Role of LXR in trout adipocytes: target genes, hormonal regulation, adipocyte differentiation and relation to lipolysis

In the present study, we describe an initial approach to investigate the role of LXR in fish adipose tissue. Rainbow trout (Oncorhynchus mykiss) isolated adipocytes were incubated with LXR agonists, unsaturated fatty acids, tumour necrosis factor-α (TNFα), insulin or growth hormone (GH) for 6 h and LXR expression was analyzed. Lipolysis was measured after incubation with one of the LXR agonists and LXR expression was compared with levels of lipolysis. LXR expression was also analyzed during the differentiation of adipocytes in culture. The incubations with agonists in isolated adipocytes indicated that ATP-binding cassette transporter A1 (ABCA1) is an LXR target gene, but lipoprotein lipase (LPL), fatty acid synthase (FAS), hormone-sensitive lipase (HSL) and peroxisome proliferator-activated receptor (PPARs) are not. LXR agonists also induced LXR expression and raised lipolysis levels. Besides, LXR expression was upregulated in parallel with basal lipolysis. LXR mRNA expression was regulated by unsaturated fatty acids, insulin, TNFα and GH in isolated adipocytes. Besides, LXR showed an upregulation during adipocyte differentiation. All these data indicate that LXR is involved in orchestrating the transcriptional regulatory network in trout adipocyte lipid metabolism, specifically, in cholesterol transport, adipocyte differentiation and lipolysis.