Chromatin complexes subunit BAP18 promotes triple-negative breast cancer progression through transcriptional activation of oncogene S100A9.

Triple-negative breast cancer (TNBC) is a highly lethal disease due to aggressive clinical phenotype and the lack of validated therapeutic targets. Our recent quantitative proteomic analysis of 90 cases of TNBC tissues and 72 cases of matched adjacent normal tissues revealed that the expression levels of BPTF-associated protein of 18 KDa (BAP18), a component of the MLL1 and NURF chromatin complexes, were upregulated in TNBC tissues relative to normal tissues. However, the biological function and the underlying mechanism of BAP18 in TNBC progression remain unexplored. Here, we report that BAP18 promoted TNBC cell proliferation, migration, and invasion in vitro and xenograft tumor growth and lung colonization in vivo. Mechanistic investigations revealed that S100 calcium-binding protein A9 (S100A9), a member of the S100 protein family that is frequently upregulated in breast tumors and acts as an oncogenic driver in breast cancer progression, was a downstream target gene of BAP18. BAP18 was recruited to histone H3 trimethylation at lysine 4 (H3K4me3)-marked promoter of S100A9 and enhanced its promoter activities. Notably, knockdown of BAP18 by short hairpin RNA in TNBC cells suppressed xenograft tumor growth in mice, the noted effect was partially reverted by re-expression of S100A9 in BAP18-depleted cells. Taken together, these results suggest that BAP18 promotes TNBC progression through, at least in part, transcriptional activation of oncogene S100A9, and represents a potential therapeutic target for TNBC.