The activated insulin-like growth factor I receptor induces depolarization in breast epithelial cells characterized by actin filament disassembly and tyrosine dephosphorylation of FAK, Cas, and paxillin.

Insulin-like growth factor I (IGF-I) promotes the motility of different cell types. We investigated the role of IGF-I receptor (IGF-IR) signaling in locomotion of MCF-7 breast cancer epithelial cells overexpressing the wild-type IGF-IR (MCF-7/IGF-IR). Stimulation of MCF-7/IGF-IR cells with 50 ng/ml IGF-I induced disruption of the polarized cell monolayer followed by morphological transition toward a mesenchymal phenotype. Immunofluorescence staining of the cells with rhodamine-phalloidin revealed rapid disassembly of actin fibers and development of a cortical actin meshwork. Activation of phosphatidylinositol (PI)3-kinase downstream of the IGF-IR was necessary for this process, as blocking PI 3-kinase activity with the specific inhibitor LY 294002 at 10 microM prevented disruption of the filamentous actin. In parallel, IGF-IR activation induced rapid and transient tyrosine dephosphorylation of focal adhesion proteins p125 focal adhesion kinase (FAK), p130 Crk-associated substrate (Cas), and paxillin. This process required phosphotyrosine phosphatase (PTP) activity, since pretreatment of the cells with 5 microM phenylarsine oxide (PAO), an inhibitor of PTPs, rescued FAK and its associated proteins Cas and paxillin from IGF-I-induced dephosphorylation. In addition, PAO-pretreated cells were refractory to IGF-I-induced morphological transition. Thus, our findings reveal a new function of the IGF-IR, the ability to depolarize epithelial cells. In MCF-7 cells, mechanisms of IGF-IR-mediated cell depolarization involve PI 3-kinase signaling and putative PTP activities.