In addition to being essential for the transcription of genes involved in cellular lipogenesis, increasing evidence associates sterol regulatory element binding proteins (SREBPs) with the transcriptional control of carbohydrate metabolism. The aim of this study was to assess the effect of overexpression SREBP1a, a potent activator of all SREBP-responsive genes, on the intermediary metabolism of Sparus aurata, a glucose-intolerant carnivorous fish. Administration of chitosan-tripolyphosphate nanoparticles complexed with a plasmid driving expression of the N-terminal transactivation domain of SREBP1a significantly increased SREBP1a mRNA and protein in the liver of S. aurata. Overexpression of SREBP1a enhanced the hepatic expression of key genes in glycolysis-gluconeogenesis (glucokinase and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase), fatty acid synthesis (acetyl-CoA carboxylase 1 and acetyl-CoA carboxylase 2), elongation (elongation of very long chain fatty acids protein 5) and desaturation (fatty acid desaturase 2) as well as reduced nicotinamide adenine dinucleotide phosphate production (glucose-6-phosphate 1-dehydrogenase) and cholesterol synthesis (3-hydroxy-3-methylglutaryl-coenzyme A reductase), leading to increased blood triglycerides and cholesterol levels. Beyond reporting the first study addressing in vivo effects of exogenous SREBP1a in a glucose-intolerant model, our findings support that SREBP1a overexpression caused multigenic effects that favoured hepatic glycolysis and lipogenesis and thus enabled protein sparing by improving dietary carbohydrate conversion into fatty acids and cholesterol.

The administration of a single dose of chitosan nanoparticles driving the expression of sterol regulatory element-binding protein 1a (SREBP1a) was recently associated with the enhanced conversion of carbohydrates into lipids. To address the effects of the long-lasting expression of SREBP1a on the growth and liver intermediary metabolism of carnivorous fish, chitosan-tripolyphosphate (TPP) nanoparticles complexed with a plasmid expressing the N terminal active domain of hamster SREBP1a (pSG5-SREBP1a) were injected intraperitoneally every 4 weeks (three doses in total) to gilthead sea bream (Sparus aurata) fed high-protein–low-carbohydrate and low-protein–high-carbohydrate diets. Following 70 days of treatment, chitosan-TPP-pSG5-SREBP1a nanoparticles led to the sustained upregulation of SREBP1a in the liver of S. aurata. Independently of the diet, SREBP1a overexpression significantly increased their weight gain, specific growth rate, and protein efficiency ratio but decreased their feed conversion ratio. In agreement with an improved conversion of dietary carbohydrates into lipids, SREBP1a expression increased serum triglycerides and cholesterol as well as hepatic glucose oxidation via glycolysis and the pentose phosphate pathway, while not affecting gluconeogenesis and transamination. Our findings support that the periodical administration of chitosan-TPP-DNA nanoparticles to overexpress SREBP1a in the liver enhanced the growth performance of S. aurata through a mechanism that enabled protein sparing by enhancing dietary carbohydrate metabolisation.

A single dose of chitosan-tripolyphosphate (TPP) nanoparticles carrying expression plasmids for fish codon-optimized Caenorhabditis elegans fat-1 and fat-2 was intraperitoneally administered to gilthead seabream (Sparus aurata) to stimulate the biosynthesis of omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) and evaluate subsequent short-term effects on liver intermediary metabolism and immunity. Seventy-two hours post-injection, the upregulation of fat-1 elevated eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and total n-3 fatty acids in the liver, while fat-2 enhanced DHA and n-3 fatty acids. Co-expression of fat-1 and fat-2 increased EPA, DHA, PUFA, and the total n-6 and n-3 LC-PUFA, while reducing plasma triglycerides. The expression of fat-1 and fat-2 suppressed hepatic lipogenesis by downregulating srebf1 and pparg, and consequently key genes in fatty acid synthesis (acaca, acacb, fasn, scd1, and fads2). In contrast, the co-expression of fat-1 and fat-2 upregulated hnf4a, chrebp, and pfkl, a rate-limiting enzyme in glycolysis. Furthermore, fat-1 and fat-2 reduced hepatic proinflammatory markers such as tnfa and nfkb1. In addition to enhancing EPA and DHA biosynthesis, promoting glycolysis, and suppressing lipogenesis, our findings suggest that the short-term expression of C. elegans fat-1 and fat-2 in the liver may also reduce inflammation and, therefore, could impact the health and growth performance of cultured fish.