Inflammatory priming of human mesenchymal stem cells induces osteogenic differentiation via the early response gene IER3.

Mesenchymal stem cells (MSCs) have gained tremendous interest due to their overall potent pro-regenerative and immunomodulatory properties. In recent years, various in vitro and preclinical studies have investigated different priming ("licensing") approaches to enhance MSC functions for specific therapeutic purposes. In this study, we primed bone marrow-derived human MSCs (hMSCs) with an inflammation cocktail designed to mimic the elevated levels of inflammatory mediators found in serum of patients with severe injuries, such as bone fractures. We observed a significantly enhanced osteogenic differentiation potential of primed hMSCs compared to untreated controls. By RNA-sequencing analysis, we identified the immediate early response 3 (IER3) gene as one of the top-regulated genes upon inflammatory priming. Small interfering RNA knockdown experiments established IER3 as a novel positive regulator of osteogenic differentiation. Mechanistic analysis further revealed that IER3 deletion significantly downregulated bone marrow stromal cell antigen 2 (BST2) expression and extracellular signal-related kinase 1/2 (ERK1/2) phosphorylation in hMSCs, suggesting that IER3 regulates osteogenic differentiation through BST2 and ERK1/2 signaling pathway activation. On the basis of these findings, we propose IER3 as a novel therapeutic target to promote hMSC osteoblastogenesis, which might be of high clinical relevance, for example, in patients with osteoporosis or compromised fracture healing.