Ets2-Dependent Stromal Regulation of Mouse Mammary Tumors

The Ets2 transcription factor is regulated by mitogen-activated protein (MAP) kinase phosphorylation of a single threonine residue. We generated by gene targeting a single codon mutation in Ets2 substituting Ala for the critical Thr-72 phosphorylation site (Ets2^A72), to investigate the importance of MAP kinase activation of Ets2 in embryo and tumor development. Ets2^A72/A72 mice are viable and develop normally. However, combining the Ets2^A72 allele with a deletion mutant of Ets2 results in lethality at E11.5 and shows that Ets2^A72 is a hypomorphic allele. Mammary tumors caused by transgenic polyomavirus middle T antigen, activated Neu (Erbb2), or the combination of Neu and transgenic VEGF (Neu; VEGF-25) were all restricted in Ets2^A72/A72 females. The Ets2^A72/A72 restriction on Neu; VEGF-25 tumor growth was associated with increased p21 ^Cip1 expression. The size of tumors transplanted into fat pads of mice with Ets2 targeted alleles was correlated directly with Ets2 activity and fewer stromal cells expressing matrix metalloproteinase 9 (MMP-9). Decreased MMP-3 and MMP-9 mRNAs were confirmed in Ets2^A72/A72 macrophages. Activation of Ets2 at Thr-72 acts in the stroma, downstream of vascular endothelial growth factor production, in part through the regulation of macrophage proteases to support the progression of Neu- and polyomavirus middle-T-initiated mammary tumors.