Chronic kidney disease (CKD) is defined as an abnormality in the structure or function of kidney, lasting more than 3 months, with specific implications for health (1). It could progress to end-stage renal disease (ESRD), which is fatal without renal replacement therapy (kidney transplantation or dialysis). Moreover, because of its effect on cardiovascular risk, CKD could directly affect the global burden of death caused by cardiovascular disease (2). Over 10% of the world’s population carries CKD and in the United States, more than 30 million people have CKD (3). In addition, with the rapid increase in the prevalence of risk factors such as hypertension, diabetes and obesity, the burden of CKD will become even heavier in the future. Therefore, CKD as a major public health problem has attracted more and more attention.

Selenium is an essential microelement for mammals and is widely distributed in all tissues in the body (4). Elemental selenium is biologically inactive by itself. It is involved in the synthesis of 25 selenoproteins in the form of selenocysteine and selenomethionine to perform its biological functions (5). Selenoproteins play important functions in the body such as combating oxidative stress, regulating immune system, regulating thyroxine metabolism and fertility (6). However, the biological activities of about 50% of selenoproteins are unknown and their roles in biology remain to be elucidated (7). Kidney is the organ with the highest selenium content (8). Many observational studies have found a strong association between selenium and CKD (9–11). Selenium levels are usually lower in CKD patients than normal people (12, 13), but whether selenium supplementation can benefit CKD patients is still controversial (14, 15).

Mendelian randomization (MR) is a research approach that utilizes genetic variation as an instrumental variables to examine whether exposure factor has a causal effect on health outcome (16). Compared to conventional methods, MR could avoid reverse causation bias and attenuates the interference of confounding factors, so it is becoming increasingly popular in epidemiologic studies (17). Using MR method, our team has previously found that increased selenium levels in the body could lead to a decrease of glomerular filtration rate and an increase in blood urea nitrogen levels, suggesting that selenium supplementation in CKD patients should be taken with great caution (18). However, the mechanism by which increased selenium levels in the body lead to the deterioration of renal function remains unclear. Genome-wide association studies (GWAS) have detected genetic variants associated with plasma proteome levels (19, 20), which provide an opportunity to utilize MR to reveal the underlying mechanisms behind the genetically causal relationship between exposure and outcome, as many circulating proteins always act as the principal regulators of molecular pathways (21).

In the current study, we further selected 3282 circulating plasma proteome levels to explore their potential mediating effect in the association of selenium levels and renal function. By evaluating these mediation effects, we can gain insights into the mechanistic pathways through which the increased selenium levels might influence the risk of renal failure, paving the way for potential therapeutic interventions.

As a trace element, selenium is essential to the human body, and due to its important role in combating oxidative stress, regulating immune system, regulating thyroxine metabolism and fertility (6), there has been an increasing focus on selenium supplementation. As selenium levels in CKD patients are usually lower than normal (12, 13), they are more likely to favor selenium supplementation without professional guidance. Notably, it is important to note that the safe intake range for selenium is very narrow, and the optimal daily intake of selenium for the human body is only 30-400 μg day⁻¹ adult⁻¹ (38), below which can lead to selenium deficiency, and above which can produce symptoms of toxicity, such as hair loss, thyroid dysfunction, and neurological damage (39). Therefore, selenium intake needs to be finely regulated. Moreover, selenium is not biologically active by itself, and it plays its biological roles mainly via selenoproteins (5). Currently, most of the functionally known selenoproteins of eukaryotes are oxidoreductases with antioxidant properties (40, 41). However, it is worth noting that taking GPX1 as an example, while it has anti-inflammatory and antioxidant effects, it has also been found to be associated with insulin resistance and type 2 diabetes (42), which fully illustrates the complexity of selenoproteins’ biological functions. Beneficial or harmful depending on the levels of selenoproteins in the body and other factors, and the biological effects of most selenoproteins, especially on kidney, remain unclear. Furthermore, the benefits and harms of selenium are dependent on its dose and form (43). With increasing selenium intake, selenoprotein expression tends to saturate and the excess selenium bind nonspecifically to human proteins through the substitution of methionine by selenomethionine (SeMet) resulting the production of non-specific selenium-containing proteins, and the non-specific selenium-containing proteins could cause damage to health (44, 45). Hence, lack or excess of selenium is a relative concept. CKD patients are characterized by decreased dietary intake, reduced intestinal resorption, increased urinary protein excretion, and reduced ability in synthesizing selenoproteins (46). Therefore, when supplemented with selenium, they are more susceptible to produce non-specific selenium-containing proteins and thus to toxic effects of selenium. Results of this study found that increased selenium levels could lead to decreased renal function, so supplementation of selenium must be taken with extreme caution, especially for patients with CKD.

In this study, we found that increased levels of selenium in body would lead to an increase of plasma protein levels such as FAS-associated factor 2 (FAF2), Retinoic acid receptor responder protein 1 (RARRES1). It may be related to the pro-oxidant effect of selenium compounds. As selenium levels increase, the selenium compounds, such as selenite, also increase, and selenite could lead to an excess of reactive oxygen species (ROS) in the cell, triggering oxidative stress (47). RARRES1 protein is involved in the regulation of signaling pathways in response to oxidative damage, so their up-regulation may reflect an adaptive reaction of the body to maintain cellular homeostasis facing oxidative challenges (48). The pro-oxidative effects of selenium compounds can also induce the activation of apoptosis and protein degradation pathways. FAF2, an apoptosis-associated protein involved in ubiquitination and protein degradation, may increase its expression in response to oxidative stress-induced protein damage (49). This reflects a complex mechanism of cellular regulation of oxidative damage and provides insight into the potential mechanisms of selenium toxicity.

Many circulating proteins are key regulators of molecular pathways, so exploring their potential mediating effect in the association of selenium levels and renal function could help to understand the underlying mechanisms of selenium’s influence on renal function. In this study, a total of 35 plasma proteins mediating between selenium levels and renal function were identified, and the three with the highest mediating effects were fibroblast growth factor receptor 4 (FGFR4), fibulin-1 (FBLN1), and cilia-and flagella-associated protein 45 (CFAP45). CKD patients have elevated serum levels of fibroblast growth factor (FGF), of which FGF23 has a strong dose-dependent association with cardiovascular morbidity, chronic inflammation and progression of kidney disease (50). FGFR4 is one of the receptors for FGF23. Activation of hepatic FGFR4 causes the production of inflammatory cytokines in the liver, thereby exacerbating the chronic inflammation in vivo and participating in the progression of CKD (51). In addition, it has been shown that FGF23 could participate in renal fibrosis by amplifying the activation of local renal fibroblast in injury and perpetuating pro-fibrotic signaling through FGFR4 (52). FBLN1 is an extracellular matrix (ECM) protein, which interacts with many ECM molecules (53). As a secreted plasma protein, FBLN1 has been found to be a good potential biomarker for renal impairment, significantly elevated in the plasma of CKD patients (54). The mechanism underlying the elevation of FBLN1 in patients with impaired renal function remains unknown. In the present study, we found that decreased plasma FBLN1 level could lead to the impairment of renal function, which is counterintuitive and suggests that, like BNP in heart failure, increased FBLN1 may be a compensatory result when renal function is impaired. However, the specific mechanisms behind this need to be studied further. CFAP45 protein is a static structural component of the ciliary axoneme and critical for motor cilia stability (55). Cilia is widely distributed in the proximal and distal tubules as well as the collecting ducts of kidney. Normal structure and function of cilia are essential for renal organogenesis and maintenance of epithelial cell differentiation and proliferation, and persistent ciliary dysfunction contributes to both early and progressive renal disease (56). However, the study on the relationship between CFAP45 and the kidney is still lacking, and its effect on renal cilia and renal function needs to be further explored in the future.

Eleven pathways, including thyroxine biosynthesis and defective SLC5A5 causes thyroid dyshormonogenesis were found to mediate the effects of selenium levels on renal function in the enrichment analysis. Selenium is important for thyroxine biosynthesis. The thyroid gland expresses selenoproteins such as glutathione peroxidase, selenoprotein P and thioredoxin reductase, which together form the antioxidant system to mitigate the damage to thyroid cells caused by reactive oxygen species generated during thyroxine biosynthesis, and thus maintains the normal function of thyroid tissue (57). In addition, iodothyronine deiodinases is also a selenoprotein. It converts the inactive precursor thyroxine to the active triiodothyronine. Abnormal selenium levels in the body affect the expression and activity of iodothyronine deiodinases, which can lead to disruption of thyroxine metabolism (58). The kidney is an important target organ for thyroxine action. Thyroxine affects protein synthesis and cell growth and therefore has an important role in the growth and development of kidney (59). Thyroxine regulates the activity of tubular ion transporters (Na+, K+, 2Cl− co-transport protein, etc.), which modulates tubuloglomerular feedback and affects glomerular filtration rate (60). It also regulates the expression of glomerular vasodilatory factors, such as vascular endothelial growth factor, and the synthesis and secretion of various components of the renin-angiotensin-aldosterone system, to modulate renal perfusion (61). Additionally, it has been found that abnormal increases or decreases of thyroxine are strongly associated with the development and progression of renal fibrosis (62, 63). These evidences support our finding that thyroxine disruption may be a potential mechanism by which selenium affects renal function.

Our two-sample MR analysis uses stringent criteria and provides compelling insights into the intricate relationship between the levels of selenium in vivo and renal function, suggesting that increased selenium levels are associated with decreased renal function. Furthermore, our mediation analysis revealed the important role of plasma proteome as mediators, depicting the causal trajectories from increased selenium levels to decreased renal function, which has profound significance in constructing a blueprint from the levels of micronutrients in vivo to the pathogenesis of CKD, thereby effectively guiding the prevention of CKD and delaying its progression.

However, it is crucial to acknowledge certain limitations in our study. First, the genetic analyses in this study were performed on individuals of European ancestry, so generalization of the findings to other ethnic populations is limited. Second, selenium levels in populations are closely related to geographic variation, thus also limiting the generalization of the findings to different geographic populations. Third, our data were derived from publicly available aggregated statistics and the raw clinical outcome data for each individual were not available, thus preventing further population stratification analyses. Fourth, this study used the relative scale of “per 1 SD” to determine the association of selenium with eGFR, and did not use an absolute scale due to limitations of the original data. Lastly, we did not perform experimental validations for the identified mediating proteins and potential mechanisms by which selenium affects renal function, and the role of environmental factors and the influence of genetic factors interacting with the environment on this causal effect should be further explored in future studies.

Our comprehensive MR analysis has revealed the complex relationship between selenium levels and renal function, and found that increased selenium level is one genetically causative factor for kidney function impairment, suggesting that supplementation of selenium must be taken with extreme caution, especially for patients with CKD. More importantly, we reveal the underlying mechanism by which increased selenium levels lead to deterioration of renal function, effectively guiding the prevention of CKD and paving the way for future studies.