Cyclin overexpression inhibits Smad 3 tumor suppression in breast cancer cells

Abstract #5033

Introduction: The transcription factor Smad 3, a member of the TGFβ signaling cascade, contributes to G1 cell cycle arrest in breast cancer cell lines. Overexpression of the cell cycle regulatory protein cyclin E, and its low molecular weight (LMW) counterparts, has been associated with poor prognosis in breast cancer. Cyclin E has been shown to mediate phosphorylation within or near the linker region of Smad 3 by cyclin dependent kinase 2 (CDK 2), and to inhibit Smad 3 activity in normal mouse fibroblasts and epithelial cells. We hypothesize that overexpression of cyclin E may exert tumorigenic effects in breast cancer cell lines through the functional inhibition of Smad 3 mediated by CDK 2 phosphorylation.
 Methods: Endogenous levels of Smad 3 and phosphorylated Smad 3 in parental, vector only control, full length (FL), and two LMW forms of cyclin E overexpressing MCF-7 breast cancer cell lines were determined by western blotting. To elucidate the impact of cyclin E overexpression on Smad 3 function, constructs containing an empty vector, WT Smad 3, Smad 3 with a CDK phosphorylation site mutation within the linker region (Thr178), and outside the linker region (Thr8) were co-transfected with a Smad 3 reporter into the panel of MCF-7 cells. CDK2 or scrambled control siRNA was also transfected into the study panel of cells. Smad 3 function was then evaluated by luciferase reporter assays.
 Results: MCF-7 cells overexpressing FL or LMW cyclin E had a higher level of P- SMAD 3 as compared to the parental and vector only control cells. Total SMAD 3 levels were similar in all cells examined. Transfection of WT Smad 3 elicited an increase in reporter activity in parental and vector control cells, while both FL and LMW forms of cyclin E overexpressing cells were resistant to WT Smad 3 reporter induction. In parental and vector control MCF-7 cells, expression of the Smad 3 Thr178 mutation resulted in a three-fold induction in Smad 3 reporter activity as compared to the WT Smad 3 response. In FL and LMW cyclin E overexpressing cells, induction by the Thr178 mutation was more pronounced, as seen by a 5-fold and 8-fold induction of reporter activity in FL and LMW cyclin E cells, respectively. Transient transfection of the Thr8 mutation failed to evoke a reporter response in any of the panel of MCF-7 cells. MCF-7, FL, and LMW cyclin E cells demonstrated the greatest fold induction of Smad 3 reporter activity when transfected with CDK 2 siRNA.
 Conclusions: Overexpression of cyclin E may circumvent the growth inhibitory effects of Smad 3 signal transduction through the modulation of Smad 3 function. Specifically, this work indicates that cyclin E mediates Smad 3 inhibition in a CDK 2 dependent fashion in breast cancer cells.

Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 5033.