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Salinity and Temperature Effects on Cholesterol Accumulation through SIRT1/LXRα/SREBP1 Pathway in Livers of the Indian Medaka (Oryzias dancena)

Being ectothermic, physiological mechanisms of fish usually change with environmental temperatures. The sterol regulatory‐element binding protein 1 (SREBP1) is an important regulator of lipid metabolism in the liver as well as other tissues of vertebrates. It is involved in cholesterol biosynthesis for the cellular processes mainly in building blocks of the cellular membrane and creation of hormones, vitamin D, and enzymes. In mammals, SREBP1 was found to be activated by the other transcription factors, i.e., SIRT1 and LXRα, in the cholesterol biosynthesis pathway. However, little is known about these transcription factors involved in the SIRT1/LXRα/SREBP1 pathway of cholesterol biosynthesis in lower vertebrates. This study used the Indian medaka (Oryzias dancena), an euryhaline model fish, to (i) compare low‐temperature effects on this pathway of the ectothermic fish between seawater (SW)‐ and freshwater (FW)‐acclimated individuals and (ii) demonstrate the involvement of SREBP1 in cholesterol biosynthesis in livers of fish by translational Vivo‐Morpholino knockdown of SREBP1. Immunoblotting analyses revealed that relative amounts of SIRT1 and SREBP1 increased significantly in livers of both SW and FW medaka when transferred from normal‐temperature (28 °C) to low‐temperature (18 °C) for 1 week. Meanwhile, significant decrease and increase in protein abundance of hepatic LXRα was found in SW and FW individuals, respectively, in the 18 °C group. In addition, after transfer to 18 ºC for 1 week from 28 ºC, the cholesterol contents in livers of SW and FW medaka were significantly increased and decreased, respectively. Under low‐temperature stress, SW‐acclimated medaka showed significantly higher levels of cholesterol in livers than FW‐acclimated fish. The results of translational Vivo‐Morpholino knockdown of SREBP1 showed significant decrease of the cholesterol accumulation and increased mRNA expression of HMG‐CO A reductase and Farnesyl diphosphate synthase, two rate limiting enzymes required for cholesterol biosynthesis by SREBP1 in medaka livers. Taken together, this study revealed that expression of transcription factors in the SIRT1/LXRα/SREBP1 pathway for cholesterol biosynthesis as well as the cholesterol contents (accumulation) in livers of the Indian medaka altered with ambient salinities and temperatures to cope with their physiological demand under environmental stress. Moreover, this study is the first one to demonstrate the involvement of SREBP1 in cholesterol biosynthesis in fish livers.