Mutually exclusive antiproliferative effect of cell line‐specific HOX inhibition in epithelial ovarian cancer cell lines: SKOV‐3 vs RMUG‐S

We aimed to discover cell line‐specific overexpressed HOX genes responsible for chemoresistance and to identify the mechanisms behind HOX‐induced cell line‐specific chemoresistance in EOC. Ten HOX genes and eight EOC cell lines were tested for any cell line‐specific overexpression that presents a mutually exclusive pattern. Cell viability was evaluated after treatment with cisplatin and/or siRNA for cell line‐specific overexpressed HOX genes. Immunohistochemical (IHC) staining for HOXB9 was performed in 84 human EOC tissues. HOXA10 and HOXB9 were identified as cell line‐specific overexpressed HOX genes for SKOV‐3 and RMUG‐S, respectively. Inhibiting the expression of cell line‐specific HOX genes, but not of other HOX genes, significantly decreased cell viability. In SKOV‐3 cells, cell viability decreased to 46.5% after initial 10 µM cisplatin treatment; however, there was no further decrease upon additional treatment with HOXA10 siRNA. In contrast, cell viability did not significantly decrease upon cisplatin treatment in RMUG‐S cells, but decreased to 65.5% after additional treatment with HOXB9 siRNA. In both cell lines, inhibiting cell line‐specific HOX expression enhanced apoptosis but suppressed the expression of epithelial‐mesenchymal transition (EMT) markers such as vimentin, MMP9, and Oct4. IHC analysis showed that platinum‐resistant cancer tissues more frequently had high HOXB9 expression than platinum‐sensitive cancer tissues. HOXB9, which is overexpressed in RMUG‐S but not in SKOV‐3 cells, appeared to be associated with cell line‐specific platinum resistance in RMUG‐S. Inhibiting HOXB9 overexpression in RMUG‐S cells may effectively eliminate platinum‐resistant ovarian cancer cells by facilitating apoptosis and inhibiting EMT.