Sortilin deficiency improves the metabolic phenotype and reduces hepatic steatosis of mice subjected to diet-induced obesity Liane Rabinowich 1,  , Sigal Fishman 1,  , Einav Hubel 1 , Tamar Thurm 2 , Woo-Jae Park 3,4 , Yael Pewzner-Jung 3 , Ashish Saroha 3 , Noam Erez 1 , Zamir Halpern 1 , Anthony H. Futerman 3 , Isabel Zvibel 1,⇑ 1 The Research Center for Digestive Tract and Liver Diseases, Tel Aviv Sourasky Medical Center and the Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel; 2 Internal Medicine D, Tel Aviv Sourasky Medical Center, Tel Aviv, Israel; 3 Department of Biological Chemistry, Weizmann Institute of Science, Rehovot, Israel; 4 Department of Biochemistry, School of Medicine, Gachon University, Incheon 406-799, Republic of Korea Background & Aims: Sortilin traffics newly synthesized mole- cules from the trans-Golgi apparatus along secretory pathways to endosomes, lysosomes or to the cell surface. Sortilin trafficking of acid sphingomyelinase (aSMase) may regulate ceramide levels, a major modulator of insulin signalling. We therefore tested whether sortilin deficiency reduces hepatic and adipose tissue aSMase activity, improving insulin sensitivity in diet-induced obesity (DIO). Methods: DIO in C57BL/6 (WT) and sortilin À/À mice was induced by high-fat diet feeding for 10 weeks. Results: Sortilin À/À mice gained less body weight and less vis- ceral fat, despite similar food intake compared to WT type mice and had enhanced glucose uptake in insulin tolerance tests, which was further corroborated by enhanced hepatic pAkt expression. Sortilin deficiency led to attenuated hepatic steatosis, reduced expression of genes involved in lipogenesis, ceramide synthesis and inflammatory cytokine production and reduced activity of ceramide synthase 5/6 (CerS5/6). Sortilin À/À mice had reduced hepatic aSMase activity under both steady-state and DIO. Likewise, sortilin À/À hepatocytes displayed hypersensi- tivity to insulin, due to enhanced insulin receptor downstream signalling. In adipose tissue, sortilin À/À mice exhibited lower expression of inflammatory cytokines and lower expression and activity of CerS5/6. As in liver, adipose tissue displayed increased insulin signalling, accompanied by attenuated aSMase activity. Conclusions: Sortilin deficiency induces a beneficial metabolic phenotype in liver and adipose tissue upon DIO, mediated in part by reduced aSMase activity. Ó 2014 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved. Introduction Obesity has become a world-wide epidemic in western countries, leading to insulin resistance, heart disease, type 2 diabetes and non-alcoholic fatty liver disease (NAFLD). NAFLD is commonly associated with insulin resistance and characterized by hepatic accumulation of triglycerides (TGs) [1]. Sortilin is a member of the vacuolar protein sorting 10 protein (VPS10P)-domain receptor family. Sortilin functions both as a co-receptor and as a trafficking molecule that binds and directs newly synthesized molecules from the trans-Golgi network (TGN) to regulated secretion pathways, including endosomes, lysosomes or to the cell surface [2]. Recent studies have implicated sortilin in several metabolic processes in the liver and adipose tissue [2–4]. Human genome wide association studies (GWAS) indicate a strong association of a specific SNP in the SORT1 gene with decreased serum levels of low-density lipoprotein (LDL) cholesterol and myocardial infarction [3]. Following this discovery, sortilin was shown to control plasma LDL by two possible mechanisms. In the first, sortilin modulates LDL secretion by binding to ApoB in the Golgi and directing it towards the cell surface and secretion, or towards a degradation pathway [3–4]. In the second, sortilin regulates LDL catabolism by uptake of circulating serum LDL and transferring it to lyso- somes for degradation [3–6]. Recently, sortilin was shown to influence sphingolipid metabolism by trafficking acid sphingo- myelinase (aSMase) to the lysosome or cell surface, as well as by participating in the trafficking of sphingolipid activation proteins (SAPs) [7–9]. ASMase and neutral-sphingomyelinase 2 (nSMase) regulate the turnover of sphingomyelin (SM), to produce ceramide in Journal of Hepatology 2015 vol. 62 j 175–181 Keywords: Hepatic steatosis; Sortilin; Insulin resistance; Acid sphingomyelinase; Ceramide synthesis. Received 24 April 2014; received in revised form 28 July 2014; accepted 12 August 2014; available online 27 August 2014 ⇑ Corresponding author. Address: Tel Aviv Sourasky Medical Center, Department of Gastroenterology, Weizman 6, Tel Aviv, Israel. Tel.: +972 3 697 4481; fax: +972 3 697 4622. E-mail address: isab@tlvmc.gov.il (I. Zvibel).   These authors contributed equally to this work. Abbreviations: DIO, diet-induced obesity; aSMase, acid sphingomyelinase; NAFLD, non-alcoholic fatty liver disease; TGN, trans-Golgi network; TGs, triglycerides; LDL, low-density lipoprotein; GWAS, genome wide association studies; VPS10P, vacuolar protein sorting 10 protein; SAP, sphingolipid activation protein; IR, insulin resistance; HFD, high fat diet. Research Article