Ski is a versatile nuclear oncoprotein that regulates transcriptions of genes involved in multiple biological processes. Ski induces oncogenic transformation of chicken embryo fibroblasts (CEF). This biological function has been implicated with Ski’s ability to inhibit transforming growth factor β (TGF-β) signaling through interactions with Smad proteins via Dachshund homology domain (DHD) and SAND (Sp100, AIRE-1, NucP41/75 and DEAF-1) domains by previous studies. In Ski transformed CEFs (Ski-CEF), we discovered that Ski reversed the normal Smad3 to Smad2 protein ratio, by increasing the Smad2 protein level while lowering the Smad3 level. Smad2 translocated from cytoplasm into nucleus in the absence of TGF-β stimulation. Moreover, Smad2 was constantly phosphorylated by some mechanism independent of TGF-β. Knocking down endogenous Smad2, but not Smad3 completely blocked the Ski induced oncogenic transformation judged by morphology and anchorage independent growth. Therefore, we conclude that Smad2 is essential to mediate the Ski induced cellular transformation of CEFs.
Ski-CEFs display transformed cell phenotypes, but not the enhanced glycolysis in the presence of adequate oxygen: the Warburg effect that is the classical metabolic feature of tumor cells. On the contrary to making a lot of lactate from glucose, Ski-CEFs are slow to acidify culture medium. We unveiled that compared to CEFs; Ski-CEFs produced less lactate and utilized less glucose by suppressing glycolysis. In parallel, Ski-CEFs stimulated fatty acid oxidation, mitochondrial respiration and increased mitochondrial number and mass. Interestingly, we found that PPARγ, a potent lipid metabolism regulator was dramatically elevated at protein and message level and activated as a transcription regulator in Ski-CEFs. PPARγ target genes involved in lipid transport, fatty acid oxidation and mitochondrial biogenesis were also up-regulated by Ski. Knocking down PPARγ by RNAi in Ski-CEF reversed Ski induced fatty acid oxidation, mitochondrial respiration, mitochondrial biogenesis and restores glucose utilization. We conclude that PPARγ mediates the energy metabolism shift induced by Ski.