Can curcumin supplementation break the vicious cycle of inflammation, oxidative stress, and uremia in patients undergoing peritoneal dialysis?

BACKGROUND & AIMS: Turmeric (a source of curcumin) is an excellent food to modulate oxidative stress, inflammation, and gut dysbiosis in patients with chronic kidney disease (CKD). However, no studies report the benefits of curcumin in patients undergoing peritoneal dialysis (PD). This study aims to evaluate the effects of curcuminoid supplementation on oxidative stress, inflammatory markers, and uremic toxins originating from gut microbiota in patients with CKD undergoing PD. METHODS: This longitudinal, randomized, single-blind, placebo-controlled trial evaluated 48 patients who were randomized into two groups: Curcumin (three capsules of 500 mg of Curcuma longa extract, with 98.42 % total curcuminoids) or placebo (three capsules of 500 mg of starch) for twelve weeks. In the peripheral blood mononuclear cells (PBMCs), the transcriptional expression levels of Nrf2, HOX-1 and NF-κB were evaluated by quantitative real-time PCR. Oxidative stress was evaluated by malondialdehyde (MDA) and total Thiol (T-SH). TNF-α and IL-6 plasma levels were measured by ELISA. P-cresyl sulphate plasma level, a uremic toxin, was evaluated by high-performance liquid chromatography (HPLC) with fluorescent detection. RESULTS: Twenty-four patients finished the study: 10 in the curcumin group (57.5 ± 11.6 years) and 14 in the placebo group (56.5 ± 10.0 years). The plasma levels of MDA were reduced after 12 weeks in the curcumin group (p = 0.01), while the placebo group remained unchanged. However, regarding the difference between the groups at the endpoint, no change was observed in MDA. Still, there was a trend to reduce the p-CS plasma levels in the curcumin group compared to the placebo group (p = 0.07). Likewise, the concentrations of protein thiols, mRNA expression of Nrf2, HOX-1, NF-κB, and cytokines plasma levels did not show significant changes. CONCLUSION: Curcuminoid supplementation for twelve weeks attenuates lipid peroxidation and might reduce uremic toxin in patients with CKD undergoing PD. This study was registered on Clinicaltrials.gov as NCT04413266.

PAS kinase is a nutrient and energy sensor in hypothalamic areas required for the normal function of AMPK and mTOR/S6K1.

The complications caused by overweight, obesity and type 2 diabetes are one of the main problems that increase morbidity and mortality in developed countries. Hypothalamic metabolic sensors play an important role in the control of feeding and energy homeostasis. PAS kinase (PASK) is a nutrient sensor proposed as a regulator of glucose metabolism and cellular energy. The role of PASK might be similar to other known metabolic sensors, such as AMP-activated protein kinase (AMPK) and the mammalian target of rapamycin (mTOR). PASK-deficient mice resist diet-induced obesity. We have recently reported that AMPK and mTOR/S6K1 pathways are regulated in the ventromedial and lateral hypothalamus in response to nutritional states, being modulated by anorexigenic glucagon-like peptide-1 (GLP-1)/exendin-4 in lean and obese rats. We identified PASK in hypothalamic areas, and its expression was regulated under fasting/re-feeding conditions and modulated by exendin-4. Furthermore, PASK-deficient mice have an impaired activation response of AMPK and mTOR/S6K1 pathways. Thus, hypothalamic AMPK and S6K1 were highly activated under fasted/re-fed conditions. Additionally, in this study, we have observed that the exendin-4 regulatory effect in the activity of metabolic sensors was lost in PASK-deficient mice, and the anorexigenic properties of exendin-4 were significantly reduced, suggesting that PASK could be a mediator in the GLP-1 signalling pathway. Our data indicated that the PASK function could be critical for preserving the nutrient effect on AMPK and mTOR/S6K1 pathways and maintain the regulatory role of exendin-4 in food intake. Some of the antidiabetogenic effects of exendin-4 might be modulated through these processes.

PAS kinase as a nutrient sensor in neuroblastoma and hypothalamic cells required for the normal expression and activity of other cellular nutrient and energy sensors.

PAS kinase (PASK) is a nutrient sensor that is highly conserved throughout evolution. PASK-deficient mice reveal a metabolic phenotype similar to that described in S6 kinase-1 S6K1-deficient mice that are protected against obesity. Hypothalamic metabolic sensors, such as AMP-activated protein kinase (AMPK) and the mammalian target of rapamycin (mTOR), play an important role in feeding behavior, the homeostasis of body weight, and energy balance. These sensors respond to changes in nutrient levels in the hypothalamic areas involved in feeding behavior and in neuroblastoma N2A cells, and we have recently reported that those effects are modulated by the anorexigenic peptide glucagon-like peptide-1 (GLP-1). Here, we identified PASK in both N2A cells and rat VMH and LH areas and found that its expression is regulated by glucose and GLP-1. High levels of glucose decreased Pask gene expression. Furthermore, PASK-silenced N2A cells record an impaired response by the AMPK and mTOR/S6K1 pathways to changes in glucose levels. Likewise, GLP-1 effect on the activity of AMPK, S6K1, and other intermediaries of both pathways and the regulatory role at the level of gene expression were also blocked in PASK-silenced cells. The absence of response to low glucose concentrations in PASK-silenced cells correlates with increased ATP content, low expression of mRNA coding for AMPK upstream kinase LKB1, and enhanced activation of S6K1. Our findings indicate that, at least in N2A cells, PASK is a key kinase in GLP-1 actions and exerts a coordinated response with the other metabolic sensors, suggesting that PASK might play an important role in feeding behavior.

Insulin-receptor substrate-2 (irs-2) is required for maintaining glucokinase and glucokinase regulatory protein expression in mouse liver.

Insulin receptor substrate (IRS) proteins play important roles in hepatic nutrient homeostasis. Since glucokinase (GK) and glucokinase regulatory protein (GKRP) function as key glucose sensors, we have investigated the expression of GK and GKRP in liver of Irs-2 deficient mice and Irs2(-/-) mice where Irs2 was reintroduced specifically into pancreatic ß-cells [RIP-Irs-2/IRS-2(-/-)]. We observed that liver GK activity was significantly lower (p<0.0001) in IRS-2(-/-) mice. However, in RIP-Irs-2/IRS-2(-/-) mice, GK activity was similar to the values observed in wild-type animals. GK activity in hypothalamus was not altered in IRS-2(-/-) mice. GK and GKRP mRNA levels in liver of IRS-2(-/-) were significantly lower, whereas in RIP-Irs-2/IRS-2(-/-) mice, both GK and GKRP mRNAs levels were comparable to wild-type animals. At the protein level, the liver content of GK was reduced in IRS-2(-/-) mice as compared with controls, although GKRP levels were similar between these experimental models. Both GK and GKRP levels were lower in RIP-Irs-2/IRS-2(-/-) mice. These results suggest that IRS-2 signalling is important for maintaining the activity of liver GK. Moreover, the differences between liver and brain GK may be explained by the fact that expression of hepatic, but not brain, GK is controlled by insulin. GK activity was restored by the ß-cell compensation in the RIP-Irs-2/IRS-2 mice. Interestingly, GK and GKRP protein expression remained low in RIP-Irs-2/IRS-2(-/-) mice, perhaps reflecting different mRNA half-lives or alterations in the process of translation and post-translational regulation.

Glucagon-like peptide 1 (GLP-1) can reverse AMP-activated protein kinase (AMPK) and S6 kinase (P70S6K) activities induced by fluctuations in glucose levels in hypothalamic areas involved in feeding behaviour.

The anorexigenic peptide, glucagon-like peptide-1 (GLP-1), reduces glucose metabolism in the human hypothalamus and brain stem. The brain activity of metabolic sensors such as AMP-activated protein kinase (AMPK) responds to changes in glucose levels. The mammalian target of rapamycin (mTOR) and its downstream target, p70S6 kinase (p70S6K), integrate nutrient and hormonal signals. The hypothalamic mTOR/p70S6K pathway has been implicated in the control of feeding and the regulation of energy balances. Therefore, we investigated the coordinated effects of glucose and GLP-1 on the expression and activity of AMPK and p70S6K in the areas involved in the control of feeding. The effect of GLP-1 on the expression and activities of AMPK and p70S6K was studied in hypothalamic slice explants exposed to low- and high-glucose concentrations by quantitative real-time RT-PCR and by the quantification of active-phosphorylated protein levels by immunoblot. In vivo, the effects of exendin-4 on hypothalamic AMPK and p70S6K activation were analysed in male obese Zucker and lean controls 1 h after exendin-4 injection to rats fasted for 48 h or after re-feeding for 2-4 h. High-glucose levels decreased the expression of Ampk in the lateral hypothalamus and treatment with GLP-1 reversed this effect. GLP-1 treatment inhibited the activities of AMPK and p70S6K when the activation of these protein kinases was maximum in both the ventromedial and lateral hypothalamic areas. Furthermore, in vivo s.c. administration of exendin-4 modulated AMPK and p70S6K activities in those areas, in both fasted and re-fed obese Zucker and lean control rats.

Leptin but not neuropeptide Y up-regulated glucagon-like peptide 1 receptor expression in GT1-7 cells and rat hypothalamic slices.

In an attempt to gain better insight into the central effects of glucagon-like peptide (GLP-1), we studied the action of glucose and of regulatory peptides on the expression of its receptor (GLP-1R) in hypothalamic GT1-7 cells and in ventromedial (VMH) and lateral (LH) rat hypothalamus slices. The promoter activity of GLP-1R in transfected GT1-7 cells increased with leptin, whereas neuropeptide Y (NPY) did not modify it. Interestingly, when cells were incubated with both NPY and leptin, NPY blocked the stimulating effect of leptin. The effects of leptin and NPY were also confirmed at messenger RNA levels. In hypothalamic slices, GLP-1R messenger RNA levels increased at higher glucose concentrations in the VMH. In addition, leptin exerted a stimulating effect; and NPY did not modify receptor expression. By contrast, in the LH, the opposite effects were found for those parameters, except at 20 mmol/L glucose. These findings suggest that the stimulating effect of leptin on GLP-1R expression, with no changes in NPY-induced activity, could enhance the anorexic actions generated through this receptor. In addition, the different responses of the VMH and LH may be related to specific functions of these structures, as already known in vivo, highlighting the interest of hypothalamic slices for this kind of study.

Effects of glucose and insulin on glucokinase activity in rat hypothalamus.

In an attempt to study the role of glucokinase (GK) and the effects of glucose and peptides on GK gene expression and on the activity of this enzyme in the hypothalamus, we used two kinds of biological models: hypothalamic GT1-7 cells and rat hypothalamic slices. The expression of the GK gene in GT1-7 cells was reduced by insulin (INS) and was not modified by different glucose concentrations, while GK enzyme activities were significantly reduced by the different peptides. Interestingly, a distinctive pattern of GK activities between the ventromedial hypothalamus (VMH) and lateral hypothalamus (LH) were found, with higher enzyme activities in the VMH as the glucose concentrations rose, while LH enzyme activities decreased at 2.8 and 20 mM glucose, the latter effect being prevented by incubation with INS. These effects were produced only by d-glucose and the modifications found were due to GK, but not to other hexokinases. In addition, GK activities in the VMH and the LH were reduced by glucagon-like peptide 1, leptin, orexin B, INS, and neuropeptide Y (NPY), but this effect was only statistically significant for NPY in LH. Our results indicate that the effects of both glucose and peptides occur on GK enzyme activities rather than on GK gene transcription. Moreover, the effects of glucose and INS on GK activity suggest that in the brain GK behaves in a manner opposite to that in the liver, which might facilitate its role in glucose sensing. Finally, hypothalamic slices seem to offer a good physiological model to discriminate the effects between different areas.

The expression of GLP-1 receptor mRNA and protein allows the effect of GLP-1 on glucose metabolism in the human hypothalamus and brainstem.

In the present work, several experimental approaches were used to determine the presence of the glucagon-like peptide-1 receptor (GLP-1R) and the biological actions of its ligand in the human brain. In situ hybridization histochemistry revealed specific labelling for GLP-1 receptor mRNA in several brain areas. In addition, GLP-1R, glucose transporter isoform (GLUT-2) and glucokinase (GK) mRNAs were identified in the same cells, especially in areas of the hypothalamus involved in feeding behaviour. GLP-1R gene expression in the human brain gave rise to a protein of 56 kDa as determined by affinity cross-linking assays. Specific binding of 125I-GLP-1(7-36) amide to the GLP-1R was detected in several brain areas and was inhibited by unlabelled GLP-1(7-36) amide, exendin-4 and exendin (9-39). A further aim of this work was to evaluate cerebral-glucose metabolism in control subjects by positron emission tomography (PET), using 2-[F-18] deoxy-D-glucose (FDG). Statistical analysis of the PET studies revealed that the administration of GLP-1(7-36) amide significantly reduced (p < 0.001) cerebral glucose metabolism in hypothalamus and brainstem. Because FDG-6-phosphate is not a substrate for subsequent metabolic reactions, the lower activity observed in these areas after peptide administration may be due to reduction of the glucose transport and/or glucose phosphorylation, which should modulate the glucose sensing process in the GLUT-2- and GK-containing cells.