Characterisation of SerpinB2 as a stress response modulator

SerpinB2 [plasminogen activator inhibitor type-2 (PAI-2)] was first described as a placental protein, which regulated plasminogen activation via the inhibition of the urokinase (uPA) or tissue-type (tPA) plasminogen activators during pregnancy. Further work has confirmed that SerpinB2 is an efficient inhibitor of cell surface, receptor bound uPA and clearance of the serpin:protease complex occurs via members of the low density lipoprotein receptor (LDLR) family, specifically very low density lipoprotein receptor (VLDLR) and low density lipoprotein-related protein receptor (LRP). This property has allowed for the development of SerpinB2 drug conjugates for the selective targeting of uPA positive carcinoma cells. However, potential utility of SerpinB2 in haematological disorders, such as acute myeloid leukaemia (AML), as well as its role as a physiological inhibitor of tPA is not well characterised. To date biochemical analysis has shown that SerpinB2 is able to inhibit the two-chain (active) form of tPA. However, whether this inhibitory function occurs within the cellular context was unknown. This thesis investigates the potential use of exogenous SerpinB2 as a delivery vehicle for the specific targeting of lymphoma or leukaemia, via cell surface uPA using the relatively low versus high uPA expressing, monocytic cell lines U-937 and THP-1, respectively, as disease models. We show here that uPA:SerpinB2 is internalised via the Low Density Lipoprotein Receptor (LDLR) family members, specifically Low Density Lipoprotein-Related Protein (LRP) and Very Low Density Lipoprotein Receptor (VLDLR) in a uPA dependent manner and these findings are consistent with those previously reported for epithelial cancer cell lines.