Involvement of Cdk5/p25 in digoxin-triggered prostate-cancer cell apoptosis

Cardiac digitalis has been considered to be a treatment for breast cancer. Our previous study indicates that digoxin, one member in digitalis, decreases the proliferation of prostate cancer cells, but the mechanisms remain unclear. In the present study, Ca²⁺ proved to be an important factor in digoxin-triggered prostate cancer cell death. Because cyclin-dependent kinase (Cdk)5 and p35 cleavage (p25 formation) have been reported to be targets of intracellular Ca²⁺, and subsequently cor- related to apoptosis, we not only demonstrated first that Cdk5, p35, and p25 proteins were all expressed in prostate cancer cells (including lymph node carcinoma of the prostate (LNCaP) and DU-145 cells), but also showed where p25 formation and Cdk5 kinase activity were affected by treatment with digoxin. The inhibitor of p35 cleavage (calpeptin) was used to reduce p25 formation, and the result suggested that p25 accumulation might be the major cause of digoxin-triggered LNCaP cell death. Butyrolactone-I and roscovitine, two Cdk5 kinase inhibitors, were also found to prevent digoxin-triggered LNCaP cell death. In addition, treatment of siRNA-Cdk5 diminished digoxin-triggered cell death, as compared with the treatments of siRNA-Cdk1 or siRNA-Cdk2, which implies the specific involvement of Cdk5 in digoxin-triggered cell death. Caspase inhibitor set and terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling assay were used to demonstrate that digoxin-triggered LNCaP cell apoptosis through Cdk5 activation. These results suggest that Cdk5/p35 and p25 are novel players in digoxin-triggered prostate cancer cell apoptosis and, therefore, become potential therapeutic targets.