Endometrial carcinoma is the leading type of gynecological cancer in the Western world. Dysregulation of apoptosis is an early event in the pathogenesis of endometrial carcinomas. Tumour cells often acquire intrinsic resistance to the growth inhibitory and pro-apoptotic effects of transforming growth factor-beta (TGF-β). In addition, the cytokine can confer invasive properties to established tumour cells. The three mammalian TGF-β isoforms (TGF-β1, -β2 and -β3) are present in endometrial tumours, and several lines of evidence suggest that endometrial carcinoma cells are resistant to TGF-β growth inhibitory effects. Underlying mechanisms, however, are poorly understood. We have examined the signaling pathways activated by each TGF-β isoform in endometrial carcinoma cells, and compared their effect on cellular proliferation, apoptosis and invasiveness, using KLE and Hec-1A endometrial carcinoma cell lines as in vitro models. Our results showed that each TGF-β isoform triggers proper Smad signaling in human endometrial carcinoma cell lines, and efficiently inhibits cellular proliferation. In addition, we found that each TGF-β isoform upregulates X-linked inhibitor of apoptosis protein (XIAP) levels in endometrial carcinoma cells, which confers resistance to TGF-β-induced apoptosis. Noteworthy, TGF-β3, but not TGF-β1 or TGF-β2, increased in vitro invasiveness of endometrial carcinoma cells. Using RNA interference (RNAi) and pharmacological inhibitors, we found that TGF-β3-induced invasion involved two distinct pathways: phosphatidylinositol 3-kinase-dependent upregulation of XIAP, and protein kinase C-dependent induction of matrix-metalloproteinase-9 expression. In addition, silencing of Smad4 using RNAi blocked the ability of TGF-β3 to upregulate XIAP and to induce invasion. Such a role for TGF-β3 in regulating survival and invasiveness of endometrial cells in vitro correlates well with in vivo data. Indeed, large-scale immunofluorescence analysis revealed the presence of TGF-β3 in human endometrial carcinoma tissue samples; the cytokine localized mostly to epithelial cells in normal endometrium, but upon progression towards a more invasive phenotype (from stage I to stage III endometrial adenocarcinoma), TGF-β3 immunoreactivity gradually extended from the epithelial compartment to the stroma, consistent with a role for TGF-β3 in cellular invasiveness. Collectively our results highlight for the first time a role for TGF-β3, Smad4 and XIAP in tumour cell invasion, and thereby identify promising targets for the treatment of endometrial carcinoma.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA