Down-regulation of hepatic urea synthesis by oxypurines: xanthine and uric acid inhibit N-acetylglutamate synthase

Excessive gluconeogenesis causes the hepatic insulin resistance paradox and its sequelae

Background

Hepatic insulin signaling suppresses gluconeogenesis but promotes de novo lipid synthesis. Paradoxically, hepatic insulin resistance (HIR) enhances both gluconeogenesis and de novo lipid synthesis. Elucidation of the etiology of this paradox, which participates in the pathogenesis of non-alcoholic fatty liver disease (NAFLD), cardiovascular disease, the metabolic syndrome and hepatocellular carcinoma, has not been fully achieved.

Scope of review

This article briefly outlines the previously proposed hypotheses on the etiology of the HIR paradox. It then discusses literature consistent with an alternative hypothesis that excessive gluconeogenesis, the direct effect of HIR, is responsible for the aberrant lipogenesis. The mechanisms involved therein are explained, involving de novo synthesis of fructose and uric acid, promotion of glutamine anaplerosis, and induction of glucagon resistance. Thus, gluconeogenesis via lipogenesis promotes hepatic steatosis, a component of NAFLD, and dyslipidemia. Gluconeogenesis-centred mechanisms for the progression of NAFLD from simple steatosis to non-alcoholic steatohepatitis (NASH) and fibrosis are suggested. That NAFLD often precedes and predicts type 2 diabetes is explained by the ability of lipogenesis to cushion against blood glucose dysregulation in the earlier stages of NAFLD.

Major conclusions

HIR-induced excessive gluconeogenesis is a major cause of the HIR paradox and its sequelae. Such involvement of gluconeogenesis in lipid synthesis rationalizes the fact that several types of antidiabetic drugs ameliorate NAFLD. Thus, dietary, lifestyle and pharmacological targeting of HIR and hepatic gluconeogenesis may be a most viable approach for the prevention and management of the HIR-associated network of diseases.

ASS1 and ASL suppress growth in clear cell renal cell carcinoma via altered nitrogen metabolism

BACKGROUND

Kidney cancer is a common adult malignancy in the USA. Clear cell renal cell carcinoma (ccRCC), the predominant subtype of kidney cancer, is characterized by widespread metabolic changes. Urea metabolism is one such altered pathway in ccRCC. The aim of this study was to elucidate the contributions of urea cycle enzymes, argininosuccinate synthase 1 (ASS1), and argininosuccinate lyase (ASL) towards ccRCC progression.

METHODS

We employed a combination of computational, genetic, and metabolomic tools along with in vivo animal models to establish a tumor-suppressive role for ASS1 and ASL in ccRCC.

RESULTS

We show that the mRNA and protein expression of urea cycle enzymes ASS1 and ASL are reduced in ccRCC tumors when compared to the normal kidney. Furthermore, the loss of ASL in HK-2 cells (immortalized renal epithelial cells) promotes growth in 2D and 3D growth assays, while combined re-expression of ASS1 and ASL in ccRCC cell lines suppresses growth in 2D, 3D, and in vivo xenograft models. We establish that this suppression is dependent on their enzymatic activity. Finally, we demonstrate that conservation of cellular aspartate, regulation of nitric oxide synthesis, and pyrimidine production play pivotal roles in ASS1+ASL-mediated growth suppression in ccRCC.

CONCLUSIONS

ccRCC tumors downregulate the components of the urea cycle including the enzymes argininosuccinate synthase 1 (ASS1) and argininosuccinate lyase (ASL). These cytosolic enzymes lie at a critical metabolic hub in the cell and are involved in aspartate catabolism and arginine and nitric oxide biosynthesis. Loss of ASS1 and ASL helps cells redirect aspartate towards pyrimidine synthesis and support enhanced proliferation. Additionally, reduced levels of ASS1 and ASL might help regulate nitric oxide (NO) generation and mitigate its cytotoxic effects. Overall, our work adds to the understanding of urea cycle enzymes in a context-independent of ureagenesis, their role in ccRCC progression, and uncovers novel potential metabolic vulnerabilities in ccRCC.

Combined intake of blueberry juice and probiotics ameliorate mitochondrial dysfunction by activating SIRT1 in alcoholic fatty liver disease

Background

Mitochondrial dysfunction has been implicated as a significant factor in the liver disease process. Blueberry juice and probiotics (BP) synergistically improve liver function in alcoholic fatty liver disease (AFLD), although the mechanism for this effect was unclear. This study aims to investigate the effect and specific mechanisms of BP on AFLD.

Methods

C57/BL6 mice were randomly divided into seven groups: CG (control), MG (AFLD model), BJ (MG mice treated with blueberry), BJB (MG mice treated with BP), SI (AFLD mice treated with SIRT1 siRNA), BJSI (SI mice treated with blueberry), and BJBSI (SI mice treated with BP). The mice were fed an alcohol liquid diet for 10 days to establish the AFLD model, and subjected to BP and SIRT1 siRNA intervention for 10 days. Liver pathology was performed on day 11, and biochemical and molecular analyses of liver mitochondria were employed on day 12.

Results

BP significantly ameliorated hepatic mitochondrial injury, mitochondrial swelling, and hepatic necrosis in AFLD. BP alleviated hepatic mitochondrial dysfunction by increasing the expression of succinate dehydrogenase and cytochrome c oxidase, increasing respiratory control rate and the ADP/O ratio, and facilitating the synthesis of energy-related molecules. Besides, BP increased the expression of glutathione and superoxide dismutase, and inhibited malondialdehyde expression and reactive oxygen species activity. BP-induced sirtuin 1 (SIRT1), which activates peroxisome proliferator-activated receptor-gamma coactivator-1α, both of which mediate mitochondrial homeostasis. SIRT1 silencing suppressed the BP-induced changes in liver mitochondria, blunting its efficacy.

Conclusions

The ingredients of BP ameliorate hepatocyte mitochondrial dysfunction in AFLD mice.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12986-021-00554-3.

Thymoquinone attenuates oxidative stress of kidney mitochondria and exerts nephroprotective effects in oxonic acid-induced hyperuricemia rats

BACKGROUND

Recent studies indicate hyperuricemia as an aggravating factor for kidney diseases progression. Basic research for novel agents to reduce hyperuricemia and kidney abnormalities will be highly rewarding. Herein, we report Thymoquinone (Tq) as an active constituent of Nigella sativa to have renal protective effective against oxonic acid (OA)-induced hyperuricemia, hypertension, and renal oxidative stress in rat models.

METHODS

OA 750 mg/kg BW for 12 weeks was used to induce uricemia in Sprague dawley rats. Tq at 10 and 20 mg/kg BW were administered along with OA for treatment groups. Plasma uric acid concentration and systolic blood pressure were measured. Oxidative stress markers, total ATP content, and membrane bound ATPases were measured in renal mitochondria. Anti-oxidant enzymes were analyzed in the renal tissues. Apoptosis in renal tissue was detected. Key signaling proteins for apoptosis, oxidative stress, and lipid oxidation pathways were determined.

RESULTS

OA induced both circulating uric acid levels and hypertension in the control group which was brought down on Tq treatments. Tq effectively prevented accumulation of uric acid and oxidative stress in the renal tissues. Tq also proved to increase the total ATP content of the renal mitochondria and prevented the apoptosis induced by OA. Tq increased the expressions of phosphorylated Akt, Nrf2, and HO-1 proteins while decreasing the levels of cleaved caspase-3 in renal cells.

CONCLUSION

In summary, Tq exhibited protective effects on hyperuricemia-mediated renal oxidative stress and mitochondrial abnormalities which could be mediated by Nrf2/HO-1, Akt signaling pathways.

Effects of N-carbamylglutamate and arginine on steroidogenesis and proliferation of pig granulosa cells in vitro

Results of in vivo studies indicate dietary N-carbamylglutamate (NCG) and arginine (ARG) can enhance reproductive performance in gilts. It was hypothesized that both NCG and ARG will alter hormone-induced estradiol (E2) production by granulosa cells (GC), explaining why these compounds could improve reproductive performance in pigs. The objective of these studies, therefore, was to evaluate the direct effects of NCG and ARG on porcine GC proliferation and steroidogenesis, using an in vitro cell culture system. The GC from small (SM; 1-5 mm) and large (LG; >5 mm) pig follicles were cultured for 2 days in 5% fetal bovine serum and 5% porcine serum-containing medium followed by 2 days in serum-free medium containing 500 ng/mL of testosterone (as an E2 precursor), and NCG or ARG at various doses in the presence of either follicle-stimulating hormone (FSH; 30 ng/mL), insulin-like growth factor-1 (IGF1; 30 ng/mL), or both. Numbers of GC were determined at the end of the experiment and concentrations of progesterone (P4) and E2 in culture medium were determined. Results indicated that LG-follicle GC were more responsive to NCG and ARG than SM-follicle GC. Specifically, in LG-follicle GC, NCG inhibited (P <  0.05) basal and FSH-induced P4 and E2 production but stimulated cell numbers; whereas ARG inhibited FSH-induced E2 production and cell numbers. In SM-follicle GC, treatment with NCG and ARG decreased IGF1 plus FSH induced P4 production, but E2 production and cell proliferation were not affected. These studies indicate that NCG and ARG may directly affect follicular function in pigs.

Arginase 2 Suppresses Renal Carcinoma Progression via Biosynthetic Cofactor Pyridoxal Phosphate Depletion and Increased Polyamine Toxicity

Kidney cancer, one of the ten most prevalent malignancies in the world, has exhibited increased incidence over the last decade. The most common subtype is "clear cell" renal cell carcinoma (ccRCC), which features consistent metabolic abnormalities, such as highly elevated glycogen and lipid deposition. By integrating metabolomics, genomic, and transcriptomic data, we determined that enzymes in multiple metabolic pathways are universally depleted in human ccRCC tumors, which are otherwise genetically heterogeneous. Notably, the expression of key urea cycle enzymes, including arginase 2 (ARG2) and argininosuccinate synthase 1 (ASS1), is strongly repressed in ccRCC. Reduced ARG2 activity promotes ccRCC tumor growth through at least two distinct mechanisms: conserving the critical biosynthetic cofactor pyridoxal phosphate and avoiding toxic polyamine accumulation. Pharmacological approaches to restore urea cycle enzyme expression would greatly expand treatment strategies for ccRCC patients, where current therapies only benefit a subset of those afflicted with renal cancer.

A role for bacterial urease in gut dysbiosis and Crohn's disease

Gut dysbiosis during inflammatory bowel disease involves alterations in the gut microbiota associated with inflammation of the host gut. We used a combination of shotgun metagenomic sequencing and metabolomics to analyze fecal samples from pediatric patients with Crohn's disease and found an association between disease severity, gut dysbiosis, and bacterial production of free amino acids. Nitrogen flux studies using 15N in mice showed that activity of bacterial urease, an enzyme that releases ammonia by hydrolysis of host urea, led to the transfer of murine host-derived nitrogen to the gut microbiota where it was used for amino acid synthesis. Inoculation of a conventional murine host (pretreated with antibiotics and polyethylene glycol) with commensal Escherichia coli engineered to express urease led to dysbiosis of the gut microbiota, resulting in a predominance of Proteobacteria species. This was associated with a worsening of immune-mediated colitis in these animals. A potential role for altered urease expression and nitrogen flux in the development of gut dysbiosis suggests that bacterial urease may be a potential therapeutic target for inflammatory bowel diseases.

Renal oxidative stress induced by long-term hyperuricemia alters mitochondrial function and maintains systemic hypertension

We addressed if oxidative stress in the renal cortex plays a role in the induction of hypertension and mitochondrial alterations in hyperuricemia. A second objective was to evaluate whether the long-term treatment with the antioxidant Tempol prevents renal oxidative stress, mitochondrial alterations, and systemic hypertension in this model. Long-term (11-12 weeks) and short-term (3 weeks) effects of oxonic acid induced hyperuricemia were studied in rats (OA, 750 mg/kg BW), OA+Allopurinol (AP, 150 mg/L drinking water), OA+Tempol (T, 15 mg/kg BW), or vehicle. Systolic blood pressure, renal blood flow, and vascular resistance were measured. Tubular damage (urine N-acetyl-β-D-glucosaminidase) and oxidative stress markers (lipid and protein oxidation) along with ATP levels were determined in kidney tissue. Oxygen consumption, aconitase activity, and uric acid were evaluated in isolated mitochondria from renal cortex. Short-term hyperuricemia resulted in hypertension without demonstrable renal oxidative stress or mitochondrial dysfunction. Long-term hyperuricemia induced hypertension, renal vasoconstriction, tubular damage, renal cortex oxidative stress, and mitochondrial dysfunction and decreased ATP levels. Treatments with Tempol and allopurinol prevented these alterations. Renal oxidative stress induced by hyperuricemia promoted mitochondrial functional disturbances and decreased ATP content, which represent an additional pathogenic mechanism induced by chronic hyperuricemia. Hyperuricemia-related hypertension occurs before these changes are evident.

The molecular and metabolic influence of long term agmatine consumption

Agmatine (AGM), a product of arginine decarboxylation, influences multiple physiologic and metabolic functions. However, the mechanism(s) of action, the impact on whole body gene expression and metabolic pathways, and the potential benefits and risks of long term AGM consumption are still a mystery. Here, we scrutinized the impact of AGM on whole body metabolic profiling and gene expression and assessed a plausible mechanism(s) of AGM action. Studies were performed in rats fed a high fat diet or standard chow. AGM was added to drinking water for 4 or 8 weeks. We used (13)C or (15)N tracers to assess metabolic reactions and fluxes and real time quantitative PCR to determine gene expression. The results demonstrate that AGM elevated the synthesis and tissue level of cAMP. Subsequently, AGM had a widespread impact on gene expression and metabolic profiling including (a) activation of peroxisomal proliferator-activated receptor-α and its coactivator, PGC1α, and (b) increased expression of peroxisomal proliferator-activated receptor-γ and genes regulating thermogenesis, gluconeogenesis, and carnitine biosynthesis and transport. The changes in gene expression were coupled with improved tissue and systemic levels of carnitine and short chain acylcarnitine, increased β-oxidation but diminished incomplete fatty acid oxidation, decreased fat but increased protein mass, and increased hepatic ureagenesis and gluconeogenesis but decreased glycolysis. These metabolic changes were coupled with reduced weight gain and a curtailment of the hormonal and metabolic derangements associated with high fat diet-induced obesity. The findings suggest that AGM elevated the synthesis and levels of cAMP, thereby mimicking the effects of caloric restriction with respect to metabolic reprogramming.

Kidney biomimicry--a rediscovered scientific field that could provide hope to patients with kidney disease

Most studies on kidney disease have relied on classic experimental studies in mice and rats or clinical studies in humans. From such studies much understanding of the physiology and pathophysiology of kidney disease has been obtained. However, breakthroughs in the prevention and treatment of kidney diseases have been relatively few, and new approaches to fight kidney disease are needed. Here we discuss kidney biomimicry as a new approach to understand kidney disease. Examples are given of how various animals have developed ways to prevent or respond to kidney failure, how to protect themselves from hypoxia or oxidative stress and from the scourge of hyperglycemia. We suggest that investigation of evolutionary biology and comparative physiology might provide new insights for the prevention and treatment of kidney disease.

Ammonium metabolism in humans

Free ammonium ions are produced and consumed during cell metabolism. Glutamine synthetase utilizes free ammonium ions to produce glutamine in the cytosol whereas glutaminase and glutamate dehydrogenase generate free ammonium ions in the mitochondria from glutamine and glutamate, respectively. Ammonia and bicarbonate are condensed in the liver mitochondria to yield carbamoylphosphate initiating the urea cycle, the major mechanism of ammonium removal in humans. Healthy kidney produces ammonium which may be released into the systemic circulation or excreted into the urine depending predominantly on acid-base status, so that metabolic acidosis increases urinary ammonium excretion while metabolic alkalosis induces the opposite effect. Brain and skeletal muscle neither remove nor produce ammonium in normal conditions, but they are able to seize ammonium during hyperammonemia, releasing glutamine. Ammonia in gas phase has been detected in exhaled breath and skin, denoting that these organs may participate in nitrogen elimination. Ammonium homeostasis is profoundly altered in liver failure resulting in hyperammonemia due to the deficient ammonium clearance by the diseased liver and to the development of portal collateral circulation that diverts portal blood with high ammonium content to the systemic blood stream. Although blood ammonium concentration is usually elevated in liver disease, a substantial role of ammonium causing hepatic encephalopathy has not been demonstrated in human clinical studies. Hyperammonemia is also produced in urea cycle disorders and other situations leading to either defective ammonium removal or overproduction of ammonium that overcomes liver clearance capacity. Most diseases resulting in hyperammonemia and cerebral edema are preceded by hyperventilation and respiratory alkalosis of unclear origin that may be caused by the intracellular acidosis occurring in these conditions.

Effects of a glucokinase activator on hepatic intermediary metabolism: study with 13C-isotopomer-based metabolomics

GKAs (glucokinase activators) are promising agents for the therapy of Type 2 diabetes, but little is known about their effects on hepatic intermediary metabolism. We monitored the fate of (13)C-labelled glucose in both a liver perfusion system and isolated hepatocytes. MS and NMR spectroscopy were deployed to measure isotopic enrichment. The results demonstrate that the stimulation of glycolysis by GKA led to numerous changes in hepatic metabolism: (i) augmented flux through the TCA (tricarboxylic acid) cycle, as evidenced by greater incorporation of (13)C into the cycle (anaplerosis) and increased generation of (13)C isotopomers of citrate, glutamate and aspartate (cataplerosis); (ii) lowering of hepatic [Pi] and elevated [ATP], denoting greater phosphorylation potential and energy state; (iii) stimulation of glycogen synthesis from glucose, but inhibition of glycogen synthesis from 3-carbon precursors; (iv) increased synthesis of N-acetylglutamate and consequently augmented ureagenesis; (v) increased synthesis of glutamine, alanine, serine and glycine; and (vi) increased production and outflow of lactate. The present study provides a deeper insight into the hepatic actions of GKAs and uncovers the potential benefits and risks of GKA for treatment of diabetes. GKA improved hepatic bioenergetics, ureagenesis and glycogenesis, but decreased gluconeogenesis with a potential risk of lactic acidosis and fatty liver.

Mitochondrial roles and cytoprotection in chronic liver injury

The liver is one of the richest organs in terms of number and density of mitochondria. Most chronic liver diseases are associated with the accumulation of damaged mitochondria. Hepatic mitochondria have unique features compared to other organs' mitochondria, since they are the hub that integrates hepatic metabolism of carbohydrates, lipids and proteins. Mitochondria are also essential in hepatocyte survival as mediator of apoptosis and necrosis. Hepatocytes have developed different mechanisms to keep mitochondrial integrity or to prevent the effects of mitochondrial lesions, in particular regulating organelle biogenesis and degradation. In this paper, we will focus on the role of mitochondria in liver physiology, such as hepatic metabolism, reactive oxygen species homeostasis and cell survival. We will also focus on chronic liver pathologies, especially those linked to alcohol, virus, drugs or metabolic syndrome and we will discuss how mitochondria could provide a promising therapeutic target in these contexts.

Carglumic acid for the treatment of N-acetylglutamate synthase deficiency and acute hyperammonemia

Carglumic acid is a structural analog and the first registered synthetic form of the naturally occurring allosteric activator of the urea cycle, N-acetylglutamate (NAG), which is the product of the enzyme NAG synthase (NAGS). Because NAG is essential for the function of carbamoylphosphate synthetase 1 as the first step of the urea cycle, a decreased availability of NAG due to primary or secondary defects of NAGS will affect ammonia detoxification in the urea cycle. Carglumic acid (Carbaglu®, Orphan Europe SARL, Paris, France) is approved for the acute and long-term treatment of primary defects of NAGS in Europe and the USA. In addition, it is approved in Europe for the treatment of acute hyperammonemia in patients with specific organic acidurias that can lead to NAG deficiency secondary to inhibition of NAGS. This article reviews the use of carglumic acid for both approved indications and considers the potential of this compound for acute hyperammonemias in general.