Nuclear translocation of small G protein RhoA via active transportation in gastric cancer cells

Recent studies have shown the localization of RhoA in the cell nucleus, in addition to its cellular distribution in the cytosol and cell membrane. Our previous results that a high amount of RhoA was detected in gastric cancer cell nucleus and application of anticancer drug Taxol could reduce RhoA nuclear localization, suggest a relationship between nuclear translocation of RhoA and tumor progression. However, the mechanism and biological function of RhoA nuclear translocation remain unclear. The aim of the present study was to explore the molecular mechanism(s) for RhoA protein entering the nucleus in the human gastric cancer cell line AGS. Using immunofluorescence microscopy we observed not only a partial colocalization of RhoA with importin alpha, mainly surrounding and on the nuclear membrane, but also an intensive colocalization of RhoA with NF-kappa B P50 in both cytoplasm and nucleus, particularly in the cell nucleoli. A strong association between RhoA and importin a as well as RhoA and NF-kappa B P50 was revealed by co-immuno-precipitation and western blotting. Moreover, AGS cells treated with the nuclear export inhibitor, leptomycin B (LMB), showed an increase of RhoA protein amount in the nucleus, indicating an active transport process for nuclear export of RhoA. Taken together, our results suggest that nuclear translocation of RhoA in AGS cells is via active transportation, a process through importin alpha in a NF-kappa B-dependent manner.