Page 1985 - Williams Hematology ( PDFDrive )

1960 Part XII: Hemostasis and Thrombosis Chapter 114: Control of Coagulation Reactions 1961

TFPIα is the full-length mature protein that contains an acidic N-termi- V, thereby binding more TFPIα and prolonging its half-life. Factor
nal sequence, three homologous but distinct Kunitz-type protease inhib- V-short may also enhance inhibition of factor Xa by TFPI with the effect
itor domains (K1, K2, K3), and a C-terminal positively charged basic of increasing bleeding risk. This genetic disorder, as well as previous
amino acid sequence (Fig. 114–7). TFPIβ contains K1 and K2 but an studies showing that inhibition of TFPI reduced bleeding in preclinical
unrelated sequence replaces the K3 domain and the C-terminus. TFPIβ hemophilia models, lends support for ongoing efforts to develop TFPI
can be covalently modified by addition of glycosylphosphatidylinositol inhibitors for reducing bleeding in some hemophilia subjects, especially
(GPI) that localizes TFPIβ to cell membranes (Fig. 114–7). Some TFPI in those with anti–factor VIII inhibitors.
plasma is present as a disulfide-linked heterodimer of TFPI–apolipopro-
tein A-II, 327,328 but the functional significance of the apoA-II appendage TFPI GENE
is unknown. TFPI in its multiple forms is a significant inhibitor of the
coagulation pathways that can function synergistically with the protein C The sequence of TFPI was established from cloning of its complemen-
pathway and antithrombin to suppress thrombin generation. tary DNA. The gene contains nine exons, spans 85 kb, and is located on
335,336
TFPI is synthesized by endothelial cells, megakaryocytes, and chromosome 2q31–32.1 (see Table 114–1).
smooth muscle cells. 301–303 Free TFPI in plasma is TFPIα but it is a
minor fraction of the amount of TFPI in blood vessels. More than half OTHER PROTEASE INHIBITORS
of TFPIα in plasma is associated with lipoproteins, especially HDL and
low-density lipoprotein. TFPIα is also the main form within platelet and HEPARIN COFACTOR II
it is secreted by activated platelets. A substantial amount of TFPIα is
329
released from the vessel wall when heparin is infused. TFPIβ is mem- Heparin cofactor II, a serpin whose inhibitory activity is enhanced by
brane bound, especially to endothelium, because of its GPI anchor. dermatan sulfate, inhibits thrombin in vivo and in vitro by an approx-
The interaction of TFPI with lipoproteins reduces its anticoagulant imation mechanism. 337–340 A few reports link heparin cofactor II defi-
activity measured in vitro though the physiologic significance of TFPI’s ciency to venous thrombosis, but no significant clinical relevance has
341
binding to various lipoproteins remains uncertain. In addition to bind- been established. Curiously, a severe heparin cofactor II deficiency
342
ing lipoproteins, TFPIα but not TFPIβ binds to protein S and to cer- was reported in an asymptomatic subject. Some studies imply that
tain forms of factor Va/factor V. 277–279,330–333 Different regions of TFPIα, heparin cofactor II may play significant roles in arterial vascular wall
namely the K3 domain or the basic amino acid cluster, respectively, are processes, but definitive mechanisms remain to be elucidated.
responsible for binding protein S or factors Va/V (see Fig. 114–7). Inhi-
bition of factor Xa by TFPIα is accelerated by protein S and by certain PROTEIN Z–DEPENDENT PROTEASE INHIBITOR
but not all forms of factor Va (see below). Protein Z–dependent protease inhibitor (ZPI) is a plasma serpin that
TFPI neutralizes factors Xa and VIIa by multiple complicated mech- inhibits factors Xa, XIa, and IXa, but not factor XIIa or thrombin. 343–350
anisms. 277,301–303 In each mechanism, the K1 domain binds and inhibits Protein Z, which is a vitamin K–dependent protein that contains a GLA
factor VIIa while the K2 domain inhibits factor Xa (see Fig. 114–7). domain, two EGF-like domains, and a protease-like domain, stimu-
351
No protease has yet been identified as the target of the K3 protease lates factor Xa inhibition by ZPI. Curiously, the protease-like domain of
inhibitor domain. In one mechanism, initially the K2 domain of TFPI protein Z lacks any protease activity because it has mutations at two of
reacts with and inhibits the enzyme activity of factor Xa. Subsequently, the three active site triad residues. The major hypothesis for stimulation
this binary complex reacts with factor VIIa in the tissue factor–factor of inhibition of factor Xa by protein Z is based on a structural model
VIIa complex to form a quaternary protein complex on a membrane in which three proteins assemble on a phospholipid membrane via the
with both proteases neutralized. In an alternative proposed scheme, two GLA domains (see Fig. 114–7). In this putative ternary complex,
351
TFPI first reacts with factor VIIa in a tissue factor–factor VIIa com- the protease-like domain and the second EGF-like domain of protein Z
plex that has generated factor Xa, and thereafter it rapidly reacts with bind ZPI in an alignment that facilitates reaction of factor Xa with the
factor Xa before it can dissociate from the ternary tissue factor–factor reactive center loop of ZPI.
VIIa–factor Xa complex. Possibly each proposal is valid. Some argue In plasma, ZPI is in slight protein molar excess over protein Z with
that because some kinetic studies showed that TFPI requires factor Xa which it associates noncovalently, and it has been speculated, but not
for kinetically favorable reactions with factor VIIa, TFPI does not shut proven, that almost all plasma protein Z is associated with ZPI. 352–357 If
off the initiation of the extrinsic pathway by tissue factor until some the ZPI is a physiologic coagulation inhibitor, the deficiency of either
significant though small amount of factor Xa is generated, in which case protein Z or ZPI might be associated with thrombosis. Knocking out
TFPI provides negative feedback inhibition of the generation of factor the protein Z gene in a mouse does not produce a remarkable pheno-
Xa by the factor VIIa–tissue factor complex. An additional property type unless protein Z deficiency coexists with factor V Leiden, in which
of TFPIα involves its inhibition of factor Xa in the absence of factor case the mouse exhibits a hypercoagulable, prothrombotic state. This
353
VIIa, and this reaction is accelerated by protein S and by certain forms murine observation is mirrored by one clinical report that subnormal
of factor Va. 277–279,330–333 In contrast to the anticoagulant factors, anti- levels of protein Z are associated with venous thrombosis in subjects
thrombin, protein C, and protein S, for which hereditary deficiencies who are heterozygous for factor V Leiden. Some associations between
354
are linked to significantly increased risk for venous thrombosis (Chap. venous thrombosis and defects in protein Z or ZPI have been reported
130), no clear pattern for increased risk of thrombosis has been defin- but not uniformly confirmed. 352,354–357 An association with peripheral
itively established for TFPI deficiency in humans. In mice, knockout arterial disease was reported. However, to date no convincing pattern
358
of TFPI is embryonically lethal. In a highly informative kindred that between thrombosis and defects in either protein Z or ZPI has been
334
presented with a serious bleeding diathesis, highly elevated plasma firmly established.
TFPI levels were linked to increased bleeding risk, indicating that TFPI
functions in man as a physiologically significant inhibitor of coagula-
tion. 322,332 The genetic mutation causing elevated plasma TFPI levels was OTHER MINOR PROTEASE INHIBITORS
in the factor V gene, not the TFPI gene. The mutated factor V, named Thrombin in plasma can be inhibited not only by antithrombin but also
“factor V-short,” has a higher affinity for TFPIα than wild-type factor by α -macroglobulin, an acute-phase reactant. No association between
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Kaushansky_chapter 114_p1949-1966.indd 1960 9/18/15 10:06 AM