Inhibition of Promyelocytic Leukemia (PML)/Retinoic Acid

The fusion protein promyelocytic leukemia (PML)/retinoic acid receptor (RAR)a is tightly linked to the pathogenesis of acute promyelocytic leukemia (APL); hence, it represents a tumor-associated, transformationrelated molecule. In this study, three anti-PML adamantyl-conjugated peptide nucleic acid (PNA) oligomers previously described as in vitro inhibitors of PML/RARa translation were combined and used to block PML/RARa synthesis in NB4 cells. Cationic liposomes were used to achieve sufficient delivery of PNAs into the cells. Upon treatment of cells with the liposome/PNA mixture, enhanced cellular uptake of PNA (approximately 5-fold compared with control) was obtained. Concomitantly, a substantial reduction (>90%) of the expression of PML/RARa was observed when all of the three PNAs were used together. This resulted in a dramatic effect on the number and viability of NB4 cells in culture after 48 h of treatment. This phenomenon was preceded by induction of apoptosis that could be observed 24 h after treatment. No sign of granulocytic differentiation was observed after treatment. These effects were also noted on other leukemic cell lines that express PML but not the fusion transcript. These results show that it is possible to deliver PNA into hematopoietic cells and obtain specific gene inhibition, and they suggest that a growth inhibitory effect on acute promyelocytic leukemia cells can be obtained through the block of PML/RARa and PML expression.