Palbociclib triggers apoptosis in bladder cancer cells by Cdk2-induced Rad9-mediated reorganization of the Bak.Bcl-xl complex.

Palbociclib is a Cdk4/6 inhibitor approved for metastatic estrogen receptor-positive breast cancer. The drug is also under clinical evaluation for metastatic urothelial cancer and other solid tumors. Preclinical studies from multiple tumor types suggest that other factors also affect the sensitivity of individual tumors to Cdk4/6 inhibitor. We show here that Cdk2 has an essential role in palbociclib antitumor effect against bladder cancers. We found that palbociclib induced apoptosis instead of cell cycle arrest to exhibit its anticancer activity in T24 cells, as was evidenced by membrane blebbing, caspase-3 activation and AIF release from mitochondria. Cdk2 activation was important to palbociclib-induced apoptotic triggering activity, since depletion of Cdk2 significantly inhibited caspase-3 activation and cell apoptosis. Cdk2 activation caused p-Rad9 translocation to the mitochondria and subsequently interaction with Bcl-xl, leading to conformational activation of Bak and cell apoptosis. The anticancer activity and Cdk2 activation of palbociclib-treated mice were finally validated in a T24 xenograft model. Collectively, these results together demonstrate that palbociclib exerts its anticancer effect in T24 cells mainly through Cdk2 activation. Our findings provide new insights into the molecular interactions and anticancer mechanisms of Cdk4/6 inhibitors.