This study has investigated the effects of prenatal ethanol exposure on serum growth factors in humans, where current knowledge is very limited.
In a previous report, a small group of short children with FAS exhibited serum IGF- I and IGFBP-3 in the low normal range [9
], and it has been suggested that newborn infants of ethanol-abusing mothers may have low serum IGF-I levels [10
]. Interestingly, in our much larger, prospective study, we observed that serum IGF-I and IGF-II levels increased during postnatal life in children exposed to ethanol in utero compared with healthy unexposed children. This was particularly true for serum IGF-II levels. IGF-II levels were weakly correlated with growth measures in the children, which may not be surprising given that IGF-II appears to be more important for prenatal growth. IGF-I was correlated more strongly and more significantly in the unexposed than in the exposed group, suggesting that alcohol disturbs the normal relationship.
Our exposed subjects were more likely to be SGA at birth than the general Chilean population. This effect did not persist. Although some studies show persistent poor growth in children exposed to large quantities of alcohol in utero, others do not [6
]. It appears that children who are reasonably well nourished can experience catch-up growth. Our population was lower middle class, not impoverished, and most had good diets. Thus, it is likely that they did experience catch-up growth and grew reasonably well since most of them did not manifest the classic FAS phenotype.
The IGF system is very important for pre- and postnatal growth. Both IGF-I and IGF-II are detected in the fetal circulation during early gestation, but their specific actions differ depending on the fetal tissue and gestational age. There is a known association of IGF-I serum levels with birth weight, ponderal index and birth length [25
], and its effects are regulated by nutrient supply.
Altered prenatal expression of these growth factors and/or its receptor may influence fetal growth. IGF-I gene polymorphisms have been associated with reduced birth size, and IGF-I receptor mutations have also been found in patients with intrauterine growth restriction [27
]. Experiments in knockout mice have shown the importance of IGFs for fetal growth, since gene deletions for IGF-I and IGF-II are associated with a birth weight approximately 40% below that of the wild type [29
]. From human studies, it is known that IGF-I gene deletions or mutations may cause intrauterine growth restriction [30
IGF-II is the primary growth factor supporting prenatal growth, but most studies have not documented an association with birth weight [32
]. IGFs, especially IGF-II, may influence placental function [33
], and reduced placental expression of IGF-II is associated with intrauterine growth restriction [12
]. Serum concentrations of IGF-II are much higher than those of IGF-I during late gestation, and tissue and circulating IGF-II concentrations are higher in the fetus than in the newborn or adult in most species. Likewise, there is a shift in IGF predominance from IGF-II during gestation to IGF-I after birth. Serum IGF-II concentrations are relatively unaffected by variations in nutritional status, but they are modified by changes in fetal glucocorticoid concentrations, suggesting that they may be affected by adverse intrauterine conditions [34
]. This growth factor may play a role in normal growth, as Bernardini et al. [35
] demonstrated that there is a progressive increase in serum IGF-II levels during the first 3 months of life in normal infants, and García et al. [36
] reported a transient increase in serum IGF-II at 3 months of age in SGA infants who experienced catch-up growth.
Very limited data are available regarding the serum concentrations of these hormones in human fetuses exposed to alcohol during pregnancy. Mauceri et al. [37
] showed that ethanol alters the pattern of IGF-II release in rat fetuses, probably by interference with the signaling mechanism. During gestation, control rats had increased tissue release of IGF-II, especially in the brain; but in ethanol-exposed rats the IGF-II content in specific tissues was not increased. Systemic IGF-II levels were elevated, which may reflect IGF-II release from the liver or a slower clearance. Whether IGF-II plays a role in ethanol-induced brain dysfunction is not clear [37
]. In addition, in agreement with our findings, Gundogan et al. [12
] recently demonstrated that prenatal exposure to ethanol in rodents increases placental IGF-II concentrations and IGF-II receptor mRNA levels. However, Singh et al. [38
] showed reduced IGF-II mRNA and serum levels in a fetal rodent model exposed to ethanol.
We found that children exposed to large amounts of alcohol prenatally, particularly girls, start with lower than expected IGF-II levels in infancy, but their levels increase very rapidly in early childhood to become significantly higher during the first few years of life. These findings in children exposed prenatally to high amounts of ethanol should prompt investigation of serum IGF-II as a potential marker for prenatal alcohol exposure. In conjunction with other data, it could potentially be a valuable marker of ethanol exposure in suspected cases in which a clear maternal history is not available.
Leptin is an adipocyte-secreted hormone, present in higher concentrations in females, which is significantly associated with newborn weight and length [39
]. In children at high risk for adult obesity, high baseline serum leptin concentration predicts greater body mass index and fat mass over time in both genders [41
]. Prenatal ethanol exposure results in decreased leptin levels in subcutaneous adipose tissue of neonatal rats, but not in adult rats [42
]. We observed lower serum leptin levels at 1 and 2 years of age in our ethanol-exposed children, which may be explained by prenatal ethanol exposure.
This study has several strengths, such as the fact that ethanol exposure was documented prenatally in a large cohort of women who came from an unselected population attending a prenatal clinic, and the exposed children were followed longitudinally over time. The study has limitations as well. Although only a small number of subjects were born early enough in the study to be followed to age 5, and the small sample size decreases the power to find differences, we were able to identify significant differences between the exposed and unexposed groups. As noted above, we had to use an external control group to provide reference data for logistic reasons. In fact, the potential problem that choosing healthy children could have made the exposed group's growth look worse by comparison turned out not to be the case. The very limited number of control subjects who provided data at more than one time point is, nonetheless, a limitation of the study.
As expected in an unselected group of children heavily exposed to alcohol prenatally, most of them did not develop full blown FAS. This may explain the limited impact that this exposure had on longitudinal growth and serum IGF-I and IGFBP-3 levels, although it is noteworthy that the normal correlation between IGF-I and growth parameters was greatly reduced in our exposed population.
Growth retardation is one of the defining features of FAS; however, the etiology of growth retardation in FAS remains unclear. Our population, which was exposed to large quantities of alcohol in utero, showed increased serum IGF-I and IGF-II levels during early childhood. In addition, we observed that ethanol-exposed children showed lower leptin levels at 2 years, which may be caused by effects of ethanol on adipose tissue. The high concentration of serum IGF-II in the ethanol-exposed children suggests that this hormone should be further investigated as a marker for prenatal alcohol exposure.