Gene expression analyses of the differentiating hMSC into Osteoblasts and Adipocytes.

Age-related skeletal degeneration in patients with osteoporosis is characterized by decreased bone mass and occurs concomitant with an increase in bone marrow adipocytes. Using microarray expression profiling with high temporal resolution, we identified gene regulatory events in early stages of osteogenic and adipogenic lineage commitment of human mesenchymal stromal cells (hMSCs). Data analysis reveal three distinct phases when cells adopt a committed expression phenotype: initiation of differentiation (0-3h, Phase I), lineage-acquisition (6-24h, Phase II) and early lineage-progression (48-96h, Phase III). Upstream regulator analysis identifies 34 transcription factors (TFs) in Phase I with a role in hMSC differentiation. Interestingly, expression levels of identified TFs did not always change and indicate additional post-transcriptional regulatory mechanisms. Functional analysis reveals that forced expression of IRF2 enhances osteogenic differentiation. Thus, IRF2 and other ‘early-responder‘ TFs may control osteogenic cell fate of MSCs and should be considered in mechanistic models that clarify bone-anabolic changes during clinical progression of osteoporosis.