No abstract

Among the different breast cancer subtypes, triple‐negative breast cancer (TNBC) is associated with a poor prognosis, low survival rates, and high expression of histone deacetylases. Treatment with histone deacetylase inhibitor trichostatin A (TSA) leads to an increased expression of potential tumor‐suppressive miRNAs. Characterization of these miRNAs can help to find new molecular targets for treatment of TNBC. We identified differentially expressed miRNAs by microarray analyses after treatment with TSA in the TNBC cell lines HCC38, HCC1395, and HCC1935. The gene locus of hsa‐miRNA‐192‐5p (miR‐192) and hsa‐miR‐194‐2 (miR‐194‐2) with its host gene, long noncoding RNA miR‐194‐2HG, has been linked to inhibition of migration in different tumor types. Therefore, we examined tumor‐relevant functional effects using WST‐1‐based proliferation, capsase‐3/7‐based apoptosis, and trans‐well migration assays after transfection with miRNA mimics or specific siRNAs. We demonstrated the tumor‐suppressive capacity of miR‐192 in TNBC cells, which was exerted through inhibition of proliferation, induction of apoptosis, and reduction of migration. Gene expression and bioinformatics analyses of TNBC cell lines transfected with miR‐192 mimics, identified a number of genes involved in migration including the Rho GTPase Activating Protein ARHGAP19. Through RNA immunoprecipitation we demonstrated the direct binding of miR‐192 and ARHGAP19. Downregulation of ARHGAP19 expression by either miR‐192 or siRNA inhibited migration of TNBC cells significantly. Our findings demonstrate that overexpression of epigenetically deregulated miR‐192 decreases proliferation, promotes apoptosis, and inhibits migration of TNBC cell lines.

Chromosomal instability (CIN) can be a driver of tumorigenesis but is also a promising therapeutic target for cancer associated with poor prognosis such as triple negative breast cancer (TNBC). The treatment of TNBC cells with defects in DNA repair genes with poly(ADP-ribose) polymerase inhibitor (PARPi) massively increases CIN, resulting in apoptosis. Here, we identified a previously unknown role of microRNA-449a in CIN. The transfection of TNBC cell lines HCC38, HCC1937 and HCC1395 with microRNA-449a mimics led to induced apoptosis, reduced cell proliferation, and reduced expression of genes in homology directed repair (HDR) in microarray analyses. EME1 was identified as a new target gene by immunoprecipitation and luciferase assays. The reduced expression of EME1 led to an increased frequency of ultrafine bridges, 53BP1 foci, and micronuclei. The induced expression of microRNA-449a elevated CIN beyond tolerable levels and induced apoptosis in TNBC cell lines by two different mechanisms: (I) promoting chromatid mis-segregation by targeting endonuclease EME1 and (II) inhibiting HDR by downregulating key players of the HDR network such as E2F3, BIRC5, BRCA2 and RAD51. The ectopic expression of microRNA-449a enhanced the toxic effect of PARPi in cells with pathogenic germline BRCA1 variants. The newly identified role makes microRNA-449a an interesting therapeutic target for TNBC.