Down-Regulation of 3-Phosphoinositide-Dependent Protein Kinase-1 Levels Inhibits Migration and Experimental Metastasis of Human Breast Cancer Cells

High expression of 3-phosphoinositide-dependent protein kinase-1 (PDK1) has been detected in various invasive cancers. In the current study, we investigated its role in cancer cell migration and experimental metastasis. Down-regulation of PDK1 expression by small interference RNA markedly inhibited spontaneous migration and epidermal growth factor (EGF)-induced chemotaxis of human breast cancer cells. The defects were rescued by expressing wild-type PDK1. PDK1-depleted cells showed impaired EGF-induced actin polymerization and adhesion, probably due to a decrease in phosphorylation of LIM kinase/cofilin and integrin beta 1. Confocal microscopy revealed that EGF induced cotranslocation of PDK1 with Akt and protein kinase C zeta (PKC zeta), regulators of LIM kinase, and integrin beta 1. Furthermore, PDK1 depletion dampened EGF-induced phosphorylation and translocation of Akt and PKC zeta, suggesting that Akt and PKC zeta functioned downstream of PDK1 in the chemotactic signaling pathway. In severe combined immunodeficiency mice, PDK1-depleted human breast cancer cells formed more slowly growing tumors and were defective in extravasation to mouse lungs after i.v. injection. Our results indicate that PDK1 plays an important role in regulating the malignant behavior of breast cancer cells, including their motility, through activation of Akt and PKC zeta. Thus, PDK1, which increases its expression in cancer cells, can be used as a target for the development of novel therapies. (Mol Cancer Res 2009;7(6):944-54)