Compartmentalization of stearoyl-coenzyme A desaturase 1 activity in HepG2 cells

Shu Lim,Virender K Rehan,Heidi S Hummel,Wai-Nang Paul Lee,Catherine S Mao,Jennifer K Yee,Sharon Sugano,Gary Xiao

doi:10.1194/jlr.m700600-jlr200

Abstract

Stearoyl-coenzyme A desaturase 1 (SCD1) catalyzes the conversion of stearate (18:0) to oleate (18:1n-9) and of palmitate (16:0) to palmitoleate (16:1), which are key steps in triglyceride synthesis in the fatty acid metabolic network. This study investigated the role of SCD1 in fatty acid metabolism in HepG2 cells using SCD1 inhibitors and stable isotope tracers. HepG2 cells were cultured with [U-(13)C]stearate, [U-(13)C]palmitate, or [1,2-(13)C]acetate and (1) DMSO, (2) compound CGX0168 or CGX0290, or (3) trans-10,cis-12 conjugated linoleic acid (CLA). (13)C incorporation into fatty acids was determined by GC-MS and desaturation indices calculated from the respective ion chromatograms. FAS, SCD1, peroxisome proliferator-activated receptor alpha, and peroxisome proliferator-activated receptor gamma mRNA levels were assessed by semiquantitative RT-PCR. The addition of CGX0168 and CGX0290 decreased the stearate and palmitate desaturation indices in HepG2 cells. CLA led to a decrease in the desaturation of stearate only, but not palmitate. Comparison of desaturation indices based on isotope enrichment ratios differed, depending on the origin of saturated fatty acid. SCD1 gene expression was not affected in any group. In conclusion, the differential effects of SCD1 inhibitors and CLA on SCD1 activity combined with the dependence of desaturation indices on the source of saturated fatty acid strongly support the compartmentalization of desaturation systems. The effects of SCD1 inhibition on fatty acid composition in HepG2 cells occurred through changes in the dynamics of the fatty acid metabolic network and not through transcriptional regulatory mechanisms.

Highlights

Stearoyl-coenzyme A desaturase 1 (SCD1) catalyzes the conversion of stearate (18:0) to oleate (18:1n-9) and of palmitate (16:0) to palmitoleate (16:1), which are key steps in triglyceride synthesis in the fatty acid metabolic network
We found that SCD1 inhibition altered the interconversion of long-chain fatty acids (LCFAs) in the fatty acid metabolic network in HepG2 cells with different effects on the SCD1 pathways, indicating compartmentalization
There was a significant decrease in the desaturation index for oleate/stearate in the SCD1 inhibitor and conjugated linoleic acid (CLA) groups compared with the control in the experiment with labeled stearate (Fig. 3A)

Summary

Introduction

Stearoyl-coenzyme A desaturase 1 (SCD1) catalyzes the conversion of stearate (18:0) to oleate (18:1n-9) and of palmitate (16:0) to palmitoleate (16:1), which are key steps in triglyceride synthesis in the fatty acid metabolic network. Because an inappropriate ratio of MUFA to saturated fatty acid can affect membrane lipid fluidity and lipoprotein metabolism, the effects of SCD have been implicated in obesity and in diabetes, atherosclerosis, and cancer [1,2,3,4,5]. Ntambi and colleagues [8] created a knockout mouse for SCD1 These mice have decreased adiposity, increased insulin sensitivity, and are resistant to diet-induced weight gain. Choi et al [11] showed some

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