Metabolism of glycerol monoethers in cultured liver cells and implications for monoglyceride pathways.

A comparative study has been made of the assimilation and metabolism of rac-1 and 2-[9, 10(-3)H]-octadec-9-enylglycerol in a clone of epithelial-like cells isolated from rabbit liver. Based on cell protein content, the free glycerol ether isomers attained equal cellular concentrations. As shown by isolation and degradation experiments, however, the incorporation of radioactive 1-monether was appreciably higher than that of radioactive 2-monoether in both the triacylglycerol and phospholipid fractions. The 1-monoether, unlike the 2-monether, was also a significant source of esterified fatty acids in both lipid fractions. In addition, the 1-monoether, but not the 2-monoether, was an active precursor of plasmalogens, particularly ethanolamine plasmalogen. In contrast to the 1-monoether, the 2-monoether was a more active precursor of triacylglycerols than it was of phospholipids. The results indicate that in the rabbit liver cells the pathway of complex lipid synthesis from 1-monoether was via 1-alkyl-sn-glycerol-3-phosphoric acid and from 2-monoether via 1-alkyl-2-acyl-sn-glycerol.

Differential labeling of glycerol moieties of phospholipids and triacylglycerols of cultured mammalian cells by [U-14 C] glucose.

Rabbit liver cells, in which fatty acid synthesis was suppressed by the rabbit serum component of the medium, were grown through 8- to 120-fold increases in cell numbers and mass of cell lipid in the presence of [U-14 C]-glucose. Triacylglycerols, phosphatidylcholine, and phosphatidylethanolamine were isolated from the total cell lipid and deacylated. Carbons 1 and 3 of the glycerol from the triacylglycerols and the no. 1 glycerol carbons of the two deacylated phospholids were oxidized by periodate and isolated as the dimedon derivative of formaldehyde. The specific activities of the glycerol carbons indicated that 58, 44, and 37 percent of the glycerol of the triacylglycerols. phosphatidylcholine, and phosphatidylethanolamine, respectively, were derived from the glucose of the medium. An additional 8 percent and 1-2 percent of the glycerol of each lipid was derived, respectively, from [U-14 C] glycerol and U14C-labeled amino acids added to the medium. In agreement with an experiment with albumin-bound [9,10- minus 3H]-oleic acid, and with smilar earlier experiments, it appears likely that appriacylglycerols originated from serum lipoproteins, or their partial hydrolysis products. An appreciable part of the ethanolamine of the cells' phosphatidylethanolamine originated from exogenous U- minus 14 C-labeled amino acids. Phosphatidyl-ethanolamine, however, was not a primary source of phosphatidylcholine. Labeling of the fatty acids of triacylglycerols and phospholipids by radioactive glucose, glycerol and amino acids was negligible.

Identification of albumin-bound fatty acids as the major factor in serum-induced lipid accumulation by cultured cells.

Factors responsible for the high lipogenic activity of rabbit serum were investigated using an assay procedure based on the gravimetric determination of the 24 hr increase in cell lipid. Cellular synthesis of fatty acids was inhibited by the presence of serum in the assay medium. Approximately 90% of the increase in cell lipid produced by serum fractions was due to triglyceride accumulation. Fractionation of rabbit serum by precipitation with ammonium sulfate or by ultracentrifugation in high density medium, both indicated that three-quarters of its lipogenic activity was associated with albumin. The lipoproteins prepared by ultracentrifugation also exhibited about one-half the activity of whole serum. The lipogenic activity of albumin was confirmed by the high potency of the albumin isolated in a nearly pure form from proteins of d>1.21 by precipitation with trichloroacetic acid and extraction with ethanol. As judged from chemical and isotopic analysis, neither the lipid content nor the lipid composition of the albumin was appreciably altered during its isolation. Of the albumin-bound lipids, only the free fatty acids, as determined by DEAE column chromatography, were present in an amount sufficient to account for the observed increase in cell triglycerides. In control experiments with horse serum of low lipogenic activity, the proteins of d>1.21 also possessed low activity in conjunction with a low content of free fatty acid. However, the albumin isolated from the latter preparation exhibited the high lipogenic activity of rabbit serum albumin. Chemical and isotopic analysis of the recovered horse serum albumin revealed that its free fatty acid content was the same as that of rabbit serum albumin. These results indicated that the isolation of horse serum albumin was attended by a substantial increase in its free fatty acid content. When the rabbit serum and horse serum content of media were adjusted to provide equivalent concentrations of albumin-bound fatty acids, the rabbit liver cells grown on the former media accumulated more lipid than cells grown on the latter media. This difference was shown to be due to the higher concentration of albumin per micro mole of fatty acid in horse serum as compared with rabbit serum. Consequently, the albumin to fatty acid ratio also controls the lipogenic activity of a serum. A linear relationship is presented which relates the cell lipid content to the molar ratio of albumin to free fatty acids and to the absolute concentration of free fatty acids in the medium.

Increase in cell lipid and cytoplasmic particles in mammalian cells cultured at reduced pH.

The hydrogen ion concentration of the medium has been shown to exert a regulatory effect on the lipid content of cultured mammalian cells. Reduction of the pH of the medium from 7.4 to 6.9 causes a significant increase in cell lipid, relative to cell protein, within 2-3 days. Triglycerides are increased twofold and account for 75% of the additional lipid. Polar lipids, on the other hand, remain nearly constant in concentration. Concurrent with the increase in lipid, particles with an average diameter of 1 micro appear in the cytoplasm. Because the density of these particles is low, ultracentrifugation of the cell homogenate separates the particles completely from the other subcellular structures. The amount of lipid in the particle fraction is approximately equal to the increase in total cell lipid. As shown by silicic acid column chromatography, the particle lipid contains about 75% triglycerides, 15% diglycerides plus an unknown substance, and smaller amounts of material in the monoglyceride and sterol ester-hydrocarbon fractions. The quantitative results indicate that the lipid accumulated at low pH is assembled into discrete cytoplasmic particles.