Abstract 1947: Divergent effects of Smad2 and Smad3 on TGF-β mediated breast cancer cell growth and expression of TMEPAI/PMEPA1
Authors: Singha Prajjal K. , Pandeswara Srilakshmi , Venkatachalam Manjeri A. , Saikumar Pothana 
Published: August 2015

We showed previously that TGF-β induces a delayed growth spurt to increase the proliferation of MDA-MB-231 breast cancer cells, not growth suppression that is the expected response in normal epithelium. We also reported that knockdown of amplified TMEPAI reversed the high rates of proliferation of breast cancer cells under basal conditions as well as in the presence of TGF-β. Despite the demonstrable effects of amplified TMEPAI on canonical signaling by TGF-β, the role played by R-Smads in their TGF-β dependent growth remained enigmatic.

MDA-MB-231 cancer cells showed higher content of Smad3 than Smad2, and TMEPAI knockdown did not modify this profile. Smad2 knockdown showed increased TMEPAI in response to TGF-β relative to cells with control shRNA. In contrast, Smad3-knockdown resulted in decreased TMEPAI, suggesting that TGF-β induced TMEPAI expression requires Smad3. Selective Smad2 or Smad3 knockdown had distinctive effects on basal and TGF-β induced growth of MDA-MB-231 cells. Smad2 knockdown that increased TMEPAI expression over basally high levels did not modify the normally high rates of basal or TGF-β induced growth of MDA-MB-231 cells. In contrast, Smad3 knockdown, which reduced TMEPAI expression, greatly decreased the growth of MDA-MB-231 cells basally and in response to TGF-β, a pattern similar to the effects of TMEPAI knockdown. In contrast to breast cancer cells, normal mammary epithelial cells HMEC have more Smad2 than Smad3 and express little TMEPAI basally that increased only modestly after TGF-β treatment. Also, unlike breast cancer cells, HMEC were growth arrested by TGF-β and the TGF-β induced growth suppression was partially reversed by exogenously expressed TMEPAI. Furthermore, knockdown of either Smad2 or Smad3 in HMEC partially rescued them from TGF-β induced growth suppression. Smad2 knockdown did not modify TGF-β-mediated growth patterns of breast cancer cells, whereas Smad3 knockdown, which is expected to increase cell growth by decreasing Smad signaling, instead dramatically decreased the growth of MDA-MB-231 cells while concomitantly decreasing TMEPAI expression. Indeed, the growth patterns of MDA-MB-231 cells with Smad3 knockdown and TMEPAI knockdown were virtually identical. Because Smad3 and TMEPAI appear to produce similar effects on MDA-MB-231 cell growth, the results suggest that subversion of the growth suppressive effects of TGF-β in MDA-MB-231 cells occurs as a result of signaling downstream of Smad3-TMEPAI, likely mediated by PTEN.

Citation Format: Prajjal K. Singha, Srilakshmi Pandeswara, Manjeri A. Venkatachalam, Pothana Saikumar. Divergent effects of Smad2 and Smad3 on TGF-β mediated breast cancer cell growth and expression of TMEPAI/PMEPA1. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1947. doi:10.1158/1538-7445.AM2015-1947