Mechanisms of Resistance of Hepatocyte Retinoid X Receptor α-Null Mice to WY-14,643-induced Hepatocyte Proliferation and Cholestasis

Abstract

Peroxisome proliferators, such as the lipid-lowering fibrates that function as agonists for peroxisome proliferator-activated receptor ␣ (PPAR␣), induce liver tumors in rodents and may produce cholestasis in humans. Considerable attention has focused on peroxisome proliferator-induced hepatocellular carcinoma, a phenomenon not noted in man, whereas limited studies examine fibrates and other therapeutic drugs that induce cholestasis, a common finding in humans. Moreover, the mechanisms by which fibrates induce hepatocyte proliferation and cholestasis are still not fully understood. We have examined the role of hepatocyte retinoid X receptor ␣ (RXR␣), an essential partner of PPAR␣, in modulating WY-14,643-induced hepatocyte proliferation and cholestasis. WY-14,643 treatment induced hepatomegaly in wild type (WT) mice that was also accompanied by induction of the expression of cyclins D1, D3, A2, and B1 and Cdc2 as well as inhibition of Wee 1. Such changes were either absent or greatly reduced in hepatocyte RXR␣-null mice. Furthermore, neither WY-14,643 treatment nor RXR␣ deficiency affected apoptosis, indicating the importance of PPAR␣/RXR␣ in regulating Wee 1-mediated Cdc2/cyclin B1 expression for cells to enter into mitosis. WY-14,643 treatment also induced cholestasis and liver injury, which is evidenced by induction of alanine aminotransferase, alkaline phosphatase, and hepatic bile acid levels in WT mice. Hepatocyte RXR␣ deficiency protected the mice from WY-14,643-induced liver injury. WY-14,643-mediated induction of the small heterodimer partner, Mrp3, and Cyp3a11 levels was greater in hepatocyte RXR␣-null than in WT mouse livers suggesting enhanced repression of bile acid synthesis and increased efflux of bile acids into blood for renal excretion as well as hydroxylation of bile acids because of hepatocyte RXR␣ deficiency. These data establish a crucial role of hepatocyte RXR␣ in regulating WY-14,643-mediated cell cycle progression as well as bile acid homeostasis.

PPARα-UGT axis activation represses intestinal FXR-FGF15 feedback signalling and exacerbates experimental colitis

“…Some studies reported that activation of PPARα by fibrates could repress the expression of Cyp7a1 (refs 41, 42), which was presumed to be a potential mechanism underlying the side effects of fibrates in causing gall stones in humans. However, other studies indicated that the activation of PPARα by endogenous agonist, such as fatty acid metabolites, could promote Cyp7a1 transcriptional activity43 and that the PPARα-specific agonist Wy-14643 increased the bile acid synthesis in mice44. However, none of the previous studies considered the potential influence of PPARα activation in regulating CYP7A1 via the intestinal FXR-FGF15 signalling.…”

Retinoic Acid-mediated Nuclear Receptor Activation and Hepatocyte Proliferation

“…Thus, PPARα activation not only activates CDC2 by preventing Wee1-mediated CDC2 phosphorylation, but also induces the transcription of the CDC2 gene, leading to G2/M transition. 44 In addition, activation of PPARα-mediated cell cycle gene expressions were either not found or greatly reduced due to hepatocyte RXRα deficiency. These findings suggest the dual role of PPARα in regulating the cell cycle; PPARα regulates G1/S transition as well as Wee1/CDC2/cyclin B1-mediated G2/M transition.…”

“…For example, the activation of PPARα, CAR, PXR, and FXR causes hepatomegaly or liver cancer. 41–44 Partial hepatectomy-induced liver regeneration is impaired in mice lacking these nuclear receptors. Since RA can activate nuclear receptor-mediated pathways, it is likely that mediation via PPARα, PXR, CAR, and FXR, RA can exert its mitogenic effect in the liver.…”

“…This indicates that the hypolipidemic effect of PPARα is dependent on the presence of hepatocyte RXRα. 44,49 …”

Regulation of hepatic ABCC transporters by xenobiotics and in disease states

“…Treatment with phenobarbital, diallyl sulfide, trans-stilbene oxide, and oltipraz induces hepatic Mrp3 mRNA levels in these null mice (Cherrington et al, 2003). WY-14643-mediated Mrp3 induction is greater in Rxrα-knockout hepatocyte than in wild-type mouse livers (Gyamfi and Wan, 2009). …”