Inhibition of hepatic glycogen synthesis by hyperhomocysteinemia mediated by TRB3

Xanthosine, a purine glycoside mediates hepatic glucose homeostasis through inhibition of gluconeogenesis and activation of glycogenesis via regulating the AMPK/ FoxO1/AKT/GSK3β signaling cascade

Type 2 Diabetes Mellitus (T2DM) is characterized by hepatic insulin resistance, which results in increased glucose production and reduced glycogen storage in the liver. There is no previous study in the literature that has explored the role of Xanthosine in hepatic insulin resistance. Moreover, mechanistic explanation for the beneficial effects of Xanthosine in lowering glucose production in diabetes is yet to be determined. This study for the first time investigated the beneficial effects of Tribulus terrestris (TT) and its active constituent, Xanthosine on gluconeogenesis and glycogenesis in Free Fatty Acid (FFA)-induced CC1 hepatocytes and streptozotocin (STZ)-induced Wistar rats. Xanthosine enhanced glucose uptake and decreased glucose production through phosphorylation of AMP-activated protein kinase (AMPK) and forkhead box transcription factor O1 (FoxO1), and downregulation of two rate limiting enzymes of gluconeogenesis, phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6Pase) expression in FFA-induced CC1 cells. Xanthosine also prevented FFA-induced decreases in the phosphorylation of AKT/Protein kinase B, glycogen synthase kinase-3β (GSK3β), and increased glycogen synthase (GS) phosphorylation to increase the glycogen content in the hepatocytes. Moreover, in STZ-induced diabetic rats, oral administration of TT n-butanol fraction (TTBF) enriched with compound Xanthosine (10, 50 & 100 mg/kg body weight) improved insulin sensitivity, reduced fasting blood glucose levels, improved glucose homeostasis by reducing gluconeogenesis via AMPK/FoxO1-mediated PEPCK and G6Pase down-regulation and increasing glycogenesis via AKT/GSK3β-mediated GS activation. Overall, Xanthosine may be developed further for treating insulin resistance and hyperglycemia in T2DM.

New insights into the regulation of Cystathionine beta synthase (CBS), an enzyme involved in intellectual deficiency in Down syndrome

Down syndrome (DS), the most frequent chromosomic aberration, results from the presence of an extra copy of chromosome 21. The identification of genes which overexpression contributes to intellectual disability (ID) in DS is important to understand the pathophysiological mechanisms involved and develop new pharmacological therapies. In particular, gene dosage of Dual specificity tyrosine phosphorylation Regulated Kinase 1A (DYRK1A) and of Cystathionine beta synthase (CBS) are crucial for cognitive function. As these two enzymes have lately been the main targets for therapeutic research on ID, we sought to decipher the genetic relationship between them. We also used a combination of genetic and drug screenings using a cellular model overexpressing CYS4, the homolog of CBS in Saccharomyces cerevisiae, to get further insights into the molecular mechanisms involved in the regulation of CBS activity. We showed that overexpression of YAK1, the homolog of DYRK1A in yeast, increased CYS4 activity whereas GSK3β was identified as a genetic suppressor of CBS. In addition, analysis of the signaling pathways targeted by the drugs identified through the yeast-based pharmacological screening, and confirmed using human HepG2 cells, emphasized the importance of Akt/GSK3β and NF-κB pathways into the regulation of CBS activity and expression. Taken together, these data provide further understanding into the regulation of CBS and in particular into the genetic relationship between DYRK1A and CBS through the Akt/GSK3β and NF-κB pathways, which should help develop more effective therapies to reduce cognitive deficits in people with DS.

Effects of myo-inositol, gymnemic acid, and L-methylfolate in polycystic ovary syndrome patients

Polycystic ovary syndrome (PCOS) is a heterogeneous endocrine and metabolic disorder, characterized by chronic anovulation/oligomenorrhea, hyperandrogenism, and insulin-resistance. Moreover, some studies propose a possible association between insulin resistance and hyperhomocysteinemia, which is a significant long-term risk for factor for atherogenesis and chronic vascular damage, especially in situations where insulin levels are increased. Insulin-sensitizing agents are used in the treatment of PCOS: in fact, inositols were shown to have insulin-mimetic properties. Synergic action to myo-inositol is that of gymnemic acids that have antidiabetic, anti-sweetener, and anti-inflammatory activities. Gymnemic acid formulations have also been found useful against obesity due to their ability to delay the glucose absorption in the blood. L-methyl-folate increases peripheral sensitivity to insulin, maintaining folatemia stable, and thus restoring normal homocysteine levels. Unlike folic acid, L-methyl folate has a higher bioavailability, no drug/food interferences, high absorption, and it is stable to UV-A exposure. The aim of our study is to compare the clinical, endocrine, and metabolic parameters in 100 PCOS women treated with myo-inositol, gymnemic acid, and l-methylfolate (Group A) or myo inositol and folic acid only (Group B), continuously for 6 months. From a clinical point of view, it was noticed a more significant improvement of the menstrual cycle regularity and a more significant reduction of BMI in Group A. Moreover, a more significant decrease of total testosterone and increase of SHBG serum levels were noticed in Group A. The metabolic assessment found a more significant decrease of total cholesterol and homocysteine levels; OGTT glycemia and insulinemia values were significantly more improved after treatment with myo-inositol + gymnemic acid. In conclusion, we can state that a good option for the treatment of PCOS is the combined administration of myo-inositol + gymnemic acid + l-methyl-folate, especially for overweight/obese patients with marked insulin resistance and with associated hyperhomocysteinemia.

Of the milk sugars, galactose, but not prebiotic galacto-oligosaccharide, improves insulin sensitivity in male Sprague-Dawley rats

BACKGROUND

Consumption of dairy products reduces risk of type 2 diabetes. Milk proteins and fats exhibit anti-diabetic properties but milk sugars have been studied little in this context. Galactose from milk lactose is readily converted to glycogen in the liver but its effects on insulin sensitivity have not been assessed. Prebiotic oligosaccharides from milk alter gut microbiota and can thereby influence host metabolism. Our objective was to assess the effect on insulin sensitivity of dietary galactose compared to glucose and fructose, and fermentable galacto-oligosaccharides compared to non-fermentable methylcellulose.

METHODS

Diets containing 15% of dry matter from glucose, fructose, galactose, galacto-oligosaccharides, or methylcellulose were fed to 36 rats per diet for 9 weeks. Hyperinsulinemic-euglycemic clamps with [3-3H]glucose infusion and a steady-state 2-[1-14C]deoxyglucose bolus injection were used to assess insulin sensitivity and glucose uptake indices. Tissue was collected in fed, fasted and fasted, insulin-stimulated states.

RESULTS

Galactose increased glucose infusion rate during the clamp by 53% and decreased endogenous glucose production by 57% compared to glucose and fructose. Fed-state hepatic glycogen content was greater with galactose compared to glucose and fructose, consistent with a potentiation of the insulin effect on glycogen synthase by dephosphorylation. Galactose decreased the fecal Firmicutes:Bacteroidetes ratio while galacto-oligosaccharides increased abundance of fecal Bifidobacterium spp. 481-fold compared to methylcellulose, and also increased abundance of Lactobacillus spp. and Bacteroidetes. Galacto-oligosaccharides did not affect glucose infusion rate or endogenous glucose production during basal or clamp periods compared to methylcellulose.

CONCLUSIONS

Galactose at 15% of daily intake improved hepatic insulin sensitivity in rats compared to glucose and fructose. Galactose caused an increase in fed-state hepatic glycogen content and a favourable shift in gut microbial populations. Intake of galacto-oligosaccharides improved the gut microbial profile but did not improve insulin sensitivity.

Exendin-4 regulates redox homeostasis in rats fed with high-fat diet

Non-alcoholic fatty liver disease (NAFLD) is associated with increased plasma homocysteine level, which is caused by down-regulation of hepatic cystathionine beta-synthase (CBS) activity. CBS catalyzes the first step in the transsulfuration of homocysteine to cysteine, which contributes ∼50% of the cysteine required for hepatic biosynthesis of glutathione (GSH), the most abundant antioxidant in cells. As the glucagon-like peptide-1 (GLP-1) receptor agonists (e.g. exendin-4) effectively reverse hepatic steatosis, the effect of exendin-4 on both homocysteine and redox status was investigated in the livers of rats fed with high-fat diet (HFD). It was found that HFD down-regulated CBS protein expression, which was probably due to induction of rno-miR-376c expression in the liver. The level of GSH was markedly reduced, whereas the level of malonydialdehyde, an indicator of lipid peroxidation, was significantly increased in the livers of rats fed with HFD. Exendin-4 treatment increased hepatic CBS protein and GSH levels, and reduced malonydialdehyde level in hyperlipidemic rats. Our findings suggest that GLP-1 receptor agonists have beneficial effects on redox homeostasis in NAFLD.

Systematic identification of signal integration by protein kinase A

Cellular processes and homeostasis control in eukaryotic cells is achieved by the action of regulatory proteins such as protein kinase A (PKA). Although the outbound signals from PKA directed to processes such as metabolism, growth, and aging have been well charted, what regulates this conserved regulator remains to be systematically identified to understand how it coordinates biological processes. Using a yeast PKA reporter assay, we identified genes that influence PKA activity by measuring protein-protein interactions between the regulatory and the two catalytic subunits of the PKA complex in 3,726 yeast genetic-deletion backgrounds grown on two carbon sources. Overall, nearly 500 genes were found to be connected directly or indirectly to PKA regulation, including 80 core regulators, denoting a wide diversity of signals regulating PKA, within and beyond the described upstream linear pathways. PKA regulators span multiple processes, including the antagonistic autophagy and methionine biosynthesis pathways. Our results converge toward mechanisms of PKA posttranslational regulation by lysine acetylation, which is conserved between yeast and humans and that, we show, regulates protein complex formation in mammals and carbohydrate storage and aging in yeast. Taken together, these results show that the extent of PKA input matches with its output, because this kinase receives information from upstream and downstream processes, and highlight how biological processes are interconnected and coordinated by PKA.

Corrective effects of hepatotoxicity by hepatic Dyrk1a gene delivery in mice with intermediate hyperhomocysteinemia

Hyperhomocysteinemia results from hepatic metabolism dysfunction and is characterized by a high plasma homocysteine level, which is also an independent risk factor for cardiovascular disease. Elevated levels of homocysteine in plasma lead to hepatic lesions and abnormal lipid metabolism. Therefore, lowering homocysteine levels might offer therapeutic benefits. Recently, we were able to lower plasma homocysteine levels in mice with moderate hyperhomocysteinemia using an adenoviral construct designed to restrict the expression of DYRK1A, a serine/threonine kinase involved in methionine metabolism (and therefore homocysteine production), to hepatocytes. Here, we aimed to extend our previous findings by analyzing the effect of hepatocyte-specific Dyrk1a gene transfer on intermediate hyperhomocysteinemia and its associated hepatic toxicity and liver dysfunction. Commensurate with decreased plasma homocysteine and alanine aminotransferase levels, targeted hepatic expression of DYRK1A in mice with intermediate hyperhomocysteinemia resulted in elevated plasma paraoxonase-1 and lecithin:cholesterol acyltransferase activities and apolipoprotein A-I levels. It also rescued hepatic apolipoprotein E, J, and D levels. Further, Akt/GSK3/cyclin D1 signaling pathways in the liver of treated mice were altered, which may help prevent homocysteine-induced cell cycle dysfunction. DYRK1A gene therapy could be useful in the treatment of hyperhomocysteinemia in populations, such as end-stage renal disease patients, who are unresponsive to B-complex vitamin therapy.

Hepatocyte-specific Dyrk1a gene transfer rescues plasma apolipoprotein A-I levels and aortic Akt/GSK3 pathways in hyperhomocysteinemic mice

Hyperhomocysteinemia, characterized by high plasma homocysteine levels, is recognized as an independent risk factor for cardiovascular diseases. The increased synthesis of homocysteine, a product of methionine metabolism involving B vitamins, and its slower intracellular utilization cause increased flux into the blood. Plasma homocysteine level is an important reflection of hepatic methionine metabolism and the rate of processes modified by B vitamins as well as different enzyme activity. Lowering homocysteine might offer therapeutic benefits. However, approximately 50% of hyperhomocysteinemic patients due to cystathionine-beta-synthase deficiency are biochemically responsive to pharmacological doses of B vitamins. Therefore, effective treatments to reduce homocysteine levels are needed, and gene therapy could provide a novel approach. We recently showed that hepatic expression of DYRK1A, a serine/threonine kinase, is negatively correlated with plasma homocysteine levels in cystathionine-beta-synthase deficient mice, a mouse model of hyperhomocysteinemia. Therefore, Dyrk1a is a good candidate for gene therapy to normalize homocysteine levels. We then used an adenoviral construct designed to restrict expression of DYRK1A to hepatocytes, and found decreased plasma homocysteine levels after hepatocyte-specific Dyrk1a gene transfer in hyperhomocysteinemic mice. The elevation of pyridoxal phosphate was consistent with the increase in cystathionine-beta-synthase activity. Commensurate with the decreased plasma homocysteine levels, targeted hepatic expression of DYRK1A resulted in elevated plasma paraoxonase-1 activity and apolipoprotein A-I levels, and rescued the Akt/GSK3 signaling pathways in aorta of mice, which can prevent homocysteine-induced endothelial dysfunction. These results demonstrate that hepatocyte-restricted Dyrk1a gene transfer can offer a useful therapeutic targets for the development of new selective homocysteine lowering therapy.

Dyrk1A, a serine/threonine kinase, is involved in ERK and Akt activation in the brain of hyperhomocysteinemic mice

Hyperhomocysteinemia due to cystathionine beta synthase (CBS) deficiency is associated with diverse brain disease. Whereas the biological actions linking hyperhomocysteinemia to the cognitive dysfunction are not well understood, we tried to establish relationships between hyperhomocysteinemia and alterations of signaling pathways. In the brain of CBS-deficient mice, a murine model of hyperhomocysteinemia, we previously found an activation of extracellular signal-regulated kinase (ERK) pathway and an increase of Dyrk1A, a serine/threonine kinase involved in diverse functions ranging from development and growth to apoptosis. We then investigated the relationship between Dyrk1A and the signaling pathways initiated by receptor tyrosine kinases (RTK), the ERK and PI3K/Akt pathways. We found a significant increase of phospho-ERK, phospho-MEK, and phospho-Akt in the brain of CBS-deficient and Dyrk1a-overexpressing mice. This increase was abolished when CBS-deficient and Dyrk1A-transgenic mice were treated with harmine, an inhibitor of Dyrk1A kinase activity, which emphasizes the role of Dyrk1A activity on ERK and Akt activation. Sprouty 2 protein level, a negative feedback loop modulator that limits the intensity and duration of RTK activation, is decreased in the brain of CBS-deficient mice, but not in the brain of Dyrk1A transgenic mice. Furthermore, a reduced Dyrk1A and Grb2 binding on sprouty 2 and an increased interaction of Dyrk1A with Grb2 were found in the brain of Dyrk1A transgenic mice. The consequence of Dyrk1A overexpression on RTK activation seems to be a decreased interaction of sprouty 2/Grb2. These observations demonstrate ERK and Akt activation induced by Dyrk1A in the brain of hyperhomocysteinemic mice and open new perspectives to understand the basis of the cognitive defects in hyperhomocysteinemia.

Homocysteine inhibits hepatocyte proliferation via endoplasmic reticulum stress

Homocysteine is an independent risk factor for coronary, cerebral, and peripheral vascular diseases. Recent studies have shown that levels of homocysteine are elevated in patients with impaired hepatic function, but the precise role of homocysteine in the development of hepatic dysfunction is unclear. In this study, we examined the effect of homocysteine on hepatocyte proliferation in vitro. Our results demonstrated that homocysteine inhibited hepatocyte proliferation by up-regulating protein levels of p53 as well as mRNA and protein levels of p21^Cip1 in primary cultured hepatocytes. Homocysteine induced cell growth arrest in p53-positive hepatocarcinoma cell line HepG2, but not in p53-null hepatocarcinoma cell line Hep3B. A p53 inhibitor pifithrin-α inhibited the expression of p21^Cip1 and attenuated homocysteine-induced cell growth arrest. Homocysteine induced TRB3 expression via endoplasmic reticulum stress pathway, resulting in Akt dephosphorylation. Knock-down of endogenous TRB3 significantly suppressed the inhibitory effect of homocysteine on cell proliferation and the phosphorylation of Akt. LiCl reversed homocysteine-mediated cell growth arrest by inhibiting TRB3-mediated Akt dephosphorylation. These results demonstrate that both TRB3 and p21^Cip1 are critical molecules in the homocysteine signaling cascade and provide a mechanistic explanation for impairment of liver regeneration in hyperhomocysteinemia.