Regulation of Nucleo-Cytoplasmic Trafficking of Human Annexin II




Liu, Jie


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Annexin II is a molecule having diversified functions in the cell. It contains a specific N terminus and a C terminal core domain that is homologous with other molecules in annexin family. To date, whether annexin II is a tumor promoter or suppressor is still a question of controversy. So the study of the functions of annexin II according to the specific cancer is of utmost importance for further manipulating this molecule to achieve the control of cancer. Nuclear export signal (NES) was found in the N terminus of annexin II, and nuclear localization of annexin II is cell cycle dependent. In addition, nuclear annexin II was shown to be phosphorylated. Based on these observations along with the previous finding that anexin II is a subunit of primer recognition proteins (PRP), it was proposed that nucleo-cytoplasmic trafficking of annexin II is a phosphorylation-regulated process which is relevant to cancer cell growth. Annexin II-null LNCaP cell line was used as a model to study the intracellular localization of annexin II without the effects of background endogenous annexin II. The stable clones expressing GFP alone, GFP fused wild type annexin II and GFP fused NES mutant annexin II were established. We observed that consistent nuclear localizing of NES mutant annexin II resulted in a decreased proliferation rate of LNCaP cells, while expression of wild type annexin II had no effect on the cell proliferation. However, the expression of wild type annexin II changed the morphology and decreased matrigel migration of LNCaP cells. These observations suggest that annexin II play a role in regulating cell proliferation, and re-expression of this molecule in annexin II-null cells can decrease the malignancy of the parental LNCaP cells. In order to investigate the effect phosphorylation on the subcellular localization of annexin II, we used site directed mutagenesis to convert the putative phosphorylation sites on the N terminus of annexin II to other amino acid to resemble the unphosphorylated/phosphorylated status of the residues. We found that double mutation of serine 11 and 25, which mimics the double phosphorylation of these residues, resulted in inhibition of nuclear entry of annexin II. So, there might be other phosphorylation sites responsible for the nuclear retention of annexin II. In highly malignant cancer cells, we found that annexin II is usually over-expressed and present in both the nucleus and the cytoplasm, i.e. although there is NES, annexin II can not be exported from the nucleus by CRM1 mediated pathway. Hela, DU-145 and PC-3 cell lines also have the same pattern of annexin II distribution. We confirmed this observation by using immunocytochemistry, Western blotting and immunoprecipitation. Using different antibody recognizing different domains of annexin II, we found that C terminus is masked in the nucleus. In order to know the relationship between the masking of C terminus and the nuclear retention of annexin II, truncation mutation was used to delete the C terminal core domain of annexin II. After we expressed the truncated annexin II in Hela, Du-145 and PC-3, we found that C terminal masking is not responsible for the nuclear retention of annexin II. So, we proposed that there are other nuclear factors interacting with the N terminus that prevent the binding of CRM1 and nuclear export of annexin II.