Cytokinetically quiescent (G0/G1) human multiple myeloma cells are susceptible to simultaneous inhibition of Chk1 and MEK1/2

Abstract

Effects of Chk1 and MEK1/2 inhibition were investigated in cytokinetically quiescent multiple myeloma (MM) and primary CD138+ cells. Coexposure 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 G 0/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 (eg, Taxol and VP-16) but significantly increased susceptibility to Chk1 ± MEK1/2 inhibitors or Chk1 shRNA knock-down. These events were associated with increased γH2A.X expression/foci formation and Bim upregulation, whereas Bim shRNA knockdown markedly attenuated lethality. Immunofluorescent analysis of G 0/G1-enriched or primary MM cells demonstrated colocalization of activated caspase-3 and the quiescent (G0) marker statin, a nuclear envelope protein. Finally, Chk1/MEK1/2 inhibition increased cell death in the Hoechst-positive (Hst+), low pyroninY(PY)-staining (2N Hst+/PY-) G0 population and in sorted small side-population (SSP) MM cells. These findings provide evidence that cytokinetically quiescentMMcells are highly susceptible to simultaneous Chk1 and MEK1/2 inhibition. © 2011 by The American Society of Hematology.