S-phase kinase-associated protein 2 impairs the inhibitory effects of miR-1236-3p on bladder tumors.

We have previously demonstrated that miR-1236-3p has the robust ability to up-regulate p21 expression by targeting the p21 promoter, thus inhibiting bladder cancer progression. Microarray experiments displayed that miR-1236-3p significantly increased the expression of the oncogenic F-box protein S-phase kinase-associate protein 2 (Skp2) while activating p21 expression in bladder cancer cells. Here, we confirmed that Skp2 was over-expressed following transfection with miR-1236-3p. Further, we demonstrated that miR-1236-3p and its sequence homology dsRNA, dsRNA-245 (which is completely complementary to the p21 promoter), both are able to potently induce p21 expression. We found that dsRNA-245 did not induce changes in Skp2 expression, while miR-1236-3p could increase Skp2 expression; this influence was independent of p21 activation. Moreover, transfection of miR-1236-3p or dsRNA-245 into bladder cancer cells significantly inhibited cell proliferation and clonegenesis and induced cell cycle arrest mainly by regulating p21 expression. However, the growth inhibition caused by dsRNA-245 was more effective than that caused by miR-1236-3p. This difference in effect size is mainly related to the miR-1236-3p-induced expression of Skp2. In summary, our results provided evidence that both endogenous and exogenous small RNAs might function to induce p21 expression by interacting with the same promoter region, therefore impeding bladder cancer cell growth. Additionally, our results indicated that microRNA activation can activate the expression of some tumor suppressor genes as well as some oncogenes. This indicated the need for the further study of clinical applications of RNA activation.