Plumbagin inhibited AKT signaling pathway in HER-2 overexpressed-endocrine resistant breast cancer cells

The important mechanism of endocrine resistance is the crosstalk between estrogen receptor (ER) and HER2 signaling pathways. Aside from ER downregulation, there was an increase in HER2 expression and increased activation of the downstream AKT/ERK pathways in endocrine-resistant cells (MCF-7/LCC2 and MCF-7/LCC9) which is similar to HER2-overexpressed (SKBR3) cells. However, nuclear receptor coactivator 3 (NCOA3), the important ER-coactivator, that upregulated in endocrine-resistant cells did not express in HER2-overexpressed (SKBR3) cells. NCOA3 was able to activate AKT/ERK signalling pathway. Our previous study reported that plumbagin (PLB), a naphthoquinone compound, had a potent cytotoxic activity against endocrine-resistant cells. This study aimed to further investigate the mechanism of anti-cancer effects of PLB on ER and HER-2 signaling. PLB can inhibit estradiol (E₂)-induced cell proliferation in MCF-7 wild-type cells but had no effect in the resistant cells. It also inhibited HER2 expression in both endocrine-resistant and HER-2 overexpressed cells. Therefore, the mechanism of PLB may be regulated through HER-2 signaling. PLB inhibited the phosphorylation of AKT (pAKT) and pERK1/2 and induced apoptosis and reduced the expression of anti-apoptotic genes Bcl-2 and pro-caspase 3 and Cleaved Caspase 3 protein in both endocrine-resistant and HER-2 overexpressed cells. However, the inhibitory effect of PLB was more obvious when pre-treated the cells with AKT inhibitor only in HER-2 overexpressed cells. In addition, the inhibitory effect of PLB on pAKT was attenuated when NCOA3 was downregulated. Our finding suggested that the inhibitory effect of PLB on AKT signaling pathways regulated through NCOA3 in HER2-overexpressed endocrine-resistant cells.