The role of Myc in bone metabolism

Various studies have proven the involvement of Myc in cell growth and proliferation but also cell transformation and tumorigenesis. Since regulation of cell proliferation and differentiation is mandatory in bone metabolism, we investigated the role of Myc in osteoblasts. Studies involving osteoblast specific deletion of c-Myc traced by EYFP revealed no difference in fluorescence comparing mice that have either two copies, one copy or that are lacking both copies of c-Myc under the microscope. FACS analysis of these same mice showed no significant difference in numbers of EYFP positive osteoblasts among these mice. Immunofluorescence stainings showed absence of c-Myc protein in the majority of EYFP positive cells. Studies involving osteoblast specific c-Myc and N-Myc deletion traced by EYFP were performed to analyze possible consequences of loss of both genes. Bones of newborn myc-deficient mice appeared EYFP fluorescent under the microscope. FACS analysis of myc-deficient mice demonstrated variable numbers of EYFP positive osteoblasts. However, numbers of EYFP positive osteoblasts were in the same range as numbers obtained by FACS for c-Myc deficient mice. Osteoblast specific overexpression of c-Myc was analyzed using a Tet-Off model in which human c-Myc is overexpressed. qRT-PCRs showed that markers of mid to late differentiation of osteoblasts are less expressed in mice overexpressing c-Myc compared to control mice. Since mutant mice develop osteosarcomas, the expression levels of osteoblast differentiation markers were tested by qRT-PCR. Tumors revealed to be composed of overall less differentiated osteoblasts. Using these two models we could show that myc is dispensable for osteoblasts in vivo, whereas ectopic expression of c-myc leads to severe changes in the transcription profile of osteoblasts with a specific loss of mature osteoblast markers, and to the development of osteosarcomas, which show a rather immature osteoblast phenotype