Metabolism of lipoprotein acylglycerols by liver parenchymal cells

Computed tomography scanning of Morris hepatomas with liver-specific polyiodinated triglycerides

RATIONALE AND OBJECTIVES

We compared the computed tomography (CT) scanning characteristics of a polyiodinated triglyceride analog with those of a urographic agent to distinguish Morris-7777 hepatoma (MH) cells from normal hepatocytes in rats.

METHODS

Eighteen Buffalo rats were laparotomized and MH cells injected directly into the hepatic parenchyma or introduced via the portal vein to produce, respectively, focal or diffuse lesions in the liver. Baseline CT scans were obtained 21 days after implantation and prior to intravenous administration of either the polyiodinated triglyceride (45-100 mg I/kg) or the nonionic contrast agent, iohexol (560 mg I/kg). Images were obtained at 0-3 hr and 24 hr. Gross pathologic inspection was performed to validate the imaging results.

RESULTS

Hepatomas were nearly isodense with normal liver parenchyma in many of the animals, rendering lesion detection difficult with no contrast agent. The bolus administration of iohexol improved lesion detection in many cases. Lesion conspicuity, however, was significantly improved with the polyiodinated triglyceride at less than one eighth the dose of iohexol.

CONCLUSION

Because of their biochemical nature, polyiodinated triglyceride analogs are specifically cleared by the liver. Consequently, they offer several advantages over nonspecific urographic agents in their ability to enhance lesion conspicuity in this hepatoma model.

Analysis of the monocyte chemotactic response to lysophosphatidylcholine: role of lysophospholipase C

Previously, we reported that lysophosphatidylcholine (lyso-PtdCho), a component of oxidized low-density lipoprotein, was a monocyte chemoattractant (M.T. Quinn et al. (1988) Proc. Natl. Acad. Sci. USA 85, 2805-2809). Monocyte chemotaxis was also stimulated by lyso-platelet activating factor but not by platelet activating factor itself. In the present studies, we used other analogs of lyso-PtdCho to determine structural and metabolic features required for chemotactic activity. Although both D- and L-lyso-PtdCho stimulated chemotaxis, suggesting a lack of stereospecificity, studies using propanediol and ethanediol analogs of lyso-PtdCho suggested that a free hydroxyl moiety or an ester-linked fatty acid vicinal to the phosphocholine group of the lysophospholipid was required for the expression of activity. Incubation of [3H]choline-labeled lyso-PtdCho with monocytes resulted in the formation of labeled PtdCho, glycerophosphocholine (GPC), phosphocholine, and free choline, while resident peritoneal macrophages, cells which we show do not respond chemotactically to lyso-PtdCho, metabolized the labeled substrate to generate only labeled PtdCho and GPC; no labeled phosphocholine was found, suggesting a possible role for lysophospholipase C activity in the monocyte chemotactic response. Although monoacylglycerol, the product of lysophospholipase C hydrolysis of lyso-PtdCho, was not chemotactic for monocytes, diacylglycerol demonstrated chemotactic activity, suggesting that the subsequent acylation to diacylglycerol may be involved in the monocyte chemotactic response to lyso-PtdCho. Indeed, monocytes incorporated [3H]glycerol from [3H]glycerol-labeled lyso-PtdCho into di- and triacylglycerol. Based on these results, a model is proposed whereby the monocyte chemotactic response to lyso-PtdCho involves a sequence of metabolic steps which includes hydrolysis of lyso-PtdCho to monoacylglycerol and phosphocholine by lysophospholipase C followed by acylation of monoacylglycerol to diacylglycerol. Diacylglycerol would then act as an intracellular second messenger that could activate or facilitate the chemotactic response.

The uptake and metabolism of chylomicron-remnant lipids by rat liver parenchymal and non-parenchymal cells in vitro

The uptake and metabolism of chylomicron-remnant lipids by individual liver cell types was examined by incubating remnants with monolayer cultures of hepatocytes, Kupffer cells, and endothelial cells from rat liver. Remnants were prepared in vitro from radiolabelled mesenteric-lymph chylomicra, utilizing either purified lipoprotein lipase from bovine milk, or plasma isolated from heparinized rats. The resulting particles contained [3H]phosphatidylcholine and cholesterol, and [14C]oleate in the acylglycerol, phospholipid, fatty-acid and cholesterol-ester fractions. The capacities of the three cell types for uptake of both [3H]lipids and [14C]lipids were determined to be, on a per-cell basis, in the order: Kupffer greater than hepatocytes greater than endothelial. The relative proportions of [3H]phospholipid and total [3H]cholesterol taken up by hepatocytes and non-parenchymal cells remained constant with time. The uptake of [14C]oleoyl lipids by all three cell types was slightly greater than that of the total [3H]cholesterol and [3H]phospholipid components. There was evidence of cholesterol-ester hydrolysis and turnover of [14C]oleate in the phospholipid fraction in hepatocytes and Kupffer cells, but not endothelial cells, over the first 2 h. With both remnant preparations, these observations indicate that significant differences exist between the three major liver cell types with respect to the uptake and metabolism of remnant lipid components.

Potential tumor- or organ-imaging agents XXIV: chylomicron remnants as carriers for hepatographic agents

This paper describes the possible utility of plasma lipoproteins for the site-specific delivery of diagnostic agents. The class of lipoproteins known as chylomicrons was selected for this preliminary study, since they are known to be rapidly metabolized and taken up by the liver. Cholesteryl iopanoate (II), an iodinated analogue of a normal constituent of the hydrophobic core of chylomicrons, was synthesized from cholesterol and iopanoic acid (I) and subsequently radiolabeled with ioidine-125. Whereas intravenous administration of II in physiological saline resulted in the appearance of approximately 31% of the dose in the liver at 0.5 hr, prior incorporation of II into chylomicrons resulted in an almost threefold (87%) increase in the liver accumulation of II in the same time period. A more gradual appearance of II in steroid-secreting tissues was consistent with the association of II with high-density lipoproteins following administration.