Hepatic triacylglycerol hydrolysis regulates peroxisome proliferator-activated receptor alpha activity - PubMed (original) (raw)

Hepatic triacylglycerol hydrolysis regulates peroxisome proliferator-activated receptor alpha activity

Jessica M Sapiro et al. J Lipid Res. 2009 Aug.

Abstract

Recent evidence suggests that fatty acids generated from intracellular triacylglycerol (TAG) hydrolysis may have important roles in intracellular signaling. This study was conducted to determine if fatty acids liberated from TAG hydrolysis regulate peroxisome proliferator-activated receptor alpha (PPARalpha). Primary rat hepatocyte cultures were treated with adenoviruses overexpressing adipose differentiation-related protein (ADRP) or adipose triacylglycerol lipase (ATGL) or treated with short interfering RNA (siRNA) targeted against ADRP. Subsequent effects on TAG metabolism and PPARalpha activity and target gene expression were determined. Overexpressing ADRP attenuated TAG hydrolysis, whereas siRNA-mediated knockdown of ADRP or ATGL overexpression resulted in enhanced TAG hydrolysis. Results from PPARalpha reporter activity assays demonstrated that decreasing TAG hydrolysis by ADRP overexpression resulted in a 35-60% reduction in reporter activity under basal conditions or in the presence of fatty acids. As expected, PPARalpha target genes were also decreased in response to ADRP overexpression. However, the PPARalpha ligand, WY-14643, was able to restore PPARalpha activity following ADRP overexpression. Despite its effects on PPARalpha, overexpressing ADRP did not affect PPARgamma activity. Enhancing TAG hydrolysis through ADRP knockdown or ATGL overexpression increased PPARalpha activity. These results indicate that TAG hydrolysis and the consequential release of fatty acids regulate PPARalpha activity.

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Figures

Fig. 1.

Fig. 1.

ADRP overexpression decreases the loss of fatty acids from TAG and inhibits PPARα activity and target gene expression. A: Western blot analysis of ADRP overexpression showing endogenous rat ADRP and overexpressed murine FLAG-tagged ADRP, which ran approximately 2 kDa higher during electrophoresis. B: Incorporation of [1-14C]oleate into cellular lipids. C–E: Chase experiments evaluating turnover of TAG, phospholipids, and cholesterol esters. Statistics for the chase period were analyzed as a percentage of the pulse. F: PPARα reporter gene activity following exposure to fatty acids. G: Expression of PPARα target genes. All data are reported as means ± SE from three to four experiments. *P < 0.05 and **P < 0.01 when compared with Ad-GFP controls.

Fig. 2.

Fig. 2.

ADRP knockdown enhances fatty acid loss from intracellular TAG and increases PPARα activity. A: ADRP protein abundance measured with Western blotting at 48 h after transfection. B: Eight hours after siRNA transfection, cells were lysed and RNA was harvested and analyzed for ADRP mRNA abundance with qRT-PCR. C: Incorporation of [1-14C]oleate into cellular lipids. D: Chase experiments evaluating turnover of lipid species. Statistics for the chase period were analyzed as a percentage of the pulse. Statistics for the chase period were analyzed as a percentage of the pulse. E: PPARα reporter gene activity following exposure to fatty acids. Data are reported as means ± SE, n = 3. *P < 0.05, **P < 0.01, and ***P < 0.001 when compared with siRNA controls.

Fig. 3.

Fig. 3.

Overexpressing ADRP decreases PPARα reporter activity following removal of exogenous fatty acids. Fourteen hours after transduction, cells were exposed to a combination of 250 μM EPA and 250 μM oleate for 6 h (pulse). The media was removed and replaced with media devoid of fatty acids for an additional 6 h (chase). Reporter gene assays were performed immediately after removal of exogenous fatty acids and 6 h later. Data are presented as PPARα reporter activity after the 6 h chase period expressed as a percentage of the pulse period. Data are reported as means ± SE, n = 3. *P < 0.05 when compared with Ad-GFP controls.

Fig. 4.

Fig. 4.

Overexpressing ATGL enhances the loss of fatty acids from TAG and increases PPARα activity and target gene expression. A: Western blot analysis of ATGL overexpression. B: Incorporation of [1-14C]oleate into cellular lipids. C: Chase experiments evaluating turnover of TAG. Statistics for the chase period were analyzed as a percentage of the pulse. D: PPARα reporter gene activity following exposure to fatty acids. E: Expression of PPARα target genes. All data are reported as means ± SE, n = 3. *P < 0.05 and **P < 0.01 when compared with Ad-GFP controls.

Fig. 5.

Fig. 5.

The PPARα ligand, WY-14643, normalizes PPARα activity in cells overexpressing ADRP. At 24 h after viral transduction, cells were treated with DMSO or 1 μM WY-14643 for 6 h prior to harvesting for reporter gene assays. Data are reported as means ± SE, n = 3. *P < 0.05 when compared with Ad-GFP controls.

Fig. 6.

Fig. 6.

ADRP overexpression does not alter PPARγ activity. PPARγ reporter gene activity following exposure to fatty acids. Data are reported as means ± SE, n = 3. There were no significant differences between Ad-GFP and Ad-ADRP transduced cells.

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