Nunzia Montuori

Multiple activities of a multifaceted receptor: roles of cleaved and soluble uPAR.

The urokinase-type plasminogen activator receptor (uPAR) is a GPI-anchored cell-surface receptor involved in many physiological and pathological events that include cell migration and tissue invasion. uPAR traditional role was considered the focusing of uPA proteolytic activity on the cell surface; however, different uPAR activities have been demonstrated in the last years. In fact, cell surface uPAR functionally interacts with integrins, fMLP-receptors (fMLP-Rs) and growth factor receptors, thus regulating cell adhesion, migration and proliferation. uPAR also exists in a soluble form (suPAR) that has been detected in human body fluids. Both cell surface and suPAR can be proteolytically cleaved, thus generating truncated forms lacking the N-terminal domain and exposing the specific sequence able to interact with the fMLP-Rs. The cleaved form of suPAR binds and activates the fMLP-Rs and regulates the activity of MCP-1, RANTES and SDF1 receptors. Here, we review the role that shedding and cleavage could play in regulating uPAR structural/functional interaction with other cell-surface receptors and in uPAR-mediated biological and pathological processes.

uPAR regulates pericellular proteolysis through a mechanism involving integrins and fMLF-receptors

The expression of the urokinase-type plasminogen activator (uPA) and its receptor (uPAR) can be regulated by several hormones, cytokines, and tumour promoters. uPAR is a glycosyl-phosphatidyl inositol (GPI)-linked cell-surface protein; however, it is capable to transduce signals inside the cell by interacting with other cell-surface proteins, such as integrins and G-protein coupled (GPC) receptors. We previously reported that uPAR cell-surface expression can be positively regulated by its ligand, uPA, independently of its proteolytic activity. We now demonstrate that uPAR overexpression induces or increases uPA secretion both in uPAR-negative and in uPAR-expressing cells. Accordingly, uPAR depletion impairs uPA expression in cells which constitutively express both uPA and its receptor. uPAR exerts its regulatory effect through the activation of the ERK mitogen-activated protein kinases (MAPKs), whereas the p-38 MAPK is not involved. Overexpression of truncated forms of uPAR, lacking the N-terminal domain (DI) and not able to interact with membrane co-receptors, failed to increase uPA expression. Inhibition of uPAR-integrin interaction by the specific P-25 peptide, as well as Gi-protein inhibition by cholera pertussin toxin or depletion of the GPC receptors for fMLF (fMLF-Rs) also impaired uPAR capability to regulate uPA expression. These findings demonstrate that uPAR, whose expression is regulated by uPA, can, in turn, regulate uPA expression through a mechanism involving its functional interaction with integrins and fMLF-Rs.

The Plasminogen Activator System in Fibroblasts from Systemic Sclerosis

Systemic sclerosis (SSc) is characterized by excessive fibrosis throughout the body. There are two major subsets of SSc, diffuse cutaneous Systemic sclerosis (dSSc) and limited cutaneous Systemic sclerosis (ISSc). Fibroblasts play a key role in SSc. The expression and function of the urokinase (uPA)-mediated plasminogen activation (PA) system, a well-characterized system of serine-proteases involved in several pathological processes, has been investigated in SSc fibroblasts. The expression of the components of the PA system, including uPA, its type-1 and type-2 inhibitors (PAI-1 and PAI-2) and its receptor (uPAR), was examined by Western blot in fibroblasts from patients affected by limited and diffuse forms of SSc. uPA and PAI-1 secretion increased only in fibroblasts from ISSc lesions compared to normal fibroblasts. PAI-2 levels were decreased in fibroblasts from both SSc forms. Interestingly, fibroblasts from areas not adjacent to the lesions (not-affected) of the diffuse form showed reduced levels of PAI-1 and increased uPAR expression. Adhesion experiments showed reduced adherence to VN of fibroblasts from ISSc lesions and from non-affected areas of the diffuse form, as compared to normal controls. These results suggest a role for uPA and PAI-1 in the ISSc form, likely related to the activation of latent forms of cytokines and to the accumulation of ECM components, whereas a role for uPAR can be hypothesized in the evolvement of the diffuse form, based on its up-regulation in the non-affected areas.