Page 1981 - Williams Hematology ( PDFDrive )

1956 Part XII: Hemostasis and Thrombosis Chapter 114: Control of Coagulation Reactions 1957

Va procoagulant activity, ablates the molecule’s anticoagulant cofactor are able to convey beneficial effects in multiple injury and disease mod-
activity. However, when factor V is cleaved at Arg506 by APC, its APC els with diminished risks for bleeding that would be anticipated with
cofactor activity is increased 10-fold. This suggests that Gln506-factor V wild-type APC therapy. 8,40–44
has two potential prothrombotic defects, namely, resistance of the vari-
ant factor Va to APC inactivation and resistance of the variant factor V Activated Protein C Neuroprotective Effects
to activation of its APC cofactor function. 23,209–211 Neuroprotective effects of APC have been convincingly demonstrated
in rodent ischemic stroke models and N-methyl-D-aspartate (NMDA)
High-Density Lipoprotein as Activated Protein C Cofactor excitotoxic injury models. 8,223,226,227,231,233–242 APC not only provides direct
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HDL can exert antithrombotic activity through multiple mechanisms. cytoprotection in vitro and in vivo for brain endothelium against ische-
HDL enhances the anticoagulant activity of APC both in plasma and in mic injury but also directly protects neurons against NMDA-induced
purified reaction mixtures, and this APC cofactor activity requires protein excitotoxic injury both in vivo and in vitro. APC mutants with reduced
S and involves, at least in part, stimulation of APC’s cleavage at Arg306 anticoagulant activity were as neuroprotective as wild-type APC, and
in factor Va. 189,190 HDL is heterogeneous in both protein and lipid com- certain cellular receptors were required for APC’s neuroprotection,
position, and the components responsible for this activity have not been strongly implying that neuroprotection by APC involves its actions
identified, although large HDL, but not small HDL, possesses APC anti- directly on the endothelium and on neurons. Remarkably, in the ische-
190
coagulant cofactor activity. Venous thrombosis in males and in subjects mic penumbra in a murine stroke model, APC caused neovasculariza-
experiencing venous thrombosis recurrence are associated with a pattern tion and neurogenesis. 223,240,243–245 The extensive preclinical studies on
of dyslipoproteinemia and low HDL, consistent with the hypothesis that APC’s neuroprotective effects paved the pathway for translation of the
deficiency of large HDL is a risk factor for venous thrombosis. 213,214 3K3A-APC variant to potential neuroprotective therapy for acute ische-
mic stroke. 246,247 Because of the greatly reduced anticoagulant activity of
Glycosphingolipids as Activated Protein C Cofactors 3K3A-APC (<10 percent of normal), high-dose bolus dosing in healthy
Although both procoagulant and anticoagulant reactions are markedly volunteers can achieve circulating APC levels that are 100-fold higher
enhanced by the presence of negatively charged phospholipid surfaces than those used in the PROWESS or PROWESS-SHOCK sepsis trials
in vitro, certain lipoproteins, for example, HDL, and certain lipids, without notable anticoagulant effects. 46,49,246
189
for example, glycosphingolipids and sphingosine, 215–218 selectively
enhance anticoagulant reactions in plasma. Plasma glucosylceramide Cellular Receptors for Physiologic Effects of Activated Protein
deficiency is a biomarker and may be a potential risk factor for venous C on Cells
215
thrombosis. Sphingosine and several of its common analogues are The ability of exogenously administered APC to alter gene-expression
potent inhibitors of thrombin generation in plasma and on cell surfaces profiles of cultured endothelial cells, to stabilize endothelial barriers, to
because they inhibit interactions between factors Va and Xa. Further reduce lethality caused by endotoxin in murine sepsis models, to pre-
218
studies are needed to characterize the anticoagulant or procoagulant vent apoptosis of stressed endothelial cells, and to provide neuroprotec-
properties of minor abundance plasma and their significance for clini- tion requires EPCR and PAR-1, strongly supporting the EPCR–PAR-1
cal thrombotic events. cell-signaling pathway as key for APC’s pharmacologic benefits (see
Fig. 114–4). 7,8,40–43,221,225,229,233,234,236,237,244,248–251
Although few details are known about intracellular mechanisms
ACTIVATED PROTEIN C DIRECT CELLULAR for APC’s multiple cytoprotective actions, some mechanistic details for
ACTIVITIES APC’s cell signaling have become clear, as depicted in Fig. 114–6. Mul-
8
As noted in Chap. 113, control of coagulation reactions does not occur tiple considerations help explain how PAR-1 can mediate thrombin’s
in the absence of an integrated host defense system that involves a num- disruption of endothelial barrier leading to vascular leakage while,
ber of biologic processes involving multiple overlapping and integrated paradoxically, the same receptor mediates APC’s endothelial barrier
pathways. Reactions of the innate and acquired immune system includ- protection, preventing vascular leakage. 249,250 First, PAR-1–mediated
ing inflammatory processes, blood coagulation reactions, fibrinolysis, APC signaling occurs in caveolae microdomains that contain EPCR
and thrombotic processes are intertwined in vivo via multiple molecular whereas PAR-1–mediated thrombin signaling is not limited to caveolae
and cellular mechanisms. 5,7,8,81,86,87,212,219,220 In addition to its anticoagu- (Fig. 114–6). 252,253 Second, different cleavages in the extracellular N-
lant activity, APC acts directly on cells to cause multiple cytoprotective terminus of PAR-1, either at the canonical Arg41 thrombin-cleavage site
effects. Cytoprotective actions of APC include antiapoptotic and anti- (i.e., widely recognized as the essential thrombin cleavage site) or at the
inflammatory activities, beneficial changes in gene-expression profiles, novel Arg46 APC-cleavage site, results in very different signaling initi-
and endothelial barrier stabilization. These cytoprotective activities of ated by different tethered N-terminal peptide sequences which begin at
APC generally require EPCR, involve APC’s ability to activate PAR- either residue 42 or residue 47. 124,254,255 Third, following thrombin cleav-
1, and may also require additional receptors such as PAR-3, sphin- age at Arg41, PAR-1 initiates signaling involving G proteins, extracel-
gosine-1-phosphate receptor 1, integrin CD11b/CD18, apoER2, EGF lular signal-regulated kinase (ERK)1/2 and RhoA, whereas, following
receptor, and/or Tie2. 7,8,86,110,221–227 APC cleavage at Arg46, PAR-1 initiates signaling involving β-arrestin-2,
Pharmacologic APC infusions showed benefits in numerous animal phosphatidylinositide 3′-kinase (PI3K)/Akt, and Rac1. Fourth, pep-
256
injury model systems, with the most informative animal studies to date tides mimicking the N-terminus of cleaved PAR-1 are peptide agonists
being in sepsis models and in neuroprotection experiments. 7,8,110,111,226,227 with pharmacologic effects resembling those of the respective proteases
Protein engineering permitted the molecular dissection of APC’s anti- that cleave PAR-1 differentially. For example, “thrombin receptor acti-
coagulant activity from its cytoprotective activities 7,8,37,40–44,228 and led to vating peptides (TRAPs)” that begin with Ser 42 promote G-protein–
proof of principle that APC’s cell signaling activities are both necessary mediated signaling similar to thrombin. In contrast, peptides that begins
and most likely sufficient for reducing lethality in murine septic shock with Asn 47 (TR47) promote APC-like signaling. TRAP but not TR47
254
models 42,229 and for providing neuroprotective effects in ischemic stroke promotes ERK1/2 phosphorylation on endothelial cells whereas TR47
models. 226,227,230–232 Notably, recombinant APC mutants that have little but not TRAP promotes Akt phosphorylation. The different and oppo-
254
anticoagulant activity (<10 percent) but normal cell-signaling activity site induction of signaling pathways is also mirrored in different and

Kaushansky_chapter 114_p1949-1966.indd 1956 9/18/15 10:05 AM