Effects of Docosahexaenoic Acid on SREBP1-Mediated Lipogenic Enzyme Expression in Hepatocytes

Abstract

Non-Alcoholic Fatty Liver Disease (NAFLD) is the most copious liver malady and if proper treatment is not applied, it may progress to cirrhosis, the end stage liver disease. Considering the diseases prevailing and vicious outcome around the world, treatment options are limited and further research is warranted. Failure to resolve inflammatory stimuli evoked by fatty acids accumulation in NAFLD can result in the dysfunction of hepatocytes and tissue damage. Thus, it is important to seek potential pharmacological target(s) to ameliorate impairments prompted by fatty acids. Omega-3 fatty acids are clinically proven to lower serum triglyceride (TG) levels. Furthermore, these omega-3 fatty acids such as long chain free fatty acids are known to activate G-protein coupled receptor 40 (GPR40) and G-protein coupled receptor 120 (GPR120), playing a pivotal role in myriad physiological homeostatic mechanisms. We found that exposure of murine primary hepatocytes to 300 M docosahexaenoic acid (DHA) for 12 hr increased mRNA levels of GPR40 and GPR120. Moreover, DHA lowered protein or mRNA expression levels of lipogenic enzymes such as fatty acid synthase (FAS), acetyl CoA carboxylase (ACC) and stearoyl-CoA desaturase-1 (SCD1) in primary hepatocytes incubated with liver X receptor (LXR) agonist or a high glucose with insulin milieu. DHA also inhibited protein expression of both nuclear and total sterol response-element binding protein (SREBP)-1, a key lipogenesis transcription factor. These anti-lipogenic effects of were also confirmed by visualization of lipid droplets (LDs). In primary hepatocytes exposed to DHA, LD size greater than 10 μm2 representing pathologic lipid accumulation disappeared and total area of LD per cell decreased. To ascertain whether DHAs anti-lipogenic effects were due to activity of GPR120 or GPR40, GPR120 knock-out (KO) or Compound A, a GPR120 specific agonist, were used. Interestingly, with CpdA treatment, SREBP-1 levels remain elevated with either an LXR agonist or a high glucose with insulin milieu. Moreover, DHAs anti-lipogenic effects were sustained in the GPR120 KO mouse. In contrast, AMG-1638, a GPR 40 specific agonist, lowered lipogenic enzyme levels as DHA did. Additionally, GW1100, a GPR40 antagonist, reversed the anti-lipogenic effects of DHA. Collectively, our findings demonstrate that the lipid sensor, GPR40, has a key role in sensing DHA in hepatocytes and, therefore, in the control of expression of lipogenic enzymes and hepatic steatosis.

Keywords: GPR40, GPR120, DHA, Omega-3 fatty acid, SREBP-1, Hepatocytes

Student ID number: 2016-25825