Anti‐proliferative effects of γ‐tocotrienol on mammary tumour cells are associated with suppression of cell cycle progression

Abstract

Objectives:  Previous studies have shown that γ‐tocotrienol induces potent anti‐proliferative effects on +SA mammary tumour cells in culture; here, investigations have been conducted to determine its effects on intracellular signalling proteins involved in regulating cell cycle progression.

Materials and methods:  +SA cells were maintained in mitogen‐free defined media containing 0 or 4 μ mγ‐tocotrienol, for 48 h to synchronize cell cycle in G ₀ phase, and then they were exposed to 100 ng/ml EGF to initiate cell cycle progression. Whole cell lysates were collected at various time points from each treatment group and were prepared for Western blot analysis.

Results and conclusions:  Treatment with 4 μ mγ‐tocotrienol significantly inhibited +SA cell proliferation over a 4‐day culture period. Moreover, this treatment resulted in a relatively large reduction in cyclin D1, cyclin dependent kinase (CDK)4, CDK2 and CDK6 levels, between 4 and 24 h after EGF exposure. Tocotrienol treatment also resulted in a relatively large increase in CDK inhibitor (CKI) p27, prior to and after EGF exposure, but had little effect on levels of CKIs, p21 and p15. Tocotrienol treatment also induced a large relative reduction in retinoblastoma (Rb) protein phosphorylation at ser780 and ser807/811. These findings strongly suggest that anti‐proliferative effects of γ‐tocotrienol are associated with reduction in cell cycle progression from G ₁ to S, as evidenced by increased p27 levels, and a corresponding decrease in cyclin D1, CDK2, CDK4, CDK6 and phosphorylated Rb levels.