Maternal Lipid Metabolism Alteration During Pregnancy Based On The Regulation Of Key Proteins In Fatty Acid Oxidation And Transport

It was known that plasma lipid levels,including triglyceride(TG)and total cholesterol(TC),were markedly higher in pregnant women than that in non-pregnant women,and were increased as gestation proceeds.A certain degree of hyperlipidemia is necessary for the growth and development of the fetus.However,High levels of TG throughout pregnancy have been shown to increase the risk of adverse pregnancy outcomes,including gestational diabetes mellitus,preeclampsia,intrahepatic cholestasis of pregnancy and preterm birth.Since the mechanism of altered lipid metabolism during pregnancy remains unknown,there is no guide for the treatment of hyperlipidemia during pregnancy.In order to avoid pregnancy complications and adverse pregnancy outcomes,it is important to clarify the mechanisms of plasma TG and TC rise during pregnancy.Liver is a very important organ of TG metabolism,and any alters in the process of hepatic fatty acid uptake,synthesis,oxidation and TG secretion will induce changes in lipid metabolism.L-Carnitine(L-Car)is a hydrophilic quaternary amine that plays a crucial role in fatty acid β-oxidation.The plasma concentration of L-Car in pregnant women is reported to be much lower than that in non-pregnant women and keeps decline as gestation proceeds.At the time of delivery,the maternal plasma L-Car concentration is only half of that in non-pregnant women.It seems that pregnancy leads to a reversible secondary deficiency of L-Car.However,the reasons have not been clarified yet,and whether such low L-Car concentration associated with plasma lipid levels rise in pregnancy also remains unknown.We hypothesized that L-Car deficiency during pregnancy might induce hepatic fatty acid oxidation disorder,and subsequently lead to the increase of plasma lipid levels.The plasma concentrations of several hormones are gradually increased during pregnancy.The plasma levels of estradiol(E2)and progesterone(P4)in pregnant women during the third trimester will reach tens to hundreds folds of the original levels.And the plasma concentrations of glucocorticoids(GC,cortisol in humans and corticosterone(CORT)in rodents)are also gradually increased during pregnancy.We speculated that alters in lipid metabolism during pregnancy might be related to the hormones.1.Maternal plasma L-Car reduction during pregnancy is mainly attributed to OCTN2 mediated placental uptake and does not result in maternal hepatic fatty acid β-oxidation declineL-Car plays a crucial role in fatty acid β-oxidation.However,the plasma L-Car concentration in women markedly declines during pregnancy,the underlying mechanism and the consequent on maternal hepatic β-oxidation has not been clarified yet.Our results showed that the plasma and liver TG levels in mice at gestation day(GD)18 were significantly higher than that in non-pregnant mice,while the plasma L-Car levels were lower.The mean fetal-to-maternal plasma L-Car ratio in GD 18 mice was 3.0.Carnitine/organic cation transporter 2(OCTN2)was highly expressed in the maternal side of mouse and human placenta and up-regulated as gestation proceeds in human placenta,while carnitine transporter(CT)1/2 and amino acid transporter B0,+(ATB0,+)were extremely low.Further study revealed that renal peroxisome proliferator-activated receptor α(PPARα)and OCTN2 were down-regulated,renal L-Car level was reduced while the urinary excretion of L-Car was lower in late pregnant mice than that in non-pregnant mice.Meanwhile,P4(pregnancy-related hormone)down-regulated the expression of renal OCTN2 via PPARα-mediated pathway,and inhibited the activity of OCTN2,but E2,CORT and cortisol did not.Unexpectedly,the maternal hepatic level of L-Car,β-hydroxybutyrate(an indicator of mitochondrialβ-oxidation)and mRNA level of several enzymes involved in fatty acid β-oxidation in GD 18 mice were higher than that in non-pregnant mice.In conclusion,OCTN2 mediated L-Car transfer across placenta played a major role in maternal plasma L-Car reduction during pregnancy,which did not subsequently result in maternal hepatic fatty acid β-oxidation decrease.2.Role of the glucocorticoid receptor-CD36 pathway in cortisol/corticosterone-induced maternal plasma lipid increase during pregnancyThe plasma lipid levels are increased as gestation proceeds;however,the underlying mechanism has not been clarified yet.The current study aimed to explore the signaling pathway involved in lipid metabolism changes during pregnancy.We examined the expression of key enzymes/transporters involved in fatty acid uptake,synthesis and TG secretion in the liver of GD 18 mice and non-pregnant mice,and found that the mRNA level of fatty acid translocase(FAT/CD36)was significantly higher in GD 18 mice than that in non-pregnant mice.Furthermore,the mRNA and protein expression of CD36 were up-regulated as gestation proceeds in mouse liver,which was consistent with the hepatic and plasma TG levels.In primary cultured mouse hepatocytes,cortisol/CORT significantly up-regulated CD36 and increased subsequent fatty acids uptake,but E2 and P4 did not;both were significantly attenuated by small interfering RNA(siRNA)knockdown of CD36 and glucocorticoid receptor(GR)or RU486(GR antagonist).The above results indicated that GR was involved in the up-regulation of CD36 induced by cortisol/CORT in hepatocytes.We further found that liver X receptor(LXR),pregnane X receptor(PXR),PPARy,aryl hydrocarbon receptor(AHR)and hepatocyte nuclear factor 4α(HNF4α)were not involved in the pathway.Luciferase reporter gene assay revealed that cortisol/CORT promoted binding of GR to the CD36 promoter,thereby increasing its transcriptional expression.The plasma and hepatic TG levels were increased in the female mice after chronic exposure with CORT,and oil red O staining also showed obvious hepatic lipid accumulation.The hepatic CD36 was up-regulated,fatty acid β-oxidation was slightly elevated,but other key enzymes/transporters involved in fatty acid uptake,synthesis and TG secretion were not altered.The hepatic TG and CD36 levels were further increased in the pregnant mice after chronic exposure with CORT.Finally,treatment with RU486 reversed CORT-induced hepatic lipid accumulation and hyperlipidemia by inhibiting the GR-CD36 pathway in mice.In conclusion,elevated cortisol/CORT promoted binding of GR to the CD36 promoter and increased the hepatic CD36 expression,thereby increasing fatty acid uptake and TG synthesis,which resulted in physiological elevation of plasma lipids during pregnancy.RU486 reversed CORT-induced hepatic lipid accumulation and hyperlipidemia by inhibiting the GR-CD36 pathway.