Anti-proliferative effects of gamma-tocotrienol are associated with suppression of c-Myc expression in mammary tumour cells

Objectives: Aberrant c-Myc activity plays a central role in cancer transformation. gamma-tocotrienol is a member of the vitamin E family that displays potent anti-cancer activity. Here, studies were conducted to determine the role of c-Myc in mediating anti-proliferative effects of gamma-tocotrienol in mammary cancer cells. Materials and methods: Treatment effects on mouse +SA and human MCF-7 mammary cancer cell proliferation were determined by MTT assay and Ki-67 staining. Protein expression was determined by western blot analysis. Immunofluorescence staining and qRT-PCR were used to characterize cellular c-Myc and MYC levels respectively. Results: Anti-proliferative effects of gamma-tocotrienol were associated with reduction in total c-Myc and phosphorylated-c-Myc-serine 62, and increase in phosphorylated-c-Myc-threonine 58 levels. gamma-tocotrienol also reduced PI3K/Akt/mTOR and Ras/MEK/Erk mitogenic signalling, cyclin D1 and cyclin-dependent kinase 4 levels, and increased p27 levels. However, gamma-tocotrienol had no effect on MYC mRNA levels. gamma-tocotrienol also increased levels of FBW7 (E3 ligase that initiates ubiquitination of c-Myc), but had no effect on serine/threonine phosphatase PP2A or isomerase Pin 1 levels. Combined treatment with GSK3 alpha/b inhibitor LiCl or proteasome inhibitor MG132 blocked gamma-tocotrienol-induced reductions in c-Myc. Conclusions: These findings indicate that anti-proliferative effects of gamma-tocotrienol are associated with reduction in c-Myc that results from increase in GSK-3 alpha/beta-dependent ubiquitination and degradation, rather than from reduction in c-Myc synthesis in +SA and MCF-7 mammary cancer cells.