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

Xin-Yan Pei, Yun Dai, Leena E. Youssefian, Shuang Chen, Wesley W. Bodie, Yukie Takabatake, Jessica Felthousen, Jorge A. Almenara, Lora B. Kramer, Paul Dent and Steven Grant


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.