Abstract 1316: Overactivation of AKT promotes hormone-independent mammary tumors

Abstract

Abstract Using a mouse mammary tumor model induced by medroxyprogesterone acetate (MPA) that transits through different stages of hormone dependence, we have previously reported that the activation of the PI3K/AKT pathway is critical for the growth of hormone-independent (HI) mammary tumors, but not for the growth of a hormone-dependent (HD) tumor variant. The main aim of this work was to explore whether the activation of the PI3K/AKT pathway per se is responsible for the transition from a HD to a HI tumor phenotype. To answer this question we infected primary cultures of epithelial cells obtained from HD tumors with a retrovirus that directs the expression of a constitutively active form of AKT1 (myristoylated AKT1, myrAKT1), or with the empty vector as control. We inoculated 103 cells in nude mice (n = 6). MyrAKT1 expressing HD-derived cells were able to form mammary tumors even in the absence of MPA, that is, they behaved as HI cells. These tumors were highly differentiated and displayed a ductal phenotype with a pattern of expression of cytokeratin 8 and laminin-1 typical of HI tumors. Ligand-independent phosphorylation of progesterone receptor (PR) in Ser 190 was also increased in myrAKT1 tumors supporting our hypothesis that overactivation of AKT is directly involved in the acquisition of hormone independence. Furthermore, comparing myrAKT tumors with uninfected HD tumors revealed that overactivation of AKT did not increase the incidence of lung metastasis, indicating that AKT1 is involved specifically with tumor differentiation and growth rather than with tumor cell invasiveness. In vitro, primary cultures of mammary tumor cells modified to express myrAKT1 showed similar levels of proliferation determined by Ki67 and H3-tymidine incorporation as compared to control (cells infected with the empty vector), suggesting that the in vivo tumorigenic effect of myrAKT1 cells described above might require cell-cell or cell-matrix interactions absent in vitro. Interestingly, we found that conditioned medium from myrAKT1 tumor cells increase endogenous AKT1 activity when tested in untransfected cells. Moreover, conditioned medium from HI carcinoma associated fibroblasts (CAF-HI) increased the level of activation of the PI3K/AKT pathway in epithelial cells. Altogether, these results indicate that paracrine signals derived from the tumor cells and/or its microenvironment participate in the activation of the PI3K/AKT pathway, ultimately leading to tissue differentiation and ligand-independent mammary tumor growth. Thus, the PI3K/AKT pathway might be a key therapeutic target to prevent progression to autonomous and endocrine resistant breast tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1316. doi:10.1158/1538-7445.AM2011-1316