ERK1/2 pathway is involved in renal gluconeogenesis inhibition under conditions of lowered NADPH oxidase activity

Liver or kidney: Who has the oar in the gluconeogenesis boat and when?

Gluconeogenesis is an endogenous process of glucose production from non-carbohydrate carbon substrates. Both the liver and kidneys express the key enzymes necessary for endogenous glucose production and its export into circulation. We would be remiss to add that more recently gluconeogenesis has been described in the small intestine, especially under high-protein, low-carbohydrate diets. The contribution of the liver glucose release, the net glucose flux, towards systemic glucose is already well known. The liver is, in most instances, the primary bulk contributor due to the sheer size of the organ (on average, over 1 kg). The contribution of the kidney (at just over 100 g each) to endogenous glucose production is often under-appreciated, especially on a weight basis. Glucose is released from the liver through the process of glycogenolysis and gluconeogenesis. Renal glucose release is almost exclusively due to gluconeogenesis, which occurs in only a fraction of the cells in that organ (proximal tubule cells). Thus, the efficiency of glucose production from other carbon sources may be superior in the kidney relative to the liver or at least on the level. In both these tissues, gluconeogenesis regulation is under tight hormonal control and depends on the availability of substrates. Liver and renal gluconeogenesis are differentially regulated under various pathological conditions. The impact of one source vs the other changes, based on post-prandial state, acid-base balance, hormonal status, and other less understood factors. Which organ has the oar (is more influential) in driving systemic glucose homeostasis is still in-conclusive and likely changes with the daily rhythms of life. We reviewed the literature on the differences in gluconeogenesis regulation between the kidneys and the liver to gain an insight into who drives the systemic glucose levels under various physiological and pathological conditions.

“Greedy Organs Hypothesis” for sugar and salt in the pathophysiology of non-communicable diseases in relation to sodium-glucose co-transporters in the intestines and the kidney

Deposition of visceral fat and insulin resistance play central role in the development of non-communicable diseases (NCDs) including obesity, hypertension and type 2 diabetes. However, we shed more light upon the intestines and the kidney as a strong driver of NCDs. Based upon unexpected outcomes of clinical trials using sodium-glucose cotransporter (SGLT) 2 inhibitors to demonstrate their actions for not only body weight reduction and blood glucose fall but also remarkable cardiorenal protection, we speculate that hyperfunction of the intestines and the kidney is one of critical contributing factors for initiation of NCDs. By detecting high amount of glucose and sodium chloride around them by sweet/salt taste sensors, the intestines and the kidney are designed to (re)absorb these nutrients by up-regulating SGLT1 or SGLT2. We designate these hyperfunctioning organs for nutrient uptake as “greedy organs”. The greedy organs can induce NCDs (“greedy organ hypothesis”). SGLTs are regulated by glucose and sodium chloride, and SGLTs or other genes can be “greedy genes.” Regulating factors for greedy organs are renin-angiotensin system, renal sympathetic nervous activity, gut inflammation/microbiota or oxidative stress. Mitigation of organ greediness by SGLT2 inhibitors, ketone bodies, bariatric surgery, and regular lifestyle to keep rhythmicity of biological clock are promising.

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We propose the concept of “Greedy Organs” hypothesis as a possible cause of NCDs.

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Clinical implication of greedy kidney is supported by the effect of SGLT2 inhibitors.

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The significance of greedy intestines is suggested by the effect of bariatric surgery.

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The intestines and kidney become hyperactive through upregulation of SGLT1 or 2.

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To mitigate “greedy organs” should be a promising strategy against NCDs.

Renal gluconeogenesis in insulin resistance: A culprit for hyperglycemia in diabetes

Renal gluconeogenesis is one of the major pathways for endogenous glucose production. Impairment in this process may contribute to hyperglycemia in cases with insulin resistance and diabetes. We reviewed pertinent studies to elucidate the role of renal gluconeogenesis regulation in insulin resistance and diabetes. A consensus on the suppressive effect of insulin on kidney gluconeogenesis has started to build up. Insulin-resistant models exhibit reduced insulin receptor (IR) expression and/or post-receptor signaling in their kidney tissue. Reduced IR expression or post-receptor signaling can cause impairment in insulin’s action on kidneys, which may increase renal gluconeogenesis in the state of insulin resistance. It is now established that the kidney contributes up to 20% of all glucose production via gluconeogenesis in the post-absorptive phase. However, the rate of renal glucose release excessively increases in diabetes. The rise in renal glucose release in diabetes may contribute to fasting hyperglycemia and increased postprandial glucose levels. Enhanced glucose release by the kidneys and renal expression of the gluconeogenic-enzyme in diabetic rodents and humans further point towards the significance of renal gluconeogenesis. Overall, the available literature suggests that impairment in renal gluconeogenesis in an insulin-resistant state may contribute to hyperglycemia in type 2 diabetes.

Potential benefits and harms of NADPH oxidase type 4 in the kidneys and cardiovascular system

The main function of NADPH oxidases is to catalyse the formation of reactive oxygen species (ROS). NADPH oxidase 4 (NOX4) is expressed at high levels in kidney tubular cells, and at lower levels in endothelial cells, cardiomyocytes and other cell types under physiological conditions. NOX4 is constitutively active producing hydrogen peroxide (H2O2) as the prevalent ROS detected, whereas other NOX isoforms present in the renal and cardiovascular systems (i.e. NOX1, NOX2 and NOX5) generate superoxide radical anions as main products. Pharmacological inhibition of NOX4 has received enormous attention for its potential therapeutic benefit in fibrotic disease and nephropathologies. Ongoing clinical trials are testing this approach in humans. Diabetes elevates NOX4 expression in podocytes and mesangial cells, which was shown to damage glomeruli leading to podocyte loss, mesangial cell hypertrophy and matrix accumulation. Consequently, NOX4 represents an interesting therapeutic target in diabetic nephropathy. On the contrary, experiments using NOX4-deficient mice have shown that NOX4 is cytoprotective in tubular cells, cardiomyocytes, endothelial cells and vascular smooth muscle cells, and has a metabolism-regulating role when these cells are subjected to injury. Mice with systemic NOX4 deletion are more susceptible to acute and chronic tubular injury, heart failure and atherosclerosis. Overall, the current literature suggests a detrimental role of increased NOX4 expression in mesangial cells and podocytes during diabetic nephropathy, but a cytoprotective role of this enzyme in other cellular types where it is expressed endogenously. We review here the recent evidence on the role of NOX4 in the kidneys and cardiovascular system. With the emergence of pharmacological NOX4 inhibitors in clinical trials, caution should be taken in identifying potential side effects in patients prone to acute kidney injury and cardiovascular disease.

Effect of Insulin on Proximal Tubules Handling of Glucose: A Systematic Review

Renal proximal tubules reabsorb glucose from the glomerular filtrate and release it back into the circulation. Modulation of glomerular filtration and renal glucose disposal are some of the insulin actions, but little is known about a possible insulin effect on tubular glucose reabsorption. This review is aimed at synthesizing the current knowledge about insulin action on glucose handling by proximal tubules. Method. A systematic article selection from Medline (PubMed) and Embase between 2008 and 2019. 180 selected articles were clustered into topics (renal insulin handling, proximal tubule glucose transport, renal gluconeogenesis, and renal insulin resistance). Summary of Results. Insulin upregulates its renal uptake and degradation, and there is probably a renal site-specific insulin action and resistance; studies in diabetic animal models suggest that insulin increases renal SGLT2 protein content; in vivo human studies on glucose transport are few, and results of glucose transporter protein and mRNA contents are conflicting in human kidney biopsies; maximum renal glucose reabsorptive capacity is higher in diabetic patients than in healthy subjects; glucose stimulates SGLT1, SGLT2, and GLUT2 in renal cell cultures while insulin raises SGLT2 protein availability and activity and seems to directly inhibit the SGLT1 activity despite it activating this transporter indirectly. Besides, insulin regulates SGLT2 inhibitor bioavailability, inhibits renal gluconeogenesis, and interferes with Na⁺K⁺ATPase activity impacting on glucose transport. Conclusion. Available data points to an important insulin participation in renal glucose handling, including tubular glucose transport, but human studies with reproducible and comparable method are still needed.

Dibutyl phthalate-induced activation of ROS and ERK1/2 causes hepatic and renal damage in Kunming mice

Dibutyl phthalate (DBP), a ubiquitous environmental contaminant, has been reported to be involved in hepatic and renal tissue damage. However, the role of DBP in oxidative stress and in extracellular signal-regulated kinase (ERK1/2) pathways remains unclear. To investigate the underlying mechanism, Kunming (KM) mice received daily doses of combinations of 50 mg/kg DBP, 50 mg/kg vitamin E (VitE), and 1 mg/kg PD98059 for 28 consecutive days. Any changes in reactive oxygen species (ROS) and malondialdehyde (MDA) levels, as well as any histopathological alterations in tissues, were observed to assess oxidative stress. In addition, the levels of alanine aminotransferase, aspartate aminotransferase, and albumin in serum were used to evaluate liver function. The levels of creatinine and urea nitrogen in serum were measured to evaluate kidney function. We found that DBP significantly increased oxidative damage and the expression of phosphorylated ERK1/2. Furthermore, pretreatment with the ERK inhibitor PD98059 followed by the antioxidant VitE attenuated the levels of ROS, MDA, ERK1/2 phosphorylation, and DBP-mediated disorders, indicating that the oxidative stress and the ERK1/2 pathways are associated with DBP-induced hepatic and renal dysfunction in KM mice.

Molecular signaling mechanisms of renal gluconeogenesis in nondiabetic and diabetic conditions

The kidneys are as involved as the liver in gluconeogenesis which can significantly contribute to hyperglycemia in the diabetic condition. Substantial evidence has demonstrated the overexpression of rate-limiting gluconeogenic enzymes, especially phosphoenolpyruvate carboxykinase and glucose 6 phosphatase, and the accelerated glucose release both in the isolated proximal tubular cells and in the kidneys of diabetic animal models and diabetic patients. The aim of this review is to provide an insight into the mechanisms that accelerate renal gluconeogenesis in the diabetic conditions and the therapeutic approaches that could affect this process in the kidney. Increase in gluconeogenic substrates, reduced insulin concentration or insulin resistance, downregulation of insulin receptors and insulin signaling, oxidative stress, and inappropriate activation of the renin-angiotensin system are likely to participate in enhancing renal gluconeogenesis in the diabetic milieu. Several studies have suggested that controlling glucose metabolism at the renal level favors effective overall glycemic control in both type 1 and type 2 diabetes. Therefore, renal gluconeogenesis may be a promising target for effective glycemic control as a therapeutic strategy in diabetes.

Schisandrin rescues depressive-like behaviors induced by chronic unpredictable mild stress via GDNF/ERK1/2/ROS and PI3K/AKT/NOX signaling pathways in mice

The current study aimed to prove the antidepressant-like effects and the probable mechanisms of Schisandrin on depression, which induced by chronic unpredictable mild stress (CUMS) in mice. Four weeks of CUMS exposure resulted in depressive-like behavior, as indicated by the significant decrease in sucrose consumption and increase the immobility time in the forced swim test, but without any influence on the locomotor activity. Further, there were significant downregulations of GDNF/ERK1/2/ROS and PI3K/AKT/NOX signaling pathways in the hippocampus and prefrontal cortex in depressed mice. Treatment of mice with Schisandrin (30mg/kg) and Fluoxetine (10mg/kg) significantly ameliorated all the behavioral and biochemical changes induced by CUMS. These results suggest that Schisandrin produces an antidepressant-like effect in CUMS-induced mice, which possibly mediated, at least in part, by rectifying the signaling pathways of GDNF/ERK1/2/ROS and PI3K/AKT/NOX.

Diabetes and Kidney Disease: Role of Oxidative Stress

Intrarenal oxidative stress plays a critical role in the initiation and progression of diabetic kidney disease (DKD). Enhanced oxidative stress results from overproduction of reactive oxygen species (ROS) in the context of concomitant, insufficient antioxidant pathways. Renal ROS production in diabetes is predominantly mediated by various NADPH oxidases (NOXs), but a defective antioxidant system as well as mitochondrial dysfunction may also contribute. Recent Advances: Effective agents targeting the source of ROS generation hold the promise to rescue the kidney from oxidative damage and prevent subsequent progression of DKD. Critical Issues and Future Directions: In the present review, we summarize and critically analyze molecular and cellular mechanisms that have been demonstrated to be involved in NOX-induced renal injury in diabetes, with particular focus on the role of increased glomerular injury, the development of albuminuria, and tubulointerstitial fibrosis, as well as mitochondrial dysfunction. Furthermore, novel agents targeting NOX isoforms are discussed. Antioxid. Redox Signal. 25, 657-684.

Neurofibromin is a novel regulator of Ras-induced reactive oxygen species production in mice and humans

Neurofibromatosis type 1 (NF1) predisposes individuals to early and debilitating cardiovascular disease. Loss of function mutations in the NF1 tumor suppressor gene, which encodes the protein neurofibromin, leads to accelerated p21(Ras) activity and phosphorylation of multiple downstream kinases, including Erk and Akt. Nf1 heterozygous (Nf1(+/-)) mice develop a robust neointima that mimics human disease. Monocytes/macrophages play a central role in NF1 arterial stenosis as Nf1 mutations in myeloid cells alone are sufficient to reproduce the enhanced neointima observed in Nf1(+/-) mice. Though the molecular mechanisms underlying NF1 arterial stenosis remain elusive, macrophages are important producers of reactive oxygen species (ROS) and Ras activity directly regulates ROS production. Here, we use compound mutant and lineage-restricted mice to demonstrate that Nf1(+/-) macrophages produce excessive ROS, which enhance Nf1(+/-) smooth muscle cell proliferation in vitro and in vivo. Further, use of a specific NADPH oxidase-2 inhibitor to limit ROS production prevents neointima formation in Nf1(+/-) mice. Finally, mononuclear cells from asymptomatic NF1 patients have increased oxidative DNA damage, an indicator of chronic exposure to oxidative stress. These data provide genetic and pharmacologic evidence that excessive exposure to oxidant species underlie NF1 arterial stenosis and provide a platform for designing novels therapies and interventions.

Enantioselective synthesis and antioxidant activity of 3,4,5-substituted piperidine derivatives

In this study, 3,4,5-trisubstituted piperidines were synthesized enantioselectively, and their antioxidant activity was evaluated. The 3,4,5-trisubstituted piperidines containing TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) and a spatially proximal hydroxy group showed good antioxidant activity. Some of these compounds showed IC50 values in a nanomolar range, comparable to that of TEMPO. Probably the TEMPO generated from the homolysis of the CON bond of 3,4,5-trisubstituted piperidines functions as a radical-scavenging entity, and the hydroxy group of piperidines has a synergistic effect to the antioxidant activity.

Melatonin nephroprotective action in Zucker diabetic fatty rats involves its inhibitory effect on NADPH oxidase

Excessive activity of NADPH oxidase (Nox) is considered to be of importance for the progress of diabetic nephropathy. The aim of the study was to elucidate the effect of melatonin, known for its nephroprotective properties, on Nox activity under diabetic conditions. The experiments were performed on three groups of animals: (i) untreated lean (?/+) Zucker diabetic fatty (ZDF) rats; (ii) untreated obese diabetic (fa/fa) ZDF rats; and (iii) ZDF fa/fa rats treated with melatonin (20 mg/L) in drinking water. Urinary albumin excretion was measured weekly. After 4 wk of the treatment, the following parameters were determined in kidney cortex: Nox activity, expression of subunits of the enzyme, their phosphorylation and subcellular distribution. Histological studies were also performed. Compared to ?/+ controls, ZDF fa/fa rats exhibited increased renal Nox activity, augmented expression of Nox4 and p47(phox) subunits, elevated level of p47(phox) phosphorylation, and enlarged phospho-p47(phox) and p67(phox) content in membrane. Melatonin administration to ZDF fa/fa rats resulted in the improvement of renal functions, as manifested by considerable attenuation of albuminuria and some amelioration of structural abnormalities. The treatment turned out to nearly normalize Nox activity, which was accompanied by considerably lowered expression and diminished membrane distribution of regulatory subunits, that is, phospho-p47(phox) and p67(phox) . Thus, it is concluded that: (i) melatonin beneficial action against diabetic nephropathy involves attenuation of the excessive activity of Nox; and (ii) the mechanism of melatonin inhibitory effect on Nox is based on the mitigation of expression and membrane translocation of its regulatory subunits.