Preferential activation of Fgf8 by proviral insertion in mammary tumors of Wnt1 transgenic mice.

Mouse mammary tumor virus (MMTV) is an insertional mutagen that has been demonstrated to transcriptionally activate flanking cellular proto-oncogenes. Previously we have used MMTV infection to accelerate mammary tumorigenesis in Wnt1 transgenic mice in order to identify genes that cooperate with the Wnt1 oncogene. Initial investigations into the resulting tumor collection, screened primarily by Southern analysis, showed that three fibroblast growth factor genes, Fgf8, Fgf3 and Fgf4, sustain activating insertion mutations in 10%, 42% and 6% of the tumors, respectively. Here, in an examination of the tumors from MMTV-infected Wnt1 transgenic mice that emphasizes Northern analysis, we report transcriptional activation of Fgf8 in 30 additional tumors (increasing the percentage of activations to 50%), while no significant changes in the activation frequency of Fgf3 or Fgf4 were found. To determine the frequency of insertional activation in normal mice, we examined tumors from MMTV-infected nontransgenic littermates of the Wnt1 transgenics and from MMTV-infected BALB/c mice. Fgf8, Fgf3 and Fgf4 were found to be activated in 11%, 80% and 5%, respectively, of the tumors in the combined nontransgenic groups. Thus, there appears to be an increased predisposition for Fgf8 activations in Wnt1 transgenic mice versus normal mice, suggesting that cells expressing Wnt1 are especially sensitized to stimulation by FGF8 compared with FGF3 or FGF4. In contrast, the activation frequency of Fgf3 in tumors from MMTV-infected Wnt1 transgenic mice was approximately one-half that of normal mice. Our results show that this in vivo model of multistep tumorigenesis reveals significant differences in the activation rates of Fgf3 and Fgf8 depending upon the status of Wnt1 expression in the mammary gland. The differential activation of these Fgfs may relate to differences in their signaling pathways.