Effect of lipopolysaccharide on in vivo and genetic regulation of rat urea synthesis

The Effects of a Meldonium Pre-Treatment on the Course of the LPS-Induced Sepsis in Rats

A dysregulated and overwhelming response to an infection accompanied by the exaggerated pro-inflammatory state and metabolism disturbance leads to the fatal outcome in sepsis. Previously we showed that meldonium, an anti-ischemic drug clinically used to treat myocardial and cerebral ischemia, strongly increases mortality in faecal-induced peritonitis (FIP) in rats. We postulated that the same mechanism that is responsible for the otherwise strong anti-inflammatory effects of meldonium could be the culprit of the increased mortality. In the present study, we applied the LPS-induced model of sepsis to explore the presence of any differences from and/or similarities to the FIP model. When it comes to energy production, despite some shared similarities, it is evident that LPS and FIP models of sepsis differ greatly. A different profile of sympathoadrenal activation may account for this observation, as it was lacking in the FIP model, whereas in the LPS model it was strong enough to overcome the effects of meldonium. Therefore, choosing the appropriate model of sepsis induction is of great importance, especially if energy homeostasis is the main focus of the study. Even when differences in the experimental design of the two models are acknowledged, the role of different patterns of energy production cannot be excluded. On that account, our results draw attention to the importance of uninterrupted energy production in sepsis but also call for much-needed revisions of the current recommendations for its treatment.

Metabolic Fingerprinting of Feces from Calves, Subjected to Gram-Negative Bacterial Endotoxin

Gram-negative bacteria have a well-known impact on the disease state of neonatal calves and their mortality. This study was the first to implement untargeted metabolomics on calves’ fecal samples to unravel the effect of Gram-negative bacterial endotoxin lipopolysaccharide (LPS). In this context, calves were challenged with LPS and administered with fish oil, nanocurcumin, or dexamethasone to evaluate treatment effects. Ultra-high-performance liquid-chromatography high-resolution mass spectrometry (UHPLC-HRMS) was employed to map fecal metabolic fingerprints from the various groups before and after LPS challenge. Based on the generated fingerprints, including 9650 unique feature ions, significant separation according to LPS group was achieved through orthogonal partial least squares discriminant analysis (Q² of 0.57 and p-value of 0.022), which allowed the selection of 37 metabolites as bacterial endotoxin markers. Tentative identification of these markers suggested that the majority belonged to the subclass of the carboxylic acid derivatives—amino acids, peptides, and analogs—and fatty amides, with these subclasses playing a role in the metabolism of steroids, histidine, glutamate, and folate. Biological interpretations supported the revealed markers’ potential to aid in disease diagnosis, whereas beneficial effects were observed following dexamethasone, fish oil, and nanocurcumin treatment.

Non-alcoholic fatty liver disease alters expression of genes governing hepatic nitrogen conversion

BACKGROUND & AIMS

We recently showed that the functional capacity for ureagenesis is deficient in non-alcoholic fatty liver disease (NAFLD) patients. The aim of this study was to assess expression of urea cycle-related genes to elucidate a possible gene regulatory basis to the functional problem.

METHODS

Liver mRNA expression analyses within the gene pathway governing hepatic nitrogen conversion were performed in 20 non-diabetic, biopsy-proven NAFLD patients (8 simple steatosis; 12 non-alcoholic steatohepatitis [NASH]) and 12 obese and 14 lean healthy individuals. Sixteen NAFLD patients were included for gene expression validation. Relationship between gene expressions and functional capacity for ureagenesis was described.

RESULTS

Gene expression of most urea cycle-related enzymes were downregulated in NAFLD vs both control groups; markedly so for the urea cycle flux-generating carbamoyl phosphate synthetase (CPS1) (~3.5-fold, P < .0001). In NASH, CPS1 downregulation paralleled the deficit in ureagenesis (P = .03). Additionally, expression of several genes involved in amino acid uptake and degradation, and the glucagon receptor gene, were downregulated in NAFLD. Conversely, glutamine synthetase (GS) expression increased >1.5-fold (P ≤ .03), inversely related to CPS1 expression (P = .004).

CONCLUSIONS

NAFLD downregulated the expression of urea cycle-related genes. Downregulation of urea cycle flux-generating CPS1 correlated with the loss of functional capacity for ureagenesis in NASH. On gene level, these changes coincided with an increase in the major ammonia scavenging enzyme GS. The effects seemed related to a fatty liver as such rather than NASH or obesity. The findings support gene regulatory mechanisms involved in the deficient ureagenesis of NAFLD, but it remains unexplained how hepatocyte fat accumulation exerts these effects.

Time course of compromised urea synthesis in patients with alcoholic hepatitis

OBJECTIVES

Alcoholic hepatitis (AH) markedly decreases the urea synthesis capacity. We aimed to investigate the time course of this compromised essential liver function in patients with AH and its relation to treatment and survival.

MATERIALS AND METHODS

Thirty patients with AH were included in a prospective cohort study. We measured the substrate-independent urea synthesis capacity, i.e., the functional hepatic nitrogen clearance (FHNC), in the patients at study entry and again at three months (survivors/available: n = 17). Patients with severe disease (Glasgow Alcoholic Hepatitis Score ≥9, n = 17) were randomized to receive either prednisolone or pentoxifylline and were in addition examined after 14 days (n = 9).

RESULTS

FHNC (normal range = 25-45 L/h) was markedly decreased at study entry (median = 5.6 (IQR = 3.0-9.6) L/h) and increased by three-fold in survivors at three months (15.1 (12.0-22.9) L/h; p < .001). In patients with severe AH, FHNC was also increased after 14 days of pharmacologic treatment and showed the greatest increase in the patients taking prednisolone (prednisolone 25.4 (20.6-26.2) L/h vs. pentoxifylline 12.3 (8.0-15.3) L/h; p = .05). FHNC at study entry was lower in 90-day non-survivors than in survivors (p = .04).

CONCLUSIONS

The decrease in the urea synthesis capacity in patients with AH was the most marked in short-term non-survivors and partly recovered in survivors at three months. In patients on pharmacologic treatment, recovery was observed already after 14 days, and it was nearly complete in those on prednisolone. Thus, metabolic liver failure in AH seems to be prognostically important, is potentially reversible, and may recover more rapidly following treatment with prednisolone.

Cerebellar neurodegeneration in a new rat model of episodic hepatic encephalopathy

Hepatic encephalopathy has traditionally been considered a reversible disorder. However, recent studies suggested that repeated episodes of hepatic encephalopathy cause persistent impairment leading to neuronal loss. The aims of our study were the development of a new animal model that reproduces the course of episodic hepatic encephalopathy and the identification of neurodegeneration evidences. Rats with portacaval anastomosis underwent simulated episodes of hepatic encephalopathy, triggered by the regular administration of ammonium acetate, and/or lipopolysaccharide. The neurological status was assessed and neuronal loss stereologically quantified in motor areas. During the simulated episodes, ammonia induced reversible motor impairment in portacaval anastomosis rats. In cerebellum, stereology showed a reduction in Purkinje cell population in portacaval anastomosis and PCA+NH₃ groups and morphological changes. An increase in astrocyte size in PCA+NH₃ group and activated microglia in groups treated with ammonium acetate and/or lipopolysaccharide was observed. A modulation of neurodegeneration-related genes and the presence of apoptosis in Bergmann glia were observed. This new animal model reproduces the clinical course of episodic hepatic encephalopathy when ammonia is the precipitant factor and demonstrates the existence of neuronal loss in cerebellum. The persistence of over-activated microglia and reactive astrocytes could participate in the apoptosis of Bergmann glia and therefore Purkinje cell degeneration.

Alcoholic Hepatitis Markedly Decreases the Capacity for Urea Synthesis

Background and Aim

Data on quantitative metabolic liver functions in the life-threatening disease alcoholic hepatitis are scarce. Urea synthesis is an essential metabolic liver function that plays a key regulatory role in nitrogen homeostasis. The urea synthesis capacity decreases in patients with compromised liver function, whereas it increases in patients with inflammation. Alcoholic hepatitis involves both mechanisms, but how these opposite effects are balanced remains unclear. Our aim was to investigate how alcoholic hepatitis affects the capacity for urea synthesis. We related these findings to another measure of metabolic liver function, the galactose elimination capacity (GEC), as well as to clinical disease severity.

Methods

We included 20 patients with alcoholic hepatitis and 7 healthy controls. The urea synthesis capacity was quantified by the functional hepatic nitrogen clearance (FHNC), i.e., the slope of the linear relationship between the blood α-amino nitrogen concentration and urea nitrogen synthesis rate during alanine infusion. The GEC was determined using blood concentration decay curves after intravenous bolus injection of galactose. Clinical disease severity was assessed by the Glasgow Alcoholic Hepatitis Score and Model for End-Stage Liver Disease (MELD) score.

Results

The FHNC was markedly decreased in the alcoholic hepatitis patients compared with the healthy controls (7.2±4.9 L/h vs. 37.4±6.8 L/h, P<0.01), and the largest decrease was observed in those with severe alcoholic hepatitis (4.9±3.6 L/h vs. 9.9±4.9 L/h, P<0.05). The GEC was less markedly reduced than the FHNC. A negative correlation was detected between the FHNC and MELD score (rho = -0.49, P<0.05).

Conclusions

Alcoholic hepatitis markedly decreases the urea synthesis capacity. This decrease is associated with an increase in clinical disease severity. Thus, the metabolic failure in alcoholic hepatitis prevails such that the liver cannot adequately perform the metabolic up-regulation observed in other stressful states, including extrahepatic inflammation, which may contribute to the patients’ poor prognosis.

Urea

Urea is generated by the urea cycle enzymes, which are mainly in the liver but are also ubiquitously expressed at low levels in other tissues. The metabolic process is altered in several conditions such as by diets, hormones, and diseases. Urea is then eliminated through fluids, especially urine. Blood urea nitrogen (BUN) has been utilized to evaluate renal function for decades. New roles for urea in the urinary system, circulation system, respiratory system, digestive system, nervous system, etc., were reported lately, which suggests clinical significance of urea.

Non-alcoholic steatohepatitis weakens the acute phase response to endotoxin in rats

BACKGROUND & AIMS

Patients with non-alcoholic steatohepatitis (NASH) have increased mortality, including from infections. We, therefore, tested in a rodent model of steatohepatitis whether the hepatic acute phase response is intact.

METHODS

Steatohepatitis was induced in rats by feeding a high-fat, high-cholesterol diet for 4 (early) and 16 weeks (advanced NASH). 2 h after low-dose LPS (0.5 mg/kg i.p.), we measured the serum concentrations of tumour necrosis factor-α (TNF-α) and interleukin-6 (IL-6). We also measured liver mRNA's and the serum concentrations of acute phase proteins 24 h after LPS.

RESULTS

Non-alcoholic steatohepatitis in itself increased the liver mRNA levels of TNF-α and IL-6 and also the liver mRNA and serum levels of the acute phase proteins. The exposure to LPS increased serum TNF-α in both early and advanced NASH and more so than in the control rats. However, the increases in acute phase protein genes in liver tissue and proteins in the blood were lower than in the control rats.

CONCLUSION

In rats with early or advanced experimental NASH, LPS despite an increased interleukin release resulted in a blunted acute phase protein response. This tachyphylaxis may be part of the mechanism for the increased infection susceptibility of patients with NASH. We speculate that the steatosis-related interleukin release desensitises the signalling pathway leading to acute phase protein synthesis.

Experimental nonalcoholic steatohepatitis compromises ureagenesis, an essential hepatic metabolic function

Nonalcoholic steatohepatitis (NASH) is increasing in prevalence, yet its consequences for liver function are unknown. We studied ureagenesis, an essential metabolic liver function of importance for whole body nitrogen homeostasis, in a rodent model of diet-induced NASH. Rats were fed a high-fat, high-cholesterol diet for 4 and 16 wk, resulting in early and advanced experimental NASH, respectively. We examined the urea cycle enzyme mRNAs in liver tissue, the hepatocyte urea cycle enzyme proteins, and the in vivo capacity of urea-nitrogen synthesis (CUNS). Early NASH decreased all of the urea cycle mRNAs to an average of 60% and the ornithine transcarbamylase protein to 10%, whereas the CUNS remained unchanged. Advanced NASH further decreased the carbamoyl phosphate synthetase protein to 63% and, in addition, decreased the CUNS by 20% [from 5.65 ± 0.23 to 4.58 ± 0.30 μmol × (min × 100 g)(-1); P = 0.01]. Early NASH compromised the genes and enzyme proteins involved in ureagenesis, whereas advanced NASH resulted in a functional reduction in the capacity for ureagenesis. The pattern of urea cycle perturbations suggests a prevailing mitochondrial impairment by NASH. The decrease in CUNS has consequences for the ability of the body to adjust to changes in the requirements for nitrogen homeostasis e.g., at stressful events. NASH, thus, in terms of metabolic consequences, is not an innocuous lesion, and the manifestations of the damage seem to be a continuum with increasing disease severity.

Prednisolone but not infliximab aggravates the upregulated hepatic nitrogen elimination in patients with active inflammatory bowel disease

BACKGROUND

Catabolism and weight loss are serious problems in patients with active inflammatory bowel disease (IBD). The body nitrogen (N) depletion is partly related to increased hepatic capacity for the elimination of N through urea synthesis. This is probably caused by the inflammation per se, and the treatment with prednisolone may aggravate the problem, whereas the effect of biological therapy is unknown. Therefore, we examined the effects of prednisolone or infliximab on the regulation of urea synthesis in patients with active IBD.

METHODS

Urea synthesis was quantified by the functional hepatic nitrogen clearance (FHNC), i.e., the slope of the linear relationship between the urea nitrogen synthesis rate and the blood α-amino nitrogen concentration during alanine infusion. Thirty-seven patients with active IBD treated with either prednisolone or infliximab were examined before and after 7 days of treatment.

RESULTS

At baseline, the FHNC was similar in the 2 treatment groups (36 L/h). After 7 days, prednisolone increased the FHNC by 40% (55 L/h) (P = 0.03), whereas infliximab tended to reduce the FHNC by 15% (30 L/h) (P = 0.09). The changes in the FHNC differed significantly between the 2 treatment groups (P < 0.01).

CONCLUSIONS

Prednisolone treatment further upregulated urea synthesis, which increases the hepatic loss of nitrogen and promotes body catabolism. In contrast, infliximab treatment caused no such aggravation and likely reduced the N loss. These results may argue in favor of infliximab therapy for IBD and add to the pathophysiological understanding of the interplay between inflammation, catabolism, and anti-inflammatory treatment.

Regulation of urea synthesis during the acute-phase response in rats

The acute-phase response is a catabolic event involving increased waste of amino-nitrogen (N) via hepatic urea synthesis, despite an increased need for amino-N incorporation into acute-phase proteins. This study aimed to clarify the regulation of N elimination via urea during different phases of the tumor necrosis factor-α (TNF-α)-induced acute-phase response in rats. We used four methods to study the regulation of urea synthesis: We examined urea cycle enzyme mRNA levels in liver tissue, the hepatocyte urea cycle enzyme proteins, the in vivo capacity of urea-N synthesis (CUNS), and known humoral regulators of CUNS at 1, 3, 24, and 72 h after TNF-α injection (25 μg/kg iv rrTNF-α) in rats. Serum acute-phase proteins and their liver mRNA levels were also measured. The urea cycle enzyme mRNA levels acutely decreased and then gradually normalized, whereas the urea cycle enzyme proteins remained essentially unchanged over time. The CUNS rose after 3 h and then normalized. The acute-phase response was fully activated at 24 h with markedly increased serum levels of the acute-phase proteins. TNF-α acutely upregulated the CUNS. Later, despite the fully established 24-h acute-phase response and the decreased activity of the urea cycle enzyme genes, there was no change in the urea cycle enzyme proteins or the CUNS. Thus in no phase after the initiation of the acute-phase response was in vivo urea synthesis orchestrated in combination with acute-phase protein synthesis so as to limit N waste.

Unchanged capacity of urea synthesis during acute phase response in rats

BACKGROUND

The acute phase response presents a catabolic event related to increased waste of amino-N via hepatic urea synthesis despite an increased need for amino-N incorporation into acute phase proteins. In our previous studies, tumour necrosis factor-α (TNF-α) acutely up-regulated the in vivo capacity of urea-nitrogen synthesis (CUNS) in rats before the hepatic acute phase response was established. To extend these observations, this study aimed to clarify the regulation of N elimination via urea during the later stages of the acute phase response.

METHODS

Twenty-four hours after i.v. injection of 25 μg kg(-1) TNF-α or placebo, we determined the in vivo CUNS, hepatocyte urea cycle enzyme protein levels and mRNA levels of the urea cycle enzyme genes in pair-fed rats. In addition, serum acute phase proteins and their liver mRNA levels were measured.

RESULTS

After TNF-α, CUNS and hepatocyte urea cycle enzyme protein expressions were unchanged while urea cycle enzyme mRNA levels decreased. Liver mRNA levels of α2MG, haptoglobin and α1AGP rose and their serum levels increased equally.

CONCLUSION

Despite a fully established 24-h acute phase response, there was no change in the in vivo capacity for disposal of amino-N by urea synthesis or in the urea cycle enzyme proteins, although the expression of the urea cycle enzyme genes was decreased. Thus, in vivo urea synthesis was not orchestrated together with acute phase protein synthesis so as to limit N waste despite genetic regulation to this effect. This may contribute towards catabolism of inflammation.

IL-6 has no acute effect on the regulation of urea synthesis in vivo in rats

BACKGROUND

Clinical or experimentally induced, active inflammation up-regulates the in vivo capacity of urea synthesis (CUNS), which promotes nitrogen removal from the body and metabolic catabolism. We have shown that tumor necrosis factor α (TNF-α) up-regulates CUNS and increases interleukin 6 expression (IL-6) within hours of administration. The described effect of TNF-α on nitrogen homeostasis may, therefore, depend on IL-6.

METHODS

Three hours after the i.v. injection of 125 μg.kg⁻¹ of IL-6 or placebo, we evaluated the CUNS, hepatocyte urea cycle enzyme protein levels and the mRNA levels of the urea cycle enzyme genes in rats. The prevailing rat serum acute phase proteins and their liver mRNA levels were also measured.

RESULTS

IL-6 did not change CUNS or hepatocyte urea cycle enzyme protein levels, whereas urea cycle enzyme mRNA levels, except for ornithine transcarbamylase (OTC), decreased by approximately 20%. The liver mRNA levels of α2MG, haptoglobin and α1AGP all increased by 1.5- to 2-fold (p < 0.001). In serum, only the α2MG concentration slightly increased (p < 0.001), whereas the levels of the other circulating acute phase proteins remained unchanged.

CONCLUSION

IL-6 is not the mediator of the in vivo CUNS up-regulation observed 3 h after TNF-α administration, but it may be involved in the down-regulation of urea cycle genes. IL-6 may also mediate TNF-α effects on acute phase protein gene expression. Thus, IL-6 did not contribute to the in vivo hepatic component of inflammation-associated catabolism.

Cirrhosis and endotoxin decrease urea synthesis in rats

AIM

In patients with cirrhosis, endotoxemia is frequent and the vitally important capacity for urea synthesis is impaired. The patients' mortality of infection is markedly increased, which could be related to adverse metabolic effects of endotoxins. The effects of endotoxins on in vivo urea synthesis and on urea cycle genes during cirrhosis are unknown.

METHODS

We examined the effects of a low dose of 0.5 mg/kg ip lipopolysaccharide (LPS) on the basal urea nitrogen synthesis rate (UNSR), the capacity of urea nitrogen synthesis (CUNS), liver tissue mRNA levels of urea cycle enzyme genes, and on the metabolic liver function measured by the galactose elimination capacity (GEC) in rats with cirrhosis induced by bile duct ligation and in control animals.

RESULTS

LPS and cirrhosis + LPS decreased UNSR by 40% (P < 0.05). Cirrhosis and LPS each tended to decrease CUNS and cirrhosis + LPS decreased CUNS by 40% (P < 0.05). Cirrhosis and LPS each decreased the mRNA level of the gene for the flux-generating urea cycle enzyme carbamoyl phosphate synthetase (CPS) and the mRNA for the rate-limiting urea cycle enzyme arginine succinate synthetase (ASS) (P < 0.05). Cirrhosis + LPS left the mRNA level of CPS unchanged and decreased that of ASS (P < 0.05). The GEC did not differ among the study groups.

CONCLUSION

Endotoxemia in rats with experimental cirrhosis markedly impaired the ability of the animals' livers to synthesize urea, suggesting a pathophysiological mechanism underlying the severe consequences of endotoxemia in human cirrhosis.

Opposite effects on regulation of urea synthesis by early and late uraemia in rats

BACKGROUND & AIMS

Acute and chronic kidney failure lead to catabolism with loss of lean body mass. Up-regulation of hepatic urea synthesis may play a role for the loss of body nitrogen and for the level of uraemia. The aims were to investigate the effects of early and late experimental renal failure on the regulation of hepatic urea synthesis and the expression of urea cycle enzyme genes in the liver.

METHODS

We examined the in vivo capacity of urea nitrogen synthesis, mRNA levels of urea cycle enzyme genes, and N-balances 6 days and 21 days after 5/6th partial nephrectomy in rats, and compared these data with pair- and free-fed control animals.

RESULTS

Compared with pair-fed animals, early uraemia halved the in vivo urea synthesis capacity and decreased urea gene expressions (P<0.05). In contrast, late uraemia up-regulated in vivo urea synthesis and expression of all urea genes (P<0.05), save that of the flux-generating enzyme carbamoyl phosphate synthetase. The N-balance in rats with early uraemia was markedly negative (P<0.05) and near zero in late uraemia.

CONCLUSIONS

Early uraemia down-regulated urea synthesis, so hepatic ureagenesis was not in itself involved in the negative N-balance. In contrast, late uraemia up-regulated urea synthesis, which probably contributed towards the reduced N-balance of this condition. These time-dependent, opposite effects on the uraemia-induced regulation of urea synthesis in vivo were not related to food restriction and probably mostly reflected regulation on gene level.

Synthesis of acute phase proteins in rats with cirrhosis exposed to lipopolysaccharide

BACKGROUND

In patients with cirrhosis, infection is frequent and a leading cause of death. This is secondary to various immunologic abnormalities in both the innate and the adaptive immune system. However, it remains unclear whether cirrhosis affects the inflammatory systemic component of the innate immunity, 'the acute phase response', mostly effectuated by the liver itself. We hypothesized that rats with cirrhosis raise a reduced acute phase response induced by lipopolysaccharide (LPS).

RESULTS

We examined the acute phase response induced by intraperitoneal injection of a low dose of LPS, in sham operated control animals and in rats with liver cirrhosis induced by bile duct ligation (BDL). We measured the serum concentrations of the most important acute phase proteins and their liver tissue gene expressions, assessed by mRNA levels. The BDL-model itself increased the serum concentration of alpha1-acid glycoprotein (alpha1AGP) and haptoglobin. LPS was lethal to 25% of the cirrhotic animals and to none of the controls. Twenty-four hours after LPS, the serum concentration of alpha1AGP and haptoglobin, the mRNA level of these acute phase proteins and of alpha2-macroglobulin and thiostatin rose to the same level in the animals with cirrhosis and in controls.

CONCLUSION

In rats with experimental cirrhosis LPS caused high mortality. In the survivors, the cirrhotic liver still synthesized acute phase proteins as the normal liver, indicating a normal hepatic contribution to this part of the acute phase response.

In vivo urea kinetic studies in conscious mice

Stable isotope studies in conscious mice have been limited by the invasive catheterization procedures and relatively large sample size required. We developed minimally invasive catheterization protocols that together with the ability to analyze small samples have allowed for the study of urea kinetics in conscious mice. A single dose of 15N15N-urea followed by multiple sampling in mice (n = 6) showed that a primary pool of urea exchanged rapidly [70.65 +/- 14.96 mmol/(kg x h)] with a secondary pool. The urea entry rate determined with this protocol was 3.36 +/- 0.30 mmol/(kg x h). Continuous infusion of 15N15N-urea (n = 6) achieved plateau enrichment values at 3.3 +/- 0.2.h from which the urea entry rate was determined by isotope dilution [3.24 +/- 0.23 mmol/(kg x h)]. The urea entry rate measured by the single dose or continuous infusion protocol did not differ (P = 0.76). The minimally invasive methods described allow us to study not only ureagenesis and urea cycle disorders in vivo, but also urea transport and transporter function and nitrogen metabolism in general in mouse models. This is especially relevant because mouse targeting technologies will likely facilitate the generation of organ and tissue specific nulls of the various urea cycle enzymes.