title: Role of the highly structured 5'-end region of MDR1 mRNA in P-glycoprotein expression.
abstract: Overexpression of P-glycoprotein, encoded by the MDR1 (multidrug resistance 1) gene, is often responsible for multidrug resistance in acute myeloid leukaemia. We have shown previously that MDR1 (P-glycoprotein) mRNA levels in K562 leukaemic cells exposed to cytotoxic drugs are up-regulated but P-glycoprotein expression is translationally blocked. In the present study we show that cytotoxic drugs down-regulate the Akt signalling pathway, leading to hypophosphorylation of the translational repressor 4E-BP [eIF (eukaryotic initiation factor) 4E-binding protein] and decreased eIF4E availability. The 5'-end of MDR1 mRNA adopts a highly-structured fold. Fusion of this structured 5'-region upstream of a reporter gene impeded its efficient translation, specifically under cytotoxic stress, by reducing its competitive ability for the translational machinery. The effect of cytotoxic stress could be mimicked in vivo by blocking the phosphorylation of 4E-BP by mTOR (mammalian target of rapamycin) using rapamycin or eIF4E siRNA (small interfering RNA), and relieved by overexpression of either eIF4E or constitutively-active Akt. Upon drug exposure MDR1 mRNA was up-regulated, apparently stochastically, in a small proportion of cells. Only in these cells could MDR1 mRNA compete successfully for the reduced amounts of eIF4E and translate P-glycoprotein. Consequent drug efflux and restoration of eIF4E availability results in a feed-forward relief from stress-induced translational repression and to the acquisition of drug resistance.
authors.affiliation: MRC Clinical Sciences Centre, Imperial College Faculty of Medicine, Hammersmith Hospital Campus, Du Cane Road, London W12 0NN, UK.
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Randle
authors.forename: Selina
authors.initials: S
authors.lastname: Raguz
authors.forename: Christopher F
authors.initials: CF
authors.lastname: Higgins
authors.forename: Ernesto
authors.initials: E
authors.lastname: Yagüe
mesh_terms: D020121:5' Untranslated Regions / Q000235:genetics* / Q000378:metabolism; D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000235:genetics / Q000378:metabolism*; D000970:Antineoplastic Agents / Q000494:pharmacology*; D015139:Blotting, Southern; D004351:Drug Resistance / Q000235:genetics*; D004791:Enzyme Inhibitors / Q000494:pharmacology; D039561:Eukaryotic Initiation Factor-4E / Q000235:genetics / Q000378:metabolism; D005434:Flow Cytometry; D005786:Gene Expression Regulation / Q000187:drug effects; D006801:Humans; D020014:K562 Cells; D008156:Luciferases / Q000378:metabolism; D010766:Phosphorylation; D016133:Polymerase Chain Reaction; D014176:Protein Biosynthesis; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism; D012333:RNA, Messenger / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism; D014158:Transcription, Genetic
chemical_list: D020121:5' Untranslated Regions; C513055:ABCB1 protein, human; D018435:ATP Binding Cassette Transporter, Subfamily B; D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000970:Antineoplastic Agents; D004791:Enzyme Inhibitors; D039561:Eukaryotic Initiation Factor-4E; D012333:RNA, Messenger; D008156:Luciferases; D051057:Proto-Oncogene Proteins c-akt