Abstract
Activation of particular glutamate (Glu) receptors is shown to promote cellular differentiation toward maturation during osteoblastogenesis. In the present study, we have evaluated the possible modulation by Glu of cellular proliferation in osteoblastic cells endowed to proliferate for self‐renewal and to differentiate toward matured osteoblasts. Exposure to Glu significantly suppressed the proliferation activity at a concentration over 500 µM without inducing cell death in osteoblastic MC3T3‐E1 cells before differentiation. The suppression by Glu occurred in a manner sensitive to the prevention by either cystine or reduced glutathione. Expression of mRNA was for the first time shown with the cystine/Glu antiporter composed of xCT and 4F2hc subunits in these undifferentiated osteoblastic cells. A significant decrease was seen in intracellular total glutathione levels in undifferentiated MC3T3‐E1 cells cultured with Glu, indeed, whereas the cellular proliferation activity was drastically decreased by the addition of the glutathione depleter cyclohexene‐1‐one and the glutathione biosynthesis inhibitor L‐buthionine‐[S,R]‐sulfoximine, respectively. Exposure to Glu led to a significant increase in mRNA expression of nuclear factor E2 p45‐related factor 2 (Nrf2) together with the generation of reactive oxygen species, while a significant decrease was seen in the proliferation activity in MC3T3‐E1 cells with stable overexpression of Nrf2. These results suggest that Glu could suppress the cellular proliferation toward self‐renewal through a mechanism associated with the upregulation of Nrf2 expression in association with the depletion of intracellular glutathione after promoting the retrograde operation of the cystine/Glu antiporter in undifferentiated MC3T3‐E1 cells. J. Cell. Physiol. 213: 105–114, 2007. © 2007 Wiley‐Liss, Inc.