Abstract P3-14-09: Targeting TMEPAI/ PMEPA1 inhibited triple negative breast cancer cell growth and metastasis through growth suppressive TGF-β signaling

Background: Triple negative breast cancers (TNBC) that lack estrogen receptor (ER), progesterone receptor (PR) and hormone epidermal growth factor receptor 2 (HER-2/neu) are aggressive and cause high mortality among breast cancer patients. Transforming growth factor beta (TGF-β) dependency for their aggressive behavior (growth and metastasis) has been well established in many of these tumors. Although targeting TGF-β signaling has major potential to treat TNBCs, however it carries the risk of disturbing the tumor suppressive effects of TGF-β in early tumors and its homeostatic control of normal tissues. Hence there is a need to identify novel targets to impede tumor progression without compromising the beneficial effects of TGF-β signaling. To satisfy this unmet need, earlier we identified that transmembrane prostate androgen induced (TMEPAI/PMEPA1), a direct target gene of TGF-β could act as a "molecular switch" that converts TGF-β from a tumor suppressor to promoter in TNBC. Thus, we undertook the present study that identified a novel compound that blocked TMEPAI expression. Materials and Methods: All cell lines were cultured according to the ATCC recommendations. Cell proliferation was measured by quantitation of total cellular DNA. Immunoblotting, migration, invasion, tumor xenografts and lung metastasis were performed using standard methods. Results: We identified a terpenoid derivative (TD) which inhibited the expression of TMEPAI, enhanced TGF-β signaling and blocked proliferation, migration and invasion of several triple negative breast cancer cells in vitro. Our results showed that TD increased phosphorylation of Smad2/3 and increased PTEN, p21 and p27 proteins that cause growth suppression. Concomitantly, TD decreased Akt phosphorylation and reduced Snail and Slug that are required for cell growth and metastasis. Interestingly, TD did not affect the growth of normal human mammary epithelial cells and failed to cause associated molecular changes that can result in growth suppression. Notably, using mice models (nude mice and syngeneic BALB/c mice), we showed that TD reduced tumor burden significantly with little or no toxicity. Consistent with its ability to inhibit Akt phosphorylation and induction of Snail and Slug proteins, TD suppressed lung metastasis of TNBC in mice model. Importantly, reduced tumors derived from TD treated mice exhibited increased expression of pSmad2/3, PTEN, p21, p27 and reduced expression of pAkt compared to tumors obtained from vehicle treated mice. Conclusions: Our results have two important dimensions: On one hand, TD inhibited TMEPAI induction to promote growth suppressive TGF-β dependent Smad signaling in TNBC but not in normal cells. On the other hand, TD also inhibited non-Smad signaling that promotes growth and metastasis of TNBC both in vitro and in vivo. Hence our findings suggest that drugs that target TMEPAI will not only inhibit growth and metastasis of TNBC but will also restore homeostatic functions of TGF-β to prevent new tumor development. Citation Format: Singha PK, Pandeswara S, Venkatachalam MA, Saikumar P. Targeting TMEPAI/ PMEPA1 inhibited triple negative breast cancer cell growth and metastasis through growth suppressive TGF-β signaling. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-14-09.