Lipid abundance in zebrafish embryos is regulated by complementary actions of the endocannabinoid system and retinoic acid pathway

The endocannabinoid system (ECS) and retinoic acid (RA) signaling have been associated with influencing lipid metabolism. We hypothesized that modulation of these pathways could modify lipid abundance in developing vertebrates and that these pathways could have a combinatorial effect on lipid levels. Zebrafish embryos were exposed to chemical treatments altering the activity of the ECS and RA pathway. Embryos were stained with the neutral lipid dye Oil-Red-O (ORO) and underwent whole-mount in situ hybridization (WISH). Mouse 3T3-L1 fibroblasts were differentiated under exposure to RA-modulating chemicals and subsequently stained with ORO and analyzed for gene expression by qRT-PCR. ECS activation and RA exposure increased lipid abundance and the expression of lipoprotein lipase. In addition, RA treatment increased expression of CCAAT/enhancer-binding protein alpha. Both ECS receptors and RA receptor subtypes were separately involved in modulating lipid abundance. Finally, increased ECS or RA activity ameliorated the reduced lipid abundance caused by peroxisome proliferator-activated receptor gamma (PPAR gamma) inhibition. Therefore, the ECS and RA pathway influence lipid abundance in zebrafish embryos and have an additive effect when treated simultaneously. Furthermore, we demonstrated that these pathways act downstream or independently of PPAR gamma to influence lipid levels. Our study shows for the first time that the RA and ECS pathways have additive function in lipid abundance during vertebrate development.