TNF-alpha's Effects on Proliferation and Apoptosis in Human Mesenchymal Stem Cells Depend on RUNX2 Expression

RUNX2 is a bone-specific transcription factor that plays a critical role in prenatal bone formation and postnatal bone development. It regulates the expression of genes that are important in committing cells into the osteoblast lineage. There is increasing evidence that RUNX2 is involved in osteoblast proliferation. RUNX2 expression increases during osteoblast differentiation, and recent data even suggest that it acts as a proapoptotic factor. The cytokine tumor necrosis factor alpha (TNF-alpha) is known to modulate osteoblast functions in a manner that depends on the differentiation stage. TNF-alpha affects the rate at which mesenchymal precursor cells differentiate into osteoblasts and induces apoptosis in mature osteoblasts. Thus we sought to establish whether or not the effects of TNF-alpha and fetal calf serum on proliferation and apoptosis in human mesenchymal stem cells (hMSCs) were dependent on RUNX2 level and activity. We transfected hMSCs with small interfering RNAs (siRNAs) directed against RUNX2 and found that they proliferated more quickly than control hMSCs transfected with a nonspecific siRNA. This increase in proliferation was accompanied by a rise in cyclin A1, B1, and E1 expression and a decrease in levels of the cyclin inhibitor p21. Moreover, we observed that RUNX2 silencing protected hMSCs from TNF-alpha's antiproliferative and apoptotic effects. This protection was accompanied by the inhibition of caspase-3 activity and Bax expression. Our results confirmed that RUNX2 is a critical link between cell fate, proliferation, and growth control. This study also suggested that, depending on the osteoblasts' differentiation stage, RUNX2 may control cell growth by regulating the expression of elements involved in hormone and cytokine sensitivity.