Liver specific-deletion of FATP4 increases hepatic triglycerides upon D8-glycerol administration: a stable isotope labelling study

Background:

Expression of fatty acid transport protein 4 (FATP4) is increased in adipose tissues of individuals with central and acquired obesity. Polymorphisms of FATP4 are also associated with insulin resistance. We have previously shown that adipose-specific FATP4 knockout (KO) mice under high fat diet feeding show adipose hypertrophy with increased triglycerides (TG) in adipose tissues and liver (Lenz et al. JBC2011). Here we hypothesize that FATP4 may play a critical role in TG metabolism in the liver and liver-specific FATP4 KO mice were used in this investigation. We here studied hepatic metabolism of newly synthesized TG by using deuterated-labelled glycerol which is a substrate of glycerol kinase. Subsequent glycerol-3 phosphate leads to the synthesis of diacylglycerol and TG.

Methods:

Male C57/BL6 (WT) and KO mice at 1 year old were used under non-starved and overnight starved conditions (n = 3). Mice were intraperitoneally injected with 20 mg/kg D8-glycerol (which is exchanged with H in water to form 1,1,2,3,3-D5 glycerol). Blood of 10 ml was collected prior to injection and every 30 min up to 180 min by bleeding through tail vein. Plasma, liver and visceral fat tissues were subjected to lipid extraction and profiling of 12 D5-labeled TG, 8 unlabeled TG, phosphatidylcholine (PC) and sphingomyelin (SM) species by liquid chromatography tandem mass spectrometry LC/MSMS analysis.

Results:

In absolute units of pmo/mg liver, we demonstrated that the livers of KO mice showed a 2-fold increase of total unlabeled TG, when mice were under starvation. With consideration that FATP4 activates fatty acyl-CoA for phospholipid syntheses, the ratio between TG and corresponding PC in each sample was calculated as pmol TG/nmol PC. Compared to WT, livers of KO mice under either non-starved or starved condition again showed a 2 – 3-fold increase of this ratio among total unlabeled and labeled TG species. No significant difference of this ratio was observed in plasma of KO mice. Also, no significant difference of pmol TG/nmol SM was observed in liver of KO mice. Interestingly, the levels of labeled and unlabeled TG species were markedly elevated in subcutaneous fat of liver-specific FATP4 KO mice. These KO mice also showed an increase of white adipose tissue weights indicating a liver and adipose interaction throughout the mouse lifetime.

Conclusion:

Upon an administration of D8-labeled glycerol, specific FATP4 deletion in the liver led to a marked increase of newly synthesized hepatic TG likely via glyceroneogenesis. This was concomitant with an increase of labeled and unlabeled TG in subcutaneous fat suggesting that newly synthesized TG was delivered from liver to fat tissues via blood. Further experiments are warranted to study this mechanism in isolated hepatocytes. Hence, FATP4 may elicit hepatic metabolic function in fatty-acid trafficking among PC and TG upon a substrate administration.