The data of our transfection experiments, http://www.selleckchem.com/products/Vorinostat-saha.html together with these previous studies, suggest that miR-143 probably inhibits tumor development by downregulating c-Myc expression in cooperation with miR-145 in our transgenic mice. Downregulation of p68/p72 could also be one of the key factors that retard tumor development in the small intestine of Tg/APC. Our results suggested the downregulation of p72 by miR-145 through binding to its 3��UTR might be at least one of the molecular basis for the decrease of its expression in the transgenic small intestine tumors. In addition, we showed that the expression of p68 was downregulated by the introduction of c-Myc siRNAs. Thus, it is possible that p68 and c-Myc might form a positive feedback loop, which suggests that miR-143 and miR-145 might inhibit p68 in part through the repression of c-Myc.
Further analysis by TargetScan also revealed the existence of possible target sites of miR-206 and miR-34a/miR-206/miR-26a in the 3��UTRof p68 and p72, respectively, in a wide range of animal species. This was intriguing, since miR-206 and miR-26a are a set of miRNAs whose processing was proven to depend on the p53/p68/p72 complex [14]. In addition, miR-34a is a well-known p53-induced gene [32], and p68 and p72 have been shown to be strong and weak co-activators of p53, respectively [33]. Crucially, all of these were miRNAs that were found to be tumor suppressors [34], [35]. Indeed, our reporter assays showed that miR-34a (Fig. S5 B and F), miR- 206 (Fig. S5 C and G), and miR-26a (Fig.
S5 D and H) mimics suppressed the reporter activity of the constructs containing each target on the p72 3��UTR, whereas miR-206 mimic also decreased that of the construct containing a target on the p68 3��UTR (Fig. S5 A and E). Although further studies are required, the p53/p68/p72 complex and a set of tumor suppressor miRNAs linked with p53 seem to constitute a novel regulatory feedback circuit. Whereas miR-145 was induced in the small intestine tumors in Tg/APC, the molecular mechanisms remain to be obscure. Our data using DLD-1 cells and Lovo cells, however, imply that the suppression of c-Myc may be at least in part involved in its induction. Hence, given qRT-PCR analysis of pri-miR-143/145 and Western blot analysis of transgenic tumors, forced-expression of miR-143 might trigger c-Myc/pri-miR-145 signal more easily in the small intestine tumors than in the colon tumors.
Interestingly, this signaling axis appeared not to exert its function efficiently in non-tumorous segments of small intestines. Other investigators recently reported that a positive feedback circuit between miR-143 and miR-145 could work through suppressing KRAS-RREB1 signaling in pancreatic cancer Batimastat cells, although they did not mention the cross-regulation at the mature miRNA levels [36].