Blood, Vol. 94 No. 4 (August 15), 1999:
pp. 1330-1336
Increased Clearance Explains Lower Plasma Levels of Tissue-Type
Plasminogen Activator by Estradiol: Evidence for Potently Enhanced
Mannose Receptor Expression in Mice
Mirian Lansink,
Miek Jong,
Martin Bijsterbosch,
Marian Bekkers,
Karin Toet,
Louis Havekes,
Jef Emeis, and
Teake Kooistra
From the Gaubius Laboratory, TNO Prevention and Health, Leiden, The
Netherlands; the Division of Pharmaceutics, Leiden/Amsterdam Center for
Drug Research, University of Leiden, Sylvius Laboratories, Leiden, The
Netherlands; and the Department of Cardiology and Internal Medicine,
Leiden University Medical Center, Leiden, The Netherlands.
Several clinical studies have demonstrated an inverse relationship
between circulating levels of estrogen and tissue-type plasminogen
activator (t-PA). The present study was designed to test the hypothesis
that estrogens lower plasma levels of t-PA by increasing its clearance
from the bloodstream. 17
-Ethinyl estradiol (EE) treatment resulted
in a significant increase in the clearance rate of recombinant human
t-PA in mice (0.46 mL/min in treated mice v 0.32 mL/min in
controls; P < .01). The clearance of endogenous,
bradykinin-released t-PA in rats was also significantly increased after
EE treatment (area under the curve [AUC], 24.9 ng/mL · min in
treated animals v 31.9 ng/mL · min in controls; P < .05). Two distinct t-PA clearance systems exist in vivo: the low-density lipoprotein receptor-related protein (LRP) on
liver parenchymal cells and the mannose receptor on mainly liver
endothelial cells. Inhibition of LRP by intravenous injection of
receptor-associated protein (RAP) as a recombinant fusion protein
with Salmonella japonicum glutathione S-transferase (GST)
significantly retarded t-PA clearance in control mice (from 0.41 to
0.25 mL/min; n = 5, P < .001) and EE-treated mice (from
0.66 to 0.35 mL/min; n = 5, P < .005), but did
not eliminate the difference in clearance capacity between the 2 experimental groups. Similar results were obtained in mice in which LRP
was inhibited via overexpression of the RAP gene in liver by adenoviral
gene transduction. In contrast, administration of mannan, a mannose
receptor antagonist, resulted in identical clearances (0.22 mL/min in
controls and 0.24 mL/min in EE-treated mice). Northern blot analysis
showed a 6-fold increase in mannose receptor mRNA expression in the
nonparenchymal liver cells of EE-treated mice, whereas the parenchymal
LRP mRNA levels remained unchanged. These findings were confirmed at
the protein level by ligand blotting and Western blotting analysis. Our
results demonstrate that EE treatment results in increased plasma
clearance rate of t-PA via induction of the mannose receptor and could
explain for the inverse relationship between estrogen status and plasma t-PA concentrations as observed in humans.
