The expression of 14-3-3 proteins is dysregulated in vari-ous types of cancer. This study was undertaken to investigate the effects of 14-3-3 ζ and 14-3-3 δ on cell growth inhibition mediated by transforming growth fac-tor-beta 1 (TGF-β1). Mouse mammary epithelial cells (Eph4) that are transformed with oncogenic c-H-Ras (EpRas) and no longer sensitive to TGF-β1-mediated growth inhibition displayed increased expression of 14-3-3 ζ and decreased expression of 14-3-3 δ compared with parental Eph4 cells. Using small interfering RNA-mediated knockdown and overexpression of 14-3-3 δ or 14-3-3 ζ, we showed that 14-3-3 δ is required for TGF-β1-mediated growth inhibition whereas 14-3-3 ζ negatively modulates this growth inhibitory response. Notably, overexpression of 14-3-3 ζ increased the level of Smad3 protein that is phosphorylated at linker regions and cannot mediate the TGF-β1 growth inhibitory response. Consistent with this finding, mutation of the 14-3-3 ζ phosphorylation sites in Smad3 markedly reduced the 14-3-3 ζ-mediated inhibition of TGF-β1-induced p15 promoter-reporter activity and cell cycle arrest, suggesting that these residues are critical targets of 14-3-3

Keywords: 14-3-3 δ, 14-3-3 &,xeta,, cell growth inhibition, phosphorylation of Smad3 linker region, TGF-β1