Synergistic Anti-Cancer Potential of Phenethyl Isothiocyanate and Curcumin Induces Apoptosis and G2/M Cell Cycle Arrest in HER2-Positive Breast Cancer Cells

HER2 expression is associated with 30% of breast cancer patients with a poor prognosis. Though Trastuzumab is approved for HER2 targeted therapy, its use is limited because of its systemic toxicity and resistance in most patients. This study evaluated the synergistic effects of Phenethyl isothiocyanate (PEITC) and Curcumin (CUR) in HER2 overexpressing SK-BR-3, BT-474, and AU-565 breast cancer cells. The cytotoxic effect of PEITC : CUR against breast cancer cells was evaluated using an MTT assay, and the Loewe additivity model was used to evaluate the synergistic effect. Apoptosis induction and cell cycle arrest over the treatment of PEITC: CUR in breast cancer cells were examined using the flow cytometric annexin-V/Propidium iodide method. Downregulation of HER2-mediated signaling was deduced from protein expression analysis using western-blot. Our results showed that treatment of PEITC : CUR at varying levels of combinations in all three breast cancer cells extensively reduced the survival of the cells with the lowest inhibitory concentrations (IC50). Cytotoxic data revealed that the 3 : 1 ratio of PEITC : CUR was the best among several (1 : 1, 3 : 1, and 1 : 3) combinations, with the maximum cytotoxicity. PEITC : CUR (3 : 1) displayed the lowest combination index (CI) against SK-BR-3, and AU-565 cells indicated its potential synergistic effect. At twice the concentration of its IC50, the 3 : 1 combination elicited 3.5 to 4.5 fold apoptosis in HER2 overexpressing cells, approximately double the effect of the individual drugs alone. In addition, the selected combination induced the G2/M cell cycle arrest in HER2 expressing cells over the treatment. Western blot protein expression analysis revealed that the PEITC : CUR combination suppressed the HER2/PI3K/Akt signaling, eventually connected to various apoptotic biological events. Our results showed the specificity of PEITC: CUR combination in inducing apoptosis and G2/M cell cycle arrest in HER2-expressing tumor cells in-vitro and enhancing the anti-cancer effect. For a subset of breast cancer patients who overexpress HER2, this combination of PEITC and CUR could be a potential treatment option.