Lipid Homeostasis in Farmed Fish: Role of Peroxisome-Proliferator Activated Receptor-Gamma (PPARg)

Abstract

Peroxisome proliferator-activated receptors (PPARs) are ligand-activated transcription factors that belong to the nuclear hormone receptor superfamily. Three PPARs (PPARα, PPARβ and PPARγ) exist in mammals, and all are activated by binding lipid molecules, including fatty acids and their derivatives, and also by synthetic drug ligands. Together, these three receptors are critical regulators of lipid and energy homeostasis in mammals. PPARγ is a central factor in fat uptake and storage and is required for adipocyte differentiation. Fish are now known to have homologues of the three PPAR isotypes, although in many species there is more than one representative of each. Piscine PPARγ is of particular interest in finfish aquaculture, since under aquaculture conditions fish often accumulate excess visceral and hepatic fat. This can affect the health and welfare of the fish, and also represents an economic waste of valuable resources that might otherwise be channelled into growth. However, piscine PPARγ has some important structural differences to the mammalian counterpart, and is not activated by fatty acids or synthetic ligands. Although presumed to have an important role in fat accumulation, further research on piscine PPARγ has been hampered by this failure to identify of activating compounds. The aim of this project is to identify activators for piscine PPARγ, and then to discover the effects of PPARγ activation on fish lipid and energy metabolism. In addition, given the variability in numbers of PPAR genes in fish species, the PPAR complement of the salmon genome was investigated. Atlantic salmon is an important aquaculture species and unlike most other vertebrates, was found to contain two PPARγ genes with distinct tissue expression profiles. To discover activating compounds for fish PPARγ, total lipid was extracted from salmon liver tissue and fractionated into different lipid classes. Lipid fractions obtained were then tested in a high-throughput cell-based transactivation screen for fish PPAR activity in a Chinook salmon embryo (CHSE-214) cell line. Two polar lipid fractions believed to contain ceramides significantly increased PPARγ-dependent transactivation of a luciferase reporter gene. The molecular species in two of these fractions were analysed by LC-MS, confirming the presence of various ceramide and sphingolipid species. Application of pure glucosylceramide (GlcCer) in the cell transfection assay resulted in PPARγ activation. The identification of activating lipids for piscine PPARγ will enable further study on the physiological functions of this receptor in fish and under aquaculture conditions. Ultimately this knowledge could lead to improvements in finfish feed formulation to better optimise the relationship between lipid input, fat accumulation and growth.