Saturated fat-induced changes in Sf 60–400 particle composition reduces uptake of LDL by HepG2 cells

Abstract

The ability of human postprandial triacylglycerolrich lipoproteins (TRLs), isolated after meals enriched in saturated fatty acids (SFAs), n-6 PUFAs, and MUFAs, to inhibit the uptake of 125 I-labeled LDL by the LDL receptor was investigated in HepG2 cells. Addition of TRLs resulted in a dose-dependent inhibition of heparin-releasable binding, cell-associated radioactivity, and degradation products of 125 I-labeled LDL (P , 0.001). SFA-rich Svedberg flotation rate (S f ) 60-400 resulted in significantly greater inhibition of cell-associated radioactivity than PUFA-rich particles (P 5 0.016) and total uptake of 125 I-labeled LDL compared with PUFA-and MUFA-rich particles (P , 0.02). Normalization of the apolipoprotein (apo)E but not apoC-III content of the TRLs removed the effect of meal fatty acid composition, and addition of an anti-apoE antibody reversed the inhibitory effect of TRLs on the total uptake of 125 I-labeled LDL. Real time RT-PCR showed that the SFArich S f 60-400 increased the expression of genes involved in hepatic lipid synthesis (P , 0.05) and decreased the expression of the LDL receptor-related protein 1 compared with MUFAs (P 5 0.008).In conclusion, these findings suggest an alternative or additional mechanism whereby acute fat ingestion can influence LDL clearance via competitive apoE-dependent effects of TRL on the LDL receptor.-Jackson, K. G., V. Maitin, D. S. Leake, P. Yaqoob, and C. M. Williams. Saturated fat-induced changes in S f 60-400 particle composition reduces uptake of LDL by HepG2 cells. The magnitude and duration of the postprandial triacylglycerol (TAG) response has been shown to play a role in the progression of coronary heart disease (1). The metabolic handling of triacylglycerol-rich lipoproteins (TRLs) is influenced by their apolipoprotein (apo) composition, in particular their apoE and apoC-III contents. ApoE plays a crucial role in mediating the hepatic recognition and receptor-mediated uptake of TRLs by the liver (2), whereas increased amounts of apoC-III on TRLs has been shown to impede their metabolism in the circulation (3). We have shown marked differences in the apolipoprotein composition of particles within two TRL fractions, Svedberg flotation rate (S f ) . 400 and S f 60-400, in response to meals of different fatty acid composition. In particular, greater enrichment of TRLs with apoC-III and apoE was observed after a single meal containing predominantly saturated fatty acids (SFAs) compared with meals containing predominantly unsaturated fatty acids (4). Given the importance of apoC-III and apoE in the uptake and removal of these lipoproteins by the liver, the physiological significance of these diet-induced differences in determining the interaction of TRLs with the liver requires further investigation.There have been some studies of the binding of radiolabeled TRLs (in particular fasting VLDL) to receptors on HepG2 cells (5-9) and more recently of their ability to compete with 125 I-labeled LDL for binding to LDL receptors (8, 10). A small number of stu...

A continuum receptor model of hepatic lipoprotein metabolism

“…This assumption reproduces the initial conditions of Jackson et al (2006) where the HepG2 cells were incubated in lipid poor medium to upregulate LDLR levels.…”

“…These results suggest that it is only possible to discern between the different hypotheses proposed by Jackson et al (2006), and the effects of VLDL blocking and internalisation, when the intracellular cholesterol concentration of the cell is considered. Measuring LDL uptake alone does not allow the effect of VLDL on LDL internalisation to be quantified.…”

“…For simplicity, as well as to be able to compare our results with the experimental data of Jackson et al (2006), we consider two measures: the relative LDL uptake; and the relative cholesterol uptake by the cell.…”

“…By comparing with experimental data from Jackson et al (2006) our model has demonstrated that in order to differentiate between the effects of LDL uptake in the case of VLDL blocking LDLR or being internalised, measuring LDL uptake alone is not sufficient in distinguishing between the different scenarios. Instead a measure of the intracellular cholesterol concentration is required.…”

“…More recently in vitro experiments by Jackson et al (2006) have demonstrated that the rate of LDL uptake is influenced by the presence of other plasma lipoproteins, namely the very low density lipoproteins (VLDL). VLDL particles isolated from individuals following consumption of meals of different fatty acid composition, compete with varying effectiveness with LDL for hepatic uptake.…”