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.

We studied endogenous substrates for P-glycoprotein (P-gp) in an oxidative reaction mixture of ceramides, phospholipids, sphingolipids, or GM1-gangliosides (GM1-G). Extracts from the reaction mixture of galactocerebrosides (GalCer), sphingomyelin (SM) , lactocerebrosides (LactoCer), and asolectine (AS) with 0.3% hydrogen peroxide exhibited significant ATPase activity of P-gp of 7.6, 7.8, 5.3, and 4.7 nmol/min/mg protein, respectively, at a concentration of 10 microg equivalent/ml, but not GalCer, SM, LactoCer, and AS themselves. Meanwhile, both GM1-G and its oxidized product showed ATPase activity of 3.7 nmol/min/mg protein at a concentration of 0.75 microM. Phosphatidylcholine, phosphatidylethanolamine, phophatidylserine, triglyceride, and cholesterol did not show P-gp activity. When reactive oxygen species, such as hydrogen peroxide, exceed the ability of antioxidant defense systems to remove it from living cells, SM, GalCer, LactoCer, and AS could react with it; therefore, it is possible for these oxidized lipids to play as substrates for P-gp in living cells. This finding should be a milestone to search a new physiological P-gp function.