Research in the field of growth and growth factors has produced outstanding achievements in basic science which may represent the premises for future translations into clinical practice. Particularly the role of the IGF system in the central nervous system has been extensively and successfully investigated, yielding preliminary results which are not only functional for understanding physiology but also potentially useful for future clinical applications. New evidence suggests a role of IGFs in the emerging field of molecular mechanisms of endoplasmic reticulum stress response which is involved in the processes of chronic inflammation, diabetes and atherosclerosis. The availability of new genetic technologies has enabled to dissect the action of GH from that of IGF-I in mammalian skeletal muscle, thus opening an avenue for more targeted interventions for attenuating muscle loss and influencing insulin sensitivity and global nutrient metabolism. In this selection of studies, which obviously represents my own bias, I have tried to provide the reader with the most provocative, promising, controversial and enjoyable research papers published in the last year.
New concerns
Serum dioxins and polychlorinated biphenyls are associated with growth among Russian boys
Burns JS, Williams PL, Sergeyev O, Korrick S, Lee MM, Revich B, Altshul L, Del Prato JT, Humblet O, Patterson DG, Turner WE, Needham LL, Starovoytov M, Hauser R
Environmental and Occupational Medicine and Epidemiology Program, Department of Environmental Health, Harvard School of Public Health, Boston, Mass.,USA.
Pediatrics 2011;127:e59-68.
Background: Limited evidence suggests that prenatal and postnatal exposures to environmental [1] chemicals such as dioxins, furans, and polychlorinated biphenyls (PCBs) may affect both linear growth and body composition. Although the use of these chemicals has decreased over time worldwide, children continue to be exposed. The contamination of vast industrial areas implies that these substances are largely stored in adipose tissue and slowly metabolized. Diet is the most significant source of exposure, with breast-feeding transferring a portion of the mother's body stores of these compounds to her infant. The aim of this study was to determine the association between serum dioxins and PCBs and growth patterns of peripubertal boys in Chapaevsk, Russia, a highly contaminated area due to extensive chemical manufacturing industry since the 1930s.
Methods: 499 boys, aged 8-9 years, were enrolled in the study and followed prospectively for 3 years. Ten children were excluded from data analysis because they were affected by chronic diseases. Questionnaires were administered to assess pregnancy and birth history, health, socioeconomic status, lifestyle and dietary intake. The boys underwent a standardized anthropometric examination at entry and annually during the study follow-up. The measurements were performed by a single study investigator blind to the subjects’ dioxin and PCB levels. Blood samples were tested for measuring serum concentrations of dioxin, furan, PCB, and lead.
Results: The medians of serum dioxin, furan, and PCB concentrations greatly exceeded those reported in the United States and Europe. Multivariate analysis revealed that boys in the highest exposure quintile of the sum of dioxin and PCB concentrations and total toxic equiva lents (TEQs) had a significant decrease in mean BMI z scores, height z scores and height velocity (cm/year) compared with boys in the lowest exposure quintile, after 3 years of follow-up.
Conclusions: High levels of dioxin and PCB in serum are associated with growth impairment. The study follow-up is ongoing to include the assessment of pubertal growth spurt which may be affected by the exposure to chemicals, thus further compromising adult stature.
The authors previously reported preliminary results from 8- and 9-year-old boys belonging to the same study cohort. These children showed high serum levels of dioxins and PCBs compared with other populations [1]. Older age of the boys, longer breast-feeding duration, maternal local gardening, consump tion of local foods, and residential proximity to and maternal employment at the chemical plant were associated with higher concentrations of the contaminants. In this study the authors report a significant inverse relationship between serum concentrations of these chemicals and growth. The finding of reduced BMI in this Russian cohort of boys exposed to dioxin and PCB is consistent with previous reports showing a detrimental effect of prenatal exposure to PCBs on lean muscle mass [2]. The peripubertal concentrations of serum dioxins and PCBs of the study population may in fact represent a surrogate for prenatal exposure, because childhood levels often track closely with prenatal levels, especially in a primarily breastfed population. There are critical time windows characterized by higher individual susceptibility to the disrupting effects of environmental chemical contaminants. The exposure to dioxins and PCBs has so far been associated with an increased risk of immune dysfunction, cancer, cardiovascular diseases, reproductive and developmental alterations, endocrine disturbances and diabetes. The results of this study provide first evidence for detrimental effects of endocrine-disrupting chemicals on growth in childhood [3].
IGF-1 can either protect against or increase LPS-induced damage in the developing rat brain
Pang Y, Zheng B, Campbell LR, Fan LW, Cai Z, Rhodes PG
Department of Pediatrics, University of Mississippi Medical Center, Jackson, Miss.,USA.
Pediatr Res 2010;67:579-84.
Background: Prematurity is often associated with periventricular leukomalacia (PVL) which represents the major cause of cerebral palsy. Hypoxic-ischemic insult and/or inflammation may contribute to the development of PVL. The intracerebral delivery of lipopolysaccharide (LPS) induces an acute brain inflammatory response and damage, mainly characterized by the loss of preoligodendrocytes (preOLs), ultimately yielding a picture similar to PVL. IGF-I plays prominent roles in axon growth and maturation in central nervous system myelination and has been proposed for therapy in different neuropathies. The aim of this study was to test the efficacy of IGF-I in preventing LPS-induced brain alterations. Methods: Sprague-Dawley rats were injected intraventricularly with LPS (10 µg) and IGF-I (0.5-25 µg) at postnatal day 5.
Results: Intracerebral injection of LPS resulted in a loss of preOLs, hypomyelination, decreased number of mature OLs, and increased number of pyknotic cells in the cortex indicating cell death in the gray matter. Co-injection of low-dose IGF-I (0.5 µg) significantly attenuated the effects of LPS. However, the low dose of IGF-I significantly enhanced polymorphonuclear leukocyte infiltration into brain parenchyma and potentiated LPS-induced blood-brain barrier disruption and brain hemorrhage. At a higher dose (5 µg) IGF-I enhanced LPS-induced brain injury and mortality rate. No detrimental effect was observed in rat brain treated with IGF-I alone.
Conclusions: These data suggest a dual effect of IGF-I in the brain. Low-dose IGF-I protects from LPS-induced loss of preoligodendrocytes, hypomyelination and gray matter damage, but, at the same time, amplifies inflammation and leads to intracerebral hemorrhage. Moreover, if injected at higher dose, IGF-I administration is associated with increased mortality rate.
The potential role of IGF-I as a neurotrophic and neuroprotective factor is debated. During development, IGF-I, its receptor, and its binding proteins (IGFBPs) are abundantly expressed in many brain regions at relatively early stages. There is abundant evidence that brain IGF-I is of paramount importance for normal brain development [4]. IGF-I plays a key role in the formation of oligodendrocytes (oligodendrogenesis) as well as in neuronal growth and differentiation. Importantly, IGF-I intervenes in the tissue response to different types of insults. Brain injury elicits glial activation and increases de novo expression of myriad cytokines and neurotrophic factors. IGF-I is usually expressed by activated microglia, and both neurons and astroglia appear to be targeted by IGF-I in the injured areas [5]. The potent effects of IGFs on the survival, proliferation, and differentiation of neurons and glia make them attractive candidates for the treatment of diseases involving neuronal death, axonal damage, and/or demyelination. Beneficial effects of IGF-I have been demonstrated in animal models of stroke, physical trauma to the brain, peripheral neuropathies, and demyelinating diseases [6]. However, despite these promising experimental results, these therapies have not translated into successful treatment for human patients [7]. The main limitations to the therapeutic use of IGF-I in central nervous system diseases are the poor central uptake of IGF-I and its potential systemic mitogenic action. Periventricular leukomalacia is the major neuropathologic form of brain injury in preterm infants, and is the leading cause of cerebral palsy. This study indicates that IGF-I at a low dose may prevent brain damage whereas at a high dose may even enhance the inflammation, ultimately increasing the damage of neonatal brain.
Serum insulin-like growth factor I, IGF-binding proteins 2 and 3, and the risk for development of malignancies in adults with growth hormone deficiency treated with GH: data from KIMS (Pfizer International Metabolic Database)
Popovic V, Mattsson AF, Gaillard RC, Wilton P, Koltowska-Häggström M, Ranke MB
Neuroendocrine Unit, Endocrinology Clinic, University Clinical Center, Belgrade, Serbia.
J Clin Endocrinol Metab 2010;95:4449-54.
Background: Experimental and epidemiological data suggest an association between circulating levels of IGF-I and cancer risk. The fact that GH therapy increases IGF-I concentrations raises concern about a possible long-term risk of developing tumors in subjects who have undergone or are undergoing GH therapy. The aim of this study was to determine the association between circulating levels of IGF-I, IGFBP-2, and IGFBP-3 in GHD adults on replacement therapy and the relative risk (RR) of cancer.
Methods: The study population was selected within the KIMS (Pfizer International Metabolic Database) survey, a pharmacoepidemiological investigation of adults with GHD. IGF-I, IGFBP-2 and IGFBP-3 were measured in serum samples from 100 patients with de novo malignancy and from 325 patients with idiopathic GHD without a tumor diagnosis. The cohort was stratified in three age groups. The RR of malignancy was controlled for the effect of age, sex, onset of GHD, and GH naivety at KIMS entry.
Results: IGF-I levels were not associated with a higher risk of malignancy, whereas both IGFBP-2 and IGFBP-3 concentrations were significantly positively associated with a higher RR of cancer (RR 1.18, 95% CI 1.07-1.30, and 1.13, 95% CI 1.02-1.26 respectively).
Conclusions: IGF-I serum levels seem not to be associated with the risk of developing malignancy, whereas this risk seems to be more elevated in GHD adults on GH replacement therapy with higher concentrations of IGFBP-2 and IGFBP-3.
The safety issue in subjects treated with GH is nowadays a hot topic, especially after the recent EMA warning about the increased mortality observed in young adults treated with high doses of GH in childhood. Although there is robust evidence from in vitro and in vivo studies indicating a link between GH-IGF axis and cancer, in humans data are conflicting [8]. The authors of this study claim that their results, showing no association between IGF-I and risk of cancer, are reassuring. However, this report suffers from the weakness of a cross-sectional study within the context of a wide international cohort of patients. Therefore, the results represent but a snapshot, not providing any dynamic information on the fluctuations of IGF-I during GH therapy. The finding of a higher relative risk associated with higher levels of IGFBP-2 and IGFBP-3 is puzzling. It is hard to extrapolate in vitro data indicating a role of IGFBP-2 in the control and regulation of tumor growth and invasion to humans [9]. IGFBP-3 has always been considered a counter-regulator of IGF-I, favoring cell apoptosis and inhibiting the IGF-I mitogenic action. A close cooperation between the tumor suppression gene p53 and IGFBP-3 in inhibiting mitogenic signaling by IGF-1 has also been reported [10]. Furthermore, reduced IGFBP-3 levels have been related to the risk of colorectal [11] and breast cancer [12]. A possible explanation of the study results is that the proportion of variance attributable to genetic effects for the IGFBP-3 concentrations is about 60% [13] suggesting a role for genetic predisposition in determining IGFBP-3 levels independently of GH therapy. Whatever the interpretation of these results may be, it is wise to continue close monitoring of IGF-I and, possibly, IGFBP-3 circulating levels, in the follow-up of patients undergoing GH therapy.
New mechanism
A critical role for IGF-II in memory consolidation and enhancement
Chen DY, Stern SA, Garcia-Osta A, Saunier-Rebori B, Pollonini G, Bambah-Mukku D, Blitzer RD, Alberini CM
Department of Neuroscience, Mount Sinai School of Medicine, New York, N.Y.,USA.
Nature 2011;469:491-7.
Background: Different families of transcription factors such as the cAMP response element-binding protein (CREB) and CCAAT enhancer-binding protein (C/EBP) intervene in memory consolidation. The identity of the target genes regulated by CREB and C/EBP is largely unknown. It has been shown that IGF-II promoters have C/EBP-binding sites. IGF-II is abundantly expressed in central nervous system (CNS), particularly in hippocampus, during development and in postnatal life, declining with aging. The aim of this study was to explore the role of IGF-II in the memory process.
Methods: Long-Evans adult male rats were studied. Inhibitory avoidance (IA) training was carried out in a chamber consisting of a box divided into a safe compartment and a shock compartment.
Results: The IA training was associated to a significant increase in both IGF-II mRNA and IGF-II protein in the hippocampus that temporarily overlapped that of C/EBPß. The hippocampal bilateral injection of C/EBPb antisense oligodeoxynucleotide (b-ODN) was able to inhibit the training-induced IGF-II increase, thus indicating that IA training led to an increase in hippocampal C/EBPb that in turn stimulated IGF-II production. Knocking down hippocampal IGF-II expression disrupted memory retention, which was restored by IGF-II administration. Furthermore, hippocampal IGF-II injection significantly enhanced memory and prevented forgetting. However, only if administered during precise time windows corresponding to 1-2 days after training, IGF-II was effective in producing such effects on memory. The IGF-II effects on memory were mediated by activation of IGF-II receptors, new protein synthesis, function of activity-regulated cytoskeletal-associated protein and glycogen-synthase kinase 3 (GSK3). Finally, IGF-II facilitated long-term memory potentiation.
Conclusions: These effects of IGF-II on hippocampal control of memory suggest that IGF-II might represent a novel molecule to be tested for cognitive enhancement therapies.
Increasing evidence strongly supports a role for the IGF system in central nervous system (CNS) development. IGF-I, IGF-II, their receptors, and some IGF-binding proteins are expressed in many regions of the CNS beginning in utero. The expression pattern of IGF system proteins during brain growth suggests highly regulated and developmentally timed IGF actions on specific neural cell populations. This elegant study highlights the critical role of IGF-II in the control of memory when administered within a sensitive period of memory consolidation. Increasing evidence suggests that IGF-I exerts mitogenic and anti-apoptotic effects in almost any region of brain. The role of IGF-II, although being the most abundant IGF in the brain, has been less investigated so far. This investigation, performed in rodents testing their capacity to develop inhibitory avoidance memory, suggests a pivotal role of IGF-II in consolidation and enhancement of memory. This action is mediated through type 2 IGF receptors which were supposed not to transduce the IGF signal. Interestingly, both IGF-II and IGF-IIR genes are epigenetically regulated. Therefore, environment-dependent epigenetic mechanisms may control memory process by regulating IGF-II and type 2 IGF receptor genes [14]. The results of this study indicate that IGF-II may represent a novel agent for cognitive enhancement therapies, but the potential beneficial effects may be counterbalanced by the potential risk of inducing tumors as the vast majority of cancers overexpress both IGF-II and type 2 IGF receptor [15].
Important for clinical practice
Insulin-like growth factor-I in cord blood is predictive of catch-up growth in monozygotic twins with discordant growth
Gohlke BC, Schreiner F, Fimmers R, Bartmann P, Woelfle J
Department of Pediatrics, University of Bonn, Bonn, Germany.
J Clin Endocrinol Metab 2010;95:5375-81.
Background: Placental blood flow undergoes a redistribution between the twins in 15% of monozygotic twin pregnancies, leading to twin-twin transfusion syndrome, and ultimately favoring the growth of one twin at the expenses of the other one who experiences intrauterine growth restriction (IUGR). The aim of this study was to test the predictive value of perinatal anthropometric and biochemical parameters for postnatal catch-up growth in a population of monozygotic twins to exclude the effects of the genetic background.
Methods: It was a prospective study of 25 monozygotic twin pairs who were longitudinally investigated from birth to the age of 4 years. Follow-up was completed in 20 out of the initial 25 twin pairs.
Results: The inter-twin difference in height decreased progressively from birth to 4 years, with the maximum catch-up growth during the first 12 months. The inter-twin differences in BMI remained unchanged from birth to 4 years. The inter-twin differences in both birth weight and cord blood IGF-I levels resulted significant predictors of height at 4 years.
Conclusions: Twins born IUGR show early catch-up growth in height, and this catch-up is predicted by birth weight and IGF-I concentrations in cord blood. Finally, the strong inter-twin correlation of IGF-I levels observed at age 4 suggests a key role played by genetic predisposition in determining IGF-I production.
Early detection of children who have experienced IUGR and who are not going to catch up their growth may allow early and more effective interventions to stimulate their growth. Two previous studies addressed this issue [16, 17] without identifying any significant predictor at birth of the growth outcome. This was probably due to the wide interindividual variability in all tested variables. The study of monozygotic twin pairs offers the unique opportunity to exclude the genetic influences and concentrate on the potential anthropometric, biochemical and endocrine predictors. In this context the main finding of the study is the predictive value of birth size and cord blood IGF-I. The authors of this study speculate that epigenetic changes induced by an adverse uterine environment may permanently affect GH-IGF-I axis, ultimately influencing postnatal growth. While the strength of this investigation resides in the unique genetically identical study cohort, the major weakness is represented by the small number of study subjects.
Postnatal decrease in circulating insulin-like growth factor-I and low brain volumes in very preterm infants
Hansen-Pupp I, Hövel H, Hellström A, Hellström-Westas L, Löfqvist C, Larsson EM, Lazeyras F, Fellman V, Hüppi PS, Ley D
Department of Clinical Sciences, Lund University Hospital, Lund, Sweden.
E-mail: [email protected]
J Clin Endocrinol Metab 2011;96:1129-35.
Background: In vitro and in vivo evidence suggests that IGF-I functions as a neurotrophic factor during brain development. Patients with IGF-I gene deletion or mutation show severe mental retardation. The aim of this study was to assess the association between IGF-I and IGFBP-3 levels and brain volume in premature infants.
Methods: The study population consisted of 51 premature babies with a mean (SD) gestational age of 26.4 (1.9) weeks who were tested weekly for IGF-I and IGFBP-3 concentrations from birth to 35 gestational weeks. Daily enteral and parenteral intakes of protein and energy were calculated. A brain MRI scan was performed at 40 gestational weeks.
Results: Gestational age at birth correlated positively with total brain volume (TBV), unmyelinated white matter volume (UWMV), gray matter volume (GMV), and cerebellar volume (CBV) at term. Preterm infants born SGA showed significantly smaller brain volumes than AGA. After stratification according to CBV percentiles, weekly IGF-I concentrations resulted significantly lower in infants with CBV below the 25th percentile, in comparison with those with CBV above the 25th percentile. Mean IGF-I and IGFBP-3 levels positively correlated with TBV, UWMV, GMV and CMV even after adjustment for the other potential confounders including gestational age. No significant relationship was found between nutrient intake and brain volumes.
Conclusions: The finding that lower IGF-I levels are associated with reduced brain volume suggests that exogenous administration of IGF-I may contribute to normal brain development in very preterm infants.
These authors previously reported a critical role of IGF-I in retinal vascularization in humans [18] and in the retinopathy of premature babies [19]. In this study they report an association between circulating IGF-I levels and brain volume in premature infants, independently of protein and calorie intake. Though an association is not indicative of cause and effect, there may be a role for IGFs in CNS development. It may even suggest a potential therapeutic role for IGF-I to support normal brain growth and development in premature infants. Exogenous IGF-I has been proven to be effective in protecting the brain after hypoxic-ischemic insult in animal models reducing neural cell apoptosis and enhancing proliferation of neuronal and oligodendroglial progenitors [20]. In humans, however, data on effectiveness of IGF-I administration on neuropathies are not encouraging [7]. The use of IGF-I may be affected by a number of limitations such as the inability to cross the brain-blood barrier, the short half-life, the mitogenic and the hypoglycemic effects. Furthermore, due to the close cooperation between IGF-I and VEGF, the individuation of the suitable time window for administering IGF-I seems to be crucial for avoiding negative effects on tissue vascularization.
11ß-Hydroxysteroid dehydrogenase type 1 activity in short small for gestational age children and in response to growth hormone therapy
Zuckerman-Levin N, Tsivlin L, Knopf C, Flor O, Shen-Orr Z, Levin M, Hochberg Z
Pediatric Endocrinology, Meyer Children's Hospital, Haifa, Israel.
Pediatr Res 2011;70:208-12.
Background: 10% of children born small for gestational age do not catch up their growth and remain permanently short. 11 ß-Hydroxysteroid dehydrogenase type 1 (11ß-HSD-1) protects the fetus from maternal corticosteroids converting cortisol into inactive cortisone. In postnatal life, 11ß-HSD-1 is expressed in visceral fat, converts cortisone to cortisol, and its activity may be linked to the higher metabolic risk of SGA subjects. The aim of this study was to test the hypothesis that short SGA (SSGA) children have increased 11ß-HSD-1 activity and that this activity is affected by GH therapy.
Methods: 20 SSGA and 12 age-matched control children were studied. Urine specimens were tested for 11ß-HSD-1 activity by gas chromatography mass spectrometry (GCMS). Glucocorticoid (GH)-induced enzyme activity was evaluated after overnight dexamethasone by measuring cortisol generation from cortisone.
Results:SSGA children showed significantly reduced 11ß-HSD-1 activity and enhanced glucorticoid-stimulated activity. Glucocorticoid-stimulated enzyme activity was inversely related to gestational age and birth weight. Three-month GH therapy suppressed the GC-stimulated 11ß-HSD-1 activity in SSGA.
Conclusions:SSGA children show increased GC-stimulated 11ß-HSD-1 activity which may represent a risk factor for developing metabolic syndrome. GH therapy normalizes 11ß-HSD-1 activity thus potentially contributing to reduce the metabolic risk.
The function of the hypothalamic-pituitary-adrenal axis (HPAA) can be permanently programmed during development. In animals, exposures to glucocorticoids in prenatal and early postnatal life may imprint the rodent HPAA, resulting in permanent modification of the neuroendocrine response to stress throughout life [21, 22]. In rats, maternal undernutrition induces IUGR and fetus overexposure to maternal corticosterone, leading to HPAA dysregulation in newborns [23]. In humans, it was sug-gested that elevated cortisol concentrations might represent a link between low birth weight and metabolic disturbances [24]. The present finding of increased GC-induced generation of cortisol from cortisone may be related to the enhanced reactivity to stress previously described in these individuals. Due to their intrinsic higher risk of developing metabolic alterations, the use of GH in SGA children may associate with a decline in insulin sensitivity, ultimately leading to impaired glucose homeostasis and type 2 diabetes [25]. With regard to the safety issue of GH therapy, the results of this study look reassuring, showing a suppressing effect of short-term GH therapy on 11 ß-HSD-1 activity.
Short stature associated with a novel heterozygous mutation in the insulin-like growth factor 1 gene
Van Duyvenvoorde HA, van Setten PA, Walenkamp MJ, van Doorn J, Koenig J, Gauguin L, Oostdijk W, Ruivenkamp CA, Losekoot M, Wade JD, De Meyts P, Karperien M, Noordam C, Wit JM
Department of Pediatrics, Leiden University Medical Center, Leiden, The Netherlands.
J Clin Endocrinol Metab 2010;95:E363-7.
Background: Homozygous deletions or missense mutations of IGF-I gene are associated with a phenotype characterized by severe intrauterine and postnatal growth failure, microcephaly, mental retardation, and sensorineural deafness. The few patients carrying heterozygous deletions or missense mutations show a milder phenotype with less severe impairment of pre- and postnatal growth retardation. In this report the authors describe two children with heterozygous missense mutations of IGF-I gene.
Methods: The 2 patients and the other family members were investigated by anthropometry, dual-energy x-ray absorptiometry, endocrine assessment and sequencing for IGF-I.
Results: The phenotype of the 2 index cases was characterized by severe short stature, microcephaly, and low IGF-I levels. Sequencing of IGF1 gene showed a maternally derived novel heterozygous mutation in exon 3. Although the mother and maternal grandfather carried the same IGF1 mutation, their adult height and head circumference were less affected. Both children accelerated growth rate in response to GH therapy.
Conclusions: Subjects carrying IGF-I haploinsufficiency may have severe short stature and impairment of head circumference growth. This condition may respond to GH treatment.
Total IGF-I deficiency due to either homozygous deletions or missense mutations leads to a phenotype characterized by severe impairment of pre- and postnatal growth and mental retardation. On the other hand, heterozygous carriers usually have a milder phenotype with stature, head circumference and IGF-I levels in the low normal range. The 2 siblings reported in this study were peculiar in that, despite having IGF-I haploinsufficiency, they showed severe postnatal growth retardation and very low levels of IGF-I. Their mother and maternal grandfather, carrying the same mutation, had a milder phenotype. This family represents another good example of phenotype heterogeneity in the presence of the same mutation. The authors speculate that placental dysfunction secondary to maternal IGF-I haploinsufficiency and the effect of other genes involved in growth may explain the more severe phenotype observed in the 2 children. From a practical point of view it is important to point out that whereas homozygous carriers of IGF-I gene mutations show well-defined clinical features, the heterozygous carriers may have different phenotypic expressions, some of them even growing normally. A common feature in all these cases was the history of feeding disturbances during infancy, which if associated with poor growth should lead to investigate the IGF-I system. Finally, GH therapy was effective in stimulating growth in the heterozygous carriers of IGF-I mutation.
Mechanism of the year
Distinct growth hormone receptor signaling modes regulate skeletal muscle development and insulin sensitivity in mice
Mavalli MD, DiGirolamo DJ, Fan Y, Riddle RC, Campbell KS, van Groen T, Frank SJ, Sperling MA, Esser KA, Bamman MM, Clemens TL
Department of Orthopedic Surgery, Johns Hopkins University School of Medicine, Baltimore, Md.,USA.
J Clin Invest 2010;120:4007-20.
Background: The GH-IGF-I axis exerts a potent effect on mammalian skeletal muscle controlling both muscle mass and strength, it is however still unclear whether these actions are induced by GH directly or indirectly through IGF-I. The aim of this study was to determine whether the effects of GH on skeletal muscle are direct or secondary to IGF-1 expression.
Methods: Mice selectively lacking either GH receptor (GHR) or IGF-I receptor (IGF-IR) in skeletal muscle were generated. Primary myoblast cultures were prepared from tibialis anterior and gastrocnemius muscles of 4-week-old wild-type, GHR, and IGF-1R mice.
Results: GH induced myoblast proliferation and fusion through the local production of IGF-I. IGF-1R appeared to be critical for the accrual of myonuclei into nascent myotubes. Mice lacking GHR failed to attain myofiber diameters comparable with those of controls for both type I and type II fibers by 16 weeks of age, thus suggesting that GHR is required for proper skeletal muscle development in vivo. The histomorphometric changes in myofiber type and size and myonuclei number associated with disruption of GHR resulted in function impairment, as shown by reduced grip strength and rotarod endurance performance. Surprisingly, mice lacking GHR in muscle showed increased body weight and fat mass associated with elevated serum glucose and triglyceride levels, with no difference in insulin concentrations. Further analysis revealed that loss of GHR in skeletal muscle was associated to desensitization of skeletal muscle to insulin action secondary to decreased abundance of insulin receptors and reduced activation of multiple pathways downstream the receptor. The phenotype originating from the loss of IGF-IR was similar to that described for the loss of GHR but without the changes in body composition, metabolism and insulin action.
Conclusions:GHR in conjunction with IGF-IR signaling promotes normal myofiber-type specification, myonuclei accumulation, and expansion of myofiber diameter, whereas, independently of IGF-IR, muscle GHR controls insulin sensitivity, global metabolism and body composition.
GH exerts multiple metabolic actions decreasing glucose utilization and increasing lipolysis and protein storage [26]. In skeletal muscle, GH promotes muscle growth and development, and facilitates nutrient uptake and utilization. As most GH actions are mediated by IGF-I via endocrine, paracrine and autocrine secretion, it is difficult to dissect pure GH actions from those actually mediated by IGF-I. Based on an innovative genetic approach leading to specific muscle deletion of GHR or IGF-IR, this study provides the first evidence for different effects of the two hormones. While IGF-I mediates, as expected, the growth-promoting actions of GH in muscle, GH exerts direct IGF-independent effects on metabolism and, consequently, body composition. The muscle insulin resistance, induced by the loss of GHR activity, leads to reduced nutrient uptake into the muscle with consequent redistribution of nutrients to adipose tissue, ultimately leading to fat mass increase. These results indicate the existence of distinct GHR signaling pathways leading specifically to either skeletal muscle development or the modulation of nutrient metabolism. Aging is associated with reduced insulin sensitivity and loss of muscle mass, suggesting that these changes may be linked to impaired activation of anabolic signaling pathways which may include the GHR and IGF1R [27]. Knowledge of the different pathways involved in the dual effects of GH in muscle may provide means for regulating insulin sensitivity and global nutrient metabolism. It is also relevant to therapy: IGF-1 therapy in GH insensitivity syndromes will not correct the IGF-independent effects of GH malfunction.
New paradigms
Insulin/IGF-1 signaling mutants reprogram ER stress response regulators to promote longevity
Henis-Korenblit S, Zhang P, Hansen M, McCormick M, Lee SJ, Cary M, Kenyon C
Department of Biochemistry and Biophysics, University of California, San Francisco, Calif.,USA.
Proc Natl Acad Sci U S A 2010;107:9730-35.
Background: The endoplasmic reticulum (ER) is the major site in the cell for protein folding and trafficking and is central to many cellular functions. Failure of the ER's adaptive capacity results in activation of the unfolded protein response (UPR), which intersects with many different inflammatory and stress signaling pathways to restore ER homeostasis. The aim of this study was to test whether selective inactivation of insulin/IGF-1 pathway affects ER stress response in Caenorhabditis elegans.
Methods: C. elegans mutants carrying reduction-of-function mutations in daf-2, which encodes the animal's insulin/IGF-1 receptor, were studied. To test whether ER stress-response proteins contribute to the increased longevity of animals with reduced insulin/IGF-1 signaling, each of the three ER stress-response genes that comprise the UPR - IRE-1, PEK-1, and ATF-6 - were individually inactivated.
Results: daf-2 mutants exhibited increased ER stress resistance and extended longevity. The loss of the ER stress response genes ire-1 or xbp-1 (ire-1-dependent transcription factor) shortened the lifespan of daf-2 mutants. The cooperation between xbp-1 and daf-16 (a stress-protective transcription factor) increased the expression of the longevity gene dox-1.
Conclusions: These data suggest that, in C. elegans, the longevity associated with the loss of function of insulin/IGF-I pathway is mediated by a reprogramming of ER stress response which acts in cooperation with other genes and transcription factors ultimately leading to prolonged lifespan.
The ER is a vast membranous network responsible for the synthesis, maturation, and trafficking of a wide range of proteins. When the ER becomes stressed due to the accumulation of newly synthesized unfolded proteins, an unfolded protein response (UPR) is activated (fig. 1). UPR is mediated by three ER membrane-associated proteins, PERK (PKR-like eukaryotic initiation factor 2a kinase), IRE1 (inositol-requiring enzyme 1), and ATF6 (activating transcription factor-6). In a well-functioning and ‘stress-free’ ER, these three transmembrane proteins are bound by a chaperone, BiP/GRP78, and kept inactive [28]. A malfunction of the ER stress response caused by aging, genetic mutations, or environmental factors can result in various diseases such as diabetes, inflammation, and neurodegenerative disorders [29, 30]. Therefore, ER is an attractive potential therapeutic target; maintenance or enhancement of proper ER function being able to prevent chronic metabolic disease. This study reveals an interplay between the insulin/IGF-I pathway and ER stress, showing that selective inactivation of daf-2, the gene encoding the Caenorhabditis elegans insulin/IGF-1 receptor, associates with extended lifespan and increased resistance to ER stress. The ER stress-related factors, however, must cooperate with a complex constellation of transcription factors and genes to determine their final effect on longevity. These results may constitute the basis for further studies in higher organisms in an attempt to identify potential targets for innovative therapeutic approaches to chronic diseases.
Activation of unfolded protein response (UPR). In response to ER stress, the three branches of the UPR are activated. In the first branch, PERK-mediated attenuation of translation results in release of NF-κB from its inhibitor kB. NF-κB moves to the nucleus and switches on expression of a variety of different genes involved in inflammatory pathways, such as those encoding the cytokines IL-1 and tumor necrosis factor-α (TNF-α). In the second branch of the UPR, activated IRE1α recruits TNF-α receptor-associated factor 2 (TRAF2), and this complex interacts with JNK and IκB kinase (IKK) and activates them. These inflammatory kinases then phosphorylate and activate downstream mediators of inflammation. The third branch of the UPR, the ATF6 pathway, also activates NF-κB. There is also crosstalk between the three branches. For example, spliced X-box binding protein 1 (XBP1s), as well as ATF4, induce production of the nflammatory cytokines IL-8, IL-6, and monocyte chemoattractant protein 1 (MCP1) by human endothelial cells. XBP1s has also been implicated in production of IFN-ß when ER stress is combined with activation of Toll-like receptor (TLR) signaling and in IFN-α production by dendritic cells. The ability of XBP1s to expand the capacity of the ER for protein folding (and ER calcium stores) enables it to mediate calcium-dependent inflammatory responses in human bronchia epithelial cells, which produce IL-8. Inset: Assembly of a putative metabolic inflammasome or metaflammasome. This protein complex integrates pathogen and nutrient sensing with ER stress, inflammatory kinases, insulin action, and metabolic homeostasis. The eIF2α kinase PKR (double-stranded RNA-activated protein kinase) is a core component of the metaflammasome and interacts directly with several inflammatory kinases such as IKK and JNK, insulin receptor signaling components such as IRS1, and the translational machinery via eIF2α. Nutrients, inflammatory mediators, and ER stress regulate PKR activity. From Hotamisligil [30], with permission
PTK6 regulates IGF-1-induced anchorage-independent survival
Irie HY, Shrestha Y, Selfors LM, Frye F, Iida N, Wang Z, Zou L, Yao J, Lu Y, Epstein CB, Natesan S, Richardson AL, Polyak K, Mills GB, Hahn WC, Brugge JS
Department of Cell Biology, Harvard Medical School, Boston, Mass.,USA.
PLoS One 2010;5:e11729.
Background: Anoikis is the process that drives epithelial cells to programmed death when they detach from the extracellular matrix. Cancer cells are characterized by the ability to resist anoikis. IGF-I receptor (IGF-IR) seems to play a critical role in the anoikis inhibition induced by various oncogenes. The aim of this study was to identify molecules able to induce resistance to anoikis through cooperation with IGF-IR.
Methods: Loss-of function screens have been used to get insights into mechanisms underlying anoikis suppression. This investigation was performed by a novel siRNA (small interfering RNA) screen designed to identify regulators of IGF-1 receptor (IGF-1R)-driven anoikis resistance of epithelial cells.
Results: PTK6 (protein-tyrosine kinase 6) resulted in an important modulator of the IGF-I-dependent anoikis resistance in both breast and ovarian cells. PTK6 regulates the expression and the activity of IGF-IR. Whereas PTK6 downregulation reduced the IGF-IR-dependent resistance to anoikis, leading to apoptosis, its overexpression enhanced the anchorage-independent survival.
Conclusions: Due to its critical role in regulating survival of tumor cells, PTK6 may represent a potential therapeutic target for breast and ovarian cancers.
Anchorage-independent survival is critical for preserving cancer cells from apoptosis. IGF-IR is expressed in several breast cancer subtypes and its expression is associated with poor outcomes. One of the mechanisms underlying this association is the inhibition of anoikis mediated by IGF-IR. Anoikis is the process that drives epithelial cells to programmed death when they detach from the extracellular matrix. However, IGF-IR exerts its anoikis suppression in the context of a complex network of regulatory molecules. This study was performed to understand the crosstalk between the tyrosine kinase PTK6 and IGF-IR signaling in epithelial cancer cells. PTK6 was demonstrated to represent one of the major controllers of IGF-IR-mediated anti-anoikis action. PTK6 is a member of the Src family of tyrosine kinases and is overexpressed in a variety of tumor types, including many breast cancer subtypes. PTK6 function has been linked to differentiation, regulation of AKT, cell growth and apoptosis [31]. PTK6 was also shown to cooperate with ErbB2 to promote breast tumor cell growth [32]. The effects of PTK6 on anchorage independence elucidated in this study may explain the adverse outcomes associated with high PTK6 expression in patients with breast cancer. The widespread overexpression of PTK6 in a variety of epithelial cancers suggests that this kinase may represent a potential target for therapies based on blocking drugs. A PTK6-targeted treatment may offer the chance of selectively targeting malignant cells, as PTK6 expression is rather restricted in normal tissues and selectively enhanced in tumors.
Insulin-like growth factor 1 treatment extends longevity in a mouse model of human premature aging by restoring somatotroph axis function
Mariño G, Ugalde AP, Fernández AF, Osorio FG, Fueyo A, Freije JM, López-Otín
Departamento de Bioquímica y Biología Molecular and Biología Funcional, Facultad de Medicina, Instituto Universitario de Oncología, Universidad de Oviedo, Oviedo, Spain.
Proc Natl Acad Sci U S A 2010;107:16268-73.
Background: Progeroid syndromes represent clinical surrogates of aging. Therefore, the knowledge of mechanisms underlying these conditions may shed light on potential strategies for preventing or attenuating the consequences of aging. Lamin A is an essential component of the nuclear envelope, mutations either in lamin A or in Zmpste24 (a gene encoding a metalloproteinase involved in the maturation of lamin A) are responsible for several human progeroid syndromes, including Hutchinson-Gilford progeria. GH-IGF-I axis function has also been repeatedly reported to be related to longevity. This study aimed at investigating the role of GH-IGF-I signaling in a murine model of Hutchinson-Gilford progeria.
Methods: Zmpste24-deficient mice with accelerated aging and phenotype resembling that of Hutchinson-Gilford progeria were studied.
Results: Progeroid mice showed progressive decline of IGF-I circulating levels coupled with parallel increase of GH concentrations, thus resembling the picture of GH resistance (of GH insensitivity, GHI). In common with GHI patients, Zmpste24-deficient mice showed reduced skeletal muscle development and function, as well as impaired bone mineralization and other features such as alopecia, skin atrophy, and hypoglycemia. A series of genes involved in GH-IGF-I axis signaling, such as IGF-I, ALS (acid-labile subunit), IGFBP-1, IGF-IR, GH-R, STAT5a, SOCS2, were transcriptionally altered in the liver of mutated mice. To explain these findings, a search of putative microRNAs (miRs) predicted to target IGF-I gene was carried out. miR-1 resulted upregulated in both progeroid mice and human Hutchinson-Gilford progeria cells, likely contributing to somatotroph axis suppression by reducing IGF-1 synthesis. Finally, administration of IGF-I normalized the balance between IGF-I and GH levels, corrected many progeroid features and extended lifespan.
Conclusions: The fine tuning of GH-IGF-I axis signaling plays a key role in the control of lifespan, and may represent a potential therapeutic target for slowing disease progression in children with progeria.
The aging process is regulated by genes encoding proteins directly or indirectly linked to GH-IGF-I axis signaling; the inactivation of IGF-IR confers greater resistance to oxidative stress and prolongs lifespan [33]. Mice with a condition mimicking human GH insensitivity (GHI), characterized by low levels of IGF-I and high GH concentrations, show greater longevity associated with a certain degree of resistance to cancer [34]. Therefore, the finding that this murine model of progeria shows endocrine features similar to GHI but is associated with accelerated aging and reduced lifespan looks contradictory. This unexpected behavior may represent an adaptive mechanism to unfavorable conditions, such as aging or progeroid syndromes, characterized by DNA damage [35]. When DNA is damaged, the organism diverts resources from growth to survival, switching off GH-IGF-I axis signaling in order to reduce cell proliferation and ultimately the risk of amplifying DNA alterations. In this perspective, the reduced levels of IGF-I observed in Zmpste24-deficient progeroid mice may represent an example of growth suppression to limit the consequences of aging-associated DNA damage. The concomitant presence of high GH concentrations may contribute to the development of the progeroid phenotype, as GH-R signaling was found intact in the mutated animals. This study strongly supports the concept that fine tuning of the somatotroph axis is essential in the regulation of longevity. The finding that IGF-I administration extends longevity and ameliorates the progeroid clinical picture of Zmpste24-mutated mice suggests that the adaptive response seen in premature aging conditions may be exogenously modulated to extend longevity. Further studies are needed in animal models to ascertain to what extent this ‘compensatory’ mechanism may be modulated without increasing the risk of amplifying DNA damage and ultimately the risk of cancer.
Clinical trials
Final adult height in children with congenital adrenal hyperplasia treated with growth hormone
Lin-Su K, Harbison MD, Lekarev O, Vogiatzi MG, New MI
Department of Pediatric Endocrinology, Weill Medical College of Cornell University, New York, N.Y.,USA.
J Clin Endocrinol Metab 2011;96:1710-7.
Background: Patients with congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency (21-OHD), despite long-term glucocorticoid therapy, fail to achieve their mid-parental height. The aim of this study was to test the effectiveness of GH in improving final height.
Methods: 34 patients with CAH on treatment with glucocorticoids and fludrocortisone, if needed, were treated with GH at an initial dose of 0.3 mg/kg per week divided into seven subcutaneous doses. The dose was increased as needed in 0.02-mg/kg/week increments up to a maximum dose of 0.45 mg/kg to maintain a growth velocity at 50th or above percentile for bone age without exceeding the normal range of IGF-I. 5 patients dropped out during follow-up. 27 subjects with either precocious or early central puberty (<11 years in males or <0 years in females) were treated with LHRHa in addition to GH. Adult height was defined as a growth velocity of <1.5 cm/year and bone age of at least 15 years in girls or at least 17 years in boys.
Results: After a mean duration of GH therapy of about 5 years, adult height significantly exceeded pre-therapy predicted final height of approximately 10 cm. LHRHa therapy did not significantly influence the outcome. The comparison with a historical untreated group of CAH patients showed that a higher proportion of GH-treated subjects achieved an adult height within 1 SD of mid-parental height.
Conclusions: GH therapy seems to be effective in increasing adult height in patients with CAH.
Many patients with CAH fail to attain an adult height within their target due to hypercortisolism and/or hyperandrogenism. However, the height outcome of CAH is variable and may be affected by a number of factors such as diagnosis (classical or non-classical CAH), age at diagnosis, treatment regimen, age of puberty onset and pubertal growth spurt [36]. Due to this heterogeneity of situations, well-designed, large, randomized, controlled studies of experimental drugs aimed at improving adult height of children with CAH would be needed. The promising results of this study are unfortunately burdened by a series of limitations such as the lack of controls, the small size and heterogeneity of the study population. The study design included patients with either classical or non-classical CAH, with good or poor adrenal control, with either precocious or ‘early’ puberty, requiring or not LHRHa therapy. Another bias is the use of baseline predicted final height to compare the adult height achieved. Adult height prediction is based on skeletal maturity assessment and can be used to predict with acceptable accuracy which adult height will be achieved by a ‘normal’ child. However, the predictions do not apply to children with endocrine diseases affecting the tempo of growth [37]. As the authors honestly admit, ‘further larger studies in a randomized controlled design are needed before definitive conclusions can be drawn’. This study cannot become the basis for GH treatment in CAH.
Effects of insulin-like growth factor-I/IGF-binding protein-3 treatment on glucose metabolism and fat distribution in human immunodeficiency virus-infected patients with abdominal obesity and insulin resistance
Rao MN, Mulligan K, Tai V, Wen MJ, Dyachenko A, Weinberg M, Li X, Lang T, Grunfeld C, Schwarz JM, Schambelan M
Department of Medicine and Center for Molecular and Functional Imaging, University of California, San Francisco, Calif.,USA.
J Clin Endocrinol Metab 2010;95:4361-6.
Background: HIV-infected patients are at high risk of developing metabolic alterations such as visceral fat accumulation and glucose homeostasis dysregulation, ultimately leading to increased cardiovascular risk. As IGF-I has been shown to improve glucose metabolism in diabetic patients and increase lean body mass in subjects with GHI, this study aimed at testing the efficacy of IGF-I/IGFBP-3 short-term therapy in ameliorating the metabolic picture of patients with HIV infection.
Methods: This was a pilot, open-label study performed on 13 HIV-infected adult patients (mean age 53 ± 2.6 years) with excess central fat, determined by assessment of waist circumference, and insulin resistance, determined by HOMA. The subjects were on stable antiretroviral therapy for at least 6 months. The enrolled patients underwent oral glucose tolerance testing, body composition measurements and hyperinsulinemic-euglycemic clamp. They were treated for 3 months with IGF-I/IGFBP-3 (mecasermin rinfate, iPLEX; Insmed, Inc., Richmond, Va., USA). Ten subjects received 0.5 mg/kg daily s.c. and 3 subjects 1.0 mg/kg daily.
Results: Three patients dropped out during the follow-up: 2 for allergic reactions and 1 for poor adherence to treatment. The remaining 10 subjects who completed the trial showed a significant improvement of glucose tolerance associated with an increase in insulin-mediated glucose uptake and a significant reduction of triglycerides. Fasting endogenous glucose production (EGP) increased by 10%, and suppression of EGP by insulin was blunted during treatment. Body composition analysis revealed an increase in lean body mass but subcutaneous and visceral adipose tissue remained unchanged. No hypoglycemic episodes were observed during treatment. One patient developed squamous cell carcinoma.
Conclusions: Short-term IGF-I/IGFBP-3 treatments were effective in improving insulin sensitivity and glucose tolerance in HIV-infected patients with insulin resistance and excess visceral adiposity.
Antiretroviral therapy in patients with HIV infection is associated with metabolic side effects including dyslipidemia, insulin resistance, and even diabetes mellitus. This metabolic pattern is associated in adult patients with increased risk of myocardial infarction. To reverse these metabolic alterations, GH was previously used. However, GH was able to reduce visceral fat but, as expected, worsened glucose tolerance. The therapeutic choice of this trial was based on the insulin-like activity of IGF-I and the ability of IGFBP-3 to prolong IGF-I half-life. Despite these preliminary promising results, showing a beneficial effect of IGF-I/IGFBP-3 therapy on insulin sensitivity and glucose tolerance, it has to be pointed out that only 10 patients completed this uncontrolled and non-randomized trial. The finding of increased gluconeogenesis during IGF-I/IGFBP-3 therapy is puzzling and might represent a compensation for the reduced glucose concentrations. Furthermore, the finding of no significant effect on visceral adipose tissue questions whether this therapeutic approach may be really effective in reducing the long-term cardiovascular risk.
