Abstract 317: Microtubule polymerizing effects of ellagic acid reduce efficacy of taxane chemotherapy

Ellagic acid (EA) is a polyphenolic compound found in berries, pomegranates and other foods, which has cancer preventive and therapeutic properties. EA inhibits proliferation, induces apoptosis, blocks cell cycle progression and inhibits ABC drug transporters. Based on these anti-cancer activities of EA, we hypothesized that EA could enhance the efficacy of taxane chemotherapy on prostate cancer cells and inhibit drug resistance. We used castration-resistant 22Rv1, VCaP, and C4-2 prostate cancer cells and androgen-independent PC-3 cells to study effects of EA on efficacy of the taxane drug cabazitaxel (CBZ). Cells were treated with EA (0.1 μM - 30 μM) and after 72 hr proliferation was measured using the SRB assay. EA inhibited proliferation at levels greater than 10 μM. At 30 μM the greatest inhibition was observed in the most proliferative cell lines (22Rv1: 66.7%, PC-3: 82.7%), with a more modest effect on the slower growing cells (VCaP: 25.2%, C4-2: 31.7%). We next tested if growth inhibitory concentrations of EA could enhance the microtubule stabilizing effects of CBZ. In a cell-free assay, CBZ (1 nM - 1 μM) dose dependently increased tubulin polymerization, and EA (10 μM) increased polymerization by 61.0%. However, we observed an antagonistic response with the combination of CBZ and EA, reducing polymerized tubulin by 13.9% compared to CBZ alone. To confirm these results in cell culture, we treated 22Rv1 cells with EA (10 μM) and CBZ (10 nM) and determined levels of polymerized tubulin by western blotting. We observed a 1.6 fold increase in polymerized tubulin by EA, a 5.5 fold increase by CBZ, which was reduced to a 4.6 fold increase by combination of CBZ with EA. Contrary to our hypothesis, these results suggest that EA may interfere with taxanes, leading to a reduction in drug effectiveness. Additionally, the ability of EA alone to induce polymerization suggests the possibility that the growth inhibitory activity of EA is a consequence of its effects on microtubule dynamics, potentially through an interaction with taxane binding site of tubulin. This is consistent with our observation that EA has the greatest growth inhibitory effect on very proliferative cell lines, since microtubule targeting agents are most effective at killing rapidly dividing cells. To our knowledge, this is the first evidence of microtubule targeting as an anticancer mechanism of EA. We also studied the effects of EA on drug efflux to determine if EA could inhibit development of taxane drug resistance. We treated C4-2 cells with EA (0.01 μM - 30 μM) and measured calcein retention as marker of the activity of the P-glycoprotein (P-gp) pump. There was a dose dependent increase in calcein retention by EA, with a 1.9 fold increase at the highest concentration (30 μM), suggesting an inhibition of P-gp pump activity with increasing amounts of EA. Thus, EA may increase intracellular taxane levels, but at the same time reduce their microtubule-polymerization effects. Citation Format: Jillian N. Eskra, Alaina Dodge, Maarten C. Bosland. Microtubule polymerizing effects of ellagic acid reduce efficacy of taxane chemotherapy. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 317.