Abstract

Activated protein C (APC) is a systemic anti-coagulant and anti-inflammatory factor^{1, 2, 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 sepsis⁶. It was not known whether APC acts as a direct cell survival factor or whether its neuroprotective effect^{5, 7} is secondary to its anti-coagulant and anti-inflammatory effects^{1, 2, 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.