Phosphorylation of IGFBP-1 at discrete sites elicits variable effects on IGF-I receptor autophosphorylation

Signaling Pathways of the Insulin-like Growth Factor Binding Proteins

The 6 high-affinity insulin-like growth factor binding proteins (IGFBPs) are multifunctional proteins that modulate cell signaling through multiple pathways. Their canonical function at the cellular level is to impede access of insulin-like growth factor (IGF)-1 and IGF-2 to their principal receptor IGF1R, but IGFBPs can also inhibit, or sometimes enhance, IGF1R signaling either through their own post-translational modifications, such as phosphorylation or limited proteolysis, or by their interactions with other regulatory proteins. Beyond the regulation of IGF1R activity, IGFBPs have been shown to modulate cell survival, migration, metabolism, and other functions through mechanisms that do not appear to involve the IGF-IGF1R system. This is achieved by interacting directly or functionally with integrins, transforming growth factor β family receptors, and other cell-surface proteins as well as intracellular ligands that are intermediates in a wide range of pathways. Within the nucleus, IGFBPs can regulate the diverse range of functions of class II nuclear hormone receptors and have roles in both cell senescence and DNA damage repair by the nonhomologous end-joining pathway, thus potentially modifying the efficacy of certain cancer therapeutics. They also modulate some immune functions and may have a role in autoimmune conditions such as rheumatoid arthritis. IGFBPs have been proposed as attractive therapeutic targets, but their ubiquity in the circulation and at the cellular level raises many challenges. By understanding the diversity of regulatory pathways with which IGFBPs interact, there may still be therapeutic opportunities based on modulation of IGFBP-dependent signaling.

Placental Remote Control of Fetal Metabolism: Trophoblast mTOR Signaling Regulates Liver IGFBP-1 Phosphorylation and IGF-1 Bioavailability

The mechanisms mediating the restricted growth in intrauterine growth restriction (IUGR) remain to be fully established. Mechanistic target of rapamycin (mTOR) signaling functions as a placental nutrient sensor, indirectly influencing fetal growth by regulating placental function. Increased secretion and the phosphorylation of fetal liver IGFBP-1 are known to markedly decrease the bioavailability of IGF-1, a major fetal growth factor. We hypothesized that an inhibition of trophoblast mTOR increases liver IGFBP-1 secretion and phosphorylation. We collected conditioned media (CM) from cultured primary human trophoblast (PHT) cells with a silenced RAPTOR (specific inhibition of mTOR Complex 1), RICTOR (inhibition of mTOR Complex 2), or DEPTOR (activates both mTOR Complexes). Subsequently, HepG2 cells, a well-established model for human fetal hepatocytes, were cultured in CM from PHT cells, and IGFBP-1 secretion and phosphorylation were determined. CM from PHT cells with either mTORC1 or mTORC2 inhibition caused the marked hyperphosphorylation of IGFBP-1 in HepG2 cells as determined by 2D-immunoblotting while Parallel Reaction Monitoring-Mass Spectrometry (PRM-MS) identified increased dually phosphorylated Ser169 + Ser174. Furthermore, using the same samples, PRM-MS identified multiple CK2 peptides coimmunoprecipitated with IGFBP-1 and greater CK2 autophosphorylation, indicating the activation of CK2, a key enzyme mediating IGFBP-1 phosphorylation. Increased IGFBP-1 phosphorylation inhibited IGF-1 function, as determined by the reduced IGF-1R autophosphorylation. Conversely, CM from PHT cells with mTOR activation decreased IGFBP-1 phosphorylation. CM from non-trophoblast cells with mTORC1 or mTORC2 inhibition had no effect on HepG2 IGFBP-1 phosphorylation. Placental mTOR signaling may regulate fetal growth by the remote control of fetal liver IGFBP-1 phosphorylation.

IGFBP-1 hyperphosphorylation in response to nutrient deprivation is mediated by activation of protein kinase Cα (PKCα)

Fetal growth restriction (FGR) is associated with decreased nutrient availability and reduced insulin-line growth factor (IGF)-I bioavailability via increased IGF binding protein (IGFBP)-1 phosphorylation. While protein kinase C (PKC) is implicated in IGFBP-1 hyperphosphorylation in nutrient deprivation, the mechanisms remain unclear. We hypothesised that the interaction of PKCα with protein kinase CK2β and activation of PKCα under leucine deprivation (L0) mediate fetal hepatic IGFBP-1 hyperphosphorylation. Parallel Reaction Monitoring Mass Spectrometry (PRM-MS) followed by PKCα knockdown demonstrated the PKCα isoform interacts with IGFBP-1 and CK2β under L0. Pharmacological PKCα activation with phorbol 12-myristate 13-acetate (PMA) increased whereas inhibition with bisindolylmaleimide II (Bis II) decreased IGFBP-1 phosphorylation (Ser101/119/169, Ser98 + 101 and Ser169 + 174), respectively. Furthermore, PMA mimicked L0-induced PKCα translocation and IGFBP-1 expression. PKCα expression was increased in baboon fetal liver in FGR, providing biological relevance in vivo. In summary, we report a novel nutrient-sensitive mechanism for PKCα in mediating IGFBP-1 hyperphosphorylation in FGR.

Mechanistic Target of Rapamycin Complex 1 Signaling Links Hypoxia to Increased IGFBP-1 Phosphorylation in Primary Human Decidualized Endometrial Stromal Cells

Insulin-like growth factor-1 (IGF-1) bioavailability in pregnancy is governed by IGF binding protein (IGFBP-1) and its phosphorylation, which enhances the affinity of IGFBP-1 for the growth factor. The decidua is the predominant source of maternal IGFBP-1; however, the mechanisms regulating decidual IGFBP-1 secretion/phosphorylation are poorly understood. Using decidualized primary human endometrial stromal cells (HESCs) from first-trimester placenta, we tested the hypothesis that mTORC1 signaling mechanistically links hypoxia to decidual IGFBP-1 secretion/phosphorylation. Hypoxia inhibited mechanistic target of rapamycin (mTORC1) (p-P70-S6K/Thr389, -47%, p = 0.038; p-4E-BP1/Thr70, -55%, p = 0.012) and increased IGFBP-1 (total, +35%, p = 0.005; phosphorylated, Ser101/+82%, p = 0.018; Ser119/+88%, p = 0.039; Ser 169/+157%, p = 0.019). Targeted parallel reaction monitoring-mass spectrometry (PRM-MS) additionally demonstrated markedly increased dual IGFBP-1 phosphorylation (pSer98+Ser101; pSer169+Ser174) in hypoxia. IGFBP-1 hyperphosphorylation inhibited IGF-1 receptor autophosphorylation/ Tyr1135 (-29%, p = 0.002). Furthermore, silencing of tuberous sclerosis complex 2 (TSC2) activated mTORC1 (p-P70-S6K/Thr389, +68%, p = 0.038; p-4E-BP1/Thr70, +30%, p = 0.002) and reduced total/site-specific IGFBP-1 phosphorylation. Importantly, TSC2 siRNA prevented inhibition of mTORC1 and the increase in secretion/site-specific IGFBP-1 phosphorylation in hypoxia. PRM-MS indicated concomitant changes in protein kinase autophosphorylation (CK2/Tyr182; PKC/Thr497; PKC/Ser657). Overall, mTORC1 signaling mechanistically links hypoxia to IGFBP-1 secretion/phosphorylation in primary HESC, implicating decidual mTORC1 inhibition as a novel mechanism linking uteroplacental hypoxia to fetal growth restriction.

Mechanisms linking hypoxia to phosphorylation of insulin-like growth factor binding protein-1 in baboon fetuses with intrauterine growth restriction and in cell culture

Hypoxia increases fetal hepatic insulin-like growth factor binding protein-1 (IGFBP-1) phosphorylation mediated by mechanistic target of rapamycin (mTOR) inhibition. Whether maternal nutrient restriction (MNR) causes fetal hypoxia remains unclear. We used fetal liver from a baboon (Papio sp.) model of intrauterine growth restriction due to MNR (70% global diet of Control) and liver hepatocellular carcinoma (HepG2) cells as a model for human fetal hepatocytes and tested the hypothesis that mTOR-mediated IGFBP-1 hyperphosphorylation in response to hypoxia requires hypoxia-inducible factor-1α (HIF-1α) and regulated in development and DNA-damage responses-1 (REDD-1) signaling. Western blotting (n = 6) and immunohistochemistry (n = 3) using fetal liver indicated greater expression of HIF-1α, REDD-1 as well as erythropoietin and its receptor, and vascular endothelial growth factor at GD120 (GD185 term) in MNR versus Control. Moreover, treatment of HepG2 cells with hypoxia (1% pO₂ ) (n = 3) induced REDD-1, inhibited mTOR complex-1 (mTORC1) activity and increased IGFBP-1 secretion/phosphorylation (Ser101/Ser119/Ser169). HIF-1α inhibition by echinomycin or small interfering RNA silencing prevented the hypoxia-mediated inhibition of mTORC1 and induction of IGFBP-1 secretion/phosphorylation. dimethyloxaloylglycine (DMOG) induced HIF-1α and also REDD-1 expression, inhibited mTORC1 and increased IGFBP-1 secretion/phosphorylation. Induction of HIF-1α (DMOG) and REDD-1 by Compound 3 inhibited mTORC1, increased IGFBP-1 secretion/ phosphorylation and protein kinase PKCα expression. Together, our data demonstrate that HIF-1α induction, increased REDD-1 expression and mTORC1 inhibition represent the mechanistic link between hypoxia and increased IGFBP-1 secretion/phosphorylation. We propose that maternal undernutrition limits fetal oxygen delivery, as demonstrated by increased fetal liver expression of hypoxia-responsive proteins in baboon MNR. These findings have important implications for our understanding of the pathophysiology of restricted fetal growth.

Cartilage intermediate layer protein affects the progression of intervertebral disc degeneration by regulating the extracellular microenvironment (Review)

Intervertebral disc degeneration (IDD), which is caused by multiple factors, affects the health of individuals and contributes to low back pain. The pathology of IDD is complicated, and changes in the extracellular microenvironment play an important role in promoting the process of degeneration. Cartilage intermediate layer protein (CILP) is a matrix protein that resides in the middle of human articular cartilage and is involved in numerous diseases that affect cartilage. However, there is no detailed review of the relationship between CILP and degenerative disc disease. Growing evidence has revealed the presence of CILP in the extracellular microenvironment of intervertebral discs (IVDs) and has suggested that there is a gradual increase in CILP in degenerative discs. Specifically, CILP plays an important role in regulating the metabolism of the extracellular matrix (ECM), an important component of the extracellular microenvironment. CILP can combine with transforming growth factor-β or insulin-like growth factor-1 to regulate the ECM synthesis of IVDs and influence the balance of ECM metabolism, which leads to changes in the extracellular microenvironment to promote the process of IDD. It may be possible to show the correlation of CILP with IDD and to target CILP to interfere with IDD. For this purpose, in the present study, the current knowledge on CILP was summarized and a detailed description of CILP in discs was provided.

Developmental origins of metabolic diseases

Almost 2 billion adults in the world are overweight, and more than half of them are classified as obese, while nearly one-third of children globally experience poor growth and development. Given the vast amount of knowledge that has been gleaned from decades of research on growth and development, a number of questions remain as to why the world is now in the midst of a global epidemic of obesity accompanied by the "double burden of malnutrition," where overweight coexists with underweight and micronutrient deficiencies. This challenge to the human condition can be attributed to nutritional and environmental exposures during pregnancy that may program a fetus to have a higher risk of chronic diseases in adulthood. To explore this concept, frequently called the developmental origins of health and disease (DOHaD), this review considers a host of factors and physiological mechanisms that drive a fetus or child toward a higher risk of obesity, fatty liver disease, hypertension, and/or type 2 diabetes (T2D). To that end, this review explores the epidemiology of DOHaD with discussions focused on adaptations to human energetics, placental development, dysmetabolism, and key environmental exposures that act to promote chronic diseases in adulthood. These areas are complementary and additive in understanding how providing the best conditions for optimal growth can create the best possible conditions for lifelong health. Moreover, understanding both physiological as well as epigenetic and molecular mechanisms for DOHaD is vital to most fully address the global issues of obesity and other chronic diseases.

Hyperphosphorylation of fetal liver IGFBP-1 precedes slowing of fetal growth in nutrient-restricted baboons and may be a mechanism underlying IUGR

In cultured fetal liver cells, insulin-like growth factor (IGF) binding protein (IGFBP)-1 hyperphosphorylation in response to hypoxia and amino acid deprivation is mediated by inhibition of mechanistic target of rapamycin (mTOR) and activation of amino acid response (AAR) signaling and casein kinase (CK)2. We hypothesized that fetal liver mTOR inhibition, activation of AAR and CK2, and IGFBP-1 hyperphosphorylation occur before development of intrauterine growth restriction (IUGR). Pregnant baboons were fed a control (C) or a maternal nutrient restriction (MNR; 70% calories of control) diet starting at gestational day (GD) 30 (term GD 185). Umbilical blood and fetal liver tissue were obtained at GD 120 (C, n = 7; MNR, n = 10) and 165 (C, n = 7; MNR, n = 8). Fetal weights were unchanged at GD 120 but decreased at GD 165 in the MNR group (-13%, P = 0.03). IGFBP-1 phosphorylation, as determined by parallel reaction monitoring mass spectrometry (PRM-MS), immunohistochemistry, and/or Western blot, was enhanced in MNR fetal liver and umbilical plasma at GD 120 and 165. IGF-I receptor autophosphorylation^Tyr1135 (-64%, P = 0.05) was reduced in MNR fetal liver at GD 120. Furthermore, fetal liver CK2 (α/α'/β) expression, CK2β colocalization, proximity with IGFBP-1, and CK2 autophosphorylation^Tyr182 were greater at GD 120 and 165 in MNR vs. C. Additionally, mTOR complex (mTORC)1 (p-P70S6K^Thr389, -52%, P = 0.05) and mTORC2 (p-Akt^Ser473, -56%, P < 0.001) activity were decreased and AAR was activated (p-GCN2^Thr898, +117%, P = 0.02; p-eIF2α^Ser51, +294%, P = 0.002; p-ERK^Thr202, +111%, P = 0.03) in MNR liver at GD 120. Our data suggest that fetal liver IGFBP-1 hyperphosphorylation, mediated by mTOR inhibition and both AAR and CK2 activation, is a key link between restricted nutrient and oxygen availability and the development of IUGR.

Inhibition of decidual IGF-1 signaling in response to hypoxia and leucine deprivation is mediated by mTOR and AAR pathways and increased IGFBP-1 phosphorylation

Decidual mechanistic target of rapamycin (mTOR) is inhibited, amino acid response (AAR) and protein kinase CK2 are activated, and IGF (insulin-like growth factor) binding protein (IGFBP)-1 is hyperphosphorylated in human intrauterine growth restriction (IUGR). Using decidualized human immortalized endometrial stromal cells (HIESC), we hypothesized that hypoxia and leucine deprivation causing inhibition of decidual IGF-1 signaling is mediated by mTOR, AAR, CK2 and IGFBP-1 phosphorylation. Mass spectrometry demonstrated that hypoxia (1% O₂) or rapamycin increased IGFBP-1 phosphorylation singly at Ser101/119/169 (confirmed using immunoblotting) and dually at pSer169 + 174. Hypoxia resulted in mTOR inhibition, AAR and CK2 activation, and decreased IGF-1 bioactivity, with no additional changes with rapamycin + hypoxia. Rapamycin and/or hypoxia promoted colocalization of IGFBP-1 and CK2 (dual-immunofluorescence and proximity ligation assay). Leucine deprivation showed similar outcomes. Changes in IGFBP-1 phosphorylation regulated by mTOR/AAR signaling and CK2 may represent a novel mechanism linking oxygen and nutrient availability to IGF-1 signaling in the decidua.

Novel roles of mechanistic target of rapamycin signaling in regulating fetal growth†

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.

IUGR Is Associated With Marked Hyperphosphorylation of Decidual and Maternal Plasma IGFBP-1

CONTEXT

The mechanisms underpinning intrauterine growth restriction (IUGR), as a result of placental insufficiency, remain poorly understood, no specific treatment is available, and clinically useful biomarkers for early detection are lacking.

OBJECTIVE

We hypothesized that human IUGR is associated with inhibition of mechanistic target of rapamycin (mTOR) and activation of amino acid response (AAR) signaling, increased protein kinase casein kinase-2 (CK2) activity, and increased insulin-like growth factor-binding protein 1 (IGFBP-1) expression and phosphorylation in decidua and that maternal plasma IGFBP-1 hyperphosphorylation in the first trimester predicts later development of IUGR.

DESIGN, SETTING, AND PARTICIPANTS

Decidua [n = 16 appropriate-for-gestational age (AGA); 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 the 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, and AAR and CK2 are 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 predictive IUGR biomarker, allowing early intervention.

Complex, coordinated and highly regulated changes in placental signaling and nutrient transport capacity in IUGR

The most common cause of intrauterine growth restriction (IUGR) in the developed world is placental insufficiency, a concept often used synonymously with reduced utero-placental and umbilical blood flows. However, placental insufficiency and IUGR are associated with complex, coordinated and highly regulated changes in placental signaling and nutrient transport including inhibition of insulin and mTOR signaling and down-regulation of specific amino acid transporters, Na⁺/K⁺-ATPase, the Na^+/H⁺-exchanger, folate and lactate transporters. In contrast, placental glucose transport capacity is unaltered and Ca²⁺-ATPase activity and the expression of proteins involved in placental lipid transport are increased in IUGR. These findings are not entirely consistent with the traditional view that the placenta is dysfunctional in IUGR, but rather suggest that the placenta adapts to reduce fetal growth in response to an inability of the mother to allocate resources to the fetus. This new model has implications for the understanding of the mechanisms underpinning IUGR and for the development of intervention strategies.

The insulin like growth factor and binding protein family: Novel therapeutic targets in obesity & diabetes

Background

Recent changes in nutrition and lifestyle have provoked an unprecedented increase in the prevalence of obesity and metabolic disorders. Recognition of the adverse effects on health has prompted intense efforts to understand the molecular determinants of insulin sensitivity and dysglycemia. In many respects, actions of insulin-like growth factors (IGFs) mirror those of insulin in metabolic regulation. Unlike insulin, however, the bioactivity of IGFs is regulated by a family of seven high-affinity binding proteins (IGFBPs) which confer temporospatial modulation with implications for metabolic homeostasis. In addition, evidence is accumulating that IGF-independent actions of certain of the IGFBPs can directly modulate insulin sensitivity.

Scope of review

In this review, we discuss the experimental data indicating a critical role for IGF/IGFBP axis in metabolic regulation. We highlight key discoveries through which IGFBPs have emerged as biomarkers or putative therapeutic targets in obesity and diabetes.

Major conclusions

Growing evidence suggests that several components of the IGF-IGFBP system could be explored for therapeutic potential in metabolic disorders. Both IGFBP-1 and IGFBP-2 have been favorably linked with insulin sensitivity in humans and preclinical data implicate direct involvement in the molecular regulation of insulin signaling and adiposity respectively. Further studies are warranted to evaluate clinical translation of these findings.

Effect of Nutrition on Statural Growth

In children, proper growth and development are often regarded as a surrogate marker for good health. A complex system controls the initiation, rate, and cessation of growth, and thus gives a wonderful example of the interactions between genetics, epigenetics, and environmental factors (especially stress and nutrition). Malnutrition is considered a leading cause of growth attenuation in children. This review summarizes our current knowledge regarding the mechanisms linking nutrition and skeletal growth, including systemic factors, such as insulin, growth hormone, insulin-like growth factor-1, fibroblast growth factor-21, etc., and local mechanisms, including mTOR, miRNAs, and epigenetics. Studying the molecular mechanisms regulating skeletal growth may lead to the establishment of better nutritional and therapeutic regimens for more effective linear growth in children with malnutrition and growth abnormalities. 
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Increased Insulin-like Growth Factor Binding Protein-1 Phosphorylation in Decidualized Stromal Mesenchymal Cells in Human Intrauterine Growth Restriction Placentas

Intrauterine growth restriction (IUGR) is often caused by placental insufficiency, which is believed to be associated with decreased delivery of oxygen and nutrients to the placental barrier. We recently reported that hypoxia and/or leucine deprivation triggered hyperphosphorylation of insulin-like growth factor binding protein-1 (IGFBP-1) in decidualized human immortalized endometrial stromal cells (HIESCs), resulting in decreased insulin-like growth factor-1 (IGF-1) bioactivity. To test the hypothesis that human IUGR is associated with increased decidual IGFBP-1 phosphorylation at discrete sites, we used IUGR and gestational age matched appropriate for gestational age (AGA) placentas ( n=5 each). We performed dual immunofluorescence immunohistochemistry (IHC) using IGFBP-1 and vimentin as decidual and mesenchymal markers, respectively. Employing a unique strategy with imaging software, we extracted signal intensity of IGFBP-1 expressed specifically from truly decidualized cells of the placenta. Relative IGFBP-1 was increased (85%; p=0.0001) and using custom phospho-site-specific antibodies, we found that IGFBP-1 phosphorylation (pSer101; +40%, p=0.0677/pSer119; +60%, p=0.0064/pSer169; +100%, p=0.0021) was markedly enhanced in IUGR. Together, our data links for the first time, increased decidual IGFBP-1 phosphorylation at discrete sites with human IUGR. These novel findings suggest that hyperphosphorylation of IGFBP-1 in decidualized stromal mesenchymal decidua basalis contributes to potentially elevated levels of phosphorylated IGFBP-1 in maternal circulation in IUGR pregnancies.

Exposure of decidualized HIESC to low oxygen tension and leucine deprivation results in increased IGFBP-1 phosphorylation and reduced IGF-I bioactivity

Phosphorylation of decidual IGFBP-1 enhances binding of IGF-I, limiting the bioavailability of this growth factor which may contribute to reduced placental and fetal growth. The mechanisms regulating decidual IGFBP-1 phosphorylation are incompletely understood. Using decidualized human immortalized endometrial stromal cells we tested the hypothesis that low oxygen tension or reduced leucine availability, believed to be common in placental insufficiency, increase the phosphorylation of decidual IGFBP-1. Multiple reaction monitoring-MS (MRM-MS) was used to quantify IGFBP-1 phosphorylation. MRM-MS validated the novel phosphorylation of IGFBP-1 at Ser58, however this site was unaffected by low oxygen tension/leucine deprivation. In contrast, significantly elevated phosphorylation was detected for pSer119, pSer98/pSer101 and pSer169/pSer174 sites. Immunoblotting and dual-immunofluorescence using phosphosite-specific IGFBP-1 antibodies further demonstrated increased IGFBP-1 phosphorylation in HIESC under both treatments which concomitantly reduced IGF-I bioactivity. These data support the hypothesis that down regulation of IGF-I signaling links decidual IGFBP-1 hyperphosphorylation to restricted fetal growth in placental insufficiency.

Which dietary components modulate longitudinal growth?

PURPOSE OF REVIEW

Linear growth in children is sensitive to nutritional status; the growth of the human skeleton requires many different nutritional factors for energy and building blocks: proteins, lipids, carbohydrates and micronutrients. However, what are the specific nutritional factors that are required for proper growth and what is the composition that will be most beneficial is still not known.

RECENT FINDINGS

Recent findings indicate that macro and micronutrients are required as building blocks and as cofactors for important enzymes. In addition, they stimulate linear growth by acting as regulatory factors and also affect gut microbiome. Some interesting studies regarding the effect of proteins and amino acids are presented.

SUMMARY

Most studies investigated the effect of replacing a single micronutrient that was deficient; however, in real life, deficiency of one nutritional element is commonly associated with other deficiencies. Therefore, it is a reasonable clinical approach, both in developing and developed countries, to use a mixture of both macro and micronutrients to support growth. How much of each of the components and what is the best composition are still open questions that require more research.

Insulin-like growth factors and their potential role in cardiac epigenetics

Cardiovascular disease (CVD) constitutes a major public health threat worldwide, accounting for 17.3 million deaths annually. Heart disease and stroke account for the majority of healthcare costs in the developed world. While much has been accomplished in understanding the pathophysiology, molecular biology and genetics underlying the diagnosis and treatment of CVD, we know less about the role of epigenetics and their molecular determinants. The impact of environmental changes and epigenetics in CVD is now emerging as critically important in understanding the origin of disease and the development of new therapeutic approaches to prevention and treatment. This review focuses on the emerging role of epigenetics mediated by insulin like-growth factors-I and -II in major CVDs such as heart failure, cardiac hypertrophy and diabetes.

Increased IGFBP-1 phosphorylation in response to leucine deprivation is mediated by CK2 and PKC

Insulin-like growth factor binding protein-1 (IGFBP-1), secreted by fetal liver, is a key regulator of IGF-I bioavailability and fetal growth. IGFBP-1 phosphorylation decreases IGF-I bioavailability and diminishes its growth-promoting effects. Growth-restricted fetuses have decreased levels of circulating essential amino acids. We recently showed that IGFBP-1 hyperphosphorylation (pSer101/119/169) in response to leucine deprivation is regulated via activation of the amino acid response (AAR) in HepG2 cells. Here we investigated nutrient-sensitive protein kinases CK2/PKC/PKA in mediating IGFBP-1 phosphorylation in leucine deprivation. We demonstrated that leucine deprivation stimulated CK2 activity (enzymatic assay) and induced IGFBP-1 phosphorylation (immunoblotting/MRM-MS). Inhibition (pharmacological/siRNA) of CK2/PKC, but not PKA, prevented IGFBP-1 hyperphosphorylation in leucine deprivation. PKC inhibition also prevented leucine deprivation-stimulated CK2 activity. Functionally, leucine deprivation decreased IGF-I-induced-IGF-1R autophosphorylation when CK2/PKC were not inhibited. Our data strongly support that PKC promotes leucine deprivation-induced IGFBP-1 hyperphosphorylation via CK2 activation, mechanistically linking decreased amino acid availability and reduced fetal growth.

Hypoxia Increases IGFBP-1 Phosphorylation Mediated by mTOR Inhibition

In fetal growth restriction (FGR), fetal growth is limited by reduced nutrient and oxygen supply. Insulin-like growth factor I (IGF-I) is a key regulator of fetal growth and IGF binding protein -1(IGFBP-1) is the principal regulator of fetal IGF-I bioavailability. Phosphorylation enhances IGFBP-1's affinity for IGF-I. Hypoxia induces IGFBP-1 hyperphosphorylation, markedly decreasing IGF-I bioavailability. We recently reported that fetal liver IGFBP-1 hyperphosphorylation is associated with inhibition of the mechanistic target of rapamycin (mTOR) in a nonhuman primate model of FGR. Here, we test the hypothesis that IGFBP-1 hyperphosphorylation in response to hypoxia is mediated by mTOR inhibition. We inhibited mTOR either by rapamycin or small interfering RNA (siRNA) targeting raptor (mTOR complex [mTORC]1) and/or rictor (mTORC2) in HepG2 cells cultured under hypoxia (1% O2) or basal (20% O2) conditions. Conversely, we activated mTORC1 or mTORC1+mTORC2 by silencing endogenous mTOR inhibitors (tuberous sclerosis complex 2/DEP-domain-containing and mTOR-interacting protein). Immunoblot analysis demonstrated that both hypoxia and inhibition of mTORC1 and/or mTORC2 induced similar degrees of IGFBP-1 phosphorylation at Ser101/119/169 and reduced IGF-I receptor autophosphorylation. Activation of mTORC1+mTORC2 or mTORC1 alone prevented IGFBP-1 hyperphosphorylation in response to hypoxia. Multiple reaction monitoring-mass spectrometry showed that rapamycin and/or hypoxia increased phosphorylation also at Ser98 and at a novel site Ser174. In silico structural analysis indicated that Ser174 was in close proximity to the IGF-binding site. Together, we demonstrate that signaling through the mTORC1 or mTORC2 pathway is sufficient to induce IGFBP-1 hyperphosphorylation in response to hypoxia. This study provides novel understanding of the cellular mechanism that controls fetal IGFBP-1 phosphorylation in hypoxia, and we propose that mTOR inhibition constitutes a mechanistic link between hypoxia, reduced IGF-I bioavailability and FGR.

IGFBP-1 hyperphosphorylation in response to leucine deprivation is mediated by the AAR pathway

Insulin-like growth factor-1 (IGF-I) is the key regulator of fetal growth. IGF-I bioavailability is markedly diminished by IGF binding protein-1 (IGFBP-1) phosphorylation. Leucine deprivation strongly induces IGFBP-1 hyperphosphorylation, and plays an important role in fetal growth restriction (FGR). FGR is characterized by decreased amino acid availability, which activates the amino acid response (AAR) and inhibits the mechanistic target of rapamycin (mTOR) pathway. We investigated the role of AAR and mTOR in mediating IGFBP-1 secretion and phosphorylation in HepG2 cells in leucine deprivation. mTOR inhibition (rapamycin or raptor + rictor siRNA), or activation (DEPTOR siRNA) demonstrated a role of mTOR in leucine deprivation-induced IGFBP-1 secretion but not phosphorylation. When the AAR was blocked (U0126, or ERK/GCN2 siRNA), both IGFBP-1 secretion and hyperphosphorylation (pSer101/pSer119/pSer169) due to leucine deprivation were prevented. CK2 inhibition by TBB also attenuated IGFBP-1 phosphorylation in leucine deprivation. These results suggest that the AAR and mTOR independently regulate IGFBP-1 secretion and phosphorylation in response to decreased amino acid availability.

The role and regulation of IGFBP-1 phosphorylation in fetal growth restriction

Fetal growth restriction (FGR) increases the risk of perinatal complications and predisposes the infant to developing metabolic, cardiovascular, and neurological diseases in childhood and adulthood. The pathophysiology underlying FGR remains poorly understood and there is no specific treatment available. Biomarkers for early detection are also lacking. The insulin-like growth factor (IGF) system is an important regulator of fetal growth. IGF-I is the primary regulator of fetal growth, and fetal circulating levels of IGF-I are decreased in FGR. IGF-I activity is influenced by a family of IGF binding proteins (IGFBPs), which bind to IGF-I and decrease its bioavailability. During fetal development the predominant IGF-I binding protein in fetal circulation is IGFBP-1, which is primarily secreted by the fetal liver. IGFBP-1 binds IGF-I and thereby inhibits its bioactivity. Fetal circulating levels of IGF-I are decreased and concentrations of IGFBP-1 are increased in FGR. Phosphorylation of human IGFBP-1 at specific sites markedly increases its binding affinity for IGF-I, further limiting IGF-I bioactivity. Recent experimental evidence suggests that IGFBP-1 phosphorylation is markedly increased in the circulation of FGR fetuses suggesting an important role of IGFBP-1 phosphorylation in the regulation of fetal growth. Understanding of the significance of site-specific IGFBP-1 phosphorylation and how it is regulated to contribute to fetal growth will be an important step in designing strategies for preventing, managing, and/or treating FGR. Furthermore, IGFBP-1 hyperphosphorylation at unique sites may serve as a valuable biomarker for FGR.

Nutritionally-induced catch-up growth

Malnutrition is considered a leading cause of growth attenuation in children. When food is replenished, spontaneous catch-up (CU) growth usually occurs, bringing the child back to its original growth trajectory. However, in some cases, the CU growth is not complete, leading to a permanent growth deficit. This review summarizes our current knowledge regarding the mechanism regulating nutrition and growth, including systemic factors, such as insulin, growth hormone, insulin- like growth factor-1, vitamin D, fibroblast growth factor-21, etc., and local mechanisms, including autophagy, as well as regulators of transcription, protein synthesis, miRNAs and epigenetics. Studying the molecular mechanisms regulating CU growth may lead to the establishment of better nutritional and therapeutic regimens for more effective CU growth in children with malnutrition and growth abnormalities. It will be fascinating to follow this research in the coming years and to translate the knowledge gained to clinical benefit.

IGF binding proteins in cancer: mechanistic and clinical insights

The six members of the family of insulin-like growth factor (IGF) binding proteins (IGFBPs) were originally characterized as passive reservoirs of circulating IGFs, but they are now understood to have many actions beyond their endocrine role in IGF transport. IGFBPs also function in the pericellular and intracellular compartments to regulate cell growth and survival - they interact with many proteins, in addition to their canonical ligands IGF-I and IGF-II. Intranuclear roles of IGFBPs in transcriptional regulation, induction of apoptosis and DNA damage repair point to their intimate involvement in tumour development, progression and resistance to treatment. Tissue or circulating IGFBPs might also be useful as prognostic biomarkers.

Liver mTOR controls IGF-I bioavailability by regulation of protein kinase CK2 and IGFBP-1 phosphorylation in fetal growth restriction

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.

Free-flow electrophoresis in proteome sample preparation

An aim of proteome research is to identify the entire complement of proteins expressed in defined cell types of humans, animals, plants, and microorganisms. The approach requires searching for low abundant or even rarely expressed proteins in many cell types, as well as the determination of the protein expression levels in subcellular compartments and organelles. In recent years, rather powerful MS technologies have been developed. At this stage of MS device development, it is of highest interest to purify intact cell types or isolate subcellular compartments, where the proteins of interest are originating from, which determine the final composition of a peptide mixture. Free-flow electrophoresis proved to be useful to prepare meaningful peptide mixtures because of its improved capabilities in particle electrophoresis and the enhanced resolution in protein separation. Sample preparation by free-flow electrophoresis mediated particle separation was preferentially performed for purification of either organelles and their subspecies or major protein complexes. Especially, the introduction of isotachophoresis and interval zone electrophoresis improved the purity of the gained analytes of interest. In addition, free-flow IEF proved to be helpful, when proteins of low solubility, obtained, e.g. from cell membranes, were investigated.