Thioesterase Superfamily Member 2 Promotes Hepatic VLDL Secretion by Channeling Fatty Acids Into Triglyceride Biosynthesis

In hepatocyte cultures maintained in the absence of extracellular fatty acids, at least 70% of the secreted very-low-density lipoprotein (VLDL) triacylglycerol was derived via lipolysis of intracellular triacylglycerol. This proportion was unchanged when the cells were exposed for 24 h to insulin or glucagon, hormones which decreased the overall secretion of intracellular triacylglycerol, or to chloroquine or tolbutamide, agents which inhibit lysosomal lipolysis. The rate of intracellular lipolysis was 2-3-fold greater than that required to maintain the observed rate of triacylglycerol secretion. Most of the fatty acids released were returned to the intracellular pool. Neither insulin nor glucagon had any significant effect on the overall lipolysis and re-esterification of intracellular triacylglycerol. In these cases a greater proportion of the released fatty acids re-entered the cellular pool, rather than being recruited for VLDL assembly. Tolbutamide inhibited intracellular lipolysis, but suppressed VLDL secretion to a greater extent. 3,5-Dimethylpyrazole did not affect lipolysis or VLDL secretion. The increased secretion of VLDL triacylglycerol observed after exposure of cells to insulin for 3 days was not accompanied by an increased rate of intracellular lipolysis. However, a larger proportion of the triacylglycerol secreted under these conditions may not have undergone prior lipolysis.

Download Full-text

Enantiomeric Diglycerides as Stereospecific Probes in Triglyceride Synthesis In Vitro

Canadian Journal of Biochemistry ◽

10.1139/o72-123 ◽

1972 ◽

Vol 50 (8) ◽

pp. 881-887 ◽

Cited By ~ 17

Author(s):

P. J. A. O'Doherty ◽

A. Kuksis ◽

D. Buchnea

Keyword(s):

Fatty Acids ◽

Thin Layer ◽

Positional Distribution ◽

Argentation Thin Layer Chromatography ◽

Glycerol Molecule ◽

Direct Acylation ◽

Triglyceride Biosynthesis ◽

Layer Chromatography ◽

Single Enzyme

The stereospecificity of diglyceride acyltransferase (EC 2.3.1.20) was studied in the microsomes of rat intestine and liver using enantiomeric diglycerides as acceptors of labeled fatty acids. The diglyceride mixtures were prepared by combining equal amounts of synthetic 1,2- and 2,3-diglycerides of different degrees of unsaturation. The labeled triglycerides formed were resolved by argentation thin-layer chromatography, whereupon the products from each enantiomeric diglyceride moved to a different spot on the thin-layer plate.It was shown that both 1,2- and 2,3-diglycerides were utilized for direct acylation to triglycerides by both tissues. The diglyceride acyltransferase, if a single enzyme, showed a definite preference for the acylation of the 1,2-diglycerides. The intestine esterified more of the 2,3-isomer than did the liver. The yields of triglyceride depended upon the nature of the fatty acids involved, and their positional distribution on the glycerol molecule. The new method of stereospecific assessment of triglyceride biosynthesis in vitro is applicable to preparations of other subcellular fractions and to other suitable combinations of fatty acid and diglyceride precursors.

Download Full-text

Impairment of VLDL Secretion by Medium-Chain Fatty Acids in Chicken Primary Hepatocytes Is Affected by the Chain Length

Journal of Nutrition ◽

10.1093/jn/135.7.1636 ◽

2005 ◽

Vol 135 (7) ◽

pp. 1636-1641 ◽

Cited By ~ 11

Author(s):

Kan Sato ◽

Yositake Cho ◽

Shizuko Tachibana ◽

Tomoyuki Chiba ◽

Wolfgang J. Schneider ◽

...

Keyword(s):

Fatty Acids ◽

Chain Length ◽

Primary Hepatocytes ◽

Medium Chain ◽

Medium Chain Fatty Acids ◽

Vldl Secretion ◽

Chain Fatty Acids

Download Full-text

Suppression of VLDL secretion by cultured hepatocytes incubated with chylomicron remnants enriched in n−3 polyunsaturated fatty acids is regulated by hepatic nuclear factor-4α

Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids ◽

10.1016/j.bbalip.2009.08.004 ◽

2009 ◽

Vol 1791 (12) ◽

pp. 1181-1189 ◽

Cited By ~ 12

Author(s):

Iliana López-Soldado ◽

Michael Avella ◽

Kathleen M. Botham

Keyword(s):

Fatty Acids ◽

Polyunsaturated Fatty Acids ◽

Nuclear Factor ◽

Cultured Hepatocytes ◽

Vldl Secretion ◽

Chylomicron Remnants

Download Full-text

The Effects of Dietary n-3 Polyunsaturated Fatty Acids Delivered in Chylomicron Remnants on the Transcription of Genes Regulating Synthesis and Secretion of Very-Low-Density Lipoprotein by the Liver: Modulation by Cellular Oxidative State

Experimental Biology and Medicine ◽

10.1177/153537020322800203 ◽

2003 ◽

Vol 228 (2) ◽

pp. 143-151 ◽

Cited By ~ 27

Author(s):

Kathleen M. Botham ◽

Xiaozhong Zheng ◽

Mariarosaria Napolitano ◽

Michael Avella ◽

Claudio Cavallari ◽

...

Keyword(s):

Fatty Acids ◽

Fish Oil ◽

Corn Oil ◽

Low Density Lipoprotein ◽

Density Lipoprotein ◽

Very Low Density Lipoprotein ◽

Low Density ◽

Vldl Secretion ◽

Chylomicron Remnants ◽

Oxidative State

The influence of chylomicron remnants enriched in n-3 or n-6 polyunsaturated fatty acids (PUFA) (derived from fish or corn oil, respectively) on the expression of mRNA for four genes Involved in the regulation of the synthesis, assembly, and secretion of very-low-density lipoprotein (VLDL) in the liver was investigated in normal rat hepatocytes and after manipulation of the cellular oxidative state by incubation with N-acetyl cysteine (NAC) or CuSO4. The four genes investigated were those encoding apolipoprotein B (apoB), the microsomal triacylglycerol transfer protein (MTP), and the enzymes acyl coenzyme A:diacylglycerol acyltransferase (DGAT) and acyl coenzyme A:cholesterol acyltransferase 2 (ACAT2), which play a role in the regulation of triacylglycerol and cholesteryl ester synthesis, respectively. mRNA levels for apoB, MTP, and DGAT were unaffected by either fish or corn oil chylomicron remnants, but the amount of ACAT2 mRNA was significantly reduced after Incubation of the hepatocytes with fish oil remnants as compared with corn oil remnants or without remnants. These findings indicate that the delivery of dietary n-3 PUFA to hepatocytes in chylomicron remnants downregulates the expression of mRNA for ACAT2, and this may play a role in their inhibition of VLDL secretion. However, when the cells were shifted into a prooxidizing or pro-reducing state by pretreatment with CuSO4 (1 mM) or NAC (5 mM) for 24 hr, levels of mRNA for MTP were increased by about 2- or 4-fold, respectively, by fish oil remnants, whereas corn oil remnants had no significant effect. Fish oil remnants also caused a smaller increase in apoB mRNA in comparison with com oil remnants in NAC-treated cells (+38%). These changes would be expected to lead to increased VLDL secretion rather than the decrease associated with dietary n-3 PUFA in normal conditions. These findings suggest that relatively minor changes in cellular redox levels can have a major influence on important liver functions such as VLDL synthesis and secretion.

Download Full-text