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Women and Infant Hospital Rhode Island (H.P., M.C.), Department Obstetrics and Gynecology, Brown University, Providence, Rhode Island 02912; and Case Western Reserve University (S.B., K.H., L.L., L.P., P.M.C., S.H.-d.M.), Department Reproductive Biology, MetroHealth Medical Center, Cleveland, Ohio 44109
Address all correspondence and requests for reprints to: S. Hauguel-de Mouzon, Case Western Reserve University, Department Reproductive Biology, MetroHealth Medical Center, 2500 MetroHealth Drive, Cleveland, Ohio 44109. E-mail: shdemouzon{at}metrohealth.org.
| Abstract |
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Objective: This study was undertaken to identify the sources and functional properties of adiponectin in utero.
Design and Methods: Tissue specimens were obtained at autopsy from 21- to 39-wk-old stillborn human fetuses. Adipose tissue and placenta were obtained at term elective cesarean section. Adiponectin complexes and expression were measured by immunodetection and real-time PCR.
Results: Adiponectin mRNA transcripts were detected in fetal sc and omental adipose depots at lower concentrations than in maternal adipose tissue. Immunoreactive adiponectin was also observed in vascular endothelial cells of fetal organs, including skeletal muscle, kidney, and brain. The absence of adiponectin in all placental cell types and lack of correlation between maternal and umbilical adiponectin indicate that umbilical adiponectin reflects its exclusive production by fetal tissues. The most prominent forms of adiponectin in fetal plasma were high and low molecular mass (HMW and LMW) multimers of 340 and 160 kDa, respectively. The proportion of the HMW complexes was 5-fold (P < 0.001) higher in umbilical plasma than in adult. The high HMW and total adiponectin levels were associated with lower insulin concentration and lower homeostasis model of assessment of insulin resistance indices in umbilical plasma, reflecting higher insulin sensitivity of the fetus compared with adult.
Conclusions: The abundance of HMW adiponectin and its vascular expression are characteristics of human fetal adiponectin. Combined with high insulin sensitivity, fetal adiponectin may be a critical determinant of in utero growth.
| Introduction |
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During pregnancy, plasma adiponectin concentrations are decreased compared with pre-gravid as a consequence of reduced gene expression in white adipose tissue (13, 14). Adiponectin concentration is lower in gestational diabetes compared with nondiabetic pregnancy, consistent with greater insulin resistance in this condition (15). The low levels of adiponectin in pregnant women contrast with the high adiponectin concentrations in the fetus (16, 17). Moreover, high adiponectin levels in neonates indicate that the hyperadiponectinemia is a common feature of early developmental stages in humans (18). In contrast to other adipocytokines that have a ubiquitous tissue distribution, adiponectin is exclusively synthesized by adipocytes in adults (1, 19), but little data are available in fetal life. The aim of this study was to identify the source of fetal plasma adiponectin.
| Patients and Methods |
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The use of human material conformed to the principal outlined in the declaration of Helsinki. Subcutaneous and omental adipose tissue specimens (150–300 mg) from eight stillborns were obtained at postmortem examination. The gestational ages of the fetuses ranged from 21–39 wk, with birth weight of 774-3183 g. None of the stillborns showed any evidence of maceration. The postmortem examinations were performed within 5 h after demise. Tissue fragments for RNA analysis were immediately snap frozen in liquid nitrogen and stored at –80 C before processing. Additional tissue fragments of kidney, skin, adrenal with adjacent adipose tissue, and from cerebral cortex were obtained from four term fetuses (38–39 wk) as well as adipose tissue from one 21-wk-old fetus. Tissues were fixed in 4–10% formalin and embedded in paraffin for use in immunohistochemistry. Subcutaneous fat tissue (1–3 g) and blood samples were obtained in the fasting state from 56 lean and 72 obese nonlaboring women without evidence of clinical infection at elective cesarean section delivery. Placenta tissue and umbilical venous cord blood were obtained upon delivery of the placenta. The study was approved by the institution review board of MetroHealth Medical Center.
Immunohistochemistry
Adiponectin was localized in maternal and fetal paraffin-embedded tissue sections by indirect immunofluorescence. Sections were rehydrated and rinsed in a 1:20 dilution of Tris-buffered saline-Triton X-100. Normal rabbit serum solutions [20% in Tris-HCl buffer (pH 7.6)] were used to reduce the background. Mouse monoclonal antiadiponectin antibody (1:250 dilution) was added for 30 min, followed by sequential 10-min incubations with secondary fluorescein isothiocyanate, and tetramethylrhodamine isothiocyanate antibodies (United States Biological, Inc., Swampscott, MA). Anti-CD34 antibody (CHEMICON International, Inc., Tremecula, CA) was used for double staining of endothelial cells. Sections were illuminated and photographed using a fluorescent microscope (Nikon, Melville, NY). The capture of digital pictures was made with SPOT software (Diagnostic Instruments, Inc., Sterling Heights, MI).
RNA purification and cDNA synthesis
Total RNA was extracted from adipose tissue using TRIZOL (Invitrogen Corp., Carlsbad, CA). Total RNA was purified from white blood cells using the RNeasy kit (QIAGEN, Inc., Valencia, CA). Integrity was verified by Agilent Technologies, Inc. (Palo Alto, CA). RNA was reversed transcribed using the Superscript II Rnase H Reverse Transcriptase system (Invitrogen). Adiponectin and actin gene expression was measured by real-time PCR using the LightCycler FastStart DNA Master SYBR Green I kit (Roche, Indianapolis, IN). Amplifications were performed in duplicate using 30 ng cDNA using the primer pairs as follows: actin (NM_001101), forward 5'-ggacttcgagcaagagatgg-3' and reverse 5'-agcactgtgttggcgtacag-3'; and adiponectin (NM_004797), forward 5'-gcagtctgtggttctgattcc-3' and reverse 5'-ccagtggagccatcatagtg-3'. Results normalized for actin were expressed as copy number per ng RNA calculated from an adipose tissue cDNA standard curve.
Western blots
Maternal and venous umbilical cord plasma (1 µl,
60 µg protein) was electrophoresed under denaturing and nondenaturing conditions, and transferred to nitrocellulose membrane. Membranes were blocked overnight then incubated for 2 h with primary antibody for rabbit polyclonal anti-adiponectin (CHEMICON International), then incubated with goat antirabbit IgG horseradish peroxidase-conjugated secondary antibody. Immunocomplexes were visualized by chemiluminescence (Amersham Biosciences Inc., Piscataway, NJ). The abundance of HMW adiponectin multimers was determined by densitometry with a gel-doc system (Bio-Rad Laboratories, Inc., Hercules, CA) and expressed as percent total adiponectin multimers. Total plasma adiponectin was determined by ELISA (LINCO Research, Inc., Billerica, MA).
Plasma assays
All plasma samples were run in duplicate in a single assay. Total plasma adiponectin was determined by ELISA. Insulin was measured by RIA kits (LINCO). Glucose was assessed by the glucose oxidase method using a glucose analyzer (Yellow Springs Instrument Inc., Yellow Springs, OH).
Data analysis
The homeostasis model of assessment (HOMA) index of insulin sensitivity was calculated as: glucose (mmol/liter) insulin (µU/ml)/22.5. Statistical significance was determined using ANOVA, followed by the Fishers projected least significant difference test. Pearsons correlation coefficient was used to determine the relationship between maternal and umbilical cord adiponectin. Values are given as mean ± SEM.
| Results |
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To investigate the origin of high adiponectin concentrations during fetal life, we have characterized adiponectin gene and protein expression in several fetal tissues as potential in utero sources of adiponectin. Adiponectin mRNA was detected by real-time PCR in white adipose tissue of the fetus, albeit at a much lower concentration than in maternal adipose tissue (3.8 ± 0.5 vs. 840 ± 82 copies per µg RNA). Adiponectin mRNA transcripts were also detected at a low level in umbilical white blood cells (0.03 ± 0.006 copies per ng RNA), and the copy numbers were at the limit of detection (0.005 ± 0.002 copies per ng RNA) in placental villous tissue (Fig. 1
), with an average Ct value of 35. The immunoreactive adiponectin protein was identified in sc and omental adipose tissue of term and preterm fetuses (Fig. 2
), and the protein identity was confirmed by Western blot of adipose homogenates (data not shown). In addition to adipose tissue, adiponectin-specific immunoreactive signals were observed in vessels and vessel walls of several fetal organs, including skeletal muscle, kidney, skin, and cortex (Fig. 3
). Double labeling with CD34 identified the endothelial cells as the source of adiponectin expression in capillaries of fetal tissues but not in placental endothelial cells or any other placental cell type, including trophoblast (Fig. 4
). Adiponectin immunoreactivity was abundant in the fetal blood trapped within the fetal vessels of the placental stem and terminal villi.
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To examine the characteristics of fetal adiponectin, we measured total plasma adiponectin and quantified the absolute amount of low (LMW) and high (HMW) adiponectin multimers in umbilical and maternal venous plasma in 56 uncomplicated term pregnancies in lean women. Fetal adiponectin was detected as a protein of approximately 30 kDa under denaturing conditions, corresponding to the monomeric form of adiponectin similar to adult (Fig. 5B
). However, the most prominent forms of circulating fetal adiponectin were multimeric complexes of 340 and 160 kDa corresponding to the assembly of 12 and six molecules of adiponectin with a minor representation of the 67-kDa trimers (Fig. 5A
). The absolute amount of HMW was higher in umbilical than in maternal plasma (20.9 ± 8.6 vs. 4.5 ± 0.2 densitometry units; P < 0.001). The increased abundance of HMW adiponectin was reflected in the higher adiponectin indices (defined as the ratio of HMW/total adiponectin) in umbilical compared with maternal plasma: 2-fold higher than in insulin-sensitive subjects (0.35 ± 0.06 vs. 0.17 ± 0.04; P < 0.01) and 20-fold higher (0.35 ± 0.06 vs. 0.05 ± 0.02; P < 0.001) than in insulin-resistant subjects (Fig. 5C
). Total adiponectin concentration was higher in umbilical compared with maternal plasma (42.4 ± 4.3 vs. 12.3 ± 1.8 µg/ml; P < 0.001), whereas HOMA indices were low in fetal plasma (1.1 ± 0.9) compared with that in insulin-sensitive and insulin-resistant pregnant women (2.2 ± 0.2 and 5.0 ± 0.4), respectively (Table 1
). There was a strong negative correlation between maternal and fetal adiponectin and insulin concentrations (r = –0.54; P < 0.0001), as well as HOMA indices (r = –0.55; P < 0.0001).
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| Discussion |
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We have addressed the possibility that multiple fetal tissues may produce adiponectin and thereby contribute to the higher concentration of circulating adiponectin during fetal life. Our results extend the original observation of Scherers group to fetal life showing adiponectin expression in several fat depots of the fetus (3). In addition, we show that adiponectin is highly expressed in vascular endothelial cells of fetal capillaries and in the media of larger fetal blood vessels of many tissues, including muscle, white adipose tissue, brain, and kidney (the only tissues studied to date in our laboratory). Interestingly, despite the fetal genotype of the placenta, adiponectin was not detected in placental endothelial cells lining the fetal circulation. These data provide definite support that the human placenta is not a site of endogenous adiponectin production, a finding in some but not all prior reports (22, 23, 24). We conclude that the adiponectin detected in human term placenta is accounted for by the adiponectin of fetal origin that circulates in the fetal vasculature of the placental villi (Fig. 3
). In addition, the absence of correlation between maternal and umbilical adiponectin concentrations (y = 30.2 + 0.30x, r = 0.10; P = 0.31) supported that umbilical adiponectin primarily reflects the production by fetal tissues for lack of placental transfer. Whether or not adiponectin is produced by fetal brown adipocytes as described in vitro (25) and may also contribute to circulating adiponectin will require further investigation.
Multimer distribution
In addition to changes in body mass, insulin sensitivity is regarded as a potential confounder in adiponectin regulation (26). Data in rodents and humans indicate that adiponectin functions as an insulin sensitizer independently of the adipose tissue size (4, 6, 27, 28). In addition, studies by Kadowaki and colleagues (9) have suggested that the multimer distribution reflects insulin sensitivity better than simply total circulating concentration. Our study shows that the HMW oligomers are the main adiponectin species circulating in fetal plasma. The higher proportion of the HMW multimers was associated with lower insulin concentrations and HOMA indices in umbilical compared with adult plasma, reflecting a high insulin sensitivity of the fetus. To the best of our knowledge, our study is the first to estimate insulin sensitivity in neonates from fasted mothers using the homeostasis model of assessment of insulin resistance (HOMA-IR). The higher insulin sensitivity at birth compared with adults is in line with that in preterm human neonates (29), suggesting that early development in humans is a period of remarkable insulin sensitivity.
Together, these data suggest that the specific fetal multimer distribution may relate to a higher bioactivity of adiponectin at early developmental stages, contributing to a higher sensitivity to insulin. The high adiponectin levels with increased bioactive HMW multimers found in the late fetus suggest that adiponectin may have different metabolic effects in utero than in the adult. Adiponectin is a potent inducer of adipogenesis, promoting preadipocyte differentiation into adipocytes, augmenting programmed gene expression for adipogenesis, and increasing lipid content and insulin responsiveness of the glucose transport system in adipocytes (30). Thus, the high adiponectin levels in the third trimester fetus may contribute to the 70% fetal fat accretion that occurs in this period (31). In addition, the angiogenic properties of adiponectin stimulating endothelial differentiation, vessel growth and remodeling (32, 33) may also regulate the development of the vascular system of the fetus.
In conclusion, we show that fetal white adipose tissue and vascular endothelial cells are primary sources for adiponectin production in utero. The absence of endogenous placental adiponectin points to a distinct control of adiponectin production in the fetoplacental and maternal compartments. The abundance of circulating HMW multimers and specific vascular expression are unique characteristics of fetal adiponectin, which in combination with high insulin sensitivity may be key metabolic parameters for in utero growth.
| Acknowledgments |
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| Footnotes |
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Disclosure Information: The authors have nothing to declare.
First Published Online April 29, 2008
Abbreviations: HMW, High molecular weight; HOMA, homeostasis model of assessment; HOMA-IR, homeostasis model of assessment of insulin resistance; LMW, low molecular weight.
Received January 3, 2008.
Accepted April 21, 2008.
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