Modulation of cell growth and apoptosis response in human prostate cancer cells supplemented with tocotrienols

Abstract

Previous studies have shown that tocotrienols are powerful growth inhibitors and potent inducers of apoptosis in human breast cancer cells. The objective of the current study was to examine effects of tocotrienols on apoptotic signals in androgen-independent PC-3 human prostate cancer cells. We investigated the effects of the tocotrienol-rich fraction (TRF) from palm oil, α-tocopherol (αT), α-tocotrienol (αT₃), γ-tocotrienol (γT₃) and δ-tocotrienol (δT₃) on PC-3 cell growth. TRF inhibited PC-3 growth with a nonlinear response, with complete growth suppression at 10 µg/mL. δT₃ and γT₃ showed complete cell inhibition at 8 µg/mL whilst αT had no effect. δT₃ and γT₃ showed the most promise in the cell growth assays, and all subsequent experiments were performed with δT₃, TRF and αT. TRF and δT₃ at 8 µg/mL induced apoptosis in PC-3 cells after 48 h of treatment. In addition, TRF and δT₃ treatments were able to affect the cell cycle, with accumulation in the S phase, G2 phase block and increases in SubG1 by 72 h. We then proceeded to investigate the expression levels of Fas receptor and Fas ligand, caspase 8, caspase 3 and bax in PC-3 cells following treatment with tocotrienols using real-time PCR and Western blot methods. TRF and δT₃ at 8 µg/mL increased Fas ligand expression levels by 368 and 456%, respectively, after 24 h and Fas receptor expression levels by 210% and 356%, respectively, after 48 h. TRF- and δT₃-treated PC-3 cells overexpressed caspase 8 and bax protein after 24 h, and caspase 3 after 48 h. In conclusion, tocotrienols are able to induce apoptosis and cell cycle arrest in PC-3 cells, with increased expression of Fas receptor, Fas ligand, caspase 8, caspase 3 and bax, suggesting a potential role in chemoprevention of prostate cancer.