Inactivation of p53 and retinoblastoma family pathways in canine osteosarcoma cell lines

Canine osteosarcoma (OS) has been used as a model system for the study of c ancer biology and treatment despite the lack of information regarding its p athogenesis. Expression of tumor suppressor genes known to participate in m alignant transformation were studied in five different OS cell lines. Each of the cell lines exhibited properties of transformed cells, and those that were tested grew in soft agarose and formed osteoid-containing tumors when injected subcutaneously into nude mice. p53 function was determined to be defective in each cell line as indicated by the lack of induction of p53-re sponsive genes, p21 and mdm2, following treatment with 5-fluorouracil. p53 mRNA and protein levels were elevated in three cell lines and were extremel y low in two cell lines, p53 protein overexpression correlated with the pre sence of mutations within the DNA binding domain. Four cell lines appeared to contain normal retinoblastoma (Rb) mRNA and Rb protein and no detectable p16 mRNA or protein. in contrast, the remaining cell line contained high l evels of p16 mRNA and protein and significantly reduced levels of Rb, p107, and p130 proteins. These results underscore the importance of inactivating p53 and RD family pathways in canine OS and suggest that unlike human OS, cells derived from canine OS contain mutations that simultaneously inactiva te all three Rb family members.