Genuine functions of P-glycoprotein (ABCB1).

P-glycoprotein (P-gp, ABCB1, MDR1) was recognized as a drug-exporting protein from cancer cells three decade ago. Apart from the multidrug transporter side effects of P-gp, normal physiological functions of P-gp have been reported. P-gp could be responsible for translocating platelet-activating factor (PAF) across the plasma membrane and PAF inhibited drug transport mediated by P-gp in cancer cells. P-gp regulated the translocation of sphingomyelin (SM) and GlcCer, and short chain C(6)-NBD-GlcCer was found in the apical medium of P-gp cells exclusively and not in the basolateral membrane. SM plays an important role in the esterification of cholesterol. High expression of P-gp prevents stem-cell differentiation, leading to the proliferation and amplification of this cell repertoire, and functional P-gp plays a fundamental role in regulating programmed cell death, apoptosis. The transporter function of P-gp is therefore necessary to protect cells from death. P-gp can translocate both C(6)-NBD-PC and C(6)-NBD-PE across the apical membrane. This PC translocation was also confirmed with [(3)H]choline radioactivity. Progesterone is not transported by P-gp, but blocks P-gp-mediated efflux of other drugs and P-gp can mediate the transport of a variety of steroids. Cells transfected with human P-gp esterified more cholesterol. P-gp might also be involved in the transport of cytokines, particularly IL-1beta, IL-2, IL-4 and IFNgamma, out of activated normal lymphocytes into the surrounding medium. P-gp expression is also associated with a volume-activated chloride channel, thus P-gp is bifunctional with both transport and channel regulators. We also present information about P-gp polymorphism and new structural concepts, "gate" and "twist", of the P-gp structure.

Induction of human UGT1A1 by bilirubin through AhR dependent pathway.

UDP-glucuronosyltransferase1A1 (UGT1A1) plays a key role to conjugate bilirubin and preventing jaundice, but there is no report showing the induction of human UGT1A1 (UGT1A1) by bilirubin. In this report, we show findings of the induction of the reporter gene (-3475/+14) of UGT1A1 in HepG2 cells by bilirubin at 50 microM, 100 microM, with human aryl hydrocarbon receptor (hAhR). We confirmed that induction of the reporter gene by bilirubin is dependent on the position of the xenobiotic responsive element (XRE) (-3328/-3319) of UGT1A1, because the XRE deletion UGT1A1 gene did not respond to stimulation by a complex of bilirubin and hAhR. alpha-Naphthoflavone (alpha-NF) of a typical AhR antagonist at 50 microM inhibited induction by bilirubin, suggesting that bilirubin stimulates through binding with hAhR. Meanwhile, bilirubin itself did not stimulate the induction of AhR, because we detected no-elevation of the mRNA level of AhR by RT-PCR. These results indicate that the induction of UGT1A1 by bilirubin-AhR did not depend on the elevation of AhR but on ligand binding. From this result, we considered that high bilirubin in neonates must induce the elevation of UGT1A1 after birth to prevent jaundice, and bilirubin in adults also regulates the level of UGT1A1. This is the first report showing direct induction of UGT1A1 by a bilirubin through AhR pathway.

Induction of human UGT1A1 by a complex of dexamethasone-GR dependent on proximal site and independent of PBREM.

UDP-glucuronosyltransferase 1A1 (UGT1A1) plays a key role to conjugate bilirubin and prevent jaundice. There are two major elements for the induction of UGT1A1, such as PBREM (-3483/-3194), far from the promoter site, and HNF1 (-75/-63), near to the promoter site. In a previous report, we showed that the proximal HNF1 site is essential for the induction of UGT1A1 by glucocorticoid receptor (GR). In this report, we investigated the influence of PBREM on the induction of the UGT1A1 reporter gene by GR and PXR with dexamethasone (DEX). We confirmed that GR was transferred from cytosol into the nucleus in 15-30 min by DEX stimulation, but HNF1 was not. We constructed a reporter gene containing PBREM to compare the induction of the reporter gene without PBREM by DEX-GR. The results show that induction of the reporter gene with PBREM by DEX at 100 muM is the same level as the induction of the reporter gene without PBREM, although PBREM contains GRE. Co-transfection of hGR with the reporter gene did not show any influence of the induction of the reporter gene between the vector with and without PBREM. Meanwhile, by co-transfection of hPXR, the induction of the reporter gene with PBREM was significantly more than the induction of the reporter gene without PBREM at 100 microM DEX. This supports that hPXR induced UGT1A1 through PBREM by DEX. These results showed that PBREM has no relation with the induction by DEX-GR but the proximal site of UGT1A1 may function in stimulation by DEX-GR.