Novel Regulators of the TGF-β Signaling Pathway

The transforming growth factor-β (TGF-β) superfamily consists of related multifunctional cytokines, which include TGF-βs, activins, and bone morphogenetic proteins (BMPs) and coordinate several biological responses in diverse cell types. The biological activity of TGF-β members is executed by transmembrane serine/threonine kinase receptors and intracellular Smad proteins. The effects of TGF-β on the epithelium are of high interest. Carcinomas (tumors of epithelial origin) are the most common type of human cancer and frequently exhibit aberrant responses to TGF-β. Therefore, TGF-β can be defined as tumor suppressor as it inhibits growth of normal epithelial cells. However, TGF-β also induces an epithelial-mesenchymal transition (EMT), a key component of metastasis, and thus promotes cancer spread. The scope of this thesis is the mechanism of TGF-β signaling in epithelial cells. We established that only TGF-β, but not BMP pathways can elicit EMT. Moreover, we found that Smad signaling is critical for regulation of EMT. In a transcriptomic analysis, we identified a large group of novel genes, whose regulation is pivotal for TGF-β-induced EMT and metastasis. We focused on two of such genes, Id2 and Id3. Interestingly, we found that TGF-β-induced repression of Ids is necessary for inducing EMT and potent cell cycle arrest. BMP increases expression of Ids and therefore it cannot induce the same biological responses as TGF-β. Hence, knock-down of endogenous Id2 and Id3 proteins sensitized epithelial cell to BMP-7. We proposed a model, in which Id2 and Id3 are important components controlling concerted regulation of cell proliferation and EMT downstream of TGF-β pathways. Furthermore, we identified a serine/threonine kinase, SNF1LK, whose mRNA is rapidly induced by TGF-β in epithelial cells. We found that SNF1LK is a negative regulator of the TGF-β pathway and it promotes TGF-β receptor turnover. Subsequently, we demonstrated that SNF1LK together with Smad7 and Smurf2 targets TGF-β receptor for ubiquitin-dependent degradation. Furthermore, SNF1LK interacts with proteasomes, suggesting that SNF1LK serves as bridge between ubiquitinated receptors and proteasomes, helping proteasomes to recognize the ubiquitinated cargo destined for degradation. We therefore established a novel negative feedback regulatory mechanism of TGF-β signaling