Rapid induction of apoptosis during Kinesin-5 inhibitor-induced mitotic arrest in HL60 cells

Small molecule inhibitors of Kinesin-5 (K5Is) that arrest cells in mitosis with monopolar spindles are promising anti-cancer drug candidates. Clinical trials of K5Is revealed dose-limiting neutropenia, or loss of neutrophils, for which the molecular mechanism is unclear. We investigated the effects of a K5I on HL60 cells, a human promyelocytic leukemia cell line that is often used to model dividing neutrophil progenitors in cell culture. We found K5I treatment caused unusually rapid death of HL60 cells exclusively during mitotic arrest. This mitotic death occurred via the intrinsic apoptosis pathway with molecular events that include cytochrome c leakage into the cytoplasm, caspase activation, and Parp1 cleavage. Bcl-2 overexpression protected from death. We probed mitochondrial physiology to find candidate triggers of cytochrome c release, and observed a decrease of membrane potential (ΔΨm) before mitochondrial outer membrane permeabilization (MOMP). Interestingly, this loss of ΔΨm was not blocked by overexpressing Bcl-2, suggesting it might be a cause of Bax/Bak activation, not a consequence. Taken together, these results show that K5I induces intrinsic apoptosis during mitotic arrest in HL60 with loss of ΔΨm as an upstream event of MOMP.

Kinesin motor proteins as targets for cancer therapy

The process of mitosis is a validated point of intervention in cancer therapy and a variety of anti-mitotic drugs are successfully being used in the clinic. To date, all approved antimitotics target the spindle microtubules, thus interfering with spindle dynamics, leading to mitotic arrest and apoptosis. While effective, these drugs are also associated with a variety of side effects, including neurotoxicity. In recent years, mitotic kinesins have attracted significant attention in the search for novel, alternative mitotic drug targets. Due to their specific function in mitosis, targeting these proteins creates an opportunity for the development of more selective antimitotics with an improved side effect profile. In addition, kinesin inhibitors may overcome resistance to microtubule targeting drugs. Drug discovery efforts in this area have initially focused on the plus-end directed kinesin spindle protein (KSP) and a variety of compounds are currently undergoing clinical testing.