Context-dependent regulation of the expression of c-Ski protein by Arkadia in human cancer cells

Arkadia is a positive regulator of transforming growth factor-beta (TGF-beta) signalling, which induces ubiquitylation and proteasome-dependent degradation of negative regulators of the TGF-beta signalling pathway, i.e. Smad7, c-Ski and SnoN. In the present study, we examined the roles of Arkadia in human cancer cells. We first examined the expression of Arkadia in 20 cancer cell lines and 2 non-cancerous cell lines, and found that it was expressed ubiquitously at both the mRNA and protein levels. Interestingly, levels of expression of c-Ski protein, one of the substrates of Arkadia, were not correlated with those of c-Ski mRNA. Arkadia induced down-regulation of c-Ski protein expression in many cell lines examined, but did not in certain cell lines with high levels of expression of c-Ski protein. We also found that knockdown of Arkadia attenuated the induction of TGF-beta target genes, whereas ectopically expressed Arkadia enhanced it. Notably, over-expression of Arkadia inhibited the growth of HepG2 cells in the presence as well as the absence of TGF-beta stimulation. Arkadia thus regulates the levels of expression of c-Ski protein in cell-type-dependent fashion, and exhibits a tumour suppressor function by inhibiting tumour cell growth.