Differential Expression of Gastrointestinal Glutathione Peroxidase (GI-GPx) Gene during Mouse Organogenesis

Biological and Catalytic Properties of Selenoproteins

Selenocysteine is a catalytic residue at the active site of all selenoenzymes in bacteria and mammals, and it is incorporated into the polypeptide backbone by a co-translational process that relies on the recoding of a UGA termination codon into a serine/selenocysteine codon. The best-characterized selenoproteins from mammalian species and bacteria are discussed with emphasis on their biological function and catalytic mechanisms. A total of 25 genes coding for selenoproteins have been identified in the genome of mammals. Unlike the selenoenzymes of anaerobic bacteria, most mammalian selenoenzymes work as antioxidants and as redox regulators of cell metabolism and functions. Selenoprotein P contains several selenocysteine residues and serves as a selenocysteine reservoir for other selenoproteins in mammals. Although extensively studied, glutathione peroxidases are incompletely understood in terms of local and time-dependent distribution, and regulatory functions. Selenoenzymes take advantage of the nucleophilic reactivity of the selenolate form of selenocysteine. It is used with peroxides and their by-products such as disulfides and sulfoxides, but also with iodine in iodinated phenolic substrates. This results in the formation of Se-X bonds (X = O, S, N, or I) from which a selenenylsulfide intermediate is invariably produced. The initial selenolate group is then recycled by thiol addition. In bacterial glycine reductase and D-proline reductase, an unusual catalytic rupture of selenium-carbon bonds is observed. The exchange of selenium for sulfur in selenoproteins, and information obtained from model reactions, suggest that a generic advantage of selenium compared with sulfur relies on faster kinetics and better reversibility of its oxidation reactions.

The beginning of GPX2 and 30 years later

We published the first paper to characterize GPX2 (aka GSHPx-GI) as a selenoenzyme with glutathione peroxidase activity in 1993. Among the four Se-GPX isozymes, GPX1-4, GPX1 and GPX2 are closely related in terms of structure, substrate specificities, and subcellular localization. What sets them apart are distinct patterns of gene regulation, tissue distribution and response to selenium. While we identified the digestive tract epithelium as the main site of GPX2 expression, later work has shown GPX2 is found more widely in epithelial tissues with concentration of expression in stem cell and proliferative compartments. GPX2 expression is regulated over a wide range of levels by many pathways, including NRF2, WNT, p53, RARE and this often results in attaching undue significance to GPX2 as GPX2 is only a part of a system of hydroperoxidase activities, including GPX1, peroxiredoxins and catalase. These other activities may play equal or greater roles, particularly in cell lines cultured without selenium supplementation and often with very low GPX2 levels. This could be assessed by examining levels of mRNA and protein among these various peroxidases at the outset of studies. As an example, it was found that GPX1 responds to the absence of GPX2 in mouse ileum and colon epithelium with higher expression. As such, both Gpx1 and Gpx2 had to be knocked out in mice to produce ileocolitis. However, we note that the actual role of GPX1 and GPX2 in relation to peroxiredoxin function is unclear. There may be an interdependence that requires only low amounts of GPX1 and/or GPX2 in a supporting role to maintain proper peroxiredoxin function. GPX2 levels may be prognostic for cancer progression in colon, breast, prostate and liver, however, there is no consistent trend for higher or lower levels to be favorable.

Selenoproteins in the Human Placenta: How Essential Is Selenium to a Healthy Start to Life?

Selenium is an essential trace element required for human health, and selenium deficiency has been associated with many diseases. The daily recommended intake of selenium is 60 µg/day for adults, which increases to 65 µg/day for women when pregnant. Selenium is incorporated into the 21st amino acid, selenocysteine (sec), a critical component of selenoproteins that plays an important role in a variety of biological responses such as antioxidant defence, reactive oxygen species (ROS) signalling, formation of thyroid hormones, DNA synthesis and the unfolded protein response in the endoplasmic reticulum (ER). Although 25 selenoproteins have been identified, the role of many of these is yet to be fully characterised. This review summarises the current evidence demonstrating that selenium is essential for a healthy pregnancy and that poor selenium status leads to gestational disorders. In particular, we focus on the importance of the placental selenoproteome, and the role these proteins may play in a healthy start to life.

Fetal Programming Is Deeply Related to Maternal Selenium Status and Oxidative Balance; Experimental Offspring Health Repercussions

Nutrients consumed by mothers during pregnancy and lactation can exert permanent effects upon infant developing tissues, which could represent an important risk factor for diseases during adulthood. One of the important nutrients that contributes to regulating the cell cycle and tissue development and functionality is the trace element selenium (Se). Maternal Se requirements increase during gestation and lactation. Se performs its biological action by forming part of 25 selenoproteins, most of which have antioxidant properties, such as glutathione peroxidases (GPxs) and selenoprotein P (SELENOP). These are also related to endocrine regulation, appetite, growth and energy homeostasis. In experimental studies, it has been found that low dietary maternal Se supply leads to an important oxidative disruption in dams and in their progeny. This oxidative stress deeply affects gestational parameters, and leads to intrauterine growth retardation and abnormal development of tissues, which is related to endocrine metabolic imbalance. Childhood pathologies related to oxidative stress during pregnancy and/or lactation, leading to metabolic programing disorders like fetal alcohol spectrum disorders (FASD), have been associated with a low maternal Se status and intrauterine growth retardation. In this context, Se supplementation therapy to alcoholic dams avoids growth retardation, hepatic oxidation and improves gestational and breastfeeding parameters in FASD pups. This review is focused on the important role that Se plays during intrauterine and breastfeeding development, in order to highlight it as a marker and/or a nutritional strategy to avoid diverse fetal programming disorders related to oxidative stress.

Regulatory Phenomena in the Glutathione Peroxidase Superfamily

Significance: The selenium-containing Glutathione peroxidases (GPxs)1-4 protect against oxidative challenge, inhibit inflammation and oxidant-induced regulated cell death. Recent Advances: GPx1 and GPx4 dampen phosphorylation cascades predominantly via prevention of inactivation of phosphatases by H₂O₂ or lipid hydroperoxides. GPx2 regulates the balance between regeneration and apoptotic cell shedding in the intestine. It inhibits inflammation-induced carcinogenesis in the gut but promotes growth of established cancers. GPx3 deficiency facilitates platelet aggregation likely via disinhibition of thromboxane biosynthesis. It is also considered a tumor suppressor. GPx4 is expressed in three different forms. The cytosolic form proved to inhibit interleukin-1-driven nuclear factor κB activation and leukotriene biosynthesis. Moreover, it is a key regulator of ferroptosis, because it reduces hydroperoxy groups of complex lipids and silences lipoxygenases. By alternate substrate use, the nuclear form contributes to chromatin compaction. Mitochondrial GPx4 forms the mitochondrial sheath of spermatozoa and, thus, guarantees male fertility. Out of the less characterized GPxs, the cysteine-containing GPx7 and GPx8 are unique in contributing to oxidative protein folding in the endoplasmic reticulum by reacting with protein isomerase as an alternate substrate. A yeast 2-Cysteine glutathione peroxidase equipped with CP and CR was reported to sense H₂O₂ for inducing an adaptive response. Critical Issues: Most of the findings compiled are derived from tissue culture and/or animal studies only. Their impact on human physiology is sometimes questionable. Future Directions: The expression of individual GPxs and GPx-dependent regulatory phenomena are to be further investigated, in particular in respect to human health.

Selenium, Selenoproteins, and Female Reproduction: A Review

Selenium (Se) is an essential micronutrient that has several important functions in animal and human health. The biological functions of Se are carried out by selenoproteins (encoded by twenty-five genes in human and twenty-four in mice), which are reportedly present in all three domains of life. As a component of selenoproteins, Se has structural and enzymatic functions; in the latter context it is best recognized for its catalytic and antioxidant activities. In this review, we highlight the biological functions of Se and selenoproteins followed by an elaborated review of the relationship between Se and female reproductive function. Data pertaining to Se status and female fertility and reproduction are sparse, with most such studies focusing on the role of Se in pregnancy. Only recently has some light been shed on its potential role in ovarian physiology. The exact underlying molecular and biochemical mechanisms through which Se or selenoproteins modulate female reproduction are largely unknown; their role in human pregnancy and related complications is not yet sufficiently understood. Properly powered, randomized, controlled trials (intervention vs. control) in populations of relatively low Se status will be essential to clarify their role. In the meantime, studies elucidating the potential effect of Se supplementation and selenoproteins (i.e., GPX1, SELENOP, and SELENOS) in ovarian function and overall female reproductive efficiency would be of great value.

MicroRNAs as Potential Regulators of Glutathione Peroxidases Expression and Their Role in Obesity and Related Pathologies

Glutathione peroxidases (GPxs) belong to the eight-member family of phylogenetically related enzymes with different cellular localization, but distinct antioxidant function. Several GPxs are important selenoproteins. Dysregulated GPx expression is connected with severe pathologies, including obesity and diabetes. We performed a comprehensive bioinformatic analysis using the programs miRDB, miRanda, TargetScan, and Diana in the search for hypothetical microRNAs targeting 3′untranslated regions (3´UTR) of GPxs. We cross-referenced the literature for possible intersections between our results and available reports on identified microRNAs, with a special focus on the microRNAs related to oxidative stress, obesity, and related pathologies. We identified many microRNAs with an association with oxidative stress and obesity as putative regulators of GPxs. In particular, miR-185-5p was predicted by a larger number of programs to target six GPxs and thus could play the role as their master regulator. This microRNA was altered by selenium deficiency and can play a role as a feedback control of selenoproteins’ expression. Through the bioinformatics analysis we revealed the potential connection of microRNAs, GPxs, obesity, and other redox imbalance related diseases.

Glutathione peroxidases

BACKGROUND

With increasing evidence that hydroperoxides are not only toxic but rather exert essential physiological functions, also hydroperoxide removing enzymes have to be re-viewed. In mammals, the peroxidases inter alia comprise the 8 glutathione peroxidases (GPx1-GPx8) so far identified.

SCOPE OF THE REVIEW

Since GPxs have recently been reviewed under various aspects, we here focus on novel findings considering their diverse physiological roles exceeding an antioxidant activity.

MAJOR CONCLUSIONS

GPxs are involved in balancing the H2O2 homeostasis in signalling cascades, e.g. in the insulin signalling pathway by GPx1; GPx2 plays a dual role in carcinogenesis depending on the mode of initiation and cancer stage; GPx3 is membrane associated possibly explaining a peroxidatic function despite low plasma concentrations of GSH; GPx4 has novel roles in the regulation of apoptosis and, together with GPx5, in male fertility. Functions of GPx6 are still unknown, and the proposed involvement of GPx7 and GPx8 in protein folding awaits elucidation.

GENERAL SIGNIFICANCE

Collectively, selenium-containing GPxs (GPx1-4 and 6) as well as their non-selenium congeners (GPx5, 7 and 8) became key players in important biological contexts far beyond the detoxification of hydroperoxides. This article is part of a Special Issue entitled Cellular functions of glutathione.

Selenoproteins in bladder cancer

Selenoproteins with genetically encoded selenium (Se) are very important in response to oxidative stress, redox balance and regulation of various metabolic and developmental processes. Although increased circulating Se has been associated with 33% risk reduction of bladder cancer, there are little data on selenoprotein expression at the protein and genetic level from both human and animal studies. Data from the Mammalian Gene Collection (MGC) Project clearly showed that highest mRNA expression in human urinary epithelium for TRXR1 (thioredoxin reductase 1), GPX1 (glutathione peroxidase 1), SEP15 (15 kDa selenoprotein), SELT (selenoprotein T) and SEPW1 (selenoprotein W1). Although bladder tumor has been characterized by increased Se, GPX and TRXR activity, circulating Se and GPX was interestingly decreased in these cancer patients. As such, selenoprotein expression in urinary epithelium may be involved in bladder cancer (development, progression and recurrence) and may play a significant role in chemotherapeutic intervention. Despite these findings, the role of selenoproteins in bladder cancer has rarely been investigated and the significance of selenoproteins in normal and malignant uroepithelium remains poorly understood.

Developmental expression of plasma glutathione peroxidase during mouse organogenesis

Plasma glutathione peroxidase (pGPx) is an extracellular antioxidative selenoenzyme which has been detected in various adult tissues, but little is known about the expression and distribution of pGPx during embryogenesis. To investigate the expression patterns of pGPx during embryogenesis, we performed quantitative real-time PCR, in situ hybridization, Western blot, and immunohistochemistry analyses in whole embryos or each developing organ of mice on embryonic days (E)7.5-18.5. In whole embryos of E7.5-8.5, pGPx mRNA was more typically expressed in extra-embryonic tissues including ectoplacental cone, trophectoderm, and decidual cells than in embryos. However, after E9.5, pGPx mRNA and protein levels were increased in the embryos with differentiation and growth, but trended to gradually decrease in the extra-embryonic tissues until E18.5. In sectioned embryonic tissues on E13.5-18.5, pGPx mRNA and protein were mainly expressed in the developing nervous tissues, the sensory organs, and the epithelia of lung, skin, and intestine, the heart and artery, and the kidney. In particular, pGPx immunoreactivity was very strong in the developing liver. These results indicate that pGPx is spatio-temporally expressed in various embryonic organs as well as extra-embryonic tissues, suggesting that pGPx may function to protect the embryos against endogenous and exogenous reactive oxygen species during organogenesis.