RESTORATION OF RETINOID SENSITIVITY BY MDR1 RIBOZYMES IN RETINOIC ACID-RESISTANT MYELOID LEUKEMIC-CELLS

Complete remission is achieved in a high proportion of patients with a cute promyelocytic leukemia (APL) after all-trans retinoic acid (RA) t reatment, but most patients relapse and develop RA-resistant APL, We h ave previously reported that both RA-resistant HL-60 (HL-60R) and APL cells express P-glycoprotein and MDR1 transcripts; and these cells dif ferentiate to mature granulocytes after culture with RA and P-glycopro tein antagonist. Ribozymes have been shown to be able to intercept a t arget RNA by catalytic activity. To address the role of MDR1 in overco ming RA-resistance in APL cells, we investigated the biologic effects of ribozymes against the MDR1 transcript in HL-60R cells. These ribozy mes efficiently cleaved MDR1 mRNA at a specific site in vitro. The 196 MDR1 ribozyme was cloned into an expression vector and stably transfe cted (HL-60R/196Rz) cells were obtained, Expression of MDR1 transcript s was decreased in HL-60R/196Rz cells compared with parental HL-60R an d empty vector-transfected (HL-60R/neo) cells, Interestingly, RA inhib ited cellular proliferation and induced differentiation of HL-60R/196R z cells in a dose-dependent manner, suggesting reversal of drug resist ance in HL-GOR cells by the MDR1 ribozyme. These data are direct evide nce that P-glycoprotein/MDR1 is responsible in part for acquired resis tance to RA in myeloid leukemic cells. The MDR1 ribozyme may be a usef ul tool for investigating the biology of retinoid resistance and may h ave therapeutic potential for patients with RA-resistant APL. (C) 1998 by The American Society of Hematology.