Increased Smad3 and reduced Smad2 levels mediate the functional switch of TGF-β from growth suppressor to growth and metastasis promoter through TMEPAI/PMEPA1 in triple negative breast cancer

Screening of several TNBC cell lines showed altered Smad2 and Smad3 protein levels compared to normal mammary epithelial cells, suggesting the possibility that it could play an important role in the escape of cancer cells from TGF-β mediated growth inhibition. To assess the functional relevance of these endogenous molecules, Smad2 or Smad3 expression was knocked down individually and assessed their effects on pro-oncogenic properties of TGF-β. Smad3 deficiency reduced growth and invasion capacity of breast cancer cells in comparison to Smad2 which had no effect. Smad3 deficiency was also found to be associated with a reduction in the expressions of TMEPAI/PMEPA1 and EMT inducing transcription factors, E-Cadherin and increased expression of cell cycle inhibitors and Vimentin. On the other hand, Smad2 deficiency had opposite effect on these regulators. Interestingly, 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. Similarly, altered levels of Smad proteins and TMEPAI were also noted in primary TNBC tumor tissues. Analysis of the existing databases provided additional support in terms of TMEPAI and Smad2 expression impacting the survival of TNBC patients. Taken together, our data demonstrate a novel role for Smad3 in cancer transformation and cancer progression through TMEPAI and further suggest that selective targeting of TGF-β-Smad3-TMEPAI axis may be beneficial in triple negative breast cancer therapy and prevention.