Activated protein C blocks p53-mediated apoptosis in ischemic human brain endothelium and is neuroprotective

Activated protein C (APC) is a systemic anti-coagulant and anti-inflammatory factor(1-3). It reduces organ damage in animal models of sepsis, ischemic injury and stroke(1,4,5) and substantially reduces mortality in patients with severe sepsis(6). It was not known whether APC acts as a direct cell survival factor or whether its neuroprotective effect(5,7) is secondary to its anticoagulant and anti-inflammatory effects(1-3). We report that APC directly prevents apoptosis in hypoxic human brain endothelium through transcriptionally dependent inhibition of tumor suppressor protein p53, normalization of the pro-apoptotic Bax/Bcl-2 ratio and reduction of caspase-3 signaling. These mechanisms are distinct from those involving upregulation of the genes encoding the anti-apoptotic Bcl-2 homolog A1 and inhibitor of apoptosis protein-1 (IAP-1) by APC in umbilical vein endothelial cells(8,9). Cytoprotection of brain endothelium by APC in vitro required endothelial protein C receptor (EPCR) and protease-activated receptor-1 (PAR-1), as did its in vivo neuroprotective activity in a stroke model of mice with a severe deficiency of EPCR10. This is consistent with work showing the direct effects of APC on cultured cells via EPCR and PAR-1 (ref. 9). Moreover, the in vivo neuroprotective effects of low-dose mouse APC seemed to be independent of its anti-coagulant activity. Thus, APC protects the brain from ischemic injury by acting directly on brain cells.