Potentiation of Topoisomerase II Inhibitor-Mediated Myeloid Leukemia Cell Kill by Farnesyltransferase Inhibitor L-744,832.

DNA topoisomerases (topo I and II) manage the topological state of cellular DNA during replication, transcription, recombination and chromatin remodelling by introducing temporary single- or double-strand breaks in the DNA. Several clinically important anti-tumor agents inhibit topo I (e.g. camptothecins) or topo II (e.g. anthracyclines like daunorubicin, idarubicin, and anthracenediones like mitoxantrone) religation activity, thus causing increased DNA scission which results in cell death. Anti-topo II drugs are used clinically against a wide range of tumors, including myeloid leukemias. Sensitivity to anti-topo II drugs is influenced by cell cycle, oncogenic signaling (especially RAS signaling), topo IIα levels and activity. Deregulated RAS signaling plays an important role in the molecular pathogenesis of myeloid leukemias, and strategies which target RAS signaling pathways are therapeutically promising. Inhibition of RAS post-translational modification (e.g. by farnesyltransferase inhibitors = FTIs) is one strategy to impede oncogenic RAS function in vivo. FTIs are currently being tested in clinical trials in acute myeloid leukemia (AML). It has been demonstrated that FTIs work synergistically in combination with microtubule-stabilizing drugs such as paclitaxel and inhibit MDR proteins. As many chemotherapeutics used in AML treatment target topo II (e.g. daunorubicin, etoposide, idarubicin, mitoxantrone), potential beneficial effects of FTIs in combination with topo II inhibitors were investigated in our laboratory. In order to assess the effects of FTIs alone and in combination with topo II inhibitors, several myeloid leukemia cell lines were assayed for viability, cell cycle progression, cell cycle-dependent activation of MAP kinase kinase (MEK1/2), expression of topo IIα(at the mRNA and protein levels), topo IIα activity, and induction of apoptosis. Co-treatment of several myeloid leukemia cell lines (K562, Kasumi-1, HL-60, NB-4, MV4–11, THP-1) with FTI L-744,832 and a panel of topo II inhibitors led to synergistic growth inhibition over much of the range of concentrations examined. Furthermore, pre-treatment of NB-4 cells with FTI L-744,832 led to increased induction of apoptosis by daunorubicin and idarubicin. In agreement with previous results, topo IIα expression levels in myeloid leukemia cells were higher in late S to G2/M phases of the cell cycle. In HL-60 cells, G2M arrests induced by idarubicin and daunorubicin were characterized by elevated expression levels of activated, diphosphorylated MEK-1/2 and topo IIα. While FTI L-744,832 did not significantly affect topo IIα mRNA or protein expression, HL-60 cells treated with L-744,832 alone (30μM) or in combination with idarubicin (15 μM and 5 nM, respectively) did exhibit higher levels of topo II activity as determined using a kinetoplast DNA decatenation assay. As topo IIα has been shown to be phosphorylated and activated by MAP kinase, it is interesting to speculate that activation of the MAP kinase pathway in G2/M may lead to an activation of topo IIα. Thus, the synergistic growth inhibition of myeloid leukemia cells by combining FTI L-744,832 and topo II inhibitors may be a consequence of an increased expression and/or activation of topo IIα.