f 14-3-3s phosphorylation at Ser69 and Ser74 residues. We also observed co-localization of Smad3 and wild-type 14-3-3s in cells using immunofluorescence staining. Thus, the interaction of Smad3 and 14-3-3s requires phosphorylation on Ser69 and Ser74 in 14-3-3s. Smad3 is a transcription factor which binds directly to a specific promoter 16494499 element CAGA. Thereby, we explored whether phosphorylation and 
interaction of 14-3-3s with Smad3 regulates TGFb/Smad3-dependent transcriptional activation of CAGA -luc luciferase reporter. Our data suggest that overexpression of the wild type or a single Ser74 mutant 14-3-3s protein could significantly increase TGFb and Smad3-dependent transactivation activity of the reporter. However, the abrogation of TGFb dependent phosphorylation at Ser69 and Ser74 abolished the ability of 14-3-3s to co-activate TGFb and Smad3-dependent transcription. Analysis of proteins bound to CAGA element showed that the presence of Smad3 and its upper migrating form correlated with stimulatory effect on transcriptional activity. No detection of the DCC-2036 double mutant of 14-3-3s also correlated with decreased transcriptional activation. Moreover, TGFb dependent transactivation of Smad3 responsive genes PAI-1 and COL7A1, possibly through CAGA box elements in their promoters, correlated with involvement of 14-3-3s in the transcriptional complex with endogenous Smad3, and confirms importance of phosphorylation of 14-3-3s at Ser69 and Ser74 for tuning of Smad3-dependent transcriptional activation. Thus, TGFb-dependent phosphorylation of 14-3-3s is a feedforward mechanism for TGFb/Smad3 transcriptional regulation. The feed-forward tuning included 14-3-3s recruitment to the Smad3-initiated transcriptional complex, which depends on the phosphorylation state of 14-3-3s at Ser69 and Ser74. Phosphoproteomics of TGFb1 Signaling TGFb1-dependent phosphorylation of 14-3-3s regulates MCF7 tumor progenitor population A 20032260 large body of evidence has demonstrated that many human tumors contain a heterogeneous mixture of different cell types and are maintained by a small cell population called cancer stem cells or tumor progenitors, which is responsible for tumor formation and metastasis, and implicated to therapy resistance. TGFb signaling has been implicated in the maintenance of both normal somatic stem cells and cancer stem cells. TGFb axis plays a dual role in cancer progression switching from tumor Phosphoproteomics of TGFb1 Signaling suppression in early stages of cancer to promoting invasion and metastasis at later stages. Therefore, TGFb may have either a CSC-suppressing or CSC-promoting functions depending on cellular context. In order to explore the impact of TGFb dependent 14-3-3s phosphorylation on the maintenance of cancer progenitor population, we analyzed the MCF7 cells overexpressing 14-3-3s wild type or mutant proteins in sphere formation assay. We found that activation of TGFb signaling reduced the sphere formation properties of MCF7 cells in the manner dependent on 14-3-3s phosphorylation. Notably that overexpression of the wild type 143-3s and mutant protein 14-3-3s Ser74Ala and 14-3-3s Ser69/ 74Ala had significantly higher impact on TGFb dependent inhibition of sphere initiating cells than expression of 14-3-3s Ser69Ala mutant protein suggesting that TGFb1 dependent phosphorylation of 14-3-3s at Ser 69 and Ser 74 can play a different role in regulation of CSCs by TGFb1 activation. Despite the marginal effect of the double mutant 14-3-