Insulin-like Growth Factor Binding Protein-6 (IGFBP-6) Interacts with DNA-end Binding Protein Ku80 to Regulate Cell Fate
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Insulin-like growth factor binding protein-6 (IGFBP-6) is a growth inhibitory protein that regulates the availability of insulin-like growth factors (IGFs). We recently reported that IGFBP-6 exerts intracellular actions via its translocation to the nucleus. We now show that IGFBP-6 co-purifies by tandem-affinity with nuclear proteins involved in DNA stability and repair such as Ku80, Ku70, histone H2B and importin-alpha. Furthermore, this report shows that IGFBP-6 and Ku80 interact specifically using two active binding sites for Ku80 in IGFBP-6. One of the binding sites [196RKR199], as part of the NLS-sequence in IGFBP-6 also binds importin-alpha which may selectively compete with Ku80 regulating its trafficking to the nucleus. Moreover, IGFBP-6 co-localized with Ku80 based on a cell cycle pattern. Overexpression of IGFBP-6 increased the nuclear Ku80 in mitotic cells and reduced it post-mitosis. It is known that if highly expressed IGFBP-6 induces apoptosis and in our model, the down-regulation of Ku80 by specific siRNAs enhanced the apoptotic effect caused by the IGFBP-6 overexpression. This study demonstrates that IGFBP-6 alters cell survival by potentially regulating the availability of Ku80 for the DNA-repair process. This action represents a novel mechanism by which growth inhibitory proteins such as IGFBP-6 regulate cell fate.