Photoprotective effects of sphingomyelin-containing milk phospholipids in ultraviolet Bâ€“irradiated hairless mice by suppressing NF-ÎoB expression

Ceramide-containing phospholipids improve skin hydration and barrier function and are ideal for use in skin care products. In this study, we evaluated the photoprotective effect of milk phospholipids on the skin condition of UVB-irradiated hairless mice. Skin parameters were assessed following oral administration of milk phospholipids. The UVB irradiation induced photoaging in mice. The animals were divided into 5 groups: a control group (oral administration of saline with no UBV irradiation), UVB group (oral administration of saline with UVB irradiation), and 3 UVB irradiation groups receiving the milk phospholipids at 3 different concentrations of oral administration, 50 mg/ kg (ML group), 100 mg/kg (MM group), and 150 mg/ kg (MH group), for 8 wk. An increase in skin hydration and transepidermal water loss were improved in the 150 mg/kg of milk phospholipid–administered group. Hematoxylin and eosin staining revealed a decrease in epidermal thickness in the milk phospholipid–administered groups (50, 100, and 150 mg/kg of body weight). In particular, the 100 and 150 mg/kg groups showed significant changes in the area, length, and depth of the wrinkles compared with the UVB group. Moreover, the gene expression of matrix metalloproteins was attenuated, and that of proinflammatory cytokines, especially tumor necrosis factor-α, was significantly reduced in the milk phospholipid–administered groups than in the UVB group. The reduced ceramide and increased sphingosine-1-phosphate levels in the skin tissue due to UVB exposure were restored to levels similar to those of the control group following milk phospholipid administration. These results were confirmed to be due to the downregulation of protein expression of nuclear factor kappa-B (NF-κB) and phosphorylated IκB-α (inhibitor of κB α). Collectively, oral administration of milk phospholipids improves skin health through a synergistic effect on photoprotective activity.


INTRODUCTION
The skin ages similar to other organs in the body. Aging is divided into intrinsic aging, which occurs when human body functions deteriorate and physiological functions such as hormones change, and extrinsic aging, which occurs upon exposure to various environmental factors including UVB rays. Among them, photoaging induced by UVB rays is the most direct cause of extrinsic aging, and typical phenomena include dry skin, wrinkles, decreased skin elasticity, and pigmentation (Woodby et al., 2020;Krutmann et al., 2021). Skin moisture is the most important factor in maintaining skin health and controlling aging. Lack of adequate skin moisture prevents activation of enzymes that are involved in the generation of lipids and natural moisturizing factors, leading to dry and thick stratum corneum of the skin (Bouwstra et al., 2021). The skin comprises the epidermis, dermis, and subcutaneous tissue. The stratum corneum of the epidermis, which is the outermost layer of the skin, is composed of the following 3 main lipids: ceramide (50%), free fatty acids (25%), and cholesterol (25%; Wartewig and Neubert, 2007). Lipids, which form a lamellar structure in the spaces between keratinocytes, act as a barrier to water permeation, preventing water transpiration and the invasion of external factors. The ceramide content in this lipid layer is extremely high. Sphingomyelin in the skin granular layer is hydrolyzed by sphingomyelinase to produce ceramide (Mizutani et al., 2009;Vávrová et al., 2017).
Milk contains approximately 4% lipids, and most of them are dispersed in milk in the form of fat globules. These fat globules are covered with a thin membrane called the milk fat globule membrane, approximately half of which consists of lipids. Of these lipids, approximately 70% are triglycerides, approximately 30% are phospholipids, and trace amounts are cholesterol. Phospholipids in milk consist of lipids with a glycerol or sphingosine backbone (Fong et al., 2007). Sphingomyelin, which accounts for a large amount of milk phospholipids, increases ceramide levels in the body (Ohlsson et al., 2010), which is not only involved in epidermal hydration but also in the maintenance of skin barrier function (Brod, 1991). Dietary sphingomyelin is hydrolyzed to ceramide and sphingosine in the small intestine after ingestion, and a portion of it is reused for sphingolipid synthesis (Schmelz et al., 1994). Sphingolipids upregulate the expression of enzymes involved in ceramide synthesis (Duan et al., 2012) and are involved in the enhancement of ceramide levels in the skin of hairless mice (Haruta-Ono et al., 2012b).
In this study, milk phospholipids used were polar lipids obtained by ethanol treatment of milk whey powder and contained high levels of phospholipids, mainly phosphatidylcholine and phosphatidylethanolamine, and sphingolipids, mainly sphingomyelin (Lee et al., 2020). Milk phospholipids are a food material rich in ceramide precursors that can protect the skin from UVB rays. Therefore, milk phospholipids were orally administered to UVB-irradiated hairless mice to evaluate its photoprotective effects.
Male hairless mice (Skh: HR-a, 8 wk old) were purchased from the Central Lab Animal Inc. and subjected to a 1-wk acclimatization period. The breeding environment of the experimental animals was maintained at 23 ± 2°C, 55 ± 10% humidity, and a 12-h light-dark cycle. Mice were fed with a mouse-only feed (Purina), and drinking water was provided ad libitum. The breeding and management of mice was based on the Guide for the Care and Use of Laboratory Animals, and this study was approved by the Institutional Animal Care and Use Committee at Korea University (KUIA-CUC-2020-0054).

Experimental Design
Hairless mice were randomly divided into 5 groups (6 mice/group) based on their average BW. Mice were divided into oral administration of saline without UVB irradiation (control), oral administration of saline with UVB irradiation (UVB), oral administration of lowdose milk phospholipids with UVB irradiation (ML), oral administration of medium-dose milk phospholipids with UVB irradiation (MM), and oral administration of high-dose milk phospholipids with UVB irradiation (MH) groups. Once daily, mice in groups ML, MM, and MH were orally administered 50, 100, and 150 mg/ kg of BW of milk phospholipids, respectively.
To induce photoaging, UVB and milk phospholipidadministered mice (ML, MM, and MH) were irradiated with UVB at the same time, 3 times per week, using a UVB lamp (UB800, Waldman Licht Technik GmbH). Mice were exposed to 1 minimal erythemal dose (MED; 75 mJ/cm 2 ) in the first and second weeks, 2 MED in the third week, 3 MED in the fourth week, and 4 MED from the fifth week onwards. Milk phospholipids were orally administered with UVB exposure for a total of 8 wk, and mice were killed using CO 2 gas after the end of the experiment.

Measurement of Skin Hydration Level, Transepidermal Water Loss, and Wrinkle Formation
Skin hydration level and transepidermal water loss (TEWL) were measured using a Corneometer CM825 (Courage and Khazaka electronic GmbH) and Tewameter TM300 (Courage and Khazaka electronic GmbH), respectively. The dorsal side of the animal was photographed with a digital camera (EOS 600D, Canon), and a replica was generated using a silicone polymer (SILFLO impression material, Flexico) for wrinkle analysis and evaluation. The skin template was fixed at an incident angle of light of 20°, and the wrinkle shadow contrast image was photographed using the Visioline program, imaged, and analyzed using the Skin Viosiometer SV600 software (Courage and Khazana Electronic GmbH). The area, number, length, and depth of the wrinkles were determined.

Histopathological Analysis
After killing the animals, the dorsal skin was incised, attached to filter paper, and fixed in 10% neutral formalin for hematoxylin and eosin and immunohistochemical staining. Tissues were paraffin-embedded by the general tissue processing method, and 6-µm sections were cut and stained with hematoxylin and eosin. Immunohistochemical analysis for collagen was performed as described by Yamamoto et al. (2008) using rabbit anti-type I collagen antibodies (1:400; Abcam). All histopathological analyses were performed using a stereomicroscope (Axio Zoom v.16; Carl Zeiss) and the ImageJ software (version 1.8.0; National Institutes of Health).

RNA Extraction and Real-Time Quantitative PCR
Total RNA was extracted from the skin tissues using TRIzol (Life Technologies Inc.) as per the manufacturer's instructions. Complementary DNA was synthesized from 1 µL of RNA of each sample using the iScript cDNA Synthesis kit (Bio-Rad Laboratories Inc.). To determine mRNA expression of genes encoding MMP, TIMP, and proinflammatory cytokines, realtime quantitative PCR was performed using primers used in a previous study  and SYBR Green (iQ SYBR Green Supermix, Bio-Rad Laboratories, Inc.). The target gene information was as fol- 2), and Tnfa (NM_013693.3). Data were analyzed by the comparative cycle threshold (2 −∆∆C T ) method (Livak and Schmittgen, 2001), and results were normalized to Gapdh mRNA levels.

Analysis of Sphingosine-1-phosphate by HPLC
Extraction and derivatization of sphingosine-1-phosphate (S1P) from skin tissues was performed as described by Min et al. (2002) and Kim et al. (2011), with some modifications. Skin tissues (100 mg) were homogenized by adding 500 µL of PBS and centrifugation at 4,800 × g for 5 min at 25°C. The pH was adjusted to 10 to 11 by adding 700 µL of methanol, 300 µL of 1 M NaCl, 600 µL of CHCl 3 , and then 35 µL of 6 N NaOH. After centrifugation of the sample (4,800 × g, 5 min at 25°C), the lower layer was obtained, and the solution was volatilized. Then, the samples were added to 500 µL of 0.15 M methanolic KOH and vortexed at 37°C for 20 min. After, we added the solution to 500 µL of CHCl 3 , 100 µL of 2 N NH 4 OH, and 400 µL of alkaline water (100 µL 2 N NH 4 OH/100 mL water), and vortexed at 37°C for 1 h. After centrifugation at 4,800 × g for 3 min at 25°C, washing twice with 800 µL of alkaline water, and volatilizing the solution, the obtained lipid extraction samples were dissolved in 120 µL of methanol. The o-phthalaldehyde derivatization reagent [Sigma; 50 mg of ortho-phthalaldehyde, 1 mL of ethanol, 100 µL of 2-mercaptoethanol, and 50 mL of 3% (wt/vol) boric acid solution] was added to 20 µL of the sample and incubated at 25°C for 30 min for derivatization.
The S1P analysis was performed using the e2695 Waters HPLC system (Waters) equipped with a YMC-Pack Pro C18 column (4.6 mm × 150 mm) and fluorescence detector (Waters, e2475). The flow rate was 0.5 mL/ min, and the injection volume was 20 µL. The mobile phase was methanol: distilled water (92:8, vol/vol) with 0.1% triethylamine, and the chromatogram of S1P was analyzed at excitation and emission wavelengths of 340 and 455 nm, respectively. The concentrations of S1P standards (Sigma) were calculated using individual calibration curves (Moon et al., 2013).

Statistical Analysis
The results are expressed as mean and standard error. The significance of data between the UVB and experimental groups was verified by the Tukey's multiple comparison test using SPSS software version 12.0 for Windows (SPSS Inc.).

Effect of Milk Phospholipids on Skin Hydration and TEWL
Skin barrier function is an important indicator of skin homeostasis. Thus, skin hydration and TEWL were analyzed. Skin hydration was significantly lower, and TEWL was significantly higher in the UVB group than in the control group (CON, P = 0.0001 and P = 0.0005, respectively; Figure 1). Milk phospholipid administration suppressed the UVB irradiation-induced decrease in skin hydration and suppressed the increase in TEWL. In particular, skin hydration and TEWL were significantly different between the MH and UVB groups (P = 0.0001 and P = 0.0314, respectively).

Effect of Milk Phospholipids on Epidermal Thickness and Collagen Distribution
Hematoxylin and eosin and immunohistochemical staining were performed to evaluate the effect of milk phospholipid intake on epidermal thickness and collagen distribution following UVB irradiation ( Figure   2). After 8 wk of UVB irradiation and administration of milk phospholipids, hematoxylin and eosin-stained sections of skin samples obtained from mice were analyzed for epidermal thickening. The thickness of the epidermis was significantly greater in the UVB group than in the nonirradiated control group (P = 0.0001, Figure 2B). Consistent with visual observations of the hematoxylin and eosin-stained samples, oral administration of milk phospholipids significantly decreased UVB irradiation-induced increase in epidermal thickness (P = 0.0004, P = 0.0002, and P = 0.0001, respectively) in a dose-dependent manner. In addition, skin of control group had collagen fibers homogenously distributed, and collagen distribution was significantly increased by UVB irradiation, as indicated by collagen distribution ratios of the UVB and control groups (P = 0.0002, Figure 2D). Collagen distribution tended to decrease in the milk phospholipid-administered groups (ML, MM, and MH), but the difference was not significant.

Effect of Milk Phospholipids on Skin Wrinkle Formation
To evaluate the inhibitory effect of milk phospholipids on UVB irradiation-induced wrinkle formation, the wrinkle area, mean wrinkle length, and mean wrinkle depth, which are wrinkle-forming factors, were measured ( Figure 3). The UVB irradiation caused significant differences in the wrinkle area, mean wrinkle length, and mean wrinkle depth, as indicated by data of the UVB and control groups (P = 0.0001, P = 0.0002, and P = 0.0003). Oral administration of medium-dose and high-dose milk phospholipids suppressed UVB irradiation-induced wrinkle formation (area, P = 0.0002 , and high-dose (150 mg/kg of BW) milk phospholipids, respectively, with UVB irradiation. AU = arbitrary units. Data are expressed as means ± SE (n = 6). *P < 0.05, **P < 0.01, and ***P < 0.001 for control vs. UVB group without milk phospholipids group using Tukey's multiple comparison test. and P = 0.0001; length, P = 0.0356 and P = 0.0002; depth, P = 0.0002 and P = 0.0002).

Effect of Milk Phospholipids on MMP and TIMP Expression
The MMP are representative enzymes that degrade extracellular matrix proteins, especially collagen, whereas TIMP inhibit the action of collagen-degrading enzymes (Fisher et al., 1996;Vincenti and Brinckerhoff, 2002). The UVB irradiation increased the expression of genes encoding MMP, promoting wrinkle formation (Figure 4). The expression of Mmp2, Mmp3, Mmp9, and Mmp13 in the UVB group was significantly increased compared with that in the control group because of UVB irradiation (Figure 4, P = 0.0424, P = 0.0005, P = 0.0007, P = 0.0001, respectively). In contrast, the expression of Timp1 and Timp2 was lower in the UVB group than in the control group. Expression of Timp1 in the UVB group was significantly decreased compared with that in the control group (P = 0.0001). The UVB irradiation-induced increase in the expression of genes encoding MMP was decreased in the ML, MM, and MH groups compared with the UVB group. In addition, Timp2 expression in the MM and MH groups were higher than that in the UVB group, but there were no significant differences. Oral administration of milk phospholipids was more effective in regulating the expression of genes encoding MMP than genes expressing TIMP.

Effect of Milk Phospholipids on Proinflammatory Cytokine Production
The mRNA expression of the proinflammatory cytokines Il4, Il6, Il10, and Tnfa after UVB irradiation were , and high-dose (150 mg/kg of BW) milk phospholipids, respectively, with UVB irradiation. Data are expressed as means ± standard error (n = 6). ***P < 0.001 for control vs. UVB group without milk phospholipids group using Tukey's multiple comparison test. significantly increased in the UVB group compared with the control group ( Figure 5). The expression of proinflammatory cytokines in the ML, MM, and MH groups was similar to that of the control group, demonstrating the suppression of UVB irradiation-induced increase in proinflammatory cytokine expression. The expression of Il10, which encodes an immunosuppressive cytokine, was significantly decreased in the ML, MM, and MH groups compared with the UVB group in a dose-dependent manner ( Figure 5C). The expression and function of cytokines that mediate the inflammatory response induced by UVB rays were restored to normal levels by milk phospholipids.

Effect of Milk Phospholipids on Ceramide and S1P Contents in Skin Tissues
When irradiated with UVB, the ceramide content in the UVB group was lower than that in the control group, but the difference was not significant ( Figure  6A). The UVB irradiation-induced decrease in the ce-ramide content was reversed by milk phospholipids, but the difference was not significant compared with that in the UVB group. Figure 6B shows that the S1P content in the UVB group was significantly increased compared with that in the control group (P = 0.0247). Furthermore, milk phospholipids considerably decreased the S1P content compared with the UVB group. , and high-dose (150 mg/kg of BW) milk phospholipids, respectively, with UVB irradiation. Data are expressed as means ± standard error (n = 6). *P < 0.05, **P < 0.01, and ***P < 0.001 for control vs. UVB group without milk phospholipids group using Tukey's multiple comparison test.

Effect of Milk Phospholipids on NF-kB Activation
UVB irradiation was also significantly decreased by milk phospholipids (ML, MM, and MH) in a dose-dependent manner (P = 0.0410, P = 0.0070, and P = 0.0001, respectively). However, the effect of milk phospholipids on the IκBα protein level showed the opposite trend to the change of the NF-κB and p-IκBα protein levels. Milk phospholipids reduced the expression of NF-κB, a photoaging inducer of reactive oxygen species (ROS) generated by UVB irradiation, at the protein level.

DISCUSSION
The milk phospholipids used in this study were polar lipids extracted from whey powder and were composed of phospholipids, mainly phosphatidylcholine and phosphatidylethanolamine, and sphingolipids, mainly sphingomyelin; thus, they were rich in ceramide precursors, which are necessary for moisturizing the skin (Lee et al., 2020). Milk fat is a dietary source of sphingomyelin, and dietary sphingomyelin raises the levels of ceramide in the body. Ceramide plays a role in maintaining the moisture supply of the epidermis and skin barrier function (Parodi, 1997;Graves et al., 2007).
Ceramide accounts for 40 to 60% of the intercellular lipids (Lodén, 2012) and is known to play a crucial role in skin barrier function and skin moisturization by forming a lamellar structure to support the space between keratinocytes (Boer et al., 2016). Additionally, ceramide levels decrease with age, and the amount also decreases in patients with dry skin or atopic dermatitis (Simpson et al., 2013). Ceramide is recognized as a crucial component in maintaining skin homeostasis.
Skin photoaging caused by UVB exposure over a long period of time causes histological changes, such as irregular changes in keratinocytes, an increase in the number of fibroblasts and inflammatory cells, and an accelerated increase in cross-linked collagen fiber levels in the dermis (Robert et al., 2009). Furthermore, exposure of the skin to UVB also causes ROS production (Ansary et al., 2021;Geng et al., 2021). Exposure to UVB promotes continuous ROS generation in keratinocytes, and ROS are rapidly removed by enzymatic and nonenzymatic methods. However, ROS overgeneration causes skin damage. Free radicals generated from ROS cause deterioration of cell structure and function, which ultimately result in abnormal gene expression in cells. UVB-mediated ROS generation affects mitogenactivated protein kinase signaling. Mitogen-activated protein kinase is known to activate the transcription factor NF-κB. Nuclear factor-κB, which mediates the , and Timp2 (F). CON = oral administration of saline with no UVB irradiation; UVB = oral administration of saline with UVB irradiation; ML, MM, and MH = oral administration of low-dose (50 mg/kg of BW), medium dose (100 mg/kg of BW), and high-dose (150 mg/kg of BW) milk phospholipids, respectively, with UVB irradiation. Data are expressed as means ± standard error (n = 6). *P < 0.05, **P < 0.01, and ***P < 0.001 for control vs. UVB group without milk phospholipids group using the Tukey's multiple comparison test; ns = not significant. inflammatory response, binds to an inhibitor of NF-κB (IκB) and is present in the cytoplasm in an inactive form (Yamazaki et al., 2001;Garg and Aggarwal, 2002). When cells are stimulated, such as UV light, IκB kinase phosphorylates the inhibitory IκBα protein. This phosphorylation results in the dissociation of IκBα from NF-κB. The liberated NF-κB enters the nucleus and activates the transcription of genes involved in immune and inflammatory responses (Yamazaki et al., 2001;Garg and Aggarwal, 2002).
Keratinocytes that are stimulated by ROS secrete proinflammatory cytokines to stimulate dermal fibroblasts by promoting DNA binding of NF-κB and AP-1, thereby increasing the expression of MMP. On the other hand, MMP attenuates the synthesis of type-1 procollagen, which in turn inhibits collagen biosynthesis and promotes degradation, forming skin wrinkles and reducing elasticity (Gao et al., 2018;Jin et al., 2021).
Oral administration of milk phospholipids significantly decreased the mRNA expression of Mmp2, Mmp3, Mmp13, and the proinflammatory cytokines Il4, Il6, Il10, and Tnfa compared with that of the UVB group.
The decrease in the expression of proinflammatory cytokines suppressed wrinkle formation caused by photoaging. Although there was no significant difference in collagen distribution, there was a considerable decrease when compared with the UVB group. As such, oral administration of milk phospholipids not only decreased the expression of proinflammatory cytokines but also decreased the expression of MMP, thereby inhibiting collagen degradation and suppressing wrinkle formation. Moreover, milk phospholipids exhibited inhibitory effect on UVB irradiation-induced decrease in the skin moisture content and increase in moisture evaporation.
Skin hydration is closely related to the moisture content of the stratum corneum, which is the outermost layer of the epidermis. The stratum corneum has an organic complex structure of proteins and lipids that forms the skin barrier; a decrease in the moisture content of the stratum corneum results in dryness of the skin and abnormalities in skin barrier function (Nisbet, 2018). Furthermore, the main role of ceramide in the stratum corneum is protecting the epidermal barrier, and a decrease in the ceramide content in the stratum , Il10 (C), and Tnfa (D). CON = oral administration of saline with no UVB irradiation; UVB = oral administration of saline with UVB irradiation; ML, MM, and MH = oral administration of low-dose (50 mg/kg of BW), medium dose (100 mg/kg of BW), and high-dose (150 mg/kg of BW) milk phospholipids, respectively, with UVB irradiation. Data are expressed as means ± standard error (n = 6). **P < 0.01 and ***P < 0.001 vs. UVB group without milk phospholipids group using the Tukey's multiple comparison test. corneum weakens the skin barrier function and acts as a cause of increased water loss between the skin. Milk ceramide used in this experiment was a raw material rich in phospholipids, and sphingomyelin accounts for 20 to 25% of milk ceramide (Lee et al., 2020). Sphingomyelin is the main component of mammalian cell membranes and a basic component of the lamellar structure responsible for the barrier function of the skin epidermis. Dietary sphingolipids, such as sphingomyelin and glucosylceramide, reportedly improve the skin barrier function (Tsuji et al., 2006;Duan et al., 2012;Haruta-Ono et al., 2012a). The content of sphingomyelin, a ceramide precursor, is higher in milk phospholipids than in phospholipids extracted from other foods. Sphingoids are a structural component of sphingomyelin. Their absorption rate based on their source was evaluated in Caco-2 cells, and it was found that the absorption rate of animal-derived sphingoids was higher than that of plant-derived sphingoids. Therefore, sphingomyelin and glucosylceramide, which are sphingolipids, may be effective in strengthening the barrier function of the skin epidermis (Potočki, 2016;Morifuji, 2019).
Ceramides are an important component of the skin barrier and are involved in the prevention of TEWL. They are also important biological metabolites that regulate cellular processes, such as proliferation, differentiation, and apoptosis (Robert et al., 2009;Simpson et al., 2013). Sphingosine-1-phosphate is a sphingolipid . Effects of milk phospholipids on ceramide (A) and sphingosine-1-phosphate (B) contents. CON = oral administration of saline with no UVB irradiation; UVB = oral administration of saline with UVB irradiation; ML, MM, and MH = oral administration of low-dose (50 mg/kg of BW), medium dose (100 mg/kg of BW), and high-dose (150 mg/kg of BW) milk phospholipids, respectively, with UVB irradiation. Data are expressed as means ± standard error (n = 6). *P < 0.05 vs. UVB group without milk phospholipids group using the Tukey's multiple comparison test; ns = not significant differences between groups. Figure 7. Effects of milk phospholipids on protein expression of IκBa (A), phosphorylated (p)-IκBa (B), and nuclear factor (NF)-κB (C). CON = oral administration of saline with no UVB irradiation; UVB = oral administration of saline with UVB irradiation; ML, MM, and MH = oral administration of low-dose (50 mg/kg of BW), medium dose (100 mg/kg of BW), and high-dose (150 mg/kg of BW) milk phospholipids, respectively, with UVB irradiation. Data are expressed as means ± standard error (n = 6). **P < 0.01 and ***P < 0.001 vs. UVB group using the Tukey's multiple comparison test. with a phosphate group attached to position 1 and is produced from ceramide by ceramidase and sphingosine kinase. It has a special function in epidermal cells and is involved in keratinocyte proliferation, apoptosis, differentiation, and migration. In keratinocytes, S1P acts as an immune response modulator by activating the NF-κB pathway to increase the production of cathelicidin antimicrobial peptides as effectors of the innate immune response (Park et al., 2013;Jeong et al., 2015). An increase in the dose of milk phospholipids decreased the S1P content, resulting in decreased NF-κB mRNA and protein expression. Interestingly, NF-κB is involved in the inflammatory response through various pathways. Therefore, a reduction in NF-κB expression may improve photoaging by reducing the production of proinflammatory cytokines.

CONCLUSIONS
Phospholipids are representative lipids constituting the skin, and milk phospholipids, with a high phospholipid content, may be used as dietary supplements for skin health. In this study, milk phospholipids were found to suppress the expression of MMP and proinflammatory cytokines through the downregulation of NF-κB and p-IκBα protein expression, demonstrating its photoprotective effect against UVB irradiation-induced skin damage in a hairless mouse model. In addition, further studies on the effect of milk phospholipids on ROS are needed in the future.