The specific consequences of c-fos expression in transgenic mice

Induction of bone tumors in transgenic mice by C-FOS depends on the presence of a retroviral long terminal repeat

Expression of the protooncogene C-FOS can lead to the development of bone tumors in transgenic mice. However, recent experiments have suggested that the tumor induction is independent of the promoter chosen for the C-FOS expression construct but dependent on modifications in the 3' untranslated region (3'UTR) of the C-FOS gene. To investigate this effect, C-FOS expression constructs have been tested in transgenic mice for their ability to induce bone tumors. Removal of 3'UTR sequences known to affect the stability of C-FOS mRNA resulted in high expression in several organs but gave no detectable bone lesions. In contrast, the presence of different long terminal repeats of murine retroviruses at the 3'UTR caused the induction of bone tumors. These data suggest that the C-FOS gene can be converted into a tumor-inducing oncogene only by retroviral control elements.

Fos and bone cell development: lessons from a nuclear oncogene

Vertebrate embryologists are beginning to understand the early developmental decisions that control the origin and patterning of skeletal elements. However, the regulators governing the development of the cells that form the skeleton, namely, bone and cartilage cells, are poorly understood. Recent studies using transgenic and knockout mice have established a unique role for the proto-oncogene and nuclear transcription factor, Fos, in regulating the differentiation and activity of specific bone cell populations, both during normal development and in bone disease.

c-fos and bone loss: a proto-oncogene regulates osteoclast lineage determination

Development of gene transfer systems provides a key tool for understanding gene function. Exciting and often unexpected consequences from embryo manipulations are yielding insights into molecular mechanisms underlying development under normal and pathogenic states, and are providing animal models for diseases. Contributing to this progress is the elegant work on c-fos, where Wagner and coworkers identify this proto-oncogene as a primary factor which directs cell differentiation along the osteoclast/macrophage lineages, and thus regulates bone remodeling. Their studies support a link between skeletogenesis, marrow formation and hematopoiesis, and may help to delineate mechanisms underlying the oncogenic transformation of skeletal and hematopoietic cells.

Osteoblasts are target cells for transformation in c-fos transgenic mice

We have generated transgenic mice expressing the proto-oncogene c-fos from an H-2Kb class I MHC promoter as a tool to identify and isolate cell populations which are sensitive to altered levels of Fos protein. All homozygous H2-c-fosLTR mice develop osteosarcomas with a short latency period. This phenotype is specific for c-fos as transgenic mice expressing the fos- and jun-related genes, fosB and c-jun, from the same regulatory elements do not develop any pathology despite high expression in bone tissues. The c-fos transgene is not expressed during embryogenesis but is expressed after birth in bone tissues before the onset of tumor formation, specifically in putative preosteoblasts, bone-forming osteoblasts, osteocytes, as well as in osteoblastic cells present within the tumors. Primary and clonal cell lines established from c-fos-induced tumors expressed high levels of exogenous c-fos as well as the bone cell marker genes, type I collagen, alkaline phosphatase, and osteopontin/2ar. In contrast, osteocalcin/BGP expression was either low or absent. All cell lines were tumorigenic in vivo, some of which gave rise to osteosarcomas, expressing exogenous c-fos mRNA, and Fos protein in osteoblastic cells. Detailed analysis of one osteogenic cell line, P1, and several P1-derived clonal cell lines indicated that bone-forming osteoblastic cells were transformed by Fos. The regulation of osteocalcin/BGP and alkaline phosphatase gene expression by 1,25-dihydroxyvitamin D3 was abrogated in P1-derived clonal cells, whereas glucocorticoid responsiveness was unaltered. These results suggest that high levels of Fos perturb the normal growth control of osteoblastic cells and exert specific effects on the expression of the osteoblast phenotype.

Establishment of an osteogenic cell line derived from adult mouse bone marrow stroma by use of a recombinant retrovirus

In order to characterize fibroblastic colony-forming units (CFU-F) from murine bone marrow in relation to osteogenesis, adherent cells of 7-day-old BALB/c mouse bone marrow cultures were infected with a recombinant retrovirus (N2/ delta fosB) containing the bacterial neomycin resistance gene. One of the G418-resistant clones, MN7, was selected for further analysis on the basis of its high expression of the bone-specific alkaline phosphatase. The cells have now been in culture for more than 1 year and maintain a stable phenotype. The osteogenic nature of the immortalized clone MN7 was demonstrated as follows: (1) Mineralization was detected by 85Sr uptake and with the Von Kossa staining method only after in vitro cultivation on a collagen type I matrix. (2) Osteoblastic phenotype markers, including the synthesis of type I collagen, osteonectin, and the bone-specific isoenzyme of alkaline phosphatase were expressed in vitro. (3) MN7 cells responded to bone effectors such as parathyroid hormone and 1,25-dihydroxyvitamin D3. (4) Intraperitoneal injection of MN7 cells into 1-day-old BALB/c mice produced typical osteosarcomas in all animals. We conclude that MN7, derived entirely in vitro from a stromal CFU-F colony, represents a stable murine osteosarcoma cell line expressing the osteoblastic phenotype and provides the first direct evidence needed to establish adult mouse marrow-derived, nonhematopoietic stromal cells as osteoprogenitors.