Phosphorothioate-capped antisense oligonucleotides to Ras GAP inhibit cell proliferation and trigger apoptosis but fail to downregulate GAP gene expression.

We have studied the effects of an antisense oligonucleotide to Ras GAP in leukaemia cell lines. When terminal phosphorothioate linkages were introduced into this oligonucleotide, it caused major growth inhibition and apoptosis in the chronic myeloid leukaemia (CML) cell line K562, but had little effect on the promyelocytic leukaemia cell line HL60. Neither the expression of Ras GAP mRNA nor p120 GAP protein was downregulated by the antisense oligonucleotide, suggesting a non-antisense mechanism for growth inhibition. The antisense oligonucleotide contained GGC triplets which have previously been reported to inhibit the activity of p210bcr-abl both in vitro and in vivo. However, cellular phosphotyrosine levels were found to be unaffected, suggesting that the activity of p210bcr-abl was normal and that the antisense oligonucleotide may be interacting aptamerically with a different cellular protein. Since K562 is very resistant to apoptotic cell death, the identity of the putative target molecule would be of considerable interest.