Differential endocytosis of tissue plasminogen activator by serpins PAI-1 and PAI-2 on human peripheral blood monocytes

Abstract

Ranson, M. (2010). Differential endocytosis of tissue plasminogen activator by serpins PAI-1 and PAI-2 on human peripheral blood monocytes. Thrombosis and Haemostasis, 104 (6), 1133-1142. Differential endocytosis of tissue plasminogen activator by serpins PAI-1 and PAI-2 on human peripheral blood monocytes AbstractGeneration of the broad spectrum protease plasmin is facilitated by the tissue (t-PA) and urokinase (u-PA) plasminogen activators, within multiple physiological and disease states. Finely tuned control of this proteolytic cascade is exerted by the plasminogen activator inhibitors type-1 (PAI-1/SERPINE1) and 2 (PAI-2/SERPINB2). Expression of this network of activators and inhibitors by cells of myeloid lineage appears to be highly interchangeable between physiological environments, and whilst the role of PAI-1 and PAI-2 in regulating u-PA-dependent functions is well established, the interaction between t-PA and PAI-2 on these cell types is poorly characterised. To this end, we used freshly isolated peripheral blood monocytes (PBM) as a model of a t-PA-dependent cellular environment. We demonstrate that while both PAI-1 and PAI-2 could inhibit surface-bound t-PA and are internalised predominately via low-density-lipoprotein receptor family members, PAI-1 enhanced the endocytosis of t-PA, whereas PAI-2 did not. Surface plasmon resonance analyses revealed differential binding affinities between the very-low-density-lipoprotein receptor and t-PA and t-PA:PAI-1 complexes in addition to those previously described with low-density-lipoprotein receptor-related protein. Moreover, t-PA:PAI-2 bound to both endocytosis receptors with similar kinetics to t-PA. These differential biochemical interactions between t-PA and the t-PA:PAI complexes may underlie the observed differences in endocytosis mechanisms on the PBMs. This suggests that while PAI-1 and PAI-2 function similarly in the control of cellular plasmin generation by t-PA, they may have disparate effects on the alternative functions of t-PA via modulation of its engagement with endocytosis receptors. SummaryGeneration of thebroad spectrumprotease plasmin is facilitated by the tissue( t-PA) and urokinase( u-PA)p lasminogena ctivators,w ithin multiple physiological anddisease states.Finely tunedcontrolofthis proteolyticcascade is exerted by the plasminogenactivator inhibitors type-1 (PAI-1/SERPINE1)a nd 2( PAI-2/SERPINB2). Expression of this network of activators and inhibitorsbycells of myeloid lineage appears to be highly interchangeable between physiologicalenvironments,and whilstthe role of PAI-1and PAI-2inregulating u-PA-dependent functions is well established, the interaction between t-PAa nd PAI-2o n thesecelltypes is poorly characterised.To this end, we used freshlyisolated peripheralbloodmonocytes (PBM)asamodelofat-PA-dependentcellularenvironment. We demonstratethat while both PAI-1and PAI-2could inhibit surface-bound t-PAand areinternalised predominatelyvia low-density-lipoproteinreceptor familymembers,PAI-1en-hanced the endocytosisoft-PA, wher...

Forty Years Later and the Role of Plasminogen Activator Inhibitor Type 2/SERPINB2 Is Still an Enigma

“…The formation of PAI-2 inhibitory complexes on cell surfaces with either uPA 40 or tPA 39 leads to rapid endocytosis of the PA:PAI-2 complexes, as is the case for PA:PAI-1 complex formation. 18 The mechanism by which PAI-2 clears uPA from the cell surface and its implications on physiological function have previously been reviewed.…”

“…Similar differences in the clearance of tPA have also been shown. 39 Interestingly, although tPA:PAI-2 complexes are still endocytosed, PAI-2 does not enhance the endocytosis of inhibited tPA, whereas uPA:PAI-2 complexes are cleared faster than uPA alone. 38,39 The downstream physiological significance of tPA:PAI-2 endocytosis via LDLRs remains to be determined.…”

“…PAI-2 has also been shown to inhibit tc-tPA bound to various cell types in vitro and to purified annexin II heterotetramer. 38,39 Where studied, PAI-2 has poor inhibitory activity against other serine proteases (reviewed in Kruithof et al 6 ).…”

Is plasminogen activator inhibitor type 2 really a plasminogen activator inhibitor after all?

“…Plasminogen activator (PA) inhibitor type 2 (PAI‐2; SERPINB2) is an atypical and enigmatic member of the Ov‐serpin family of serine protease inhibitors (SERPINS) . PAI‐2 was originally identified as an inhibitor of urokinase PA (u‐PA) , but it also inhibits soluble and cell surface‐bound tissue‐type PA (t‐PA) . However, from the outset, questions were raised about its real biological role and significance, given that the vast majority of PAI‐2 is located in the intracellular compartment of leukocytes and a restricted number of other cells.…”

Plasminogen Activator Inhibitor-2 Polymorphism Associates with Recurrent Coronary Event Risk in Patients with High HDL and C-Reactive Protein Levels

“…In contrast, for PAI-2, complexes bind to LDLRs via the PA portion of complexes but with less strong binding such that although internalization still occurs, cell signaling events do not. Differential actions resulting from such differences have been proposed to underlie recently reported non-traditional functionalities of PAI-1 and to allow for PAI-2 a more targeted approach for inhibition of plasminogen activation without concomitant cell signaling events [20,24]. …”