Mechanism Of ErbB2 Nuclear Localization

ErbB2 is a member of the epidermal growth factor receptor (EGFR or ErbB) family, which functions as a universal ErbB co-receptor. Recently, it was reported that ErbB2 could enter the nucleus and involve in gene transcription as a transcriptional factor. However, its transportation to the nucleus is still under debate.In the first part, the cellular localization of the full-length ErbB2, its varous mutants and the full-length ErbB2-GFP fusion protein were examined. The data presented here demonstrated that the full-length ErbB2 entered the nucleus inefficiently. The NLS of PF-8 could not drive the full-length ErbB2 into the nucleus. The intrinsic features of ErbB2 may make it localize on the cell membrane. However, ErbB2 ICD was localized in the nucleus predominantly. The NLS sequence within ErbB2 ICD was identified. It may play a role in ErbB2 nuclear translocation. In addition, the arginine 897 was conservative among different ErbB proteins. A region (residues 721-970) containing the arginine triplet might be essential for the cytoplasmic trafficking of ErbB2.Until now, nuclear localization of ErbB2 was only detected in tumor tissues, but rarely in tumor cell lines overexpressing ErbB2. Tumor microenvironment was important for tumor development. Cross-talk of tumor cells and interstitial cells directly impacted attachment, movement, proliferation, differentiation and invasion of tumor cells. In the second part, by co-culturing the breast cell line SKBR3 and fibrosarcoma cell line HT1080, the increased expression and activation of MMP-2 and MMP-9 was observed. Separate culture of SKBR3 cells or HT1080 cells did not induce the expression and activation of MMP-2 and MMP-9. Meanwhile, co-culture resulted in the morphological changes of SKBR3 cells. The Matrigel assay showed that the enhanced migration of SKBR3 cells, suggesting that interstitial cells were strongly associated with the invasion and metastasis of tumor cells. ErbB2 ECD of SKBR3 cells could be cleaved by co-culture with HT1080 cells or by the treatment of the cultural supernatant from HT1080 cells. The cleavage of the ErbB2 ECD could be inhibited by EDTA, indicating that the proteinases, possibly MMPs, released by HT1080 cells were closely correlated with the cleavage of ErbB2 ECD.Hypoxia has an important role in tumor development. The tumor tissues are usually under hypoxia environment. To observe whether hypoxia have any effects on the nuclear localization of ErbB2, we incubated SKBR3 cells under hypoxia condition. Suprisingly, we found that a large amount of ErbB2 molecules accumulated at one side of the nucleus. Nuclear entry of ErbB2 could be seen clearly. It suggests that the intereaction of SKBR3 cells with HT1080 cells and hypoxia were prerequisite of the nuclear localization of ErbB2.In summary, we demonstrated that the nuclear translocation of ErbB2 could be induced by co-culture of SKBR3 and HT1080 cells or the treament of the cultural media from HT1080 cells under hypoxia condition. But it is still not clear in which form ErbB2 enters nucleus. The mechanism of nuclear localization of ErbB2 and its association with the biological behaviour of tumor cells still need to be further investigated. Elucidation of the mechanism of ErbB2 nuclear translocation will provide some clues for the therapy and drug resistance of tumor cells.