Liver proteomic response to hypertriglyceridemia in human-apolipoprotein C-III transgenic mice at cellular and mitochondrial compartment levels

Abstract

Background: Hypertriglyceridemia (HTG) is defined as a triglyceride (TG) plasma level exceeding 150 mg/dl and is tightly associated with atherosclerosis, metabolic syndrome, obesity, diabetes and acute pancreatitis. The present study was undertaken to investigate the mitochondrial, sub-mitochondrial and cellular proteomic impact of hypertriglyceridemia in the hepatocytes of hypertriglyceridemic transgenic mice (overexpressing the human apolipoproteinC-III).

Methods: Quantitative proteomics (2D-DIGE) analysis was carried out on both "low-expressor" (LE) and "high-expressor" (HE) mice, respectively exhibiting moderate and severe HTG, to characterize the effect of the TG plasma level on the proteomic response.

Results: The mitoproteome analysis has revealed a large-scale phenomenon in transgenic mice, i.e. a general down-regulation of matricial proteins and up-regulation of inner membrane proteins. These data also demonstrate that the magnitude of proteomic changes strongly depends on the TG plasma level. Our different analyses indicate that, in HE mice, the capacity of several metabolic pathways is altered to promote the availability of acetyl-CoA, glycerol-3-phosphate, ATP and NADPH for TG de novo biosynthesis. The up-regulation of several cytosolic ROS detoxifying enzymes has also been observed, suggesting that the cytoplasm of HTG mice is subjected to oxidative stress. Moreover, our results suggest that iron over-accumulation takes place in the cytosol of HE mice hepatocytes and may contribute to enhance oxidative stress and to promote cellular proliferation.

Conclusions: These results indicate that the metabolic response to HTG in human apolipoprotein C-III overexpressing mice may support a high TG production rate and that the cytosol of hepatocytes is subjected to an important oxidative stress, probably as a result of FFA over-accumulation, iron overload and enhanced activity of some ROS-producing catabolic enzymes.

Figure 1

Matricial and inner membrane PSOI expression variations in LE and HE mice compared to WT. Panel A presents the number of down- (white) and up- (black) regulated PSOI. Panel B shows the average magnitude of PSOI expression variations. Matricial and inner membrane proteins were sorted out on the basis of localization data reported in UniProtKB/Swiss-Prot and KEGG Pathway. MAT, matrix; IM, inner membrane.

Figure 2

Distribution charts of PSOI logarithmic expression variations obtained after excluding outliers (Additional files3and4). In red, the normal adjustment curve and a straight line indicating the average of the distribution (μ). Panel A, before re-normalization; Panel B, after re-normalization. MAT, matrix; IM, inner membrane.

Figure 3

Histogram classifying PSOI according to the direction of their re-normalized expression variation. White, down-regulation; Black, up-regulation; Grey, absence of regulation. MAT, matrix; IM, inner membrane.

Figure 4

Histogram illustrating the proportion of PSOI exhibiting a biologically relevant re-normalized expression variation (+/- 20%: ≤ 0.83 or ≥ 1.2). Black, spots with a relevant expression variation; White, spots without a relevant expression variation. MAT, matrix; IM, inner membrane.

Figure 5

Scatter plot comparing the expression variations obtained by re-normalization and experimental approaches. Comparison has been performed for HE mouse matricial proteome (x axis: experimental approach; y axis, re-normalized values). Dotted lines indicate the biological significance cutoffs.

Figure 6

Main adaptations of the mitochondrial and cellular proteomes in response to HTG in HE mice. Pathways in red and blue respectively indicate up-regulation or down-regulation as found in our proteomic analyses. Dashed lines indicate an assumed capacity alteration. Stoichiometries of the reactions are not showed. TAG, Triglycerides; CI-III-IV, Complexes I-III-IV; ATPs, ATP Synthase; UQ, Ubiquinone; UQH₂, Ubiquinol; FFA, Free Fatty Acids, ETFDH, Electron-transferring-flavoprotein dehydrogenase.