Mechanistic target of rapamycin (mTOR) signaling functions as a central regulator of cellular metabolism, growth, and survival in response to hormones, growth factors, nutrients, energy, and stress signals. Mechanistic TOR is therefore critical for the growth of most fetal organs, and global mTOR deletion is embryonic lethal. This review discusses emerging evidence suggesting that mTOR signaling also has a role as a critical hub in the overall homeostatic control of fetal growth, adjusting the fetal growth trajectory according to the ability of the maternal supply line to support fetal growth. In the fetus, liver mTOR governs the secretion and phosphorylation of insulin-like growth factor binding protein 1 (IGFBP-1) thereby controlling the bioavailability of insulin-like growth factors (IGF-I and IGF-II), which function as important growth hormones during fetal life. In the placenta, mTOR responds to a large number of growth-related signals, including amino acids, glucose, oxygen, folate, and growth factors, to regulate trophoblast mitochondrial respiration, nutrient transport, and protein synthesis, thereby influencing fetal growth. In the maternal compartment, mTOR is an integral part of a decidual nutrient sensor which links oxygen and nutrient availability to the phosphorylation of IGFBP-1 with preferential effects on the bioavailability of IGF-I in the maternal–fetal interface and in the maternal circulation. These new roles of mTOR signaling in the regulation fetal growth will help us better understand the molecular underpinnings of abnormal fetal growth, such as intrauterine growth restriction and fetal overgrowth, and may represent novel avenues for diagnostics and intervention in important pregnancy complications.

Fetal growth restriction (FGR) increases the risk for perinatal complications and predisposes the infant to diabetes and cardiovascular disease later in life. No treatment for FGR is available, and the underlying pathophysiology remains poorly understood. Increased IGFBP-1 phosphorylation has been implicated as an important mechanism by which fetal growth is reduced. However, to what extent circulating IGFBP-1 is phosphorylated in FGR is unknown, and the molecular mechanisms linking FGR to IGFBP-1 phosphorylation have not been established. We used umbilical cord plasma of appropriate for gestational age (AGA) and growth–restricted human fetuses and determined IGFBP-1 and IGF-I concentrations (ELISA) and site-specific IGFBP-1 phosphorylation (Western blotting using IGFBP-1 phospho-site specific antibodies). In addition, we used a baboon model of FGR produced by 30% maternal nutrient restriction and determined mammalian target of rapamycin (mTOR)C1 activity, CK2 expression/activity, IGFBP-1 expression and phosphorylation, and IGF-I levels in baboon fetal liver by Western blot, enzymatic assay, and ELISA. HepG2 cells and primary fetal baboon hepatocytes were used to explore mechanistic links between mTORC1 signaling and IGFBP-1 phosphorylation. IGFBP-1 was hyperphosphorylated at Ser101, Ser119, and Ser169 in umbilical plasma of human FGR fetuses. IGFBP-1 was also hyperphosphorylated at Ser101, Ser119, and Ser169 in the liver of growth–restricted baboon fetus. mTOR signaling was markedly inhibited, whereas expression and activity of CK2 was increased in growth–restricted baboon fetal liver in vivo. Using HepG2 cells and primary fetal baboon hepatocytes, we established a mechanistic link between mTOR inhibition, CK2 activation, IGFBP-1 hyperphosphorylation, and decreased IGF-I–induced IGF-I receptor autophosphorylation. We provide clear evidence for IGFBP-1 hyperphosphorylation in FGR and identified an mTOR and CK2-mediated mechanism for regulation of IGF-I bioavailability. Our findings are consistent with the model that inhibition of mTOR in the fetal liver, resulting in increased CK2 activity and IGFBP-1 hyperphosphorylation, constitutes a novel mechanistic link between nutrient deprivation and restricted fetal growth.

Fetal growth restriction is often caused by uteroplacental insufficiency that leads to fetal hypoxia and nutrient deprivation. Elevated IGF binding protein (IGFBP)-1 expression associated with fetal growth restriction has been documented. In this study we tested the hypothesis that hypoxia and nutrient deprivation induce IGFBP-1 phosphorylation and increase its biological potency in inhibiting IGF actions. HepG2 cells were subjected to hypoxia and leucine deprivation to mimic the deprivation of metabolic substrates. The total IGFBP-1 levels measured by ELISA were approximately 2- to 2.5-fold higher in hypoxia and leucine deprivation-treated cells compared with the controls. Two-dimensional immunoblotting showed that whereas the nonphosphorylated isoform is the predominant IGFBP-1 in the controls, the highly phosphorylated isoforms were dominant in hypoxia and leucine deprivation-treated cells. Liquid chromatography-tandem mass spectrometry analysis revealed four serine phosphorylation sites: three known sites (pSer 101, pSer 119, and pSer 169); and a novel site (pSer 98). Liquid chromatography-mass spectrometry was used to estimate the changes of phosphorylation upon treatment. Biacore analysis indicated that the highly phosphorylated IGFBP-1 isoforms found in hypoxia and leucine deprivation-treated cells had greater affinity for IGF-I [dissociation constant 5.83E (times 10 to the power)−10 m and 6.40E−09 m] relative to the IGFBP-1 from the controls (dissociation constant ∼1.54E−07 m). Furthermore, the highly phosphorylated IGFBP-1 had a stronger effect in inhibiting IGF-I-stimulated cell proliferation. These findings suggest that IGFBP-1 phosphorylation may be a novel mechanism of fetal adaptive response to hypoxia and nutrient restriction.The use of HepG2 cells demonstrates that IGF binding protein-1 hyper-phosphorylation may be a novel mechanism of fetal adaptive response to hypoxia and under-nutrition.

Context The mechanisms underpinning intrauterine growth restriction (IUGR) due to placental insufficiency remain poorly understood, no specific treatment is available, and clinically useful biomarkers for early detection are lacking. Objective We hypothesised that human IUGR is associated with inhibition of mechanistic target of rapamycin (mTOR) and activation of the amino acid response (AAR) signaling, increased protein kinase CK2 activity, and increased IGFBP-1 expression and phosphorylation in decidua, and that maternal plasma IGFBP-1 hyperphosphorylation in first trimester predicts later development of IUGR. Design, Setting and Participants Decidua (n=16 AGA, Appropriate-for-Gestational-Age; n=16 IUGR) and maternal plasma (n=13 AGA, n=13 IUGR) were collected at delivery from two different cohorts. In addition, maternal plasma was obtained in late first trimester from a third cohort of women (n=7) who later delivered an AGA or IUGR infant. Main Outcome Measures Total IGFBP-1 expression and phosphorylation (Ser101/Ser119/Ser169), mTOR, AAR and CK2 activity in decidua and IGFBP-1 concentration and phosphorylation in maternal plasma. Results We show that decidual IGFBP-1 expression and phosphorylation are increased, mTOR is markedly inhibited, AAR and CK2 is activated in IUGR. Moreover, IGFBP-1 hyperphosphorylation in first trimester maternal plasma is associated with the development of IUGR. Conclusions These data are consistent with the possibility that the decidua functions as a nutrient sensor linking limited oxygen and nutrient availability to increased IGFBP-1 phosphorylation, possibly mediated by mTOR and AAR signaling. IGFBP-1 hyperphosphorylation in first trimester maternal plasma may serve as a novel predictive IUGR biomarker allowing early intervention.