The cell cycle progression is tightly controlled by a series of positive and negative key regulators. When these proteins are abnormally regulated or expressed, they may disturb the precise cell cycle progression and frequently result in the events of tumorigenesis. Presently, the well-characterized function of a transcription factor, Slug, is to promote tumor invasion/metastasis by down-regulating E-cadherin expression. In addition, Slug also functions on cell cycle but the underlying mechanisms are still unclear. In our previous results, we found that Slug overexpression promoted tumorigenicity by the in vivo tumor growth assay. We thus infer that the invasion enhancer, Slug, may affect the normal cell cycle control. In this study, we addressed how Slug proteins are regulated during cell cycle and the effects of Slug on cell cycle. First, we measured the levels of Slug protein in the cells released from nocodazole block and double thymidine block. We found that the endogenous Slug protein levels in CL1-5 lung cancer or HeLa cells were fluctuated during the cell cycle progression, which principally decreased at two stages: G1/S transition and from late M phase to early G1 phase. Next, Slug protein at the G1-S transition, in prometaphase cells, and in cells released from a 1 h prometaphase block was substantially unstable using the cycloheximide inhibition assay. Nonetheless, treatment of proteasome inhibitor MG132 with those cells could restore the reduced level of Slug proteins. In addition, overexpression of Slug in HEK293 and CL1-5 cells markedly increased the intensity of BrdU incorporation, whereas knockdown of Slug in CL1-5 cells reduced it. However, these Slug-manipulated cells could not obviously increase the cell populations of S phase analyzed by flow cytometry and cell growth rates determined by hemacytometer. Therefore, downregulation of Slug protein at G1-S, prometaphase and late M/early G1 phase was possibly attributed to protein destabilization mediated by the ubiquitin-proteasome degradation system and maintaining Slug within normal levels may be critical for the regulation of DNA synthesis. Overall, this study will provide the molecular mechanism of Slug, suggesting that in addition to invasive functions, it may also control cell cycle and play a regulatory role in cancer progression.
Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 3325.
100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO