The primary source of insulin-like growth factor binding protein-1 (IGFBP-1) in maternal serum is the decidua. 1,2 The IGFBP-1 is believed to play a role in endometrial development and in interactions between the decidua and the invading trophoblast. 3 Human IGFBP-1 undergoes serine phosphorylation, which enhances its affinity for insulin-like growth factor-1 by six- to eight-fold. 4–6 In vitro studies show that excess amount of IGFBP-1 has the potential to inhibit trophoblast invasion. 7 Failed trophoblast invasion results in fetal growth restriction and preeclampsia. 8–10 In these conditions, maternal serum IGFBP-1 concentration is increased compared with healthy controls. 11–15 This increase is believed to result from hypoxia at the maternal-fetal interface, caused by failure of trophoblast invasion of the maternal spiral arteries.
Another cause of fetoplacental hypoxia is exposure to high altitude. At 4300-m altitude, the barometric pressure and therefore the partial pressure of oxygen are about 40% lower than at sea level, and as a consequence arterial pO2 in adults is only about 50 mmHg. 16 The aim of this study is to investigate the effect of environmental hypoxia on the maternal serum concentration of IGFBP-1.
MATERIALS AND METHODS
This was a cross-sectional study of 108 pregnant women in Peru, 62 from high altitude (4300 m, 14100 ft) and 46 from sea level at 14–42 weeks' gestation. For comparison, 20 healthy nonpregnant women (ten from high altitude and ten from sea level) were also examined. Subjects were recruited both in Lima and in Cerro de Pasco from hospital antenatal and health clinics, attending for routine visits. Local physicians, midwives, and nursing students carried out the recruitment from September 1998 to December 1998. All women were ethnic Mestizo, a mixture between native Quechua and Spanish, and they were permanently resident and lived at the altitude at which they and their parents and grandparents were born. The inclusion criteria were healthy women with normal singleton pregnancies at 14–42 weeks. Gestational age was determined by menstrual history and confirmed by ultrasound biometry at the time of the examination. In the first and second trimester, dates were changed if there was a discrepancy of more than 1 week, and normal fetal growth was confirmed by a follow-up appointment. In the third trimester, patients were only considered for serum analysis if ultrasound biometry confirmed the dates and if there was no ultrasound evidence of intrauterine growth restriction. Ethical approval was obtained from the Peruvian Ministry of Health, and all subjects gave informed written consent to participate in the study.
Total and nonphosphorylated IGFBP-1 were measured in maternal serum. Blood was taken from the antecubital vein between 8 AM and 6 PM, which avoids the nocturnal peak of IGFBP-1 levels. 17 Commercially available immunoradiometric assays (IRMA) (DSL, TX) were used. The total IGFBP-1 IRMA has a sensitivity of 0.33 μg/L and intra- and interassay coefficient of variation of 2.5% at 10.0 μg/L and 6.9% at 123 μg/L, respectively. The nonphosphorylated IGFBP-1 IRMA measures levels of primarily nonphosphorylated IGFBP-1 isoforms in serum, without reactivity to the highly phosphorylated variants. The sensitivity of the assay to the state of IGFBP-1 phosphorylation is such that, in serum samples from normal adults, the assay detects only 5–10% of the total IGFBP-1 measured by the DSL TOTAL IGFBP-1 IRMA kit, but registers a rapid rise in levels after sample treatment with alkaline phosphatase, which dephosphorylates IGFBP-1. The lower limit of detection is 0.20 μg/L. Intra- and interassay coefficients of variation are 4% and 7.6% at about 30 μg/L. There is no cross-reactivity with IGFBP-2 through IGFBP-6.
The Kolmogorov-Smirnov test was used to assess the normality of the data. The distributions of gestational age, maternal age, and body mass index were not significantly different from normal (Table 1). Total and nonphosphorylated IGFBP-1 were positively skewed and were therefore log-transformed for regression analysis. To assess the relative extent of phosphorylation, the ratio of total and nonphosphorylated IGFBP-1 was calculated and analyzed. For comparisons between sea level and high altitude in the pregnant group, multiple regression analysis was used; the coefficient of altitude (at sea level = 0 and high altitude = 1) estimated the difference between high altitude and sea level. Previous publications have shown that insulin-like growth factors and IGFBPs change with body mass index and age. In our data, there was a significant correlation between IGFBP-1 and body mass index, and the maternal age was significantly different between high altitude and sea level (Table 1). Therefore, regression analyses were also adjusted for maternal age, body mass index, and gestation. 12 For the log-transformed variables (total and nonphosphorylated IGFBP-1), antilog transformation of the “altitude” coefficient yielded the high altitude/sea level ratio. Interaction between the population and gestational age was calculated by adding a gestational age × group term to the regression model to assess whether the difference between the groups changed with gestational age. An altitude effect threshold model was also applied. 18 Two variables were defined, each of which was always zero for fetuses from sea level. For fetuses from high altitude, the variable was zero unless the gestational age was within a certain range (14–24, 25–40). Regression analysis with these variables, gestational age, maternal age, and body mass index as predictors was then performed. This allowed different slopes to be estimated for each interval of gestational age. If there were a threshold effect, these variables would only affect concentrations of IGFBP-1 at a later gestational age. Nonpregnant groups were compared between high altitude and sea level and with pregnant groups using the Mann-Whitney U test. A t test was used to compare maternal characteristics. Analyses were performed with SPSS 8.0 (SPSS for Windows, Rel. 8.0.0, 1997, SPSS, Inc., Chicago, IL).
The concentrations of both total and nonphosphorylated IGFBP-1 did not change significantly with gestation at sea level (r = 0.123, P = .415, and r = 0.167, P = .266, respectively). However, they increased with gestation at high altitude (r = 0.387, P = .002, and r = 0.379, P = .002, respectively) (Figures 1 and 2). Allowing for maternal age and body mass index, the levels were higher at high altitude than at sea level for both total and nonphosphorylated IGFBP-1 (ratio = 1.28, P = .008, and ratio = 1.45, P = .003, respectively). Also, there was significant interaction between high altitude and sea level (P = .037 and P = .043, respectively). Hence, the ratio was not the same at each gestational age. The threshold model showed that the difference became significant from 25 weeks' gestation onwards (Table 2). The ratio of total and nonphosphorylated IGFBP-1 did not change with gestation, neither at sea level (r = 0.012, P = .939) nor at high altitude (r = 0.204, P = .112). Allowing for maternal age and body mass index, there was no significant difference between high altitude and sea level (r2 = 0.148, b = 0.005, 95% confidence interval 0.09, 1.0, P = .305).
The concentrations of both total and nonphosphorylated IGFBP-1 were lower in nonpregnant controls than in the pregnant groups, and the concentrations were higher at high altitude than at sea level (Table 3).
Pregnancy outcomes were available in 87% (40 of 46 at sea level and 54 of 62 at high altitude). All women delivered at the altitude where they had been assessed during pregnancy. Fourteen were lost at follow-up because of home deliveries or because they had left the health district. There was no significant difference in gestation at delivery (39.7 ± 1.4 weeks and 39.3 ± 1.7 weeks, respectively), but birth weight was significantly lower at high altitude than at sea level (2963 ± 385 g and 3250 ± 394 g, respectively). None of the 94 women with outcome information available developed preeclampsia.
The data of this study demonstrate that from 25 weeks of pregnancy onwards, the maternal serum concentration of total and nonphosphorylated IGFBP-1 in women native at high altitude is higher than in those living at sea level. We also found that at sea level, the concentration of IGFBP-1 does not change with gestation, which is compatible with previous reports. 19,20 The ratio of total and nonphosphorylated IGFBP-1 was similar at high altitude compared with sea level, suggesting that there is no mechanism, such as reduced phosphorylation, which would counterbalance the increased IGFBP-1 production at high altitude to limit the binding capacity of the molecule.
The IGFBP-1 is primarily expressed in the adult liver and in the decidua. 21 We found that in nonpregnant controls, the concentrations were much lower than in the pregnant groups, and the difference between high altitude and sea level was minimal. It is therefore likely that the source of the increased IGFBP-1 concentration at high altitude is mainly the decidua; however, the contribution of the liver cannot be determined from our data.
A recent Doppler study has demonstrated that at high altitude, impedance to flow in the uterine arteries may actually be lower than at sea level. 22 Therefore, unlike what has been hypothesized for sea-level pregnancies, 11–15 the increased IGFBP-1 at high altitude is unlikely to be caused by inadequate trophoblast invasion resulting in placental hypoxia.
In the second half of pregnancy, the maternal and fetal demands increase dramatically, and low atmospheric oxygen with resulting maternal systemic hypoxemic hypoxia 23 may cause placental hypoxia, as suggested by animal studies. 24 This stimulates increased production of IGFBP-1, 25 which in turn restricts the insulin-like growth factor-mediated fetal growth as an adaptive mechanism to prevent worsening of the fetoplacental hypoxia. We have reported that fetal size at high altitude is lower than at sea level only from 25 weeks onwards. 18 High altitude is associated with lower birth weight, which decreases by 100 g per 1000-m altitude. 26 Our outcome data are in accordance with these findings. However, the smallness seems to be a physiologic adaptation to high altitude rather than pathologic growth restriction, as the mortality rate in low birth weight infants born at high altitude is lower than in low birth weight infants born at sea level. 27,28
An alternative mechanism to placental hypoxia for the increased IGFBP-1 at high altitude is decreased inhibition by insulin. The production of IGFBP-1 is known to be inhibited by insulin, 29 and pregnant women with diabetes mellitus have lower serum levels of IGFBP-1. 30 We have previously found in the same population that in the third trimester, maternal plasma insulin concentration was lower at high altitude compared with sea level, 31 and it is therefore possible that the higher IGFBP-1 is a consequence of the lower insulin.
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