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Lysophosphatidylcholine metabolism and lipoprotein secretion by cultured rat hepatocytes deficient in choline.

ABSTRACT: The metabolism of lysophosphatidylcholine was studied in cultured rat hepatocytes deficient in choline and methionine. Even though the cells were defective in phosphatidylcholine biosynthesis, the albumin-stimulated release of lysophosphatidylcholine (1.9 nmol/h per mg of cellular protein) was similar to that in hepatocytes supplemented with choline. Albumin also stimulated (1.4-fold) the release of phosphatidylcholine from the deficient cells. The extra phosphatidylcholine and lysophosphatidylcholine in the medium were largely recovered in the albumin fraction (density greater than 1.18 g/ml), suggesting that albumin released these lipids from hepatocytes because of binding to this protein. The secretion of glycerophosphocholine was decreased by about 40% by the addition of albumin. When choline-deficient hepatocytes were supplemented with lysophosphatidylcholine, it was transported into the cells and mainly acylated to form phosphatidylcholine, which increased in mass by 30-35% in the first 4 h of incubation. Lysophosphatidylcholine was shown to be as effective as choline in restoring the secretion of very-low-density lipoproteins to normal amounts, as judged by the secretion of triacylglycerol, phosphatidylcholine and the apolipoproteins associated with very-low-density lipoproteins. Thus phosphatidylcholine synthesis via reacylation of lysophosphatidylcholine, via the CDP-choline pathway or via methylation of phosphatidylethanolamine, will satisfy the requirements for secretion of very-low-density lipoprotein from hepatocytes.

SUBMITTER: Robinson BS

PROVIDER: S-EPMC1138647 | biostudies-other | 1989 May

REPOSITORIES: biostudies-other

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Project description:Biomethylation is the major pathway for the metabolism of inorganic arsenic (iAs) in many mammalian species, including the human. However, significant interspecies differences have been reported in the rate of in vivo metabolism of iAs and in yields of iAs metabolites found in urine. Liver is considered the primary site for the methylation of iAs and arsenic (+3 oxidation state) methyltransferase (As3mt) is the key enzyme in this pathway. Thus, the As3mt-catalyzed methylation of iAs in the liver determines in part the rate and the pattern of iAs metabolism in various species. We examined kinetics and concentration-response patterns for iAs methylation by cultured primary hepatocytes derived from human, rat, mice, dog, rabbit, and rhesus monkey. Hepatocytes were exposed to [(73)As]arsenite (iAs(III); 0.3, 0.9, 3.0, 9.0 or 30 nmol As/mg protein) for 24 h and radiolabeled metabolites were analyzed in cells and culture media. Hepatocytes from all six species methylated iAs(III) to methylarsenic (MAs) and dimethylarsenic (DMAs). Notably, dog, rat and monkey hepatocytes were considerably more efficient methylators of iAs(III) than mouse, rabbit or human hepatocytes. The low efficiency of mouse, rabbit and human hepatocytes to methylate iAs(III) was associated with inhibition of DMAs production by moderate concentrations of iAs(III) and with retention of iAs and MAs in cells. No significant correlations were found between the rate of iAs methylation and the thioredoxin reductase activity or glutathione concentration, two factors that modulate the activity of recombinant As3mt. No associations between the rates of iAs methylation and As3mt protein structures were found for the six species examined. Immunoblot analyses indicate that the superior arsenic methylation capacities of dog, rat and monkey hepatocytes examined in this study may be associated with a higher As3mt expression. However, factors other than As3mt expression may also contribute to the interspecies differences in the hepatocyte capacity to methylate iAs.

Project description:Intersections in hepatic methyl group metabolism pathways highlights potential competition or compensation of methyl donors. The objective of this experiment was to examine the expression of genes related to methyl group transfer and lipid metabolism in response to increasing concentrations of choline chloride (CC) and DL-methionine (DLM) in primary neonatal hepatocytes that were or were not exposed to fatty acids (FA). Primary hepatocytes isolated from 4 neonatal Holstein calves were maintained as monolayer cultures for 24 h before treatment with CC (61, 128, 2028, and 4528 μmol/L) and DLM (16, 30, 100, 300 μmol/L), with or without a 1 mmol/L FA cocktail in a factorial arrangement. After 24 h of treatment, media was collected for quantification of reactive oxygen species (ROS) and very low-density lipoprotein (VLDL), and cell lysates were collected for quantification of gene expression. No interactions were detected between CC, DLM, or FA. Both CC and DLM decreased the expression of methionine adenosyltransferase 1A (MAT1A). Increasing CC did not alter betaine-homocysteine S-methyltranferase (BHMT) but did increase 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR) and methylenetetrahydrofolate reductase (MTHFR) expression. Increasing DLM decreased expression of BHMT and MTR, but did not affect MTHFR. Expression of both phosphatidylethanolamine N-methyltransferase (PEMT) and microsomal triglyceride transfer protein (MTTP) were decreased by increasing CC and DLM, while carnitine palmitoyltransferase 1A (CPT1A) was unaffected by either. Treatment with FA decreased the expression of MAT1A, MTR, MTHFR and tended to decrease PEMT but did not affect BHMT and MTTP. Treatment with FA increased CPT1A expression. Increasing CC increased secretion of VLDL and decreased the accumulation of ROS in media. Within neonatal bovine hepatocytes, choline and methionine differentially regulate methyl carbon pathways and suggest that choline may play a critical role in donating methyl groups to support methionine regeneration. Stimulating VLDL export and decreasing ROS accumulation suggests that increasing CC is hepato-protective.

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Lysophosphatidylcholine metabolism and lipoprotein secretion by cultured rat hepatocytes deficient in choline.

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