Fig. 4.

Gossypol enhances cisplatin sensitivity through inhibition of NRF2/G6PD axis in cisplatin resistant HNSCC cells.

(A) The box plot of G6PD expression in different cancer cell lines. The expression levels of G6PD in cisplatin-resistant (left plot) and cisplatin-sensitive cancer cells (right plot) were retrieved from the Oncomine database. (B) Dose-dependent effect of gossypol on protein levels of NRF2, G6PD and NQO1. After treatment with 0.625, 1.25, 2.5, and 5 μM of gossypol for 24 h, the expression levels of NRF2, G6PD and NQO1 proteins were detected by Western blot analysis. β-Actin was used as the internal control. (C) The IC₅₀ values of cisplatin in parental HONE-1 cell and the two resistant sub-lines, cis6 and cis15. The Resistance Index was calculated by dividing the IC₅₀ value of cisplatin in resistant sub-line by the IC₅₀ value of cisplatin in parental HONE-1 cell. (D) Total and phospho-NRF2 levels in parental and cisplatin-resistant HONE-1 cells. β-Actin was used as the internal control. (E ~ G) Cytotoxicity effects of gossypol combined with cisplatin in HONE-1-derieved cells. The cells were co-treated with non-toxic concentration of gossypol (0.5 μM) with cisplatin for 72 h, and cell viability was determined by methylene blue assays (E: parental HONE-1 cells; F: cisplatin-resistant cis6 cells; G: cisplatin-resistant cis15 cells).