Date Published: January 22, 2010
Publisher: Public Library of Science
Author(s): Rebecca Baker, Claire V. Kent, Rachel A. Silbermann, John A. Hassell, Lawrence J. T. Young, Louise R. Howe, Mikhail V. Blagosklonny. http://doi.org/10.1371/journal.pone.0008854
Abstract: The role of the PEA3 subfamily of Ets transcription factors in breast neoplasia is controversial. Although overexpression of PEA3 (E1AF/ETV4), and of the related factors ERM (ETV5) and ER81 (ETV1), have been observed in human and mouse breast tumors, PEA3 factors have also been ascribed a tumor suppressor function. Here, we utilized the MMTV/Wnt1 mouse strain to further interrogate the role of PEA3 transcription factors in mammary tumorigenesis based on our previous observation that Pea3 is highly expressed in MMTV/Wnt1 mammary tumors. Pea3 expression in mouse mammary tissues was visualized using a Pea3NLSlacZ reporter strain. In normal mammary glands, Pea3 expression is predominantly confined to myoepithelial cells. Wnt1 transgene expression induced marked amplification of this cell compartment in nontumorous mammary glands, accompanied by an apparent increase in Pea3 expression. The pattern of Pea3 expression in MMTV/Wnt1 mammary glands recapitulated the cellular profile of activated β-catenin/TCF signaling, which was visualized using both β-catenin immunohistochemistry and the β-catenin/TCF-responsive reporter Axin2NLSlacZ. To test the requirement for PEA3 factors in Wnt1-induced tumorigenesis, we employed a mammary-targeted dominant negative PEA3 transgene, ΔNPEA3En. Expression of ΔNPEA3En delayed early-onset tumor formation in MMTV/Wnt1 virgin females (P = 0.03), suggesting a requirement for PEA3 factor function for Wnt1-driven tumor formation. Consistent with this observation, expression of the ΔNPEA3En transgene was profoundly reduced in mammary tumors compared to nontumorous mammary glands from bigenic MMTV/Wnt1, MMTV/ΔNPEA3En mice (P = 0.01). Our data provide the first description of Wnt1-mediated expansion of the Pea3-expressing myoepithelial compartment in nontumorous mammary glands. Consistent with this observation, mammary myoepithelium was selectively responsive to Wnt1. Together these data suggest the MMTV/Wnt1 strain as a potential model of basal breast cancer. Furthermore, this study provides evidence for a protumorigenic role of PEA3 factors in breast neoplasia, and supports targeting the PEA3 transcription factor family in breast cancer.
Partial Text: The mammalian Ets transcription factor superfamily comprises around 26 proteins characterized by highly related DNA-binding domains containing winged-helix-turn-helix motifs –. Ets factors regulate transcription through binding of this ETS DNA binding domain directly to Ets binding sites in the promoters of target genes. The Ets factor PEA3 (also called E1AF and ETV4) was originally identified through its ability to bind a motif in the polyomavirus enhancer and mediate oncogene-dependent activation, and was subsequently assigned to a subfamily of three closely related Ets proteins: PEA3, ERM (ETV5) and ER81 (ETV1) –. PEA3 family members exhibit high sequence homology within their ETS domains, and also have conserved regulatory domains –. Several PEA3-interacting proteins have been described that function as allosteric regulators and transcriptional coactivators , , . Additionally, both the activity and expression of PEA3 factors can be regulated by receptor tyrosine kinases through MAP kinase signaling , . PEA3 factors are expressed in discrete patterns during embryogenesis, and contribute to branching morphogenesis in epithelia as well as to the establishment of motor-neuronal circuitry –. Functional overlap between individual PEA3 factors is suggested not only by their overlapping target specificities, but also by the modest phenotype of Pea3 knockout mice .
We and others have shown that Pea3 is highly expressed in mouse breast tumors, and PEA3 overexpression also occurs in human breast carcinomas –, –, . Furthermore, Pea3 activates transcription of multiple neoplasia-associated genes, including MMP genes, COX-2, Twist, osteopontin, uPA, vimentin, MUC4, WT1, mammaglobin, cyclin D3 and HER2, , , –. For example, we have shown PEA3 to be a potent activator of COX-2 transcription, and in turn have shown Cox-2 to be an important contributor to HER2/neu-induced mammary neoplasia and angiogenesis in mouse models , , . Nevertheless, other investigators have argued in favor of PEA3 functioning as a tumor suppressor, based in part on the ability of high level PEA3 expression to suppress HER2 promoter activity in vitro and HER2-expressing tumor growth in vivo, , , . Consistent with a tumor-promoting function for PEA3, one study identified a positive association between increased patient survival and PEA3 expression in human breast cancers .