AIM: To investigate the mechanism of calcyclin binding protein/Siah-1 interacting protein(Cacy BP/SIP) nuclear translocation in promoting the proliferation of gastric cancer(GC) cells. METHODS: The effect of Cacy BP/SIP nuclear translocation on cell cycle was investigated by cell cycle analysis. Western blot analysis was used to assess the change in expression of cell cycle regulatory proteins and proteasome-mediated degradation of p27Kip1. Coimmunoprecipitation(co-IP) analysis was performed to examine the binding of Cacy BP/SIP with Skp1. A Cacy BP/SIP truncation mutant which lacked the Skp1 binding site was constructed and fused to a fluorescent protein. Subsequently, the effect on Skp1 binding with the fusion protein was examined by co-IP, while localization of fluorescent fusion protein observed by confocal laser microscopy, and change in p27Kip1protein expression assessed by Western blot analysis.RESULTS: Cacy BP/SIP nuclear translocation induced by gastrin promoted progression of GC cells from G1 phase. However, while Cacy BP/SIP nuclear translocation was inhibited using si RNA to suppress Cacy BP/SIP expression, cell cycle was clearly inhibited. Cacy BP/SIP nuclear translocation significantly decreased the level of cell cycle inhibitor p27Kip1, increased Cyclin E protein expression whereas the levels of Skp1, Skp2, and CDK2 were not affected. Upon inhibition of Cacy BP/SIP nuclear translocation, there were no changes in protein levels of p27Kip1 and Cyclin E, while p27Kip1 decrease could be prevented by the proteasome inhibitor MG132. Moreover, Cacy BP/SIP was found to bind to Skp1 by immunoprecipitation, an event that was abolished by mutant Cacy BP/SIP, which also failed to stimulate p27Kip1 degradation, even though the mutant could still translocate into the nucleus.CONCLUSION: Cacy BP/SIP nuclear translocation contributes to the proliferation of GC cells, and Cacy BP/SIP exerts this effect, at least in part, by stimulating ubiquitin-mediated degradation of p27Kip1. A: To investigate the mechanism of calcyclin binding protein / Siah-1 interacting protein (Cacy BP / SIP) nuclear translocation in promoting the proliferation of gastric cancer (GC) cells. METHODS: The effect of Cacy BP / SIP nuclear translocation on cell cycle was investigated by cell cycle analysis. Western blot analysis was used to assess the change in expression of cell cycle regulatory proteins and proteasome-mediated degradation of p27Kip1. Coimmunoprecipitation (co-IP) analysis was performed to examine the binding of Cacy BP / SIP with Skp1. A Cacy BP / SIP truncation mutant which lacked the Skp1 binding site was constructed and fused to a fluorescent protein. The effect on Skp1 binding with the fusion protein was examined by co-IP, while localization of fluorescent fusion protein observed by confocal laser microscopy, and change in p27Kiplprotein expression assessed by Western blot analysis .RESULTS: Cacy BP / SIP nuclear translocation induced by gastrin promoted progression of However, while Cacy BP / SIP nuclear translocation was inhibited using si RNA to suppress Cacy BP / SIP expression, cell cycle was clearly inhibited. Cacy BP / SIP nuclear translocation significantly decreased the level of cell cycle inhibitor p27Kip1, Increased Cyclin E protein expression versus the levels of Skp1, Skp2, and CDK2 were not affected. Upon inhibition of Cacy BP / SIP nuclear translocation, there were no changes in protein levels of p27Kip1 and Cyclin E, while p27Kip1 decrease could be prevented by the proteasome inhibitor MG132. Moreover, Cacy BP / SIP was found to bind to Skp1 by immunoprecipitation, an event that was abolished by mutant Cacy BP / SIP, which also failed to stimulate p27Kip1 degradation, even though the mutant could even translocate into the nucleus. CONCLUSION: Cacy BP / SIP nuclear translocation contributes to the proliferation of GC cells, and Cacy BP / SIP exerts this effect, at least in part, by stimulating ubiquitin-mediated degradation of p27Kip1.