Unphosphorylated HSP27 (HSPB1) regulates the translation initiation process via a direct association with eIF4E in osteoblasts

Heat-shock protein 27 (HSP27/HSPB1) and its phosphorylation are implicated in multiple physiological and pathophysiological cell functions. Our previous study reported that unphosphorylated HSP27 has an inhibitory role in triiodothyronine (T3)induced osteocalcin (OC) synthesis in osteoblasts. However, the mechanisms behind the HSP27mediated effects on osteoblasts remain to be clarified. In the present study, to investigate the exact mechanism of HSP27 and its phosphorylation in osteoblasts, the molecular targets of HSP27 were explored using osteoblastlike MC3T3E1 cells. The levels of OC mRNA induced by T3 in the HSP27overexpressing cells did not show any significant differences compared with those in the control empty vectortransfected cells. Therefore, the interactions between HSP27 and translational molecules were focused on, including eukaryotic translation initiation factor 4E (eIF4E), eIF4G and 4Ebinding protein 1 (4EBP1). The HSP27 protein in the unstimulated cells coimmunoprecipitated with eIF4E, but not eIF4G or 4EBP1. In addition, the association of eIF4E with 4EBP1 was observed in the HSP27overexpressing cells, as well as in the control cells. Under T3 stimulation, the binding of eIF4E to eIF4G was markedly attenuated in the HSP27overexpressing cells compared with the control cells. In addition, the binding of HSP27 to eIF4E in the unstimulated cells was diminished by the phosphorylation of HSP27. In response to T3 stimulation, the association of eIF4E with eIF4G in the unphosphorylatable HSP27overexpressing cells was markedly reduced compared with the phosphomimic HSP27overexpressing cells. Taken together, these findings strongly suggest that unphosphorylated HSP27 associates with eIF4E in osteoblasts and suppresses the translation initiation process.