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Apolipoprotein D Transgenic Mice Develop Hepatic Steatosis through Activation of PPARγ and Fatty Acid Uptake.

Labrie M, Lalonde S, Najyb O, Thiery M, Daneault C, Des Rosiers C, Rassart E, Mounier C - PLoS ONE (2015)

Bottom Line: Elevated expression of AMPK in the liver of Tg leads to phosphorylation of acetyl CoA carboxylase, indicating a decreased activity of the enzyme.In addition, expression of carnitine palmitoyl transferase 1, the rate-limiting enzyme of beta-oxidation, is slightly upregulated.Finally, we show that overexpressing H-apoD in HepG2 cells in presence of arachidonic acid (AA), the main apoD ligand, increases the transcriptional activity of PPARγ.

View Article: PubMed Central - PubMed

Affiliation: Centre de recherche BioMed, Département des Sciences Biologiques, Université du Québec, Montréal, Québec, H3C 3P8, Canada.

ABSTRACT
Transgenic mice (Tg) overexpressing human apolipoprotein D (H-apoD) in the brain are resistant to neurodegeneration. Despite the use of a neuron-specific promoter to generate the Tg mice, they expressed significant levels of H-apoD in both plasma and liver and they slowly develop hepatic steatosis and insulin resistance. We show here that hepatic PPARγ expression in Tg mice is increased by 2-fold compared to wild type (WT) mice. Consequently, PPARγ target genes Plin2 and Cide A/C are overexpressed, leading to increased lipid droplets formation. Expression of the fatty acid transporter CD36, another PPARgamma target, is also increased in Tg mice associated with elevated fatty acid uptake as measured in primary hepatocytes. Elevated expression of AMPK in the liver of Tg leads to phosphorylation of acetyl CoA carboxylase, indicating a decreased activity of the enzyme. Fatty acid synthase expression is also induced but the hepatic lipogenesis measured in vivo is not significantly different between WT and Tg mice. In addition, expression of carnitine palmitoyl transferase 1, the rate-limiting enzyme of beta-oxidation, is slightly upregulated. Finally, we show that overexpressing H-apoD in HepG2 cells in presence of arachidonic acid (AA), the main apoD ligand, increases the transcriptional activity of PPARγ. Supporting the role of apoD in AA transport, we observed enrichment in hepatic AA and a decrease in plasmatic AA concentration. Taken together, our results demonstrate that the hepatic steatosis observed in apoD Tg mice is a consequence of increased PPARγ transcriptional activity by AA leading to increased fatty acid uptake by the liver.

No MeSH data available.


Related in: MedlinePlus

Lipogenesis in the liver of H-apoD Tg mice.Western blot analysis of total and phospho-AMPKα (A), total and phospho-ACC (B) and FAS (C) protein expression in the liver of WT and H-apoD Tg mice. The graphs represent the levels of protein expressions standardized by amidoblack staining. Representative gels are presented. D- Semi-quantitative RT-PCR analysis of ACC, SCD1, DGAT and LXRα mRNA expression. The graph represents the level of mRNA normalized by HPRT. Representative gels are presented. E- In vivo lipogenesis measured in 1 year old mice. The values represent the amount of 3H2O incorporated into triglycerides. Values are expressed relatively to the WT mice and are the means ± SD of 4 mice per group. *P<0.05, **P<0.01 vs WT mice.
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pone.0130230.g004: Lipogenesis in the liver of H-apoD Tg mice.Western blot analysis of total and phospho-AMPKα (A), total and phospho-ACC (B) and FAS (C) protein expression in the liver of WT and H-apoD Tg mice. The graphs represent the levels of protein expressions standardized by amidoblack staining. Representative gels are presented. D- Semi-quantitative RT-PCR analysis of ACC, SCD1, DGAT and LXRα mRNA expression. The graph represents the level of mRNA normalized by HPRT. Representative gels are presented. E- In vivo lipogenesis measured in 1 year old mice. The values represent the amount of 3H2O incorporated into triglycerides. Values are expressed relatively to the WT mice and are the means ± SD of 4 mice per group. *P<0.05, **P<0.01 vs WT mice.

Mentions: We previously demonstrated that the mRNA levels of SREBP-1c and FAS were increased in the liver of H-apoD Tg compared to WT mice [15]. Since elevated lipogenesis has also been associated with hepatic steatosis, we evaluated the expression levels and the activity of several other key proteins involved in hepatic lipogenesis. We showed that the AMPK expression was increased by 1.82-fold in the liver of Tg mice. Its phosphorylation on Thr172 residue was also increased (2.63 fold), but the AMPK activity was unaffected as evaluated by the ratio of P-AMPα/AMPKα (Fig 4A). As a consequence of increased AMPKα phosphorylation in Tg mice, one of the AMPKα target proteins, ACC displayed higher phosphorylation levels on Ser79 (1.7 fold, measured by the ratio P-ACC/ACC) suggesting a reduced activity of the first lipogenic enzyme (Fig 4B). On the other hand, a significant increase in FAS protein expression was observed (1.97-fold) complementing our previous observation at the mRNA level [15] (Fig 4C). However, the mRNA expression of ACC, SCD1, DGAT and LXRα remained unaffected in the liver of H-apoD Tg mice compared to WT (Fig 4D).


Apolipoprotein D Transgenic Mice Develop Hepatic Steatosis through Activation of PPARγ and Fatty Acid Uptake.

Labrie M, Lalonde S, Najyb O, Thiery M, Daneault C, Des Rosiers C, Rassart E, Mounier C - PLoS ONE (2015)

Lipogenesis in the liver of H-apoD Tg mice.Western blot analysis of total and phospho-AMPKα (A), total and phospho-ACC (B) and FAS (C) protein expression in the liver of WT and H-apoD Tg mice. The graphs represent the levels of protein expressions standardized by amidoblack staining. Representative gels are presented. D- Semi-quantitative RT-PCR analysis of ACC, SCD1, DGAT and LXRα mRNA expression. The graph represents the level of mRNA normalized by HPRT. Representative gels are presented. E- In vivo lipogenesis measured in 1 year old mice. The values represent the amount of 3H2O incorporated into triglycerides. Values are expressed relatively to the WT mice and are the means ± SD of 4 mice per group. *P<0.05, **P<0.01 vs WT mice.
© Copyright Policy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4470830&req=5

pone.0130230.g004: Lipogenesis in the liver of H-apoD Tg mice.Western blot analysis of total and phospho-AMPKα (A), total and phospho-ACC (B) and FAS (C) protein expression in the liver of WT and H-apoD Tg mice. The graphs represent the levels of protein expressions standardized by amidoblack staining. Representative gels are presented. D- Semi-quantitative RT-PCR analysis of ACC, SCD1, DGAT and LXRα mRNA expression. The graph represents the level of mRNA normalized by HPRT. Representative gels are presented. E- In vivo lipogenesis measured in 1 year old mice. The values represent the amount of 3H2O incorporated into triglycerides. Values are expressed relatively to the WT mice and are the means ± SD of 4 mice per group. *P<0.05, **P<0.01 vs WT mice.
Mentions: We previously demonstrated that the mRNA levels of SREBP-1c and FAS were increased in the liver of H-apoD Tg compared to WT mice [15]. Since elevated lipogenesis has also been associated with hepatic steatosis, we evaluated the expression levels and the activity of several other key proteins involved in hepatic lipogenesis. We showed that the AMPK expression was increased by 1.82-fold in the liver of Tg mice. Its phosphorylation on Thr172 residue was also increased (2.63 fold), but the AMPK activity was unaffected as evaluated by the ratio of P-AMPα/AMPKα (Fig 4A). As a consequence of increased AMPKα phosphorylation in Tg mice, one of the AMPKα target proteins, ACC displayed higher phosphorylation levels on Ser79 (1.7 fold, measured by the ratio P-ACC/ACC) suggesting a reduced activity of the first lipogenic enzyme (Fig 4B). On the other hand, a significant increase in FAS protein expression was observed (1.97-fold) complementing our previous observation at the mRNA level [15] (Fig 4C). However, the mRNA expression of ACC, SCD1, DGAT and LXRα remained unaffected in the liver of H-apoD Tg mice compared to WT (Fig 4D).

Bottom Line: Elevated expression of AMPK in the liver of Tg leads to phosphorylation of acetyl CoA carboxylase, indicating a decreased activity of the enzyme.In addition, expression of carnitine palmitoyl transferase 1, the rate-limiting enzyme of beta-oxidation, is slightly upregulated.Finally, we show that overexpressing H-apoD in HepG2 cells in presence of arachidonic acid (AA), the main apoD ligand, increases the transcriptional activity of PPARγ.

View Article: PubMed Central - PubMed

Affiliation: Centre de recherche BioMed, Département des Sciences Biologiques, Université du Québec, Montréal, Québec, H3C 3P8, Canada.

ABSTRACT
Transgenic mice (Tg) overexpressing human apolipoprotein D (H-apoD) in the brain are resistant to neurodegeneration. Despite the use of a neuron-specific promoter to generate the Tg mice, they expressed significant levels of H-apoD in both plasma and liver and they slowly develop hepatic steatosis and insulin resistance. We show here that hepatic PPARγ expression in Tg mice is increased by 2-fold compared to wild type (WT) mice. Consequently, PPARγ target genes Plin2 and Cide A/C are overexpressed, leading to increased lipid droplets formation. Expression of the fatty acid transporter CD36, another PPARgamma target, is also increased in Tg mice associated with elevated fatty acid uptake as measured in primary hepatocytes. Elevated expression of AMPK in the liver of Tg leads to phosphorylation of acetyl CoA carboxylase, indicating a decreased activity of the enzyme. Fatty acid synthase expression is also induced but the hepatic lipogenesis measured in vivo is not significantly different between WT and Tg mice. In addition, expression of carnitine palmitoyl transferase 1, the rate-limiting enzyme of beta-oxidation, is slightly upregulated. Finally, we show that overexpressing H-apoD in HepG2 cells in presence of arachidonic acid (AA), the main apoD ligand, increases the transcriptional activity of PPARγ. Supporting the role of apoD in AA transport, we observed enrichment in hepatic AA and a decrease in plasmatic AA concentration. Taken together, our results demonstrate that the hepatic steatosis observed in apoD Tg mice is a consequence of increased PPARγ transcriptional activity by AA leading to increased fatty acid uptake by the liver.

No MeSH data available.


Related in: MedlinePlus