Multidrug resistance (MDR) in cancer cells is a phenotype whereby cells display reduced sensitivity to anticancer drugs, based on a variety of mechanisms, including an increase in drug efflux, the reduction of drug uptake, the activation of cell growth and survival signaling, the promotion of DNA repair, and the inhibition of apoptosis signaling. Increased expression of the plasma membrane drug efflux pumps, the ATP-binding cassette (ABC) transporters, is involved in MDR. P-Glycoprotein/ABCB1 is a member of the ABC transporter family, and facilitates the efflux of various anticancer drugs, including anthracyclines,vincaalkaloids, epipodophyllotoxins, taxanes, and kinase inhibitors, from cells. P-Glycoprotein is also expressed in normal tissues and cells, including the kidney, liver, colon, and adrenal gland, to transport and/or secrete substrates and at the blood-brain, blood-placenta, and blood-testis barriers to protect these tissues from toxic substances. To understand the mechanistic functions of P-glycoprotein and to overcome MDR, investigators have identified the substrates and competitive inhibitors of P-glycoprotein. Recently, we and other groups reported associations between cellular signaling pathways and the expression, stability, degradation, localization, and activity of P-glycoprotein. The present review summarizes the currently available information about the transcriptional and posttranslational regulation of P-glycoprotein expression and function.

Expression of P‐glycoprotein (P‐gp)/ABCB1 on cancer cell surfaces is a critical determinant of anticancer drug resistance. Regulators of P‐gp expression and function are key molecules controlling drug resistance. Here we report the mechanism underlying the ubiquitin–proteasome pathway‐mediated degradation of P‐gp. The proteasome inhibitor MG132 increased the P‐gp level, enhanced its ubiquitination, and delayed the disappearance of the ubiquitinated P‐gp. To search for regulators of P‐gp ubiquitination, MALDI–time of flight mass spectrometry analyses were carried out, and 22 candidates were identified as P‐gp binding partners. Among them, FBXO15/Fbx15 is known as an F‐box protein in the ubiquitin E3 ligase complex, Skp1‐Cullin1‐FBXO15 (SCF^Fbx15); therefore, we further studied the involvement of FBXO15 on P‐gp degradation. Coprecipitation assays revealed that FBXO15 bound to P‐gp. We screened ubiquitin‐conjugating enzyme E2s that bind to FBXO15 and P‐gp; Ube2r1/Cdc34/Ubc3 was found to be a binding partner. Exogenous FBXO15 expression enhanced P‐gp ubiquitination, but FBXO15 knockdown suppressed it. FBXO15 knockdown increased P‐gp expression without affecting its mRNA level. Ube2r1 knockdown decreased P‐gp ubiquitination, and simultaneous knockdown of Ube2r1 with FBXO15 further suppressed the ubiquitination. Ube2r1 knockdown increased P‐gp expression, suggesting that Ube2r1 is a partner of FBXO15 in P‐gp ubiquitination. FBXO15 knockdown enhanced vincristine resistance and lowered intracellular levels of rhodamine 123. These data suggest that FBXO15 and Ube2r1 regulate P‐gp expression through the ubiquitin–proteasome pathway.

Epstein–Barr virus (EBV) transforms human B lymphocytes into immortalized cells in vitro and is associated with various malignancies in vivo. EBNA1, which is expressed in the majority of EBV‐infected cells, recognizes specific DNA sequences at the cis‐acting latent origin of plasmid replication (oriP) element of the EBV genome. EBNA1 plays a critical role in the viral episome maintenance and transactivates viral transforming genes in latently infected cells. Therefore, DNA‐targeting agents that can disrupt the EBNA1–oriP interaction will offer novel functional inhibitors of EBNA1. Pyrrole–imidazole polyamides, sequence‐specific DNA ligands, can be designed to interfere with the binding of various transcriptional factors. Here, we synthesized pyrrole–imidazole polyamides targeting EBNA1‐bound DNA sequences and developed an inhibitor for the EBNA1–oriP interaction. A pyrrole‐imidazole polyamide, designated as DSE‐3, bound adjacent to the EBNA1 recognition sequences located in the dyad symmetry element of oriP, and selectively inhibited EBNA1–oriP binding both in vitro and in vivo. DSE‐3 also inhibited the proliferation of established lymphoblastoid cell lines by eradicating EBV episomes from the cells. In addition, DSE‐3 repressed the expression of viral transforming genes after infecting human peripheral blood mononuclear cells with EBV and, as a consequence, inhibited EBV‐mediated B‐cell immortalization. These results suggest that EBNA1 functions will be an attractive pharmacological target for EBV‐associated diseases. (Cancer Sci 2011; 102: 2221–2230)

Background: The ATP-binding cassette (ABC) transporters P-glycoprotein (P-gp)/ABCB1 and breast cancer resistance protein (BCRP)/ABCG2 are involved in the intestinal absorption and renal excretion of various substrate drugs. Their activities affect sub-therapeutic drug concentrations and excretion of natural transporter substrates. The new oral anti-HCV drug telaprevir has dramatically improved the efficacy of hepatitis-C virus (HCV) treatment, and recent studies have suggested a possible pharmacological interaction between telaprevir and P-gp. We studied the kinetics ofin vitrointeractions between telaprevir and P-gp and BCRP to understand the molecular basis of that interaction.Findings: The effect of telaprevir on P-gp- and BCRP-mediated transport was evaluated by anin vitrovesicle transporter assay using different transport substrates, and the kinetics of transporter inhibition was determined. The results showed that telaprevir could inhibit P-gp- and BCRP-mediated transport in thein vitrovesicle transport assay, with each IC₅₀values of ≈ 7 μmol/L and ≈ 30 μmol/L, respectively. Analyses of Lineweaver–Burk plots showed that telaprevir was likely to be a competitive inhibitor against P-gp and BCRP. Photoaffinity labeling experiments were employed to observe competitive inhibition by telaprevir using iodoarylazidoprazosin (IAAP) as a binding substrate for P-gp and BCRP. These experiments revealed that telaprevir inhibited [¹²⁵I]-IAAP-binding with P-gp and BCRP.Conclusion: Telaprevir competitively inhibited P-gp and BCRP, and P-gp-mediated transport was more sensitive to telaprevir compared with BCRP-mediated transport. These data suggest that telaprevir represses the transporter functions of P-gp and BCRPviadirect inhibition.