Role of the highly structured 5‘-end region of MDR1 mRNA in P-glycoprotein expression
Résumé
Overexpression of P-glycoprotein, encoded by the MDR1 gene, is often responsible for multidrug resistance in acute myeloid leukemia. We have shown previously that MDR1 (P-glycoprotein) mRNA levels in K562 leukemic cells exposed to cytotoxic drugs are up-regulated but P-glycoprotein expression is translationally blocked. Here we show that cytotoxic drugs down-regulate the Akt signaling pathway leading to hypophosphorylation of the translational repressor 4E-BP 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 blocking the phosphorylation of 4E-BP by mTOR using rapamycin or eIF4E siRNA, and relieved by over-expression 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.
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