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Constitutive androstane receptor (CAR) is a transcription factor regulating the expression of several genes related to drug metabolism. CAR expression was elevated in human HepG2 and SW480 cells by serum starvation. From reporter gene assays, mutagenesis, RNA interference, and chromatin immunoprecipitation assays, we identified the serum response element at -142/-139 in the CAR gene transactivated by Elk-1. Whereas treatment with U0126 (ERK inhibitor) enhanced CAR expression, SP600125 (stress-activated protein kinase inhibitor, SAPK) suppressed the phosphorylation of Elk-1 caused by serum-starvation stress and the elevation of CAR mRNA, suggesting that CAR expression may be mediated by phosphorylated Elk-1 via the SAPK signaling pathway.

Constitutive androstane receptor (CAR) is a transcription factor to regulate the expression of several genes related to drug-metabolism. Here, we demonstrate that CAR protein accumulates during G1 in human SW480 and HepG2 cells. After the G1/S phase transition, CAR protein levels decreased, and CAR was hardly detected in cells by the late M phase. CAR expression in both cell lines was suppressed by RNA interference-mediated suppression of CDK4. Depletion of CAR by RNA interference in both cells and by hepatocyte growth factor treatment in HepG2 cells resulted in decreased MDM2 expression that led to p21 upregulation and repression of HepG2 cell growth. Thus, our results demonstrate that CAR expression is an early G1 event regulated by CDK4 that contributes to MDM2 expression; these findings suggest that CAR may influence the expression of genes involved in not only the metabolism of endogenous and exogenous substances but also in the cell proliferation.

Rats that consumed a high-fat and high-sucrose (HF1) diet or a high-fat (HF2) diet developed hepatic steatosis. The alteration in nutritional status affected hepatic cytochrome P450 and UDP-glucuronosyltransferase (UGT) levels. Messenger RNA and protein levels of UGT1A1 and UGT1A6 in the liver but not the jejunum were increased in male rats fed the HF1 diet. These protein levels did not increase in HF2-fed male rats or HF1-fed female rats. In contrast, the CYP1A2 protein level was decreased in the HF1 but not HF2 diet group, whereas CYP2E1 and CYP4A protein levels were elevated in the HF2 but not HF1 diet group. No significant difference in the organic anion transporter polypeptide (Oatp) 1, Oatp2, multidrug resistance-associated protein (Mrp) 2, or Mrp3 protein levels was found between the standard and HF1 diet groups of male rats. Consumption of the HF1 diet affected the in vivo metabolism of acetaminophen (APAP) such that the area under the APAP-glucuronide plasma concentration-time curve was elevated 2.1-fold in male rats but not female rats. In liver cell nuclei of male rats but not female rats, constitutive androstane receptor (CAR) and proliferator-activated receptor alpha (PPARalpha) protein levels were significantly enhanced by intake of the HF1 diet. Additionally, administration of the PPARalpha agonist clofibrate to male rats up-regulated UGT1A1 and UGT1A6 and down-regulated CYP1A2 in the liver. Taken together, these results indicate that nutritional status may gender-specifically influence the expression and activation of CAR and PPARalpha in liver cell nuclei, and this effect appears to be associated with alterations in UGT1A1 and UGT1A6 expression.

Human UDP-glucuronosyltransferase (UGT) 1A1 is the enzyme that detoxifies neurotoxic bilirubin by conjugating it with glucuronic acid. In addition to bilirubin, UGT1A1 conjugates various endogenous and exogenous lipophilic compounds such as estrogens and the active metabolite of the anticancer drug irinotecan SN-38. Thus, activation by specific inducers of the UGT1A1 gene is critical in treating patients with unconjugated hyperbili-rubinemia and in preventing side effects of drug treatment such as SN-38-induced toxicity. This chapter describes the experimental processes used to identify the 290-bp distal enhancer module at -3499/-3210 of the UGT1A1 gene and to characterize its regulation by nuclear receptors: constitutive active/androstane receptor, pregnane X receptor, and glucocorticoid receptor.

Abstract: Epidermal growth factor (EGF) increases claudin-4 expression in Madin–Darby canine kidney (MDCK) cells. Here we examined what regulatory mechanisms are involved in the EGF-induced claudin-4 elevation. EGF transiently increased claudin-4 mRNA at 3h and persistently increased its protein for 24h without affecting claudin-1 expression. EGF increased p-ERK1/2 levels, which were inhibited by U0126, a MEK inhibitor. The exogenous expression of constitutively activated MEK increased claudin-4 expression. These results indicate that the activation of ERK1/2 is involved in the EGF-induced claudin-4 elevation. EGF increased Sp1 expression within 1h, which was inhibited by U0126. In immunocytochemistry, Sp1 was distributed in nucleus in control and the EGF-treated cells. The EGF-induced claudin-4 elevation was inhibited by mithramycin, a Sp1 inhibitor, and Sp1 small interfering RNA. We suggest that EGF activates a MEK/ERK pathway and increases Sp1 expression, resulting in an elevation of claudin-4 expression. [Copyright &y& Elsevier]