Abstract LB-103: Conditional loss of ErbB3 delays mammary gland hyperplasias induced by mutant PIK3CA in transgenic mice, but does not affect mammary tumor latency, gene expression or signaling

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL The phosphatidylinositol-3-kinase (PI3K) pathway is the most frequently mutated pathway in breast cancer. Signaling through the ErbB family of receptor tyrosine kinases (RTKs) strongly activates PI3K. Six YXXM PI3K binding motifs in ErbB3 make this RTK a strong upstream activator of PI3K. Mutations in PIK3CA, the gene encoding the p110α catalytic subunit of PI3K, are associated with increased PI3K activity, enhanced transformation, and drug resistance. We hypothesized that upstream receptors, such as ErbB3, are still required to localize PI3K to the plasma membrane and maximally drive tumor formation induced by mutant PI3K. To test this hypothesis, we generated a doxycycline (Dox)-inducible system in triple transgenic mice (MMTV-rtTA, Tet-Op-HA-PIK3CAH1047R-IRES-Luc and Tet-Op-Cre) which are homozygous or heterozygous for floxed ErbB3 alleles. In this model, referred to as iPI3K.iCre.ErbB3FL/FL, Dox induces the expression of PIK3CAH1047R and Cre recombinase. This results in Cre-mediated deletion of floxed ErbB3 alleles in mammary cells expressing PIK3CAH1047R. Loss of ErbB3 delayed mammary hyperplasias induced by PIK3CAH1047R in 12-week old animals. The mean mammary tumor latency of iPI3K.iCre.ErbB3FL/+ animals was 398 days compared to 419 days in iPI3K.iCre.ErbB3FL/FL animals (p=0.93). Both ErbB3FL/Fl and ErbB3+ tumors showed mixed pathologies of solid, papillary or cribiform adenocarcinomas (with and without squamous metaplasia). The heterogeneity of PI3K mutant tumors was also shown by the mixed expression of cytokeratin 8-positive (luminal-like) and cytokeratin 5-positive (basal-like) epithelial cells, which was not altered by loss of ErbB3. Microarray analysis of tumors revealed similar gene expression patterns in tumors with or without ErbB3. Immunoblot analysis of tumor lysates confirmed the absence of ErbB3 protein in iPI3K.iCre.ErbB3FL/FL tumors. However, downstream effectors of PI3K such as Akt, PDK1, SGK1, GSK3, 4EBP1 and S6 were similarly phosphorylated in tumors with and without ErbB3 expression. RTKs other than ErbB3, such as ErbB2, PDGFR, EGFR and MSPR, were phosphorylated in both tumor types. In ErbB3+ tumors, P-ErbB3 associated with p85, the regulatory subunit of PI3K, and coprecipitated with p85 antibodies. In ErbB3-deficient tumors and in breast cancer cell lines where ErbB3 was knocked down with RNAi, several P-Tyr proteins remained bound to p85, including the adaptor molecules IRS-1 and Gab2. We speculate these molecules circumvent a potential requirement for ErbB3 in cancers induced and driven by mutant PIK3CA. However, ErbB3 is required for the early mouse mammary hyperplasias induced by mutant PI3K. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-103. doi:1538-7445.AM2012-LB-103