Glutamate Suppresses Osteoclastogenesis through the Cystine/Glutamate Antiporter

Abstract

Previous studies have demonstrated functional expression of different glutamate receptor subtypes (GluRs) in both osteoblasts and osteoclasts. In the present study, we investigated the possible functional expression by osteoclasts of different glutamatergic signaling machineries including GluRs. In disagreement with the aforementioned prevailing view, no mRNA expression was found for all GluRs examined in primary cultured mouse osteoclasts differentiated from bone marrow precursors. Constitutive expression of mRNA was seen with glutamate transporters, such as excitatory amino acid transporters and cystine/glutamate antiporter, in primary osteoclasts. Glutamate significantly inhibited osteoclastogenesis at a concentration over 500 mol/L in both primary osteoclasts and preosteoclastic RAW264.7 cells without affecting the cell viability in a manner sensitive to the antiporter inhibitor. In RAW264.7 cells stably overexpressing the cystine/glutamate antiporter, the inhibition by glutamate was more conspicuous than in cells transfected with empty vector alone. The systemic administration of glutamate significantly prevented the decreased bone mineral density in both femur and tibia in addition to increased osteoclastic indices in ovariectomized mice in vivo. These results suggest that glutamate may play a pivotal role in mechanisms associated with osteoclastogenesis through the cystine/glutamate antiporter functionally expressed by osteoclasts devoid of any GluRs cloned to date. (Am J

Osteoclastogenesis is negatively regulated by D‐serine produced by osteoblasts

“…In our previous studies, functional NMDAR channels are expressed by bone‐forming osteoblasts (Hinoi et al, 2003), but not by bone‐resorbing osteoclasts devoid of contamination with both osteoblasts and stromal cells (Hinoi et al, 2007). Cellular differentiation is drastically inhibited in cultured rat calvarial osteoblasts exposed to different NMDAR antagonists in a manner dependent on the master regulator of osteoblast differentiation Runx2 (Hinoi et al, 2003), whereas mRNA expression is not seen for either ionotropic glutamate receptors or metabotropic glutamate receptors in primary cultured mouse osteoclasts differentiated from monocyte/macrophage progenitor cells in the presence of M‐CSF and RANKL (Hinoi et al, 2007). D ‐Ser is believed to be a potent agonist at the Gly binding site on conventional NMDAR channels in a manner insensitive to the classical Gly antagonist strychnine (Johnson and Ascher, 1987; Mothet et al, 2000).…”

Suppression by glutamate of proliferative activity through glutathione depletion mediated by the cystine/glutamate antiporter in mesenchymal C3H10T1/2 stem cells

“…Taken together, the cystine/Glu antiporter could be a target for the development of an innovative drug useful for the regenerative medicine in mesenchymal stem cells. In ovariectomized mice, the systemic administration of Glu significantly prevents decreases in bone mineral density and trabecular bone in both femur and tibia, with a concomitant increase of the endogenous Glu level in tibial bone marrows (Hinoi et al, 2007).…”

Pharmacological Topics of Bone Metabolism: Glutamate as a Signal Mediator in Bone

“…The daily administration of Glu for 28 consecutive days significantly prevented the reduction of bone mineral density in total tibia at a dose over 10 mg / kg and in total femur at a dose over 100 mg / kg, respectively. No significant alternation of bone mineral density was observed in total femur of sham-operated mice with the daily intraperitoneal administration of Glu at a dose of 1,000 mg / kg for 28 consecutive days (33).…”