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¹ Department of Pharmacology and University of Pittsburgh Cancer Institute, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania and ² Roswell Park Cancer Institute, Buffalo, New York

Requests for Reprints: Shivendra V. Singh, Hillman Cancer Center Research Pavilion, Suite 2.32A, University of Pittsburgh Cancer Institute, 5117 Center Avenue, Pittsburgh, PA 15213. Phone: (412) 623-3263; Fax: (412) 623-7828. E-mail: singhs{at}msx.upmc.edu

Phenethyl isothiocyanate (PEITC), a constituent of many cruciferous vegetables, offers significant protection against cancer in animals induced by a variety of carcinogens. The present study demonstrates that PEITC suppresses proliferation of PC-3 cells in a dose-dependent manner by causing G₂-M-phase cell cycle arrest and apoptosis. Interestingly, phenyl isothiocyanate (PITC), which is a structural analogue of PEITC but lacks the -CH₂ spacers that link the aromatic ring to the -N=C=S group, neither inhibited PC-3 cell viability nor caused cell cycle arrest or apoptosis. These results indicated that even a subtle change in isothiocyanate (ITC) structure could have a significant impact on its biological activity. The PEITC-induced cell cycle arrest was associated with a >80% reduction in the protein levels of cyclin-dependent kinase 1 (Cdk1) and cell division cycle 25C (Cdc25C; 24 h after treatment with 10 µM PEITC), which led to an accumulation of Tyr¹⁵ phosphorylated (inactive) Cdk1. On the other hand, PITC treatment neither reduced protein levels of Cdk1 or Cdc25C nor affected Cdk1 phosphorylation. The PEITC-induced decline in Cdk1 and Cdc25C protein levels and cell cycle arrest were significantly blocked on pretreatment of PC-3 cells with proteasome inhibitor lactacystin. A 24 h exposure of PC-3 cells to 10 µM PEITC, but not PITC, resulted in about 56% and 44% decrease in the levels of antiapoptotic proteins Bcl-2 and Bcl-X_L, respectively. However, ectopic expression of Bcl-2 failed to alter sensitivity of PC-3 cells to growth inhibition or apoptosis induction by PEITC. Treatment of cells with PEITC, but not PITC, also resulted in cleavage of procaspase-3, procaspase-9, and procaspase-8. Moreover, the PEITC-induced apoptosis was significantly attenuated in the presence of general caspase inhibitor and specific inhibitors of caspase-8 and caspase-9. In conclusion, our data indicate that PEITC-induced cell cycle arrest in PC-3 cells is likely due to proteasome-mediated degradation of Cdc25C and Cdk1, and ectopic expression of Bcl-2 fails to confer resistance to PEITC-induced apoptosis. Furthermore, the results of the present study point toward involvement of both caspase-8- and caspase-9-mediated pathways in apoptosis induction by PEITC.

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Received 12/19/03; revised 2/ 6/04; accepted 2/13/04.