The multidrug resistance transporter P-glycoprotein confers resistance to ferroptosis inducers

Aim:Ferroptosis is a non-apoptotic form of cell death caused by lethal lipid peroxidation.Several small molecule ferroptosis inducers(FINs)have been reported,yet little information is available regarding their interaction with the ATP-binding cassette(ABC)transporters P-glycoprotein(P-gp,ABCB1)and ABCG2.We thus sought to characterize the interactions of FINs with P-gp and ABCG2,which may provide information regarding oral bioavailability and brain penetration and predict drug-drug interactions.Methods:Cytotoxicity assays with ferroptosis-sensitive A673 cells transfected to express P-gp or ABCG2 were used to determine the ability of the transporters to confer resistance to FINs;confirmatory studies were performed in OVCAR8 and NCI/ADR-RES cells.The ability of FINs to inhibit P-gp or ABCG2 was determined using the fluorescent substrates rhodamine 123 or purpuin-18,respectively.Results:P-gp overexpression conferred resistance to FIN56 and the erastin derivatives imidazole ketone erastin and piperazine erastin.P-gp-mediated resistance to imidazole ketone erastin and piperazine erastin was also reversed in UO-31 renal cancer cells by CRISPR-mediated knockout of ABCB1.The FINs ML-162,GPX inhibitor 26a,and PACMA31 at 10µM were able to increase intracellular rhodamine 123 fluorescence over 10-fold in P-gp-expressing MDR-19 cells.GPX inhibitor 26a was able to increase intracellular purpurin-18 fluorescence over 4-fold in ABCG2-expressing R-5 cells.Conclusion:Expression of P-gp may reduce the efficacy of these FINs in cancers that express the transporter and may prevent access to sanctuary sites such as the brain.The ability of some FINs to inhibit P-gp and ABCG2 suggests potential drug-drug interactions.

Genomic stability at the coding regions of the multidrug transporter gene ABCB1: insights into the development of alternative drug resistance mechanisms in human leukemia cells

Aim:Despite considerable efforts to reverse clinical multidrug resistance(MDR),targeting the predominant multidrug transporter ABCB1/P-glycoprotein(P-gp)using small molecule inhibitors has been unsuccessful,possibly due to the emergence of alternative drug resistance mechanisms.However,the non-specific P-gp inhibitor cyclosporine(CsA)showed significant clinical benefits in patients with acute myeloid leukemia(AML),which likely represents the only proof-of-principle clinical trial using several generations of MDR inhibitors.Nevertheless,the mutational mechanisms that may underlie unsuccessful MDR modulation by CsA are not elucidated because of the absence of CsA-relevant cellular models.In this study,our aims were to establish CsA-resistant leukemia models and to examine the presence or absence of ABCB1 exonic mutations in these models as well as in diverse types of human cancer samples including AMLs.Methods:Drug-resistant lines were established by stepwise drug co-selection and characterized by drug sensitivity assay,rhodamine-123 accumulation,[3H]-labeled drug export,ABCB1 cDNA sequencing,and RNase protection assay.The genomic stability of the ABCB1 coding regions was evaluated by exome sequencing analysis of variant allele frequencies in human populations.Moreover,the mutational spectrum of ABCB1 was further assessed in diverse types of cancer samples including AMLs in the Cancer Genome Atlas(TCGA)at the National Cancer Institute.Results:We report the development of two erythroleukemia variants,RVC and RDC,which were derived by stepwise co-selection of K562/R7 drug-resistant leukemia cells with the etoposide-CsA and doxorubicin-CsA drug combinations,respectively.Interestingly,both RVC and RDC cell lines,which retained P-gp expression,showed altered multidrug-resistant phenotypes that were resistant to CsA modulation.Strikingly,no mutations were found in the ABCB1 coding regions in these variant cells even under long-term stringent drug selection.Genomically,ABCB1 displayed relatively low variant allele frequencies in human populations when compared with several ABC superfamily members.Moreover,ABCB1 also exhibited a very low mutational frequency in AMLs compared with all types of human cancer.In addition,we found that CsA played a role in undermining the selection of highly drug-resistant cells via induction of low-level and unstable drug resistance.Conclusion:Our data indicate that ABCB1 coding regions are genomically stable and relatively resistant to drug-induced mutations.Non-ABCB1 mutational mechanisms are responsible for the drug-resistant phenotypes in both RVC and RDC cell lines,which are also prevalent in clinical AML patients.Accordingly,we propose several relevant models that account for the development of alternative drug resistance mechanisms in the absence of ABCB1 mutations.