E3ANDTGFBINCANCER · Role of E3 Ub-ligases in TGF-beta signaling and tumorigenesis

The binding of TGF-b to its receptors induces phosphorylation of Smad2 and Smad3, their subsequent association to Smad4 and their nuclear translocation. In the nucleus, the Smad3/Smad4 and Smad2/Smad4 heterodimers act as transcription factors and induce specific target genes. TGF-b pathway has a tumor suppressor function characterized by its ability to inhibit cell growth. Many cancer cell lines have acquired resistance to the antiproliferative effect of TGF-b. This is mainly achieved through loss of function mutations or deletions that affect TGF-b receptors or Smad4, particularly in colon cancer and pancreatic cancer. During my Postdoctoral training in Caroline Hill’s laboratory, I have shown that the E3 Ub-ligase Arkadia is involved in the degradation of the transcriptional repressor SnoN and is therefore absolutely required for the activation of Smad3/Smad4 target genes such as PAI-1. The laboratory of Azeddine Atfi has recently performed a two-hybrid screen using another repressor of the pathway, TGIF, as a bait. This study has led to the identification of a new partner of TGIF, TIP11 (TGIF interacting protein 11), which is a new potential E3 Ub-ligase. Preliminary results indicate that TIP11 also contributes to TGF-b responses. Recently, we both have independently identified that Arkadia and TIP11 can be altered in tumor cell lines. Our study aimed to determine whether these two E3 Ub-ligases, essential for the TGF-b pathway, are novel tumor suppressor genes. Our objectives are: (1) to identify Alterations of Arkadia and TIP11 in human carcinoma cell lines and tumor samples. (2) to study the function of Arkadia and TIP11 by reintroducing Arkadia or TIP11 in deficient cell lines, in order to better understand their role in TGF-b signaling and tumorigenesis. (3) to unravel the mechanism of action of these two E3 Ub-ligases by a biochemistry approach. (4) to identify novel binding partners of Arkadia and TIP11 by performing a two-hybrid screen.