Andreas S. Mueller

Redox Regulation of Protein Tyrosine Phosphatase 1B by Manipulation of Dietary Selenium Affects the Triglyceride Concentration in Rat Liver

Protein tyrosine phosphatase 1B (PTP1B) is a key enzyme in the counter-regulation of insulin signaling and in the stimulation of fatty acid synthesis. Selenium (Se), via the activities of glutathione peroxidase (GPx) and thioredoxin reductase (TrxR), is involved in the removal of H(2)O(2) and organic peroxides, which are critical compounds in the modulation of PTP1B activity via glutathionylation. Our study with growing rats investigated how the manipulation of dietary Se concentration influences the regulation of PTP1B and lipogenic effects mediated by PTP1B. Weanling albino rats were divided into 3 groups of 10. The negative control group (NC) was fed a Se-deficient diet for 8 wk. Rats in groups Se75 and Se150 received diets supplemented with 75 or 150 microg Se/kg. Se supplementation of the rats strongly influenced expression and activity of the selenoenzymes cytosolic GPx, plasma GPx, phospholipidhydroperoxide GPx, and cytosolic TrxR, and liver PTP1B. Liver PTP1B activity was significantly higher in groups Se75 and Se150 than in the NC group and this was attributed to a lowered inhibition of the enzyme by glutathionylation. The increased liver PTP1B activity in groups Se75 and Se150 resulted in 1.1- and 1.4-fold higher liver triglyceride concentrations than in the NC rats. The upregulation of the sterol regulatory element binding protein-1c and of fatty acid synthase, 2 PTP1B targets, provided a possible explanation for the lipogenic effect of PTP1B due to the manipulation of dietary Se. We therefore conclude that redox-regulated proteins, such as PTP1B, represent important interfaces between dietary antioxidants such as Se and the regulation of metabolic processes.

Selenium and diabetes: an enigma?

In recent years diabetes has become one of the most common metabolic diseases in developed countries and it is closely related to supernutrition and obesity. Since untreated diabetes produces oxidative stress responsible for secondary complications of the disease, antioxidant supplements were considered as being favourable for the therapy of diabetes. However, the situation has changed recently, since large cross-sectional and interventional trials revealed a positive correlation between a high Se status and diabetes incidence in humans. Thus, currently available data on the role of Se in diabetes are inconsistent and an enigma appears to exist for the relation between selenium and diabetes. This review summarizes selected human and animal studies, pointing to beneficial and critical virtues of Se in diabetes. Moreover, the review discusses possible underlying mechanisms how Se may influence diabetes in both directions. From the current literature, the following information can be extracted: (1) In populations with a high Se status, with the single exception of pregnant women, Se supplements cannot be recommended for the prevention of diabetes; (2) Anti-diabetic effects of Se seem to be restricted to high and nearly toxic doses which cannot be used in humans; and (3) Future investigations should consider the stage of the disease.

Antidiabetic effects of bitter gourd extracts in insulin-resistant db/db mice.

Bitter gourd (BG, Momordica charantia) exerts proven blood glucose- and body weight-lowering effects. To develop an effective and safe application, it is necessary to identify the bioactive compounds and biochemical mechanisms responsible for these effects in type 2 diabetes. A total of forty-five 4-week-old male db/db mice were assigned to five groups of nine each. The mice were given sterile tap water as a control, a whole fruit powder, the lipid fraction, the saponin fraction or the hydrophilic residue of BG at a daily oral dosage of 150 mg/kg body weight for 5 weeks, respectively. Weight gain was significantly decreased in all the BG-treated groups (P ≤ 0.05). Glycated Hb levels were the highest in the control mice compared with all the four BG-treated mice (P = 0.02). The lipid fraction had the strongest effect, and it tended (P = 0.075) to reduce glycated Hb levels from 9.3 % (control mice) to 8.0 % (lipid fraction-treated mice). The lipid and saponin fractions reduced lipid peroxidation of adipose tissue significantly (P ≤ 0.01). Additionally, the saponin fraction and the lipid fraction reduced protein tyrosine phosphatase 1B (PTP 1B) activity in skeletal muscle cytosol by 25 % (P = 0.05) and 23 % (P = 0.07), respectively. PTP 1B is the physiological antagonist of the insulin signalling pathway. Inhibition of PTP 1B increases insulin sensitivity. This is the first study to demonstrate that BG is involved in PTP 1B regulation, and thus explains one possible biochemical mechanism underlying the antidiabetic effects of BG in insulin resistance and type 2 diabetes.

Impact of selenite and selenate on differentially expressed genes in rat liver examined by microarray analysis

Sodium selenite and sodium selenate are approved inorganic Se (selenium) compounds in human and animal nutrition serving as precursors for selenoprotein synthesis. In recent years, numerous additional biological effects over and above their functions in selenoproteins have been reported. For greater insight into these effects, our present study examined the influence of selenite and selenate on the differential expression of genes encoding non-selenoproteins in the rat liver using microarray technology. Five groups of nine growing male rats were fed with an Se-deficient diet or diets supplemented with 0.20 or 1.0 mg of Se/kg as sodium selenite or sodium selenate for 8 weeks. Genes that were more than 2.5-fold up- or down-regulated by selenite or selenate compared with Se deficiency were selected. GPx1 (glutathione peroxidase 1) was up-regulated 5.5-fold by both Se compounds, whereas GPx4 was up-regulated by only 1.4-fold. Selenite and selenate down-regulated three phase II enzymes. Despite the regulation of many other genes in an analogous manner, frequently only selenate changed the expression of these genes significantly. In particular, genes involved in the regulation of the cell cycle, apoptosis, intermediary metabolism and those involved in Se-deficiency disorders were more strongly influenced by selenate. The comparison of selenite- and selenate-regulated genes revealed that selenate may have additional functions in the protection of the liver, and that it may be more active in metabolic regulation. In our opinion the more pronounced influence of selenate compared with selenite on differential gene expression results from fundamental differences in the metabolism of these two Se compounds.

Glutathione deficiency down-regulates hepatic lipogenesis in rats

BackgroundOxidative stress is supposed to increase lipid accumulation by stimulation of hepatic lipogenesis at transcriptional level. This study was performed to investigate the role of glutathione in the regulation of this process. For that purpose, male rats were treated with buthionine sulfoximine (BSO), a specific inhibitor of γ-glutamylcysteine synthetase, for 7 days and compared with untreated control rats.ResultsBSO treatment caused a significant reduction of total glutathione in liver (-70%), which was attributable to diminished levels of reduced glutathione (GSH, -71%). Glutathione-deficient rats had lower triglyceride concentrations in their livers than the control rats (-23%), whereas the circulating triglycerides and the cholesterol concentrations in plasma and liver were not different between the two groups of rats. Livers of glutathione-deficient rats had lower mRNA abundance of sterol regulatory element-binding protein (SREBP)-1c (-47%), Spot (S)14 (-29%) and diacylglycerol acyltransferase 2 (DGAT-2, -27%) and a lower enzyme activity of fatty acid synthase (FAS, -26%) than livers of the control rats. Glutathione-deficient rats had also a lower hepatic activity of the redox-sensitive protein-tyrosine phosphatase (PTP)1B, and a higher concentration of irreversible oxidized PTP1B than control rats. No differences were observed in protein expression of total PTP1B and the mature mRNA encoding active XBP1s, a key regulator of unfolded protein and ER stress response.ConclusionThis study shows that glutathione deficiency lowers hepatic triglyceride concentrations via influencing lipogenesis. The reduced activity of PTP1B and the higher concentration of irreversible oxidized PTP1B could be, at least in part, responsible for this effect.

Study of molecular targets influencing homocysteine and cholesterol metabolism in growing rats by manipulation of dietary selenium and methionine concentrations

Inconsistent results exist from human and animal studies for Se and methionine (Met) regarding their influence on homocysteine (HCys) and cholesterol (Chol) metabolism. To elucidate these contradictions, sixty-four weanling albino rats were divided into eight groups of 8, and were fed diets containing four different Se levels (15, 50, 150 and 450 microg/kg) either in combination with the recommended Met level of 3 g/kg (C15, C50, C150 and C450) or with an increased Met concentration of 15 g/kg (M15, M50, M150 and M450) for 8 weeks. Plasma HCys was twofold higher in the Se-supplemented C groups than in group C15. Met addition also doubled plasma HCys compared with the respective C groups. In contrast, the expression of the key enzymes of glutathione biosynthesis in the liver was significantly lowered by Se and in particular by Met. Liver Chol concentration was significantly higher in all the Se-supplemented C and M groups than in groups C15 and M15. Plasma Chol was, however, lowered. The uninfluenced expression of sterol-regulatory element-binding protein 2 and of hydroxymethyl-glutaryl-CoA reductase, the increased LDL receptor expression and the reduced expression of the hepatobiliary Chol exporter ATP-binding-cassette-transporter 8 (ABCG8) by Se and/or Met explain these findings. We conclude that the elevation of plasma HCys in rats by Se and Met results from a higher export into plasma. The fact that Se in particular combined with Met increases liver Chol but reduces plasma Chol should be addressed in future investigations focussing on the regulation of ABCG8, which is also selectively involved in the reverse transport of phytosterols in the small intestine.

Examination of the Anti-Inflammatory, Antioxidant, and Xenobiotic-Inducing Potential of Broccoli Extract and Various Essential Oils during a Mild DSS-Induced Colitis in Rats

Phytogenic compounds with antioxidant and anti-inflammatory properties are currently discussed as promising complementary agents in prevention and treatment of inflammatory bowel disease (IBD). Our study aimed to evaluate possible protective and curative effects of broccoli extract (BE) and of the essential oils of turmeric (Cuo), thyme (To), and rosemary (Ro) in a rat model with a mild dextran sulphate sodium- (DSS-) induced colitis. Therefore Wistar rats were fed a diet without an additive (Con) or diets with the addition of BE, Cuo, To, and Ro during the whole experiment. Pretreatment with Ro, Cuo, and To increased the expression of the tight junction protein Cldn3. All additives reduced mRNA of VCAM-1 which plays a crucial role in the first state of inflammatory response. Only Ro pretreatment affected the expression of the antioxidant enzymes HO1, GPx2, and of glutathione-S-transferases. All additives counteracted the DSS-induced rise in COX2 and VCAM-1 expression. Colonic IL-10 was increased by Cuo, To, and Ro. During the recovery phase DSS pretreatment increased NFκB, VCAM-1, and MCP-1: This response was counter-regulated by all additives. We conclude that the phytogenic additives tested have a promising anti-inflammatory potential in vivo and a particular role in the prevention of IBD.