Drug resistance is a serious challenge in
cancer treatment and can be acquired through multiple mechanisms. These molecular changes may introduce varied extents of resistance to different
therapies and need to be characterized for optimal
therapy choice. A recently discovered small molecule, ethyl-2-amino-6-(3,5-dimethoxyphenyl)-4-(2-ethoxy-2-oxoethyl)-4H-
chromene-3-carboxylate) (
CXL017), reveals selective cytotoxicity toward
drug-resistant
leukemia. A
drug-resistant
acute myeloid leukemia cell line, HL60/MX2, also failed to acquire resistance to
CXL017 upon chronic exposure and regained sensitivity toward standard
therapies. In this study, we investigated the mechanisms responsible for HL60/MX2 cells' drug resistance and the molecular basis for its resensitization. Results show that the HL60/MX2 cell line has an elevated level of Mcl-1
protein relative to the parental cell line, HL60, and its resensitized cell line, HL60/MX2/
CXL017, whereas it has a reduced level of topoisomerase IIβ. Mcl-1 overexpression in HL60/MX2 cells is mainly regulated through phospho-extracellular signal-regulated
protein kinases 1 and 2-mediated Mcl-1 stabilization, whereas the reduction of topoisomerase IIβ in HL60/MX2 cells is controlled through genetic downregulation. Upregulating Mcl-1 introduces multidrug resistance to standard
therapies, whereas its downregulation results in significant cell death. Downregulating topoisomerase IIβ confers resistance specifically to
mitoxantrone, not to other
topoisomerase II inhibitors. Overall, these data suggest that Mcl-1 overexpression is a critical determinant for cross-resistance to standard
therapies, whereas topoisomerase IIβ downregulation is specific to
mitoxantrone resistance.