Abstract 1958: Opposing roles of Smad2 and Smad3 in the regulation of growth and invasion of TNBC through TMEPAI
Authors: Singha Prajjal K. , Pandeswara Srilakshmi , Venkatachalam Manjeri A. , Saikumar Pothana 
Published: July 2017

The dependency of triple negative breast cancers (TNBC) on TGF-β signaling activity for their growth and metastasis is well documented. Breast tumors develop resistance to TGF-β-induced growth inhibition, which helps in tumor progression. Additionally, TGF-β is enriched in the tumor microenvironment and is a key inducer of epithelial to mesenchymal transition (EMT) in breast cancer. In an effort to understand the mechanisms by which TGF-β induces breast tumorigenesis, we sought to identify the relative individual contributions of Smad2 and Smad3 in mediating cancer cell growth and metastasis by TGF-β. Screening of several TNBC cell lines showed increased Smad3/Smad2 expression ratio compared to normal mammary epithelial cells, which suggested breast cancer cells may escape TGF-β mediated cell cycle arrest by altering their relative expressions of Smad3 and Smad2. Since overexpression of R-Smads may not always accurately reflect the functional status of the endogenous molecules, we individually knocked down each R-Smad and tested its effect on pro-oncogenic behavior of TGF-β. Although Smad2 deficiency has no effect on breast cancer cell behavior, Smad3 deficiency reduced growth and invasion capacity of breast cancer cells. Interestingly, while Smad3 deficiency was associated with reduced TMEPAI/PMEPA1 and EMT gene expressions and increased expression of cell cycle inhibitors, Smad2 deficiency had opposite effect on these regulators. Moreover, the decreased growth, invasion and associated gene expressions were largely reversed by overexpressing TMEPAI in Smad3 knockdown cells, suggesting that Smad3-TMEPAI axis may be involved in subverting growth suppressive effects of TGF-β into growth promotion. Compared to control and Smad2 deficient cells, Smad3 deficient cells had markedly elevated PTEN protein levels and suppressed Akt phosphorylation both under basal conditions and TGF-β stimulation. We identified that Smad3 but not Smad2 plays an important role in stimulating TMEPAI expression, growth and EMT in triple negative breast cancer cells by promoting TGF-β dependent non-canonical signaling by decreasing the cellular content of PTEN and p27 in a TMEPAI dependent manner. Taken together, the results demonstrate a novel role for Smad3 in cancer transformation and cancer progression through TMEPAI. Current therapeutic strategies are aiming at antagonizing the whole TGF-β signaling thereby completely blocking both Smad2 and Smad3 signaling pathways. Selective targeting of TGF-β-Smad3-TMEPAI axis may be more beneficial in triple negative breast cancer therapy and prevention.

Citation Format: Prajjal K. Singha, Srilakshmi Pandeswara, Manjeri A. Venkatachalam, Pothana Saikumar. Opposing roles of Smad2 and Smad3 in the regulation of growth and invasion of TNBC through TMEPAI [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1958. doi:10.1158/1538-7445.AM2017-1958