Summary
The association between use of oral contraceptives (OCs) and increased risk of thromboembolic
disease has been firmly established. This risk increases when use of OCs is combined
with cigarette smoking. The cellular mechanism favoring an hypercoagulable state under
these behaviours is not known. Circulating monocytes are potent activators of the
coagulation cascade through their ability to synthesize procoagulant tissue factor
(TF). In the present study we report that monocyte TF expression is increased in women
who use OCs and smoke.
We studied monocyte TF expression in 4 groups of healthy pre-menopausal women (n =
15 each): (1) non-smoking OC non-users, (2) nonsmoking current OC users, (3) smoking
OC non-users and (4) smoking OC users. TF expression was assessed on both mRNA and
protein levels in unstimulated and LPS-stimulated cells. Transcriptional activation
of the TF gene was assessed by analysis of the transcription factor NF-κB and its
inhibitor molecule IκBα. Monocyte TF generation was significantly higher in OC users
than in women who did not use OCs. Enhanced monocyte TF generation was also observed
in smoking women when compared to non-smokers. Strongest monocyte TF expression occurred
in women with combined smoking and use of OCs. The enhanced TF expression in monocytes
from women using OCs or smoking was based on an increased TF gene transcription following
activation of NF-κB. Experiments on cultured monocytes/macrophages demonstrated enhanced
IκBα degradation in the presence of estradiol, suggesting that a direct hormone effect
is responsible for the observed increase in monocyte TF expression.
This study demonstrates that use of OCs and smoking is associated with an increase
in monocyte TF expression in pre-menopausal women. Aberrant TF expression by blood
monocytes may favour intravascular clotting activation in women with OC therapy. The
further enhancement of TF activity observed in women who smoke and use OCs may explain
the synergistic effect of smoking on risk of thromboembolic events associated with
contraceptive use.
Key Words
Tissue factor - monocytes - oral contraception - smoking