Intracellular serpins in haemopoietic and peripheral blood cells

The repertoire of activities shown by serpins has increased interest in the members of the family expressed by blood cells. The themes emerging so far support the role of serpins controlling endogenous proteases in myeloid precursors and in activated T cells and natural killer (NK) cells. There is mounting evidence to suggest anti-apoptotic activity for some intracellular serpins and some support for their involvement in modulation of haemopoiesis. This review will deal with the molecular and cellular biology of the intracellular serpins and their cognate proteases produced by blood cells. Serpins (serine protease inhibitors) are expressed in virtually all multicellular organisms from Drosophila to nematodes through to man with more than 200 different family members identifiable in gene databases. Some viruses also appear to have borrowed serpins and use them to suppress host inflammatory response and inhibit apoptosis. The key to the evolutionary success of the serpins lies in their mechanism of action. Unlike other families of protease inhibitors that act as steric lock and key agents that block the active site of targets, the serpins behave like a mousetrap, with the entrapment process involving a spring-like change from a metastable baited-loop state to a stable inserted-loop form. This mechanism has the advantage of being readily adaptable to modulating interactions. For example, the plasma fibrinolysis inhibitor PAI-1 can readily be switched off by the spontaneous insertion of its reactive site loop into the body of the molecule to give the inactive latent form. Moreover, this switching-off mechanism can be prevented by the interaction of PAI-1 with another plasma protein ± vitronectin ± which stabilizes the reactive centre loop in its active exposed position (Declerck et al, 1988). Similarly, antithrombin circulates within the plasma with its reactive loop in a partially inserted and hence inactive form. It is only when the antithrombin molecule binds to the heparans of the microcirculation that it becomes fully activated as an inhibitor of coagulation proteases. Another special feature of the serpins has been revealed by the structure of the final serpin±protease complex (Fig 1) (Huntington et al, 2000). The surprise from the structure was the degree of disruption of the ordered conformation of the protease resulting from the movement of the loop. This gross loss of structure of the protease makes it readily susceptible to proteolytic destruction. As a consequence, the serpins are able to give total and irreversible inhibition of proteases. The unique advantage of the serpins is that they provide the absolute inhibition that is necessary for the efficient control of the many pathways of life that are initiated by proteolysis. Until recently, most interest has focused on the control of extracellular proteolytic cascades by serpins. Over the last 10 years a new subfamily of at least 13 intracellular serpins has emerged with six of these prominently expressed in haemopoietic precursors or mature peripheral blood cells. Designated the ov-serpins because of their similarity to the prototypical ovalbumin, the intracellular serpins demonstrate a high degree of overall sequence homology, lack a cleavable N-terminal secretion-signal peptide and often possess oxidizable residues at or adjacent to their reactive site loop. Studies of some non-haemopoietic intracellular serpins suggest diverse biological activities beyond regulation of target proteases. An example is provided by the ovserpin maspin, which was originally identified as a tumour suppressor (Zou et al, 1994). Its expression is frequently downregulated in breast carcinomas and re-introduction into malignant mammary epithelial cells reduces their ability to migrate through extracellular matrix (Sheng et al, 1996; Zhang et al, 1997). Although it may be that some of its tumour-suppressing activity relates to protease inhibition (Sheng et al, 1998), recently, maspin has been reported to have potent anti-angiogenic activity independent of any protease inhibition (Zhang et al, 2000). Similarly the noninhibitory serpin, pigment epithelial derived factor (PEDF), which acts as a neurotrophic and anti-apoptotic factor, also exhibits anti-angiogenic activity and appears to play a key role in the control of retinal vascularization (Dawson et al, 1999). Angiostatic activity has also been described for proteolytically cleaved antithrombin (O'Reilly et al, 1999) ± this unexpected finding illustrates a mechanism whereby conformational change induced by cleavage of the reactive site loop uncovers a novel non-inhibitory function.