Ruitang Deng

The expression of antiapoptotic protein survivin is transcriptionally upregulated by DEC1 primarily through multiple sp1 binding sites in the proximal promoter.

Human differentially expressed in chondrocytes (DEC), mouse stimulated with retinoic acid and rat split and hairy related proteins constitute a structurally distinct class of the basic helix-loop-helix proteins. DEC1 is abundantly expressed in tumors and protects against apoptosis induced by serum starvation. In this study, we report that DEC1 antiapoptosis is achieved by inducing survivin, an antiapoptotic protein. In paired tumor–normal tissues, survivin and DEC1 exhibited a paralleled expression pattern. Tetracycline-induced expression of DEC1 in stable lines proportionally increased the expression of survivin. In reporter assays, DEC1 transactivated the survivin promoter but repressed the DEC2 promoter. In contrast to the repression, the activation was delayed and varied depending on serum concentrations and cycle blockers. Studies with reporter mutants located, in the survivin promoter, two Sp1 sites that supported DEC1 transactivation. Electrophoretic mobility shift assay and chromatin immunoprecipitation detected the presence of DEC1 in the survivin promoter. These findings establish that the survivin gene is a transcription target of DEC1, and induction of survivin is at least in part responsible for DEC1 antiapoptosis.

PYRETHROID INSECTICIDES: ISOFORM-DEPENDENT HYDROLYSIS, INDUCTION OF CYTOCHROME P450 3A4 AND EVIDENCE ON THE INVOLVEMENT OF THE PREGNANE X RECEPTOR

Pyrethroids account for more than one-third of the insecticides currently marketed in the world. In mammals, these insecticides undergo extensive metabolism by carboxylesterases and cytochrome P450s (CYPs). In addition, some pyrethroids are found to induce the expression of CYPs. The aim of this study was to determine whether pyrethroids induce carboxylesterases and CYP3A4, and whether the induction is correlated inversely with their hydrolysis. Human liver microsomes were pooled and tested for the hydrolysis of 11 pyrethroids. All pyrethroids were hydrolyzed by the pooled microsomes, but the hydrolytic rates varied by as many as 14 fold. Some pyrethroids such as bioresmethrin were preferably hydrolyzed by carboxylesterase HCE1, whereas others such as bifenthrin preferably by HCE2. In primary human hepatocytes, all pyrethroids except tetramethrin significantly induced CYP3A4. In contrast, insignificant changes were detected on the expression of carboxylesterases. The induction of CYP3A4 was confirmed in multiple cell lines including HepG2, Hop92 and LS180. Overall, the magnitude of the induction was correlated inversely with the rates of hydrolysis, but positively with the activation of the pregnane X receptor (PXR). Transfection of a carboxylesterase markedly decreased the activation of PXR, and the decrease was in agreement with carboxylesterase-based preference for hydrolysis. In addition, human PXR variants as well as rat PXR differed from human PXR (wild-type) in responding to certain pyrethroids (e.g., lambda-cyhalothrin), suggesting that induction of PXR target genes by these pyrethroids varies depending on polymorphic variants and the PXR species identity.

The far and distal enhancers in the CYP3A4 gene co-ordinate the proximal promoter in responding similarly to the pregnane X receptor but differentially to hepatocyte nuclear factor-4α

CYP3A4 (cytochrome P450 3A4) is involved in the metabolism of more than 50% of drugs and other xenobiotics. The expression of CYP3A4 is induced by many structurally dissimilar compounds. The PXR (pregnane X receptor) is recognized as a key regulator for the induction, and the PXR-directed transactivation of the CYP3A4 gene is achieved through a co-ordinated mechanism of the distal module with the proximal promoter. Recently, a far module was found to support constitutive expression of CYP3A4. The far module, like the distal module, is structurally clustered by a PXR response element (F-ER6) and elements recognized by HNF-4alpha (hepatocyte nuclear receptor-4alpha). We hypothesized that the far module supports PXR transactivation of the CYP3A4 gene. Consistent with the hypothesis, fusion of the far module to the proximal promoter of CYP3A4 markedly increased rifampicin-induced reporter activity. The increase was synergistically enhanced when both the far and distal modules were fused to the proximal promoter. The increase, however, was significantly reduced when the F-ER6 was disrupted. Chromatin immunoprecipitation detected the presence of PXR in the far module. Interestingly, HNF-4alpha increased the activity of the distal-proximal fused promoter, but decreased the activity of the far-proximal fused promoter. Given the fact that induction of CYP3A4 represents an important detoxification mechanism, the functional redundancy and synergistic interaction in supporting PXR transactivation suggest that the far and distal modules ensure the induction of CYP3A4 during chemical insults. The difference in responding to HNF-4alpha suggests that the magnitude of the induction is under control through various transcriptional networks.

Bile salt export pump is dysregulated with altered farnesoid X receptor isoform expression in patients with hepatocellular carcinoma

As a canalicular bile acid effluxer, the bile salt export pump (BSEP) plays a vital role in maintaining bile acid homeostasis. BSEP deficiency leads to severe cholestasis and hepatocellular carcinoma (HCC) in young children. Regardless of the etiology, chronic inflammation is the common pathological process for HCC development. Clinical studies have shown that bile acid homeostasis is disrupted in HCC patients with elevated serum bile acid level as a proposed marker for HCC. However, the underlying mechanisms remain largely unknown. In this study, we found that BSEP expression was severely diminished in HCC tissues and markedly reduced in adjacent nontumor tissues. In contrast to mice, human BSEP was regulated by farnesoid X receptor (FXR) in an isoform‐dependent manner. FXR‐α2 exhibited a much more potent activity than FXR‐α1 in transactivating human BSEP in vitro and in vivo. The decreased BSEP expression in HCC was associated with altered relative expression of FXR‐α1 and FXR‐α2. FXR‐α1/FXR‐α2 ratios were significantly increased, with undetectable FXR‐α2 expression in one third of the HCC tumor samples. A similar correlation between BSEP and FXR isoform expression was confirmed in hepatoma Huh7 and HepG2 cells. Further studies showed that intrahepatic proinflammatory cytokines, such as interleukin‐6 (IL‐6) and tumor necrosis factor alpha (TNF‐α), were significantly elevated in HCC tissues. Treatment of Huh7 cells with IL‐6 and TNF‐α resulted in a marked increase in FXR‐α1/FXR‐α2 ratio, concurrent with a significant decrease in BSEP expression. Conclusion: BSEP expression is severely diminished in HCC patients associated with alteration of FXR isoform expression induced by inflammation. Restoration of BSEP expression through suppressing inflammation in the liver may reestablish bile acid homeostasis. (HEPATOLOGY 2013)

Differential Modulation of Farnesoid X Receptor Signaling Pathway by the Thiazolidinediones

Thiazolidinediones (TZD), including troglitazone, rosiglitazone, and pioglitazone, are agonists of peroxisome proliferator-activated receptor (PPAR)-γ and belong to a class of insulin-sensitizing drugs for type 2 diabetes mellitus. However, member-specific, PPARγ-independent activities and toxicity have been reported, especially for troglitazone. Currently, the underlying mechanisms are not fully understood. In this study, we demonstrated that troglitazone but not rosiglitazone or pioglitazone modulated expression of farnesoid X receptor (FXR) target genes bile salt export pump (BSEP) and small heterodimer partner (SHP) in Huh-7 cells. More specifically, troglitazone acted as a partial agonist of FXR to weakly increase BSEP and SHP expression but functioned as a potent antagonist to significantly suppress bile acid-induced expression. Consistent with the finding, troglitazone partially induced but markedly antagonized bile acid-mediated BSEP promoter transactivation. However, such modulating effects were not detected with rosiglitazone or pioglitazone. Using the crystal structure of ligand-bound FXR ligand binding domain (LBD), molecular docking predicted that troglitazone, but not rosiglitazone or pioglitazone, could form a stable complex with FXR LBD. The specific α-tocopherol side chain of troglitazone significantly contributed to the formation of such a stable complex through extensive interactions with FXR LBD. The docking model was further validated by functional analyses of a series of docking-guided FXR mutants. In summary, the data demonstrated that troglitazone, but not rosiglitazone or pioglitazone, was an FXR modulator and potently antagonized bile acid-induced expression of FXR target genes. Such differential modulation of FXR signaling pathway by TZDs may represent one of the mechanisms for member-specific, PPARγ-independent activities and toxicity.

Transcriptional dynamics of bile salt export pump during pregnancy: mechanisms and implications in intrahepatic cholestasis of pregnancy.

Bile salt export pump (BSEP) is responsible for biliary secretion of bile acids, a rate‐limiting step in the enterohepatic circulation of bile acids and transactivated by nuclear receptor farnesoid X receptor (FXR). Intrahepatic cholestasis of pregnancy (ICP) is the most prevalent disorder among diseases unique to pregnancy and primarily occurs in the third trimester of pregnancy, with a hallmark of elevated serum bile acids. Currently, the transcriptional regulation of BSEP during pregnancy and its underlying mechanisms and involvement in ICP are not fully understood. In this study the dynamics of BSEP transcription in vivo in the same group of pregnant mice before, during, and after gestation were established with an in vivo imaging system (IVIS). BSEP transcription was markedly repressed in the later stages of pregnancy and immediately recovered after parturition, resembling the clinical course of ICP in human. The transcriptional dynamics of BSEP was inversely correlated with serum 17β‐estradiol (E2) levels before, during, and after gestation. Further studies showed that E2 repressed BSEP expression in human primary hepatocytes, Huh 7 cells, and in vivo in mice. Such transrepression of BSEP by E2 in vitro and in vivo required estrogen receptor α (ERα). Mechanistic studies with chromatin immunoprecipitation (ChIP), protein coimmunoprecipitation (Co‐IP), and bimolecular fluorescence complementation (BiFC) assays demonstrated that ERα directly interacted with FXR in living cells and in vivo in mice. Conclusion: BSEP expression was repressed by E2 in the late stages of pregnancy through a nonclassical E2/ERα transrepressive pathway, directly interacting with FXR. E2‐mediated repression of BSEP expression represents an etiological contributing factor to ICP and therapies targeting the ERα/FXR interaction may be developed for prevention and treatment of ICP. (Hepatology 2014;60:1992–2006)

The Hypolipidemic Agent Guggulsterone Regulates the Expression of Human Bile Salt Export Pump: Dominance of Transactivation over Farsenoid X Receptor-Mediated Antagonism

Conversion of cholesterol to bile acids in the liver is initiated by the rate-limiting enzyme cholesterol 7α-hydroxylase (CYP7A1) and excretion of bile acids from the liver is mediated by the bile salt export pump (BSEP). The expression of CYP7A1 and BSEP is coordinately regulated by a negative feedback and positive feed-forward mechanism, respectively, through bile acid-mediated activation of farsenoid X receptor (FXR). It is well established that hypolipidemic agent guggulsterone is an FXR antagonist and down-regulates FXR target genes. In this study, however, we have demonstrated that guggulsterone synergistically induced the expression of BSEP in cells treated with FXR agonist bile acids. A dissection study located in the BSEP promoter an activating protein (AP)-1 site supporting the action of guggulsterone. Deletion or mutation of the AP-1 element was diminished, whereas insertion of the AP-1 element into a heterologous promoter enhanced activation of the promoter by guggulsterone. Selective c-Jun N-terminal kinase and extracellular signal-regulated kinase inhibitors markedly decreased the transactivation, suggesting an involvement of AP-1 activation pathway in the up-regulation of BSEP by guggulsterone. Consistent with its FXR antagonism, guggulsterone antagonized bile acid-mediated transactivation of BSEP promoter when the AP-1 element was disrupted. In conclusion, guggulsterone regulates BSEP expression through composite mechanisms, and the transactivation through the AP-1 element is dominant over the FXR-mediated antagonism. The up-regulation of BSEP expression by guggulsterone without activating FXR pathway as an FXR agonist to suppress CYP7A1 expression represents a possible mechanism for guggulsterone-mediated hypolipidemic effect.

Food and food supplements with hypocholesterolemic effects.

Hypercholesterolemia is a predominant risk factor for atherosclerosis and associated coronary and cerebrovascular diseases. Control of cholesterol levels through therapeutic drugs, notably statins, have significantly reduced the risk for developing atherosclerosis and associated cardiovascular diseases. However, adverse effects associated with therapeutic drugs warrant to find other alternative approaches for managing hypercholesterolemia, especially for those with borderline cholesterol levels. Food supplements have increasingly become attractive alternatives to prevent or treat hypercholesterolemia and reduce the risk for cardiovascular diseases. This review summarized current patents on food supplements with claims of hypocholesterolemic effects. They can be mainly divided into four categories based on the active ingredients in the supplements: 1) plant sterols or stanols; 2) fiber or polysaccharides; 3) microorganism-derived; and 4) soy protein and phytoestrogens. The efficacy, mechanisms of action and potential side effects are reviewed for each of the four categories. The hypocholesterolemic effects of plant sterols, fiber, Monascus products and soy protein preparations have been consistently demonstrated in clinical trails whereas the efficacy of some probiotic bacteria and phytoestrogens-containing supplements remains to be established. Accumulative clinical data show that plant sterols, fiber, soy protein and phytoestrogen are generally considered safe and cause no obvious side effects. However, additional clinical studies are required to establish the safety profiles of certain probiotic bacteria as food supplements.

Scoparone potentiates transactivation of the bile salt export pump gene and this effect is enhanced by cytochrome P450 metabolism but abolished by a PKC inhibitor

BACKGROUND AND PURPOSE Hyperbilirubinaemia and cholestasis are two major forms of liver abnormality. The Chinese herb Yin Chin has been used for thousands of years to treat liver dysfunctions. In mice, this herb and its principal ingredient scoparone were found to accelerate the clearance of bilirubin accompanied by the induction of uridine diphosphate‐5′‐glucuronosyltransferase‐1A1 (UGT1A1), a bilirubin processing enzyme. The aim of this study was to determine whether scoparone induces the expression of human UGT1A1. In addition, the expression of the bile salt export pump (BSEP), a transporter of bile acids, was determined.

Hypolipidemic agent Z-guggulsterone: metabolism interplays with induction of carboxylesterase and bile salt export pump.

Z-Guggulsterone is a major ingredient in the Indian traditional hypolipidemic remedy guggul. A study in mice has established that its hypolipidemic effect involves the farnesoid X receptor (FXR), presumably by acting as an antagonist of this receptor. It is generally assumed that the antagonism leads to induction of cytochrome P450 7A1 (CYP7A1), the rate-limiting enzyme converting free cholesterol to bile acids. In this study, we tested whether Z-guggulsterone indeed induces human CYP7A1. In addition, the expression of cholesteryl ester hydrolase CES1 and bile salt export pump (BSEP) was monitored. Contrary to the general assumption, Z-guggulsterone did not induce CYP7A1. Instead, this phytosterol significantly induced CES1 and BSEP through transactivation. Z-Guggulsterone underwent metabolism by CYP3A4, and the metabolites greatly increased the induction potency on BSEP but not on CES1. BSEP induction favors cholesterol elimination, whereas CES1 involves both elimination and retention (probably when excessively induced). Interestingly, clinical trials reported the hypolipidemic response rates from 18% to 80% and showed that higher dosages actually increased VLDL cholesterol. Our findings predict that better hypolipidemic outcomes likely occur in individuals who have a relatively higher capacity of metabolizing Z-guggulsterone with moderate CES1 induction, a scenario possibly achieved by lowering the dosing regimens.