Tumor dormancy induced by downregulation of urokinase receptor in human carcinoma involves integrin and MAPK signaling

Mechanisms that regulate the transition of metastases from clinically undet ectable and dormant to progressively growing are the least understood aspec ts of cancer biology. Here, we show that a large (similar to 70%) reduction in the urokinase plasminogen activator receptor (uPAR) level in human carc inoma HEp3 cells, while not affecting their in vitro growth, induced a prot racted state of tumor dormancy in vivo, with G(0),/G(1) arrest. We have now identified the mechanism responsible for the induction of dormancy. We fou nd that uPA/ uPAR proteins were physically associated with alpha 5 beta 1, and that in cells with low uPAR the frequency of this association was signi ficantly reduced, leading to a reduced avidity of alpha 5 beta 1 and a lowe r adhesion of cells to the frbronectin (FN). Adhesion to FN resulted in a r obust and persistent ERK1/2 activation and serum-independent growth stimula tion of only uPAR-rich cells. Compared with uPAR-rich tumorigenic cells, th e basal level of active extracellular regulated kinase (ERK) was four to si xfold reduced in uPAR-poor dormant cells and its stimulation by single chai n uPA (scuPA) was weak and showed slow kinetics, The high basal level of ac tive ERK in uPAR-rich cells could be strongly and rapidly stimulated by scu PA. Disruption of uPAR-alpha 5 beta 1 complexes in uPAR-rich cells with ant ibodies or a peptide that disrupts uPAR-beta 1 interactions, reduced the FN -dependent ERK1/2 activation. These results indicate that dormancy of low u PAR cells may be the consequence of insufficient uPA/uPAR/alpha 5 beta 1 co mplexes, which cannot induce ERK1/2 activity above a threshold needed to su stain tumor growth in vivo. In support of this conclusion we found that tre atment of uPAR-rich cells, which maintain high ERK activity in vivo, with r eagents interfering with the uPAR/beta 1 signal to ERK activation, mimic th e in vivo dormancy induced by downregulation of uPAR.