Contributions of growth hormone receptor and postreceptor defects to growth hormone resistance in malnutrition

Dietary restriction reveals sex-specific expression of the mTOR pathway genes in Japanese quails

Limited resources affect an organism's physiology through the conserved metabolic pathway, the mechanistic target of rapamycin (mTOR). Males and females often react differently to nutritional limitation, but whether it leads to differential mTOR pathway expression remains unknown. Recently, we found that dietary restriction (DR) induced significant changes in the expression of mTOR pathway genes in female Japanese quails (Coturnix japonica). We simultaneously exposed 32 male and female Japanese quails to either 20%, 30%, 40% restriction or ad libitum feeding for 14 days and determined the expression of six key genes of the mTOR pathway in the liver to investigate sex differences in the expression patterns. We found that DR significantly reduced body mass, albeit the effect was milder in males compared to females. We observed sex-specific liver gene expression. DR downregulated mTOR expression more in females than in males. Under moderate DR, ATG9A and RPS6K1 expressions were increased more in males than in females. Like females, body mass in males was correlated positively with mTOR and IGF1, but negatively with ATG9A and RS6K1 expressions. Our findings highlight that sexes may cope with nutritional deficits differently and emphasise the importance of considering sexual differences in studies of dietary restriction.

Undernutrition and growth in the developing world

PURPOSE OF REVIEW

Despite targeted interventions, an estimated 150.8 million children under 5 years globally are still stunted, of which more than half live in Asia and more than one-third live in Africa. This review summarizes our current knowledge regarding how longitudinal bone growth is regulated by nutritional intake in the developing world. Dietary macronutrients and micronutrients necessary for growth are also briefly reviewed.

RECENT FINDINGS

Recent advances include investigations of nutritionally sensitive regulators of growth as well as prospective evaluations of the role of specific dietary components on growth in order to better assess their impact.

SUMMARY

Further investigation is required to understand how nutrition impacts growth, the mechanisms underlying stunting and to optimize therapeutic strategies for children who are at risk for growth attenuation or are stunted in low and middle-income countries (LMICs).

Environmental Enteric Dysfunction and Growth Failure/Stunting in Global Child Health

Approximately 25% of the world's children aged <5 years have stunted growth, which is associated with increased mortality, cognitive dysfunction, and loss of productivity. Reducing by 40% the number of stunted children is a global target for 2030. The pathogenesis of stunting is poorly understood. Prenatal and postnatal nutritional deficits and enteric and systemic infections clearly contribute, but recent findings implicate a central role for environmental enteric dysfunction (EED), a generalized disturbance of small intestinal structure and function found at a high prevalence in children living under unsanitary conditions. Mechanisms contributing to growth failure in EED include intestinal leakiness and heightened permeability, gut inflammation, dysbiosis and bacterial translocation, systemic inflammation, and nutrient malabsorption. Because EED has multiple causal pathways, approaches to manage it need to be multifaceted. Potential interventions to tackle EED include: (1) reduction of exposure to feces and contact with animals through programs such as improved water, sanitation, and hygiene; (2) breastfeeding and enhanced dietary diversity; (3) probiotics and prebiotics; (4) nutrient supplements, including zinc, polyunsaturated fatty acids, and amino acids; (5) antiinflammatory agents such as 5-aminosalicyclic acid; and (6) antibiotics in the context of acute malnutrition and infection. Better understanding of the underlying causes of EED and development of noninvasive, practical, simple, and affordable point-of-care diagnostic tools remain key gaps. "Omics" technologies (genomics, epigenomics, transcriptomics, proteomics, and metabolomics) and stable isotope techniques (eg, ¹³C breath tests) targeted at children and their intestinal microbiota will enhance our ability to successfully identify, manage, and prevent this disorder.

Rapid increase in fibroblast growth factor 21 in protein malnutrition and its impact on growth and lipid metabolism

Protein malnutrition promotes hepatic steatosis, decreases insulin-like growth factor (IGF)-I production and retards growth. To identify new molecules involved in such changes, we conducted DNA microarray analysis on liver samples from rats fed an isoenergetic low-protein diet for 8 h. We identified the fibroblast growth factor 21 gene (Fgf21) as one of the most strongly up-regulated genes under conditions of acute protein malnutrition (P<0·05, false-discovery rate<0·001). In addition, amino acid deprivation increased Fgf21 mRNA levels in rat liver-derived RL-34 cells (P<0·01). These results suggested that amino acid limitation directly increases Fgf21 expression. FGF21 is a polypeptide hormone that regulates glucose and lipid metabolism. FGF21 also promotes a growth hormone-resistance state and suppresses IGF-I in transgenic mice. Therefore, to determine further whether Fgf21 up-regulation causes hepatic steatosis and growth retardation after IGF-I decrease in protein malnutrition, we fed an isoenergetic low-protein diet to Fgf21-knockout (KO) mice. Fgf21-KO did not rescue growth retardation and reduced plasma IGF-I concentration in these mice. Fgf21-KO mice showed greater epididymal white adipose tissue weight and increased hepatic TAG and cholesterol levels under protein malnutrition conditions (P<0·05). Overall, the results showed that protein deprivation directly increased Fgf21 expression. However, growth retardation and decreased IGF-I were not mediated by increased FGF21 expression in protein malnutrition. Furthermore, FGF21 up-regulation rather appears to have a protective effect against obesity and hepatic steatosis in protein-malnourished animals.

MicroRNAs in the growth plate are responsive to nutritional cues: association between miR-140 and SIRT1

MicroRNAs (miRNAs) have been reported to be involved in a variety of functions, including skeletal development and longitudinal growth. The aim of this study was to investigate the role of miRNAs in food-restriction-induced growth attenuation and nutrition-induced catch-up growth in the epiphyseal growth plate (EGP). Prepubertal rats were fed ad libitum or were subjected to 40% food restriction for 10 days followed by a renewal of the regular food supply. At sacrifice, tibial EGPs were excised, and the total RNA was extracted and loaded on miRNA microarrays. The miRNA microarray yielded more than 400 miRNAs that are expressed in the EGP of mature animals. Results were confirmed by quantitative polymerase chain reaction. Chondrocyte-specific miR-140-3p showed the highest expression in the mature EGP, and it was one of the few miRNAs that were significantly reduced following nutrition restriction. Changes in predicted miRNA targets were then followed with Western immunoblotting. Direct binding was demonstrated using exogenous miRNA, the 3'UTR of the target mRNA and a luciferase reporter assay. Nutrition restriction induced an increase in the level of the miR-140-3p target, NAD+-dependent SIRT1. This study is the first to show that SIRT1 and miRNAs expressed in the mature EGP are responsive to nutritional cues. Nutrition-induced epigenetic regulation of growth activates two parts of the epigenetic world - miRNAs and histone deacetylases - that are interconnected. Deciphering the role of epigenetic regulation in growth may open a new era of research and pave the way for the development of new treatments for children with growth disorders.

Interaction between body composition, leptin and growth hormone status

Administration of growth hormone (GH) induces changes in body composition, namely, increases in both bone and lean mass and a decrease in fatty tissue. However, the contrary issue, i.e. the way in which body composition affects the secretion of GH, is highly controversial. Disease states such as obesity and chronic hypercortisolism are associated with increased adiposity and/or the central distribution of fat. Ageing, characterized by excess adiposity, is also associated with impaired secretion of GH. In these states, both spontaneous and stimulated secretion of GH is severely impeded. At the other extreme, malnutrition and fasting are both associated with increased secretion of GH when confronted with most, if not all, stimuli. As the common factor in all of these situations is the increased or decreased adiposity, or the changes in energy homeostasis, it has been postulated that adipose tissue exerts a relevant role in the control of GH secretion in man. The link between adipose tissue and GH seems to be exerted through at least two signals produced by adipocytes: free fatty acids (FFA) and the recently cloned protein, leptin. An increase in FFA blocks secretion of GH, while a decrease in FFA enhances secretion. Leptin, a hormone whose main role is to regulate the intake of food and energy expenditure, seems to regulate GH secretion by acting at the hypothalamic level. In summary, body composition affects GH secretion by way of the degree of adiposity, and free fatty acids and leptin would appear to be the messages through which adipocytes participate in the regulation of GH secretion. This framework clarifies the metabolic control of GH, a hormone with profound metabolic activities.

Insulin and insulin-like growth factor-I acutely inhibit surface translocation of growth hormone receptors in osteoblasts: a novel mechanism of growth hormone receptor regulation

We previously have demonstrated that insulin and insulin-like growth factor-I (IGF-I) down-regulate growth hormone (GH) binding in osteoblasts by reducing the number of surface GH receptors (GHRs). The present study was undertaken to investigate the mechanism of GHR down-regulation. Treatment with 5 nM insulin or IGF-I for 18 hr significantly decreased surface GH binding to 26.4 +/- 2.9% and 23.0 +/- 2.7% of control (mean +/- SE; P < 0.05), respectively. No corresponding reductions in the mRNA level and total cellular content of GHR were found, nor was the rate of receptor internalization affected. The effects on GHR translocation were assessed by measuring the reappearance of GH binding of whole cells after trypsinization to remove the surface receptors. GH binding of control cultures significantly increased (P < 0.05) over 2 hr after trypsinization, whereas no recovery of binding activity was detected in insulin and IGF-I-treated cultures, indicating that GHR translocation was impaired. Studies on the time course of GHR down-regulation revealed that surface GH binding was reduced significantly by 3-hr treatment (P </= 0.0005), whereas GHR translocation was completely abolished by 75-90 min with insulin and IGF-I. The inhibition of receptor translocation by insulin, but not IGF-I, was attenuated by wortmannin. In conclusion, insulin and IGF-I down-regulated GH binding in osteoblasts by acutely impairing GHR translocation, with their effects exerted through distinct postreceptor signaling pathways.

Growth hormone insensitivity

Growth hormone insensitivity (GHI) may be primary, caused by defects in the GH receptor, or further along the GH-insulin-like growth factor-I (IGF-I) axis, or secondary, resulting from a variety of illnesses or malnutrition affecting various steps in the pathway from the GH binding to IGF-I action. GH receptor deficiency, although rare, with only 229 cases reported, is the most common cause of primary GHI. Most patients are of Jewish, Arab, or other Mediterranean origin, the largest cohort being Catholics of Jewish origin coming from a small area in southern Ecuador, who account for one third of known cases. This large cohort has provided insight into the clinical features, growth characteristics, biochemical features, and effects of treatment with recombinant IGF-I. The Ecuadorian patients share a splice site mutation in the GH receptor gene with at least one Israeli patient of Iberian origin; 27 other mutations and a major deletion have been described in other affected patients.

Partial growth hormone insensitivity in childhood

There appears to be a continuum of GH-responsiveness, ranging from complete GH-resistance in Laron syndrome to normal GH-sensitivity. Partial GHI is suggested by findings at both ends of this spectrum; some patients with Laron-type dwarfism are less severely affected by their GHI than others (Savage et al, 1993; for a review, see Savage et al, 1995) and some short, non-GH-deficient, children have reduced responsiveness to GH. Among children with ISS, we have identified a subgroup where defects at the level of the GH receptor lead to a partial-GHI syndrome (Carlsson et al, 1994; Attie et al, 1995; Goddard et al, 1995). Partial-GHI may explain the growth failure in some children who do not meet the criteria for GHD. So far, the clinical evaluation of short children has been focused on the exclusion or demonstration of GHD. The diagnosis of GHD at present requires that stimulated or spontaneous GH concentrations should fail to reach a certain, arbitrarily determined level. This assumes that GH-sensitivity is equal in all subjects, with the exception of rare cases with Laron syndrome. The diagnosis of GHD is in itself controversial, and it has been suggested that GH testing should be supplemented by other measures such as auxological evaluation and measurement of other components of the GH/IGF-I axis (Rosenfeld et al, 1995). However, the fact that some short children have partial-GHI suggests that both GH secretion and GH responsiveness should be taken into consideration when investigating the cause of short stature (Figure 5). Many short children do not have GHD per se, but may be short due to inadequate GH stimulation because of reduced GH-sensitivity.

Influence of metabolic substrates and obesity on growth hormone secretion

In addition to stimulating body growth, GH plays an important role in metabolism. In turn, various products of intermediary metabolism, such as glucose, free fatty acids, dietary proteins, and amino acids feed back on both the hypothalamus and the anterior pituitary to control the function of the somatotroph cell. Alterations in nutritional status, such as malnutrition and obesity, markedly influence GH secretion and/or GH actions at tissue level. Therefore, the interaction between metabolic substrates and GH secretion can be viewed as part of the overall regulation of feeding and fasting in order to maintain an adequate body weight and body composition.

Growth in growth hormone insensitivity

Primary GH insensitivity due to GH receptor deficiency (GHRD) provides a model for studying the discrete effects of severe IGF-I deficiency on growth and body composition. Growth failure in utero is doubtful, but postpartum growth proceeds at rates that result in adult statures 4-12 standard deviations (SDs) below the normal mean. Wide variability in statural effect, even in a genetically homogeneous population, is partly explained by correlation of SD score with biochemical measures of GH effect (IGF-I, IGF-II, and IGFBP-3). Growth and changes in body composition (decreased fat/lean) in patients with GHRD in response to exogenous IGF-I indicate that direct local effects of GH are not necessary for these responses.

Growth and skeletal development in the dog in relation to nutrition; a review

Longitudinal growth of the appendicular skeleton in the growth plates and the adjacent metaphyseal area includes chondrocyte differentiation, proliferation, maturation, and hypertrophy in the physis and bone (re-) modelling in the metaphysis. The rate and extent of longitudinal growth are regulated by interactions between biomechanical factors and endogenous growth regulators, i.e., systemic endocrine factors, and local para or autocrine factors, that act on the growth plate chondrocytes. The most important endogenous regulators of growth and skeletal development are growth hormone (GH) and insulin-like growth factors (IGFs), and calciotropic hormones, i.e., parathyroid hormone (PTH), vitamin D (vitD), and calcitonin (CT). The biochemistry, synthesis, secretion, target organs, and effects of these endogenous factors are reviewed, and the calcium homeostatic mechanisms, dietary intake, bone turnover, and calcium excretion are discussed. Energy, protein, and calcium are nutritional factors of great importance to (skeletal) growth. The effects of low and high dietary intake of these nutrients are discussed, especially with reference to longitudinal growth and disturbances in endochondral ossification.