In our last post, we discussed the role of plasminogen activator inhibitor-1 (PAI-1) in helping to maintain the balance between the clotting and fibrinolytic (clot-dissolving) sides of the coagulation system. The primary function of PAI-1 is to inhibit plasminogen activators (t-PA and u-PA) from converting plasminogen to plasmin which is responsible for initiating fibrinolysis. The premise is that if there is too much PAI-1 activity, clots will tend to hang around longer and if there is too little, the individual would be at increased risk for bleeding problems. Before we can address possible roles of abnormalities of PAI-1 production and activity in adverse pregnancy outcome and recurrent pregnancy loss (RPL), it would be helpful to understand changes that might occur in these parameters during normal pregnancy.
Kruithof and colleagues (Blood 1987;69:460-6) reported that both plasminogen activators (t-PA and u-PA) and plasminogen activator inhibitors increased during pregnancy. t-PA and u-PA increased 50% and 200%, respectively, throughout normal pregnancy. They also found that PAI-1, produced predominantly by endothelial cells lining blood vessels, increased nearly 10-fold by term over that found in nonpregnant women and a second plasminogen activator inhibitor, PAI-2, not found in nonpregnant women, but produced by the placenta, was present in very high concentrations by term. The increase in both activators and inhibitors appeared to maintain the balance between the clotting and fibrinolytic systems during normal pregnancy because no changes in plasminogen or the overall fibrinolytic activity were found. Within “three to five days after delivery most parameters of the fibrinolytic system were normal again.”
In 1989, Estelles and colleagues (Blood 1989;74:1332-8) reported that women with severe preeclampsia in third trimester had significantly higher levels of PAI-1 than nonhypertensive women. Interestingly, PAI-2 levels were significantly lower in the preeclamptic women and a positive correlation between birth weight and PAI-2 levels was found (in other words, the higher the PAI-2, the greater the birth weight); and birth weight was inversely correlated with PAI-1 levels (higher the PAI-1 activity, the lower the birth weight). The presumption is that the lower PAI-2 levels correlated with a decreased placental mass or function in preeclamptic women. Regardless, the high levels of PAI activity in severe preeclampsia appear to be solely related to the increased activity of PAI-1. And, as many of our readers are aware, this might account in part for the coagulation abnormalities frequently accompanying the more severe forms of preeclampsia.
Unfortunately, these observations late in pregnancy don’t really tell us whether elevated levels of PAI-1 in preeclampsia are a cause, an effect, a response, or a contributor to the disease process itself. Based on several observations by other investigators, and the putative role of PAI-1 in placentation early in pregnancy (which we will eventually get to here), perhaps it is all the above. There does appear to be a genetic predisposition/association with abnormalities in PAI-1 production and later pregnancy complications. Yamada and colleagues (J Hum Genet 2000;45:138-41) evaluated the association between preeclampsia and deletion/insertion polymorphisms (4G or 5G) in the promoter of the PAI-1 gene. The 4G/5G polymorphism was assessed in 115 women with preeclampsia, 210 normotensive pregnant women and 298 nonpregnant controls. The frequency of the 4G allele (which results in increased production of PAI-1) and of 4G/4G homozygosity was significantly higher in the preeclamptic women than either the normal pregnant or nonpregnant controls, suggesting that the presence of 4G is one risk factor for preeclampsia and perhaps more severe manifestations of the disease.
Along the same lines, Glueck, et al. (Metabolism 2000;49:845-52) evaluated complications in 133 women with at least one pregnancy, and found a significant association of the 4G/4G PAI-1 polymorphism with prematurity, intrauterine growth restriction (IUGR), and “total complications of pregnancy” that was independent of the presence of other genetic thrombophilias (factor V Leiden, MTHFR C677T, and prothrombin G20210A mutations). In a subsequent study (Glueck, et al., Obstet Gynecol 2001;97:44-8), they reaffirmed the presence of the 4G/4G genotype as a risk factor for IUGR and extended their findings to include associations with severe preeclampsia, placental abruption, and stillbirth. They also reported that “the hypofibrinolytic 4G/4G mutation of the PAI-1 gene…is frequently associated with the thrombophilic factor V Leiden mutation” which would further increase the risk of problems related to clotting.
Over the years, PAI-1 made by vascular endothelial cells was found to be induced by angiotensin II which is produced by the action of the angiotensin I-converting enzyme (ACE). In a fascinating paper published in 2003, Xia and colleagues (J Soc Gynecol Invest 2003;10:82-93) reported that 18 of 20 women with severe preeclampsia were found to have IgG antibodies to the angiotensin II type 1 (AT1) receptor. None of 18 normotensive pregnant women had these autoantibodies. They also found that the serum from the same 18 of 20 women with these AT1 receptor autoantibodies stimulated PAI-1 secretion by trophoblasts (placental cells) in culture. Activation of the trophoblast AT1 receptors was also correlated with decreased trophoblast migration and invasion in tissue culture models and this, too, was directly correlated with PAI-1 production. We will return to this point in our subsequent discussion of the role of PAI-1 in recurrent early pregnancy loss. Bobst and colleagues (Am J Hypertens 2005;18:330-6) further reported that AT1 receptor autoantibodies found in preeclamptic patients stimulated PAI-1 (and the cytokine IL-6) production by human kidney (mesangial) cells in culture. Reversible ‘damage’ to the kidney is one of the events which characterize preeclampsia and the more severe the kidney impairment, generally, the more severe the preeclampsia with regard to hypertension and decreased urine production...(more to follow!)...