organoid models.

Oxidative stress in chronic kidney disease

Oxidative stress (OS), defined as disturbances in the pro-/antioxidant balance, is harmful to cells due to the excessive generation of highly reactive oxygen (ROS) and nitrogen (RNS) species. When the balance is not disturbed, OS has a role in physiological adaptations and signal transduction. However, an excessive amount of ROS and RNS results in the oxidation of biological molecules such as lipids, proteins, and DNA. Oxidative stress has been reported in kidney disease, due to both antioxidant depletions as well as increased ROS production. The kidney is a highly metabolic organ, rich in oxidation reactions in mitochondria, which makes it vulnerable to damage caused by OS, and several studies have shown that OS can accelerate kidney disease progression. Also, in patients at advanced stages of chronic kidney disease (CKD), increased OS is associated with complications such as hypertension, atherosclerosis, inflammation, and anemia. In this review, we aim to describe OS and its influence on CKD progression and its complications. We also discuss the potential role of various antioxidants and pharmacological agents, which may represent potential therapeutic targets to reduce OS in both pediatric and adult CKD patients.

Nrf2-AKT interactions regulate heme oxygenase 1 expression in kidney epithelia during hypoxia and hypoxia-reoxygenation

Ischemia-reperfusion (IR)-induced kidney injury is a major clinical problem, but its underlying mechanisms remain unclear. The transcription factor known as nuclear factor, erythroid 2-like 2 (NFE2L2 or Nrf2) is crucial for protection against oxidative stress generated by pro-oxidant insults. We have previously shown that Nrf2 deficiency enhances susceptibility to IR-induced kidney injury in mice and that its upregulation is protective. Here, we examined Nrf2 target antioxidant gene expression and the mechanisms of its activation in both human and murine kidney epithelia following acute (2 h) and chronic (12 h) hypoxia and reoxygenation conditions. We found that acute hypoxia modestly stimulates and chronic hypoxia strongly stimulates Nrf2 putative target HMOX1 expression, but not that of other antioxidant genes. Inhibition of AKT1/2 or ERK1/2 signaling blocked this induction; AKT1/2 but not ERK1/2 inhibition affected Nrf2 levels in basal and acute hypoxia-reoxygenation states. Unexpectedly, chromatin immunoprecipitation assays revealed reduced levels of Nrf2 binding at the distal AB1 and SX2 enhancers and proximal promoter of HMOX1 in acute hypoxia, accompanied by diminished levels of nuclear Nrf2. In contrast, Nrf2 binding at the AB1 and SX2 enhancers significantly but differentially increased during chronic hypoxia and reoxygenation, with reaccumulation of nuclear Nrf2 levels. Small interfering-RNA-mediated Nrf2 depletion attenuated acute and chronic hypoxia-inducible HMOX1 expression, and primary Nrf2-null kidney epithelia showed reduced levels of HMOX1 induction in response to both acute and chronic hypoxia. Collectively, our data demonstrate that Nrf2 upregulates HMOX1 expression in kidney epithelia through a distinct mechanism during acute and chronic hypoxia reoxygenation, and that both AKT1/2 and ERK1/2 signaling are required for this process.

Morin downregulates nitric oxide and prostaglandin E2 production in LPS-stimulated BV2 microglial cells by suppressing NF-κB activity and activating HO-1 induction

Morin possesses anti-inflammatory activity against septic shock and allergic responses, and prevents acute liver damage. However, the biological mechanism of action of morin in neuroinflammation remains largely unknown. Therefore, the present study investigated whether morin has the ability to attenuate expression of proinflammatory mediators such as nitric oxide (NO) and prostaglandin E2 (PGE2) in lipopolysaccharide (LPS)-stimulated BV2 microglial cells. Morin inhibited the expression of LPS-induced proinflammatory mediators such as NO and PGE2, without any cytotoxic effects. Furthermore, LPS-induced inducible NO synthase (iNOS) and cyclooxygenase-2 (COX-2) were inhibited both at the mRNA and protein levels in response to morin. Morin also attenuated LPS-induced DNA-binding activity of nuclear transcription factor-κB (NF-κB) and its promoter activity. Pyrrolidine dithiocarbamate (PDTC), a specific NF-κB inhibitor, downregulated the expression of LPS-induced iNOS and COX-2, which suggests that morin-mediated NF-κB inhibition is the main signaling pathway responsible for the inhibition of iNOS and COX-2 expression. Additionally, morin increased induction of heme oxygenase-1 (HO-1) activity, leading to the suppression of NO and PGE2 production. Our results indicate that morin downregulates the expression of proinflammatory genes, such as iNOS and COX-2, involved in the synthesis of NO and PGE2 in LPS-stimulated BV2 microglial cells by suppressing NF-κB activity and activation of HO-1. Taken together, the findings of the present study suggest that morin may have potential as a therapeutic for the prevention of neuroinflammation.

Association of HO-1 (GT)n Promoter Polymorphism and Cardiovascular Disease: A Reanalysis of the Literature

BACKGROUND

Heme-oxygenase 1 (HO-1), an inducible heme-degrading enzyme, has antiatherogenic effects through its enzymatic end products. HO-1 gene expression is modulated by a guanidine thymidine dinucleotide ([GT]n) repeat polymorphism in the promoter region. Shorter repeats with (GT)n < 25 are associated with higher inducibility and activity of HO-1.

METHODS

We performed a systematic review of all literature from 1997 to 2013 on the association of the HO-1 (GT)n and cardiovascular disease (CVD). On the basis of predefined criteria (patient characteristics, genotype data format, allelic distribution, repeat length cutoff) 41 articles were selected. Patients were redistributed into 4 homogeneous subpopulations: patients with CVD (CVD group), patients without CVD (nonCVD), 'controls' with unknown cardiovascular status (unspecified) and children younger than 20 years of age (unselected). Genotype distributions (homozygous short [SS] or long [LL], and heterozygous) of the 4 patient categories were compared and odds ratios (ORs) for CVD were calculated using logistic regression analysis.

RESULTS

Overall, the proportion of the SS genotype was lower in CVD compared with nonCVD and unspecified. The ORs for CVD was highest in patients carrying the LL genotype (OR LL vs SS, 1.769 [95% confidence interval, 1.594-1.963]). Furthermore, genotype distribution differed between Caucasian and Asian individuals, the latter having a much higher proportion of the SS genotype (22% vs 11%).

CONCLUSIONS

This review of the available literature on the epidemiological association between the HO-1 (GT)n repeat polymorphism and CVD supports the presumed protective effects of HO-1. The second but probably even more relevant finding of our review is that racial disparities in HO-1 (GT)n repeat length distribution exist and might influence the associations of the genotype with CVD status.

Negative health implications of sickle cell trait in high income countries: from the football field to the laboratory

Worldwide, sickle cell trait is a highly prevalent gene carrier state. While generally a benign condition with a normal life expectancy, it is becoming increasingly clear that the sickle trait is associated with certain adverse outcomes. This article will focus on three of these outcomes, namely exertional rhabdomyolysis and sudden death, chronic renal dysfunction, and venous thromboembolism. In each case, the epidemiological evidence for the association is reviewed, together with the existing data on potential underlying mechanisms. Because newborn screening programmes for sickle cell anaemia also identify those with sickle cell trait, it is imperative that further studies determine what, if any, preventive measures can be taken to reduce the burden of these uncommon but potentially morbid complications in affected individuals.

Heme oxygenase-1 and acute kidney injury

PURPOSE OF REVIEW

Heme oxygenase activity, possessed by an inducible heme oxygenase-1 (HO-1) and a constitutive isoform (HO-2), catalyzes the conversion of heme to biliverdin, liberates iron, and generates carbon monoxide. First shown in acute kidney injury (AKI), HO-1 is now recognized as a protectant against diverse insults in assorted tissues. This review summarizes recent contributions to the field of HO-1 and AKI.

RECENT FINDINGS

Recent findings elucidate the following: the transcriptional regulation and significance of human HO-1 in AKI; the protective effects of HO-1 in age-dependent and sepsis-related AKI, cardiorenal syndromes, and acute vascular rejection in renal xenografts; the role of heme oxygenase in tubuloglomerular feedback and renal resistance to injury; the basis for cytoprotection by HO-1; the protective properties of ferritin and carbon monoxide; HO-1 and the AKI-chronic kidney disease transition; HO-1 as a biomarker in AKI; the role of HO-1 in mediating the protective effects of specific cytokines, stem cells, and therapeutic agents in AKI; and HO-2 as a protectant in AKI.

SUMMARY

Recent contributions support, and elucidate the basis for, the induction of HO-1 as a protectant against AKI. Translating such therapeutic potential into a therapeutic reality requires well tolerated and effective modalities for upregulating HO-1 and/or administering its products, which, optimally, should be salutary even when AKI is already established.

Heme Oxygenase-1 and Acute Kidney Injury following Cardiac Surgery

BACKGROUND

Intraoperative hemolysis and inflammation are associated with acute kidney injury (AKI) following cardiac surgery. Plasma-free hemoglobin induces heme oxygenase-1 (HO-1) expression. HO-1 degrades heme but increases in experimental models of AKI. This study tested the hypothesis that plasma HO-1 concentrations are associated with intraoperative hemolysis and are increased in patients that develop AKI following cardiac surgery.

METHODS

We measured plasma HO-1, free hemoglobin, and inflammatory markers in 74 patients undergoing cardiopulmonary bypass (CPB). AKI was defined as an increase in serum creatinine concentration of 50% or 0.3 mg/dl within 72 h of surgery.

RESULTS

Twenty-eight percent of patients developed AKI. HO-1 concentrations increased from 4.2 ± 0.2 ng/ml at baseline to 6.6 ± 0.5 ng/ml on postoperative day (POD) 1 (p < 0.001). POD1 HO-1 concentrations were 3.1 ng/ml higher (95% CI 1.1-5.1) in AKI patients, as was the change in HO-1 from baseline to POD1 (4.4 ± 1.3 ng/ml in AKI patients vs. 1.5 ± 0.3 ng/ml in no-AKI patients, p = 0.006). HO-1 concentrations remained elevated in AKI patients even after controlling for AKI risk factors and preoperative drug therapy. Peak-free hemoglobin concentrations correlated with peak HO-1 concentrations on POD1 in patients that developed AKI (p = 0.02). Duration of CPB and post-CPB IL-6 and IL-10 concentrations were also associated with increased HO-1 on POD1.

CONCLUSION

Plasma HO-1 is increased in patients that develop AKI, and CPB duration, hemolysis, and inflammation are associated with increased HO-1 concentrations following cardiac surgery. Strategies that alter hemolysis and HO-1 expression during cardiac surgery may affect risk for AKI.

Renal cortical hemopexin accumulation in response to acute kidney injury

Hemopexin (Hpx) is a liver-generated acute phase reactant that binds and neutralizes prooxidant free heme. This study tested whether acute kidney injury (AKI) triggers renal Hpx accumulation, potentially impacting heme Fe-mediated tubular injury. Mice were subjected to glycerol, cisplatin, ischemia-reperfusion (I/R), or endotoxemic [lipopolysaccharide (LPS)] AKI. In each instance, 3- to 30-fold renal cortical and isolated proximal tubule segment (PTS) Hpx increases resulted. Although renal cortex and PTS showed variable Hpx mRNA increases, due, in part, to increased mRNA stability, mRNA levels did not correlate with renal Hpx protein accumulation. Conversely, AKI evoked three- to fourfold increases in hepatic Hpx gene induction, which corresponded with three- to fourfold plasma Hpx increases. Renal immunohistochemistry, and increased urinary Hpx excretion, indicated that circulating Hpx gains tubule luminal/urinary access, followed by proximal tubule endocytic uptake. Paradoxically, in cultured renal cells (HK-2, HEK-293), Fe depletion, and not free heme excess, increased Hpx mRNA. LPS acutely increased HK-2 cell Hpx mRNA. This finding, coupled with observations that LPS evoked ∼30-fold greater renal Hpx mRNA increases than any other AKI model, suggests that inflammation, not heme exposure, activates the renal Hpx gene. Each form of AKI evoked early increases in circulating free heme, which subsequently fell to subnormal levels as plasma Hpx rose. In addition, purified Hpx blunted free Fe-mediated HK-2 cell death. In sum, these data indicated that AKI-associated hepatic stress generates Hpx, which gains renal tubule access. Given its ability to bind free heme and mitigate free Fe toxicity, Hpx loading can potentially confer cytoprotective effects.