MicroRNA-145 Inhibits Growth Of Breast Cancer Cell Line MCF7 Through RTKN

MicroRNAs are small, non-coding RNAs that regulate gene expression by targeting the 3'untranslated region (3'UTR) of mRNAs, inducing RNA degradation or interfering with translation. This class of regulatory transcripts has diverse functions, including the regulation of cellular differentiation, proliferation, and apoptosis. Hence, desregulation of miRNA expression may lead to a variety of disorders, including human cancer.Emerging evidences indicate that many microRNAs are dysregulated in human malignancies and it has been proposed that some microRNAs may have oncogenic or tumor suppressor functions. Accordingly, both inactivation and overexpression of specific microRNAs have been described in a number of cancer types. For instance, miR-15a and miR-16 are down-regulated by hemizygous or homozygous deletion or other unknown mechanisms in 68% of chronic lyphomatic leukemia CLLs and miR-17-92 cluster is markedly overexpressed in B-cell lymphomas.Also in a large-scale analysis of 540 tumor samples from lung, breast, stomach, prostate, colon, and pancreatic tumors, a so-called solid cancer microRNA signature was identified. However, although miRNAs have been the subject of extensive research in recent years, the molecular basis of miRNA-mediated gene regulation and the effect of these genes on tumor growth remain largely unknown because of our limited understanding of miRNA target genes.Down-regulation of miR-145 also has been found in several cancers, including colorectal cancer, B-cell lymphoma, cervical cancer, hepatocellular carcinoma, and ovarian cancer, suggesting that it may act as a tumor suppressor. In this study, we found that miR-145 was downregulated in human breast tumor cell line MCF7, consistent with previous reports.Overexpression of miR-145 suppressed MCF-7 cell growth and induced apoptosis in vitro. To understand the mechanisms underlying this phenomenon, we used several computational methods to help identify miR-145 targets in humans. Among the-100 targets predicted by both the TargetScan and PicTar search programs, RTKN was of particular interest, because it has been found to be overexpressed in many cancer-derived cell lines. More importantly, inhibition of RTKN sensitized cells to apoptotic stimuli. RTKN was initially isolated as a scaffold protein interacting with GTP bound form of Rho. It links the Rho signal to nuclear factor-KB (NFκB) activation, leading to increased cell survival by transactivating antiapoptotic genes downstream of NFκB. In MCF-7 cells, we confirmed that miR-145 modulated RTKN expression by directly targeting the binding site within the 3'UTR. Elevated levels of miR-145 repressed the cellular mRNA and protein levels of RTKN. Furthermore, down-regulation of RTKN by siRNA can inhibit MCF-7 cell growth, although to a less extent, compared to miR-145 overexpression. Taken together, we found that miR-145 could inhibit MCF-7 cell growth by targeting RTKN...