Glutamate inhibits chondral mineralization through apoptotic cell death mediated by retrograde operation of the cystine/glutamate antiporter

Although we have previously demonstrated the functional significance of excitatory amino acid transporters as well as glutamate (Glu) receptors (GluRs) expressed by chondrocytes, little attention has been paid to the possible expression of the cystine/Glu antiporter responsible for the bi-directional transmembrane transport of Glu in chondrocytes to date. In organotypic cultured mouse embryonic metatarsals isolated before vascularization, the chondral mineralization was significantly decreased in the presence of Glu at a high concentration. Apoptotic cells were detected within the late proliferating and prehypertrophic chondrocytic layers in metatarsals cultured in the presence of Glu. A group III metabotropic GluR (mGluR) antagonist partially, but significantly, prevented the inhibition of mineralization by Glu in metatarsals without affecting the number of apoptotic cells. Both decreased mineralization and apoptosis by Glu were significantly prevented by the addition of the cystine/Glu antiporter inhibitor homocysteic acid, as well as reduced glutathione (GSH) and cystine. Expression of mRNA for xCT and 4F2hc subunits, which are components of the cystine/Glu antiporter, was seen in both cultured mouse metatarsals and rat costal chondrocytes. In chondrocytes cultured with Glu, a significant decrease was seen in intracellular GSH levels, together with increases in the number of apoptotic cells and the level of intracellular reactive oxygen species. These results suggest that Glu could regulate chondrogenic differentiation toward mineralization through a mechanism associated with apoptosis mediated by the depletion of intracellular GSH after the retrograde operation of the cystine/ Glu antiporter, in addition to the activation of group III mGluR, in chondrocytes.