Plasminogen binding and activation during late-stage cell death

The plasminogen (Pig) activation cascade is focussed at the cell surface by both protein C-terminal Lys clusters and non-proteinaceous moieties and Pig activators to specific receptors (e.g. u-PAR). Once bound to surfaces (cells, fibrin clots or C-terminal Lys cluster-containing proteins e.g. a enolase), the majority of plasmin formed is refractory to inhibition by a,-antiplasmin. Collectively, these interactions ensure plasmin is generated only when and where necessary (e.g. cell migration & tissue remodeling). Pig binding and activation is elevated on aggressive breast cancer cells,1 and in discrete tissues zones (e.g. ruptured fetal membranes, necrotic and invasive regions of tumours). Recently, we have discovered2-' that spontaneous loss of cell viability in culture or cycloheximide- or TNFa-induced apoptosis dramatically elevates (-200 fold) lysine-dependent FITC-Plg binding. Cell surface plasmin formation (FITC-aprotinin binding) is predominantly restricted to the apoptotic cell subpopulation, where a modest increase in cell-surface u-PA expression is concomitant. Phosphatidylserine externalisation, propidium iodide uptake and DNA laddering show that Pig activation occurs in late degradative phase(s) of cellular apoptosis and that the presence of Pig increases the % of cells entering apoptosis. Pig binds to cellular proteins (e.g. recombinant N-terminally 6-His tagged a-enolase) via lysine binding sites and BIACore and fluorescence spectroscopy analyses both Glu- and lys-plasminogen binding suggest that dissociation is mediated by at least two components. Glu-Plg undergoes lysine dependent confirmational changes after binding to this normally intracellular glycolytic protein to a more open conformation. We hypothesise that Pig activation plays some role in the degradative phase(s) of cellular apoptosis. The observation that CC14 liver injury does not resolve in Plg-/-mice by a fibrin-dependent process adds weight to our suggestion that Pig plays a key role in tissue protein degradation associated with cellular injury.