MicroRNA‐34a Inhibits Osteoblast Differentiation and In Vivo Bone Formation of Human Stromal Stem Cells

Osteoblast differentiation and bone formation (osteogenesis) are regulated by transcriptional and post‐transcriptional mechanisms. Recently, microRNAs (miRNAs) were identified as novel key regulators of human stromal (skeletal, mesenchymal) stem cells (hMSC) differentiation. Here, we identified miRNA‐34a (miR‐34a) and its target protein networks as modulator of osteoblastic (OB) differentiation of hMSC. miRNA array profiling and further validation by quantitative RT‐PCR revealed that miR‐34a was upregulated during OB differentiation of hMSC, and in situ hybridization confirmed its OB expression in vivo. Overexpression of miR‐34a inhibited early commitment and late OB differentiation of hMSC in vitro, whereas inhibition of miR‐34a by anti‐miR‐34a enhanced these processes. Target prediction analysis and experimental validation confirmed Jagged1 (JAG1), a ligand for Notch 1, as a bona fide target of miR‐34a. siRNA‐mediated reduction of JAG1 expression inhibited OB differentiation. Moreover, a number of known cell cycle regulator and cell proliferation proteins, such as cyclin D1, cyclin‐dependent kinase 4 and 6 (CDK4 and CDK6), E2F transcription factor three, and cell division cycle 25 homolog A were among miR‐34a targets. Furthermore, in a preclinical model of in vivo bone formation, overexpression of miR‐34a in hMSC reduced heterotopic bone formation by 60%, and conversely, in vivo bone formation was increased by 200% in miR‐34a‐deficient hMSC. miRNA‐34a exhibited unique dual regulatory effects controlling both hMSC proliferation and OB differentiation. Tissue‐specific inhibition of miR‐34a might be a potential novel therapeutic strategy for enhancing in vivo bone formation. Stem Cells 2014;32:902–912