Summary
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.
Keywords
Cell surface protein - endocytosis - protein-protein interactions - serpin - tissue
plasminogen activator (t-PA) - peripheral blood monocytes - plasminogen activator
inhibitor type-1 - plasminogen activator inhibitor type-2
