Functional relevance of urinary-type plasminogen activator receptor-α3β1 integrin association in proteinase regulatory pathways

Squamous cell carcinoma of the oral cavity is characterized by persistent, disorganized expression of integrin alpha 3 beta 1 and enhanced production of urinary-type plasminogen activator (uPA) and its receptor ( uPAR) relative to normal oral mucosa. Because multivalent aggregation of alpha 3 beta 1 integrin up-regulates uPA and induces a dramatic co-clustering of uPAR, we explored the hypothesis that lateral ligation of alpha 3 beta 1 integrin by uPAR contributes to uPA regulation in oral mucosal cells. To investigate mechanisms by which uPAR/alpha 3 beta 1 binding enhances uPA expression, integrin-dependent signal activation was assessed. Both Src and ERK1/2 were phosphorylated in response to integrin aggregation, and blocking Src kinase activity completely abrogated ERK1/2 activation and uPA induction, whereas inhibition of epidermal growth factor receptor tyrosine kinase activity did not alter uPA expression. Proteinase up-regulation occurred at the transcriptional level and mutation of the AP1 (-1967) site in the uPA promoter blocked the uPAR/integrin-mediated transcriptional activation. Because uPAR is redistributed to clustered alpha 3 beta 1 integrins, the requirement for uPAR/alpha 3 beta 1 interaction in uPA regulation was assessed. Clustering of alpha 3 beta 1 in the presence of a peptide (alpha 325) that disrupts uPAR/alpha 3 beta 1 integrin binding prevented uPA induction. Depletion of cell surface uPAR using small interfering RNA also blocked uPA induction following integrin alpha 3 beta 1 clustering. These results were confirmed using a genetic strategy in which alpha 3 null epithelial cells reconstituted with wild type alpha 3 integrin, but not a mutant alpha 3 unable to bind uPAR, induced uPA expression upon integrin clustering, confirming the critical role of uPAR in integrin-regulated proteinase expression. Disruption of uPAR/alpha 3 beta 1 binding using peptide alpha 325 or small interfering RNA blocked filopodia formation and matrix invasion, indicating that this interaction stimulates invasive behavior. Together these data support a model wherein matrix-induced clustering of alpha 3 beta 1 integrin promotes uPAR/alpha 3 beta 1 interaction, thereby potentiating cellular signal transduction pathways culminating in activation of uPA expression and enhanced uPA-dependent invasive behavior.