Ribozyme and free alkylated base: a dual approach for sensitizing Mex+ cells to the alkylating antineoplastic drug.

N-alkyl-nitrosoureas and alkyl-triazenes are alkylating antineoplastic drugs, the efficacy of which is strongly affected by the level of expression of the DNA-repair enzyme O6-methylguanine-DNA methyltransferase (MGMT). In tumors, MGMT activity reduces the chemotherapeutic potential of alkylating drugs; therefore, efforts have been made to down-regulate the protein. A partial sensitization of Mex+ cells to alkylating drugs has been obtained using either free alkylated bases or oligonucleotides targeted against MGMT mRNA. In the present work, O6-methylguanine and a chemically modified ribozyme, without a cationic liposome as a carrier, were coadministered to CHO47 cells, which express a high level of human MGMT protein. The reduction of MGMT mRNA and protein enhanced the genotoxicity of the alkylating drug mitozolomide. Furthermore, the sensitivity of CHO47 cells is the same as that of CHO5 cells, which lack MGMT protein. These data indicate that a strategy in which both mRNA and protein are degradation targets can be successfully applied to down-regulate the MGMT gene.

Transient transfection of a synthetic hammerhead ribozyme targeted against human MGMT gene to cells in culture potentiates the genotoxicity of the alkylation damage induced by mitozolomide.

Unmodified and chemically modified forms of a synthetic hammerhead ribozyme with the mRNA of methylguanine-DNA methyltransferase (MGMT) gene as substrate were characterized for their in vitro and in vivo activities. The unmodified ribozyme efficiently cleaved in vitro a short synthetic substrate, and it was rapidly degraded in fetal bovine serum (FBS). The introduction of phosphorothioates and the substitution of uridine with thymidine at probable nuclease-sensitive sites slightly increased the nuclease resistance of the ribozyme. Conversely, pyrimidine nucleoside substitution with 2'NH2 and 2'F nucleosides strongly enhanced nuclease resistance. The in vivo activity was determined by measuring the genotoxicity induced by the alkylating drug mitozolomide, the damage of which is repaired by MGMT enzyme. CHO/47 cells, temporarily depleted of the MGMT protein, were first transfected with the various synthetic ribozymes and subsequently treated with mitozolomide. At equivalent concentration of the drug, the induction of sister chromatid exchanges was higher in ribozyme-transfected than in untransfected cells, indicating that the synthetic ribozymes potentiated the genotoxicity of mitozolomide. Moreover, the concomitant occurrence of messenger RNA reduction in ribozyme-transfected cells indicated that the inhibition of MGMT resynthesis was the basis of the enhanced genotoxicity.

Quantitation of in vitro activity of synthetic trans-acting ribozymes using HPLC.

The cleavage activity of synthetic ribozymes needs to be characterized by reliable and rapid methods. A chromatographic method to simultaneously quantitate the amounts of substrate, cleavage fragments and ribozyme is described. The method allows the rapid normalization of analytical data because the sum of the 260-nm peak areas of remaining substrate and obtained fragments is essentially equal to the initial substrate peak area. Moreover, the simultaneous determination of the ribozyme content improves the accuracy of the evaluation of kinetic parameters compared with conventional densitometric methods. Finally, the characterization of two different hammerhead motifs indicated that the method is suitable for a rapid screening of synthetic ribozyme activity.