EGFR Signaling Promotes Proliferation And Survival In Osteoprogenitors By Increasing Egr2 Expression

The maintainance of bone architecture requires continuous generation of osteoblasts from osteoprognitor pool. Epidermal growth factor receptor (EGFR) ligands are well known mitogens for osteoprogenitors, however the underlying mechanisms by which EGFR signaling regulates osteoprogenitor pool are largely unknown. Here, we demonstrated that the activation of EGFR increasesd the number of osteoprogenitors by promoting both cell proliferation and survival. This result was further confirmed by calvarial organ culture in which EGF elevated the number of proliferative cells and suppressed the number of apoptotic cells, resulting in an overall increase in total osteoblast lineage cells. Microarray analysis of an osteoprogenitor cell line MC3T3 cells revealed that EGFR signaling stimulated the expression of an anti-apoptotic protein Mcl1 and a family of Egr transcription factors (Egr1,2, and 3). Overexpression of Egrs' co-repressor Nab2 attenuated the EGF-induced increase in osteoprogenitor number. Interestingly, knockdown of Egr2 by SiRNA, but not Egr1 or 3, exhibited a similar effect. Further analyses demonstrated that Egr2 was a major mediator for EGF-induced cell proliferation and survival. Using inhibitor, adenovirus overexpression, and siRNA approaches, we demonstrated that EGFR signaling activated MAPK/Erk pathway to stimulate the expression of Egr2, which in turn led to the increase of anti-apoptotic protein Mcl1 and supported cell survival. Fibroblast growth factor (FGF) 2, another mitogen and survival factor for osteoprogenitors, also stimulatesd proliferation and suppressed apoptosis in an Egr2-dependent manner, which indicated a general role of Egr2 mediating growth factor-regulated osteoprogenitor population. Taken together, our data clearly demonstrated that EGFR signaling was an important regulator of osteoprogenitor pool by affecting both cell proliferation and survival via promoting the expression of Egr2.