Effects of leukemogenic retroviruses on condylar cartilage in vitro: an ultrastructural study

Onset and dynamics of osteosclerosis in mice induced by Reilly-Finkel-Biskis (RFB) murine leukemia virus. Increase in bone mass precedes lymphomagenesis

Newborn NMRI strain mice were infected with Reilly-Finkel-Biskis (RFB) murine leukemia virus (MuLV), a murine leukemia virus that has been shown to induce lymphomas, osteosclerosis, and osteomas in susceptible strains of mice. Bone histomorphometry of the distal femoral metaphyses at 3-month intervals showed osteosclerosis 3 (100%), 6 (100%), and 9 (93%) months after infection. This was represented by significantly augmented cancellous bone mass and accompanied by distinct changes in bone architecture. High numbers of provirus copies were detected at 2-4 weeks in femora, humeri, and calvaria, and viral protein was highly expressed in trabecular and cortical bone cells, particularly in osteocytes. Infected mice showed enhanced bone formation and smaller numbers of osteoclasts relative to sex- and age-matched controls. Osteoclastic differentiation was significantly reduced in cocultures of spleen or bone marrow cells with RFB MuLV-infected osteoclastogenic, osteoblast-like cells. However, RFB MuLV did not impair the activity of mature osteoclasts. In infected mice lymphomas were only observed at 6 (22%) and 9 months (40%) of age. At 3 months, IgG gene and TCR-beta gene rearrangements were not detectable, and new proviruses showed a heterogeneous integration pattern, indicating the absence of lymphoma in early osteosclerotic mice. In contrast, lymphomas, which developed in 8- to 9-month-old infected mice, showed IgG rearrangements indicating development of B-cell lymphomas, together with mono- or oligoclonal expansion of distinct patterns of proviral integrations. These results indicate that RFB MuLV-induced osteosclerosis develops within 3 months after infection and precedes lymphomagenesis. It may therefore be considered an independent skeletal lesion in MuLV-infected mice.

Establishment and characterization of osteoblast-like cell lines from retrovirus (RFB MuLV)-induced osteomas in mice

Cell lines were established by a two-step method from osteomas which had been induced by infection of mice with RFB MuLV, a bone-pathogenic, replication-competent murine retrovirus. The benign tumors, consisting of mature lamellar bone and surrounded by a thin periosteum, were cultured on sponges of denatured collagen type I fibres for up to 4 weeks. At this time osteoma cells had grown into the collagenous matrix. After release and further cultivation in monolayers, the cell lines established from these cultures varied in morphology; they expressed T1, collagen type I and type III, alkaline phosphatase, osteonectin and osteopontin mRNAs at variable levels, but not osteocalcin/BGP. They also showed alkaline phosphatase activity, but lacked responsiveness to parathyroid hormone. All cell lines established from infected mice expressed retroviral and c-myc mRNA and viral protein. In contrast to cells from control mice they showed an extended life span in culture. After growth in a three-dimensional (3-D) collagen sponge culture the cells formed an extracellular matrix containing collagen type I, alkaline phosphatase and osteocalcin/BGP. These data indicate that the two-step method facilitates the establishment of osteoblast-like cell lines from osteomas and calvaria of old mice, and provides means for further analyses of retrovirus-induced skeletal pathogenesis and bone induction.

Osteosarcomagenic doses of radium (224Ra) and infectious endogenous retroviruses enhance proliferation and osteogenic differentiation of skeletal tissue differentiating in vitro

Cartilage tissue from embryonic mice which undergoes osteogenic differentiation during in vitro cultivation was used to study the effect of osteosarcomagenic doses of alpha-irradiation and bone-tumor-inducing retroviruses on proliferation and phenotypic differentiation of skeletal cells in a defined tissue culture model. Irradiated mandibular condyles showed dose-dependent enhancement of cell proliferation at day 7 of the culture and increased osteogenic differentiation at day 14. Maximal effects were found with 7.4 Bq/ml of 224Ra-labeled medium. Doses of 740 and 7400 Bq/ml of 224Ra-labeled medium induced increasing cell death. Retrovirus infection enhanced osteogenic differentiation and extended the viability of irradiated cells. After transplantation none of the treated tissues developed tumors in syngeneic mice.

Gene expression during osteogenic differentiation in mandibular condyles in vitro

The cartilagenous tissue of mandibular condyles of newborn mice contains progenitor cells as well as young and mature chondrogenic cells. During in vitro cultivation of the tissue, progenitor cells undergo osteogenic differentiation and form new bone (Silbermann, M., D. Lewinson, H. Gonen, M. A. Lizarbe, and K. von der Mark. 1983. Anat. Rec. 206:373-383). We have studied the expression of genes that typify osteogenic differentiation in mandibular condyles during in vitro cultivation. RNAs of the genes for collagen type I, osteonectin, alkaline phosphatase, and bone gla protein were sequentially expressed in progenitor cells and hypertrophic chondrocytes during culture. Osteopontin expression peaked in both the early and the late phase of the differentiation process. The data indicate a distinct sequence of expression of osteoblast-specific genes during osteogenic differentiation and new bone formation in mandibular condyles.

The molecular biology of radiation-induced carcinogenesis: thymic lymphoma, myeloid leukaemia and osteosarcoma

In mice, external X- or gamma-irradiation may induce thymic lymphomas or myeloid leukaemias, while bone-seeking alpha-emitters may induce osteosarcomas and, to a lesser extent, acute myeloid leukaemia. The present paper aims to review briefly some of the experimental data with respect to the molecular mechanisms underlying these radiation-induced carcinogenic processes. Thymic lymphomagenesis proceeds through an indirect mechanism. Recombinant proviruses often occur in the tumour cell DNA, favouring the idea that they might be involved. However, there are indications that they might mediate tumour growth rather than induction. It is plausible that activation of ras oncogenes by somatic point mutations might play a role in the carcinogenic process, although at a yet undetermined stage. Myeloid leukaemogenesis is characterized by a very early, putative initiating event, consisting of non-random rearrangements and/or deletions of chromosome 2. These may be related to deletions in the developmentally important homeobox gene clusters and to rearrangements of the sequences flanking the IL-1 beta gene. Either a gene of the homeobox family or IL-1 beta might be considered as potentially involved in the induction process. Osteosarcomagenesis in mice is often associated with the expression of proviruses, and the tumours often contain somatically acquired proviruses. These viruses may contribute to tumour development by affecting various growth-suppressor genes. Viruses isolated from bone tumours, although non-sarcomagenic, induce osteopetrosis, osteomas and lymphomas upon infection of newborn mice. Osteogenic tumours frequently display amplification of a region on mouse chromosome 15, which encompasses c-myc and Mlvi-1 sequences. Enhanced transcription of various oncogenes is found in individual tumours, but no specificity for osteosarcomas has been identified. In vitro systems of skeletoblast differentiation are being developed to study tumour induction in vitro.

Bone formation by osteoblast-like cells in a three-dimensional cell culture

Cells of the clonal osteogenic cell line MC3T3-E1 were seeded onto a three-dimensional matrix of denatured collagen type 1 and cultured for a period of up to 8 weeks. Specimens were analyzed by histological, enzyme histochemical, immunocytochemical, and ultrastructural methods and by in situ hybridization between day 7 and day 56 after seeding. In 56-day cultures, the MC3T3-E1 cells were arranged in a three-dimensional network and formation of bone-like tissue was indicated by calcification of a newly synthesized collagen type I matrix resembling osteoid and surrounding osteocyte-like cells. The differentiating culture showed high expression of osteocalcin and alkaline phosphatase activity. NIH3T3 fibroblasts used as control cells passed through the network of the substrate forming a confluent monolayer underneath. This culture system offers a potentially powerful model for bone formation in vitro and for investigating the osteogenic potential of bone-derived cells.

Computer-assisted imaging cytometry of nuclear chromatin reveals bone tumor virus infection and neoplastic transformation of adherent osteoblast-like cells

Established osteoblast-like (OB) cells infected with the bone tumor-inducing C-type retrovirus OA MuLV remained nontumorigenic over 104 cell culture passages. DNA histograms revealed a new cell population with a stem line peak at 5c. A second OA MuLV-infected OB cell line underwent neoplastic transformation with increasing passage level. These cells showed diffuse aneuploidy. Stepwise linear discriminant analysis of the chromatin structure of control, OA MuLV-infected, and FBR osteosarcoma virus-transformed cell lines resulted in various levels of discrimination ranging between 79.6% for control cells versus nontumorigenic OA MuLV-infected cells, and 96.6% for nontumorigenic OA MuLV-infected cells versus FBR osteosarcoma virus-transformed cells. OA MuLV-infected tumorigenic cells and FBR osteosarcoma virus-transformed cells were discriminated at a 93.6% level.

Biochemical characterization of a virus-induced osteosarcoma-like osseous lesion in vitro

Chondroprogenitor cells present in the apical and lateral parts of the mandibular condyle from neonatal mice differentiate towards the osteoblastic lineage and form bone within 7 days in culture. Infection of condylar explants with the FBR osteosarcoma virus (FBR MSV) results in the transformation of cells in the progenitor zone, previously identified as the target for the virus, and the formation of a transplantable osteosarcoma-like lesion. Morphological and biochemical changes in this system were investigated in the course of tumor development. Virus infection was followed by a significant increase in cell density and 3H-thymidine incorporation within the progenitor zone at the early stage of culture. In later stages, cell density and 3H-thymidine incorporation were lower than in control tissue. The 3H-thymidine labeling index gave similar results in infected and control tissues until day 7. Then, a significantly higher labeling index was found in the progenitor zone of infected condyles. At this stage, the proliferative effect of the virus even affected the cartilagenous core of the tissue. Quantitative alkaline phosphatase activity increased between day 3 and day 7 and was particularly high in the zone of infected cells. In addition, infected tissues consistently revealed a higher uptake of 45Ca, and deposition of the radioisotope along irregularly formed bone trabecules in the transformed tissue. The results suggest that there is an enhancement of tissue maturation following infection with the FBR osteosarcoma virus. Although biochemical investigations of whole condyles showed few differences in the total values of alkaline phosphatase activity, 3H-thymidine incorporation, DNA content, and 45Ca uptake, the histochemical assays revealed clear differences in the distributional pattern of these parameters within infected and control condyles.