Endoplasmic reticulum-resident selenoproteins and their roles in glucose and lipid metabolic disorders

Glucose and lipid metabolic disorders (GLMDs), such as diabetes, dyslipidemia, metabolic syndrome, nonalcoholic fatty liver disease, and obesity, are significant public health issues that negatively impact human health. The endoplasmic reticulum (ER) plays a crucial role at the cellular level for lipid and sterol biosynthesis, intracellular calcium storage, and protein post-translational modifications. Imbalance and dysfunction of the ER can affect glucose and lipid metabolism. As an essential trace element, selenium contributes to various human physiological functions mainly through 25 types of selenoproteins (SELENOs). At least 10 SELENOs, with experimental and/or computational evidence, are predominantly found on the ER membrane or within its lumen. Two iodothyronine deiodinases (DIOs), DIO1 and DIO2, regulate the thyroid hormone deiodination in the thyroid and some external thyroid tissues, influencing glucose and lipid metabolism. Most of the other eight members maintain redox homeostasis in the ER. Especially, SELENOF, SELENOM, and SELENOS are involved in unfolded protein responses; SELENOI catalyzes phosphatidylethanolamine synthesis; SELENOK, SELENON, and SELENOT participate in calcium homeostasis regulation; and the biological significance of thioredoxin reductase 3 in the ER remains unexplored despite its established function in the thioredoxin system. This review examines recent research advances regarding ER SELENOs in GLMDs and aims to provide insights on ER-related pathology through SELENOs regulation.

Molecular Mechanisms of the Therapeutic Effect of Selenium Nanoparticles in Hepatocellular Carcinoma

This review describes and summarizes, for the first time, the molecular mechanisms of the cytotoxic effect of selenium nanoparticles of various origins on hepatocellular carcinoma cells. The text provides information from recent years indicating the regulation of various signaling pathways and endoplasmic reticulum stress by selenium nanoparticles; the pathways of cell death of liver cancer cells as a result of exposure to selenium nanoparticles are considered. Particular attention is paid to the participation of selenoproteins and selenium-containing thioredoxin reductases and glutathione peroxidases in these processes. Previously, there were no reviews that fully reflected the cytotoxic effects of selenium nanoparticles specifically in hepatocellular carcinoma, despite the fact that many reviews and experimental articles have been devoted to the causes of this disease and the molecular mechanisms of regulation of cytotoxic effects by other agents. The relevance of this review is primarily explained by the fact that despite the development of various drugs and approaches for the treatment and prevention of hepatocellular carcinoma, this disease is still the fourth leading cause of death in the world. For this reason, a complete understanding of the latest trends in the treatment of oncology of various etiologies, especially hepatocellular carcinoma, is extremely important.

Expanding the Frontiers of Guardian Antioxidant Selenoproteins in Cardiovascular Pathophysiology

Significance: Physiological levels of reactive oxygen and nitrogen species (ROS/RNS) function as fundamental messengers for many cellular and developmental processes in the cardiovascular system. ROS/RNS involved in cardiac redox-signaling originate from diverse sources, and their levels are tightly controlled by key endogenous antioxidant systems that counteract their accumulation. However, dysregulated redox-stress resulting from inefficient removal of ROS/RNS leads to inflammation, mitochondrial dysfunction, and cell death, contributing to the development and progression of cardiovascular disease (CVD). Recent Advances: Basic and clinical studies demonstrate the critical role of selenium (Se) and selenoproteins (unique proteins that incorporate Se into their active site in the form of the 21st proteinogenic amino acid selenocysteine [Sec]), including glutathione peroxidase and thioredoxin reductase, in cardiovascular redox homeostasis, representing a first-line enzymatic antioxidant defense of the heart. Increasing attention has been paid to emerging selenoproteins in the endoplasmic reticulum (ER) (i.e., a multifunctional intracellular organelle whose disruption triggers cardiac inflammation and oxidative stress, leading to multiple CVD), which are crucially involved in redox balance, antioxidant activity, and calcium and ER homeostasis. Critical Issues: This review focuses on endogenous antioxidant strategies with therapeutic potential, particularly selenoproteins, which are very promising but deserve more detailed and clinical studies. Future Directions: The importance of selective selenoproteins in embryonic development and the consequences of their mutations and inborn errors highlight the need to improve knowledge of their biological function in myocardial redox signaling. This could facilitate the development of personalized approaches for the diagnosis, prevention, and treatment of CVD. Antioxid. Redox Signal. 40, 369-432.

Biology and Roles in Diseases of Selenoprotein I Characterized by Ethanolamine Phosphotransferase Activity and Antioxidant Potential

Selenoprotein I (SELENOI) has been demonstrated to be an ethanolamine phosphotransferase (EPT) characterized by a nonselenoenzymatic domain and to be involved in the main synthetic branch of phosphatidylethanolamine (PE) in the endoplasmic reticulum. Therefore, defects of SELENOI may affect the health status through the multiple functions of PE. On the other hand, selenium (Se) is covalently incorporated into SELENOI as selenocysteine (Sec) in its peptide, which forms a Sec-centered domain as in the other members of the selenoprotein family. Unlike other selenoproteins, Sec-containing SELENOI was formed at a later stage of animal evolution, and the high conservation of the structural domain for PE synthesis across a wide range of species suggests the importance of EPT activity in supporting the survival and evolution of organisms. A variety of factors, such as species characteristics (age and sex), diet and nutrition (dietary Se and fat intakes), SELENOI-specific properties (tissue distribution and rank in the selenoproteome), etc., synergistically regulate the expression of SELENOI in a tentatively unclear interaction. The N- and C-terminal domains confer 2 distinct biochemical functions to SELENOI, namely PE regulation and antioxidant potential, which may allow it to be involved in numerous physiological processes, including neurological diseases (especially hereditary spastic paraplegia), T cell activation, tumorigenesis, and adipocyte differentiation. In this review, we summarize advances in the biology and roles of SELENOI, shedding light on the precise regulation of SELENOI expression and PE homeostasis by dietary Se intake and pharmaceutical or transgenic approaches to modulate the corresponding pathological status.

Systematic crosstalk in plasmalogen and diacyl lipid biosynthesis for their differential yet concerted molecular functions in the cell

Plasmalogen is a major phospholipid of mammalian cell membranes. Recently it is becoming evident that the sn-1 vinyl-ether linkage in plasmalogen, contrasting to the ester linkage in the counterpart diacyl glycerophospholipid, yields differential molecular characteristics for these lipids especially related to hydrocarbon-chain order, so as to concertedly regulate biological membrane processes. A role played by NMR in gaining information in this respect, ranging from molecular to tissue levels, draws particular attention. We note here that a broad range of enzymes in de novo synthesis pathway of plasmalogen commonly constitute that of diacyl glycerophospholipid. This fact forms the basis for systematic crosstalk that not only controls a quantitative balance between these lipids, but also senses a defect causing loss of lipid in either pathway for compensation by increase of the counterpart lipid. However, this inherent counterbalancing mechanism paradoxically amplifies imbalance in differential effects of these lipids in a diseased state on membrane processes. While sharing of enzymes has been recognized, it is now possible to overview the crosstalk with growing information for specific enzymes involved. The overview provides a fundamental clue to consider cell and tissue type-dependent schemes in regulating membrane processes by plasmalogen and diacyl glycerophospholipid in health and disease.

Molecular Mechanisms of the Cytotoxic Effect of Recombinant Selenoprotein SELENOM on Human Glioblastoma Cells

Currently, selenobiology is an actively developing area, primarily due to the study of the role of the trace element selenium and its organic and inorganic compounds in the regulation of vital processes occurring in the cell. In particular, the study of the functions of selenium nanoparticles has gained great popularity in recent years. However, a weak point in this area of biology is the study of the functions of selenoproteins, of which 25 have been identified in mammals to date. First of all, this is due to the difficulties in obtaining native forms of selenoproteins in preparative quantities, due to the fact that the amino acid selenocysteine is encoded by one of the three stop codons of the TGA universal genetic code. A complex system for recognizing a given codon as a selenocysteine codon has a number of features in pro- and eukaryotes. The selenoprotein SELENOM is one of the least studied mammalian selenoproteins. In this work, for the first time, studies of the molecular mechanisms of regulation of the cytotoxic effect of this protein on human glioblastoma cells were carried out. The cytotoxicity of cancer cells in our experiments was already observed when cells were exposed to 50 μg of SELENOM and increased in proportion to the increase in protein concentration. Apoptosis of human glioblastoma cells was accompanied by an increase in mRNA expression of a number of pro-apoptotic genes, an increase in endoplasmic reticulum stress, and activation of the UPR IRE1α signaling pathway. The results obtained also demonstrate a dose-dependent depletion of the Ca2+ pool under the action of SELENOM, which proves the important role of this protein in the regulation of calcium homeostasis in the cell.

Emerging roles of ER-resident selenoproteins in brain physiology and physiopathology

The brain has a very high oxygen consumption rate and is particularly sensitive to oxidative stress. It is also the last organ to suffer from a loss of selenium (Se) in case of deficiency. Se is a crucial trace element present in the form of selenocysteine, the 21st proteinogenic amino acid present in selenoproteins, an essential protein family in the brain that participates in redox signaling. Among the most abundant selenoproteins in the brain are glutathione peroxidase 4 (GPX4), which reduces lipid peroxides and prevents ferroptosis, and selenoproteins W, I, F, K, M, O and T. Remarkably, more than half of them are proteins present in the ER and recent studies have shown their involvement in the maintenance of ER homeostasis, glycoprotein folding and quality control, redox balance, ER stress response signaling pathways and Ca2+ homeostasis. However, their molecular functions remain mostly undetermined. The ER is a highly specialized organelle in neurons that maintains the physical continuity of axons over long distances through its continuous distribution from the cell body to the nerve terminals. Alteration of this continuity can lead to degeneration of distal axons and subsequent neuronal death. Elucidation of the function of ER-resident selenoproteins in neuronal pathophysiology may therefore become a new perspective for understanding the pathophysiology of neurological diseases. Here we summarize what is currently known about each of their molecular functions and their impact on the nervous system during development and stress.

The Role of Selenoproteins SELENOM and SELENOT in the Regulation of Apoptosis, ER Stress, and Calcium Homeostasis in the A-172 Human Glioblastoma Cell Line

Simple Summary

In this work, we present for the first time the effects of the suppression of the activity of poorly studied selenoproteins SELENOM and SELENOT in human glioblastoma cells, which is extremely important for understanding the functions of these proteins in brain cells. It has been shown that despite the structural similarity of these proteins, they affect the viability of these cancer cells in different ways, affecting various molecular mechanisms of regulation of pro-apoptotic genes, ER stress markers, and their physiological partners, as well as the regulation of cytosolic calcium.

Abstract

It is known that seven mammalian selenoproteins are localized in the endoplasmic reticulum: SELENOM, SELENOT, SELENOF, SELENOK, SELENOS, SELENON, and DIO2. Among them, SELENOM and SELENOT are the least studied; therefore, the study of their function using the widespread method of suppressing the expression of genes encoding these proteins and the activity of the enzymes themselves by RNA interference is of great interest. We have shown that a decrease in the expression of SELENOM and SELENOT mRNA in the A-172 human glioblastoma cell line by more than 10 times and the quantitative content of enzymes by more than 3 times leads to ER stress, expressed as a decrease in the ER capacity for storing Ca²⁺ ions. At the level of regulation of apoptotic processes, SELENOM knockdown leads to an increase in the expression of pro-apoptotic CHOP, GADD34, PUMA, and BIM genes, but a compensatory increase in the levels of SELENOT and antioxidant genes from the group of glutathione peroxidases and thioredoxins did not induce cell death. Knockdown of SELENOT had the opposite effect, reducing the expression of pro-apoptotic proteins and regulating the level of a smaller number of genes encoding antioxidant enzymes, which also did not affect the baseline level of apoptosis in the studied cells. At the same time, ER stress induced by MSA or SeNPs induced a more pronounced pro-apoptotic effect in SELENOT knockdown cells through suppression of the expression of selenium-containing antioxidant proteins. Thus, in this work, for the first time, the mechanisms of fine regulation of the processes of apoptosis, cell proliferation, and ER stress by two ER resident proteins, SELENOM and SELENOT, are touched upon, which is not only fundamental but also applied to clinical importance due to the close relationship between the calcium signaling system of cells, folding proteins-regulators of apoptosis and cell survival pathways.

A novel therapeutic strategy for hepatocellular carcinoma: Immunomodulatory mechanisms of selenium and/or selenoproteins on a shift towards anti-cancer

Selenium (Se) is an essential trace chemical element that is widely distributed worldwide. Se exerts its immunomodulatory and nutritional activities in the human body in the form of selenoproteins. Se has increasingly appeared as a potential trace element associated with many human diseases, including hepatocellular carcinoma (HCC). Recently, increasing evidence has suggested that Se and selenoproteins exert their immunomodulatory effects on HCC by regulating the molecules of oxidative stress, inflammation, immune response, cell proliferation and growth, angiogenesis, signaling pathways, apoptosis, and other processes in vitro cell studies and in vivo animal studies. Se concentrations are generally low in tissues of patients with HCC, such as blood, serum, scalp hair, and toenail. However, Se concentrations were higher in HCC patient tissues after Se supplementation than before supplementation. This review summarizes the significant relationship between Se and HCC, and details the role of Se as a novel immunomodulatory or immunotherapeutic approach against HCC.

Expression of ER-resident selenoproteins and activation of cancer cells apoptosis mechanisms under ER-stress conditions caused by methylseleninic acid

The aim of this work was to study changes in gene expression levels of 7 ER-resident selenoproteins under ER-stress caused by the action of a selenium-containing compound of organic nature, methylselenic acid using three human cancer cell lines DU 145 (prostate carcinoma), MCF 7 (breast adenocarcinoma)and HT-1080 (fibrosarcoma). According to the obtained results, we can speak of a synchronous changes in the expression of SELT and SEP15 mRNA depending on the concentration of MSA for 24 h, while the pattern of SELM expression was completely opposite and was radically different from other selenoproteins. It should be noted that in HT-1080 cells, the expression pattern of SELM differed from the expression pattern in two other cancer cells, while the expression patterns of other ER-resident selenoproteins (SELT, SEP15, SELK, SELS, SELN and DIO2) differed slightly depending on the cell line. Also we investigated the molecular mechanisms of UPR caused by MSA-induced ER stress in three cancer cell lines. According to the obtained results, it can be assumed that in DU 145 cells, MSA promotes activation of the PERK signaling pathway of UPR. In fibrosarcoma cells MSA was promoted the activation of ATF-6 UPR signaling pathway. In MCF 7 cells, MSA promoted the activation of two pro-apoptotic UPR signaling pathways at once: IRE1 and ATF-6.The results of this work once again demonstrate that the mechanisms of ER-stress regulation caused by the same agent, in this case, MSA, lead to the activation of different UPR signaling pathways in different cancer cells, and about their relationship.

Activation of Signal Pathways of Apoptosis under Conditions of Prolonged ER-Stress Caused by Exposure of Mouse Testicular Teratoma Cells to Selenium-Containing Compounds

Aim to study the molecular mechanisms of apoptotic death of mouse testicular teratocarcinoma cells (line F-9) under exposure to the widely used selenium-containing compounds with antitumor activity, sodium selenite and methylseleninic acid. Methods fluorescence microscopy, MTT assay, Western blotting. Results It was shown that sodium selenite at a concentration of 10 μM and methylseleninic acid at concentrations of 1 and 10 μM cause apoptosis-dependent death of F-9 cells, excluding necrotic death. Western blotting showed an increase in the expression of XBP1s when treating F-9 cells with 1 μM methylseleninic acid. Conclusions 10 μM methylseleninic acid leads to cell apoptosis, most likely by activation of the IRE1 signaling pathway under prolonged stress of the endoplasmic reticulum.

Protein-protein interactions of ER-resident selenoproteins with their physiological partners

ER is a highly specialized complex of branched microtubules enclosed in a membrane and communicating with each other, its functions in the cell are important and very diverse: lipid and phospholipid synthesis, calcium storage, hormone synthesis, protein synthesis and maturation, membrane production, toxin neutralization, etc. The high concentration of calcium ions and the oxidizing properties of the contents of the ER cavities contribute to the proper synthesis and folding of proteins designed for secretion or exposure on the surface of the cell membrane. However, disturbance of redox regulation can lead to the accumulation of improperly folded proteins in the ER, disruption of calcium regulation, which can cause ER-stress. This review is devoted to the role of ER-resident selenoproteins in the processes occurring in this organelle of a cell. The main emphasis is placed on the study of protein-protein interactions of selenoproteins with their physiological partners; this will facilitate understanding of their functional purpose in this organelle. Currently, 7 selenoproteins are known that are localized in the ER, but the functions of most of them are not at all clear, for some, physiological partners have been identified. It is known that selenoproteins are oxidoreductases with antioxidant properties, this is extremely important for the normal functioning of ER. Therefore, this review can be very useful for understanding the full picture of the functions of ER-resident selenoproteins obtained on the basis of recent data.

Selenoproteins in colon cancer

Selenocysteine-containing proteins (selenoproteins) have been implicated in the regulation of various cell signaling pathways, many of which are linked to colorectal malignancies. In this in-depth excurse into the selenoprotein literature, we review possible roles for human selenoproteins in colorectal cancer, focusing on the typical hallmarks of cancer cells and their tumor-enabling characteristics. Human genome studies of single nucleotide polymorphisms in various genes coding for selenoproteins have revealed potential involvement of glutathione peroxidases, thioredoxin reductases, and other proteins. Cell culture studies with targeted down-regulation of selenoproteins and studies utilizing knockout/transgenic animal models have helped elucidate the potential roles of individual selenoproteins in this malignancy. Those selenoproteins, for which strong links to development or progression of colorectal cancer have been described, may be potential future targets for clinical interventions.

Participation of selenoproteins localized in the ER in the processes occurring in this organelle and in the regulation of carcinogenesis-associated processes

The functions performed by the ER are diverse: synthesis of steroid hormones, synthesis of proteins for the plasma membrane, lysosomes, as well as proteins meant for exocytosis, protein folding, formation of disulfide bonds, N-linked glycosylation, etc. Selenoproteins localized in this organelle are definitely involved in the processes occurring in it, and the most common of them include participation in protein degradation, regulation of ER stress and redox metabolism. ER stress has been registered in many types of cancer cells. The ability to persist under prolonged ER stress increases their survival, resistance to drugs and immunity. Disturbances in the redox regulation of the cell cycle, which result in the accumulation of misfolded proteins in the ER, viral infection, disruption of Ca²⁺ regulation, are known to cause an evolutionarily conserved reaction - unfolded protein response (UPR) and, ultimately, lead to ER stress. Since selenoproteins, as oxidoreductases, possess antioxidant properties, and their role in the regulation of important processes, such as carcinogenesis and ER stress, has been actively studied in the recent decades, the subject of this review is highly relevant.