A "multitarget multiribozyme" (MTMR) was constructed. It consists of three trans-acting hammerhead ribozymes directed against the transcripts of the ABC transporters MDR1/P-gp, BCRP, and MRP2; three cis-acting MDR1/P-gp-specific ribozymes; and three MDR1/P-gp-homologous spacer sequences. The trans-acting hammerhead ribozymes are liberated from the MTMR through autocatalytic self-cleavage by the cis-acting ribozymes. The MTMR was characterized with regard to its kinetic parameters. Comparison of the MTMR-specific kinetic values with those of the corresponding monoribozymes demonstrated that MTMR fragments could cleave their specific substrates without loss of efficiency. The MTMR was applied to three cell models, each overexpressing another ABC transporter, i.e., the gastric carcinoma cell line EPG85-257RDB expresses MDR1/P-gp, the cell variant EPG85-257RNOV synthesizes BCRP, and the ovarian carcinoma line A2780RCIS produces MRP2. In all cellular systems, the MTMR could specifically decrease the expression of the respective ABC transporter at the mRNA level (97% decrease in the MDR1/P-gp mRNA, 80% decrease in the BCRP mRNA, 96% decrease in the MRP2 mRNA) and the protein level. Resistance against the selection drug was reversed completely (100% in EPG85-257RDB) or by 94 (EPG85-257RNOV) or 63% (A2780RCIS). Thus, the MTMR technology provides a novel tool for gene therapeutic applications to reverse different ABC-transporter-dependent drug-resistant phenotypes.
The phenomenon of multidrug resistance ( MDR ) in human cancers is one of the major causes of failure of chemotherapy. A recently identified new member of the superfamily of ATP -binding cassette transporters, breast cancer resistance protein ( BCRP ), was demonstrated to confer an atypical multidrug -resistant phenotype to tumor cells. To overcome the BCRP -mediated drug resistance, a specific anti -BCRP hammerhead ribozyme was introduced into the human gastric carcinoma cell line, EPG85 -257RNOV, exhibiting an atypical MDR phenotype. By this approach, the expression levels of the targeted BCRP -encoding mRNA and the BCRP transport protein were decreased to the low constitutive expression level that was observed in highly drug -sensitive parental gastric carcinoma cells. In addition, in the anti -BCRP ribozyme -treated cells, the cellular drug accumulation was dramatically increased to the level measured in drug -sensitive cells. These effects were accompanied by an extensive reversal of the drug -resistant phenotype of more than 80%. Because additional mechanisms contribute to the multimodal -mediated MDR phenotype exhibited by this gastric carcinoma cell line, the data suggest that the BCRP -mediated contingent to the drug resistance was overcome nearly completely. Moreover, the data indicate that ribozyme -based gene therapy may be clinically applicable in preventing and reversing BCRPmediated atypical MDR.
Breast cancer resistance protein ( BCRP ) is a recently identified new member of the superfamily of ATP -binding cassette transporters. BCRP is a``half transporter'' that may homo -or heterodimerize to form an active transport complex. A considerable overexpression of BCRP was reported from various atypical multidrug -resistant tumor cell lines, in particular from those which were established by treatment with mitoxantrone. Thus, BCRP represents a very interesting candidate molecule for reversal of a drug -resistant phenotype. Six hammerhead ribozymes directed against the BCRP -encoding mRNA were designed and tested for their ability to cleave their target molecule. The anti -BCRP ribozymes were in vitro synthesized using bacteriophage T7 RNA polymerase and oligonucleotide primers whereby one primer contains a T7 RNA polymerase promoter sequence. BCRP -encoding substrate RNA molecules were created by a reverse transcription polymerase chain reaction using total RNA prepared from the atypical multidrug -resistant gastric carcinoma cell line EPG85 -257RNOV exhibiting a high BCRP mRNA expression level. One anti -BCRP ribozyme was found to show a very high endoribonucleolytic cleavage activity at physiologic pH and temperature. This ribozyme was characterized in a cell -free system with regard to its specific kinetic parameters using large target molecules.
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