Bodo Speckmann

High selenium intake and increased diabetes risk: experimental evidence for interplay between selenium and carbohydrate metabolism

The essential trace element selenium has long been considered to exhibit anti-diabetic and insulin-mimetic properties, but recent epidemiological studies indicated supranutritional selenium intake and high plasma selenium levels as possible risk factors for development of type 2 diabetes, pointing to adverse effects of selenium on carbohydrate metabolism in humans. However, increased plasma selenium levels might be both a consequence and a cause of diabetes. We summarize current evidence for an interference of selenium compounds with insulin-regulated molecular pathways, most notably the phosphoinositide-3-kinase/protein kinase B signaling cascade, which may underlie some of the pro- and anti-diabetic actions of selenium. Furthermore, we discuss reports of hyperinsulinemia, hyperglycemia and insulin resistance in mice overexpressing the selenoenzyme glutathione peroxidase 1. The peroxisomal proliferator-activated receptor gamma coactivator 1α represents a key regulator for biosynthesis of the physiological selenium transporter, selenoprotein P, as well as for hepatic gluconeogenesis. As proliferator-activated receptor gamma coactivator 1α has been shown to be up-regulated in livers of diabetic animals and to promote insulin resistance, we hypothesize that dysregulated pathways in carbohydrate metabolism and a disturbance of selenium homeostasis are linked via proliferator-activated receptor gamma coactivator 1α.

Selenoprotein P expression is controlled through interaction of the coactivator PGC‐1α with FoxO1a and hepatocyte nuclear factor 4α transcription factors

Selenoprotein P (SeP), the major selenoprotein in plasma, is produced mainly by the liver, although SeP expression is detected in many organs. Recently, we reported stimulation of SeP promoter activity by the forkhead box transcription factor FoxO1a in hepatoma cells and its attenuation by insulin. Here, we demonstrate that this translates into fine‐tuning of SeP production and secretion by insulin. Overexpression of peroxisomal proliferator activated receptor‐γ coactivator 1α (PGC‐1α) enhanced the stimulatory effect of FoxO1a on SeP promoter activity. We identified a novel functional binding site for hepatocyte nuclear factor (HNF)‐4α, termed hepatocyte nuclear factor binding element 1, in the human SeP promoter directly upstream of the FoxO‐responsive element daf16‐binding element 2 (DBE2). Point mutations in hepatocyte nuclear factor binding element 1 alone or together with DBE2 decreased basal activity and responsiveness of the SeP promoter to PGC‐1α. Moreover, the PGC‐1α‐inducing glucocorticoid dexamethasone strongly enhanced SeP messenger RNA levels and protein secretion in cultured rat hepatocytes, whereas insulin suppressed the stimulation of both PGC‐1α and SeP caused by dexamethasone treatment. In a brain‐derived neuroblastoma cell line with low basal SeP expression, SeP transcription was stimulated by PGC‐1α together with FoxO1a, and overexpression of HNF‐4α potentiated this effect. Conclusion: High‐level expression of SeP in liver is ensured by concerted action of the coactivator PGC‐1α and the transcription factors FoxO1a and HNF‐4α. Hence, the production of SeP is regulated similarly to that of the gluconeogenic enzyme glucose‐6‐phosphatase. As hepatic SeP production is crucial for selenium distribution throughout the body, the present study establishes PGC‐1α as a key regulator of selenium homeostasis. (HEPATOLOGY 2008;48:1998‐2006.)

Toward Understanding Success and Failures in the Use of Selenium for Cancer Prevention

SIGNIFICANCE Adequate and supranutritional selenium (Se) intake, maintaining full expression of selenoproteins, has been assumed to be beneficial for human health with respect to prevention of cancer. Strikingly, the effectiveness of dietary Se supplementation depends on many factors: baseline Se status, age, gender, and genetic background of an individual; type of cancer; and time point of intervention in addition to metabolic conversion and dose of applied Se compounds. RECENT ADVANCES Se intake levels for optimization of plasma selenoproteins in humans have been delineated. Regulation, function, and genetic variants of several selenoproteins have been characterized in the intestine, where Se-mediated prevention of colorectal cancer appears to be particularly promising. CRITICAL ISSUES Numerous cell culture and animal studies indicate anticarcinogenic capacity of various Se compounds but, at present, the outcome of human studies is inconsistent and, in large part, disappointing. Moreover, supranutritional Se intake may even trigger adverse health effects, possibly increasing the risk for Type 2 diabetes in Se-replete populations. FUTURE DIRECTIONS To improve protocols for the use of Se in cancer prevention, knowledge on cellular and systemic actions of Se compounds needs to be broadened and linked to individual-related determinants such as the occurrence of variants in selenoprotein genes and the Se status. Based on better mechanistic insight, populations and individuals that may benefit most from dietary Se supplementation need to be defined and studied in suitably planned intervention trials.

Epigenetic effects of selenium and their implications for health.

Alterations of epigenetic marks are linked to normal development and cellular differentiation as well as to the progression of common chronic diseases. The plasticity of these marks provides potential for disease therapies and prevention strategies. Macro- and micro-nutrients have been shown to modulate disease risk in part via effects on the epigenome. The essential micronutrient selenium affects human health outcomes, e.g., cancers, cardiovascular and autoimmune diseases, via selenoproteins and through a range of biologically active dietary selenocompounds and metabolism products thereof. This review provides an assessment of the current literature regarding epigenetic effects of dietary and synthetic selenocompounds, which include the modulation of marks and editors of epigenetic information and interference with one-carbon metabolism, which provides the methyl donor for DNA methylation. The relevance of a selenium-epigenome interaction for human health is discussed, and we also indicate where future studies will be helpful to gain a deeper understanding of epigenetic effects elicited by selenium.

Delaying of insulin signal transduction in skeletal muscle cells by selenium compounds

Supranutritional selenium (Se) intake and high serum Se levels have been associated epidemiologically with increased risk for type 2 diabetes, suggesting adverse effects of dietary Se compounds and/or antioxidant selenoenzymes on the sensitivity of target tissues for insulin. Here, we compared the capability of inorganic (sodium selenite and sodium selenate) and organic (selenomethionine and methylseleninic acid (MSeA)) Se compounds to interfere with insulin signaling in rat L6 myotubes, differentiated skeletal muscle cells. When applied at doses of 1 μM, only selenite and MSeA were capable of delaying insulin-induced phosphorylation of protein kinase B (Akt) and attenuating insulin-induced phosphorylation of forkhead box class O transcription factors FoxO1a and FoxO3. Insulin-stimulated glucose uptake was lowered by selenite and MSeA as well. Even though all tested Se compounds strongly stimulated expression/activity of the cellular selenoproteins glutathione peroxidase 1 and selenoprotein W, selenite and MSeA were the most efficiently utilized Se donors. Moreover, at doses of 1 μM, only selenite and MSeA had a significant inhibitory effect on generation of intracellular reactive oxygen species (ROS). These results suggest that the Se(IV) compounds selenite and MSeA may impair the insulin sensitivity of myocytes by influencing cellular redox homeostasis.

Proinflammatory cytokines down-regulate intestinal selenoprotein P biosynthesis via NOS2 induction

Selenoprotein P (SeP), serving as selenium transporter and extracellular antioxidant, is assumed to have a protective role in the gastrointestinal tract, which is particularly susceptible to oxidative damage. Decreased SeP mRNA levels have been found in colon cancer; however, information on the control of intestinal SeP biosynthesis is scarce. We analyzed SeP biosynthesis in human intestinal epithelial Caco-2 cells subject to differentiation from crypt- to villous-like enterocytes. In the course of Caco-2 cell differentiation, SeP mRNA expression and secretion increased concomitant with three regulators of SeP transcription: hepatocyte nuclear factor-4alpha, forkhead box class O1a, and peroxisomal proliferator-activated receptor-gamma coactivator 1alpha. Treatment of differentiated Caco-2 cells with the proinflammatory cytokines IL-1beta, TNF-alpha, and IFN-gamma caused a down-regulation of SeP biosynthesis, resulting from induction of nitric oxide synthase 2. These observations were corroborated by decreased SeP mRNA levels in the colon of dextran sodium sulfate-treated mice, an animal model of experimental colitis. We conclude that inflammation of the intestinal mucosa causes a decline in locally produced selenoprotein P in the colon that eventually may contribute to the emergence of inflammatory bowel disease-related colorectal cancer.

Induction of Glutathione Peroxidase 4 Expression during Enterocytic Cell Differentiation

Glutathione peroxidase 4 (GPx4), an abundant selenoenzyme, is ubiquitously expressed in a tissue-, cell- and differentiation-dependent manner, and it is localized in cytoplasmic, mitochondrial, and nuclear cellular compartments. Here, we report cytoplasmic and nuclear localization of GPx4 in Caco-2 intestinal epithelial cells. Enterocytic differentiation of Caco-2 cells triggers an increase in GPx4 mRNA and protein levels, mediated by enhanced promoter activity. We identified a combined cAMP response element (CREB) and CCAAT/enhancer binding protein (C/EBP) site as critical for the differentiation-triggered GPx4 promoter activity. Induction of GPx4 correlated with C/EBPα transcript levels during differentiation, suggesting a role of C/EBPα as regulator of enterocytic GPx4 expression. Consistent with the in vitro results, GPx4 protein was detected in cytoplasmic and nuclear compartments of enterocytes in human intestinal epithelia. GPx4 is uniformly expressed in colonic crypts and is differentially expressed along the crypt-to-villus axis in the small intestine with a more pronounced expression of GPx4 in the upper villi, which contain fully differentiated enterocytes. These data suggest that intestinal GPx4 expression is modulated by the enterocytic differentiation program, and the results support a direct role of nuclear GPx4 in the (selenium-dependent) prevention of oxidative damage in the gastrointestinal tract.

Attenuation of hepatic expression and secretion of selenoprotein P by metformin

High serum selenium levels have been associated epidemiologically with increased incidence of type 2 diabetes. The major fraction of total selenium in serum is represented by liver-derived selenoprotein P (SeP). This study was undertaken to test for a hypothesized effect of hyperglycemia and the antihyperglycemic drug metformin on hepatic selenoprotein P biosynthesis. Cultivation of rat hepatocytes in the presence of high glucose concentrations (25 mmol/l) resulted in increased selenoprotein P mRNA expression and secretion. Treatment with metformin dose-dependently downregulated SeP mRNA expression and secretion, and suppressed glucocorticoid-stimulated production of SeP. Moreover, metformin strongly decreased mRNA levels of selenophosphate synthetase 2 (SPS-2), an enzyme essential for selenoprotein biosynthesis. Taken together, these results indicate an influence of metformin on selenium metabolism in hepatocytes. As selenoprotein P is the major transport form of selenium, metformin treatment may thereby diminish selenium supply to extrahepatic tissues.

Selenium and selenoproteins in inflammatory bowel diseases and experimental colitis.

Abstract:Inadequate dietary intake of the essential trace element selenium (Se) is thought to be a risk factor for several chronic diseases associated with oxidative stress and inflammation. Biological actions of Se occur through low-molecular weight metabolites and through selenoproteins. Several key selenoproteins including glutathione peroxidases; selenoproteins M, P, and S; and selenium-binding protein 1 have been detected in the intestine. Interestingly, Se and antioxidant selenoproteins are known to modulate differentiation and function of immune cells and contribute to avoid excessive immune responses. This review discusses the role of Se and intestinal selenoproteins in inflammatory bowel diseases, based on data from human, animal, and in vitro studies. In humans, Se deficiency is commonly observed in patients with Crohns disease. In animal models of experimental colitis, the Se status was negatively correlated with the severity of the disease. While the cause–effect relationship of these observations remains to be clarified, the beneficial outcome of dietary Se supplementation and an optimization of selenoprotein biosynthesis in murine inflammatory bowel disease models have led to investigations of targets and actions of Se in the gastrointestinal tract. The Se status affects gene expression, signaling pathways, and cellular functions in the small and large intestine as well as the gut microbiome composition. This data, particularly from animal experiments, hold promise that adequate dietary Se supply may counteract chronic intestinal inflammation in humans.

Localization and regulation of pancreatic selenoprotein P

Progressive loss of pancreatic β-cell mass is a crucial feature of type 2 diabetes mellitus. As β-cells express very low amounts of the antioxidant enzymes catalase and glutathione peroxidase (GPx), they appear to be particularly vulnerable to oxidative damage in the pathogenesis of diabetes. Here, we investigated the pancreatic expression pattern and regulation of selenoprotein P (Sepp1), which may serve as an additional antioxidant enzyme inside and outside of cells. Sepp1 was detected in rodent pancreas by immunofluorescence and real-time RT-PCR. Regulation of Sepp1 biosynthesis in INS-1 rat insulinoma cells was investigated by real-time RT-PCR, luciferase gene reporter assay, and immunoblotting. Sepp1 and Gpx1 gene expressions in rat pancreas were 58 and 22% respectively of the liver values. Pancreatic Sepp1 expression was restricted to the endocrine tissue, with Sepp1 being present in the α- and β-cells of mouse islets. In INS-1 insulinoma cells, Sepp1 expression was stimulated by the selenium compound sodium selenate and diminished in the presence of high glucose (16.7 vs 5  mM) concentrations. Sepp1 mRNA stability was also lowered at 16.7  mM glucose. Moreover, Sepp1 mRNA levels were decreased in isolated murine islets cultured in high-glucose (22  mM) medium compared with normal glucose (5.5  mM) medium. Pancreatic Sepp1 expression was elevated upon treatment of mice with the β-cell toxin streptozotocin. This study shows that pancreatic islets express relatively high levels of Sepp1 that may fulfill a function in antioxidant protection of β-cells. Downregulation of Sepp1 expression by high glucose might thus contribute to glucotoxicity in β-cells.