Effects of ionizing radiation in breast cancer stem cellsmiRNAs as radio-response biomarkers

Abstract

In breast cancer (BC), the presence of cancer stem cells (CSCs) has been related to relapse, metastasis, self-renew and radioresistance. Radiotherapy (RT) is an extended treatment for this tumour, but is not always effective. CSCs are thought to be directly responsible of the relapse in a tumour process after having received RT. Ionizing Radiation (IR) enriches the fraction of cells expressing CSC markers, which also have an enhanced self-renewal capacity and tumorigenicity compared to the tumour bulk. Different tumour types, and CSC markers associated with them, support this hypothesis. It has been shown that CD133 positive cells, mostly associated to brain cancer, are found in a greater proportion after receiving a fractionated radiation both in vitro and in vivo. Cells with high aldehyde dehydrogenase (ALDH1) activity had increased radioresistance, whose inhibition resulted in a sensitization to the same. The mechanisms by which CSCs may be resistant to RT can be framed into four groups: repair mechanisms of DNA damage, redistribution of the cell cycle, cells tumour repopulation, and level of intratumor hypoxia. MicroRNA (miRNA) is a small endogenous non-coding RNA molecule that regulates gene expression in transcriptional and post-transcriptional specific sequences, many of which have been shown to play important roles in a variety of biological processes, like development, differentiation, apoptosis, proliferation, and cell death. It is now clear, that miRNAs contribute to carcinogenesis. miRNA deregulation is involved in initiation and progression of cancer. They modulate the expression of their target genes by either degrading their target mRNA or inhibiting their translation through pairing of miRNA sequences to complementary bases on the target mRNA. They also play a crucial role in the cellular response to ionizing radiation (IR). The main aim of this work was to demonstrate how IR affects the expression of miRNAs related to stemness properties in different molecular BC subtypes. Our results showed that irradiation at 2, 4 and 6 Gy affects the phenotype, functional characteristics, pluripotency genes and in vivo tumorigenic capacity of different molecular subtypes of BC cells (according to ER, PR and HER-2 status) plated in monolayer and suspension as mammospheres (BCSCs). The effect of IR over eight miRNAs (miR-210, miR-10b, miR-182, miR-142, miR-221, miR-21, miR-93, miR-15b) that play an important role in tumorigenesis, stemness and radioresistance showed a variability of expression depending on cell line subpopulation and clinicopathological features of BC patients. Therefore, a future clinical implementation of miRNA signature determination as a liquid biopsy, for personalized and precision RT dosage regimes, could be necessary to improve prognosis, treatments and survival of BC patients due to miRNAs involvement in cancer biology.