1 Introduction

Obesity has become one of the most severe social problems in recent decades. Obesity is associated with several chronic diseases including hyperlipidemia, hypertension, type 2 diabetes, sleep disorders, cardiovascular diseases, and arteriosclerosis (Guh et al., 2009, Pi-Sunyer, 2009, Payab et al., 2014, Payab et al., 2020). Thus, the amelioration of obesity, induced by adipogenesis (adipocyte growth) and lipogenesis (intracellular lipid accumulation), is regarded as a crucial task in modern society (Tang et al., 2003, Payab et al., 2020). Adipogenesis is the process by which preadipocytes differentiate into mature adipocytes. Insulin stimulates the activity of CCAAT/enhancer-binding protein α (C/EBPα) and peroxisome proliferator-activated receptor γ (PPARγ) to trigger adipogenesis. In mature adipocytes, triglycerides are synthesized and accumulated by the secretion of fatty acid synthase (Fasn) and transportation of fatty acid-binding protein 4 (FABP4), which are downstream targets of PPARγ (Ockner et al., 1972, Payab et al., 2020). PPARγ is a key transcriptional factor that controls glucose and the metabolism of fatty acids. PPARγ plays a pivotal role in increasing the expression of adipogenic genes and inducing the differentiation of preadipocytes into fully differentiated adipocytes by promoting the transcription of its target genes. In addition, PPARγ is an indispensable factor in maintaining the physiological function of mature adipocytes (Jiang et al., 2014). However, upon phosphorylation, PPARγ undergoes cytoplasmic degradation via poly-ubiquitination. This process suppresses the translocation of PPARγ into the nucleus and its binding with the peroxisome proliferator-response element (PPRE) promoter located in the PPARγ target genes (Moya and Marquez-Aguirre, 2016). Therefore, the induction of PPARγ phosphorylation may be an effective strategy for preventing obesity.

Sunlight, a major factor that affects the skin, can be categorized as ultraviolet (UV), visible, and infrared light based on its wavelength and amount of energy (Liebel et al., 2012, Moya and Marquez-Aguirre, 2016, Byrne et al., 2018, Park et al., 2022). Blue light, also known as high-energy visible (HEV) light, is emitted at wavelengths of 400–500 nm. Blue light is emitted not only from the sun, but also from digital devices, including televisions, smartphones, and computers (Schalka et al., 2019, Campiche et al., 2020, Park et al., 2022). Moreover, blue light has the shortest wavelength in the visible light spectrum, leading to a high-energy release. Hence, blue light can penetrate the skin deeper than UV light, indicating that blue light affects the adipose tissue in the hypodermis (the innermost layer of the skin) more effectively than UV light (Kang et al., 2018). Research on the effects of UV light irradiation on adipocytes has been widely reported; however, the effects of blue light irradiation on adipocytes remain unclear. In this study, we examined the effects of blue light on adipogenic differentiation.

Transient receptor potential vanilloid 1 (TRPV1) is a member of the transient receptor potential (TRP) cation channel family. TRPV1 is expressed in keratinocytes, kidney cells, melanocytes, sensory neurons, bronchial epithelial cells, and adipocytes (Lee et al., 2012, Chen et al., 2015, Caterina and Pang, 2016). The TRPV1 channel is controlled by capsaicin, the key component that imparts pungency to chili peppers. TRPV1 is also stimulated by heat (>43 °C), low pH (<4.5), and UV light (Tominaga and Tominaga, 2005). In our previous studies, we have shown that blue light induces intracellular calcium ion influx into human keratinocytes by promoting TRPV1 upregulation (Yoo et al., 2020, Park et al., 2022). In addition, TRPV1 has been suggested to be involved in adipogenesis (Zhang et al., 2007). However, the involvement of blue light in adipocyte biology has not been reported. Collectively, these two factors helped us develop our research hypotheses. One was a previous study by our research team on the relationship between blue light and TRPV1 and the other was a previous study on the relationship between TRPV1 and adipocyte differentiation. This motivated us to examine the effects of blue light on adipocyte biology and its underlying mechanisms of action.

AMP-activated protein kinase (AMPK) signaling contributes to the regulation of lipid metabolism, such as β-oxidation of fatty acids and lipolysis of triglycerides (Ma et al., 2015, Bu et al., 2019). The AMPK-dependent pathway is activated when AMPK is phosphorylated, leading to the suppression of lipogenesis, β-oxidation of fatty acids, and stimulation of lipolysis (Li et al., 2016, Bu et al., 2019). In addition, AMPK phosphorylation induces phosphorylation of PPARγ, transcription factors of adipogenic differentiation. Phosphorylation of PPARγ leads to suppression of the expression of adipogenic markers including glucose transporter type 4 (GLUT4), Fasn, and FABP4 (Lim et al., 2021). p38 mitogen-activated protein kinase (p38 MAPK), c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK) are major regulators of several cellular events, such as cell proliferation, autophagy, and adipocyte differentiation (Lim et al., 2021, Yu et al., 2021). MAPKs are also involved in PPARγ phosphorylation.

In the present study, we examined the effects of blue light on adipogenesis and its mechanism of action in 3T3-L1 adipocytes. Specifically, we determined the link between TRPV1 signaling and the AMPK and MAPK signaling pathways in the context of the impact of blue light on adipogenesis.

2 Materials and methods

3 Results

3.1

3.1 Blue light inhibits cellular adipogenesis

We examined the anti-adipogenic effects of blue light during the differentiation of 3T3-L1 preadipocytes into mature adipocytes. Oil Red O staining, triglyceride assays, and glutathione peroxidase (GPDH) activity assays were performed to demonstrate the effects of blue light on adipogenic inhibition. Two days after the 3T3-L1 preadipocytes reached approximately 70–80 % confluence, they were irradiated with blue light for 10, 15, and 30 min before changing the differentiation medium. Oil Red O staining was performed on the eighth day after initiating cell differentiation to examine the proportion of intracellular lipid droplets. Compared to that in undifferentiated cells, lipid accumulation in fully differentiated cells increased significantly. Lipid accumulation in differentiated cells exposed to blue light significantly reduced in a time-dependent manner (Fig. 1A). To further test the hypothesis that blue light inhibits adipocyte differentiation, we measured the intracellular levels of GPDH activity and triglyceride content. The triglyceride levels of cells treated with the differentiation cocktail were much higher than those of undifferentiated cells. However, blue light irradiation during adipogenesis of 3T3-L1 preadipocytes reduced the intracellular triglyceride content in a time-dependent manner (Fig. 1B). Evaluation of GPDH activity showed the same trend, as shown in Fig. 1A and 1B. Compared with pre-adipocytes, the GPDH activity of mature adipocytes was greatly enhanced, but blue light repressed GPDH activity in mature adipocytes. These data indicated that blue light suppressed adipogenic differentiation by inhibiting cellular lipid droplet accumulation and GPDH activity.

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Fig. 1

Blue light inhibits 3T3-L1 adipogenesis. Each time, before differentiation medium replacement, the confluent 3T3-LI cells preadipocytes (approximately 70–80 % confluent were irradiated with blue light at three doses: 76 W × 10 min/m² (4.56 J /cm² for 10 min), 76 W × 15 min/m² (6.84 J/cm² for 15 min), and 76 W × 30 min/m² (13.68 J/cm² for 30 min)). Undifferentiated cells were used as a negative control, and cells treated with a differentiation cocktail were used as a positive control. The assays were conducted on the eighth day after differentiation initiation. (A) Oil Red O staining of lipid droplets. (B) Total level of intracellular triglycerides. (C) GPDH activity in the 3T3-L1 cells. Results are shown as mean ± SEM of at least three different experiments. #: vs preadipocytes, *: vs mature adipocytes, #p < 0.05, ##p < 0 0.01, ###p < 0 0.001, *p < 0.05, **p < 0.01, and ***p < 0.001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3.2

3.2 Blue light promotes the degradation of PPARγ protein by inducing PPARγ phosphorylation

PPAR-γ is an important regulator of adipogenic differentiation. When PPARγ moves into the nucleus and binds to PPRE in the target molecules of PPARγ, it stimulates the transcription of PPARγ-downstream targets such as Fasn and FABP4. Phosphorylation of PPARγ induces proteasomal degradation in the cytosol, preventing nuclear translocation of PPARγ (Moya and Marquez-Aguirre, 2016). PPARγ expression in mature adipocytes was much higher than that in preadipocytes. In contrast, the protein level of PPARγ in 3T3-L1 mature adipocytes treated with blue light three times during adipogenesis was time-dependently downregulated. Moreover, the phosphorylation level of PPARγ was decreased by treatment with the MDI cocktail and was recovered upon exposure to blue light (Fig. 2A). The transcriptional level of PPARγ showed the same trend as the PPARγ protein expression (Fig. 2B). Similarly, the activity of the PPRE-luciferase reporter increased when adipogenesis was initiated but was suppressed by blue light irradiation in a time-dependent manner (Fig. 2C). Taken together, blue light inhibits the expression and transcriptional activity of PPARγ; however, it upregulates PPARγ phosphorylation. Moreover, blue light inhibits the luciferase activity of PPRE. These results indicated that blue light irradiation lowered the expression and transcriptional activity of PPARγ. Furthermore, blue light blocked the differentiation of 3T3-L1 preadipocytes by promoting the phosphorylation of PPARγ, leading to the degradation of PPARγ protein.

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Fig. 2

Blue light promotes the degradation of PPARγ protein by inducing PPARγ phosphorylation in 3T3-L1 cells. Each time, before differentiation medium replacement, the confluent 3 T3-LI cells preadipocytes (approximately 70–80 % confluent) were irradiated with blue light at three doses: 76 W × 10 min/m² (4.56 J /cm² for 10 min), 76 W × 15 min/m² (6.84 J/cm² for 15 min), and 76 W × 30 min/m² (13.68 J/cm² for 30 min). The undifferentiated cells were used as a negative control, and the fully differentiated cells were used as a positive control. (A) Western blotting analysis of PPARγ and its phosphorylated form on the eighth day after differentiation initiation. (B) Quantitative PCR analysis of the mRNA level of PPARγ on the eighth day after differentiation initiation. (C) The confluent 3T3-L1 cells (approximately 70–80 % confluence) were transfected with PPRE-luciferase reporter plasmid and β-galactosidase. The cells were exposed to blue light the next day at three doses. The undifferentiated cells were used as a negative control, and the fully differentiated cells were used as a positive control. Then, the cells were treated with an MDI cocktail and incubated for 24 h to induce differentiation. Finally, the adipocytes were harvested and used to perform a luciferase reporter assay for evaluating the luciferase activity of PPRE. The results are shown as mean ± SEM of at least three different experiments. #: vs preadipocytes, *: vs mature adipocytes, #p < 0.05, ##p < 0.01, ###p < 0.001, *p < 0.05, **p < 0.01, and ***p < 0.001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3.3

3.3 Effect of blue light on PPARγ downstream targets

Fasn and FABP4 are typical adipogenic genes activated mainly via PPARγ induction. The expression levels of Fasn and FABP4 were examined on day 8 after the induction of cell differentiation. Blue light attenuated the differentiation cocktail-induced increases in FASN and FABP4 mRNA levels (Fig. 3A). Similarly, exposure to blue light reduced Fasn and FABP4 protein levels, which were increased by the differentiation medium (Fig. 3B). These findings imply that blue light exerts an inhibitory effect on the downstream targets of PPARγ, such as Fasn and FABP4, through the inhibition of the PPARγ signaling pathway, as shown in Fig. 2A and Fig. 2B.

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Fig. 3

Effect of blue light on PPARγ downstream targets. Each time, before differentiation medium replacement, the confluent 3 T3-LI cells preadipocytes (approximately 70–80 % confluent) were irradiated with blue light at three doses: 76 W × 10 min/m² (4.56 J /cm² for 10 min), 76 W × 15 min/m² (6.84 J/cm² for 15 min), and 76 W × 30 min/m² (13.68 J/cm² for 30 min). The undifferentiated cells were used as a negative control, and the fully differentiated cells were used as a positive control. (A) Quantitative PCR analysis of the mRNA levels of Fasn and FABP4 on the eighth day after differentiation initiation. (B) Western blotting analysis of Fasn and FABP4 on the eighth day after differentiation initiation. The results are shown as mean ± SEM of at least three different experiments. #: vs preadipocytes, *: vs mature adipocytes, #p < 0.05, ##p < 0.01, ###p < 0.001, *p < 0.05, **p < 0.01, and ***p < 0.001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3.4

3.4 Blue light recovers TRPV1 expression and calcium influx in adipocyte differentiation

It has already been reported that the TRPV1 channel is involved in adipogenic differentiation (Zhang et al., 2007). In a previous study, we showed that blue light regulates the activation and expression of TRPV1 in human keratinocytes and contributes to increased cellular calcium influx [18]. Therefore, we hypothesized that blue light would recover the expression of TRPV1 and that intracellular calcium influx would be inhibited by adipogenic differentiation. We found that protein expression of TRPV1 decreased in cells treated with the differentiation cocktail, whereas it increased upon irradiation with blue light in a time-dependent manner (Fig. 4A). The levels of the phosphorylated form of TRPV1 showed a trend similar to that of TRPV1. The phosphorylation level of TRPV1 was repressed during adipogenesis and was reversed by exposure to blue light for 15 and 30 min (Fig. 4B). Additionally, we found that the cellular influx of calcium was attenuated by adipogenic differentiation but was enhanced again by 10 min irradiation with blue light. However, the recovery of calcium influx by blue light irradiation was reduced by co-treatment with capsazepine, a TRPV1 antagonist (Fig. 4C). These results indicate that blue light irradiation restores TRPV1 expression, leading to the stimulation of intracellular calcium influx in 3T3-L1 adipocytes.

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Fig. 4

Blue light recovers TRPV1 expression and calcium influx during 3T3-L1 adipocyte differentiation. Each time, before differentiation medium replacement, the confluent 3 T3-LI cells preadipocytes (approximately 70–80 % confluent) were irradiated with blue light at three doses: 76 W × 10 min/m² (4.56 J /cm² for 10 min), 76 W × 15 min/m² (6.84 J/cm² for 15 min), and 76 W × 30 min/m² (13.68 J/cm² for 30 min). The undifferentiated cells were used as a negative control, and the fully differentiated cells were used as a positive control. (A-B) Western blotting analysis of TRPV1 and its phosphorylated form on the eighth day after differentiation initiation. (C) Each time, prior to the differentiation medium replacement, the confluent 3 T3-LI cells preadipocytes (approximately 70–80 % confluent) were irradiated with blue light at 4.56 J/cm². The cells were co-treated with capsazepine (10 μM) and the differentiation cocktail. Analysis of calcium influx was performed on the eighth day. The results are shown as mean ± SEM in at least three different experiments. #: vs preadipocytes, *: vs mature adipocytes, #p < 0.05, ##p < 0.01, ###p < 0.001, *p < 0.05, **p < 0.01, and ***p < 0.001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3.5

3.5 Effect of TRPV1 ablation on intracellular levels of GPDH activity and lipid accumulation

In our previous experiments, we found that TRPV1 might be involved in the anti-adipogenic effects of blue light. To verify the contribution of TRPV1 to blue light-induced inhibition of adipogenesis, capsazepine, a well-known TRPV1 agonist, was used for Oil Red O staining and GPDH activity assays. As shown in Fig. 5A, lipid droplets stained with Oil Red O were denser in fully differentiated cells than in undifferentiated cells. Blue light suppresses lipid formation during adipogenesis, whereas capsazepine restores it. Additionally, the increased GPDH activity induced by the MDI cocktail was blocked by exposure to blue light; however, capsazepine enhanced GPDH activity (Fig. 5B). These data indicate that TRPV1 is directly associated with the attenuation of adipocyte differentiation by blue light.

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Fig. 5

Effect of TRPV1 ablation on intracellular lipid accumulation and GPDH activity. Each time, prior to the differentiation medium replacement, the confluent 3 T3-LI cells preadipocytes (approximately 70–80 % confluent) were irradiated with blue light at 13.68 J cm⁻². The cells were co-treated with capsazepine (10 μM) and the differentiation cocktail. Preadipocytes were used as a negative control, and mature adipocytes were used as a positive control. The assays were performed on the eighth day after differentiation initiation. (A) Oil Red O staining of lipid accumulation in the cells. (B) GPDH activity of 3 T3-LI cells. The results are shown as mean ± SEM of at least three different experiments. #: vs preadipocytes, *: vs mature adipocytes, #p < 0.05, ##p < 0.01, ###p < 0.001, *p < 0.05, **p < 0.01, and ***p < 0.001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3.6

3.6 Blue light increases the intracellular phosphorylation levels of MAPK and AMPK

MAPK pathway proteins (p38, JNK, and ERK) contribute to various cellular processes, including transcription, DNA replication, and differentiation (Lim et al., 2021, Yu et al., 2021). AMPK signaling is also associated with lipid metabolism, including lipolysis of triglycerides and β-oxidation of fatty acids (Ma et al., 2015, Bu et al., 2019). Activation of AMPK and its downstream signaling, acetyl-CoA carboxylase (ACC), is induced when the proteins are phosphorylated and contribute to the suppression of adipogenesis by inhibiting key proteins (PPARγ and C/EBPα), which are transcription factors involved in adipogenic differentiation (Lim et al., 2021). In this study, the phosphorylation levels of MAPKs, AMPK, and ACC were measured in 3T3-L1 cells by western blotting. Blue light did not alter the total expression of total ERK, JNK, and p38. Compared to undifferentiated cells, the phosphorylation levels of MAPKs were noticeably decreased in differentiated cells. In contrast, levels of phosphorylated MAPKs increased after exposure to blue light during adipogenesis (Fig. 6A). The phosphorylation levels of AMPK and ACC were also reduced upon differentiation induction and recovered upon blue light irradiation (Fig. 6B). Therefore, the results proved that blue light activates the MAPK and AMPK signaling pathways.

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Fig. 6

Blue light increases the phosphorylation of MAPK and AMPK in 3T3-L1 cells. Each time, before differentiation medium replacement, the confluent 3 T3-LI cells preadipocytes (approximately 70–80 % confluent) were irradiated with blue light at three doses: 76 W × 10 min/m² (4.56 J /cm² for 10 min), 76 W × 15 min/m² (6.84 J/cm² for 15 min), and 76 W × 30 min/m² (13.68 J/cm² for 30 min). The preadipocytes were used as a negative control, and the mature adipocytes were used as a positive control. (A-B) Western blotting analysis of MAPKs, AMPK, and ACC and their phosphorylated forms on the eighth day after differentiation initiation. The data are shown as mean ± SEM of at least three different experiments. #: vs preadipocytes, *: vs mature adipocytes, #p < 0.05, ##p < 0.01, ###p < 0.001, *p < 0.05, **p < 0.01, and ***p < 0.001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3.7

3.7 Effect of TRPV1 inhibition on MAPK and AMPK signalings

In previous experiments, we demonstrated that blue light activates the TRPV1 channel as well as MAPKs and AMPK. Therefore, we examined the relationship between TRPV1, MAPKs, and AMPK. As shown in Fig. 7A, although the phosphorylation levels of p38 MAPK, JNK, and ERK were decreased in the differentiation medium, blue light increased their phosphorylation levels. However, treatment with capsazepine blocked this effect. AMPK signaling also showed results similar to those of MAPKs (Fig. 7B). These results indicated that TRPV1 inhibition reduced the phosphorylation levels of MAPK, AMPK, and ACC proteins, leading to a disturbance in the anti-adipogenic effect of blue light. In addition, these results suggest that TRPV1 operates upstream of AMPK and MAPKs in blue-light-induced effects.

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Fig. 7

Effect of TRPV1 inhibition on MAPK and AMPK signaling pathways. Each time, prior to the differentiation medium replacement, the confluent 3 T3-LI cells preadipocytes (approximately 70–80 % confluent) were irradiated with blue light at 13.68 J cm⁻². The cells were co-treated with capsazepine (10 μM) and the differentiation cocktail. Preadipocytes were used as a negative control, and mature adipocytes were used as a positive control. (A-B) Western blotting analysis of MAPKs, AMPK, and ACC and their phosphorylated forms on the eighth day after differentiation initiation. The data are represented as mean ± SEM of at least three different experiments. #: vs preadipocytes, *: vs mature adipocytes, #p < 0.05, ##p < 0.01, ###p < 0.001, *p < 0.05, **p < 0.01, and ***p < 0.001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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3.8

3.8 Inhibition of AMPK and MAPK downregulates PPARγ phosphorylation level

Previous results have reported that AMPK and MAPKs induce the phosphorylation of PPARγ, contributing to its degradation (Moya and Marquez-Aguirre, 2016). In addition, we demonstrated the activation of AMPK-MAPK signaling pathways and the suppression of PPARγ expression and phosphorylation by blue light. Therefore, to test whether AMPK and MAPK suppress PPARγ, SP600125 (inhibitor of JNK), PD0325901 (inhibitor of ERK), SB203580 (inhibitor of p38), and compound C (AMPK inhibitor) were used for western blot analysis. As shown in Fig. 8A, protein levels of PPARγ in mature adipocytes increased remarkably compared to that in preadipocytes but decreased in cells exposed to blue light for 30 min. However, the group treated with SP600125, PD0325901, SB203580, and compound C showed recovery from the blue light-induced reduction of PPARγ. In contrast, phosphorylated PPARγ levels were reduced by adipogenesis induction, whereas they were recovered by blue light irradiation. However, the phosphorylation level of PPARγ was reduced by inhibiting ERK, JNK, p38 MAPK, and AMPK (Fig. 8A and Fig. 8B). These data indicate that AMPK and MAPKs activation have negative impacts on PPARγ protein level but positively affect PPARγ phosphorylation in the blue light-induced anti-adipogenesis.

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Fig. 8

Inhibition of AMPK and MAPK downregulates PPARγ phosphorylation level. Each time, prior to the differentiation medium replacement, the confluent 3 T3-LI cells preadipocytes (approximately 70–80 % confluent) were irradiated with blue light at 13.68 J cm⁻². The cells were co-treated with the differentiation cocktail and Compound C (10 μM), PD0325901 (10 μM), SP600125 (20 μM), or SB203580 (10 μM). Preadipocytes were used as a negative control and mature cells were used as a positive control. (A-B) Western blotting analysis of PPARγ and its phosphorylated form on the eighth day after differentiation initiation. The data are shown as mean ± SEM of at least three different experiments. #: vs preadipocytes, *: vs mature adipocytes, #p < 0.05, ##p < 0.01, ###p < 0.001, *p < 0.05, **p < 0.01, and ***p < 0.001. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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4 Discussion

Obesity is a major global social issue. Obesity is associated with several metabolic diseases, including arteriosclerosis, cardiovascular disease, and type 2 diabetes (Guh et al., 2009, Pi-Sunyer, 2009, Payab et al., 2014, Payab et al., 2020). Obesity typically occurs through the sequential processes of adipocyte formation, proliferation, and differentiation (Oh et al., 2018). Today, inhibition of adipogenic differentiation is on the rise as an essential strategy for improving obesity.

Sunlight is divided into infrared, visible, and ultraviolet. Blue light (high-energy visible (HEV) light) is short-wavelength light with high energy in the 380–500 nm range of visible light. In addition, LED lights such as smartphones, tablet PCs, TV screens, and monitors emit blue light, which can be detrimental to the health of people who spend more than 12 h a day using digital devices. Negative effects of blue light on the skin have been widely suggested, including reducing proliferation and inducing oxidative stress in human keratinocytes (Park et al., 2022), whereas few reports have suggested beneficial effects of blue light on the skin. In our previous study, blue light was shown to play a role in the upregulation of TRPV1 in epithelial cells. The TRPV1 channel is generally activated by high temperature, low pH, and capsaicin, an active component of chili peppers, leading to the enhancement of intracellular calcium ion levels in cells such as kidney cells, bronchial epithelial cells, sensory nerves, and adipose cells (Yoo et al., 2020). Adipose cells form adipose tissue in the subcutaneous layer, which is the deepest layer of the skin. TRPV1 is also suggested to be involved in adipose cell differentiation (Moya and Marquez-Aguirre, 2016). Therefore, based on our previous studies on the relationship between blue light and TRPV1 and existing reports on the relationship between TRPV1 and adipocyte differentiation, we evaluated in the present study the effects of blue light on adipocyte differentiation and its mechanisms of action.

The TRPV1 channel plays multiple roles in obesity, including the regulation of lipid homeostasis, improvement of diabetes, and amelioration of hypertension (Sun et al., 2013). In our previous studies, we showed that blue light stimulates the expression of TRPV1, resulting in the induction of intracellular calcium influx into other skin cells, such as keratinocytes (Yoo et al., 2020, Park et al., 2022). In this study, we confirmed that blue light induces activation of the TRPV1 receptor in 3T3-L1 mouse cells as well as phosphorylation of the TRPV1 channel. In addition, the MDI cocktail-induced inhibition of calcium influx was reversed by TRPV1 activation stimulated by blue light irradiation. These results indicate that blue light-induced anti-adipogenesis involves TRPV1 signaling in adipocytes.

GPDH promotes the formation of glycerol-3-phosphate and regulates lipid metabolism (Zhao et al., 2018). The GPDH enzyme is involved in triglyceride synthesis and the conversion of adipose cells, both morphologically and characteristically (Kozak and Jensen, 1974, Lee et al., 2009). In this study, we confirmed that blue light greatly reduces both intracellular triglyceride accumulation and GPDH activity in 3T3-L1 mature cells. These results demonstrated the inhibitory effect of blue light on adipogenic differentiation. In addition, we examined the involvement of TRPV1, the main molecule activated by blue light, in blue light-induced inhibition of adipogenesis. In this study, we found that the suppression of TRPV1 by capsazepine, an antagonist of TRPV1, interfered with the blue light-induced inhibition of differentiation. These results indicate that the anti-adipogenic effect of blue light occurs through TRPV1 channel activation.

Adipogenesis is closely associated with the activity of adipocyte-specific genes, including PPARγ. PPARγ is a transcription factor with ligand-dependence and belongs to the nuclear hormone receptor gene superfamily (Peverelli et al., 2013). PPARγ is widely known to be a major regulator of adipogenic differentiation. In addition, PPARγ has the potential to drive adipogenic differentiation by expressing downstream targets, such as Fasn and FABP4. Fasn is involved in the synthesis of fatty acids, and FABP4 plays a critical role in transporting fatty acids within cells (Soret et al., 2016). Therefore, developing a therapeutic agent targeting PPARγ may be an effective strategy for obesity prevention. In contrast, phosphorylation of PPARγ causes degradation of the protein through polyubiquitination; therefore, the phosphorylated form of PPARγ can be conjugated as a novel PPARγ inhibitor (Moya and Marquez-Aguirre, 2016). In this study, we found that PPARγ expression was suppressed by exposure to blue light. We also found that the luciferase activity of PPRE, a promoter of PPARγ target genes that forms a complex with PPARγ in the nucleus, is decreased by blue light irradiation. Furthermore, the mRNA and protein levels of PPARγ target molecules FASN and FABP4 were attenuated by blue light irradiation. However, the phosphorylation level of the PPARγ protein shows an opposite trend to the expression of PPARγ. These results suggest that blue light can act as a potential inhibitor of PPARγ and its target molecules by stimulating the phosphorylation or lowering the expression of the gene.

ERK, JNK, and p38 are widely known to be involved in adipogenesis (Oh et al., 2018). Although there have been differences in opinions regarding the positive and negative effects of MAPKs on adipogenesis, the phosphorylation of MAPK proteins, particularly ERK and JNK, contributes to the phosphorylation of PPARγ (Moya and Marquez-Aguirre, 2016). AMPK is an important modulator of energy homeostasis in adipose cells. According to several studies, in adipose tissue, the phosphorylation of AMPK stimulates catabolic pathways, including the breakdown of glucose, fatty acid oxidation, and energy expenditure via lipolysis (Oh et al., 2018, Lim et al., 2021, Yang et al., 2021). In addition, phosphorylated forms of AMPK and its substrate ACC are involved in increasing the phosphorylation of PPARγ (Xu et al., 2021). Therefore, in this study, we hypothesized that the induction of MAPKs and AMPK phosphorylation may be a crucial upstream event in the blue light-mediated inhibition of 3T3-L1 preadipocyte differentiation. In this study, we found that blue light influenced the activation of AMPK, ERK, JNK, and p38, leading to attenuation of adipogenic differentiation by increasing the phosphorylation level of PPARγ. In addition, blue light-induced phosphorylation of AMPK and MAPKs was attenuated by capsazepine treatment. These results indicate that blue light regulates adipogenesis through TRPV1-dependent activation of MAPKs and AMPK.

Taken together, these findings indicate that blue light inhibits adipogenic differentiation and that its anti-adipogenic effect is mediated through TRPV1-dependent activation of the AMPK and MAPK signaling pathways (Fig. 9). These results suggest that blue light could be a therapeutic agent for improving obesity-associated disorders. However, as these findings were demonstrated through in vitro research using mouse cell line, additional research, including clinical trials in animals and humans, is necessary to verify the clinical applicability and safety of blue light irradiation.

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Fig. 9

Schematic diagram showing the action mechanism of blue light-induced anti-adipogenesis in 3T3-L1 cells. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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5 Conclusion

This is the first study to report the anti-adipogenic effect of blue light, along with its anti-adipogenic mechanism, in a mouse preadipocyte cell line. Specifically, blue light negatively regulates adipogenesis, and its anti-adipogenic effect is mediated through TRPV1-dependent activation of the AMPK and MAPK signaling pathways. This study also suggests that blue light can be used as a therapeutic agent to treat obesity-associated disorders.

Funding

This research was supported by a grant from the Medical Device Technology Development Program (Grant No. 20008861) funded by the Ministry of Trade, Industry, and Energy (MOTIE), Republic of Korea, and a grant from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and Technology Information and Communication (Grant No. RS-2023-00246887).

CRediT authorship contribution statement

Seoyoun Yang: Conceptualization, Methodology, Formal analysis, Writing – original draft. Eunbi Yu: Conceptualization, Methodology, Formal analysis, Writing – original draft. See-Hyoung Park: Conceptualization, Methodology, Formal analysis, Writing – original draft. Sae Woong Oh: Conceptualization, Methodology. Kitae Kwon: Conceptualization, Methodology. Su Bin Han: Conceptualization, Methodology. Soo Hyun Kang: Conceptualization, Methodology. Jung Hyun Lee: Conceptualization, Methodology. Heejun Ha: Conceptualization, Methodology. Minkyung Song: Conceptualization, Methodology, Formal analysis, Writing – original draft. Jae Youl Cho: Conceptualization, Methodology, Formal analysis, Writing – original draft, Funding acquisition, Supervision, Writing – review & editing. Jongsung Lee: Conceptualization, Methodology, Formal analysis, Writing – original draft, Funding acquisition, Supervision, Writing – review & editing.