New insights into lipid and carbohydrate metabolism in teleost fish: transcriptional and functional characterization of adipocytes

Abstract

The aim of the present thesis is directed towards the comprehension of the mechanisms involved in the regulation of lipid and carbohydrate metabolism in relevant species in European aquaculture. Special attention has been placed on adipose tissue due to its key role in the control of the overall energy homeostasis and its endocrine functions. Six diets were formulated to determine the feasibility of replacing dietary lipids (from 23 to 17%) by starch (from 12 to 28%) and subsequent replacements in starch by fibre up to levels of 18% in gilthead sea bream. The dietary treatments did not affect fish growth nor feed utilization efficiency and triggered only moderate changes in lipid and carbohydrate metabolism. Gilthead sea bream adipose tissue exhibited an activation of the lipogenic pathway and a transcriptional regulation of relevant transcription factors directed to modulate the preferential use of lipids or carbohydrates according to availability. Despite of a lack of hepatic regulation of gluconeogenesis, an efficient metabolic hepatic adaptation to dietary starch was found through the modulation of enzymes directed to increase glucose metabolization and balance fatty acid uptake and lipolysis. These metabolic adaptations encourage the formulation of diets with a relatively high content of starch or fibre. In addition, the capacity of Atlantic salmon adipocytes to synthesise fatty acids from glucose has been demonstrated for the first time. However, the capacity of the pathway is relatively low, which might be contributing to the previously reported glucose intolerance in salmonids. Atlantic salmon adipocytes mainly transformed glucose to glycerol and Krebs-cycle intermediates. The effect of glucose on the transcriptional regulation of enzymes involved in glycolysis, the pentose phosphate pathway and glyceroneogenesis, highlights the relevance of carbohydrate metabolism in fish adipocytes as well as the metabolic adaptation of these cells to ensure the availability of necessary factors and other substrates for lipid synthesis and storage. The implication of adiponectin and TNFα in the regulation of glucose uptake in rainbow trout adipocytes has been explored, showing that both adipokines stimulated glucose uptake without modifying AKT or TOR phosphorylation. However, adiponectin failed to stimulate glucose uptake in insulin-treated adipocytes. Insulin regulated the transcription of the adiponectin system enhancing the mRNA levels of the peptide and reducing those of adiponectin receptors, following the mammalian model. However, neither adiponectin nor TNFα mRNA levels were affected by each other, concluding that the reciprocal suppressive effects reported in mammals are not conserved in fish. TNFα supressed PPARγ at a protein level, corroborating its anti-adipogenic role. However, TNFα enhanced glucose uptake in insulin-stimulated adipocytes, suggesting that the interactions between this cytokine and insulin are not so well defined in fish. Finally, the transcriptomic profile along primary culture of rainbow trout preadipocytes has been characterized revealing that adipogenesis is a complex process involving the coordinated activation or repression of genes associated to the different stages of cell development. It presented high similarities to the mammalian model, being characterized by two main phases: proliferation and differentiation. Proliferation phase was enriched with genes involved in cellular remodelling, autophagy and angiogenesis. In addition, the implication of the eicosanoid signalling pathway was highlighted during this phase. Differentiation phase was enriched with genes involved in energy production, lipid and carbohydrate metabolism. During this phase, the activation of the TR/RXR and the PPAR signalling pathways were revealed. The whole process was driven by a coordinated activation of transcription factors and epigenetic modulators. Overall, these studies have contributed to better understand the mechanisms regulating lipid and carbohydrate metabolisms in teleost fish, as well as adipose tissue growth and development. All this information might be applied to modulate and optimize farming practices.