Adult human mesenchymal stem cell differentiation to the osteogenic or adipogenic lineage is regulated by mitogen-activated protein kinase

Adult human mesenchymal stem cells are primary, multipotent cells capable o f differentiating to osteocytic, chondrocytic, and adipocytic lineages when stimulated under appropriate conditions. To characterize the molecular mec hanisms that regulate osteogenic differentiation, we examined the contribut ion of mitogen-activated protein kinase family members, ERK, JNK, and p38, Treatment of these stem cells with osteogenic supplements resulted in a sus tained phase of ERK activation from day 7 to day 11 that coincided with dif ferentiation, before decreasing to basal levels. Activation of JNK occurred much later (day 13 to day 17) in the osteogenic differentiation process. T his JNK activation was associated with extracellular matrix synthesis and i ncreased calcium deposition, the two hallmarks of bone formation. Inhibitio n of ERK activation by PD98059, a specific inhibitor of the ERK signaling p athway, blocked the osteogenic differentiation in a dose-dependent manner, as did transfection with a dominant negative form of MAP kinase kinase (MEK -1), Significantly, the blockage of osteogenic differentiation resulted in the adipogenic differentiation of the stem cells and the expression of adip ose-specific mRNAs peroxisome proliferator-activated receptor gamma 2, aP2, and lipoprotein lipase. These observations provide a potential mechanism i nvolving MAP kinase activation in osteogenic differentiation of adult stem cells and suggest that commitment of hMSCs into osteogenic or adipogenic li neages is governed by activation or inhibition of ERK, respectively.