MicroRNAs (miRNAs) constitute a class of evolutionarily conserved small non-coding RNAs which regulates gene expression at the post-transcriptional level. They were at first assigned to target complementary sequences in the 3′UTR of mRNAs, only requiring a continuous base-pairing of miRNA nucleotides 2 to 8, known as the seed sequence to subsequently direct mRNAs for translational inhibition and decay [
1]. Later, recognition sites located outside 3′UTR have been observed by computational tools and validated by functional approaches [
2,
3], rendering these small RNAs to be involved in the regulation of virtually all cellular processes [
4,
5]. The global action of miRNAs is to repress gene expression and, by this way, they are involved in important cellular processes including DNA damage response (DDR) [
6,
7]. Ionizing radiation such as X-rays can harm cells by direct DNA breakage or indirectly through the creation of free radicals which will contribute to increase and prolong DNA damage [
8]. Increasing evidence indicates that such injurious effects are not linear with the radiation dose mainly due to different cellular mechanisms to adapt or die, which are highly dependent on cell type and cell environmental conditions [
8]. Hence, irradiated cells may activate DNA repair system or apoptosis. Initial cell responses to genotoxic stress occur through molecular sensors, usually kinases that trigger DDRs. For example, the ATM-mediated DDR activates the tumor suppressor protein p53, a transcription factor critical for genomic stability, regulating cell cycle progression and DNA repair, as well as apoptosis. In this same sense, it is expected that miRNAs are involved in mechanisms such those regulated by p53 [
9,
10]. Apoptosis and DNA repair are intricately connected with DNA damage and cancer. Irradiation leads to a massive change in miRNA expression pattern [
11,
12], unfortunately the roles of specific miRNAs in radiation response are not yet clearly identified. We decided to concentrate on three miRNAs, miR-34a, let-7a and miR-21 since they are consistently associated with the modulation of cell damage response pathways [
13]. MiR-34a is a direct p53 target gene and its ectopic expression induces apoptosis, cell-cycle arrest in G1 or senescence [
14].
In vitro experiments using
C elegans and breast cancer cells as models showed that loss of function mutations in miR-34a gene generated an abnormal cellular survival response to radiation [
15]. Validated miR-34a targets include several genes involved in DDR as Bcl-2, Notch1, Cyclin D1, Cyclin E2, CDK4, MET and SIRT1 [
16‐
18], suggesting that miR-34a may serve as a marker of radiation injury and as a therapeutic target [
14,
19]. Let-7a is a member of a family which comprises 12 miRNAs with tumor suppressor activities that can be regulated in response to ionizing radiation. Among let-7a targets there are molecules involved in such important cellular activities as proliferation (K-ras; c-myc; E2F2) and cell cycle control (Cdc25a; Cyclin D1). Let-7a is usually down-regulated after ionizing radiation exposure, however its overexpression can increase radiosensitivity
in vivo and in different tumor types mainly by downregulation of K-Ras [
20,
21]. Finally, miR-21, classified as an oncogenic miRNA, was described as a negative regulator of some suppressor genes related to proliferation, apoptosis and invasion such as PTEN, PDCD4, Tropomyosin-1 and Bcl-2 [
22‐
24]. MiR-21 is often up-regulated in tumors and its overexpression is associated with a more proliferative and aggressive phenotype [
25].
In vivo and
in vitro studies suggest a role for miR-21 in tumor initiation and progression and as a possible diagnostic and prognostic marker for human malignancies. In breast cancer, miR-21 knockdown cells can trigger apoptotic cell death followed by a decrease in cell proliferation suggesting a function as anti-apoptotic factor [
26]. MiR-21 is usually up-regulated after irradiation and its inactivation can contribute to radiation induced apoptosis [
27,
28]. Several miRNAs with aberrant expression are present ubiquitously in breast and other cancers. Microarray analysis shows a global change in miRNA expression in the presence of genotoxic agents including ionizing radiation [
29]. To test the hypothesis that miR-34a is involved in the DDR after X-ray irradiation of breast cells, we determined relative expression of miR-34a, let-7a and miR-21, in the non-cancerous breast cell line MCF-10A and the breast cancer cell lines MCF-7 and T47-D, 4 and 24 hours after X-ray exposure at a high dose (5 Gy). We have also applied X-ray irradiation doses rate and energy equivalent to those utilized in mammographic exams, usually 10 mGy/s for 28 kV [
30] in breast cells MCF-10A and MCF-7. Our results show an overexpression of miR-34a in the non-cancerous MCF-10A cells in response to DNA damage caused by low-doses of X-ray radiation.