Extracellular fatty acids are not utilized directly for the synthesis of very-low-density lipoprotein in primary cultures of rat hepatocytes

Abstract

In hepatocytes cultured in the presence of oleate (initial concn. 0.75 mM), the secretion of very-low-density lipoprotein (VLDL) triacylglycerol and, to a lesser extent, apoprotein B (apoB) increased with time, whereas there was a large decline in the extracellular concentration of fatty acid. There was thus no synchronous relationship between the extracellular fatty acid concentration and the secretion of VLDL. Rather, the appearance of VLDL in the medium was dependent on the intracellular triacylglycerol concentration. At a given concentration of extracellular fatty acid, cells depleted of triacylglycerol secreted less VLDL triacylglycerol and apoB than did control cells. A similar pattern was observed for triacylglycerol newly synthesized from extracellular [3H]oleate. By contrast, the synthesis and output of ketone bodies were directly dependent on the fatty acid concentration of the medium. These results suggest that, at least for oleic acid, extracellular fatty acids are not utilized directly for VLDL assembly, but first enter a temporary intracellular storage pool of triacylglycerol, which is the immediate precursor of secreted triacylglycerol. The size of this pool then determines the rate of secretion of VLDL triacylglycerol and apoB. Ketogenesis, on the other hand, relies mainly on the direct utilization of extracellular fatty acids.

In vitro metabolism of rumenic acid in bovine liver slices

“…In our experimental conditions, the uptake of oleic acid by bovine liver slices amounted to about 1.08% of FA added to the medium whereas it amounted to 2.5% of FA in rat liver slices medium [21]. The incorporation of these FA was lower than that reported in previous experiments in the bovine liver in vivo (7 to 25% of circulating non esterified FA) [38] or using isolated rat hepatocytes (50% after 6 h of incubation) [39], or on perfused rat liver (90% after 2 h of incubation) [40]. These discrepancies could be explained by a lower contact between the cells and medium in the in vitro model with liver slices than in the in vivo or the in vitro model with isolated hepatocytes or in the ex vivo model with perfused liver.…”

Glucose Phosphorylation Is Essential for the Turnover of Neutral Lipid and the Second Stage Assembly of Triacylglycerol-Rich ApoB-Containing Lipoproteins in Primary Hepatocyte Cultures

“…It is generally agreed that glucose suppresses endogenous ketogenesis in the livers of starved rats as first shown by Krebs and Hems 71 almost 30 years ago. However, endogenous ketogenesis in hepatocytes from fed rats is low 25 and in the current work the difference between ketone body production at 0 and 25 mmol/L glucose was sufficient to account for a maximum diversion of only 23 nmol endogenous TAG/mg cell protein away from the ketogenic pathway at the higher glucose concentration (results not shown). This value is far too low to account for the large increase in lipolytic TAG turnover of Ϸ150 nmol/mg protein ( Figure 6).…”

“…18 -22 It is not yet clear whether this step requires microsomal triglyceride transfer protein (MTP). 23,24 Nevertheless, it seems likely that this stage of assembly is dependent on the efficient mobilization and transfer of endogenous neutral lipid 25 to appropriate intracellular site(s) in the secretory apparatus. 22,26 -30 This latter step may require efficient cytosolic TAG lipolysis and reesterification.…”

Suppression of Long Chain Acyl-CoA Synthetase 3 Decreases Hepatic de Novo Fatty Acid Synthesis through Decreased Transcriptional Activity

“…Fatty acids derived from de novo lipogenesis are preferentially incorporated into storage lipids such as TAG and secreted as very low density lipoprotein rather than mitochondrial oxidation (19,42). Additionally, an intermediate in de novo fatty acid synthesis, malonyl-CoA, allosterically inhibits carnitine palmitoyl transferase to suppress ␤-oxidation (43), which may further exacerbate the development of steatosis.…”