STI-571 (Imatinib Mesylate) Enhances the Apoptotic Efficacy of Pyrrolo-1,5-Benzoxazepine-6, a Novel Microtubule-Targeting Agent, in Both STI-571-Sensitive and -Resistant Bcr-Abl-Positive Human Chronic Myeloid Leukemia Cells

Abstract

Interactions between the Bcr-Abl kinase inhibitor STI-571 (imatinib mesylate) and a novel microtubule-targeting agent (MTA), pyrrolo-1,5-benzoxazepine (PBOX)-6, were investigated in STI-571-sensitive and -resistant human chronic myeloid leukemia (CML) cells. Cotreatment of PBOX-6 with STI-571 induced significantly more apoptosis in Bcr-Abl-positive CML cell lines (K562 and LAMA-84) than either drug alone (P Ͻ 0.01). Cell cycle analysis of propidium iodide-stained cells showed that STI-571 significantly reduced PBOX-6-induced G 2 M arrest and polyploid formation with a concomitant increase in apoptosis. Similar results were obtained in K562 CML cells using lead MTAs (paclitaxel and nocodazole) in combination with STI-571.Potentiation of PBOX-6-induced apoptosis by STI-571 was specific to Bcr-Abl-positive leukemia cells with no cytoxic effects observed on normal peripheral blood cells. The combined treatment of STI-571 and PBOX-6 was associated with the down-regulation of Bcr-Abl and repression of proteins involved in Bcr-Abl transformation, namely the antiapoptotic proteins Bcl-x L and Mcl-1. Importantly, PBOX-6/STI-571 combinations were also effective in STI-571-resistant cells. Together, these findings highlight the potential clinical benefits in simultaneously targeting the microtubules and the Bcr-Abl oncoprotein in STI-571-sensitive and -resistant CML cells.

Sphingosine kinase-1 is a downstream regulator of imatinib-induced apoptosis in chronic myeloid leukemia cells

“…The expression of Bcl-2 family membersFupstream regulators of apoptogenic mitochondrial proteins releaseFwas examined. As previously reported in primary CML cells or other Bcr-Abl-overexpressing cell lines, [25][26][27][28] imatinib treatment was associated with downregulation of both cytoprotective Bcl-xL and Mcl-1 but not Bcl-2 and increased expression of the pro-apoptotic Bcl-2 member Bim in LAMA84/Neo cells. Noteworthy, these effects were remarkably inhibited in LAMA84/SphK1 cells (Figure 2f).…”

“…30 Herein, we establish that SphK1 overexpression can interfere with the expression of Bcl-2 family members known to be downstream effectors of imatinib. [25][26][27][28] On the one hand, imatinib-induced Bcl-xL and Mcl-1 downregulation was inhibited by SphK1 overexpression. On the other hand, the pro-apoptotic Bcl-2 member Bim upregulation induced by imatinib was inhibited by SphK1 overexpression.…”

Treatment of Chronic Myeloid Leukaemia: Current Practice and Future Prospects

“…The PBOX compounds also induced apoptosis in primary CML samples including those resistant to imatinib. In addition we have shown that the PBOXs enhance the apoptotic efficacy of imatinib in CML cell lines (Bright et al, 2009;Greene et al, 2007). Furthermore we have demonstrated the in vivo efficacy of a representative pro-apoptotic PBOX compound, PBOX-6, in a CML mouse model of the T315I Bcr-Abl mutant.…”

BCR-ABL Hits at Mitosis; Implications for Chromosomal Instability, Aneuploidy and Therapeutic Strategy

“…There were indications that CML cells could have a dysfunctional mitotic checkpoint, as their resistance to spindle poisons was reported previously. In the K562 and Lama-84 CML cell lines, microtubule disruption caused either by paclitaxel, nocodazole or novel microtubule-targeting agent PBOX-6 led to polyploidization without the presence of significant apoptosis (Greene et al, 2007). Imatinib treatment minimized the formation of polyploid cells and enhanced the apoptotic index upon treatment of CML cells with spindle poisons.…”