Muscadine Grape Extract Prevents Proliferation of HER2 Positive Breast Cancer Cells in Association with a Decrease in Phosphorylation of AKT and mTOR

Abstract

Breast cancer is the most common cancer among American women and the second leading cause of cancer death. 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 about 20% of diagnosed cases. Although targeted HER2 drugs are successful in treating HER2 overexpressing breast cancer with up to 70% of patients responding to treatment, less than 10% of the patients achieve total remission and many women become resistant, highlighting the need for additional therapies for this disease. Grape seed and/or skin extracts are popular dietary supplements due to their high polyphenol content and potent anti‐oxidant and anti‐inflammatory properties. Recently, numerous studies showed the cancer chemotherapeutic potential of extracts prepared from or active components isolated from a number of different species of grapes. The muscadine grape (Vitis rotundifolia), found in the warm, humid climate of the southeast United States, has a distinct phytochemical composition as compared to other grape varieties. The muscadine grape contains a high concentration of anthocyanin 3,5‐diglucosides, ellagic acid, ellagic acid precursors, gallic acid, flavan‐3‐ols and flavonols. Extracts from muscadine grapes have not been 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 HER2 positive breast cancer cells. Treatment with increasing concentrations of MGE (a proprietary MGE from Piedmont Research & Development Corp.; 10 – 40 μg/mL) significantly inhibited proliferation of human SKBR3 HER2 overexpressing breast cancer cells. The reduction in growth was dependent upon the dose of the MGE with an observed maximal effect at 40 μg of phenolics/mL (58.4% inhibition; n=3, p < 0.0001). In addition, incubation of human SKBR3 cells with 30 μg/mL of MGE attenuated cell proliferation in a time‐dependent manner with a maximal 49.2% reduction after 48 h (n=3; p < 0.0001). Protein kinase B (AKT) and its downstream effector, mechanistic target of rapamycin (mTOR), are implicated in the regulation of breast cancer proliferation and cell survival. Treatment of SKBR3 cells with MGE (20 μg of phenolics/mL) for 16 h significantly reduced phospho‐AKT by approximately 70% (n=4, p ≤ 0.01) and phospho‐mTOR by greater than 50% (n = 5, p ≤ 0.05) compared to untreated SKBR3 cells. These results suggest that MGE attenuates proliferation in HER2 positive breast cancer cells by reducing phospho‐AKT which inhibits phosphorylation of mTOR. Thus, MGE may serve as an effective therapeutic either administered singly or in a combination regimen with targeted drugs for the treatment of HER2 positive breast cancer.