1. Introduction
Changes in modern lifestyles are important factors contributing to many metabolic disorders. The excessive intake of lipids and simple carbohydrates in the diet, combined with oxidative stress caused by surrounding factors, could result in metabolic abnormalities and chronic inflammatory damage to tissues throughout the body. The prevalence of nonalcoholic fatty liver disease (NAFLD) has shown a rapid increase over the past two decades, and the age of onset has started to show a trend toward younger people, making it an important public health issue that cannot be ignored [
1]. The pathogenesis of NAFLD is a complex process that can potentially progress from simple hepatic lipid accumulation to mild or severe inflammation, ultimately resulting in tissue damage and fibrosis. In recent years, the multiple hit hypothesis has been the most reported, indicating that NAFLD may be caused by the interaction of various factors, including many genetic and external environmental factors, the release of pro-inflammatory substances, and the influence of the gut-liver axis, all of which may play roles in the progression of NAFLD [
2,
3] (
Figure 1).
The increase in oxidative stress is a key factor accelerating the progression of NAFLD, and it also represents the result of an imbalance between reactive oxygen species (ROS) and the body’s antioxidant defense system. Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that regulates redox reactions in the body to modulate oxidative stress in vivo, and it can activate the downstream anti-oxidative system when the internal environment changes and ROS increases, achieving the effect of regulating oxidative stress and reducing lipid peroxidation [
4]. Gut dysbiosis may be another important factor in the progression of NAFLD. When excessive endotoxins in the intestinal lumen pass through the intestinal mucosa into the blood circulation, caused by diet or other environmental factors, they flow into the liver via the portal vein, thereby activating the Toll-like receptor 4 (TLR4) signaling pathway; this results in the activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κBs) via the downstream myeloid differentiation primary response gene 88 (MyD88) pathway, increasing the secretion of pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α and interleukin (IL)-1β, resulting in inflammatory reactions in the liver. Many experiments have reported that long-term high-fat or high-fructose diets may lead to an imbalance in the gut microbiota and impair normal physiological functions [
5,
6].
Additionally, light-emitting diodes (LEDs) have been widely used in various indoor lighting equipment in recent years. Because LED is a light source rich in blue light, which is more likely to penetrate eye tissue and reach the retina, high-intensity or prolonged exposure may cause tissue damage due to its short wavelength and high energy [
7]. Due to the global COVID-19 pandemic, the public has become more accustomed to using online media for learning or working, which has also indirectly altered lifestyles and increased the time of exposure to blue light. Therefore, the harm that excessive exposure may cause to physiology is also very worthy of attention. In addition to the damage to eye tissue, some studies have begun to find that blue light exposure may also increase oxidative stress and inflammatory reactions in the body, and even affect the composition of the gut microbiota, thereby exacerbating metabolic disorders and other tissue damage [
8,
9,
10]. However, current research on the relationship between blue light exposure and diet-induced NAFLD is still quite limited. Therefore, this study aims to clarify the potential effects of long-term exposure to blue light on hepatic injury. Firstly, we investigated whether exposure to blue light alone would affect liver health in mice of different genders and ovariectomy (OVX) status. Secondly, we further evaluate the effects of blue light exposure on hepatic lipid accumulation, oxidative stress, inflammatory responses, and gut microbiota in mice consuming an unhealthy, high-fat, high-fructose diet.
2. Materials and Methods
2.1. Experimental Design
Experiment 1: Seven-week-old male ICR mice were purchased from BioLASCO Taiwan Co., Ltd. (Yilan, Taiwan), comprising 12 males, 12 females, and 12 ovariectomized mice. All study protocols were reviewed and approved by the Institutional Animal Care and Use Committee of the National Taiwan Normal University (No. 114007 and 114013). The environmental conditions for the mice were maintained in a room at 22 °C, 55% humidity, and a 12 h light/dark cycle. After 1-week adaptation, the mice were randomly assigned to a control group or a BL group, in which the mice were exposed to blue light (465 nm BL LED light, Philips, Amsterdam, The Netherlands) at an intensity of 0.8 μW/cm
2 (37.7 lux) for 6 h per day as previously reported, for a 16-week experiment [
11]. All mice were fed a standard rat chow diet (Rodent Laboratory Chow 5001, Purina Mills, St. Louis, MO, USA) throughout the experimental period with free access to food. Body weight and food intake were recorded, and the mice were anesthetized with isoflurane and euthanized at the 16th week. Liver tissues were collected for further analysis. The experimental design is shown in
Figure 2.
Experiment 2: Eighteen 6-week-old male ICR mice were purchased from BioLASCO Taiwan Co., Ltd. (Yilan, Taiwan). All study protocols were reviewed and approved by the Institutional Animal Care and Use Committee of the National Taiwan Normal University (No. 112027). The environmental conditions for the mice were maintained in a room at 22 °C, 55% humidity, and a 12 h light/dark cycle. The mice were fed a standard rodent chow diet (Rodent Laboratory Chow 5001, Purina Mills) for 1 week and then switched to a control liquid diet based on AIN93M for an additional 1 week to facilitate adaptation. Then, during the 16-week experimental period, the H and HB groups of mice were fed a high-fat, high-fructose (HFHF) diet [
12], with some modifications using fructose instead of dextrin, while the C group of mice was fed a control liquid diet (
Table 1). The mice in the HB group were exposed to blue light under the conditions described in Experiment 1. Body weight and food intake were recorded. At the end of the study, fresh fecal samples were collected from 4 randomly chosen mice in each group for microbiota analysis. Then, the mice were anesthetized with isoflurane and euthanized at the 16th week. Abdominal epididymal, perirenal, and mesenteric fat pads were collected and weighed. Blood and liver tissues were collected for further analysis.
2.2. Blood Analysis
The mice were sacrificed after an 8 h fasting, and we collected blood from the inferior vena cava. Serum was collected after centrifugation (1200× g at 4 °C for 15 min) for analyzing glucose, total cholesterol (TC), triglycerides (TGs), HDL-cholesterol (HDL-C), LDL cholesterol (LDL-C), aspartate transaminase (AST), alanine transaminase (ALT) and creatinine levels, all of which were analyzed using an autoanalyzer (Roche 110 Modular P800; Diamond Diagnostics, Holliston, MA, USA).
2.3. Liver Sample Analysis
For pathohistological analysis, the resected rat liver sections were fixed in formaldehyde (10% [
v/
v]) and then stained with hematoxylin and eosin. Tissue morphological changes were examined by a pathologist, and fat accumulation and damage were scored according to Brunt’s method [
13]. The NAFLD activity scores (NASs) were calculated as the sum of scores for lipid accumulation (steatosis) degree, (lobular) inflammation severity, and hepatocellular ballooning degree. Immunohistochemistry was performed on the BenchMark Ultra machine (Ventana Inc., Tucson, AZ, USA). Then, 4-μm-thick sections were obtained from each paraffin block and mounted on positively charged glass slides for immunostaining with anti-NF-κB (Cell Signaling Technology, Inc., Danvers, MA, USA) and anti-NRF2 (Proteintech Group, Inc., Rosemont, IL, USA, 16396-1-AP) antibodies. Slides were pre-treated for antigen retrieval in Benchmark Ultra using the Ultra Cell Conditioning 1 reagent (Ventana Medical Systems, 06414575001) for Nrf2 (16 min) and Ultra Cell Conditioning 2 reagent (Ventana Medical Systems, 06414575001) for NF-κB (92 min). Dilutions of 1:200 for NF-κB and 1:400 for NRF2 were used. The immunostaining was performed using a Ventana BenchMark Ultra and OptiView DAB IHC Detection Kit system. The presence of a brownish color in the immunohistochemistry staining of NF-κB and NRF2 was considered positive.
To measure hepatic lipid levels, samples were homogenized and extracted using a chloroform/methanol mixture [
12]. Triglyceride and cholesterol concentrations were then determined using commercial kits (Randox TR210 and Fortress BXC0271, Antrim, UK). In addition, liver tissues were homogenized in a buffer (50 mM Tris-HCl, 150 mM NaCl, 1% NP-40 (Sigma-Aldrich, I8896, St. Louis, MO, USA), and 0.1% sodium dodecyl sulfate (Bio-Rad Laboratories, 1610416, Hercules, CA, USA)) containing a protease inhibitor (Roche, 04693116001, Mannheim, Germany). After centrifugation, the supernatant was collected for further analysis. Lipid peroxidation marker malondialdehyde (MDA) levels were determined spectrophotometrically using the thiobarbituric acid reactive substances assay [
14]. TNF-α and IL-1β, concentrations were measured using commercial ELISA kits (Mouse TNF-α DuoSet ELISA, DY-410-05; Mouse IL-1β/IL-1F2 DuoSet ELISA, DY-401-05, R&D Systems, Minneapolis, MN, USA) and adjusted by protein concentration measured with a Bio-Rad protein assay dye (Bio-Rad Laboratories, 5000006).
Western blotting was utilized to analyze the expression of proteins involved in the hepatic TLR4 pathway. Samples were initially prepared by homogenizing them in a lysis buffer (50 mM Tris-HCl, 50 mM NaCl, 1% NP-40, 0.1% SDS) supplemented with protease inhibitors. Following centrifugation, 30 μg of supernatant protein was loaded onto an 8% gel made with SDS-polyacrylamide (1610158, Bio-Rad Laboratories) for separation, after which the proteins were transferred to a polyvinylidene difluoride (PVDF) membrane. The membrane was initially blocked by incubation in non-fat milk to prevent the non-specific binding of antibodies. After washing with PBS/Tween-20, the membrane was combined with an anti-Toll-like receptor 4 (TLR4) antibody (monoclonal antibody to TLR4, IMG-5031A, Novus Biologicals, Centennial, CO, USA), anti-myeloid differentiation primary response 88 (MyD88) antibody (MyD88, D80F5 rabbit mAb, Cell Signaling Technology), and anti-TIR-domain-containing adapter-inducing interferon-β (TRIF) antibody (TRIF/TICAMI antibody, N8120-13810, Novus Biologicals) and secondary antibodies (horseradish peroxidase [HRP] donkey anti-rabbit immunoglobulin G [IgG] antibody 406401; HRP goat anti-mouse IgG antibody 405306, Biolegend, San Diego, CA, USA). After washing, the membranes were treated using a chemiluminescence detection system (PerkinElmer, Waltham, MA, USA) to visualize the immune complexes. The resulting bands were then quantified using the BioSpectrum AC image system, UVP Visionwork LS software, and Image-Pro Plus 4.5 (Media Cybernetic, Rockville, MD, USA). β-actin (Proteintech, 20536-1-AP, IL, USA) served as the total protein loading control. Data are ultimately reported as the relative proportion of the protein compared to the control protein.
2.4. Fecal Analysis
We randomly selected fresh fecal samples and extracted DNA with a QIAamp DNA Stool Mini Kit (Qiagen, Germantown, MD, USA) according to Godon’s method [
15]. Illumina HiSeq Sequencing (Illumina, San Diego, CA, USA) was performed on the amplified 16S rDNA V3-V4 regions to generate raw data post-extraction. High-quality sequences (effective tags) were obtained by filtering chimeras using UCHIME and then clustering them at a sequence identity of greater than 97% [
16]. To normalize for sampling depth differences across samples, abundance data were retrieved from operational taxonomic units (OTUs), ensuring that all samples met a minimum sequence count. Alpha and beta diversity were analyzed as previously reported, and the
Firmicutes-to-
Bacteroidetes (F-B) ratio was also calculated [
17]. The Shannon index was implemented to measure alpha diversity, whereas beta diversity was visualized through PCA based on OTU abundance. Taxonomical profiling was conducted at the phylum, class, family, and genus levels to assess relative abundance distributions.
2.5. Statistical Analysis
GraphPad Prismatic 10.4.0 (GraphPad Software, San Diego, CA, USA) and Microsoft Excel 2016 (Microsoft Corporation, Redmond, WA, USA) were used for statistical analysis and figure preparation, with values expressed as the mean and standard deviation (SD). Two-way analysis of variance (ANOVA) and Tukey’s multiple range test were performed to compare data among groups following Experiment 1. Two-way ANOVA was used to evaluate the main effects of sex/OVX status (male, female without OVX, and female with OVX) and light exposure (with or without blue light exposure), as well as their interaction. One-way ANOVA and Tukey’s multiple range test were performed to compare data among groups following experiment 2. Homogeneity of variances was assessed using the Brown-Forsythe test before conducting the ANOVA. To identify variations in microbial composition, the Wilcoxon rank-sum test was employed to detect absolute abundance shifts among the top 10 significantly different genera. Furthermore, the relative abundance of five key genera was compared across groups using STAMP analysis. A p value < 0.05 was considered significant. We determined the required sample size using G*Power 3.1.9.7 for a three-group one-way ANOVA comparison. Targeting a statistical power of 0.80 and an alpha level of 0.05 and considering the hepatic TG level as the primary outcome, the analysis showed that a minimum of 6 animals per group was required to detect an effect size (Cohen’s f) of 0.75.
4. Discussion
Studies have proposed various hypotheses to explain the pathogenesis of fatty liver disease progression and the related metabolic disorders. Currently, the most widely accepted hypothesis of fatty liver disease’s pathological etiology is that it may be caused by the interaction of multiple factors, known as the multiple hit hypothesis. Therefore, for fatty liver disease induced by unhealthy dietary patterns, in addition to modifying eating habits, effectively controlling external environmental factors is crucial to reduce pro-oxidative reactions, the release of pro-inflammatory substances, and the potential impact of the gut-liver axis, all of which play quite an important role [
2]. Recent studies have found that irregular light exposure may be important in modulating lipid metabolism in vivo, and the study by Guan et al. indeed revealed that, compared to green light and white light, 24 h continuous blue light exposure may increase oxidative stress and liver damage in the body by disrupting biological rhythm [
19]. To simulate the current reality that might occur in everyday life, in this experiment, we adopted a long-term (16 weeks) exposure of 6 h per day. Since our previous study has already confirmed that this type of exposure, combined with an unhealthy diet, indeed causes visual damage [
10], we further investigated potential effects on other body tissues, beyond the eyes.
In this study, we utilized animals of different genders and physiological conditions for research and found that, under a normal diet, blue light exposure only had a slight effect on liver TNF-α in female animals. However, when combined with a high-fat, high-fructose diet, we observed that blue light indeed increased the damage caused by an unhealthy dietary pattern. Modern lifestyles involve extensive exposure to artificial light sources and technological products, which increase the eye’s exposure to light and blue light. Some research suggests that blue light can accelerate retinal aging and may be a contributing factor to age-related macular degeneration [
20]. However, besides affecting eye and retinal function, in 2013, the American Medical Association also suggested that nighttime exposure to light sources, such as electric lights or other electronic products with light sources, may increase the risk of many diseases, including cancer, obesity, diabetes, and psychiatric disorders, by disrupting the circadian rhythm [
21]. Another study also found a positive correlation between greater exposure to artificial light at night and the incidence of obesity among middle-aged and older adults in the United States [
22].
In animal experiments, mice housed in a 24 h light exposure environment showed obvious signs of fatty liver after 8 weeks compared to mice under a normal light-dark cycle [
23]. In a study where rats were divided into two groups, one receiving a normal diet and the other a high-fat diet, they were housed under light-dark cycles and continuous light exposure for 12 weeks, respectively, to observe the effects on indicators related to fatty liver disease. Their results showed that continuous light exposure led to abnormal blood glucose and lipid concentrations in the animals, and feeding a high-fat diet exacerbated these abnormalities, resulting in conditions such as insulin resistance and steatohepatitis [
24]. Recently, studies have further compared the effects of specific wavelengths of light on physiological functions. Blue light is a short-wavelength light with relatively high energy in the visible spectrum. Currently, we frequently use LEDs for lighting, mobile phones, computer liquid crystal displays, and tablets, among other applications [
25]. However, the blue light power of white LED lights used for daily illumination is much higher than that of traditional light sources such as incandescent or fluorescent lamps [
26]. This is because white light is ultimately created by combining LED blue light with excited yellow phosphor, and it indeed contains a strong blue light band (450–470 nm) despite appearing as white on the surface [
25].
Animal experiments have indicated that a high-fat diet, combined with ten weeks of blue light exposure, could result in greater body weight, hyperlipidemia, and reduced insulin sensitivity compared to the control group, leading to metabolic abnormalities [
10]. In addition, a high-fat diet combined with long-term exposure to blue light (24 h, 12 weeks, 150 lux) can decrease the total antioxidant capacity in mice and may exacerbate renal tissue damage by affecting the Nrf2/HO-1 signaling pathway and activating pro-inflammatory cytokines [
9]. In our study, although blue light exposure did not significantly affect body weight or the lipid metabolic abnormalities induced by the HFHF diet during the experiment, we observed a higher NAS score and more pronounced Nrf-2 and NF-κB expression in the pathological sections. Furthermore, the analysis also revealed that the HB group indeed had higher hepatic MDA and inflammatory cytokine concentrations. When dietary factors (such as excessive fat) and environmental factors lead to an imbalance between ROS and the antioxidant system in vivo, the body needs to rely on its own antioxidant system to clear excessive free radicals to maintain physiological stability [
27]. Nrf2 is an important upstream transcription factor that regulates redox reactions, and it is typically stable in the cytoplasm and bound to the Kelch-like ECH-associated protein 1 (Keap1). When the internal environment changes and ROS increases, Nrf2 dissociates from Keap1, resulting in the activation and translocation of Nrf2 into the nucleus, where it interacts with the antioxidant response element, and this subsequently activates downstream antioxidant enzymes to reduce ROS generation in the body, regulating oxidative stress and reducing lipid peroxidation [
4]. These results suggest that the combination of HFHF and blue exposure may indeed cause an increase in hepatic oxidative stress and inflammation, thereby upregulating the expression of Nrf2 in response to tissue damage.
On the other hand, some studies have also found that blue light exposure may cause gut microbiota dysbiosis and potentially lead to neuroinflammation by affecting the MyD88/NFκB pathways [
8]. Gut dysbiosis has a significant impact on the progression of diet-induced fatty liver disease. The composition of the human gut microbiota is diverse, and numerous recent studies have suggested that the gut microbiota may influence physiological functions and even be associated with various clinical symptoms [
28]. For example, an imbalance between
Firmicutes and
Bacteroidetes is considered to be associated with various metabolic diseases [
29]. When excessive endotoxin in the intestinal lumen passes through the intestinal mucosa and enters the bloodstream, it activates the TLR4 signaling pathway in the liver. This process proceeds through the downstream MyD88 pathway, resulting in the activation of NF-κB and the increased secretion of pro-inflammatory cytokines, such as TNF-α and IL-1β, which results in liver inflammation. Furthermore, TLR4 can also act through another downstream pathway, TRIF, which in addition to potentially activating the expression of interferon (IFN)-related factors via interferon regulatory factor 3 (IRF3), also causes the activation of the common downstream NF-κB signal transduction, further promoting tissue inflammation and damage [
29]. Although some animal studies have suggested that a long-term high-fat or high-fructose diet may result in gut microbiota imbalances, thereby affecting normal physiological functions [
4,
5], we did not observe a difference in the TLR4 signaling pathway among groups. These results suggest that the effect of this study may not be through this pathway, or the experimental conditions are still insufficient for influencing this pathway.
In the gut microbiota analysis, we found that both analysis methods showed significantly changed genera, including Bacteroides, Corynebacterium, and Muribaculum, reflecting the impact of a high-fat diet and blue light exposure on the gut microbial composition. The genus Bacteroides belongs to the phylum Bacteroidetes, order Bacteroidales, and family Bacteroidaceae, and it is one of the main dominant bacterial groups in the mammalian gut. This genus plays a crucial role in energy metabolism, the breakdown of complex carbohydrates, and host immune regulation, and it is considered a core bacterium for maintaining intestinal stability [
30]; however, the effects of different species may vary, and some strains may also be pathogenic [
31]. In this study, we found that the relative abundance of Bacteroides was increased in both the H and the HB groups, suggesting that Bacteroides may exhibit a compensatory growth advantage under an HFHF diet; however, the functional differences at the species level still require further clarification. Most Corynebacteria are commonly found on the skin, but they have also been detected in gut samples recently, and some strains are believed to be involved in lipid metabolism and cholesterol conversion [
32]. The present study showed that the presence of this genus was significantly higher in the HB group, suggesting that blue light exposure might affect this bacterium under the HFHF background; however, further experiments are needed to verify its resulting physiological function. The genus Muribaculum has been reported to be able to decompose dietary fiber and polysaccharides, producing short-chain fatty acids (SCFAs), which suggests that it may play a crucial role in maintaining the intestinal barrier, immune stability, and energy balance [
33]. Muribaculum was found to be lower in both the H and HB groups in our study. These results suggest that both a long-term HFHF diet and changes in the blue light environment may weaken the dominant gut bacteria that metabolize fiber, thereby affecting overall microecological stability and energy metabolism.
In this study, we obtained interesting results that differ from previous research on the potential eye damage caused by blue light. In addition to consuming an unhealthy diet that is high in fat and sugar, daily exposure to a certain amount of blue light may have the potential to increase oxidative and inflammatory responses in the body, which may accelerate the progression of liver disease. However, the present study still has some limitations. For example, considering the acceptance of blue light stimulation, we prioritized selecting an albino animal strain, but this strain may not be the most sensitive choice for liver or metabolic disorders. Although the daily blue light exposure time in this experiment was not as long as in other experiments, the possible impact on the circadian rhythm still cannot be completely ruled out. Additionally, since neither the diet nor the blue light exposure dose in this experiment is considered to cause acute damage, the study duration may not be long enough, and the sample size for gut microbiota analysis may have affected statistical power. A longer experimental period might be needed to observe more pronounced results. Due to changes in modern living environments and dietary patterns, many metabolic abnormalities have arisen. Besides diets rich in high-fat and high-sugar foods, the effects of extensive blue light exposure in modern life cannot be ignored. Therefore, understanding the potential combined effects and mechanisms under conditions that involve both dietary and environmental factors is crucial for developing future prevention and improvement strategies, as well as for furthering clinical research. Our present study has established preliminary data, and future studies may consider more related issues to further clarify the underlying mechanisms.