Hormone-sensitive lipase in Japanese flounder Paralichthys olivaceus: the potential function of the inclinator muscle of fin as a lipid storage site

Abstract

Hormone-sensitive lipase (HSL) is an important regulator of lipolysis in mammals, but little is known about its function in other animals. For a better understanding of fish HSL and potential application of the HSL-mediated lipolysis in aquaculture, we cloned HSL cDNAs from Japanese flounder Paralichthys olivaceus. The full-length cDNAs of two HSL genes, designated HSL1 and HSL2, consisted of 2,922 and 2,832 bp, respectively. RT-PCR showed that their transcripts were abundant in the inclinator muscle of fin, liver, and skeletal muscle. Quantitative real-time PCR further revealed that the transcripts of HSL1 and HSL2 genes were broadly expressed in all tissues tested, with the highest abundance in the inclinator muscle of fin. The inclinator muscle of fin also contained many adipocytes. In addition to triacylglycerols, considerable amounts of cholesteryl esters and non-esterified fatty acids were detected in the lipids extracted from the inclinator muscle of fin. Finally, in situ hybridization localized the HSL1 transcripts to the adipocytes observed in the inclinator muscle of fin. Together, these results suggest that the inclinator muscle of fin is a lipid storage site that releases non-esterified fatty acids for aerobic fin movement, possibly through HSL-mediated lipolysis.