Background: Triple negative (ER, PR and Her2 –negative) breast cancers (TNBC) depend on transforming growth factor beta (TGF-b) signaling activity for their late growth and metastasis. Thus TGF-β signaling antagonism has major potential in treating TNBC. However, this approach carries the risk of disturbing the tumor suppressive homeostatic control of TGF-β in normal tissues and early tumors. In this context, we identified that Transmembrane prostate androgen induced (TMEPAI) could act as a “molecular switch” that converts TGF-β from a tumor suppressor to promoter. Therefore, we hypothesized that therapeutic targeting of TMEPAI will impede tumor progression without compromising TGF-β dependent homeostasis of normal cells. Hence, we undertook the present study to evaluate the role of TMEPAI in the regulation of TGF-β signaling in breast cancer progression and screened drugs that can block TMEPAI expression as well as breast cancer cell growth and metastasis.

Materials and Methods: All cell lines were cultured according to the recommended standard procedures. Lentiviral mediated expression vector was used to stably knockdown endogenous TMEPAI expression. DNA transfections and luciferase assays were performed according to vendor instructions. Cell proliferation was measured by quantitation of total cellular DNA. Immunoblotting, tumor xenografts, migration, invasion and immunohistochemical assays were performed using standard methods.

Results: Our results showed increased stimulation and prolonged sustenance of Smad2 and Smad3 phosphorylation and Smad-driven luciferase reporter activity by TGF-β in TMEPAI knock-down MDA-MB-231 triple negative breast cancer cells. In contrast, exogenous expression of TMEPAI in normal human mammary epithelial cells (HMEC) or TMEPAI-deficient MDA-MB-231 cells reduced Smad dependent TGF-β signaling. While proliferation of HMEC is inhibited by TGF-β, MDA-MB-231 cells showed a biphasic growth response to TGF-β, which is inhibited by TMEPAI-deficiency. Moreover, TMEPAI subverts tumor suppressive TGF-β mediated Smad signaling into tumor promotive non-Smad signaling through stimulation of mammalian mitogen-activated protein kinases (MAPKs), pAkt and reduced PTEN while TMEPAI-deficiency prevented MAPKs, Akt activation and enhanced PTEN. Furthermore, immunohistochemistry revealed high TMEPAI expression in several aggressive human breast tumor samples but not in the matched normal human breast tissue specimens. Interestingly, drug screening resulted in the identification of a terpenoid derivative (TD) that blocked the expression of TMEPAI and inhibited the proliferation, migration and invasion of cancer cells in vitro and lung metastasis of breast cancer cells in vivo. Notably, TD did not affect the growth of normal human mammary epithelial cells. Remarkably, TD enhanced TGF-β mediated Smad signaling and inhibited growth promoting non-Smad-signaling in breast cancer cells.

Conclusions: Hence our findings suggest that TMEPAI not only serves as a novel biomarker but also as a suitable therapeutic target for aggressive breast cancers. Drugs that target TMEPAI may develop an effective anti-TGF-β therapy against aggressive breast cancers without disturbing the growth suppressive homeostatic control of TGF-β in normal tissues and early tumors.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P4-15-06.