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Peroxisome proliferator-activated receptor alpha (PPARalpha)-mediated regulation of multidrug resistance 2 (Mdr2)
Tineke Kok1, Vincent W Bloks, Henk Wolters
1Groningen University Institute for Drug Exploration, Center for Liver, Digestive and Metabolic Diseases, Laboratory of Pediatrics, University Hospital Groningen, The Netherlands. T.Kok@med.rug.nl
Abstract:
Peroxisome proliferator-activated receptor alpha (PPARalpha) is a nuclear receptor that controls expression of genes involved in lipid metabolism and is activated by fatty acids and hypolipidaemic fibrates. Fibrates induce the hepatic expression of murine multidrug resistance 2 ( Mdr2 ), encoding the canalicular phospholipid translocator. The physiological role of PPARalpha in regulation of Mdr2 and other genes involved in bile formation is unknown. We found no differences in hepatic expression of the ATP binding cassette transporter genes Mdr2, Bsep (bile salt export pump), Mdr1a / 1b, Abca1 and Abcg5 / Abcg8 (implicated in cholesterol transport), the bile salt-uptake systems Ntcp (Na(+)-taurocholate co-transporting polypeptide gene) and Oatp1 (organic anion-transporting polypeptide 1 gene) or in bile formation between wild-type and Ppar alpha((-/-)) mice. Upon treatment of wild-type mice with ciprofibrate (0.05%, w/w, in diet for 2 weeks), the expression of Mdr2 (+3-fold), Mdr1a (+6-fold) and Mdr1b (+11-fold) mRNAs was clearly induced, while that of Oatp1 (-5-fold) was reduced. Mdr2 protein levels were increased, whereas Bsep, Ntcp and Oatp1 were drastically decreased. Exposure of cultured wild-type mouse hepatocytes to PPARalpha agonists specifically induced Mdr2 mRNA levels and did not affect expression of Mdr1a / 1b. Altered transporter expression in fibrate-treated wild-type mice was associated with a approximately 400% increase in bile flow: secretion of phospholipids and cholesterol was increased only during high-bile-salt infusions. No fibrate effects were observed in Ppar alpha((-/-)) mice. In conclusion, our results show that basal bile formation is not affected by PPARalpha deficiency in mice. The induction of Mdr2 mRNA and Mdr2 protein levels by fibrates is mediated by PPARalpha, while the induction of Mdr1a / 1b in vivo probably reflects a secondary phenomenon related to chronic PPARalpha activation.
Insights
Peroxisome proliferator-activated receptor alpha (PPARalpha) regulates lipid metabolism. PPARalpha mediates fibrate-induced expression of Mdr2, a key transporter in bile formation, but does not affect basal bile flow in mice.
Area of Science:
- Biochemistry
- Molecular Biology
- Hepatology
Background:
- Peroxisome proliferator-activated receptor alpha (PPARalpha) is a nuclear receptor crucial for lipid metabolism.
- PPARalpha is activated by fatty acids and hypolipidaemic fibrates, influencing gene expression.
- The role of PPARalpha in regulating bile formation, particularly Mdr2 (murine multidrug resistance 2), is not fully understood.
Purpose of the Study:
- To investigate the physiological role of PPARalpha in the regulation of genes involved in bile formation, including Mdr2.
- To determine the effect of PPARalpha deficiency and activation on bile composition and flow.
- To elucidate the mechanism by which fibrates influence hepatic transporter expression and bile secretion.
Main Methods:
- Comparison of hepatic gene expression and bile formation in wild-type and PPARalpha-deficient (Ppar alpha((-/-))) mice.
- Treatment of wild-type mice with the PPARalpha agonist ciprofibrate and analysis of transporter gene and protein expression.
- In vitro studies using cultured wild-type mouse hepatocytes exposed to PPARalpha agonists.
- Assessment of bile flow, phospholipid, and cholesterol secretion under varying conditions.
Main Results:
- No significant differences in basal bile formation or expression of key ATP binding cassette transporter genes were observed between wild-type and Ppar alpha((-/-))) mice.
- Ciprofibrate treatment in wild-type mice significantly induced Mdr2, Mdr1a, and Mdr1b mRNA levels and increased Mdr2 protein, while decreasing Oatp1.
- PPARalpha agonists specifically induced Mdr2 mRNA in cultured hepatocytes, but did not affect Mdr1a/1b expression.
- Fibrate treatment led to a ~400% increase in bile flow in wild-type mice, with increased phospholipid and cholesterol secretion during high bile salt infusions.
- No fibrate effects were observed in Ppar alpha((-/-))) mice, confirming PPARalpha mediation.
Conclusions:
- Basal bile formation in mice is not significantly affected by PPARalpha deficiency.
- PPARalpha is essential for the fibrate-induced upregulation of Mdr2 mRNA and protein levels, playing a key role in bile phospholipid transport.
- The induction of Mdr1a/1b expression by chronic PPARalpha activation in vivo appears to be a secondary effect, not directly mediated by PPARalpha.