MELK and Del-1 as druggable targets in TNBC.

Abstract

e15100 Background: In our previous study, the researchers found that Del-1 was a promising predictive marker for breast cancer. However, the Del-1 downstream targets and biological effectors remain unknown and undruggable in breast cancer. We used RNA-seq to analyze the transcriptome and investigate the mechanism of Del-1 in regulating the course of breast cancer to uncover a druggable target. Methods: To evaluate the expression levels of Del-1 and MELK, mRNA levels in several breast cancer cell lines were analyzed. Due to the heterogeneity of TNBC, eight different TNBC cell lines were analyzed (BT20, TNBC; MDA-MB-468, basal-like 1; HCC-1806, basal-like 2; DU4475, Immunomodulatory; BT549, mesenchymal-like; MDA-MB-231, mesenchymal stem-like; HS578T, mesenchymal stem-like; MDA-MB-453, TNBC Luminal androgen receptor). Total RNA was isolated using RNAiso Plus (TaKaRa, Otsu, Shiga, Japan). Libraries were prepared from total RNA using the NEBNext Ultra II Directional RNA-Seq Kit. High-throughput sequencing was performed as paired-end 100 sequencing using NovaSeq 6000 (Illumina, Inc., USA). OTS167 was treated for inhibition of MELK. The effects of MELK on cell proliferation and invasion were determined using MTT and Matrigel Transwell assays. To further investigate the relationship between DEL-1 and MELK, the researchers performed dual inhibition of DEL-1 & MELK. Results: Del-1 and MELK expression were significantly increased in TNBC cell lines compared to luminal or HER 2 positive breast cancer cell lines. Expression of Del-1 and MELK was dramatically enhanced in MB 468, HCC-1806, and MB 231 TNBC cell lines. Three TNBC cell lines were transfected with Del-1-specific siRNA to examine the effect of Del-1 downregulation on breast cancer. Del-1 knockdown HCC-1806 & MDA-MB 231 cells significantly decreased MELK expression, suggesting the possible relation between Del-1 and MELK. OTS 167, a MELK inhibitor, significantly inhibits breast cell proliferation and promotes cell apoptosis (MBA-MB-468, p < 0.001). Furthermore, when the OTS 167 was treated, the cell viability decreased as the dose of the MELK inhibitor increased, and the degree was the greatest in MBA-MB 468, basal-like 1 TNBC cell line. To further investigate the relationship between DEL-1 and MELK, the researchers inhibited DEL-1 and then MELK in MTT assay. Dual inhibition significantly reduced cell viability in MDA-MB468 and MDA-MB 231 cell lines, with the basal-like 1 cell lines showing the most significant effect. Conclusions: In this study, we sought to investigate the association between De1-1 and MELK in TNBC cell lines and tumor tissue. We found that MELK is downstream of del-1 and is a promising target, especially in the basal-like 1 subtype. Further research is warranted.