VLDL hydrolysis by LPL activates PPAR-alpha through generation of unbound fatty acids - PubMed (original) (raw)

VLDL hydrolysis by LPL activates PPAR-alpha through generation of unbound fatty acids

Maxwell A Ruby et al. J Lipid Res. 2010 Aug.

Abstract

Recent evidence suggests that lipoproteins serve as circulating reservoirs of peroxisomal proliferator activated receptor (PPAR) ligands that are accessible through lipolysis. The present study was conducted to determine the biochemical basis of PPAR-alpha activation by lipolysis products and their contribution to PPAR-alpha function in vivo. PPAR-alpha activation was measured in bovine aortic endothelial cells following treatment with human plasma, VLDL lipolysis products, or oleic acid. While plasma failed to activate PPAR-alpha, oleic acid performed similarly to VLDL lipolysis products. Therefore, fatty acids are likely to be the PPAR-alpha ligands generated by VLDL lipolysis. Indeed, unbound fatty acid concentration determined PPAR-alpha activation regardless of fatty acid source, with PPAR-alpha activation occurring only at unbound fatty acid concentrations that are unachievable under physiological conditions without lipase action. In mice, a synthetic lipase inhibitor (poloxamer-407) attenuated fasting-induced changes in expression of PPAR-alpha target genes. Apolipoprotein CIII (apoCIII), an endogenous inhibitor of lipoprotein and hepatic lipase, regulated access to the lipoprotein pool of PPAR-alpha ligands, because addition of exogenous apoCIII inhibited, and removal of endogenous apoCIII potentiated, lipolytic PPAR-alpha activation. These data suggest that the PPAR-alpha response is generated by unbound fatty acids released locally by lipase activity and not by circulating plasma fatty acids.

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Figures

Fig. 1.

Fig. 1.

VLDL-derived fatty acids serve as potent PPAR-α ligands due to efficient delivery. BAEC were transfected with the PPAR α-LBD-GAL4 reporter system, as described in “Methods,” treated for 18 h, and cell lysate assayed for luciferase and β-galoctosidase activity. A: BAEC were exposed to various concentrations of LPL (1, 3, or 10 units/ml) and VLDL (1, 3, 10, or 30 μg/ml), oleic acid (0, 5, 10, or 20 μM at an unbound oleic acid concentration of 2450 nM), or plasma (0–5% v/v). This produced a range of NEFA concentrations, as measured in the cell culture media at the end of treatment. For oleic acid, a linear relationship exists between oleic acid added and NEFA concentration in the cell culture media (data not shown), such that ∼60% of NEFA added remains in the media following incubation with cells. PPAR-α activity is presented as percentage of activation by 10 µM Wy14643, a synthetic PPAR-α ligand. B: Transfected BAEC were incubated with VLDL (10 µg protein/ml) and LPL (10 units/ml) and increasing concentrations of albumin in triplicate. PPAR-α activity is expressed as fold activation over control.

Fig. 2.

Fig. 2.

Fatty acid uptake determines PPAR-α activation. A: In parallel experiments, BAECs were treated with oleic acid (90 μM) and varying concentrations of albumin, and PPAR-α activation and fatty acid uptake were determined as described in Methods. B: Varying total (0–180 μM) and unbound oleic acid concentrations (0–2,450 nM) were used to generate a range of fatty acid uptake. Fatty acid uptake above 300 pmol/mg protein/min displayed a strong linear relationship with PPAR-α activation (_r_2 = 0.98; P < 0.05).

Fig. 3.

Fig. 3.

P-407 inhibits the transcriptional response to fasting in vivo. Following an initial 2 h fast, 9-week-old male C57Bl6 mice were treated with saline or P-407 (500 mg/kg, i.p.) and fasted for an additional 24 h. A group of saline-injected animals was fed ad libitum for the 24 h period. Abundance of mRNA for PPAR-α target genes was determined by RT-PCR and normalized to a control gene (gusb) in the liver (A) and heart (B). Groups not sharing a common superscript letter are significantly different (P < 0.05).

Fig. 4.

Fig. 4.

ApoCIII regulates access to lipoprotein-derived PPAR-α ligands. A: PPAR-α activation (solid bars) in BAEC were transfected as described in Fig. 1 and treated with LPL (10 units/ml), VLDL (10 µg protein/ml), Orlistat (10 µM), or apoCIII (3 μg/ml) as indicated for 18 h. Cell culture media was assayed for fatty acids. Groups not sharing a common superscript letter are significantly different (P < 0.05). B: BAEC were transfected as described in Fig. 1 and treated with total VLDL or apoCIII-depleted VLDL (10 μg protein/ml) and LPL (3 units/ml). The difference between the VLDL treatments was significant (P = 0.002) by paired two-tailed _t_-test.

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References

    1. Brown J. D., Plutzky J. 2007. Peroxisome proliferator-activated receptors as transcriptional nodal points and therapeutic targets. Circulation. 115: 518–533. - PubMed
    1. Kliewer S. A., Sundseth S. S., Jones S. A., Brown P. J., Wisely G. B., Koble C. S., Devchand P., Wahli W., Willson T. M., Lenhard J. M., et al. 1997. Fatty acids and eicosanoids regulate gene expression through direct interactions with peroxisome proliferator-activated receptors alpha and gamma. Proc. Natl. Acad. Sci. USA. 94: 4318–4323. - PMC - PubMed
    1. Forman B. M., Chen J., Evans R. M. 1997. Hypolipidemic drugs, polyunsaturated fatty acids, and eicosanoids are ligands for peroxisome proliferator-activated receptors alpha and delta. Proc. Natl. Acad. Sci. USA. 94: 4312–4317. - PMC - PubMed
    1. Chawla A., Lee C. H., Barak Y., He W., Rosenfeld J., Liao D., Han J., Kang H., Evans R. M. 2003. PPARdelta is a very low-density lipoprotein sensor in macrophages. Proc. Natl. Acad. Sci. USA. 100: 1268–1273. - PMC - PubMed
    1. Lee C. H., Kang K., Mehl I. R., Nofsinger R., Alaynick W. A., Chong L. W., Rosenfeld J. M., Evans R. M. 2006. Peroxisome proliferator-activated receptor delta promotes very low-density lipoprotein-derived fatty acid catabolism in the macrophage. Proc. Natl. Acad. Sci. USA. 103: 2434–2439. - PMC - PubMed

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