Effects of Chk1 and MEK1/2 inhibition were investigated in cytokinetically quiescent multiple myeloma (MM) and primary CD138+ cells. Co-exposure to the Chk1 and MEK1/2 inhibitors AZD7762 and selumetinib (AZD6244) robustly induced apoptosis in various MM cells and CD138+ primary samples but spared normal CD138- and CD34+ cells. Furthermore, Chk1/MEK1/2 inhibitor treatment of asynchronized cells induced G0/G1 arrest and increased apoptosis in all cell cycle phases, including G0/G1. To determine whether this regimen is active against quiescent G0/G1 MM cells, cells were cultured in low serum medium to enrich the G0/G1 population. G0/G1-enriched cells exhibited diminished sensitivity to conventional agents (e.g., taxol, VP-16) but significantly increased susceptibility to Chk1 ± MEK1/2 inhibitors or Chk1 shRNA knockdown. These events were associated with increased γH2A.X expression/foci formation and Bim up-regulation, whereas Bim shRNA knockdown markedly attenuated lethality. Immunofluorescent analysis of G0/G1-enriched or primary MM cells demonstrated co-localization of activated caspase-3 and the quiescent (G0) marker statin, a nuclear envelope protein. Finally, Chk1/MEK1/2 inhibition increased cell death in the Hst+, low pyronin-staining (2N Hst+/PY-) G0 population and in sorted small side-population (SSP) MM cells. Collectively, these findings provide evidence arguing that cytokinetically quiescent MM cells are highly susceptible to simultaneous Chk1 and MEK1/2 inhibition.
- Submitted February 24, 2011.
- Accepted August 30, 2011.
- Copyright © 2005 American Society of Hematology