Characterization of a novel KRAB/C2H2 zinc finger transcription factor involved in bone development

Osteogenic differentiation involves a cascade of coordinated gene expressio n that regulates cell proliferation and matrix protein formation in a defin ed temporo-spatial manner. Here we have used differential display to identi fy a novel zinc finger transcription factor (AJ18) that is induced during d ifferentiation of bone cells in vitro and in vivo, The 64-kDa protein, enco ded by a 7-kilobase mRNA, contains a Kruppel-associated box (KRAB) domain f ollowed by 11 successive C2H2 zinc finger motifs, AJ18 mRNA, which is also expressed in kidney and brain, is developmentally regulated in embryonic ti biae and calvariae, with little expression in neonate and adult animals. Du ring osteogenic differentiation in vitro AJ18 mRNA is expressed as cells ap proach confluence and declines as bone formation occurs. Using bacterially expressed, His-tagged AJ18 in a target detection assay, we identified a con sensus binding sequence of 5'-CCACA-3', which forms part of the consensus e lement for Runx2, a master gene for osteogenic differentiation. Overexpress ion of AJ18 suppressed Runx2-mediated transactivation of an osteocalcin pro moter construct in transient transfection assays and reduced alkaline phosp hatase activity in bone morphogenetic protein-induced C3H10T1/2 cells. Thes e studies, therefore, have identified a novel zinc finger transcription fac tor in bone that can modulate Runx2 activity and osteogenic differentiation .