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.