A Study On The Roles Of MiR-17Family Members In Carcinogenesis

miR-17family, which is also known as miR-106b family, is made up of6family members, they are:miR-17-5p, miR-20a (mature miRNAs arising from the miR-17-92cluster located at13q31.3), miR-106b, miR-93(from the miR-106b-25cluster at7q22.1), miR-106a,and miR-20b (from the miR-106a-363cluster at Xq26.2). They not only share the similar sequence and structure, but also own the identical "seed region"(the most important sequence of miRNA for target selection). And moreover, the three clusters which they locate are paralogs to each other; studies reveal that they have played essential roles during normal development of the heart, lungs and immune system. Furthermore, expression of miR-17family members in different kinds of tumors has attracted more attention due to their oncogenic potential. They could not only promote cell proliferation, but also suppress cancer cell apoptosis and induce tumor angiogenesis by binding to the3'untranslated regions of different target messenger RNAs.miR-106a is located at human chromosome Xq26.2, processed from miR-106a-363cluster. Although this cluster is undetectable or found at trace level in many tissues and cell types, altered expression of miR-106a has been identified in many kinds of malignancies, especially gastrointestinal tumors; as an indicator, it could be detected not only in solid tumors, but also in plasma and stool. Nevertheless, the underlying mechanism of ectopically overexpressed miR-106a has scarcely been explored.In order to illuminate the potential role of miR-106a in gastric carcinogenesis, we examined miR-106a expression in50pairs of gastric cancer tissues and their matched non-tumor adjacent tissues by quantitative Real-time PCR at the beginning. Obvious upregulated miR-106a expression in gastric cancers compared with their non-tumor counterparts urged us to look for the underlying mechanism. Suppression of miR-106a significantly inhibited gastric cancer cell proliferation and triggered apoptosis. Bioinformatic analysis combining with validation experiments identified FAS as a direct target of miR-106a. Rescue experiments and examination of Caspase-8, PARP and Caspase-3further approved that miR-106a could inhibit gastric cancer cell apoptosis through interfering with FAS-mediated apoptotic pathway. Intratumoral delivery of antagomir-106a to xenografts formed by BGC823cells in nude mice could not only attenuate tumorigenesis but also elevate FAS expression. Moreover, a significant inverse correlation was found between miR-106a and FAS expression not only in gastric cancer cell lines but also in gastric cancer specimens. Taken together, these findings suggest that ectopically overexpressed miR-106a may play an oncogenic role in gastric carcinogenesis and impair extrinsic apoptotic pathway through targeting FAS.As the study develops in depth, more and more target genes of miR-17family have been uncovered; meanwhile, its upstream regulatory mechanism has drawn much attention, as well. Except somatic gene lesions (such as chromosomal amplification) and epigenetic lesions of miR-17-92cluster and its paralogs, the aberrant activities of transcription factors that control their expression are also involved. So, could these ectopically expressed transcription factors strengthen their regulatory function through activating miRNA?Previous studies have shown that the expression of miR-17family members has close relationship with the expression of transcription factor c-Myc. Another study has reported p21is a direct target of miR-17family and their silencing of p21contributes to tumor cell proliferation in part. Since c-Myc is a potent transcriptional repressor of p21, these findings suggest that c-Myc may negatively modulate p21expression through an additional pathway except transcriptional suppression. In the study presented here, we compared p21mRNA in the nucleus and cytoplasm of c-Myc stable transfectants with its control, which indicated further repression of p21by c-Myc at the post-transcriptional level. Stem-loop and conventional Real-time PCR showed elevated expression of some members in miR-17family and their primary transcripts when c-Myc was stably overexpressed. To further investigate the relationship of c-Myc, miR-17family and p21, we antagonized the expression of each miR-17family member by transfection of their antisense oligonucleotides respectively in transfectants constitutive ly expressed c-Myc or not, and found that the restoration of p21expression by treatment above was much stronger in the presence of c-Myc. These results suggest c-Myc could further repress p21expression at the post-transcriptional level through some, but not all, members of miR-17family.