Insulin-like growth factor binding proteins (IGFBPs) are a group of six structurally similar proteins that modulates the mitogenic actions of IGFs through high binding affinity. IGFBP-5 and -3 has been reported to function independently of their IGF-binding ability and this involves interaction with several non-IGF proteins.
Yeast two-hybrid and co immunoprecipitation (co-IP) assays identified SPRY2 and GADD34 as novel IGFBP-5-interacting partners. In MCF 10A cells, IGFBP 5:SPRY2 interaction potentiated SPRY2-mediated inhibition to EGF-stimulated DNA synthesis while IGFBP-5 expression alone had no effect. Under EGF-stimulation, cell numbers was decreased in SPRY2 and IGFBP 5 transfected cells to 24 % and 52 %, respectively, and a further reduction to 80 % was observed when cells co-expressed SPRY2 and IGFBP-5. These findings suggest a synergistic effect of the IGFBP-5:SPRY2 interaction.
Biochemical analysis demonstrated that GADD34 binds to IGFBP 5 and -3, but not -1, -2, -4 and -6, and that amino acids between 320 and 400 were required for the interaction. Further co-IP showed that IGFBP-5 forms a complex with GADD34 and protein phosphate 1 alpha and in return, accelerate the survival of breast cancer MCF-7 cells from stress stimuli.
In summary, we have characterised two novel mechanisms and expanded the current understanding of IGF-independent functions of IGFBP-5.