Interaction of non-esterified fatty acid and insulin in control of triacylglycerol secretion by Hep G2 cells

Abstract

The role of insulin in the regulation of plasma triacylglycerol is poorly understood. Conflicting actions of insulin on rat liver cells have been reported, insulin inhibiting triacylglycerol secretion in short incubations (less than 24 h) and stimulating triacylglycerol secretion in longer incubations (48-72 h). The present study was undertaken to examine regulation of triacylglycerol secretion by insulin and investigate the interaction between insulin and non-esterified fatty acid over 72 h in human hepatoblastoma (Hep G2) cells. Insulin inhibited triacylglycerol secretion throughout the 72 h period. The inhibition increased from 66% in the first 24 h to 88% in the final 24 h. Increasing the initial concentration of oleic acid from 200 microM to 1000 microM resulted in a 358% increase in triacylglycerol secretion and a 712% increase in accumulation over 24 h. Oleic acid uptake by the cells was rapid, with only 2.4% of the initial concentration (500 microM) remaining after 24 h. Supplementation of the medium with oleic acid to maintain the concentration between 750 microM and 1000 microM throughout a 5 h period resulted in a 350% increase in triacylglycerol secretion. Supplementation also decreased the insulin-induced inhibition of triacylglycerol secretion (18.2 to 7.8%; P less than 0.001). These results demonstrate that there is not a biphasic action of insulin on triacylglycerol secretion by Hep G2 cells. Experiments of this nature have not previously taken into account the rapid uptake of non-esterified fatty acid by hepatocytes and have consequently underestimated the effect of a sustained concentration on triacylglycerol metabolism. Oleic acid is therefore an even more potent stimulus to triacylglycerol synthesis and secretion than has previously been recognized. In addition, in the presence of a sustained increase in oleic acid concentration, there is a decrease in the action of insulin to inhibit triacylglycerol secretion.

Insulin and non-esterified fatty acid relations to alimentary lipaemia and plasma concentrations of postprandial triglyceride-rich lipoproteins in healthy middle-aged men

“…Thus, the NEFA effects on the postprandial metabolism of large TRLs appear to be not solely a consequence of the degree of insulin sensitivity of the individual person. The rate of hepatic VLDL secretion is strongly dependent on the NEFA supply [21]. Failure to suppress the NEFA supply to the liver in the postprandial state could lead to sustained VLDL production and impaired clearance of postprandial TRLs through competition of the large VLDLs with chylomicrons and large chylomicron remnants for hydrolysis by LPL.…”

Role of insulin in hepatic fatty acid partitioning: emerging concepts

“…Thus, whereas experiments conducted with isolated perfused rat liver showed that insulin stimulates both synthesis and secretion of TAG [136,137], many subsequent studies conducted on cultured hepatocytes have found that insulin, although promoting synthesis of TAG, inhibits its secretion [6,8,133,138,139]. The reasons for this discrepancy between the two sets of data may reside in the use of different experimental systems (perfused liver, cultured cells) and in the choice of the precise perfusion or culture conditions, by different laboratories.…”

“…Culture of hepatocytes with insulin for 6-48 h results in the inhibition of secretion, whereas more prolonged culture (up to 72 h) results in stimulation of secretion (over a diminishing control value) [142][143][144]. Although this phenomenon has not always been observed [6,145], it has been interpreted as indicating that insulin acts acutely on the liver to inhibit TAG secretion, whereas, when the liver is chronically exposed to high levels of insulin, it becomes resistant to the hormone and the effect is lost. Consequently, it has been suggested that acute exposure of the liver to surges in portal insulin concentration (such as would occur during the prandial period) would result in the inhibition of TAG secretion, whereas more prolonged exposure of the liver to hyperinsulinaemic conditions (e.g.…”

“…In all liver preparations used in itro, insulin has always been found to stimulate TAG synthesis [5][6][7][8][9]. The enzymes that catalyse the first reactions that commit long-chain acyl-CoA either to glycerolipid formation (the glycerol-3-phosphate acyltransferases, GPATs) or to intramitochondrial β-oxidation (overt carnitine palmitoyltransferase, CPT I) necessarily compete for their common substrate, and the outcome of this competition is directly influenced by insulin [10].…”

Définition des limites de flexibilité des apports en acides oléique, linoléique et alphalinolénique sur la lipidémie et les paramètres d’athérothrombose chez l’homme : intérêt des huiles végétales combinées

“…Or, des études in vitro et chez l'animal ont également montré que l'acide oléique augmente la production et la sécrétion hépatique des triglycérides associés à l'apo B, sous forme de lipoprotéines de type VLDL riches en triglycérides (TGRL) [25][26][27][28][29][30][31]. D'autre part, si l'apport en acide linoléique est trop faible, l'expression minimale du LDL-récepteur pourrait limiter la dégradation post-traductionnelle des apo-B nouvellement sécrétées [32].…”