Peroxisome proliferator-activated receptor alpha is involved in the regulation of lipid metabolism by ginseng

Abstract

1. Peroxisome proliferator-activated receptor alpha (PPARalpha) regulates the expression of the key genes involved in lipid metabolism following activation of this receptor by various ligands. Ginseng, a highly valuable medicine in oriental societies, is also reported to modulate lipid metabolism, although the mechanism of its action remains unknown. In order to test our hypothesis that ginseng exerts its effects by altering PPARalpha-mediated pathways, the effects of Korean red ginseng on PPARalpha function and serum lipid profiles were investigated using in vivo and in vitro approaches. 2. In vivo administration of ginseng extract (GE) and ginsenosides (GS) not only inhibited mRNA levels of acyl-CoA oxidase, a rate-limiting enzyme for PPARalpha-mediated peroxisomal fatty acid beta-oxidation, induced by the potent PPARalpha ligand Wy14,643 in a dose- and time-dependent manner, but also inhibited the induction of PPARalpha target genes expected following treatment with Wy14,643. 3. Consistent with the in vivo data, both GE and GS caused dose-dependent decreases in the endogenous expression of a luciferase reporter gene containing the PPAR responsive element (PPRE), while GS significantly decreased the magnitude of reporter gene activation in the presence of Wy14,643. 4. Serological studies demonstrated that, compared with vehicle-treated mice, treatment with GS significantly increased serum concentrations of total cholesterol, triglycerides, and high-density lipoprotein (HDL) cholesterol. Compared to groups treated with Wy14,643 alone, which significantly decreased serum triglyceride and HDL cholesterol levels versus controls, coadministration of either GE or GS with Wy14,643 modestly increased serum triglycerides and HDL cholesterol. 5. These results indicate that the effects of ginseng on serum lipid profiles may be mediated by changes in the expression of PPARalpha target genes, providing the first evidence that in vivo and in vitro treatments of ginseng modulate PPARalpha action. In addition, these data suggest that ginseng can act as an inhibitor of PPARalpha function, which may have therapeutic implications.

Figure 1

Dose–response effects of (a) GE and (b) GS on ACOX mRNA levels by Wy14,643. Adult male mice received intraperitoneal injections of GE or GS at the indicated doses for 10 days followed by a diet containing 0.1% (w w⁻¹) Wy14,643 for 1 day. Chow diet-fed control mice (CON) were administered saline. At the end of the experiment, samples were collected and analyzed as described under ‘Methods'. The mean±s.d. for three animals is shown and all values are expressed as relative density units using β-actin as reference. Insets show representative autoradiograms of Northern blots used for quantitation. #Significantly different versus control, P<0.001. ^*Significantly different versus Wy14,643 diet-only, P<0.05. ^**Significantly different versus Wy14,643 diet-only, P<0.005.

Figure 2

Time-dependent effects of (a) GE and (b) GS on ACOX mRNA levels by Wy14,643. Adult male mice received intraperitoneal injections of GE or GS at the indicated doses for 10 days followed by a diet containing 0.1% (w w⁻¹) Wy14,643 for 1 day. Chow diet-fed control mice (CON) were administered saline. At the end of the experiment, samples were collected and analyzed as described under ‘Methods'. All values are expressed as the mean±s.d. of three animals and are expressed as relative density units using β-actin as reference. #Significantly different versus control, P<0.001. ^*Significantly different versus Wy14,643 diet-only, P<0.05. ^**Significantly different versus Wy14,643 diet-only, P<0.005.

Figure 3

Inhibition of Wy14,643-induced mRNA levels of PPARα target genes by GE and GS. Mice received intraperitoneal injections of either saline (control), GE (5 mg), and GS (0.5 mg) for 10 days, or a 0.1% Wy14,643 diet for 1 day. For cotreatment studies, mice were treated with either GE (5 mg) for 10 days followed by a 0.1% Wy14,643 diet for 1 day, or GS (0.5 mg) for 10 days followed by a 0.1% Wy14,643 diet for 1 day. At the end of the experiment, samples were collected and analyzed as described under ‘Methods'. PPARα, peroxisome proliferator-activated receptor α; RXRα, retinoid X receptor α; ACOX, acyl-CoA oxidase; HD, enoyl-CoA hydratase/3-hydroxyacyl-CoA dehydrogenase; thiolase, 3-ketoacyl-CoA thiolase; CYP4A1, cytochrome P450 4A1; CYP4A3, cytochrome P450 4A3; VLCAD, very-long-chain acyl-CoA dehydrogenase; LCAD, long-chain acyl-CoA dehydrogenase.

Figure 4

Dose-dependent inhibitions of PPARα reporter gene expression by (a) GE and (b) GS. NMu2Li cells were transiently transfected with expression plasmids for PPARα, a luciferase reporter gene construct containing three copies of the PPRE from the rat ACOX gene and β-galactosidase gene. Cells were treated with various concentrations of GE and GS at the initial time of culture. Following incubation for 24 h, cells were harvested, lysed, and subsequently assayed for luciferase and β-galactosidase activities. All values are expressed as the mean±s.d. of relative luciferase units/β-galactosidase activity. Experiments were performed at least three times. ^*Significantly different versus vehicle, P<0.05.

Figure 5

Inhibition of Wy14,643-induced PPARα reporter gene expression by GE and GS. NMu2Li cells were transiently transfected with pSG5-mPPARα, reporter plasmid PPRE-TK-Luc, β-galactosidase gene, and pBSK. Cells were treated with 10 μM Wy-14,643 (Wy) alone or concomitantly treated with Wy and GE or Wy and GS. After incubation for 24 h, cells were harvested, lysed and subsequently assayed for luciferase and β-galactosidase activities. All values are expressed as the mean±s.d. of relative luciferase units/β-galactosidase activity. Experiments were performed at least three times. #Significantly different versus vehicle, P<0.01. ^*Significantly different versus Wy, P<0.05. ^**Significantly different versus Wy, P<0.01.

Figure 6

Changes in circulating (a) total cholesterol, (b) triglycerides, and (c) HDL cholesterol by GE and GS. Mice received intraperitoneal injections of either saline (control), GE (5 mg), and GS (0.5 mg) for 10 days, or a 0.1% Wy14,643 (Wy) diet for 1 day. For cotreatment studies, mice were treated with either GE (5 mg) for 10 days followed by a 0.1% Wy diet for 1 day, or GS (0.5 mg) for 10 days followed by a 0.1% Wy diet for 1 day. Serum concentrations of total cholesterol, triglycerides, and HDL cholesterol were measured, and all values are expressed as the mean±s.d. of four animals. ^*Significantly different versus control, P<0.05.