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Blood, Vol. 95 No. 6 (March 15), 2000:
pp. 2008-2014
The anticoagulant factor, protein S, is produced by cultured human
vascular smooth muscle cells and its expression is up-regulated by
thrombin
Omar Benzakour and
Chryso Kanthou
From the Molecular Cell Biology Laboratory, Thrombosis Research
Institute, London, UK.
The anticoagulant factor protein S is a secreted vitamin K-dependent
 -carboxylated protein that is mainly made in the liver. Protein S is
homologous to the growth arrest specific protein, Gas6, the expression
of which is up-regulated in cultured fibroblasts upon serum withdrawal.
We report here the synthesis and secretion of protein S by cultured
human vascular smooth muscle cells (HVSMCs). Western blot analysis
revealed that similar amounts of protein S are secreted by both
growing and growth-arrested HVSMCs. HVSMC-derived protein S was
found to be -carboxylated as it was precipitated by barium citrate
and was shown to possess protein C cofactor activity. Treatment with
the vitamin K antagonist warfarin led to the accumulation of
intracellular undercarboxylated protein S forms that were rapidly
secreted upon the reintroduction of vitamin K. Northern blotting
analysis showed that cultured HVSMCs express a protein S
transcript. The expression of protein S messenger RNA was
unaffected by either warfarin, growth arrest, or various VSMC mitogens,
such as platelet-derived growth factor-BB, basic fibroblast growth
factor, transforming growth factor- , or hepatocyte growth factor.
Thrombin, however, induced an up-regulation of protein S expression at
both messenger RNA and protein levels. The evidence we provide for
protein S secretion by cultured HVSMCs and its up-regulation by
thrombin, together with earlier reports showing that protein S acts as
a mitogen for these cells, suggests that, in addition to its known role
in regulating blood clotting, protein S may also be an important
autocrine factor in the pathophysiology of the vasculature.
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