Cross-talk between Integrin α6β4 and Insulin-like Growth Factor-1 Receptor (IGF1R) through Direct α6β4 Binding to IGF1 and Subsequent α6β4-IGF1-IGF1R Ternary Complex Formation in Anchorage-independent Conditions

Integrin alpha v beta 3 plays a role in insulin-like growth factor-1 (IGF1) signaling (integrin-IGF1 receptor (IGF1R) cross-talk). The specifics of the cross-talk are, however, unclear. In a current model, ligand occupancy of alpha v beta 3 (i.e. the binding of extracellular matrix proteins) enhances signaling induced by IGF1 binding to IGF1R. We recently reported that IGF1 directly binds to alpha v beta 3 and induces alpha v beta 3-IGF1-IGF1R ternary complex formation. Consistently, the integrin binding-defective IGF1 mutant (R36E/R37E) is defective in inducing ternary complex formation and IGF signaling, but it still binds to IGF1R. Like alpha v beta 3, integrin alpha 6 beta 4 is overexpressed in many cancers and is implicated in cancer progression. Here, we discovered that alpha 6 beta 4 directly bound to IGF1, but not to R36E/R37E. Grafting the beta 4 sequence WPNSDP (residues 167-172), which corresponds to the specificity loop of beta 3, to integrin beta 1 markedly enhanced IGF1 binding to beta 1, suggesting that the WPNSDP sequence is involved in IGF1 recognition. WT IGF1 induced alpha 6 beta 4-IGF1-IGF1R ternary complex formation, whereas R36E/R37E did not. When cells were attached to matrix, exogenous IGF1 or alpha 6 beta 4 expression had little or no effect on intracellular signaling. When cell-matrix adhesion was reduced (in poly(2-hydroxyethyl methacrylate-coated plates), IGF1 induced intracellular signaling and enhanced cell survival in an alpha 6 beta 4-dependent manner. Also IGF1 enhanced colony formation in soft agar in an alpha 6 beta 4-dependent manner. These results suggest that IGF binding to alpha 6 beta 4 plays a major role in IGF signaling in anchorage-independent conditions, which mimic the in vivo environment, and is a novel therapeutic target.