Virus production (lentivirus). Production of miR-424(322)/503 and CDC25A-containing lentivirus was achieved by transfecting Phoenix packaging cells with jetPEI in combination with lentiviral plasmids, pCMV-dR8.91, and pMD.G helper plasmids at a ratio of 2:1:1, respectively. Similarly, the same conditions were employed to produce hTERT retrovirus by combining the retroviral plasmid pBABE-hTERT with the retroviral helper plasmids pMSCV-Psi and pCMV-VSV-G.

Recently, we demonstrated that the microRNA 424(322)/503 [miR-424(322)/503] cluster is transcriptionally controlled by transforming growth factor beta (TGF-beta) in the mammary epithelium. Induction of this microRNA cluster impacts mammary epithelium fate by regulating apoptosis and insulin-like growth factor 1 (IGF1) signaling. Here, we expanded our finding to demonstrate that miR-424(322)/503 is an integral component of the cell cycle arrest mediated by TGF-beta. Mechanistically, we showed that after TGF-beta exposure, increased levels of miR-424(322)/503 reduce the expression of the cell cycle regulator CDC25A. miR-424(322)/503-dependent posttranscriptional downregulation of CDC25A cooperates with previously described transcriptional repression of the CDC25A promoter and proteasome-mediated degradation to reduce the levels of CDC25A expression and to induce cell cycle arrest. We also provide evidence that the TGF-beta/miR-424(322)/503 axis is part of the mechanism that regulates the proliferation of hormone receptor-positive (HR(+)) mammary epithelial cells in vivo.