Effect of UV irradiation on Energy Homeostasis of Hairless Rats and Mice

Abstract

Ultraviolet (UV) light is an electromagnetic radiation with wavelengths ranged from 200 nm~400 nm, and human skin is continuously exposed to UV light. Previous studies suggest that lipid metabolism in human subcutaneous (SC) fat tissues can be regulated by UV. In particular, UV radiation significantly reduces the lipids synthesis and lipolysis in the human SC fat tissues. Accordingly, this excess of energy derived from reduced SC fat is supposed to move to other organs instead of being accumulated in SC fat tissues. In this study, changes in energy homeostasis induced by UV irradiation were examined in major organs which participate in energy homeostasis of hairless rats and mice in vivo. Fifteen week-old rats (Rattus rattus) were separated into two groups, and 6 week-old mice (Mus musculus) were divided into two groups as well. One group was irradiated with UV light, and the other group was sham-irradiated for 8 weeks (3 times / week), (UV dose

200~600 mJ/cm2, total amount of UV radiation

6000~8000 mJ/cm2). After 8-week UV irradiation, serum, dorsal and abdominal skin including SC fat tissue, visceral fat, liver, and muscle tissues were obtained from seventeen rats and twenty mice. Although, there was no significant difference in food intake between two groups, the average body weight at 8 weeks was slightly decreased in the UV-irradiated group in rats [1]. There was no significant difference in food intake and body weight between two groups in mice. To investigate changes of lipid contents in major organs, we determined levels of triglyceride (TG), total cholesterol, and glucose by fluorescent enzymatic assay. Besides to identify their regulation factors, the expression of lipogenic enzymes, such as fatty acid synthase (FAS), and sterol regulated element-binding protein (SREBP)-1c and (SREBP)-2 were quantified. Moreover, expressions of glucose transporter type 4 (GLUT-4), a glucose metabolism gene, Fas receptor (TNF- α receptor), and suppressor of cytokine signal (SOCS)-3, which are known to mediate inflammation and insulin resistance, were also examined by real-time quantitative polymerase chain reaction (PCR) or Western blot analysis. Overall, this study demonstrated that UV irradiation affects impaired SC fat storage ability. The UV-irradiated rats showed a decreased tendency of SC fat accumulation in back and abdomen skin. In contrast, other energy metabolism in visceral fat and liver tissues tended to be enhanced in UV-irradiated group as compared to sham-irradiated one. Similarly, whereas SC fat from abdominal skin of UV-irradiated mice tended to decrease, their visceral fat increased after exposure to UV. In conclusion, this study suggests that excess energy which could not be stored in SC fat tissues due to UV-induced impairment of SC fat storage could be stored in other organs, such as visceral fat or liver, probably through dynamic regulation of enzymes and transcription factors associated with glucose and lipids metabolism in those organs.