The Role of Beta-1 Integrin Signaling in Regulating the Phosphorylation and Recycling of MT1-MMP at Invadopodia

Malignant cancer cells can invade extracellular matrix (ECM) through the formation of F-actin-rich subcellular structures, termed invadopodia. ECM degradation at invadopodia is mediated by matrix metalloproteinases (MMPs), and recent findings indicate that membrane-anchored membrane type-1 matrix metalloproteinase (MT1-MMP) has a primary role in this process. Maintenance of an invasive phenotype is dependent on internalization of MT1-MMP from the plasma membrane and its recycling to sites of ECM remodeling, which requires phosphorylation of the enzyme’s cytoplasmic domain. In addition to ECM degradation, invadopodia are sites where ECM adhesion is upregulated through increased localization of integrin receptors. The expression of integrins containing the β1 subunit has been found to be upregulated in numerous invasive cancer cells, and downstream signaling from integrins has been found to play a role in cellular behaviours such as migration, proliferation, and survival. ECM adhesion and degradation at invadopodia are tightly regulated processes which must be balanced for cells to efficiently invade and migrate. The goal of this study was to identify and characterize a signaling pathway that links β1 integrin and MT1-MMP at sites of invadopodia in invasive cancer cells. We utilized an antibody-mediated approach, both to activate and inhibit β1 integrin, and monitored the resulting changes in MT1-MMP activity and invadopodia formation. The activation of β1 integrin was found to be crucial for MT1-MMP phosphorylation which facilitated the internalization of the enzyme from the cell surface. It was also found that MT1-MMP was recycled in Rab5-positive early endosomes and Rab7-positive late endosomes prior to delivery to newly forming invadopodia at the migration front. The activation of focal adhesion kinase (FAK), Src, and the epidermal growth factor receptor (EGFR), in response to β1 integrin activation, was also observed and it was determined that these signaling molecules act in a pathway to promote phosphorylation of MT1-MMP on Thr567. Furthermore, inhibition of β1 integrin did not result in the phosphorylation of MT1-MMP, instead causing its accumulation at the cell surface and a loss of long-term invasiveness. Together, these results define the importance of a β1 integrin signaling axis in the regulation of MT1-MMP activity at invadopodia.