Rhinacanthin-C enhances chemosensitivity of breast cancer cells via the downregulation of P-glycoprotein through inhibition of Akt/NF-kappa B signaling pathway

Introduction: High expression of P-glycoprotein (P-gp) has been linked to multidrug resistance (MDR) and chemotherapeutic failure. Previously, we demonstrated that rhinacanthin-C, a naphthoquinone from Rhinacanthus nasutus, was able to enhance the cytotoxicity of doxorubicin against breast cancer MCF-۷ cells via direct P-gp inhibition. In this study, we looked at its effect on P-gp downregulation and the mechanism involved in the resistance of MCF-۷ cells to doxorubicin. Methods: Doxorubicin-resistant MCF-۷ (MCF-۷/DOX) cells were exposed to rhinacanthin-C for ۲۴-۴۸ hours prior to the assessment of their chemosensitivity via MTT assay, P-gp activity via calcein-AM uptake assay, P-gp expression, and signaling via qRT-PCR and western blot analyses. Results: Pretreatment with ۱ µM of rhinacanthin-C for ۴۸ hours significantly enhanced cytotoxicity of doxorubicin, as well as camptothecin and etoposide, to MCF-۷/DOX cells. In the rhinacanthin-C-treated cells, reduction of MDR۱ mRNA and P-gp levels and increased intracellular calcein were observed. Moreover, phosphorylation of Akt, NF-ᴋB and IκB-α, along with YB-۱ expression, significantly decreased after ۲۴-hour treatment with rhinacanthin-C. In contrast, the naphthoquinone had no effect on expression levels of ERK۱/۲ and phosphorylated ERK۱/۲ under similar conditions. Conclusion: Rhinacanthin-C, at a non-cytotoxic concentration (۱ µM), could downregulate P-gp expression in MCF-۷/DOX cells via the inhibition of the Akt/NF-ᴋB signaling pathway and YB-۱ expression. Long-term exposure to this natural naphthoquinone may increase the chemosensitivity of cancer cells with MDR phenotype.