Hepatic ketogenesis is not required for starvation adaptation in mice.

OBJECTIVE: In response to bacterial inflammation, anorexia of acute illness is protective and is associated with the induction of fasting metabolic programs such as ketogenesis. Forced feeding during the anorectic period induced by bacterial inflammation is associated with suppressed ketogenesis and increased mortality. As ketogenesis is considered essential in fasting adaptation, we sought to determine the role of ketogenesis in illness-induced anorexia. METHODS: A mouse model of inducible hepatic specific deletion of the rate limiting enzyme for ketogenesis (HMG-CoA synthase 2, Hmgcs2) was used to investigate the role of ketogenesis in endotoxemia, a model of bacterial inflammation, and in prolonged starvation. RESULTS: Mice deficient of hepatic Hmgcs2 failed to develop ketosis during endotoxemia and during prolonged fasting. Surprisingly, hepatic HMGCS2 deficiency and the lack of ketosis did not affect survival, glycemia, or body temperature in response to endotoxemia. Mice with hepatic ketogenic deficiency also did not exhibit any defects in starvation adaptation and were able to maintain blood glucose, body temperature, and lean mass compared to littermate wild-type controls. Mice with hepatic HMGCS2 deficiency exhibited higher levels of plasma acetate levels in response to fasting. CONCLUSIONS: Circulating hepatic-derived ketones do not provide protection against endotoxemia, suggesting that alternative mechanisms drive the increased mortality from forced feeding during illness-induced anorexia. Hepatic ketones are also dispensable for surviving prolonged starvation in the absence of inflammation. Our study challenges the notion that hepatic ketogenesis is required to maintain blood glucose and preserve lean mass during starvation, raising the possibility of extrahepatic ketogenesis and use of alternative fuels as potential means of metabolic compensation.

Metabolism and Nutrition in Sepsis: In Need of a Paradigm Shift.

BACKGROUND: Sepsis continues to cause significant morbidity and mortality despite technological advancements in medical management. While sepsis is defined as organ dysfunction owing to the dysregulated host response to infection, our understanding of the dysregulation of the host response remains incomplete. SUMMARY: Many metabolic derangements that occur during sepsis, including those associated with anorexia, hyperglycemia, and proteolysis, have largely been considered maladaptive. Supportive medical and nutritional interventions targeted at correcting these metabolic derangements have not led to improved outcomes, suggesting a reappraisal of our approach to metabolism and nutrition in critically ill septic patients is needed. KEY MESSAGES: Explanations of the lack of efficacy of these clinical interventions may include targeting the wrong metric or patient population, or the possibility that some of these metabolic changes could be protective. In this mini-review, we propose a paradigm shift that is needed in metabolism and nutrition management in sepsis.

When a calorie isn't just a calorie: a revised look at nutrition in critically ill patients with sepsis and acute kidney injury.

PURPOSE OF REVIEW: To discuss how nutritional management could be optimized to promote protective metabolism in sepsis and associated acute kidney injury. RECENT FINDINGS: Recent evidence suggests that sepsis is a metabolically distinct critical illness and that certain metabolic alterations, such as activation of fasting metabolism, may be protective in bacterial sepsis. These findings may explain the lack of survival benefit in recent randomized controlled trials of nutrition therapy for critical illness. These trials are limited by cohort heterogeneity, combining both septic and nonseptic critical illness, and the use of inaccurate caloric estimates to determine energy requirements. These energy estimates are also unable to provide information on specific substrate preferences or the capacity for substrate utilization. As a result, high protein feeding beyond the capacity for protein synthesis could cause harm in septic patients. Excess glucose and insulin exposures suppress fatty acid oxidation, ketogenesis and autophagy, of which emerging evidence suggest are protective against sepsis associated organ damage such as acute kidney injury. SUMMARY: Distinguishing pathogenic and protective sepsis-related metabolic changes are critical to enhancing and individualizing nutrition management for critically ill patients.

Fasting-induced HMGCS2 expression in the kidney does not contribute to circulating ketones.

Mitochondrial hydroxymethylglutaryl-CoA synthase 2 (HMGCS2) is the rate-limiting enzyme in ketogenesis. The liver expresses high levels of HMGCS2 constitutively as the main ketogenic organ. It has been suggested that the kidney could be ketogenic as HMGCS2 is expressed in the kidney during fasting and diabetic conditions. However, definitive proof of the capacity for the kidney to produce ketones is lacking. We demonstrated that during fasting, HMGCS2 expression is induced in the proximal tubule of the kidney and is peroxisome proliferator activated receptor-α dependent. Mice with kidney-specific Hmgcs2 deletion showed a minor, likely physiologically insignificant, decrease in circulating ketones during fasting. Conversely, liver-specific Hmgcs2 knockout mice exhibited a complete loss of fasting ketosis. Together, these findings indicate that renal HMGCS2 does not significantly contribute to global ketone production and that during fasting, the increase in circulating ketones is solely dependent on hepatic HMGCS2. Proximal tubule HMGCS2 serves functions other than systemic ketone provision.NEW & NOTEWORTHY The mitochondrial enzyme hydroxymethylglutaryl-CoA synthase 2 (HMGCS2) catalyzes the rate-limiting step of ketogenesis. Although the liver constitutively expresses HMGCS2 and is considered the main ketogenic organ, HMGCS2 is induced in the kidney during fasting, leading to the proposal that the kidney contributes to fasting ketosis. We showed kidney HMGCS2 does not contribute to circulating ketones during fasting and cannot compensate for hepatic ketogenic insufficiency.

Metabolism as Disease Tolerance: Implications for Sepsis-Associated Acute Kidney Injury.

Sepsis is a significant cause for mortality among critically ill patients. Metabolic derangements that develop in sepsis are often considered to be pathologic, contributing to sepsis morbidity and mortality. However, alterations in metabolism during sepsis are multifaceted and are incompletely understood. Acute anorexia during infection is an evolutionarily conserved response, suggesting a potential protective role of anorexia in the host response to infection. In animal models of bacterial inflammation, fasting metabolic programs associated with acute anorexia such as those regulated by fibroblast growth factor 21 and ketogenesis are associated with improved survival. Other fasting metabolic pathways such as fatty acid oxidation and autophagy are also implicated in preventing acute kidney injury (AKI). Global metabolic changes during sepsis and current clinical interventions can potentially affect disease tolerance mechanisms and modify the risk of AKI.

Hepatic FGF21 preserves thermoregulation and cardiovascular function during bacterial inflammation.

Sickness behaviors, including anorexia, are evolutionarily conserved responses to acute infections. Inflammation-induced anorexia causes dramatic metabolic changes, of which components critical to survival are unique depending on the type of inflammation. Glucose supplementation during the anorectic period induced by bacterial inflammation suppresses adaptive fasting metabolic pathways, including fibroblast growth factor 21 (FGF21), and decreases survival. Consistent with this observation, FGF21-deficient mice are more susceptible to mortality from endotoxemia and polybacterial peritonitis. Here, we report that increased circulating FGF21 during bacterial inflammation is hepatic derived and required for survival through the maintenance of thermogenesis, energy expenditure, and cardiac function. FGF21 signaling downstream of its obligate coreceptor, ß-Klotho (KLB), is required in bacterial sepsis. However, FGF21 modulates thermogenesis and chronotropy independent of the adipose, forebrain, and hypothalamus, which are operative in cold adaptation, suggesting that in bacterial inflammation, either FGF21 signals through a novel, undescribed target tissue or concurrent signaling of multiple KLB-expressing tissues is required.

Renal tubular cell spliced X-box binding protein 1 (Xbp1s) has a unique role in sepsis-induced acute kidney injury and inflammation.

Sepsis is a systemic inflammatory state in response to infection, and concomitant acute kidney injury (AKI) increases mortality significantly. Endoplasmic reticulum stress is activated in many cell types upon microbial infection and modulates inflammation. The role of endoplasmic reticulum signaling in the kidney during septic AKI is unknown. Here we tested the role of the spliced X-box binding protein 1 (Xbp1s), a key component of the endoplasmic reticulum stress-activated pathways, in the renal response to sepsis in the lipopolysaccharide (LPS) model. Xbp1s was increased in the kidneys of mice treated with LPS but not in other models of AKI, or several chronic kidney disease models. The functional significance of Xbp1s induction was examined by genetic manipulation in renal tubules. Renal tubule-specific overexpression of Xbp1s caused severe tubule dilation and vacuolation with expression of the injury markers Kim1 and Ngal, the pro-inflammatory molecules interleukin-6 (Il6) and Toll-like receptor 4 (Tlr4), decreased kidney function and 50% mortality in five days. Renal tubule-specific genetic ablation of Xbp1 had no phenotype at baseline. However, after LPS, Xbp1 knockdown mice displayed lower renal NGAL, pro-apoptotic factor CHOP, serum creatinine levels, and a tendency towards lower Tlr4 compared to LPS-treated mice with intact Xbp1s. LPS treatment in Xbp1s-overexpressing mice caused a mild increase in NGAL and CHOP compared to LPS-treated mice without genetic Xbp1s overexpression. Thus, increased Xbp1s signaling in renal tubules is unique to sepsis-induced AKI and contributes to renal inflammation and injury. Inhibition of this pathway may be a potential portal to alleviate injury.

Targeting protein translation to prevent septic kidney injury.

The development of acute kidney injury (AKI) in patients with sepsis causes significant morbidity and mortality. The pathogenesis of AKI in sepsis is incompletely understood. In this issue of the JCI, Hato et al. investigate the renal translatome during bacterial sepsis and identify the global shutdown of renal protein translation mediated by the eukaryotic translation initiation factor 2-α kinase 2/eukaryotic translation initiation factor 2α (EIF2AK2/eIF2α) axis as a major pathway in mediating septic AKI. The results of this study suggest that inhibiting this pathway could be a potential therapeutic strategy for preventing septic AKI.

Acute Kidney Injury After Burn: A Cohort Study From the Parkland Burn Intensive Care Unit.

Acute kidney injury (AKI) is a common and morbid complication in patients with severe burn. The reported incidence of AKI and mortality in this population varies widely due to inconsistent and changing definitions. They aimed to examine the incidence, severity, and hospital mortality of patients with AKI after burn using consensus criteria. This is a retrospective cohort study of adults with thermal injury admitted to the Parkland burn intensive care unit (ICU) from 2008 to 2015. One thousand forty adult patients with burn were admitted to the burn ICU. AKI was defined by KDIGO serum creatinine criteria. Primary outcome includes hospital death and secondary outcome includes length of mechanical ventilation, ICU, and hospital stay. All available serum creatinine measurements were used to determine the occurrence of AKI during the hospitalization. All relevant clinical data were collected. The median total body surface area (TBSA) of burn was 16% (IQR: 6%-29%). AKI occurred in 601 patients (58%; AKI stage 1, 60%; stage 2, 19.8%; stage 3, 10.5%; and stage 3 requiring renal replacement therapy [3-RRT], 9.7%). Patients with AKI had larger TBSA burn (median 20.5% vs 11.0%; P < .001) and more mechanical ventilation and hospitalization days than patients without AKI. The hospital death rate was higher in those with AKI vs those without AKI (19.7% vs 3.9%; P < .001) and increased by each AKI severity stage (P trend < .001). AKI severity was independently associated with hospital mortality in the small burn group (for TBSA ≤ 10%: stage 1 adjusted OR 9.3; 95% CI, 2.6-33.0; stage 2-3 OR, 35.0; 95% CI, 9.0-136.8; stage 3-RRT OR, 30.7; 95% CI, 4.2-226.4) and medium burn group (TBSA 10%-40%: stage 2-3 OR, 6.5; 95% CI, 1.9-22.1; stage 3-RRT OR, 35.1; 95% CI, 8.2-150.3). AKI was not independently associated with hospital death in the large burn group (TBSA > 40%). Urine output data were unavailable. AKI occurs frequently in patients after burn. Presence of and increasing severity of AKI are associated with increased hospital mortality. AKI appears to be independently and strongly associated with mortality in patients with TBSA ≤ 40%. Further investigation to develop risk-stratification tools tailoring this susceptible population is direly needed.

Glucose metabolism mediates disease tolerance in cerebral malaria.

Sickness behaviors are a conserved set of stereotypic responses to inflammatory diseases. We recently demonstrated that interfering with inflammation-induced anorexia led to metabolic changes that had profound effects on survival of acute inflammatory conditions. We found that different inflammatory states needed to be coordinated with corresponding metabolic programs to actuate tissue-protective mechanisms. Survival of viral inflammation required intact glucose utilization pathways, whereas survival of bacterial inflammation required alternative fuel substrates and ketogenic programs. We thus hypothesized that organismal metabolism would be important in other classes of infectious inflammation and sought to understand its role in the prototypic parasitic disease malaria. Utilizing the cerebral malaria model, Plasmodium berghei ANKA (PbA) infection in C57BL/6J male mice, we unexpectedly found that inhibition of glycolysis using 2-deoxy glucose (2DG) conferred protection from cerebral malaria. Unlike vehicle-treated animals, 2DG-treated animals did not develop cerebral malaria and survived until ultimately succumbing to fatal anemia. We did not find any differences in parasitemia or pathogen load in affected tissues. There were no differences in the kinetics of anemia. We also did not detect differences in immune infiltration in the brain or in blood-brain barrier permeability. Rather, on pathological analyses performed on the entire brain, we found that 2DG prevented the formation of thrombi and thrombotic complications. Using thromboelastography (TEG), we found that 2DG-treated animals formed clots that were significantly less strong and stable. Together, these data suggest that glucose metabolism is involved in inflammation-induced hemostasis and provide a potential therapeutic target in treatment of cerebral malaria.

Macrophages in Renal Injury and Repair.

Acute kidney injury (AKI) is a growing global health concern, yet no treatment is currently available to prevent it or to promote kidney repair after injury. Animal models demonstrate that the macrophage is a major contributor to the inflammatory response to AKI. Emerging data from human biopsies also corroborate the presence of macrophages in AKI and their persistence in progressive chronic kidney disease. Macrophages are phagocytic innate immune cells that are important mediators of tissue homeostasis and host defense. In response to tissue injury, macrophages become activated based on specific signals from the damaged microenvironment. The activation and functional state of the macrophage depends on the stage of tissue injury and repair, reflecting a dynamic and diverse spectrum of macrophage phenotypes. In this review, we highlight our current understanding of the mechanisms by which macrophages contribute to injury and repair after AKI.

Opposing Effects of Fasting Metabolism on Tissue Tolerance in Bacterial and Viral Inflammation.

Acute infections are associated with a set of stereotypic behavioral responses, including anorexia, lethargy, and social withdrawal. Although these so-called sickness behaviors are the most common and familiar symptoms of infections, their roles in host defense are largely unknown. Here, we investigated the role of anorexia in models of bacterial and viral infections. We found that anorexia was protective while nutritional supplementation was detrimental in bacterial sepsis. Furthermore, glucose was necessary and sufficient for these effects. In contrast, nutritional supplementation protected against mortality from influenza infection and viral sepsis, whereas blocking glucose utilization was lethal. In both bacterial and viral models, these effects were largely independent of pathogen load and magnitude of inflammation. Instead, we identify opposing metabolic requirements tied to cellular stress adaptations critical for tolerance of differential inflammatory states. VIDEO ABSTRACT.

Molecular phenotyping of clinical AKI with novel urinary biomarkers.

Acute kidney injury (AKI) is a common hospital complication. There are no effective treatments to minimize kidney injury or limit associated morbidity and mortality. Currently, serum creatinine and urine output remain the gold standard used clinically in the diagnosis of AKI. Several novel biomarkers can diagnose AKI earlier than elevations of serum creatinine and changes in urine output. Recent long-term observational studies have elucidated a subgroup of patients who have positive biomarkers of AKI but do not meet criteria for AKI by serum creatinine or urine output, termed subclinical AKI. These patients with subclinical AKI have increased risk of both short- and long-term mortality. In this review, we will highlight the implications of what these patients may represent and the need for better phenotyping of AKI by etiology, severity of injury, and ability to recover. We will discuss two AKI biomarkers, neutrophil gelatinase-associated lipocalin (NGAL) and breast regression protein-39 (BRP-39)/YKL-40, that exemplify the need to characterize the complexity of the biological meaning behind the biomarker, beyond elevated levels reporting on tissue injury. Ultimately, careful phenotyping of AKI will lead to identification of therapeutic targets and appropriate patient populations for clinical trials.

Macrophage-mediated injury and repair after ischemic kidney injury.

Acute ischemic kidney injury is a common complication in hospitalized patients. No treatment is yet available for augmenting kidney repair or preventing progressive kidney fibrosis. Animal models of acute kidney injury demonstrate that activation of the innate immune system plays a major role in the systemic response to ischemia/reperfusion injury. Macrophage depletion studies suggest that macrophages, key participants in the innate immune response, augment the initial injury after reperfusion but also promote tubular repair and contribute to long-term kidney fibrosis after ischemic injury. The distinct functional outcomes seen following macrophage depletion at different time points after ischemia/reperfusion injury suggest heterogeneity in macrophage activation states. Identifying the pathways that regulate the transitions of macrophage activation is thus critical for understanding the mechanisms that govern both macrophage-mediated injury and repair in the postischemic kidney. This review examines our understanding of the complex and intricately controlled pathways that determine monocyte recruitment, macrophage activation, and macrophage effector functions after renal ischemia/reperfusion injury. Careful delineation of repair and resolution pathways could provide therapeutic targets for the development of effective treatments to offer patients with acute kidney injury.

GM-CSF Promotes Macrophage Alternative Activation after Renal Ischemia/Reperfusion Injury.

After kidney ischemia/reperfusion (I/R) injury, monocytes home to the kidney and differentiate into activated macrophages. Whereas proinflammatory macrophages contribute to the initial kidney damage, an alternatively activated phenotype can promote normal renal repair. The microenvironment of the kidney during the repair phase mediates the transition of macrophage activation from a proinflammatory to a reparative phenotype. In this study, we show that macrophages isolated from murine kidneys during the tubular repair phase after I/R exhibit an alternative activation gene profile that differs from the canonical alternative activation induced by IL-4-stimulated STAT6 signaling. This unique activation profile can be reproduced in vitro by stimulation of bone marrow-derived macrophages with conditioned media from serum-starved mouse proximal tubule cells. Secreted tubular factors were found to activate macrophage STAT3 and STAT5 but not STAT6, leading to induction of the unique alternative activation pattern. Using STAT3-deficient bone marrow-derived macrophages and pharmacologic inhibition of STAT5, we found that tubular cell-mediated macrophage alternative activation is regulated by STAT5 activation. Both in vitro and after renal I/R, tubular cells expressed GM-CSF, a known STAT5 activator, and this pathway was required for in vitro alternative activation of macrophages by tubular cells. Furthermore, administration of a neutralizing antibody against GM-CSF after renal I/R attenuated kidney macrophage alternative activation and suppressed tubular proliferation. Taken together, these data show that tubular cells can instruct macrophage activation by secreting GM-CSF, leading to a unique macrophage reparative phenotype that supports tubular proliferation after sterile ischemic injury.

Macrophage-specific deletion of transforming growth factor-ß1 does not prevent renal fibrosis after severe ischemia-reperfusion or obstructive injury.

Macrophage infiltration is a prominent feature of the innate immune response to kidney injury. The persistence of macrophages is associated with tubulointerstitial fibrosis and progression of chronic kidney disease. Macrophages are known to be major producers of transforming growth factor-ß1 (TGF-ß1), especially in the setting of phagocytosis of apoptotic cells. TGF-ß1 has long been implicated as a central mediator of tissue scarring and fibrosis in many organ disease models, including kidney disease. In this study, we show that homozygous deletion of Tgfb1 in myeloid lineage cells in mice heterozygous for Tgfb1 significantly reduces kidney Tgfb1 mRNA expression and Smad activation at late time points after renal ischemia-reperfusion injury. However, this reduction in kidney Tgfb1 expression and signaling results in only a modest reduction of isolated fibrosis markers and does not lead to decreased interstitial fibrosis in either ischemic or obstructive injury models. Thus, targeting macrophage-derived TGF-ß1 does not appear to be an effective therapy for attenuating progressive renal fibrosis after kidney injury.

Predicting acute kidney injury after cardiac surgery: a systematic review.

Acute kidney injury (AKI) after cardiac surgery confers a significant increased risk of death. Several risk models have been developed to predict postoperative kidney failure after cardiac surgery. This systematic review evaluated the available risk models for AKI after cardiac surgery. Literature searches were performed in the Web of Science/Knowledge, Scopus, and MEDLINE databases for articles reporting the primary development of a risk model and articles reporting validation of existing risk models for AKI after cardiac surgery. Data on model variables, internal or external validation (or both), measures of discrimination, and measures of calibration were extracted. The systematic review included 7 articles with a primary development of a prediction score for AKI after cardiac surgery and 8 articles with external validation of established models. The models for AKI requiring dialysis are the most robust and externally validated. Among the prediction rules for AKI requiring dialysis after cardiac surgery, the Cleveland Clinic model has been the most widely tested thus far and has shown high discrimination in most of the tested populations. A validated score to predict AKI not requiring dialysis is lacking. Further studies are required to develop risk models to predict milder AKI not requiring dialysis after cardiac surgery. Standardizing risk factor and AKI definitions will facilitate the development and validation of risk models predicting AKI.

Macrophages promote cyst growth in polycystic kidney disease.

Polycystic kidney disease (PKD) exhibits an inflammatory component, but the contribution of inflammation to cyst progression is unknown. Macrophages promote the proliferation of tubular cells following ischemic injury, suggesting that they may have a role in cystogenesis. Furthermore, cultured Pkd1-deficient cells express the macrophage chemoattractants Mcp1 and Cxcl16 and stimulate macrophage migration. Here, in orthologous models of both PKD1 and PKD2, abnormally large numbers of alternatively activated macrophages surrounded the cysts. To determine whether pericystic macrophages contribute to the proliferation of cyst-lining cells, we depleted phagocytic cells from Pkd1(fl/fl);Pkhd1-Cre mice by treating with liposomal clodronate from postnatal day 10 until day 24. Compared with vehicle-treated controls, macrophage-depleted mice had a significantly lower cystic index, reduced proliferation of cyst-lining cells, better-preserved renal parenchyma, and improved renal function. In conclusion, these data suggest that macrophages home to cystic areas and contribute to cyst growth. Interruption of these homing and proliferative signals could have therapeutic potential for PKD.

Successful use of intraperitoneal daptomycin in the treatment of vancomycin-resistant enterococcus peritonitis.

Peritoneal dialysis-associated peritonitis from such resistant organisms as vancomycin-resistant enterococci increasingly is occurring and is challenging to treat. We describe 2 cases of vancomycin-resistant entercoccus peritonitis successfully treated with intraperitoneal daptomycin. Both patients were on automated peritoneal dialysis therapy with culture-positive vancomycin-resistant Enterococcus faecium peritonitis and were treated with 10 to 14 days of intraperitoneal daptomycin given every 4 hours through manual peritoneal dialysate exchanges. Despite the known degradation in dextrose solutions, intraperitoneal daptomycin was effective in clearing both infections. Neither patient experienced a relapse or repeated peritonitis. Additional studies of dosing and pharmacokinetics of intraperitoneal daptomycin in the treatment of patients with vancomycin-resistant enterococcus peritonitis are needed.