Abstract 5179: TMEPAI, a negative regulator of Smad signaling switches tumor suppressive TGF- β to a tumor promoter in breast tumorigenesis
Authors: Singha Prajjal Kanti , Pandeswara Srilakshmi , Venkatachalam Manjeri A. , Saikumar Pothana 
Published: June 2012

Despite major advances in hormonal, surgical and chemical therapies, triple negative breast cancers and therapy resistant breast cancers still pose a challenge due to lack of suitable treatment targets. Dependency on transforming growth factor beta (TGF-β) signaling activity for their late growth and metastasis is one of the hallmarks in several of these patients. Thus targeting TGF-β signaling has major potential in treating theses patients. However, antagonists of TGF-β signaling pathway carry the risk of disturbing the tumor suppressive homeostatic control of TGF-β in normal tissues and early cancers. Hence, identification of a marker which will make it feasible to develop drugs that retard tumor progression without compromising normal growth suppression by TGF-β in normal tissues.

Earlier we have shown that TMEPAI, a TGF-β inducible gene has potential role as a molecular switch that converts tumor suppressive TGF-β into a tumor promoter role. In the present study, we show that TMEPAI knockdown has increased TGF-β signaling as evidenced by increased stimulation of Smad signaling measured by Smad binding elements (CAGAx12) driven reporter (luciferase) activity. While CAGA driven luciferase activity is modestly stimulated (∼5 fold) by TGF-β in wild-type MDA-MB-231 (Wt) cells, a robust stimulation (∼96 fold) by TGF-β was observed in TMEPAI knockdown MDA-MB-231 cells (KD). Exogenous expression of mouse TMEPAI in these knockdown cells reduced Smad stimulation by TGF-β to modest levels (∼8 fold). Although Wt cells showed enhanced Smad2 and Smad3 phosphorylation and increased expression of TMEPAI in response to TGF-β, TMEPAI deficiency in KD cells resulted in increased phosphorylation of Smad2 and Smad3 by TGF-β. Moreover, while proliferation of mammary epithelial cells (HME) from normal human mammary epithelium, immortalized by hTERT, is inhibited by TGF-β, Wt cancer cells showed a biphasic growth response to TGF-β. TMEPAI-deficiency blocked the growth response to TGF-β in KD cells. Interestingly, both HME and TMEPAI-deficient KD cells showed robust stimulation (∼95 fold) of Smad signaling that is reversed by exogenous expression of TMEPAI. Furthermore, our studies identified that TMEPAI subverts tumor suppressive TGF-β dependent Smad signaling into tumor promotive non-Smad signaling through stimulation of stress activated protein kinases. Inhibition of stress kinases also reduced the growth of cancer cells just as TMEPAI deficiency. In summary, our results show that TMEPAI subverts Smad signaling into non-smad signaling to promote growth and motility of cancer cells and provide new clues to develop an effective and decisive anti-TGF-β therapy against aggressive breast cancers.

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 5179. doi:10.1158/1538-7445.AM2011-5179