Combination
therapies that display
cancer-killing activities through either coexistent targeting of several cellular factors or more efficient suppression of a specific pathway are generally used in
cancer treatment.
Sildenafil, a specific
phosphodiesterase type 5 (
PDE5) inhibitor, has been suggested to display both cardioprotective and neuroprotective activities that provide a rationale for the combination with
vincristine on the treatment against
castration-resistant
prostate cancer (CRPC). In the present work,
vincristine arrested cells in the metaphase stage of mitosis.
Vincristine-induced mitotic arrest was identified by Cdk1 activation (i.e., increased Cdk1Thr161 phosphorylation and decreased Cdk1Tyr15 phosphorylation),
cyclin B1 upregulation, and increased phosphorylation of multiple mitotic
proteins and
stathmin.
Sildenafil synergistically potentiated
vincristine-induced mitotic arrest and a dramatic increase of mitotic index. Furthermore,
sildenafil potentiated
vincristine-induced mitochondrial damage, including Mcl-1 downregulation, Bcl-2 phosphorylation and downregulation, Bak upregulation and loss of mitochondrial membrane potential, and sensitized
caspase-dependent apoptotic cell death.
Sildenafil-mediated synergistic effects were mimicked by other
PDE5 inhibitors including
vardenafil and
tadalafil, and also by PDE5A knockdown in cells, suggesting PDE5-involved mechanism. Notably,
sildenafil amplified
vincristine-induced phosphorylation and cleavage of BUBR1, a
protein kinase in spindle assembly checkpoint (SAC) function and chromosome segregation.
Sildenafil also significantly decreased kinetochore tension during SAC activation. Moreover,
sildenafil synergized with
vincristine on suppressing
tumor growth in an in vivo model. In conclusion, the data suggest that
sildenafil, in a PDE5-dependent manner, potentiates
vincristine-induced mitotic arrest signaling, and sensitizes mitochondria damage-involved apoptosis in CRPC. Both in vitro and in vivo data suggest the combination potential of
PDE5 inhibitors and
vincristine on CRPC treatment.