Introduction
Insulin-like growth factor-1 (IGF-1) is one of the most important endocrine regulators of fetal growth and also plays an essential role in linear growth during childhood. Previous studies have shown that IGF-1 levels were reduced in fetuses with intrauterine growth restriction (IUGR) and in children with decreased rates of growth.Reference Wollmann1, Reference Suwa, Katsumata and Maesaka2 In addition, children with IGF-1 gene deletion are characterized by severe prenatal and postnatal growth retardation,Reference Woods, Camacho-Hubner and Savage3 and recombinant human IGF-1 (rhIGF-1) treatment has improved their linear growth.Reference Camacho-Hübner, Woods and Miraki-Moud4
IGF-1 also has important metabolic actions throughout life.Reference Takahashi5 IGF-1 levels were investigated in adults with metabolic syndrome or cardiovascular diseases, demonstrating that low IGF-1 level was associated with cardiometabolic risks.Reference Conti, Musumeci and De Giusti6, Reference Ungvari and Csiszar7 In healthy adults, IGF-1 level was negatively correlated with total cholesterol (TC) and triglyceride (TG) levelsReference Colao, Spiezia and Di Somma8, Reference Succurro, Arturi and Grembiale9 and positively with high-density lipoprotein cholesterol (HDLC).Reference Succurro, Arturi and Grembiale9 In addition, rhIGF-1 administration was attempted to clarify the acute effect on serum lipids. In patients with type 2 diabetes mellitus, subcutaneously administered rhIGF-1 caused a drop in plasma TG and very low-density lipoprotein (VLDL) levels, as well as TC and low-density lipoprotein cholesterol (LDLC) levels directly or indirectly or both because of decreased glucose and insulin levels, and the effects were also confirmed in healthy adults.Reference Zenobi, Holzmann and Glatz10
Although many studies have been carried out to investigate the roles of IGF-1 for fetal growth, there has been limited information about its association with serum lipids in neonates. Nelson et al.Reference Nelson, Freeman and Sattar11 reported that IGF-1 level had a positive association with HDLC level and an inverse association with TG level in offspring of mothers with type 1 diabetes and in control term infants. However, the association has not been elucidated in preterm infants or small for gestational age (SGA) infants. In particular, it is poorly understood whether in cord blood IGF-1 level was associated with serum lipid profile independently from gestational age or birth weight. Many epidemiological studies demonstrated that low birth weight was associated with later cardiometabolic diseases in adult age, probably under the mechanism of nutritional imprinting.Reference Guilloteau, Zabielski and Hammon12, Reference Hanley, Dijane and Fewtrell13 Furthermore, animal studies confirmed that IGF-1 might have a crucial role in the regulation of imprinting by prenatal and postnatal nutrition.Reference Tosh, Fu and Callaway14, Reference Fu, Yu and Callaway15 Therefore, it is important to elucidate the association between IGF-1 and serum lipids in preterm infants. In the present study, we measured IGF-1 level in cord blood and analyzed its relationship with serum lipoprotein profile in preterm infants born SGA and appropriate for gestational age (AGA).
Subjects and methods
Subjects
Informed consent was obtained from all parents, and the study was approved by the University Ethics Committee (Nihon University, Itabashi Hospital).
The subjects were 41 consecutive, healthy preterm neonates (27 male, 14 female) born at <37-week gestational age, including 10 SGA infants diagnosed as having a birth weight below 10th percentile and 31 AGA infants with birth weight from 10th to 90th percentile.Reference Itabashi, Fujimura and Kusuda16 The inclusion criteria were: neonates that were born by uneventful vaginal delivery or caesarean operation in the maternity ward of our university hospital. The exclusion criteria were: infants whose mothers had experienced complications, except premature rupture of the membranes or threatened premature delivery and infants that had birth weight over 90th percentile for gestational age, asphyxia at birth, intrauterine infections or organic disorders.
The body weight of each neonate was determined to the nearest gram with an electronic scale. The body length was measured to the nearest 0.1 cm in the supine position with a length board.
Cord blood sampling was performed from the umbilical vein after double clamping of the umbilical cord at birth. Serum IGF-1 levels were analyzed by immunoradiometric assay kit (FUJIFILM RI Pharma Co. Ltd., Tokyo, Japan). Serum TC and TG levels were determined by enzymatic methods, and HDLC, LDLC and VLDL-TG levels were measured using HPLC with gel permeation columns (LipoSEARCH; Skylight-Biotec Inc., Akita, Japan).Reference Okazaki, Usui and Ishigami17 Apolipoprotein A-1 (apoA-1), apolipoprotein B (apoB) and apolioprotein E (apoE) levels were measured by turbidimetric immunoassay (Daiichi Pure Chemicals, Tokyo, Japan).Reference Sakurabayashi, Saito and Kita18
All statistical analyses were performed using the statistical package, STATVIEW, version 4.5 (Abacus Concepts, Berkeley, CA, USA). Data are given as the mean ± s.e. The group differences in measured parameters were analyzed by Mann–Whitney U-test. Simple and multiple regressions were used to assess the correlation between variables. In regression analysis, parameters with nonnormal distributions (IGF-1, TG, VLDL-TG) were log transformed. A P value <0.05 was considered to indicate statistical significance.
Results
Characteristics of preterm SGA infants
Table 1 shows the characteristics of the subjects. Both in SGA and AGA groups no sex difference was demonstrated in serum lipids or apolipoproteins. In groups combining both the sexes, SGA infants had lower IGF-1 levels (13.1 ± 5.3 ng/ml) than AGA infants (53.6 ± 25.6 ng/ml). TC, LDLC and HDLC levels (mg/dl) were significantly lower in SGA infants (55.0 ± 14.8, 21.6 ± 8.3, 26.3 ± 11.3) than in AGA infants (83.4 ± 18.9, 36.6 ± 11.1, 38.5 ± 11.6, respectively), whereas TG and VLDL-TG levels (mg/dl) were significantly higher in SGA infants (40.7 ± 17.1, 19.0 ± 12.7) than in AGA infants (25.6 ± 13.7, 8.1 ± 7.0, respectively). ApoA-1, B and E levels were significantly lower in SGA infants (66.4 ± 9.2, 22.0 ± 3.8, 3.0 ± 1.4) than in AGA infants (80.3 ± 17.7, 28.3 ± 10.5, 5.2 ± 2.2, respectively).
Characteristics of the subjects
SGA, small for gestational age; AGA, appropriate for gestational age; LDL, low-density lipoprotein; HDL, high-density lipoprotein; VLDL, very low-density lipoprotein.
Mean ± s.e.
Mann–Whitney U-test.
Male v. Female.
*P < 0.05.
Relationship between log IGF-1 and parameters
The relationship between log IGF-1 and parameters analyzed by simple regression was shown in Table 2. In male infants (n = 27), log IGF-1 had positive associations with birth weight (r = 0.814, P < 0.001), birth height (r = 0.416, P = 0.031), TC (r = 0.720, P < 0.001), LDLC (r = 0.494, P = 0.014), HDLC (r = 0.695, P < 0.001) and apoA-1 levels (r = 0.684, P < 0.001). Whereas in female infants (n = 14) log IGF-1 had positive associations with birth weight (r = 0.650, P = 0.012), TC (r = 0.537, P = 0.048) and HDLC (r = 0.666, P = 0.018) levels, and negatively with log TG (r = −0.571, P = 0.033). In the total subjects combined, log IGF-1 had positive associations with birth weight (r = 0.721, P < 0.001) and birth height (r = 0.457, P = 0.003), TC (r = 0.636, P < 0.001), LDLC (r = 0.453, P = 0.006), HDLC (r = 0.648, P < 0.001) and apoA-1 levels (r = 0.589, P < 0.001), and negatively with log TG (r = −0.484, P = 0.002) and log VLDL-TG (r = −0.393, P = 0.018). ApoB and apoE levels showed no association with log IGF-1.
Relationship between log IGF-1 and the variables
LDL, low-density lipoprotein; HDL, high-density lipoprotein; TG, triglyceride; VLDL, very low-density lipoprotein
In multiple regression analyses (Table 3), IGF-1 emerged as the only independent determinant explaining 43.4% of TC, 45.6% of HDLC, 39.5% of apoA-1 and 26.1% of TG variability after the gestational age and birth weight were taken into account, whereas IGF-1 was not a significant determinant for LDLC, VLDL-TG, apoB or apoE level.
Independent predictors for serum lipoprotein profile
IGF, insulin-like growth factor; LDL, low-density lipoprotein; HDL, high-density lipoprotein; TG, triglyceride; VLDL, very low-density lipoprotein.
Discussion
In the present study, we have investigated whether IGF-1 levels are associated with serum lipoprotein alteration in cord blood of preterm infants and have confirmed that reduced IGF-1 level is associated with decreased TC, HDLC and elevated TG levels.
Previous studies demonstrated that preterm infants had higher cord blood TC and LDLC and lower cord blood TG levels than term infants.Reference Bansal, Cruickshank and McElduff19–Reference Kelishadi, Badiee and Adeli21 Some mechanisms of this alteration observed in preterm infants have been suggested.Reference Bansal, Cruickshank and McElduff19 In human fetuses, gestational age was positively correlated with hepatic LDL receptor activity and thus negatively with LDLC level.Reference Cai, Xie and Chen22 In addition, the major fetal organ to utilize cholesterol is the adrenal gland and its development may affect LDLC level.Reference Diaz, Leal and Ramon23 Although IGF-1 promoted growth of liver and adrenal grand,Reference Lok, Owens and Mundy24 IGF-1 level, in the present study, had a positive association with LDLC level and the association was not significant after the gestational age and birth weight were taken into account. Therefore, LDLC level in cord blood may represent maturational status rather than nutritional status, because in fetus IGF-1 level is influenced by nutrients delivered through placenta rather than by fetal pituitary growth hormone.Reference Fowden25 Moreover, it may be partly explained by the finding that IGF-1 had no effect for steroidogenesis in adrenal grand of the fetus.Reference Ross, McMillen and Lok26 On the other hand, in our study, HDLC level may have a direct positive association with IGF-1 level independent of gestational age and body weight. Nelson et al. also reported that HDLC correlated positively with IGF-1 but not with birth weight in term infants.Reference Nelson, Freeman and Sattar11 Another study showed that HDLC level remained constant from 30 to 42 weeks of gestation.Reference Parker, Carr and Simpson27 Therefore, HDLC level in cord blood may represent nutritional status rather than maturational status. In nondiabetic adults, the positive association between HDLC and IGF-1 levels was also demonstrated independent of gender, adiposity, TC, TG and glucose levels, suggesting that IGF-1 may be an independent modulator for HDLC level.Reference Succurro, Arturi and Grembiale9
Another novel finding of the present study is that cord blood lipid alteration observed in preterm SGA infants was associated with decreased IGF-1. Intrauterine undernutrition with decreased transport of nutrients through placenta may cause IUGR with reduced IGF-1 level.Reference Wollmann1 In SGA infants, cord blood demonstrates lower TC and HDLC levels and higher TG level than in AGA infants.Reference Bansal, Cruickshank and McElduff19 Our results in preterm infants were compatible with these previous findings, although their intrauterine growth could not be evaluated. The possible mechanisms of reduced HDLC level may be an increased cholesterol ester transferReference Kaser, Ebenbichler and Wolf28 and a decreased lecithin:cholesterol acyltransferase activity.Reference Loukidi-Bouchenak, Lamri-Senhadji and Merzouk29 In addition, impaired TG clearance may contribute the elevated TG level observed in SGA infants.Reference Bansal, Cruickshank and McElduff19, Reference Yoshikawa, Okada and Munakata30 Other hormones such as leptin, adiponectin and insulin may play an important role in the regulation of lipids. Nelson et al. investigated the relationship between cord lipids and these hormones and concluded that IGF-1, leptin and male sex, rather than insulin, may be the major determinants of HDLC and TG in cord blood. In the present study, IGF-1 level in SGA infants was markedly reduced about one-fourth of that in AGA infants. In addition, IGF-1 had independent associations with TC, HDLC and TG. These findings suggest that IUGR may affect IGF-1 level in cord blood with concomitant serum lipid alteration.
It has been confirmed that there is a link between birth weight and cardiovascular diseases, as well as serum lipid levels in later life.Reference Kajantie, Barker and Osmond31 The fetal origins hypothesis suggests that the link is because of a programmed response to intrauterine undernutrition.Reference Guilloteau, Zabielski and Hammon12 Intrauterine programming of the GH/IGF axis may influence postnatal growth, insulin resistance and consequently the risk of cardiovascular disease, and thus IGF-1 may serve as a link between fetal growth and adult-onset cardiometabolic disease.Reference Tosh, Fu and Callaway14, Reference Fu, Yu and Callaway15, Reference Jensen, Chellakooty and Vielwerth32 In addition, studies of adolescent twin pairs suggest that genetic factors account for the association of low birth weight with high levels of TC and LDLC, whereas intrauterine factors, rather than genetic factors, possibly play a role in the association of low birth weight with low levels of HDLC.Reference IJzerman, Stehouwer and Van Weissenbruch33 In the present study, preterm SGA infants demonstrated lower cord blood HDLC with reduced IGF-1 levels. Whether IGF-1 is one of the key players for the link between intrauterine factors and HDLC level in later life remains to be studied.
The present study had some limitations. First, this is a cross-sectional study with limited sample size. Therefore, we were unable to divide our regression analyses into SGA or AGA infants. Furthermore, we could not obtain the blood samples from healthy term infants that may provide information to understand the underlying mechanisms of the lipid alteration in preterm SGA infants. Second, we obtained no information on the characteristics of intrauterine growth. Thus, several distinct patterns of IUGR may be identified among the heterogeneous SGA infants. Intrauterine undernutrition should be evaluated in future studies. Third, this was a cross-sectional study at birth. Longitudinal studies including term control infants are necessary to evaluate whether IGF-1 plays a role in the development of reduced HDLC levels in adults born SGA. Furthermore, IGF-binding proteins, which influence the biological availability of IGF-1, should also be investigated, because IGF-binding protein-3 itself has significant association with serum lipid levels in adults.Reference Colangelo, Liu and Gapstur34
In conclusion, IGF-1 level had a significant association with serum lipid profile in cord blood of preterm infants independent from gestational age and birth weight. SGA infants had decreased TC and HDLC, and elevated TG levels with low IGF-1 level.
Acknowledgments
The authors thank the medical and nursing staff of the ward for their assistance in this study. T.O., S.H. and S.T. designed the study; H.M. and T.Y. supervised research; N.N., K.Y., S.M., Y.U. and M.M. helped data collection; R.F. and T.O. analyzed data; and R.F. and T.O. drafted the paper. This study was partly supported by Health and Labour Science Research Grants: Comprehensive Research on Cardiovascular Disease, 17160501, in Japan.
Financial Support
This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Conflicts of Interest
None.
Ethical Standards
The authors assert that all procedures contributing to this work comply with the Helsinki Declaration of 1975, as revised in 2008, and has been approved by the University Ethics Committee (Nihon University, Itabashi Hospital).
