Hepatokine Selenoprotein P-Mediated Reductive Stress Causes Resistance to Intracellular Signal Transduction

Differential associations between selenoprotein P and distal sensorimotor polyneuropathy in people with and without diabetes: KORA F4/FF4 study

BACKGROUND

Oxidative stress is a risk factor for distal sensorimotor polyneuropathy (DSPN). Selenoprotein P is a protein with antioxidant properties but has not been investigated in the context of DSPN. This study aimed to assess the associations between selenoprotein P and DSPN in people without and with type 2 diabetes (T2D).

METHODS

Cross-sectional and prospective analyses were based on 1053 (including 217 with T2D) and 513 participants (including 79 with T2D), respectively, aged 61-82 years from the population-based KORA F4 survey. DSPN at baseline (KORA F4) and in the follow-up survey KORA FF4 was defined based on the Michigan Neuropathy Screening Instrument. Serum levels of full-length selenoprotein P were quantified by ELISA. Associations between selenoprotein P and prevalent or incident DSPN were estimated using logistic regression analysis adjusting for multiple confounders.

RESULTS

Selenoprotein P levels were not associated with prevalent DSPN in the total sample. However, there was a significant interaction by diabetes status. Higher levels of selenoprotein P were associated with lower odds of prevalent DSPN in individuals without T2D (fully adjusted model: OR 0.825 [95 % CI 0.682, 0.998], p = 0.0476), but not in those with T2D (OR [95 % CI] 1.098 [0.829, 1.454], p = 0.5132; pinteraction = 0.0488). Selenoprotein P levels were not associated with incident DSPN over a follow-up of 6.5 years.

CONCLUSION

In individuals without T2D from the older general population, lower selenoprotein P levels were associated with a higher prevalence of DSPN. Whether the antioxidant properties of selenoprotein P are responsible for the observed associations remains to be elucidated in future research.

The Role of Selected Elements in Oxidative Stress Protection: Key to Healthy Fertility and Reproduction

Oxidative stress and its relationship to fertility and reproduction is a topic of interest in medicine, especially in the context of the effects of trace elements and micronutrients. Oxidative stress occurs when there is an excess of free radicals in the body, which can lead to cell and tissue damage. Free radicals are reactive oxygen species (ROS) that can be formed as a result of normal metabolic processes, as well as under the influence of external factors such as environmental pollution, UV radiation, and diet. Oxidative stress has a significant impact on fertility. In men, it can lead to DNA damage in sperm, which can result in reduced semen quality, reduced sperm motility and increased numbers of defective sperm, and free radical damage to sperm cell membranes causing a reduction in the number of available sperm. In women, oxidative stress can affect the quality of female reproductive cells, which can lead to problems with their maturation and with embryo implantation in the uterus and can also affect ovarian function and disrupt hormonal regulation of the menstrual cycle. A proper balance of trace elements and micronutrients is key to protecting against oxidative stress and maintaining reproductive health. Supplementation with appropriate elements such as zinc, selenium, copper, manganese, chromium, and iron can help reduce oxidative stress and improve fertility. This work discusses the effects of selected elements on oxidative stress parameters specifically in terms of fertility and reproduction.

Mechanisms of exercise intervention in type 2 diabetes: a bibliometric and visualization analysis based on CiteSpace

Objective

Type 2 diabetes (T2D) is a common chronic metabolic disease, and its prevalence is increasing globally. Exercise is crucial for T2D management, yet many aspects of its mechanisms remain unclear. This study employs CiteSpace to reveal research hotspots and frontier issues in exercise intervention for T2D.

Method

A literature review spanning from January 1, 2013 to December 31, 2022, was conducted using the Web of Science Core Collection (WoSCC), with keywords including "exercise," "type 2 diabetes," and "mechanisms." We analyzed network diagrams generated by CiteSpace, which depicted relationships among countries, authors, and keywords.

Results

This study includes 1,210 English papers from 555 journals, affiliated with 348 institutions across 80 countries/regions. Notably, the United States, China, and the United Kingdom account for nearly half of all publications. The University of Copenhagen leads in publication volume, followed by Harvard Medical School and the University of Colorado. Key authors include Kirwan, John P (Case Western Reserve University), Malin, Steven K (Rutgers University), and Pedersen, Bente Klarlund (University of Copenhagen). Based on co-occurrence analysis of keywords, it is evident that terms such as "disease," "glucagon-like peptide 1," and "cardiovascular risk factor" exhibit high intermediary centrality.

Conclusion

The analysis highlights ongoing investigations into molecular mechanisms, such as β-cell function enhancement, exerkines, and epigenetic mechanisms. Emerging areas include exercise response heterogeneity, circadian rhythm regulation, transcription factors, neurotrophic factors, and mitochondrial function. Future studies should prioritize understanding interactions between different exercise mechanisms and optimizing exercise prescriptions for T2D. Exercise prescriptions are crucial for effective interventions. Collaboration between countries and institutions is essential to understand the influences of different genetic backgrounds and environmental factors. Currently, a combination of aerobic and resistance training is considered the optimal form of exercise. However, considering time efficiency, high-intensity interval training (HIIT) has gained widespread attention and research due to its ability to achieve similar exercise effects in a shorter duration. Additionally, circadian rhythm regulation may affect the exercise outcomes of diabetic individuals at different times of the day, particularly concerning the specific types, doses, and intensities used for precision intervention in T2D.

Research progress on the biological regulatory mechanisms of selenium on skeletal muscle in broilers

As one of the indispensable trace elements for both humans and animals, selenium widely participates in multiple physiological processes and facilitates strong anti-inflammatory, antioxidant, and immune enhancing abilities. The biological functions of selenium are primarily driven by its presence in selenoproteins as a form of selenocysteine. Broilers are highly sensitive to selenium intake. Recent reports have demonstrated that selenium deficiency can adversely affect the quality of skeletal muscles and the economic value of broilers; the regulatory roles of several key selenoproteins (e.g., GPX1, GPX4, TXNRD1, TXNRD3, SelK, SelT, and SelW) have been identified. Starting from the selenium metabolism and its biological utilization in the skeletal muscle, the effect of the selenium antioxidant function on broiler meat quality is discussed in detail. The progress of research into the prevention of skeletal muscle injury by selenium and selenoproteins is also summarized. The findings emphasize the necessity of in vivo and in vitro research, and certain mechanism problems are identified, which aids their further examination. This mini-review will be helpful to provide a theoretical basis for the further study of regulatory mechanisms of selenium nutrition in edible poultry.

Inter-organ crosstalk during development and progression of type 2 diabetes mellitus

Type 2 diabetes mellitus (T2DM) is characterized by tissue-specific insulin resistance and pancreatic β-cell dysfunction, which result from the interplay of local abnormalities within different tissues and systemic dysregulation of tissue crosstalk. The main local mechanisms comprise metabolic (lipid) signalling, altered mitochondrial metabolism with oxidative stress, endoplasmic reticulum stress and local inflammation. While the role of endocrine dysregulation in T2DM pathogenesis is well established, other forms of inter-organ crosstalk deserve closer investigation to better understand the multifactorial transition from normoglycaemia to hyperglycaemia. This narrative Review addresses the impact of certain tissue-specific messenger systems, such as metabolites, peptides and proteins and microRNAs, their secretion patterns and possible alternative transport mechanisms, such as extracellular vesicles (exosomes). The focus is on the effects of these messengers on distant organs during the development of T2DM and progression to its complications. Starting from the adipose tissue as a major organ relevant to T2DM pathophysiology, the discussion is expanded to other key tissues, such as skeletal muscle, liver, the endocrine pancreas and the intestine. Subsequently, this Review also sheds light on the potential of multimarker panels derived from these biomarkers and related multi-omics for the prediction of risk and progression of T2DM, novel diabetes mellitus subtypes and/or endotypes and T2DM-related complications.

Astaxanthin: Past, Present, and Future

Astaxanthin (AX), a lipid-soluble pigment belonging to the xanthophyll carotenoids family, has recently garnered significant attention due to its unique physical properties, biochemical attributes, and physiological effects. Originally recognized primarily for its role in imparting the characteristic red-pink color to various organisms, AX is currently experiencing a surge in interest and research. The growing body of literature in this field predominantly focuses on AXs distinctive bioactivities and properties. However, the potential of algae-derived AX as a solution to various global environmental and societal challenges that threaten life on our planet has not received extensive attention. Furthermore, the historical context and the role of AX in nature, as well as its significance in diverse cultures and traditional health practices, have not been comprehensively explored in previous works. This review article embarks on a comprehensive journey through the history leading up to the present, offering insights into the discovery of AX, its chemical and physical attributes, distribution in organisms, and biosynthesis. Additionally, it delves into the intricate realm of health benefits, biofunctional characteristics, and the current market status of AX. By encompassing these multifaceted aspects, this review aims to provide readers with a more profound understanding and a robust foundation for future scientific endeavors directed at addressing societal needs for sustainable nutritional and medicinal solutions. An updated summary of AXs health benefits, its present market status, and potential future applications are also included for a well-rounded perspective.

An efficient selenium transport pathway of selenoprotein P utilizing a high-affinity ApoER2 receptor variant and being independent of selenocysteine lyase

Selenoprotein P (SeP, encoded by the SELENOP gene) is a plasma protein that contains selenium in the form of selenocysteine residues (Sec, a cysteine analog containing selenium instead of sulfur). SeP functions for the transport of selenium to specific tissues in a receptor-dependent manner. Apolipoprotein E receptor 2 (ApoER2) has been identified as a SeP receptor. However, diverse variants of ApoER2 have been reported, and the details of its tissue specificity and the molecular mechanism of its efficiency remain unclear. In the present study, we found that human T lymphoma Jurkat cells have a high ability to utilize selenium via SeP, while this ability was low in human rhabdomyosarcoma cells. We identified an ApoER2 variant with a high affinity for SeP in Jurkat cells. This variant had a dissociation constant value of 0.67 nM and a highly glycosylated O-linked sugar domain. Moreover, the acidification of intracellular vesicles was necessary for selenium transport via SeP in both cell types. In rhabdomyosarcoma cells, SeP underwent proteolytic degradation in lysosomes and transported selenium in a Sec lyase-dependent manner. However, in Jurkat cells, SeP transported selenium in Sec lyase-independent manner. These findings indicate a preferential selenium transport pathway involving SeP and high-affinity ApoER2 in a Sec lyase-independent manner. Herein, we provide a novel dynamic transport pathway for selenium via SeP.

Redox Balance in Type 2 Diabetes: Therapeutic Potential and the Challenge of Antioxidant-Based Therapy

Oxidative stress is an important factor in the development of type 2 diabetes (T2D) and associated complications. Unfortunately, most clinical studies have failed to provide sufficient evidence regarding the benefits of antioxidants (AOXs) in treating this disease. Based on the known complexity of reactive oxygen species (ROS) functions in both the physiology and pathophysiology of glucose homeostasis, it is suggested that inappropriate dosing leads to the failure of AOXs in T2D treatment. To support this hypothesis, the role of oxidative stress in the pathophysiology of T2D is described, together with a summary of the evidence for the failure of AOXs in the management of diabetes. A comparison of preclinical and clinical studies indicates that suboptimal dosing of AOXs might explain the lack of benefits of AOXs. Conversely, the possibility that glycemic control might be adversely affected by excess AOXs is also considered, based on the role of ROS in insulin signaling. We suggest that AOX therapy should be given in a personalized manner according to the need, which is the presence and severity of oxidative stress. With the development of gold-standard biomarkers for oxidative stress, optimization of AOX therapy may be achieved to maximize the therapeutic potential of these agents.

The Role of Selected Trace Elements in Oxidoreductive Homeostasis in Patients with Thyroid Diseases

Impaired levels of selenium (Se), zinc (Zn), copper (Cu), iron (Fe), manganese (Mn) and iodine (I) in the organism may adversely affect the thyroid endocrine system. These trace elements play a role in the fight against oxidative stress as components of enzymes. Oxidative-antioxidant imbalance is considered a possible factor in many pathological conditions, including various thyroid diseases. In the available literature, there are few scientific studies showing a direct correlation of the effect of supplementation of trace elements on slowing down or preventing the occurrence of thyroid diseases in combination with the improvement of the antioxidant profile, or through the action of these elements as antioxidants. Among the available studies, it has been shown that an increase in lipid peroxidation levels and a decrease in the overall antioxidant defense status occur during such thyroid diseases as thyroid cancer, Hashimoto's thyroiditis and dysthyroidism. In studies in which trace elements were supplemented, the following were observed: a decrease in the level of malondialdehyde after supplementation with Zn during hypothyroidism and reduction in the malondialdehyde level after Se supplementation with a simultaneous increase in the total activity status and activity of antioxidant defense enzymes in the course of autoimmune thyroiditis. This systematic review aimed to present the current state of knowledge about the relationship between trace elements and thyroid diseases in terms of oxidoreductive homeostasis.

Selenium Protects Mouse Hypothalamic Cells from Glucocorticoid-Induced Endoplasmic Reticulum Stress Vulnerability and Insulin Signaling Impairment

The use of glucocorticoid medications is known to cause metabolic side effects such as overeating, excess weight gain, and insulin resistance. The hypothalamus, a central regulator of feeding behavior and energy expenditure, is highly responsive to glucocorticoids, and it has been proposed that it plays a role in glucocorticoid-induced metabolic defects. Glucocorticoids can alter the expression and activity of antioxidant enzymes and promote the accumulation of reactive oxygen species. Recent evidence indicates that selenium can counter the effects of glucocorticoids, and selenium is critical for proper hypothalamic function. This study sought to determine whether selenium is capable of protecting hypothalamic cells from dysfunction caused by glucocorticoid exposure. We treated mHypoE-44 mouse hypothalamic cells with corticosterone to study the effects on cellular physiology and the involvement of selenium. We found that corticosterone administration rendered cells more vulnerable to endoplasmic reticulum stress and the subsequent impairment of insulin signaling. Supplementing the cell culture media with additional selenium alleviated endoplasmic reticulum stress and promoted insulin signaling. These findings implicate a protective role of selenium against chronic glucocorticoid-induced hypothalamic dysfunction.

Cardiac Hepatopathy: New Perspectives on Old Problems through a Prism of Endogenous Metabolic Regulations by Hepatokines

Cardiac hepatopathy refers to acute or chronic liver damage caused by cardiac dysfunction in the absence of any other possible causative reasons of liver injury. There is a large number of evidence of the fact that cardiac hepatopathy is associated with poor clinical outcomes in patients with acute or actually decompensated heart failure (HF). However, the currently dominated pathophysiological background does not explain a role of metabolic regulative proteins secreted by hepatocytes in progression of HF, including adverse cardiac remodeling, kidney injury, skeletal muscle dysfunction, osteopenia, sarcopenia and cardiac cachexia. The aim of this narrative review was to accumulate knowledge of hepatokines (adropin; fetuin-A, selenoprotein P, fibroblast growth factor-21, and alpha-1-microglobulin) as adaptive regulators of metabolic homeostasis in patients with HF. It is suggested that hepatokines play a crucial, causative role in inter-organ interactions and mediate tissue protective effects counteracting oxidative stress, inflammation, mitochondrial dysfunction, apoptosis and necrosis. The discriminative potencies of hepatokines for HF and damage of target organs in patients with known HF is under on-going scientific discussion and requires more investigations in the future.

Relationship between selenium status, selenoproteins and COVID-19 and other inflammatory diseases: A critical review

The antioxidant effects of selenium as a component of selenoproteins has been thought to modulate host immunity and viral pathogenesis. Accordingly, the association of low dietary selenium status with inflammatory and immunodeficiency has been reported in the literature; however, the causal role of selenium deficiency in chronic inflammatory diseases and viral infection is still undefined. The COVID-19, characterized by acute respiratory syndrome and caused by the novel coronavirus 2, SARS-CoV-2, has infected millions of individuals worldwide since late 2019. The severity and mortality from COVID-19 have been associated with several factor, including age, sex and selenium deficiency. However, available data on selenium status and COVID-19 are limited, and a possible causative role for selenium deficiency in COVID-19 severity has yet to be fully addressed. In this context, we review the relationship between selenium, selenoproteins, COVID-19, immune and inflammatory responses, viral infection, and aging. Regardless of the role of selenium in immune and inflammatory responses, we emphasize that selenium supplementation should be indicated after a selenium deficiency be detected, particularly, in view of the critical role played by selenoproteins in human health. In addition, the levels of selenium should be monitored after the start of supplementation and discontinued as soon as normal levels are reached. Periodic assessment of selenium levels after supplementation is a critical issue to avoid over production of toxic metabolites of selenide because under normal conditions, selenoproteins attain saturated expression levels that limits their potential deleterious metabolic effects.

Circulating selenoprotein P levels predict glucose-lowering and insulinotropic effects of metformin, but not alogliptin: A post-hoc analysis

AIMS/INTRODUCTION

Selenoprotein P (SeP; encoded by SEPP1 in humans) is a hepatokine that causes impaired insulin secretion and insulin resistance. Metformin downregulates SELENOP promoter activity through an adenosine monophosphate-activated kinase-forkhead box protein O3a pathway in hepatocytes. This study aimed to test our hypothesis that circulating SeP levels are associated with the glucose-lowering effect of metformin in humans.

MATERIALS AND METHODS

A total of 84 participants with poorly controlled type 2 diabetes were randomly assigned to receive metformin (1,000 mg, twice daily) or a dipeptidyl peptidase-4 inhibitor, alogliptin (25 mg, once daily) for 12 weeks. We tested metformin and alogliptin on SeP levels and factors associated therewith as a post-hoc analysis.

RESULTS

Both metformin and aloglipitin did not change the SeP levels. Although metformin significantly increased the insulin secretory index secretory units of islets in transplantation only in participants with higher baseline SeP (>3.87), both agents similarly reduced fasting plasma glucose and glycated hemoglobin. SeP levels at baseline were correlated negatively with changes in SeP (r = -0.484, P = 0.004) and fasting plasma glucose (r = -0.433, P = 0.011), and positively with changes in C-peptide immunoreactivity (r = 0.420, P = 0.017) and secretory units of islets in transplantation (r = 0.388, P = 0.028) in the metformin, but not alogliptin, group.

CONCLUSIONS

Higher baseline levels of SeP significantly predicted metformin-mediated, but not alogliptin-mediated, glucose-lowering and insulinotropic effects. Serum SeP levels might be a novel biomarker for predicting the outcomes of metformin therapy, which might be helpful in tailoring diabetes medication.

Selenium in Bodily Homeostasis: Hypothalamus, Hormones, and Highways of Communication

The ability of the body to maintain homeostasis requires constant communication between the brain and peripheral tissues. Different organs produce signals, often in the form of hormones, which are detected by the hypothalamus. In response, the hypothalamus alters its regulation of bodily processes, which is achieved through its own pathways of hormonal communication. The generation and transmission of the molecules involved in these bi-directional axes can be affected by redox balance. The essential trace element selenium is known to influence numerous physiological processes, including energy homeostasis, through its various redox functions. Selenium must be obtained through the diet and is used to synthesize selenoproteins, a family of proteins with mainly antioxidant functions. Alterations in selenium status have been correlated with homeostatic disturbances in humans and studies with animal models of selenoprotein dysfunction indicate a strong influence on energy balance. The relationship between selenium and energy metabolism is complicated, however, as selenium has been shown to participate in multiple levels of homeostatic communication. This review discusses the role of selenium in the various pathways of communication between the body and the brain that are essential for maintaining homeostasis.

Antioxidant effects of Se-glutathione peroxidase in alcoholic liver disease

Oxidative damage induced by ethanol and its metabolites is one of the factors that fuels the development of alcoholic liver disease (ALD). Selenium (Se) is an effective cofactor for glutathione peroxidase (GPx), and has antioxidant effects that improve ALD. In patients with ALD, ethanol-induced oxidative damage inhibits the synthesis of related Se-containing proteins such as: selenoprotein P (Sepp1), albumin (ALB), and GPx in the liver, thus decreasing the overall Se level in patients. Both Se deficiency and excess can affect the expression of GPx, resulting in damage to the antioxidant defense system. This damage enhances oxidative stress by increasing the levels of reactive oxygen species (ROS) in the body, which aggravates the inflammatory response, lipid metabolism disorder, and lipid peroxidation and worsens ALD symptoms. A cascade of oxidative damages caused by ALD will deplete selenium deposition in the body, stimulate the expression of Gpx1, Sepp1, and Gpx4, and thus mobilize systemic selenoproteins, which can restore GPx activity in the hepatocytes of ALD patients, reduce the levels of reactive oxygen species and alleviate oxidative stress, the inflammatory response, lipid metabolism disorder, and lipid peroxidation, thus helping to mitigate ALD. This review provides a reference for future ALD studies that evaluate the regulation of Se levels and contributes to studies on the potential pathological mechanisms of Se imbalance in ALD.

Mineral metabolism and ferroptosis in non-alcoholic fatty liver diseases

Nonalcoholic fatty liver disease (NAFLD) has become the most prevalent chronic liver disease worldwide. Minerals including iron, copper, zinc, and selenium, fulfil an essential role in various biochemical processes. Moreover, the identification of ferroptosis and cuproptosis further underscores the importance of intracellular mineral homeostasis. However, perturbation of minerals has been frequently reported in patients with NAFLD and related diseases. Interestingly, studies have attempted to establish an association between mineral disorders and NAFLD pathological features, including oxidative stress, mitochondrial dysfunction, inflammatory response, and fibrogenesis. In this review, we aim to provide an overview of the current understanding of mineral metabolism (i.e., absorption, utilization, and transport) and mineral interactions in the pathogenesis of NAFLD. More importantly, this review highlights potential therapeutic strategies, challenges, future directions for targeting mineral metabolism in the treatment of NAFLD.

Cyclosporine A Downregulates Selenoprotein P Expression via a Signal Transducer and Activator of Transcription 3-Forkhead Box Protein O1 Pathway in Hepatocytes In Vitro

Cyclosporine A (CsA) is an immunosuppressant applied worldwide for preventing graft rejection and autoimmune diseases. However, CsA elevates oxidative stress, which can lead to liver injuries. The present study aimed to clarify the mechanisms underlying the CsA-mediated oxidative stress. Among the redox proteins, CsA concentration-dependently downregulated Selenop-encoding selenoprotein P, a major circulating antioxidant protein reducing reactive oxygen species, in hepatocytes cell lines and primary hepatocytes. The luciferase assay identified the CsA-responsive element in the SELENOP promoter containing a putative binding site for forkhead box protein O (FoxO) 1. The CsA-mediated suppression on the SELENOP promoter was independent of the nuclear factor of activated T-cell, a classic target repressed by CsA. A chromatin immunoprecipitation assay showed that CsA suppressed the FoxO1 binding to the SELENOP promoter. Foxo1 knockdown significantly downregulated Selenop expression in H4IIEC3 cells. Furthermore, CsA downregulated FoxO1 by inactivating its upstream signal transducer and activator of transcription 3 (STAT3). Knockdown of Stat3 downregulated Foxo1 and Selenop expression in hepatocytes. These findings revealed a novel mechanism underlying CsA-induced oxidative stress by downregulating the STAT3-FoxO1-Selenop pathway in hepatocytes. SIGNIFICANCE STATEMENT: This study shows that Cyclosporine A (CsA) downregulates Selenop, an antioxidant protein, by suppressing the signal transducer and activator of transcription 3-forkhead box protein O1 pathway in hepatocytes, possibly one of the causations of CsA-induced oxidative stress in hepatocytes. The present study sheds light on the previously unrecognized CsA-redox axis.

Selenium Status and Its Antioxidant Role in Metabolic Diseases

Selenium (Se), in the form of selenoproteins, is an essential micronutrient that plays an important role in human health and disease. To date, there are at least 25 selenoproteins in humans involved in a wide variety of biological functions, including mammalian development, metabolic progress, inflammation response, chemoprotective properties, and most notably, oxidoreductase functions. In recent years, numerous studies have reported that low Se levels are associated with increased risk, poor outcome, and mortality of metabolic disorders, mainly related to the limited antioxidant defense resulting from Se deficiency. Moreover, the correlation between Se deficiency and Keshan disease has received considerable attention. Therefore, Se supplementation as a therapeutic strategy for preventing the occurrence, delaying the progression, and alleviating the outcomes of some diseases has been widely studied. However, supranutritional levels of serum Se may have adverse effects, including Se poisoning. This review evaluates the correlation between Se status and human health, with particular emphasis on the antioxidant benefits of Se in metabolic disorders, shedding light on clinical treatment.

Cyclosporine A downregulates selenoprotein P expression via a STAT3-FoxO1 pathway in hepatocytes in vitro

Cyclosporine A (CsA) is a worldwide applied immunosuppressant for preventing graft rejection and autoimmune diseases. However, CsA elevates oxidative stress leading to liver injuries. The present study aimed to clarify the mechanisms underlying the CsA-mediated oxidative stress. Among the redox proteins, CsA concentration-dependently downregulated Selenop encoding selenoprotein P (SeP), a major circulating antioxidant protein reducing reactive oxygen species (ROS), in hepatocytes cell lines and primary hepatocytes. The luciferase assay identified the CsA-responsive element in the SELENOP promoter containing a putative binding site for FoxO1. The CsA-mediated suppression on the SELENOP promoter was independent of NFAT, a classic target repressed by CsA. A ChIP assay showed that CsA suppressed the FoxO1 binding to the SELENOP promoter. Foxo1 knockdown significantly downregulated Selenop expression in H4IIEC3 cells. Furthermore, CsA downregulated FoxO1 by inactivating its upstream signal transducer and activator of transcription 3 (STAT3). Knockdown of Stat3 downregulated Foxo1 and Selenop expression in hepatocytes. These findings revealed a novel mechanism underlying CsA-induced oxidative stress via downregulating the STAT3-FoxO1-Selenop pathway in hepatocytes. Significance Statement Our study shows that CsA downregulates Selenop, an antioxidant protein, via suppressing the STAT3-FoxO1 pathway in hepatocytes, possibly one of the causations of CsA-induced oxidative stress in hepatocytes. The present study sheds light on the previously unrecognized CsA-redox axis.

Associations between Circulating SELENOP Level and Disorders of Glucose and Lipid Metabolism: A Meta-Analysis

Selenoprotein P (SELENOP) is an extracellular antioxidant, selenium transporter, and hepatokine interfering with glucose and lipid metabolism. To study the association between the circulating SELENOP concentration and glucose and lipid metabolic diseases (GLMDs), including gestational diabetes (GD), metabolic syndrome (MetS), non-alcoholic fatty liver disease, obesity, and type 2 diabetes, as well as the individual markers, a meta-analysis was conducted by searching multiple databases from their establishment through March 2022 and including 27 articles published between October 2010 and May 2021, involving 4033 participants. Participants with GLMDs had higher levels of SELENOP than those without GLMDs (standardized mean difference = 0.84, 95% CI: 0.16 to 1.51), and the SELENOP levels were positively correlated with the markers of GLMDs (pooled effect size = 0.09, 95% CI: 0.02 to 0.15). Subgroup analyses showed that the SELENOP concentrations were higher in women with GD and lower in individuals with MetS than their counterparts, respectively. Moreover, SELENOP was positively correlated with low-density lipoprotein cholesterol, but not with the other markers of GLMDs. Thus, the heterogenicity derived from diseases or disease markers should be carefully considered while interpreting the overall positive association between SELENOP and GLMDs. Studies with a larger sample size and advanced design are warranted to confirm these findings.

Selenium and Selenoproteins at the Intersection of Type 2 Diabetes and Thyroid Pathophysiology

Type 2 diabetes (T2D) is considered one of the largest global public-health concerns, affecting approximately more than 400 million individuals worldwide. The pathogenesis of T2D is very complex and, among the modifiable risk factors, selenium (Se) has recently emerged as a determinant of T2D pathogenesis and progression. Selenium is considered an essential element with antioxidant properties, and is incorporated into the selenoproteins involved in the antioxidant response. Furthermore, deiodinases, the enzymes responsible for homeostasis and for controlling the activity of thyroid hormones (THs), contain Se. Given the crucial action of oxidative stress in the onset of insulin resistance (IR) and T2D, and the close connection between THs and glucose metabolism, Se may be involved in these fundamental relationships; it may cover a dual role, both as a protective factor and as a risk factor of T2D, depending on its basal plasma concentration and the individual’s diet intake. In this review we discuss the current evidence (from experimental, observational and randomized clinical studies) on how Se is associated with the occurrence of T2D and its influence on the relationship between thyroid pathophysiology, IR and T2D.

NADPH Oxidases Connecting Fatty Liver Disease, Insulin Resistance and Type 2 Diabetes: Current Knowledge and Therapeutic Outlook

Nonalcoholic fatty liver disease (NAFLD), characterized by ectopic fat accumulation in hepatocytes, is closely linked to insulin resistance and is the most frequent complication of type 2 diabetes mellitus (T2DM). One of the features connecting NAFLD, insulin resistance and T2DM is cellular oxidative stress. Oxidative stress refers to a redox imbalance due to an inequity between the capacity of production and the elimination of reactive oxygen species (ROS). One of the major cellular ROS sources is NADPH oxidase enzymes (NOX-es). In physiological conditions, NOX-es produce ROS purposefully in a timely and spatially regulated manner and are crucial regulators of various cellular events linked to metabolism, receptor signal transmission, proliferation and apoptosis. In contrast, dysregulated NOX-derived ROS production is related to the onset of diverse pathologies. This review provides a synopsis of current knowledge concerning NOX enzymes as connective elements between NAFLD, insulin resistance and T2DM and weighs their potential relevance as pharmacological targets to alleviate fatty liver disease.

Role of selenoprotein P expression in the function of pancreatic β cells: Prevention of ferroptosis-like cell death and stress-induced nascent granule degradation

Selenoprotein P (SELENOP) is a major selenium (Se)-containing protein (selenoprotein) in human plasma that is mainly synthesized in the liver. SELENOP transports Se to the cells, while SELENOP synthesized in peripheral tissues is incorporated in a paracrine/autocrine manner to maintain the levels of cellular selenoproteins, called the SELENOP cycle. Pancreatic β cells, responsible for the synthesis and secretion of insulin, are known to express SELENOP. Here, using MIN6 cells as a mouse model for pancreatic β cells and Selenop small interfering (si)RNA, we found that Selenop gene knockdown (KD) resulted in decreased cell viability, cellular pro/insulin levels, insulin secretion, and levels of several cellular selenoproteins, including glutathione peroxidase 4 (Gpx4) and selenoprotein K (Selenok). These dysfunctions induced by Selenop siRNA were recovered by the addition of Se. Ferroptosis-like cell death, regulated by Gpx4, was involved in the decrease of cell viability by Selenop KD, while stress-induced nascent granule degradation (SINGD), regulated by Selenok, was responsible for the decrease in proinsulin. SINGD was also observed in the pancreatic β cells of Selenop knockout mice. These findings indicate a significant role of SELENOP expression for the function of pancreatic β cells by maintaining the levels of cellular selenoproteins such as GPX4 and SELENOK.

Selenoprotein P-mediated reductive stress impairs cold-induced thermogenesis in brown fat

Reactive oxygen species (ROS) activate uncoupler protein 1 (UCP1) in brown adipose tissue (BAT) under physiological cold exposure and noradrenaline (NA) stimulation to increase thermogenesis. However, the endogenous regulator of ROS in activated BAT and its role in pathological conditions remain unclear. We show that serum levels of selenoprotein P (SeP; encoded by SELENOP) negatively correlate with BAT activity in humans. Physiological cold exposure downregulates Selenop in BAT. Selenop knockout mice show higher rectal temperatures and UCP1 sulfenylation during cold exposure. SeP treatment to brown adipocytes eliminated the NA-induced mitochondrial ROS by upregulating glutathione peroxidase 4 and impaired cellular thermogenesis. A high-fat/high-sucrose diet elevates serum SeP levels and diminishes the elevated NA-induced thermogenesis in BAT-Selenop KO mice. Therefore, SeP is the intrinsic factor inducing reductive stress that impairs thermogenesis in BAT and may be a potential therapeutic target for obesity and diabetes.

Safety of selenium exposure and limitations of selenoprotein maximization: Molecular and epidemiologic perspectives

Recent evidence from laboratory and epidemiologic studies has shed a different light on selenium health effects and its recommended range of environmental exposure, compared with earlier research. Specifically, epidemiologic studies in Western populations have shown adverse effects of selenium exposure at low levels, sometimes below or slightly above selenium intakes needed to maximize selenoprotein expression and activity. In addition, three recent lines of evidence in molecular and biochemical studies suggest some potential drawbacks associated with selenoprotein maximization: 1) the possibility that selenoprotein upregulation is a compensatory response to oxidative challenge, induced by selenium itself or other oxidants; 2) the capacity of selenoproteins to trigger tumor growth in some circumstances; and 3) the deleterious metabolic effects of selenoproteins and particularly of selenoprotein P. The last observation provides a toxicological basis to explain why in humans selenium intake levels as low as 60 μg/day, still in the range of selenium exposure upregulating selenoprotein expression, might start to increase risk of type 2 diabetes. Overall, these new pieces of evidence from the literature call into question the purported benefit of selenoprotein maximization, and indicate the need to reassess selenium dietary reference values and upper intake level. This reassessment should clarify which range of selenoprotein upregulation follows restoration of adequate selenium availability and which range is driven by a compensatory response to selenium toxicity and oxidative stress.

Natural Autoimmunity to Selenoprotein P Impairs Selenium Transport in Hashimoto's Thyroiditis

The essential trace element selenium (Se) is needed for the biosynthesis of selenocysteine-containing selenoproteins, including the secreted enzyme glutathione peroxidase 3 (GPX3) and the Se-transporter selenoprotein P (SELENOP). Both are found in blood and thyroid colloid, where they serve protective functions. Serum SELENOP derives mainly from hepatocytes, whereas the kidney contributes most serum GPX3. Studies using transgenic mice indicated that renal GPX3 biosynthesis depends on Se supply by hepatic SELENOP, which is produced in protein variants with varying Se contents. Low Se status is an established risk factor for autoimmune thyroid disease, and thyroid autoimmunity generates novel autoantigens. We hypothesized that natural autoantibodies to SELENOP are prevalent in thyroid patients, impair Se transport, and negatively affect GPX3 biosynthesis. Using a newly established quantitative immunoassay, SELENOP autoantibodies were particularly prevalent in Hashimoto’s thyroiditis as compared with healthy control subjects (6.6% versus 0.3%). Serum samples rich in SELENOP autoantibodies displayed relatively high total Se and SELENOP concentrations in comparison with autoantibody-negative samples ([Se]; 85.3 vs. 77.1 µg/L, p = 0.0178, and [SELENOP]; 5.1 vs. 3.5 mg/L, p = 0.001), while GPX3 activity was low and correlated inversely to SELENOP autoantibody concentrations. In renal cells in culture, antibodies to SELENOP inhibited Se uptake. Our results indicate an impairment of SELENOP-dependent Se transport by natural SELENOP autoantibodies, suggesting that the characterization of health risk from Se deficiency may need to include autoimmunity to SELENOP as additional biomarker of Se status.

Hepatic Proteomic Analysis of Selenoprotein T Knockout Mice by TMT: Implications for the Role of Selenoprotein T in Glucose and Lipid Metabolism

Selenoprotein T (SELENOT, SelT), a thioredoxin-like enzyme, exerts an essential oxidoreductase activity in the endoplasmic reticulum. However, its precise function remains unknown. To gain more understanding of SELENOT function, a conventional global Selenot knockout (KO) mouse model was constructed for the first time using the CRISPR/Cas9 technique. Deletion of SELENOT caused male sterility, reduced size/body weight, lower fed and/or fasting blood glucose levels and lower fasting serum insulin levels, and improved blood lipid profile. Tandem mass tag (TMT) proteomics analysis was conducted to explore the differentially expressed proteins (DEPs) in the liver of male mice, revealing 60 up-regulated and 94 down-regulated DEPs in KO mice. The proteomic results were validated by western blot of three selected DEPs. The elevated expression of Glycogen [starch] synthase, liver (Gys2) is consistent with the hypoglycemic phenotype in KO mice. Furthermore, the bioinformatics analysis showed that Selenot-KO-induced DEPs were mainly related to lipid metabolism, cancer, peroxisome proliferator-activated receptor (PPAR) signaling pathway, complement and coagulation cascades, and protein digestion and absorption. Overall, these findings provide a holistic perspective into SELENOT function and novel insights into the role of SELENOT in glucose and lipid metabolism, and thus, enhance our understanding of SELENOT function.

Dietary selenium intake and risk of hospitalization for type 2 diabetes in the Moli-sani study cohort

BACKGROUND AND AIMS

Experimental and non-experimental human studies have consistently shown a positive association between exposure to the trace element selenium, which occurs primarily through diet, and risk of type 2 diabetes mellitus. Plausible biological mechanisms include adverse effects of selenium and selenium-containing proteins on glucose metabolism. However, the levels of exposure above which risk increases are uncertain.

METHODS AND RESULTS

We examined the association between selenium intake and first hospitalization for type 2 diabetes during a median follow-up period of 8.2 years among 21,335 diabetes-free participants in the Moli-sani cohort, Italy. Selenium intake was ascertained at baseline using a food frequency questionnaire, showing a median value of 59 μg/day. During follow-up, we identified 135 incident cases of hospitalization for diabetes, based on population-based hospital discharge data. We used a Cox proportional hazards model to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for hospitalization for diabetes, adjusting for potential confounders. HRs (95% CIs) were 1.01 (0.60-1.70), 1.13 (0.66-1.96) and 1.75 (0.99-3.10) comparing the second, third, and fourth sex-specific quartiles with the first quartile, respectively. Risk was 64% greater in the fourth quartile as compared with the previous three. Spline regression analysis also indicated a steeper increase in risk occurring among men compared with women.

CONCLUSIONS

In a large population of Italian adults free of type 2 diabetes at cohort entry, high dietary selenium intake was associated with increased risk of hospitalization for diabetes.

A systematic review and dose-response meta-analysis of exposure to environmental selenium and the risk of type 2 diabetes in nonexperimental studies

Accumulating evidence from both experimental and nonexperimental human studies in the last 15 years indicates that exposure to high levels of the trace element selenium increases the risk of type 2 diabetes. However, the relation of dose to effect is not well understood because randomized controlled trials used only one dose (200 μg/day) of selenium supplementation. While no new trial on this topic has been published since 2018, several nonexperimental studies have appeared. We therefore updated a previous meta-analysis to include recently published observational studies, and incorporated the recently developed one-stage random-effects model to display the dose-response relation between selenium and type 2 diabetes. We retrieved 34 potentially eligible nonexperimental studies on selenium and diabetes risk up to April 15, 2021. The bulk of the evidence indicates a direct relation between blood, dietary and urinary levels of selenium and risk of diabetes, but not with nail selenium, which may be considered a less reliable biomarker. The association was nonlinear, with risk increasing above 80 μg/day of dietary selenium. Whole blood/plasma/serum selenium concentrations of 160 μg/L corresponded to a risk ratio of 1.96 (95% CI 1.27-3.03) compared with a concentration of 90 μg/L (approximately 60 μg of daily selenium intake). The cohort studies, which are less susceptible to reverse causation bias, indicated increased risk for both blood and urine selenium levels and dietary selenium intake, whereas no such pattern emerged from studies relying on nail selenium content. Overall, the nonexperimental studies agree with findings from randomized controlled trials, indicating that moderate to high levels of selenium exposure are associated with increased risk for type 2 diabetes.

Selenium Transport Mechanism via Selenoprotein P-Its Physiological Role and Related Diseases

Selenoprotein P (SELENOP) is selenium (Se)-containing protein in plasma, which is primarily produced in the liver. The “P” in SELENOP originated from the presence in plasma. SELENOP contains selenocysteine, a cysteine analog containing Se instead of sulfur. SELENOP is a multi-functional protein to reduce phospholipid hydroperoxides and to deliver Se from the liver to other tissues, such as those of the brain and testis, playing a pivotal role in Se metabolism and antioxidative defense. Decrease in SELENOP causes various dysfunctions related to Se deficiency and oxidative stress, while excessive SELENOP causes insulin resistance. This review focuses on the Se transport system of SELENOP, particularly its molecular mechanism and physiological role in Se metabolism. Furthermore, the chemical form of Se and its biological meaning is discussed.

Reductive stress in cancer

Reductive stress is defined as a condition characterized by excess accumulation of reducing equivalents (e.g., NADH, NADPH, GSH), surpassing the activity of endogenous oxidoreductases. Excessive reducing equivalents can perturb cell signaling pathways, change the formation of disulfide bonding in proteins, disturb mitochondrial homeostasis or decrease metabolism. Reductive stress is influenced by cellular antioxidant load, its flux and a subverted homeostasis that paradoxically can result in excess ROS induction. Balanced reducing equivalents and antioxidant enzymes that contribute to reductive stress can be regulated by Nrf2, typically considered as an oxidative stress induced transcription factor. Cancer cells may coordinate distinct pools of redox couples under reductive stress and these may link to biological consequences from both molecular and translational standpoints. In cancer, there is recent interest in understanding how selective induction of reductive stress may influence therapeutic management and disease progression.

Exercise-Stimulated ROS Sensitive Signaling Pathways in Skeletal Muscle

Physical exercise represents a major challenge to whole-body homeostasis, provoking acute and adaptative responses at the cellular and systemic levels. Different sources of reactive oxygen species (ROS) have been described in skeletal muscle (e.g., NADPH oxidases, xanthine oxidase, and mitochondria) and are closely related to the physiological changes induced by physical exercise through the modulation of several signaling pathways. Many signaling pathways that are regulated by exercise-induced ROS generation, such as adenosine monophosphate-activated protein kinase (AMPK), mitogen activated protein kinase (MAPK), nuclear respiratory factor2 (NRF2), and PGC-1α are involved in skeletal muscle responses to physical exercise, such as increased glucose uptake, mitochondriogenesis, and hypertrophy, among others. Most of these adaptations are blunted by antioxidants, revealing the crucial role played by ROS during and after physical exercise. When ROS generation is either insufficient or exacerbated, ROS-mediated signaling is disrupted, as well as physical exercise adaptations. Thus, an understanding the limit between "ROS that can promote beneficial effects" and "ROS that can promote harmful effects" is a challenging question in exercise biology. The identification of new mediators that cause reductive stress and thereby disrupt exercise-stimulated ROS signaling is a trending on this topic and are covered in this current review.

Alcohol Intake Is Associated With Elevated Serum Levels of Selenium and Selenoprotein P in Humans

Selenoprotein P is a hepatokine with antioxidative properties that eliminate a physiologic burst of reactive oxygen species required for intracellular signal transduction. Serum levels of selenoprotein P are elevated during aging and in people with type 2 diabetes, non-alcoholic fatty liver disease, and hepatitis C. However, how serum levels of full-length selenoprotein P are regulated largely remains unknown, especially in the general population. To understand the significance of serum selenoprotein P levels in the general population, we evaluated intrinsic and environmental factors associated with serum levels of full-length selenoprotein P in 1,183 subjects participating in the Shika-health checkup cohort. Serum levels of selenium were positively correlated with liver enzymes and alcohol intake and negatively correlated with body mass index. Serum levels of selenoprotein P were positively correlated with age, liver enzymes, and alcohol intake. In multiple regression analyses, alcohol intake was positively correlated with serum levels of both selenium and selenoprotein P independently of age, gender, liver enzymes, and fatty liver on ultrasonography. In conclusion, alcohol intake is associated with elevated serum levels of selenium and selenoprotein P independently of liver enzyme levels and liver fat in the general population. Moderate alcohol intake may exert beneficial or harmful effects on health, at least partly by upregulating selenoprotein P. These findings increase our understanding of alcohol-mediated redox regulation and form the basis for the adoption of appropriate drinking guidelines.

The Impaired Bioenergetics of Diabetic Cardiac Microvascular Endothelial Cells

Diabetes causes hyperglycemia, which can create a stressful environment for cardiac microvascular endothelial cells (CMECs). To investigate the impact of diabetes on the cellular metabolism of CMECs, we assessed glycolysis by quantifying the extracellular acidification rate (ECAR), and mitochondrial oxidative phosphorylation (OXPHOS) by measuring cellular oxygen consumption rate (OCR), in isolated CMECs from wild-type (WT) hearts and diabetic hearts (db/db) using an extracellular flux analyzer. Diabetic CMECs exhibited a higher level of intracellular reactive oxygen species (ROS), and significantly reduced glycolytic reserve and non-glycolytic acidification, as compared to WT CMECs. In addition, OCR assay showed that diabetic CMECs had increased maximal respiration, and significantly reduced non-mitochondrial oxygen consumption and proton leak. Quantitative PCR (qPCR) showed no difference in copy number of mitochondrial DNA (mtDNA) between diabetic and WT CMECs. In addition, gene expression profiling analysis showed an overall decrease in the expression of essential genes related to β-oxidation (Sirt1, Acox1, Acox3, Hadha, and Hadhb), tricarboxylic acid cycle (TCA) (Idh-3a and Ogdh), and electron transport chain (ETC) (Sdhd and Uqcrq) in diabetic CMECs compared to WT CMECs. Western blot confirmed that the protein expression of Hadha, Acox1, and Uqcrq was decreased in diabetic CMECs. Although lectin staining demonstrated no significant difference in capillary density between the hearts of WT mice and db/db mice, diabetic CMECs showed a lower percentage of cell proliferation by Ki67 staining, and a higher percentage of cellular apoptosis by TUNEL staining, compared with WT CMECs. In conclusion, excessive ROS caused by hyperglycemia is associated with impaired glycolysis and mitochondrial function in diabetic CMECs, which in turn may reduce proliferation and promote CMEC apoptosis.