The anti-diabetic drug metformin (MET) and the anti-epileptic drug valproic acid (VPA), when used alone, have demonstrated anti-cancer effects in prostate cancer (PCa). We have demonstrated that the combination of MET and VPA (MET+VPA) can induce synergistic apoptosis in the human prostate cancer cell line LNCaP (p53+) but not in PC3 (p53-), and that the response is dependent on functional p53. Here, we aimed to (1) investigate the mechanism of synergistic apoptosis induced by MET+VPA in LNCaP and (2) propose patient subsets for clinical applications of MET+VPA based on the mechanism involved.

LNCaP and PC3 cells were treated with vehicle, 2.5 mM MET, 2.5 mM VAL, or 2.5 mM MET+VAL and proteins extracted from cell fractions. Western blots were used to confirm cytoplasmic and mitochondrial fractions by measuring the presence of β-actin, pyruvate dehydrogenase subunit E1-α, and ATP synthase subunit-α protein, as well as the release of cytochrome c from mitochondria to the cytoplasm. The mechanism of apoptosis via p53 activation is proposed based on our findings and current understanding from literature.

The release of cytochrome c from the mitochondria to the cytoplasm was significantly increased up to 12-fold (p<0.05) in LNCaP, but not in PC3, in response to MET+VPA compared to vehicle, MET alone, and VPA alone. This finding suggests that both MET and VPA are required to induce an intrinsic apoptosis response in LNCaP via p53. The mechanism proposed is that MET can inhibit the activity of mitochondrial complex 1 which alters the AMP/ATP ratio. This change can activate adenosine monophosphate protein kinase which phosphorylates p53 at Ser15 and leads to p53-dependent cell death. As a non-specific histone deacetylation inhibitor, VPA causes protein acetylation including p53 at lysine residues 373 and 382. By combining metformin and VPA, increased p53 protein activation can occur through phosphorylation of p53 at different sites driving p53-dependent cell apoptosis via the intrinsic pathway, as evidenced by release of cytochrome c into the cytoplasm.

Our results suggest that MET+VPA provides a useful treatment for PCa at a clinically localized stage where p53 is present in most patients or advanced stage where p53 has not been mutated. Additionally, MET+VPA is potentially an alternative to androgen deprivation therapy, particularly in tumors with functional p53. A phase I neo-adjuvant clinical trial will start in early 2017 to evaluate the response of high risk localized PCa to MET+VPA (ANZCTR trial ID: ACTRN12616001021460).

Citation Format: Linh N.K. Tran, Ganessan Kichenadasse, Katherine L. Morel, Pamela J. Sykes. The combination of metformin and valproic acid induces synergistic apoptosis of prostate cancer cells via p53 activation and the intrinsic pathway [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1062. doi:10.1158/1538-7445.AM2017-1062

©2017 American Association for Cancer Research.
2017
American Association for Cancer Research.