Function of Atf4 in Endochondral Bone Formation
Activating transcription factor 4 (Atf4) is a leucine zip transcription factor. Atf4 mutant (Atf4-/-) mice show severe low bone mass and short stature phenotype. Atf4 is expressed in growth plate chondrocytes. Atf4-/- growth plate shows disturbed and shortened proliferative chondrocyte zone, expanded hypertrophic zone and decreased expression of Indian hedgehog (Ihh). Atf4 directly binds to the Ihh promoter and activates its transcription. Reactivation of Hh signaling pharmacologically in mouse limb explants corrects the Atf4-/- chondrocyte proliferation and short limb phenotypes. Chondrocyte-specific overexpression of Atf4 in Atf4-/- mice restores Ihh expression and Hh signaling, and completely rescues the cartilage defects presented in Atf4-/- embryos. These data demonstrate a chondrocyte-autonomous function of Atf4 in chondrogenesis. Unexpectedly, Atf4 overexpression in chondrocytes of Atf4-/-;Col2α1-Atf4 mice also rescues the Osteocalcin expression and bone formation normalize in Atf4-/- mice. The differentiation of Atf4-/- osteoblasts induced by the conditioned medium of Atf4-/-;Col2a1-Atf4 cartilage can be blocked by a neutralizing Ihh antibody, indicating that Atf4 in chondrocytes indirectly regulates osteoblast differentiation via Ihh. My study thus identifies a novel autonomous function of Atf4 in chondrocytes for regulating both chondrogenesis and osteogenesis in endochondral ossification.