Hepatic Overexpression of Hormone-sensitive Lipase and Adipose Triglyceride Lipase Promotes Fatty Acid Oxidation, Stimulates Direct Release of Free Fatty Acids, and Ameliorates Steatosis

Brendan N Reid,Gene P Ables,Oleg A Otlivanchik,Gabriele Schoiswohl,Rudolf Zechner,William S Blaner,Ira J Goldberg,Robert F Schwabe,Streamson C Chua,Li-Shin Huang

doi:10.1074/jbc.m800533200

Abstract

Hepatic steatosis is often associated with insulin resistance and obesity and can lead to steatohepatitis and cirrhosis. In this study, we have demonstrated that hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL), two enzymes critical for lipolysis in adipose tissues, also contribute to lipolysis in the liver and can mobilize hepatic triglycerides in vivo and in vitro. Adenoviral overexpression of HSL and/or ATGL reduced liver triglycerides by 40-60% in both ob/ob mice and mice with high fat diet-induced obesity. However, these enzymes did not affect fasting plasma triglyceride and free fatty acid levels or triglyceride and apolipoprotein B secretion rates. Plasma 3-beta-hydroxybutyrate levels were increased 3-5 days after infection in both HSL- and ATGL-overexpressing male mice, suggesting an increase in beta-oxidation. Expression of genes involved in fatty acid transport and synthesis, lipid storage, and mitochondrial bioenergetics was unchanged. Mechanistic studies in oleate-supplemented McA-RH7777 cells with adenoviral overexpression of HSL or ATGL showed that reduced cellular triglycerides could be attributed to increases in beta-oxidation as well as direct release of free fatty acids into the medium. In summary, hepatic overexpression of HSL or ATGL can promote fatty acid oxidation, stimulate direct release of free fatty acid, and ameliorate hepatic steatosis. This study suggests a direct functional role for both HSL and ATGL in hepatic lipid homeostasis and identifies these enzymes as potential therapeutic targets for ameliorating hepatic steatosis associated with insulin resistance and obesity.

Highlights

hormone-sensitive lipase (HSL) and adipose triglyceride lipase (ATGL) Overexpression Ameliorates Hepatic Steatosis apolipoprotein B proteins or deficiency of microsomal triglyceride transfer protein), fatty acid synthesis, and FA uptake (e.g. CD36 deficiency)
We determined the role of HSL and ATGL in hepatic lipolysis, and we evaluated the potential of adenoviral overexpression of these enzymes as a means to ameliorate hepatic steatosis in ob/ob mice [27] and mice with diet-induced insulin resistance and obesity
Because these data confirmed that HSL and ATGL were functionally active in cultured liver cells, we proceeded to test whether these enzymes exerted similar effects in vivo

Summary

EXPERIMENTAL PROCEDURES

Generation of Recombinant Adenoviruses—Full-length cDNA fragments of mouse HSL (2.3 kb) and ATGL (1.5 kb) were amplified from adipose cDNA isolated from C57BL/6 (B6) mice and cloned into a pCRII vector using a TA cloning kit as described [28]. HSL-deficient and ATGL-deficient mice were generated by targeted homologous recombination as described [26, 30] These animals (3– 4 months of age) were sacrificed between 8 and 12 a.m., and liver tissues were collected for use in TG hydrolase activity assays. Measurement of Cellular TG Mass—McA-RH7777 cells were washed with PBS 40 h post-infection, and lipids were extracted with hexane/isopropyl alcohol (3/2, v/v) (4 ml/well) at room temperature for 3 h as described [33]. Cellular FA Oxidation—McA-RH7777 cells were infected with recombinant adenoviruses for 24 h and washed with PBS prior to labeling in DMEM (1 ml/well) containing 0.4 mM OA, 1.5% BSA, and [14C]OA (1 ␮Ci/ml) (NEC317, PerkinElmer Life Sciences) for 16 h. Significant differences between two groups were defined as those giving a value of p Ͻ 0.05

RESULTS

75 Ϯ 34 100 Ϯ 21 121 Ϯ 27 120 Ϯ 50 100 Ϯ 3 92 Ϯ 36

DISCUSSION