SELENOS ssociated with pathogenic factors for endothelial dysfunction
Research on the polygenic animal model of type 2 diabetes, Psammomys obesus, has shown that the expression of SELENOS in the liver is inversely correlated with circulating glucose and insulin levels and directly proportional with plasma TG concentrations [
6]. In hepatoma H4IIE cells, overexpression of SELENOS led to decreased hepatic glucose utilization by reducing glucose uptake, glycogen synthesis and content. It also mitigated the suppressive effect of insulin on gluconeogenesis, leading to increased hepatic glucose output [
47].
Interestingly, SELENOS was secreted from hepatoma HepG2 cells, but not from a variety of other examined cell types. These include human embryonic kidney 293 cells, kidney Cos7 cells, 3T3-L1 pre-adipocytes, skeletal muscle L6 cells, macrophage RAW264.7 cells, HUVECs, and HA/VSMCs [
12,
48]. When serum SELENOS levels were measured in healthy human subjects and those with type 1 and type 2 diabetes, the protein was detected in 65 out of 209 subjects, a detection rate of 31.1%. The average levels of these positive subjects across the three groups were not statistically different [
48].
However, these results appear inconsistent with other research. In a previous study, all tested subjects (100%) showed detectable serum SELENOS, and levels in type 2 diabetic patients were lower compared to healthy controls. In this research, there was a negative correlation observed between SELENOS levels and waist circumference (WC), as well as fasting plasma glucose (FPG) [
12]. Consistently, another study found lower serum SELENOS in metabolic syndrome patients and cardiovascular disease patients compared to patients without metabolic syndrome, and a negative relationship was noted between the levels of SELENOS, WC, and fasting blood sugar (FBS) [
49]. The disparities could possibly be attributed to the variations in race and geographic location of the human participants involved in the studies, as well as the different enzyme-linked immunosorbent assay (ELISA) systems employed [
12,
48,
49]. The connection between SELENOS, LDL, and very low-density lipoprotein (VLDL) was suggested in Gao's study, which found SELENOS in human serum fractionated into HDL, LDL, and VLDL [
48]. There was also a positive correlation found between SELENOS levels and HDL [
49]. In line with these findings, a study on apolipoprotein E deficient (ApoE
−/−) mice suggested that hepatic SELENOS might be associated with dyslipidemia, as selenium nanoparticles (SeNPs) significantly decreased total cholesterol (TC), TG, and Low-density lipoprotein cholesterol, L (LDL-C) levels and increased serum HDL-C. Additionally, SeNPs enhanced the expression levels of SELENOS in the liver [
50].
In Swedish obese subjects, with an average BMI of 37.7 kg/m
2, SELENOS gene expression in subcutaneous adipose tissue was higher than in lean counterparts, who exhibited an average BMI of 22.0 kg/m
2. Furthermore, in these obese subjects, SELENOS expression level correlated positively with BMI, fat mass, serum levels of HDL-C, TG, and HOMA-IR. Additionally, a positive correlation was noted between SELENOS expression and waist circumference, as well as fat-free mass [
7]. Echoing these findings, our team recently established that in human subjects, levels of SELENOS in both subcutaneous and omental fat were elevated in the obese group (BMI ≥ 28.0 kg/m
2) compared to the non-obese group (BMI < 28.0 kg/m
2) [
8]. Furthermore, our research identified SELENOS as a positive regulator for the process of fat cell differentiation, known as adipogenesis, in 3T3-L1 preadipocytes. This regulation occurs through the IRE1α-X-box-binding protein 1 (XBP1) pathway [
8,
51].
However, other studies have pointed to an anti-adipogenic role of SELENOS. Reduction in SELENOS expression, mediated by peroxisome proliferator-activated receptor γ (PPARγ)-induced ubiquitination, was shown to promote adipocyte differentiation, potentially through modulation of ER stress and its related ubiquitin–proteasome system (UPS) [
52‐
54].
The absence of impact on obesity and body composition observed in SELENOS knockout mice sharply contrasts with the findings in both in-vivo adipose tissue and in-vitro 3T3-L1 preadipocytes. In SELENOS-deficient (SELENOS
−/−), heterozygous (SELENOS
−/+), and wild-type mice, neither genetic reduction nor deletion of SELENOS had any notable impact on whole-body metabolism, body weight, fat mass, or lean mass. These mice displayed similar oxygen consumption (VO
2), carbon dioxide production (VCO
2), and respiratory exchange ratio (RER), as measured using metabolic cages. Furthermore, body composition, which is determined by magnetic resonance imaging (MRI), was also alike across all groups [
55,
56].
The discrepancies between these findings underscore the complexity of deciphering SELENOS function and highlight the necessity for further in-vivo and in-vitro studies. Considering that SELENOS performs diverse biological functions across different tissues and organs [
57], it might explain why no discernible differences were observed in global SELENOS knockout mice. Thus, the creation of tissue-specific knockout mice could provide more in-depth insights into SELENOS's role. To this end, our group recently engineered hepatic-specific SELENOS knockout mice (SelS
H−KO). Indeed, our findings indicated that hepatic-specific deletion of SELENOS accelerated the onset and progression of obesity, impaired glucose tolerance and insulin sensitivity, and increased hepatic TG and diacylglycerol (DAG) accumulation. This seemed to be regulated by encouraging fatty acid absorption and lessening fatty acid oxidation [
58].
The compromised function of pancreatic β-cells and a decrease in β-cell mass, frequently as a result of excessive β-cell apoptosis and reduced β-cell proliferation, are fundamental factors contributing to insulin resistance and the emergence of T2DM [
59,
60].
Overexpression of SELENOS was shown to protect Min6 β-cells, a mouse insulinoma cell line, from oxidative stress-induced apoptosis, suggesting that SELENOS could be important for insulin secretion and insulin sensitivity [
61]. On the other hand, the knockdown of SELENOS in Min6 cells induced β-cell apoptosis and reduced cell proliferation. This effect was associated with a decrease in the activation of the unfolded protein response (UPR), ultimately leading to the endoplasmic reticulum (ER) stress [
62].
However, it appears confusing that SELENOS knockdown increased insulin production and secretion in Men's study. The researchers speculated that this might be a feedback reaction to the decline in cell survival and proliferation, considering that insulin is essential for β-cell survival and proliferation [
62]. Furthermore, malfunction of pancreatic β-cells can be triggered by a flaw in insulin signaling within the β-cells, which leads to β-cell insulin resistance [
63]. Hence, it's plausible to theorize that the knockdown of SELENOS might instigate β-cell insulin resistance, necessitating a compensatory rise in insulin secretion. This, in turn, could result in the observed escalation in insulin production and secretion.
Accumulation of lipids, particularly saturated fatty acids, in the liver, adipose tissue, and skeletal muscle, has been associated with IR and T2DM. Several mechanisms have been implicated in this process, including oxidative stress, inflammatory signaling, ER stress, and cell death [
64‐
66]. In a hepatic steatosis model using pigs, selenium supplementation alleviated oxidative damage and apoptosis induced by a high-fat diet (HFD), alongside an increase in SELENOS expression in the liver [
67]. Furthermore, the silencing of SELENOS via small interference RNA (siRNA) was found to significantly exacerbate the inflammatory response, apoptosis, and oxidative stress in hepatoma HepG2 and Hepa1-6 cells induced by β-mercaptoethanol (an ER stress agent) and lipopolysaccharide (LPS) [
68‐
70]. Conversely, the overexpression of SELENOS in HepG2 cells was observed to mitigate ER stress and reduce NF-κB activity [
71].
Our group demonstrated that SELENOS mRNA expression in human omental adipose tissues was higher in individuals with T2DM than in those without the condition, with SELENOS levels positively correlated with HOMA-IR [
9]. Furthermore, SELENOS knockdown in murine C2C12 myoblasts decreased cell viability and exacerbated ER and oxidative stress responses in the presence of palmitate, suggesting a role for SELENOS in skeletal muscle insulin resistance [
72].
Genetic polymorphisms of SELENOS, such as single nucleotide polymorphisms (SNPs), have been linked with metabolic disorders and DM. For instance, the SELENOS SNP rs4965373 was linked to increased serum insulin levels and HOMA-IR in a cohort of 618 Swedish patients with acute coronary symptoms and 618 healthy controls [
7]. Another SNP, rs12910524, was discovered to be linked with increased TG concentrations in both Han and Uygur ethnic groups of nondiabetic Chinese subjects, even after adjusting for sex, age, alcohol intake, smoking, BMI, and plasma glucose levels [
73].
In a study by Zhao et al. [
74] comprising 1947 T2DM patients and 1639 control subjects, four SELENOS SNPs were genotyped (rs12910524, rs1384565, rs2101171, rs4965814), and rs1384565 was found to be an independent risk factor for T2DM in a Chinese population.
However, other studies have yielded negative results. No significant differences were observed in the SELENOS SNPs (rs28665122 and rs4965373) between subjects with metabolic syndrome (n = 71) and without metabolic syndrome (n = 65) in an Iranian population [
75]. In a similar vein, there was no significant disparity in the genotype and allele distribution of SELENOS SNPs (rs4965814, rs28665122, rs34713741, and rs4965373) between type 2 diabetes mellitus patients (n = 170) and healthy controls (n = 100) in a Chinese population [
76]. Additionally, no association was detected between SELENOS SNPs (rs11327127, rs28665122, rs4965814, rs12917258, rs4965373, and rs2101171) and type 1 DM (n = 311) compared to healthy controls (n = 550) in Spanish subjects [
77,
78].
It is important to consider that factors such as ethnicity, sex, age, SNP genotyping methods, and the number of subjects can confound results from genome-wide association studies, potentially leading to discrepancies between studies. Therefore, to further elucidate the relationship between SELENOS gene variation and the risk of metabolic diseases, more studies should be conducted, ideally using stratified sub-group analysis and larger cohorts. Further investigation is also needed to understand the mechanisms that link SELENOS SNPs to metabolic diseases.
SELENOS involved in vascular endothelial dysfunction
SELENOS has been identified as a potential receptor for SAA [
6], which is a key player in promoting vascular ED and AS development [
11,
79‐
81]. Studies have shown positive correlations between SELENOS expression in skeletal muscle and adipose tissue and SAA [
9,
82].
Atherosclerotic lesions in ApoE
−/− mice were found to be alleviated by SeNPs, with increased SELENOS expression observed in the liver [
50]. Moreover, the expression of SELENOS was heightened in the vascular wall intima of streptozotocin (STZ)-induced diabetic rats and low-density lipoprotein receptor (LDLR) knockout mice induced by a high-fat diet (HFD) [
83,
84]. These findings suggest that SELENOS is involved in vascular ED.
Indeed, an increasing amount of research underscores the protective role of SELENOS in vascular endothelial cells. Our group discovered that overexpressing SELENOS in HUVECs significantly bolstered cell viability and superoxide dismutase (SOD) activity, while simultaneously reducing malondialdehyde (MDA) production and caveolin-1 (Cav-1) expression in response to hydrogen peroxide (H
2O
2) treatment. In contrast, the silencing of SELENOS was associated with decreased cell viability, reduced SOD activity, and diminished protein kinase Cα (PKCα) expression, while MDA production and Cav-1 expression were increased [
85].
Following this, our study employed an integrated microfluidic chip that was designed to simulate the diabetic vascular endothelial microenvironment. This was established with accurate concentrations of glucose and oxidized-LDL (ox-LDL). We delved deeper into understanding the role and mechanism of SELENOS in oxidative damage to human aortic endothelial cells (HAECs), which was caused by the combined impact of high glucose levels and/or ox-LDL [
86].
The results demonstrated that SELENOS provided protection to HAECs against oxidative stress injury induced by multiple factors, evidenced by increased cell viability, reduced ET-1 and reactive oxygen species (ROS) levels, and augmented SOD1 and SOD2 expression. These findings align with our previous study. Further, it was confirmed that the antioxidant protective effect of SELENOS within the diabetic vascular endothelial microenvironment was facilitated through inhibiting PKCα and subsequently activating the PI3K/protein kinase B (Akt)/eNOS signaling pathway [
86]. Moreover, SELENOS was shown to protect against endothelial injury in HAECs prompted by high glucose and/or ox-LDL, with the underlying mechanisms potentially associated with its regulation of autophagy through the activation of the Akt/mammalian target of rapamycin (mTOR) signaling pathway [
87]. Furthermore, SELENOS inhibited the growth in endothelial apoptosis and cleaved caspase3 levels induced by high glucose, which coincided with the suppression of the PKCβII/JNK/B-cell lymphoma/leukemia-2 (Bcl-2) pathway. The protective effects of SELENOS were countered, and apoptosis and cleaved caspase3 levels increased when HUVECs were pretreated with PKC activators [
83]. Additionally, overexpression of SELENOS prevented the reduction of NO and eNOS, as well as the rise of ET-1 and ROS triggered by TNF-α [
84].
The levels of TNF-α-induced ICAM-1 and VCAM-1 expression were found to be reduced, along with the adhesion of THP-1 cells to HUVECs. Additionally, there was observed suppression of inflammatory factors, including interleukin-1β (IL-1β), interleukin-6 (IL-6), interleukin-8 (IL-8), and MCP-1. These discoveries imply the potential function of SELENOS in mitigating leukocyte adhesion by suppressing adhesion molecules [
84]. Further, overexpression of SELENOS was shown to mitigate TNF-α-induced activation of the MAPK and NF-κB pathways. In contrast, the silencing of SELENOS resulted in amplified TNF-α-induced damage in HUVECs. Aligning with our results, the suppression of SELENOS significantly induced an inflammatory response as the expression levels of TNF-α and IL-1β were elevated in arterial endothelial cells, and enhanced neutrophil adhesion was observed [
88].
In conclusion, SELENOS appears to be a promising contender for the early prevention and management of macrovascular complications associated with diabetes.
SELENOS effect on the advanced atherosclerotic lesion
VSMCs have a notable role in the formation and structure of advanced AS lesions, as well as in vascular calcification [
36,
89]. Both intimal and medial calcification in arteries, primarily driven by VSMCs, are associated with atherosclerotic plaque rupture and vessel stiffness [
36,
89]. Additionally, apoptosis of VSMCs contributes to the destabilization and rupture of atherosclerotic plaques and promotes vascular calcification [
36,
89].
A study by Ye et al. demonstrated that silencing SELENOS through siRNA makes VSMCs more susceptible to oxidative injury and apoptosis, triggered by H
2O
2 or tunicamycin [
90]. It also enhances the phosphorylation of MAPK and JNK in VSMCs. Moreover, SELENOS silence exacerbates ER stress induced by H2O2 or tunicamycin, as indicated by elevated protein levels of ER stress transducer phosphorylated protein kinase RNA-like ER kinase (PERK), ER chaperone glucose-regulated protein 78 (GRP78), and the proapoptotic transcription factor CCAAT/enhancer-binding-protein (C/EBP) homologous protein (CHOP) [
90].
Furthermore, they investigated SELENOS's role in inflammation-induced vascular calcification. SELENOS knockdown worsened LPS- or TNF-α-induced osteoblastic differentiation and calcification of VSMCs. This was evidenced by the increased levels of key osteogenic transcription factors like bone-related proteins and runt-related transcription factor 2 (Runx2), including alkaline phosphatase and type I collagen, along with calcium deposition content. Both the classical and alternative pathways of NF-κB signaling were activated, with increases in ER stress markers GRP78 and IRE1α expression observed in calcifying VSMCs [
91].
These findings provide new insights into SELENOS's effect on VSMCs apoptosis and vascular calcification, which could be potentially beneficial for preventing and treating ASCVD.
Macrophages and CD4
+ T cells play pivotal roles in the inflammatory response seen throughout all stages of atherosclerotic lesion development [
27‐
29]. The stimulation of macrophages with LPS is frequently used as an effective model for studying inflammatory responses and for evaluating potential anti-inflammatory agents [
92,
93].
SELENOS has been linked to inflammation induced by LPS-stimulated RAW264.7 macrophages. Specifically, it has been observed that selenium pretreatment alleviated immunological stress in these cells, reducing inflammation cytokines such as IL-6, IL-1β, IL-10, TNF-α, and MCP-1, while simultaneously increasing SELENOS expression [
94].
In advanced atherosclerotic lesions, the apoptosis of lipid-engorged foam cells, whether originating from macrophages or VSMCs, contributes to the generation and development of the pro-inflammatory necrotic lipid core [
95]. Kim et al. discovered that overexpression of SELENOS protected RAW264.7 macrophages against ER stress-induced cytotoxicity and apoptosis, thereby promoting cell survival [
96]. In contrast, suppression of SELENOS sensitized cells to ER stress-induced cell death. These findings suggest that SELENOS could be a promising therapeutic target for atherosclerosis.
Furthermore, SELENOS has been identified as a gene regulating the effector functions of CD4
+ T cells. After SELENOS knockdown, increased levels of IL-2, IL-21, and granulocyte–macrophage colony-stimulating factor (GM-CSF) were observed in the culture media. This effect was found to be regulated via both the early 2 factor (E2F) transcription factor 5 (E2F5) regulatory pathway and the Ca
2+/ immune transcription factor nuclear factor of activated T cells, cytoplasmic 2 (NFATC2) signaling pathway [
97]. This adds another layer of complexity to our understanding of the role of SELENOS in immune responses and inflammation.
SELENOS SNPs maybe gene markers for atherosclerosis
Multiple studies have suggested a significant correlation between SELENOS SNPs and susceptibility to atherosclerosis-related diseases. These findings suggest that SELENOS gene polymorphisms could serve as genetic markers for predicting the risk of atherosclerosis.
Moreover, an increasing amount of research has unveiled a significant correlation between SELENOS SNPs and susceptibility to AS-related diseases. This suggests that SELENOS gene polymorphisms might serve as promising genetic markers for predicting the risk of AS. For instance, in a case–control study composed of 2,222 subjects from the FINRISK Study in Finland, the SELENOS SNP rs8025174 was projected to enhance the risk of CHD in females by 2.95 times [
98]. Furthermore, the SNP rs7178239 was found to elevate the risk of ischemic stroke by 1.75 times across both genders and 3.35 times in females [
98]. However, no connection was observed between the SNP rs28665122 and CHD or ischemic stroke [
98,
99]. Subsequent studies discovered that carrying the SELENOS SNP rs4965814 and rs9874 escalated the risk of ischemic stroke in Finnish women by 2.89 and 3.32 times, respectively [
100]. These findings were echoed by Li et al. and Qiu et al. [
101,
102], who found that the SELENOS SNP rs4965814 could amplify the risk of ischemic stroke by 1.54 times in both genders and 2.43 times in females within a Chinese sample population of 239 ischemic stroke patients and 240 non-ischemic stroke control subjects. (Table
1).
Table 1
Association between SELENOS SNPs and the risk of metabolic disorders, diabetes and AS-related diseases
1 | rs28665122 | Chr15:101277522 | C/T | Subclinical CVD in T2DM | |
2 | rs8025174 | Chr15:101279548 | C/A | Coronary heart disease | |
3 | rs4965814 | Chr15:101273712 | T/C | Ischemic stroke | |
| | | T/C | Subclinical CVD in T2DM | |
| | | T/C | CVD in T2DM | |
4 | rs12917258 | Chr15:101273134 | G/C | Subclinical CVD in T2DM | |
5 | rs4965373 | Chr15:101272190 | G/A | Serum insulin, HOMA-IR | |
6 | rs9874 | Chr15:101271199 | T/C | Ischemic stroke | |
7 | rs28628459 | Chr15:101272152 | T/C | Subclinical CVD in T2DM | |
| | | | CVD in T2DM | |
8 | rs7178239 | Chr15:101267907 | C/G | Ischemic stroke | |
| | | | Subclinical CVD in T2DM | |
9 | rs9806366 | Chr15:101262752 | C/T | CVD in T2DM | |
10 | rs12910524 | Chr15:101262360 | C/T | TG concentration | |
11 | rs1384565 | Chr15:101264707 | T/C | T2DM | |
However, in a case–control study conducted in Germany, which comprised 470 ischemic stroke patients and 807 population controls, no significant interaction effects of the SELENOS SNP rs9874 were found [
103]. This discrepancy may be partially due to the absence of sex-stratified sub-group analysis in the study. Further, Cox et al. examined the correlation between ten types of SELENOS SNPs and the risk of AS in T2DM patients, using a sample of 1220 European American T2DM subjects from the Diabetes Heart Disease Study [
104]. This study discovered that the SELENOS SNPs rs28665122, rs4965814, rs28628459, rs7178239, and rs12917258 were associated with SAS, while the SNPs rs4965814, rs28628459, and rs9806366 were associated with clinical AS. Additionally, the SELENOS SNP rs34713741 was linked to a 1.49-fold increase in the risk of PAD among Polish subjects (PAD group n = 664, control group n = 543) [
105]. Recently, Wang et al. reported that the SELENOS SNP rs117613208 raised the risk of coronary artery disease (CAD) by 2.107-fold in a Chinese population-based case–control study (576 CAD cases and 452 control subjects) [
106]. Furthermore, leveraging this locus, they developed a diagnostic model for CAD, referred to as the GASDLY score. This model exhibited a sensitivity of 74.7% and a specificity of 75.5%. The GASDLY score is calculated using the following formula: GASDLY score = −2.145 + (age × 0.59) + (smoking × 1.675) + (diabetes × 0.724) + (rs117613208 TT genotype × 0.745) + (lipoprotein A × 0.002)—(1.817 × apolipoprotein A1) [
106]. (Table
1).
These findings suggest that SELENOS gene polymorphisms may serve as valuable genetic indicators for screening and evaluating the risk of macroangiopathy in both non-diabetic and T2DM patients.