Humanin Peptide Binds to Insulin-Like Growth Factor-Binding Protein 3 (IGFBP3) and Regulates Its Interaction with Importin-β.

Nuclear translocation of IGFBP3 by importin-β1 is a prerequisite for IGFBP3-induced apoptosis. The neuroprotective peptide humanin (HN) counteracts IGFBP3-induced cell death. However, the mechanism by which humanin protects cells is currently unknown. The natural synthesis of this peptide decreases with age, coincident with the likelihood for the development of Alzheimer's Disease, making it a promising target for therapeutics. We have examined the effect of full-length humanin and a synthetic analogue (HN 3-19), known to be sufficient for its neuroprotective function, on the interaction between IGFBP3 and importin-β1. Using competitive ligand dot blotting, co-immunoprecipitation, and an ELISA-based binding assay, we determined that 1) humanin binds to IGFBP3 with a Kd of 5.05 µM and 2) both humanin (IC50 of 18.1 µM) and HN 3-19 (IC50 of 10.3 μM) interfere with the binding of importin-β1 to IGFBP3 in vitro. We also demonstrated that HN 3-19 is able to reduce the rate of apoptosis in a human lung adenocarcinoma cell line, suggesting a possible mechanism of action for humanin as an inhibitor of IGFBP3 nuclear translocation. Understanding the exact mechanism by which humanin and its analogue, HN 3-19, bind to IGFPB3 and regulate its interaction with importin-β1 will open the door to modulating the protein-protein interactions involved in neuronal cell death.