Abstract 1860: Muscadine grape extract prevents proliferation of trastuzumab-sensitive and -resistant HER2 positive breast cancer cells and has synergistic effects when combined with trastuzumab

Human epidermal growth factor receptor 2 (HER2) positive breast cancer, characterized by overexpression of the HER2 protein, is an aggressive type of breast cancer comprising ~20% of diagnosed cases. Although targeted HER2 drugs, such as trastuzumab (TRZ), have reduced mortality by about one third, many patients still experience primary or acquired resistance. Grape seed/skin extracts are popular dietary supplements due to their potent antioxidant and anti-inflammatory properties. However, extracts from muscadine grapes are not extensively studied, and information regarding their therapeutic effects on cancer is limited. The objective of this study was to determine whether a muscadine grape skin and seed extract (MGE) inhibits the proliferation of TRZ-sensitive and -resistant HER2 positive breast cancer cells, and determine if combination treatment with MGE and TRZ has synergistic effects. Treatment with increasing concentrations of MGE for 48h significantly inhibited proliferation of TRZ-sensitive SKBR3 and -resistant HCC1954 human HER2 overexpressing breast cancer cells in a time and dose-dependent manner. HER2 heterodimerization and phosphorylation results in activation of the protein kinase B (AKT) pathway, which regulates breast cancer proliferation and cell survival. Treatment with 20 μg/mL MGE for 24h reduced HER2 protein expression by 52% (p≤0.01) in SKBR3 and 60% (p≤0.01) in HCC1954 cells. AKT activation was also reduced by 63% (p≤0.001) and 65% (p≤0.0001) in SKBR3 and HCC1954 cell lines, respectively. Active AKT stabilizes s-phase kinase-associated protein 2 (SKP2), ultimately leading to increased ubiquitination and degradation of cell-cycle inhibitor cyclin-dependent kinase inhibitor 1B (p27) and its transcription factor forkhead box protein 1 (FOXO1). Treatment with 20 μg/mL MGE for 24h increased total FOXO1 and p27 levels by 4-fold (p≤0.01) and 2.5-fold (p≤0.05), respectively, in SKBR3 cells. In addition, FOXO1 was increased 5-fold (p≤0.0001) and p27 4-fold (p≤0.05) in HCC1954 cells. Furthermore, SKP2 protein expression was reduced by 66% (p≤0.01) in SKBR3 and 73% (p≤0.01) in HCC1954 cells. MGE, in combination with TRZ, was synergistic at five different dose combinations (determined using Chou and Talalay method of synergy), and significantly inhibited proliferation to a greater extent than either agent alone in TRZ-sensitive SKBR3 cells. In summary, treatment with MGE reduced proliferation, inhibited components of the AKT pro-proliferation/survival pathway, and had synergistic effects when combined with TRZ in HER2 positive breast cancer cells. Thus, MGE may serve as an effective therapeutic either administered singly or in combination with targeted therapies for the treatment of HER2 positive breast cancer. Citation Format: Jessica Mackert, Patricia E. Gallagher, E. Ann Tallant. Muscadine grape extract prevents proliferation of trastuzumab-sensitive and -resistant HER2 positive breast cancer cells and has synergistic effects when combined with trastuzumab [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1860.