Bcl-2 expression in F-MuLV-induced erythroleukemias: a role for the anti-apoptotic action of Bcl-2 during tumor progression.

Erythroleukemias induced by various strains of Friend virus are multistage malignancies that result from the accumulation of genetic mutations, including the activation of proto-oncogenes and the inactivation of tumor suppressor genes. In this study, we demonstrate that Bcl-2 expression is activated in the majority of F-MuLV-induced erythroleukemia cell lines. In contrast, Bcl-2 was not expressed in any of the FV-P-induced erythroleukemia cell lines and protein levels were low or negligible in FV-A-induced erythroleukemia cell lines examined. In vivo, Bcl-2 expression levels gradually increased in F-MuLV-induced erythroleukemic cells prior to adaptation to culture. High expression of Bcl-2 in F-MuLV-induced erythroleukemic cells was shown to proceed the emergence of p53 mutation suggesting that Bcl-2 expression may delay p53 mutation in the leukemic cells. This is further supported by the demonstration that the majority of F-MuLV-induced erythroleukemia cell lines established from primary tumors induced in p53 mutant mice express low to negligible levels of Bcl-2. We have shown that the high levels of Bcl-2 expression in FV-P-induced erythroleukemic cells inhibited apoptosis induced by etoposide, low serum and p53 expression. Similarly, ectopic Bcl-2 expression within these cells also provided protection from apoptosis induced by etoposide and growth in low serum. These results suggest that the anti-apoptotic action of Bcl-2 may confer a selective in vivo and in vitro growth advantage to F-MuLV-induced erythroleukemic cells, which is not shared by FV-P/FV-A-induced erythroleukemic cells. The observed induction of Bcl-2 expression in vivo constitutes a novel but late oncogenic event associated with the progression of F-MuLV-induced erythroleukemias.