SerpinB2 (PAI-2) Modulates Proteostasis via Binding Misfolded Proteins and Promotion of Cytoprotective Inclusion Formation.

SerpinB2 (PAI-2), a member of the clade B family of serine protease inhibitors, is one of the most upregulated proteins following cellular stress. Originally described as an inhibitor of urokinase plasminogen activator, its predominant cytoplasmic localisation suggests an intracellular function. SerpinB2 has been reported to display cytoprotective properties in neurons and to interact with intracellular proteins including components of the ubiquitin-proteasome system (UPS). In the current study we explored the potential role of SerpinB2 as a modulator of proteotoxic stress. Initially, we transiently transfected wild-type SerpinB2 and SerpinB2-/- murine embryonic fibroblasts (MEFs) with Huntingtin exon1-polyglutamine (fused C-terminally to mCherry). Inclusion body formation as result of Huntingtin aggregation was evident in the SerpinB2 expressing cells but significantly impaired in the SerpinB2-/- cells, the latter concomitant with loss in cell viability. Importantly, recovery of the wild-type phenotype and cell viability was rescued by retroviral transduction of SerpinB2 expression. SerpinB2 modestly attenuated Huntingtin and amyloid beta fibril formation in vitro and was able to bind preferentially to misfolded proteins. Given the modest chaperone-like activity of SerpinB2 we tested the ability of SerpinB2 to modulate UPS and autophagy activity using a GFP reporter system and autophagy reporter, respectively. Activity of the UPS was reduced and autophagy was dysregulated in SerpinB2-/- compared to wild-type MEFs. Moreover, we observed a non-covalent interaction between ubiquitin and SerpinB2 in cells using GFP-pulldown assays and bimolecular fluorescence complementation. We conclude that SerpinB2 plays an important role in proteostasis as its loss leads to a proteotoxic phenotype associated with an inability to compartmentalize aggregating proteins and a reduced capacity of the UPS.

The small heat shock proteins αB-crystallin and Hsp27 suppress SOD1 aggregation in vitro.

Amyotrophic lateral sclerosis is a devastating neurodegenerative disease. The mechanism that underlies amyotrophic lateral sclerosis (ALS) pathology remains unclear, but protein inclusions are associated with all forms of the disease. Apart from pathogenic proteins, such as TDP-43 and SOD1, other proteins are associated with ALS inclusions including small heat shock proteins. However, whether small heat shock proteins have a direct effect on SOD1 aggregation remains unknown. In this study, we have examined the ability of small heat shock proteins αB-crystallin and Hsp27 to inhibit the aggregation of SOD1 in vitro. We show that these chaperone proteins suppress the increase in thioflavin T fluorescence associated with SOD1 aggregation, primarily through inhibiting aggregate growth, not the lag phase in which nuclei are formed. αB-crystallin forms high molecular mass complexes with SOD1 and binds directly to SOD1 aggregates. Our data are consistent with an overload of proteostasis systems being associated with pathology in ALS.

Forty years later and the role of plasminogen activator inhibitor type 2/SERPINB2 is still an enigma.

Plasminogen activator inhibitor (PAI)-2 expression is acutely upregulated in pregnancy, inflammation, infection, and other pathophysiological conditions. Circumstances that prevent PAI-2 upregulation are associated with chronic pathology. Altogether this strongly suggests that PAI-2 is one of the many proteins that maintain homeostasis during damage or stress. However, several functions ranging from a classical serpin to various intracellular roles have been ascribed to PAI-2 and, because none of these have been definitively proven in vivo, to this day its precise role or roles remains an enigma. This review readdresses the evidence supporting a role for PAI-2 in fibrinolysis and proteolysis within extracellular environments and includes a review of the many potential intracellular functions attributed to PAI-2.

Small molecule antagonists of the urokinase (uPA): urokinase receptor (uPAR) interaction with high reported potencies show only weak effects in cell-based competition assays employing the native uPAR ligand.

Binding of the urokinase-type plasminogen activator (uPA) to its cell-surface-bound receptor uPAR and upregulation of the plasminogen activation system (PAS) correlates with increased metastasis and poor prognosis in several tumour types. Disruptors of the uPA:uPAR interaction represent promising anti-tumour/metastasis agents and several approaches have been explored for this purpose, including the use of small molecule antagonists. Two highly potent non-peptidic antagonists 1 and 2 (IC(50)1=0.8 nM, IC(50)2=33 nM) from the patent literature were reportedly identified using competition assays employing radiolabelled uPAR-binding uPA fragments and appeared as useful pharmacological tools for studying the PAS. Before proceeding to such studies, confirmation was sought that 1 and 2 retained their potencies in physiologically relevant cell-based competition assays employing uPAR's native binding partner high molecular weight uPA (HMW-uPA). This study describes a new solution phase synthesis of 1, a mixed solid/solution phase synthesis of 2 and reports the activities of 1 and 2 in semi-quantitative competition flow cytometry assays and quantitative cell-based uPA activity assays that employed HMW-uPA as the competing ligand. The flow cytometry experiments revealed that high concentrations of 2 (10-100 µM) are required to compete with HMW-uPA for uPAR binding and that 1 shows no antagonist effects at 100 µM. The cell-based enzyme activity assays similarly revealed that 1 and 2 are poor inhibitors of cell surface-bound HMW-uPA activity (IC(50) >100 µM for 1 and 2). The report highlights the dangers of identifying false-positive lead uPAR antagonists from competition assays employing labelled competing ligands other than the native HMW-uPA.

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

Generation 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.

The CD-loop of PAI-2 (SERPINB2) is redundant in the targeting, inhibition and clearance of cell surface uPA activity.

BACKGROUND: Plasminogen activator inhibitor type-2 (PAI-2, SERPINB2) is an irreversible, specific inhibitor of the urokinase plasminogen activator (uPA). Since overexpression of uPA at the surface of cancer cells is linked to malignancy, targeting of uPA by exogenous recombinant PAI-2 has been proposed as the basis of potential cancer therapies. To this end, reproducible yields of high purity protein that maintains this targeting ability is required. Herein we validate the use in vitro of recombinant 6 x His-tagged-PAI-2 lacking the intrahelical loop between C and D alpha-helices (PAI-2 Delta CD-loop) for these purposes. RESULTS: We show that PAI-2 Delta CD-loop expressed and purified from the pQE9 vector system presents an easier purification target than the previously used pET15b system. Additionally, PAI-2 Delta CD-loop gave both higher yield and purity than wild-type PAI-2 expressed and purified under identical conditions. Importantly, absence of the CD-loop had no impact on the inhibition of both solution phase and cell surface uPA or on the clearance of receptor bound uPA from the cell surface. Furthermore, uPA:PAI-2 Delta CD-loop complexes had similar binding kinetics (KD approximately 5 nM) with the endocytosis receptor Very Low Density Lipoprotein Receptor (VLDLR) to that previously published for uPA:PAI-2 complexes. CONCLUSION: We demonstrate that the CD-loop is redundant for the purposes of cellular uPA inhibition and cell surface clearance (endocytosis) and is thus suitable for the development of anti-uPA targeted cancer therapeutics.