The Over-Expression of miR-34a Fails to Block DoHH2 Lymphoma Cell Proliferation by Reducing p53 via c-MYC Down-Regulation

MicroRNAs (miRNAs) might behave as tumor suppressors and for that they are under consideration as novel therapeutic drugs. We tested the tumor suppressor activity of miRNA-34a (miR-34a) by measuring cell proliferation of the follicular lymphoma cell line DoHH2 transfected with this miRNA. We report that miR-34a did not inhibit cell proliferation notwithstanding a marked down-regulation of c-MYC. Interestingly, DoHH2 transfected cells showed a significant p53 down-regulation, suggesting that c-MYC positively controls p53 and the failed inhibition of cell proliferation is probably due to the down-regulation of the c-MYC/p53 axis. In keeping with this, c-MYC silencing also down-regulated p53 and had no effect on cell proliferation. In accordance with this hypothesis, etoposide or nutlin-3 treatment or a small interfering RNA (siRNA) against BCL6 (B-cell lymphoma 6) inhibited the proliferation of DoHH2 cells by up-regulating p53 without affecting either miR-34a or c-MYC levels. These results indicate that the proliferation is controlled by the regulatory axis c-MYC/p53 and suggest that paradoxically miR-34a behaves as a pro-proliferative rather than an anti-proliferative miRNA in DoHH2 cells.