TNBC is a subtype of breast cancer with high heterogeneity, high recurrence, early metastasis and poor prognosis [4]. Due to the lack of hormone receptors and HER-2, endocrine therapy, targeted therapy and chemotherapy often fail to be effective treatments for TNBC [43, 44]. It is urgent to find potential molecular targets to improve the prognosis of TNBC. In our study, we found MKLP1 was upregulated in TNBC and associated with prognosis of TNBC by bioinformatic analysis. Several studies showed MKLP1 plays an oncogenic role in several cancer types [11, 45]. Moreover, our results illustrated MKLP1 was upregulated in TNBC cell lines and tissues. Knockdown of MKLP1 inhibited TNBC proliferation and metastasis while upregulated MKLP1 caused opposite effects. Previous studies have demonstrated Wnt/β-catenin signaling pathway is involved in different types of tumors progression [20, 46], including breast cancer [47, 48], gastric cancer [11], hepatocellular carcinoma [49], lung cancer [50]. And aberrant Wnt signaling pathway was associated with tumor proliferation, metastasis, stemness and drug resistance [23, 24, 51, 52]. It was reported MKLP1 could activate Wnt/β-catenin signaling pathway in gastric and colorectal cancer [11, 45], and then we explored whether MKLP1 affected Wnt/β-catenin signaling pathway in TNBC. Our results showed the related genes of Wnt/β-catenin signaling pathway, such as β-catenin, CyclinD1 and c-myc, were upregulated due to the MKLP1 overexpression, indicating MKLP1 promotes TNBC progression via activating this pathway. In addition, Wnt/β-catenin signaling pathway can promote EMT progression [49], which is associated with tumor progression [23, 50]. We also found knockdown of MKLP1 decreased EMT marker proteins expression, suggesting MKLP1 could promote EMT progression in TNBC.
FOXM1 is a transcription factor and plays a crucial role in different types of cancers, including gastric cancer, lung cancer and prostate cancer [53-56]. In breast cancer, FOXM1 was identified as a specific marker for TNBC and enhanced paclitaxel resistance in TNBC [57, 58]. In the present study, we found FOXM1 might be recruited and bind to the promoter region of MKLP1 via analyzing public CHIP-Seq datasets and FOXM1 expression is positively correlated with MKLP1 expression in TNBC tissues. Inhibition of FOXM1 with siRNA or thiostrepton [59] significantly suppressed TNBC cells proliferation and migration, while overexpression of MKLP1 could partly rescue the effects caused by FOXM1 inhibition. Meanwhile, FOXM1 could promote EMT progression and activate Wnt/β-catenin pathway, which is in accord with previous study [60]. Thus, FOXM1 could accelerate TNBC progression via regulating transcriptional activity of MKLP1.Additionaly, thiostrepton, the selective inhibitor of FOXM1, might be the potential therapy drug for TNBC.
Epigenetics refers to the reversible and heritable changes in gene expression and function without changes in the DNA sequence, including DNA modification, various modifications of histones, and so on [61]. H3K4me3 modification is a common modification of histones, which affects transcription of genes[41]. In the present study, we detected histone modification of FOXM1 by bioinformatics analysis and found H3K4me3 modification might affect FOXM1 expression. Subsequent experiments demonstrated WDR5 upregulated FOXM1 expression via promoting FOXM1 H3K4me3 modification. In addition, the effects caused by inhibition of WDR5 on proliferation and metastasis of TNBC cells could be partly restored by the upregulating FOXM1 expression. Our results illustrated WDR5 increased H3K4me3 modification to upregulate FOXM1 expression. Knockdown of WDR5 or small molecule antagonist of WDR5-MLL complex could significantly inhibit TNBC cells proliferation and metastasis, suggesting OICR-9429 might be used to treat TNBC.