Högskolan i Skövde

Transforming growth factor beta (TGF-β) is a multifunctional cytokine that regulates cell differentiation, proliferation and migration depending on cell context. TGF-β has a growth inhibitory effect on epithelial cells via activation of Smad proteins (R-Smad/Co-Smad complex). TGF-β signaling is negatively regulated by the proto-oncogene Ski (Sloan Kettering Institute). Ski in turn is degraded by TGF-β signaling (Smads) via ubiquitin proteasome system. TGF-β acts like a tumor suppressor in early stages of cancer due to its inhibitory growth effect, but in late stages it acts like a tumor promoter. The main aim of this thesis was to study the regulation of corepressor Ski stability in normal and transformed hepatic cells by TGF-β signaling and Smad proteins as well as determine Ski protein subcellular localization by using immunoprecipitation and Western blot assays for these studies. The obtained data showed that the TGF-β/Smad signaling pathway caused a downregulation of Ski protein through its degradation via proteasome in HepG2 cells. In addition, the Ski protein levels were restored when Smads were restored in C9 and hepatocytes even though the activated Smads were still present. In the subcellular fractionation studies it was observed that Ski protein was mainly localized in the nucleus of HepG2 cells, whereas it was localized in both nucleus and cytoplasm in hepatocytes.  The presence of Ski in the cell cytoplasm could be explained because of low sensitivity in TGF-β signaling or translocation of Ski from nucleus to cytoplasm. C184M is a protein that binds to Ski in the cytoplasm and Ski in turn binds to Smads and inhibits their translocation from cytoplasm to nucleus, which in turn means inhibition of gene transcription that instead results in growth stimulation.