Abstract 3977: γ-Tocotrienol and metformin are cytotoxic to prostate cancer cell lines and exhibit synergy

Metformin and γ-tocotrienol (GT3) are promising non-cytotoxic drugs for prostate cancer chemoprevention and as adjunctive treatment options for prostate cancer. Multiple mechanistic pathways may be modulated. Studies suggest that GT3, a member of the vitamin E family, and the antidiabetic agent metformin have antineoplastic properties but their effects in prostate cancer are unclear. We compared the abilities of GT3 and metformin in preventing growth of two prostate cancer cells lines (LNCaP and PC-3) and a control prostate cell line (RWPE-1) by quantifying their effects on three signaling pathways known to be pivotal in prostate carcinogenesis (AKT, MAP Kinase [pERK], and p-c-JUN).Two prostate cancer cell lines, LNCaP (androgen dependent) and PC-3 (androgen independent), and prostate control cell line RWPE-1 were treated with increasing concentrations of GT3 and/or metformin in DMEM, and cytotoxicity determined by MTS cell culture experiments. Synergy was determined using the CompuSyn program. IC50 of LNCaP with GT3 and metformin were 76.6 µM and 100.6 mM, respectively. IC50 of PC-3 with GT3 and metformin were 53.8 µM and 73.5 mM, respectively. We found synergy in LNCaP at 5 µM/mM and 10 µM/mM GT3/metformin. We found synergy in PC-3 at 5 µM/mM GT3/metformin. RWPE-1 were unaffected. We determined the effect of GT3 and metformin on signaling pathways by analyzing β-actin, p-AKT, p-c-JUN, and p-ERK in combination via Western immunoblot. Initial experiments with LNCaP and PC-3 cells showed metformin inhibits the expression of p-c-JUN and p-ERK. GT3 inhibits the expression of p-ERK but not p-c-JUN in both prostate cancer cell lines. P-AKT was not activated by GT3 or metformin. GT3 and metformin inhibit cell growth in both prostate cancer cell lines, greater in combination compared to separately (more apparent in LNCaP compared to PC-3). GT3 and/or metformin caused less growth inhibition and no effects on expression of proteins p-AKT, p-c-JUN or p-ERK in the RWPE-1 prostate control cells. There is synergy with GT3 and metformin in inhibiting prostate cancer cell growth. GT3 inhibits expression of p-ERK, and metformin inhibits expression of both p-c-JUN and p-ERK. Although AKT is an important target in prostate cancer therapy, AKT is not modulated by either metformin and GT3 but modulates p-c-JUN and p-ERK. Cytotoxic effects of GT3 and metformin are independent of androgen sensitivity although androgen-independent PC-3 cell lines are more sensitive. GT3 and metf