Suppression of osteoclastogenesis through phosphorylation of eukaryotic translation initiation factor 2 alpha

In response to various stresses including viral infection, nutrient deprivation, and stress to the endoplasmic reticulum, eukaryotic translation initiation factor 2 alpha (eIF2 alpha) is phosphorylated to cope with stress induced apoptosis. Although bone cells are sensitive to environmental stresses that alter the phosphorylation level of eIF2 alpha, little is known about the role of eIF2 alpha mediated signaling during the development of bone-resorbing osteoclasts. Using two chemical agents (salubrinal and guanabenz) that selectively inhibit de-phosphorylation of eIF2 alpha, we evaluated the effects of phosphorylation of eIF2 alpha on osteoclastogenesis of RAW264.7 pre-osteoclasts as well as development of MC3T3 E1 osteoblast-like cells. The result showed that salubrinal and guanabenz stimulated matrix deposition of osteoblasts through upregulation of activating transcription factor 4 (ATF4). The result also revealed that these agents reduced expression of the nuclear factor of activated T cells c1 (NFATc1) and inhibited differentiation of RAW264.7 cells to multi-nucleated osteoclasts. Partial silencing of eIF2 alpha with RNA interference reduced suppression of salubrinal/guanabenz-driven downregulation of NFATc1. Collectively, we demonstrated that the elevated phosphorylation level of eIF2 alpha not only stimulates osteoblastogenesis but also inhibit osteoclastogenesis through regulation of ATF4 and NFATc1. The results suggest that eIF2 alpha-mediated signaling might provide a novel therapeutic target for preventing bone loss in osteoporosis.