| Overexpression of the AP-1 transcription factors DeltaFosB or Delta2DeltaFosB in transgenic mice results in a severe and progressive osteosclerosis, due to a cell-autonomous effect on the osteoblast cells. To elucidate the mechanism of this DeltaFosB effect, a yeast two-hybrid library of osteoblast cDNA was screened with Delta2DeltaFosB and seven Delta2DeltaFosB-interacting proteins were identified, as described in Chapter 2 of this dissertation. Two of these proteins are involved in human diseases: Wolf-Hirschhorn syndrome candidate 1 (whsc1) and acute lymphoblastic leukemia 1 (ALL1). Two are factors involved in the ubiquitin-proteosomal degradation pathway: nuclear zinc finger protein 95 (NP95) and E2I ligase. One, the F-box and leucine-rich repeat protein 11 (Fbox11) has no known function. One clone encoded the runt domain-containing transcription factor Runx2, a well-characterized and essential factor for osteoblast commitment and differentiation. Chapter 3 of this dissertation describes the interaction between Runx2 and DeltaFosB isoforms and the enhancement of Runx2-mediated promoter activation and DNA binding by DeltaFosB isoforms. Finally, the screen identified a clone that encoded a novel protein that consists of 30 C2H2 Kruppel-like zinc fingers, which we named MC-33. Chapter 4 of this dissertation describes the cloning and characterization of MC-33 and its role in the regulation of bone formation and osteoblastogenesis. We found that MC-33 inhibits the expression of the early osteoblast markers and transcriptional activation of the osteocalcin promoter. In addition, MC-33 overexpression resulted in reduced nodule formation and mineralization in vitro. However, contrary to the in vitro results, in vivo overexpression of MC-33 in transgenic mice resulted in an increase in bone mass. Thus, we conclude the MC-33 is a complex regulator of osteoblastogenesis with possible differences in its effect dependent on the specific location and time of expression. |
