Page 1978 - Williams Hematology ( PDFDrive )

1952 Part XII: Hemostasis and Thrombosis Chapter 114: Control of Coagulation Reactions 1953

by hepatocytes, neuroblastoma cells, kidney cells, testis, megakaryo- protein C activation by thrombin or by accelerating neutralization of
cytes, and endothelial cells, and is also found in platelet α-granules. 62 thrombin by protease inhibitors. Modulation of the substrate specificity
Protein S is synthesized as a precursor protein of 676 amino acids, of thrombin by thrombomodulin involves conformational changes in
which gives rise to a mature secreted single-chain glycoprotein of 635 res- thrombin caused by binding of thrombomodulin.
idues with three N-linked carbohydrate side chains (see Fig. 114–5). 63,64 Low levels of soluble thrombomodulin circulate in plasma, pre-
Eleven GLA residues in the N-terminal region of mature protein S con- sumably as a result of limited proteolysis of the protein near its trans-
2+
tribute to Ca -mediated binding of the protein to phospholipid mem- membrane cell surface anchor. The functional significance of circulating
branes. The thrombin-sensitive region, residues 47 to 72, follows the thrombomodulin is unknown, although variations in its plasma level
GLA-domain (see Fig. 114–5). arise in different clinical conditions.
The C-terminal region of protein S, residues 270 to 635, the sex Recombinant soluble thrombomodulin has been developed for its
hormone–binding globulin-like (SHBG) region contains binding sites potential therapeutic value for disseminated intravascular coagulation
for C4b-binding protein (see “Activated Protein C–Independent Anti- and has been approved for this indication in Japan. 77,84
65
coagulant Activity of Protein S” below) and for factor V, as well as fac-
tor Va. 66,67 Protein S, like the homologous gas6, also binds to receptor Thrombomodulin Gene
tyrosine kinases, for example, Axl, and initiates cell signaling, and the The thrombomodulin gene, which lacks introns, is located on chromo-
SHBG region binds the receptor. Thus, for the expression of its multi- some 20p11.2 and spans 3.7 kb (see Chap. 113, Fig. 113–19, Fig. 114–5,
68
ple activities, different domains of protein S exhibit a number of differ- and Table 114–1). 77,82 Deletion of the thrombomodulin gene in mice
85
ent binding sites for different proteins. is embryonically lethal. Downregulation of thrombomodulin gene
expression is promoted by a variety of inflammatory agents, including
Protein S Gene endotoxin, interleukin-1, and tumor necrosis factor (TNF)-α, whereas
The protein S gene, comprising 15 exons and 14 introns, is located on its expression is upregulated by retinoic acid. 5–8,86,87 Generally, throm-
chromosome 3p11.1–11.2 and spans 80 kb (see Fig. 113–16 and Ta ble bomodulin is a key member among the counterbalancing factors that
114–1). 69,70 The protein S gene has limited homology with other genes contribute to inflammation, thrombin generation, and coagulation in
for vitamin K–dependent factors in the GLA and EGF domains and the endothelium.
notable homology of the region coding for residues 240 to 635 with
genes of the SHBG family. Humans contain a protein S pseudogene that Thrombomodulin Mutations
contains several stop codons and is not translated and that is located Thrombomodulin mutations are well documented in atypical hemolytic
very near the normal protein S gene on chromosome 3. uremic syndrome patients, 78,88 and they may also be associated with an
increased risk of arterial thrombosis and myocardial infarction. In contrast,
Protein S Mutations there is less supportive data for association with risk for venous thrombosis
The molecular basis for hereditary protein S deficiency associated (Chap. 130). 89–92 Atypical hemolytic uremic syndrome is strongly linked
with venous thrombosis (Chap. 130) is linked to more than 100 differ- to excessive complement activation, and thrombomodulin’s lectin-like
71
ent mutations. A protein S polymorphism that is strongly linked to domain inhibits complement activation. 77,93 Furthermore, besides pro-
risk for venous thrombosis in Japanese subjects is known as protein S moting protein C activation by thrombin, thrombomodulin also supports
Tokushima. It involves K155E, which ablates APC-cofactor activity. 57,72 activation of the carboxypeptidase, also known as thrombin-activatable
But the K155E apparently is not present in Americans of European fibrinolysis inhibitor (TAFI) that is a potent inactivator of bradykinin and
ancestry or Chinese populations, thus mirroring the presence of factor of the activated complement components, C3a and C5a. 94–96
V Leiden and prothrombin G20210A that are risk factors in Americans
of European decent but not in the Japanese, Chinese, or Americans
of African descent populations. Another single nucleotide polymor- ENDOTHELIAL PROTEIN C RECEPTOR
73
phism present in approximately 1 percent of Americans of European EPCR binds both protein C and APC with similar affinities through
descent is S460P, which is designated protein S Heerlen; it results in their GLA domains and mediates multiple activities of this zymogen or
absence of N-linked carbohydrate on Asn458 but has no accepted sig- its activated protease, APC. 9,10,33,86,97–107 The mature EPCR glycoprotein
nificant functional consequence. 74 contains 221-amino-acid residues and N-linked carbohydrate, giving
an Mr of 46,000. EPCR is an integral membrane protein that is homol-
THROMBOMODULIN ogous to CD1/major histocompatibility complex class I molecules. The
Thrombomodulin was discovered as an endothelial cell surface receptor N-terminus is part of an extracellular domain, which is connected to
that binds protein C and thrombin, thereby accelerating protein C acti- a single transmembrane sequence that is followed by a short Arg-Arg-
vation. 75–78 Binding of thrombin to thrombomodulin converts thrombin Cys-COOH cytoplasmic tail (see Fig. 114–5). The cytoplasmic tail can be
from a procoagulant enzyme to an anticoagulant enzyme because palmitoylated, and this modification may help localize EPCR to certain
thrombomodulin-bound thrombin loses its normal ability to clot lipid rafts or caveolae. The three-dimensional structure of EPCR deter-
fibrinogen or activate platelets. 79,80 Thrombomodulin is a multidomain mined by X-ray crystallography or inferred by molecular modeling estab-
transmembrane protein comprising an N-terminal lectin-like domain, lished that the GLA domain of protein C and APC binds to EPCR. 107,108
six EGF domains, a Ser/Thr-rich region, a single membrane-spanning EPCR on endothelial surfaces enhances by greater than fivefold the rate of
sequence, and an intracellular C-terminal tail (see Fig. 114–5). 5–8,75–83 activation of protein C by thrombin–thrombomodulin (see Fig. 114–2).
EGF domains 4, 5, and 6 are essential for activation of protein C, with EPCR is also required for the cytoprotective activities of APC by promot-
the latter two domains binding thrombin and the first domain binding ing the cleavage of PAR-1 by APC to induce cell-signaling pathways (see
protein C. The mature protein has 557-amino-acid residues and vari- Fig. 114–4). Notably, the cytoprotective actions of APC are completely
able amounts of N- and O-linked carbohydrate modifications that cause independent of its anticoagulant activity and are based on cell signaling
variability in molecular size. Glycosaminoglycans, notably chondroitin actions (see “Activated Protein C Direct Cellular Activities” below). 7,8,109–111
sulfate, covalently attached to the Ser/Thr-rich region, contribute to The presence of functional EPCR on the cell surface is regulated by
the functional properties of thrombomodulin by enhancing either two mechanisms, namely generation of EPCR and clearance of EPCR.

Kaushansky_chapter 114_p1949-1966.indd 1953 9/18/15 10:05 AM