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Introduction: Tumour-associated urokinase plasminogen activator (uPA) is a critical marker of invasion and metastasis, and it is recognised as having strong prognostic relevance as well as being a therapeutic target. The specific uPA inhibitor plasminogen activator inhibitor type-2 (PAI-2, SerpinB2) specifically targets cell bound uPA and is internalised. Furthermore, preclinical studies have established the “proof-of-principle” of uPA-targeting by PAI-2-cytotoxin conjugates in human carcinoma models. However, these studies also suggest that PAI-2 is rapidly cleared via the renal system with low total dose reaching the tumour. In this study, a comparative single photon emission computed tomography (SPECT) and biodistribution (BD) analysis of different forms of PAI-2 labelled with the radioisotopes iodine-123 (123I) and technetium-99m (99mTc) was undertaken. Methods: The pharmacokinetic (PK) properties and BD of wild-type, ΔCD-loop and PEGylated ΔCD-loop PAI-2 labelled with the commonly used diagnostic SPECT radioisotopes 99mTc or 123I were compared in mouse models of human prostate carcinoma. Whole body SPECT imaging was also performed. Results: Both wild-type or the shorter but active ΔCD-loop form of PAI-2 123I-labelled indirectly via conjugation to free amine groups (termed 123I-Bn-PAI-2) exhibited low tumour uptake, rapid excretion and similar PK profiles. Preliminary studies with a short branched-chain PEGylated 123I-Bn-PAI-2 ΔCD-loop indicated an increase in blood retention time and tumour uptake. All 123I-Bn-labelled radiotracers were largely excreted through the kidneys. By comparison, both wild-type 123I-PAI-2 (labelled directly via tyrosine residues) and 99mTc-PAI-2 displayed different PK/BD patterns compared to 123IBn- PAI-2, suggesting greater liver based catabolism and thus slower elimination. SPECT imaging mimicked the BD results of all radiotracers. Conclusion: The different labelling methods gave distinct PAI-2 BD and tumour uptake profiles, with rad

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.

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.

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.

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.

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 × His-tagged-PAI-2 lacking the intrahelical loop between C and D alpha-helices (PAI- 2 ΔCD-loop) for these purposes. Results: We show that PAI-2 ΔCD-loop expressed and purified from the pQE9 vector system presents an easier purification target than the previously used pET15b system. Additionally, PAI-2 ΔCD-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 ΔCD-loop complexes had similar binding kinetics (KD ~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 antiuPA targeted cancer therapeutics.

PAI-1 and PAI-2 (plasminogen-activator inibitor types 1 and 2) are inhibitors of cell surface uPA (urokinase plasminogen activator). However, tumour expression of PAI-1 and PAI-2 correlates with poor compared with good patient prognosis in breast cancer respectively. This biological divergence may be related to additional functional roles of PAI-1. For example, the inhibition of uPA by PAI-1 reveals a cryptic high-affinity site within the PAI-1 moiety for the VLDLr (very-low-density-lipoprotein receptor), which sustains cell signalling events initiated by binding of uPA to its receptor. These interactions and subsequent signalling events promote proliferation of breast cancer cells. Biochemical and structural analyses show that, unlike PAI-1, the PAI-2 moiety of uPA–PAI-2 does not contain a high-affinity-binding site for VLDLr, although uPA–PAI-2 is still efficiently endocytosed via this receptor in breast cancer cells. Furthermore, global protein tyrosine phosphorylation events were not sustained by uPA–PAI-2 and cell proliferation was not affected. We thus propose a structurally based mechanism for these differences between PAI-1 and PAI-2 and suggest that PAI-2 is able to inhibit and clear uPA activity without initiating mitogenic signalling events through VLDLr.

PAI-1 and PAI-2 (plasminogen-activator inibitor types 1 and 2) are inhibitors of cell surface uPA (urokinase plasminogen activator). However, tumour expression of PAI-1 and PAI-2 correlates with poor compared with good patient prognosis in breast cancer respectively. This biological divergence may be related to additional functional roles of PAI-1. For example, the inhibition of uPA by PAI-1 reveals a cryptic high-affinity site within the PAI-1 moiety for the VLDLr (very-low-density-lipoprotein receptor), which sustains cell signalling events initiated by binding of uPA to its receptor. These interactions and subsequent signalling events promote proliferation of breast cancer cells. Biochemical and structural analyses show that, unlike PAI-1, the PAI-2 moiety of uPA–PAI-2 does not contain a high-affinity-binding site for VLDLr, although uPA–PAI-2 is still efficiently endocytosed via this receptor in breast cancer cells. Furthermore, global protein tyrosine phosphorylation events were not sustained by uPA–PAI-2 and cell proliferation was not affected. We thus propose a structurally based mechanism for these differences between PAI-1 and PAI-2 and suggest that PAI-2 is able to inhibit and clear uPA activity without initiating mitogenic signalling events through VLDLr.

PAI-1 and PAI-2 (plasminogen-activator inibitor types 1 and 2) are inhibitors of cell surface uPA (urokinase plasminogen activator). However, tumour expression of PAI-1 and PAI-2 correlates with poor compared with good patient prognosis in breast cancer respectively. This biological divergence may be related to additional functional roles of PAI-1. For example, the inhibition of uPA by PAI-1 reveals a cryptic high-affinity site within the PAI-1 moiety for the VLDLr (very-low-density-lipoprotein receptor), which sustains cell signalling events initiated by binding of uPA to its receptor. These interactions and subsequent signalling events promote proliferation of breast cancer cells. Biochemical and structural analyses show that, unlike PAI-1, the PAI-2 moiety of uPA–PAI-2 does not contain a high-affinity-binding site for VLDLr, although uPA–PAI-2 is still efficiently endocytosed via this receptor in breast cancer cells. Furthermore, global protein tyrosine phosphorylation events were not sustained by uPA–PAI-2 and cell proliferation was not affected. We thus propose a structurally based mechanism for these differences between PAI-1 and PAI-2 and suggest that PAI-2 is able to inhibit and clear uPA activity without initiating mitogenic signalling events through VLDLr.