FAK potentiates Rac1 activation and localization to matrix adhesion sites: a role for betaPIX

Mammary epithelial-specific deletion of the focal adhesion kinase gene leads to severe lobulo-alveolar hypoplasia and secretory immaturity of the murine mammary gland

Integrin-mediated cell adhesion and signaling is required for mammary gland development and functions. As a major mediator of integrin signaling, focal adhesion kinase (FAK) has been implicated to play a role in the survival, proliferation, and differentiation of mammary epithelial cells in previously studies in vitro. To assess the role of FAK in vivo, we created mice in which FAK is selectively deleted in mammary epithelial cells. The mammary gland FAK conditional knock-out (MFCKO) mice are viable, fertile, and macroscopically indistinguishable from the control littermates. In virgin MFCKO mice, mammary ductal elongation is retarded at 5 weeks of age but reaches the full extent by 8 weeks of age compared with the control mice. However, the MFCKO females are unable to nurse their pups due to severe lobulo-alveolar hypoplasia and secretory immaturity during pregnancy and lactation. Analysis of the mammary epithelial cells in MFCKO mice showed reduced Erk phosphorylation, expression of cyclin D1, and a corresponding decrease in proliferative capability compared with the littermate controls. In addition, phosphorylation of STAT5 and expression of whey acidic protein are significantly reduced in the mammary glands of MFCKO mice, suggesting defective secretory maturation in these mice. Therefore, the combination of the severe lobulo-alveolar hypoplasia and defective secretory differentiation is responsible for the inability of the MFCKO females to nurse their pups. Together, these results provide strong support for a role of FAK in the mammary gland development and function in vivo.

Phospholipase C-gamma1 potentiates integrin-dependent cell spreading and migration through Pyk2/paxillin activation

Phospholipase C-gamma1 (PLC-gamma1), which generates two second messengers, namely, inositol-1, 4, 5-trisphosphate and diacylglycerol, is implicated in growth factor-mediated chemotaxis. However, the exact role of PLC-gamma1 in integrin-mediated cell adhesion and migration remains poorly understood. In this study, we demonstrate that PLC-gamma1 is required for actin cytoskeletal organization and cell motility through the regulation of Pyk2 and paxillin activation. After fibronectin stimulation, PLC-gamma1 directly interacted with the cytoplasmic tail of integrin beta1. In PLC-gamma1-silenced cells, integrin-induced Pyk2 and paxillin phosphorylation were significantly reduced and PLC-gamma1 potentiated the integrin-induced Pyk2/paxillin activation in its enzymatic activity-dependent manner. In addition, specific knock-down of PLC-gamma1 resulted in a failure to form focal adhesions dependent on fibronectin stimulation, which appeared to be caused by the suppression of Pyk2 and paxillin phosphorylation. Interestingly, PLC-gamma1 potentiated the activations of Rac, thus integrin-induced lamellipodia formation was up-regulated. Consequently, the strength of cell-substratum interaction and cell motility were profoundly up-regulated by PLC-gamma1. Taken together, these results suggest that PLC-gamma1 is a key player in integrin-mediated cell spreading and motility achieved by the activation of Pyk2/paxillin/Rac signaling.