Despite efforts to discover the cellular pathways regulating
breast cancer metastasis, little is known as to how
prolactin (PRL) cooperates with extracellular environment and
cytoskeletal proteins to regulate
breast cancer cell motility and invasion. We implicated
serine-threonine kinase p21-activated kinase 1 (PAK1) as a novel target for PRL-activated
Janus-kinase 2 (JAK2). JAK2-dependent PAK1 tyrosyl phosphorylation plays a critical role in regulation of both
PAK1 kinase activity and scaffolding properties of PAK1. Tyrosyl phosphorylated PAK1 facilitates PRL-dependent motility via at least two mechanisms: formation of
paxillin/GIT1/βPIX/pTyr-PAK1 complexes resulting in increased adhesion turnover and phosphorylation of
actin-binding protein filamin A. Increased adhesion turnover is the basis for cell migration and phosphorylated
filamin A stimulates the
kinase activity of PAK1 and increases actin-regulating activity to facilitate cell motility. Tyrosyl phosphorylated PAK1 also stimulates invasion of
breast cancer cells in response to PRL and three-dimensional (3D)
collagen IV via transcription and secretion of MMP-1 and MMP-3 in a MAPK-dependent manner. These data illustrate the complex interaction between PRL and the cell microenvironment in
breast cancer cells and suggest a pivotal role for PRL/PAK1 signaling in
breast cancer metastasis.