Reversal Of Breast Cancer Resistance Protein-Mediated Resistance To Vp16 By Tamoxifen In JAR/Vp16 Cell Line

Objective Human breast cancer resistance protein (BCRP) is usuallyexpressed in many normal tissues and a variety of tumor cells.Overexpression of BCRP may cause tumor cells to cross-resist to avariety of chemotherapeutic drugs. The phenomenon of multidrugresistance (MDR) was found on Human chorioepithelioma cell lineJAR/Vp16 whose BCRP was overexpressed. Tamoxifen (TAM), anantiestrogen drug, can be used clinically mainly as endocrine therapeuticdrug in breast cancer with estrogen receptor(ER) and/or progesteronereceptor (PR) positive. This study was designed to investigate the reversalof the breast cancer resistance protein-mediated resistance to Vp16 byTAM in JAR/Vp 16 and the sensitization by TAM in JAR and MCF-7 cellline when JAR, JAR/Vp16 and MCF-7 were exposed to TAM and/orVp16. And the preliminary mechanisms were discussed. MethodsToxicities of tamoxifen, Vp16 and Vp16 combining with TAM in JARand JAR/Vp16 cell lines were detected by MTT. Toxicities of TAM,pharmorubicin (EPI) and tamoxifen plus EPI were also detected by MTT.Then the effects of colony forming units in JAR/Vp16 exposed to TAM,Vp16 and Vp16 combining with TAM by clone formation experimentwere compared further. Intracelluar fluorescence intensity of hochest 33342 and PI were observed by flow cytometry. Expressiones of BCRPmRNA and protein in JAR and JAR/Vp16 cell line were detected byRT-PCR and Western blot, respectively. Results The sensibilities of Vp16in JAR and JAR/Vp16 were different (P＜0.01). In JAR/Vp16, IC₅₀ ofJAR/Vp16 to Vp16 was 71.24-fold times that of JAR. The mutation ofBCRP amino acids located at 482 was not detected in both JAR andJAR/Vp16. The expression of BCRP was obviously up-regulated inJAR/Vp16 in comparision with that in JAR. Effect of Vp16 can beenhanced by tamoxifen both in JAR and in JAR/Vp16, and TAM canreverse the resistance to Vp16 in JAR/Vp16 (P＜0.001). Cell inhibitionratio was significant increased when Vp16 was combined with tamoxifen(P＜0.05). Accumulation of hochest 33342 was increased and excretion ofhochest 33342 was decreased.when JAR and JAR/Vp16 were exposed toTAM 10μM (P＜0.01). Expression of BCRP can be down-regulated bytamoxifen in both mRNA level and protein level (P＜0.05), andup-regulation of BCRP by Vp16 can be blocked by TAM (P＜0.05).Additionally, we also observed that sensibility to pharmorubicin inMCF-7 expressing BCRP could be enhanced by TAM (P＜0.001).Conelusion 1.Overexpression of BCRP may mediate the resistance toVp16 in JAR/Vp16. 2. TAM can increase the antitumor effect of Vp16both on JAR and JAR/Vp16 The sensitization on JAR and reversalresistance on JAR／Vp16 by TAM may be due to inhibit the function of BCRP so that drug-pumping out decreased and intracelluar accumulationincreased. 3. TAM could down-regulate the expresssion of BCRP, whichmay be one of the mechanisms that TAM reverses the resistance to Vp16on JAR/Vp16.4. Besides, TAM mainly enhanced the sensibility to EPI inMCF-7, which espresses BCRP possibly, via inhibiting the function ofBCRP and by down-regulating expression of BCRP. 5. TAM may beclinically used as a chemotherapy sensitizer in tumor that expressesBCRP, and may be used as one of the reversal drugs in the MDRmediated by BCRP.