Department of Molecular Pathology, Graduate School of Medicine, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo, Japan.
J Biochem. 2010 Apr;147(4):545-54. doi: 10.1093/jb/mvp202. Epub 2009 Dec 2.
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.
Arkadia 是转化生长因子-β（TGF-β）信号的正调控因子，可诱导 TGF-β信号通路的负调控因子 Smad7、c-Ski 和 SnoN 的泛素化和蛋白酶体依赖性降解。在本研究中，我们研究了 Arkadia 在人类癌细胞中的作用。我们首先在 20 种癌细胞系和 2 种非癌细胞系中检测了 Arkadia 的表达，发现其在 mRNA 和蛋白质水平上均广泛表达。有趣的是，Arkadia 的底物之一 c-Ski 蛋白的表达水平与 c-Ski mRNA 的表达水平不相关。Arkadia 在许多检测的细胞系中诱导 c-Ski 蛋白表达下调，但在某些 c-Ski 蛋白高表达的细胞系中没有。我们还发现，Arkadia 的敲低减弱了 TGF-β靶基因的诱导，而外源性表达 Arkadia 则增强了它。值得注意的是，Arkadia 的过表达抑制了 HepG2 细胞在 TGF-β刺激存在和不存在的情况下的生长。因此，Arkadia 以细胞类型依赖的方式调节 c-Ski 蛋白的表达水平，并通过抑制肿瘤细胞生长发挥肿瘤抑制功能。
