GPx2 counteracts PGE2 production by dampening COX-2 and mPGES-1 expression in human colon cancer cells

GPx2, the gastrointestinal glutathione peroxidase, is a selenoprotein predominantly expressed in the intestine. An anti-inflammatory and anticarcinogenic potential has been inferred from the development of colitis and intestinal cancer in GPx1 and GPx2 double knockout mice. Further, induction by Nrf2 activators classifies GPx2 as a protective enzyme. In contrast, enhanced COX-2 expression is consistently associated with inflammation. The antagonistic roles and an intriguing co-localization of GPx2 and COX-2 prompted us to investigate their possible mutual regulation. Both enzymes were upregulated in tissues of patients with colorectal cancer and colitis, and co-localized in the endoplasmic reticulum. A stable knockdown of GPx2 in HT-29 cells by siRNA resulted in a high basal and IL-1-induced expression of COX-2 and mPGES-1, enzymes required for the production of the pro-inflammatory PGE(2). Accordingly, si-GPx2 cells released high concentrations of PGE(2). Observed effects were specific for GPx2, since COX-2 and mPGES-1 expression was not affected by selenium-deprivation which resulted in the disappearance of GPx1. It is concluded that GPx2 by compartmentalized removal of hydroperoxides silences COX-2 activity and suppresses PGE(2)-dependent COX-2 expression. Thus, GPx2 may prevent undue responses to inflammatory stimuli and, in consequence, inflammation-driven initiation of carcinogenesis.

Pectin does not inhibit intestinal carcinogenesis in APC-deficient Min/+ mice

APC-germline mutation creates predisposition for intestinal tumorigenesis. APCMin/+ mice, developing tumors preferentially in the small intestine and only minimally in the colon, were fed pectin-enriched diets (10% galacturonan; degree of methoxylation=37.0 and 70.4%) or standard diet. Pectins used in the present study do not inhibit intestinal tumorigenesis and rather accelerate it in APCMin/+ mice. Both pectins exhibited prebiotic effects associated with high fermentative formation of acetate but producing low butyrate. The differences of the short-chain fatty acid concentrations between cecum and colon and those between colon and feces were larger than expected and increased with cancer progression, indicating an inhibition of butyrate absorption. Pectins transported more bile acids toward the colon than the standard diet and caused a higher generation of secondary bile acids despite lower pH values. Overexpression of COX-2 resulted in lower antioxidative capacity, thus promoting cancer. Apoptosis increased in hyperplasia but decreased in late adenomas. When biological modular design principles are taken into consideration, it can be expected that pectin also reinforces colorectal tumorigenesis of patients suffering from APC gene defects.

Hydrothermal treatment of Novelose 330 results in high yield of resistant starch type 3 with beneficial prebiotic properties and decreased secondary bile acid formation in rats

Annealing and heat-moisture treatment (HMT) are shown to be suitable methods to increase the yield of resistant starch type 3 (RS3) from Novelose 330 by up to 75%. Peak temperatures of approximately 121 degrees C were used to produce to a sufficiently high thermal stability of the hydrothermal modified RS3 products for a wide range of applications. HMT significantly increased the crystallinity up to 40%. An in vivo feeding experiment with Wistar rats showed that fermentation of Novelose 330 dominated in the proximal colon, but degradation of HMT-Novelose was more dominant in the distal colon, leading to higher butyrate concentrations in this segment of the large bowel. Large-bowel surface and crypt length increased in the proximal colon in rats fed the Novelose 330-containing diet. In contrast, after the intake of HMT-Novelose, maximal values were found in the distal segment. The lower pH and higher butyrate concentration of the caecal and colonic contents significantly suppressed the formation of secondary bile acids in RS3-fed rats. The formation of secondary bile acids was inhibited more strongly by HMT-Novelose than by Novelose 330. The Ki-67-immunopositive epithelial cells in the colon of RS3-fed rats indicated the establishment of an optimal balance in the dynamic process of mucosal regeneration. HMT provides a method for the economical production of a high-quality RS3 with dominating prebiotic properties in the distal colon for health-promoting applications.

Dietary resistant starch type 3 prevents tumor induction by 1,2-dimethylhydrazine and alters proliferation, apoptosis and dedifferentiation in rat colon

Some epidemiological and experimental studies suggest that consumption of resistant starch is preventive against colon cancer. Resistant starch leads to a fermentation-mediated increase in the formation of short-chain fatty acids, with a particularly high butyrate fraction in large bowel. Butyrate is considered to be protective against colon cancer because it causes growth arrest and apoptosis and regulates expression of proteins involved in cellular dedifferentiation in various tumor cell lines in culture. We sought to investigate these processes under conditions of a carcinogenicity experiment in vivo. In the present study, 1,2-dimethylhydrazine-treated Sprague-Dawley rats were fed standard diet (n=12) or diet with 10% hydrothermally modified Novelose 330, a resistant starch type 3 (RS3), replacing digestible starch (n=8). After 20 weeks tumor number, epithelial proliferation, apoptosis, immunoreactivity of carcinogenesis-related proteins [protein kinase C-delta (PKC-delta), heat shock protein 25 (HSP25) and gastrointestinal glutathione peroxidase (GI-GPx)], as well as mucin properties were evaluated in proximal and distal colon in situ. No tumors developed under RS3 diet, compared to a tumor incidence of 0.6+/-0.6 (P<0.05) under the standard diet. RS3 decreased the number of proliferating cells, the length of the proliferation zone and the total length of the crypt in the distal colon, but not proximal colon, and enhanced apoptosis in both colonic segments. It induced PKC-delta and HSP25 expression, but inhibited GI-GPx expression in the epithelium of distal colon. RS3 increased the number of predominantly acidic mucin containing goblet cells in the distal colon, but had no effect on the goblet cell count. We conclude that hydrothermally treated RS3 prevented colon carcinogenesis, and that this effect was mediated by enhanced apoptosis of damaged cells accompanied by changes in parameters of dedifferentiation in colonic mucosa.

Modulation of Cyp3a11 mRNA expression by alpha-tocopherol but not gamma-tocotrienol in mice

Metabolism of vitamin E is initiated by cytochrome P450 (CYP) enzymes usually involved in the metabolism of xenobiotics. Like other CYP substrates, vitamin E induced a reporter gene under the control of the pregnane X receptor (PXR) which regulates the expression of CYPs including CYP3A4. gamma-Tocotrienol, the most effective PXR activator, also induced endogenous CYP3A4 mRNA in HepG2 cells. Since these findings imply an interference of vitamin E with drug metabolism it was deemed necessary to investigate their in vivo relevance. Therefore, mice were grown for 3 months with alpha-tocopherol-deficient, -adequate, and -supranutritional diet, i.e. 2, 20 and 200 mg RRR-alpha-tocopheryl acetate/kg diet, respectively. Half of them received 250 microg gamma-tocotrienol/day for the last 7 days. After 3 months, hepatic levels of Cyp3a11 mRNA, the murine homolog to human CYP3A4, were about 2.5-fold higher in the 20 and 200 mg alpha-tocopherol groups than in the 2 mg group. After feeding 200 mg alpha-tocopherol for 9 months, Cyp3a11 mRNA was 1.7-fold higher than after 3 months. In contrast, gamma-tocotrienol did not induce Cyp3a11 mRNA. This could be explained by its high metabolism as demonstrated by the 20- to 25-fold increase in the urinary excretion of gamma-CEHC, the final metabolite of gamma-tocotrienol degradation. In conclusion, alpha-tocopherol maintains an adequate level of xenobiotic-metabolizing enzymes. If fed in supranutritional dosages, especially for longer times, alpha-tocopherol induces Cyp3a11 to levels which might interfere with drug metabolism.

Localization of alpha-tocopherol transfer protein in trophoblast, fetal capillaries' endothelium and amnion epithelium of human term placenta

Vitamin E has been linked to fertility since its discovery in 1922. However, the exact mechanism by which alpha-tocopherol allows pregnancy to continue until term has remained puzzling over the last 80 years. Alpha-tocopherol transfer protein (TTPA) is expressed in liver and in Purkinje cells of the cerebellum. TTPA is suggested to be responsible for the transfer of alpha-tocopherol across barrier membranes. Ttpa-knockout mice are infertile and show symptoms similar to those observed in severe vitamin E deficiency. We thus investigated TTPA expression in human placenta and whether clues from its localization in different parts of the placenta might be of functional significance. TTPA-mRNA transcripts were quantified with a fluorescent 5'-nuclease assay (TaqMan) in five different tissues. Placental expression ranged second behind that of liver. Immunohistochemistry identified TTPA in the cytosol but also in nuclei of the trophoblast and in the endothelium of the fetal capillaries. Expression in trophoblast and fetal capillaries' endothelium indicates a role of TTPA in the stereoselective transport of RRR-alpha-tocopherol from the maternal to the fetal plasma. In amnion epithelial cells, however, TTPA was predominantly located in the nuclei. Nuclear localization of the protein may represent a novel function of TTPA.

Inhibition of basal and interleukin-1-induced VCAM-1 expression by phospholipid hydroperoxide glutathione peroxidase and 15-lipoxygenase in rabbit aortic smooth muscle cells

Cytokines or hydroperoxides upregulate cell adhesion molecules (CAM) in early stages of atherosclerosis. VCAM-1 expression was therefore investigated in rabbit aortic smooth muscle cells (SMC) stably transfected either with phospholipid hydroperoxide glutathione peroxidase (PHGPx; SMCPHGPx) as a hydroperoxide-reducing enzyme or with 15-lipoxygenase (15-LOX; SMCLOX) as a hydroperoxide-producing enzyme. Transfected cells showed up to 3-fold enhanced PHGPx and a marked LOX activity, respectively, that was absent in controls. Intracellular hydroperoxides were 6-fold higher in SMCLOX than in SMC or SMCPHGPx. Intracellular protein thiols were decreased by 50 and 90% in SMCPHGPx and SMCLOX, respectively. Glutathione mixed disulfides were tentatively increased from SMC via SMCPHGPx to SMCLOX, accordingly. Thiol reduction with tris(2-carboxyethyl)phosphine completely restored protein thiols in SMCPHGPx, whereas in SMCLOX only 60% of control values were recovered. Basal VCAM-1 mRNA levels were decreased by 50% in SMCPHGPx and 75% in SMCLOX. VCAM-1-inducibility was abrogated in SMCLOX but not in SMCPHGPx. Accordingly, NFkappaB-driven reporter gene activation by IL-1 was unaffected in SMCPHGPx but abolished in SMCLOX. The data confirm that PHGPx overexpression dampens CAM expression either by lowering stimulatory hydroperoxides or by using hydroperoxides for protein modification. But hydroperoxides, when constitutively overproduced as in SMCLOX, inhibit CAM expression and render cells refractory to IL-1 stimulation likely due to oxidation of protein thiols of the signaling system.

Frataxin deficiency in pancreatic islets causes diabetes due to loss of beta cell mass

Diabetes is caused by an absolute (type 1) or relative (type 2) deficiency of insulin-producing beta cells. We have disrupted expression of the mitochondrial protein frataxin selectively in pancreatic beta cells. Mice were born healthy but subsequently developed impaired glucose tolerance progressing to overt diabetes mellitus. These observations were explained by impairment of insulin secretion due to a loss of beta cell mass in knockout animals. This phenotype was preceded by elevated levels of reactive oxygen species in knockout islets, an increased frequency of apoptosis, and a decreased number of proliferating beta cells. Hence, disruption of the frataxin gene in pancreatic beta cells causes diabetes following cellular growth arrest and apoptosis, paralleled by an increase in reactive oxygen species in islets. These observations might provide insight into the deterioration of beta cell function observed in different subtypes of diabetes in humans.