Effects of elaidic acid on lipid metabolism in HepG2 cells, investigated by an integrated approach of lipidomics, transcriptomics and proteomics

Abstract

Trans fatty acid consumption in the human diet can cause adverse health effects, such as cardiovascular disease, which is associated with higher total cholesterol, a higher low density lipoprotein-cholesterol level and a decreased high density lipoprotein-cholesterol level. The aim of the study was to elucidate the hepatic response to the most abundant trans fatty acid in the human diet, elaidic acid, to help explain clinical findings on the relationship between trans fatty acids and cardiovascular disease. The human HepG2 cell line was used as a model to investigate the hepatic response to elaidic acid in a combined proteomic, transcriptomic and lipidomic approach. We found many of the proteins responsible for cholesterol synthesis up-regulated together with several proteins involved in the esterification and hepatic import/export of cholesterol. Furthermore, a profound remodeling of the cellular membrane occurred at the phospholipid level. Our findings contribute to the explanation on how trans fatty acids from the diet can cause modifications in plasma cholesterol levels by inducing abundance changes in several hepatic proteins and the hepatic membrane composition.

Figure 1. HepG2-SF cell proliferation during FA incubation.

Proliferation of HepG2-SF cells in supplemented medium (100 µM FFA) was followed during six days of incubation and measured on day 0, 2, 4 and 6 by CyQuant cell proliferation assay, where fluorescence (y-axis) is a measure of cell numbers. HepG2-SF cell proliferation is differentially affected by EA, OA and SA. EA supplemented cells appear to be compromised on their growth rate when compared to Control, OA and SA supplemented cells. * marks measurements significantly differing from controls using the unpaired t-test, two-tailed with a 95% confidence interval.

Figure 2. The FA composition of HepG2-SF phospholipids analyzed by gas chromatography.

The composition of phospholipids from HepG2-SF cells incubated 6 days in 100 µM oleic, elaidic, or stearic acid supplemented medium were determined. Panel A and Panel B are the high and low abundant FAs present in the PLs, respectively. Each bar represents the average of three biological replicas. The y-axis is the percent of total FA methyl esters measured. The x-axis indicates the different PL FAs measured as their methyl esters. Color coding corresponds to the different supplemented FFAs. Panel C shows a heatmap representation of the distribution of FAs in PLs of the HepG2-SF cells after supplementation. The FA composition of HepG2-SF phospholipids change depending on the different supplemented fatty acids, Stearic and Oleic profiles are alike whereas the Elaidic FA PL profile differs from all the other groups. * marks measurements significantly differing from controls using the unpaired t-test, two-tailed with a 95% confidence interval.

Figure 3. Search space comparison of genes and proteins for the comparison of EvsO differentially expressed genes/proteins in SILAC and GEMA.

Panel A. Comparison of search spaces containing all genes/proteins quantified in SILAC and GEMA analyses. Panel B. Numbers of differentially expressed genes/proteins in the EvsO comparison according to method (SILAC fold change >1.3, p-value <0.01; GEMA fold change >1.5, BH-p-value <0.01). Red numbers indicate up-regulations while green indicates down-regulations. The brown number indicates genes/proteins found down-regulated in SILAC while up-regulated in GEMA. In both GEMA and SILAC, 7% of the genes/proteins in the respective search spaces were found to be regulated in the EvsO comparison.

Figure 4. IPA categories and functions specifically perturbed by EA as compared to OA, comparing GEMA and SILAC data.

Panel A displays perturbed IPA categories. The y-axis reports the significances of the perturbation and is plotted as minus log of the BH multiple testing corrected p-value calculated by IPA based on the specific data set. Panel B displays the 24 significantly perturbed functions found in the top three IPA categories for both SILAC and GEMA. For both panels, the dotted line is the threshold BH-p-value 0.01, with orange bars representing the GEMA data and the blue bars representing the SILAC data.