Analysis of microRNA expression in canine mammary cancer stem-like cells indicates epigenetic regulation of transforming growth factor-beta signaling.

Cancer stem cells (CSCs) display both unique self-renewal ability as well as the ability to differentiate into many kinds of cancer cells. They are supposed to be responsible for cancer initiation, recurrence and drug resistance. Despite the fact that a variety of methods are currently employed in order to target CSCs, little is known about the regulation of their phenotype and biology by miRNAs. The aim of our study was to assess miRNA expression in canine mammary cancer stem-like cells (expressing stem cell antigen 1, Sca-1; CD44 and EpCAM) sorted from canine mammary tumour cell lines (CMT-U27, CMT-309 and P114). In order to prove their stem-like phenotype, we conducted a colony formation assay that confirmed their ability to form colonies from a single cell. Profiles of miRNA expression were investigated using Agilent custom-designed microarrays. The results were further validated by real-time rt-PCR analysis of expression of randomly selected miRNAs. Target genes were indicated and analysed using Kioto Encyclopedia of Genes and Genomes (KEGG) and BioCarta databases. The results revealed 24 down-regulated and nine up-regulated miRNAs in cancer stem-like cells compared to differentiated tumour cells. According to KEGG and BioCarta databases, target genes (n=240) of significantly down-regulated miRNAs were involved in transforming growth factor-beta signaling, mitogen-activated protein kinases (MAPK) signaling pathway, anaplastic lymphoma receptor tyrosine kinase (ALK) and peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC1A) pathways. The analysis of single-gene overlapping with different pathways showed that the most important genes were: TGFBR1, TGFBR2, SOS1, CHUK, PDGFRA, SMAD2, MEF2A, MEF2C and MEF2D. All of them are involved in tumor necrosis factor-beta signaling and may indicate its important role in cancer stem cell biology. Increased expression of TGFBR2, SMAD2, MEF2A and MEF2D in canine mammary cancer stem-like cells was further confirmed by real-time-qPCR. The results of our study point at epigenetic differences between cancer stem-like cells and differentiated tumour cells, which may be important not only for veterinary medicine but also for comparative oncology.