The role of IGFs in catabolism

Exploring the mechanistic link between corticosterone and insulin-like growth factor-1 in a wild passerine bird

Background

Physiological regulators of life history trade-offs need to be responsive to sudden changes of resource availability. When homeostasis is challenged by unpredictable stressors, vertebrates respond through a set of physiological reactions, which can promote organismal survival. Glucocorticoids have been traditionally recognized as one of the main regulators of the physiological stress response, but the role of an evolutionarily more conserved pathway, the hypothalamic-pituitary-somatotropic (HPS) axis producing insulin-like growth factor-1 (IGF-1) has received much less attention. Although IGF-1 is known to affect several life history traits, little is known about its role in the physiological stress response and it has never been studied directly in adult wild animals.

Methods

In this study, we combined field observations with a controlled experiment to investigate how circulating levels of IGF-1 change in response to stress and whether this change is due to concomitant change in glucocorticoids in a free-living songbird, the bearded reedling Panurus biarmicus. We used a standard capture-restraint protocol in field observation, in which we took first and second (stress induced: 15 minutes later) samples. In a follow-up experiment, we used a minimally invasive oral corticosterone manipulation.

Results

We showed that corticosterone levels significantly increased while IGF-1 levels significantly decreased during capture and handling stress. However, change in corticosterone levels were not related to change in IGF-1 levels. We found that experimentally elevated corticosterone levels did not affect IGF-1 levels.

Discussion

Our results are the first to highlight that circulating IGF-1 levels are responsive to stress independently from glucocorticoids and suggest that the HPS axis is an autonomous physiological pathway that may play an important role as regulator of life-history decisions.

Wnt7a-Fzd7 signalling directly activates the Akt/mTOR anabolic growth pathway in skeletal muscle

Wnt7a signals through its receptor Fzd7 to activate the planar-cell-polarity pathway and drive the symmetric expansion of satellite stem cells resulting in enhanced repair of skeletal muscle. In differentiated myofibres, we observed that Wnt7a binding to Fzd7 directly activates the Akt/mTOR growth pathway thereby inducing myofibre hypertrophy. Notably, the Fzd7 receptor complex was associated with Gαs and PI3kinase and these components were required for Wnt7a to activate the Akt/mTOR growth pathway in myotubes. Wnt7a/Fzd7 activation of this pathway was completely independent of IGF-receptor activation. Together, these experiments demonstrate that Wnt7a/Fzd7 activates distinct pathways at different developmental stages during myogenic lineage progression, and together identify a novel non-canonical anabolic signalling pathway for Wnt7a and its receptor Fzd7 in skeletal muscle.

Systemic response of the GH/IGF-I axis in timely versus delayed fracture healing

The GH-IGF axis has profound effects on the local and systemic regulation of bone metabolism and may be important for quality of fracture healing. To test the hypothesis that deficiency of the GH/IGF axis may play a role in the pathogenesis of fracture non-union we investigated whether alterations of serum concentrations of the GH-IGF axis could be related to failed fracture healing compared to timely fracture healing in trauma patients. Serum probes were prospectively collected from 186 patients with surgical treatment of long bone fractures up to 6 months after surgery. Samples from 14 patients with atrophic type of non-union have been compared to 14 matched patients with normal bone healing. Postoperative time courses of serum concentrations have been analyzed using commercially available chemiluminescence sandwich assays (GH), fully automated assay systems (IGF-I, IGFBP-3) or sandwich immunometric assays (ALS). Comparison between both collectives revealed significantly lower serum concentrations of GH dependent ALS during early (1st week after surgery) and of both IGFBP-3 and ALS during late stages of fracture healing (6 and 8 weeks after surgery) in non-union patients, coinciding clinically with failed fracture healing. Tendentially lower serum levels of IGF-I in the non-union group over the entire investigation period were statistically not significant. We have been able to show time courses of serum concentrations of the GH/IGF-I axis during normal and failed fracture healing in humans. An impairment of the GH/IGF-I axis might be involved in the biochemical mechanisms determining delayed or failed fracture healing.

Pharmacogenomics and end-organ susceptibility to injury in the perioperative period

Genomic medicine has provided new mechanistic understanding for many complex diseases over the last 5-10 years. More recently genomic approaches have been applied to the perioperative paradigm, facilitating identification of patients at high risk for adverse events, as well as those who will respond better/worse to specific pharmacologic therapies. The consistent biological theme emerging is that while inflammation is important in healing from surgical trauma, patients who are too robustly proinflammatory appear to be at higher risk for adverse perioperative events. Precise predictors of each adverse event are being elucidated so that corrective therapeutics can be instituted to improve outcomes in high-risk patients. While the field of perioperative genomics could be considered in its infancy, such approaches are the wave of the future.

Protein kinase B/Akt: a nexus of growth factor and cytokine signaling in determining muscle mass

Although the boundaries of skeletal muscle size are fundamentally determined by genetics, this dynamic tissue also demonstrates great plasticity in response to environmental and hormonal factors. Recent work indicates that contractile activity, nutrients, growth factors, and cytokines all contribute to determining muscle mass. Muscle responds not only to endocrine hormones but also to the autocrine production of growth factors and cytokines. Skeletal muscle synthesizes anabolic growth factors such as insulin-like growth factor (IGF)-I and potentially inhibitory cytokines such as interleukin (IL)-6, tumor necrosis factor (TNF)-alpha, and myostatin. These self-regulating inputs in turn influence muscle metabolism, including the use of nutrients such as glucose and amino acids. These changes are principally achieved by altering the activity of the protein kinase known as protein kinase B or Akt. Akt plays a central role in integrating anabolic and catabolic responses by transducing growth factor and cytokine signals via changes in the phosphorylation of its numerous substrates. Activation of Akt stimulates muscle hypertrophy and antagonizes the loss of muscle protein. Here we review the many signals that funnel through Akt to alter muscle mass.

Hypopituitarism after traumatic brain injury

Traumatic brain injury (TBI) is one of the main causes of death and disability in young adults, with consequences ranging from physical disabilities to long-term cognitive, behavioural, psychological and social defects. Post-traumatic hypopituitarism (PTHP) was recognized more than 80 years ago, but it was thought to be a rare occurrence. Recently, clinical evidence has demonstrated that TBI may frequently cause hypothalamic-pituitary dysfunction, probably contributing to a delayed or hampered recovery from TBI. Changes in pituitary hormone secretion may be observed during the acute phase post-TBI, representing part of the acute adaptive response to the injury. Moreover, diminished pituitary hormone secretion, caused by damage to the pituitary and/or hypothalamus, may occur at any time after TBI. PTHP is observed in about 40% of patients with a history of TBI, presenting as an isolated deficiency in most cases, and more rarely as complete pituitary failure. The most common alterations appear to be gonadotropin and somatotropin deficiency, followed by corticotropin and thyrotropin deficiency. Hyper- or hypoprolactinemia may also be present. Diabetes insipidus may be frequent in the early, acute phase post-TBI, but it is rarely permanent. Severity of TBI seems to be an important risk factor for developing PTHP; however, PTHP can also manifest after mild TBI. Accurate evaluation and long-term follow-up of all TBI patients are necessary in order to detect the occurrence of PTHP, regardless of clinical evidence for pituitary dysfunction. In order to improve outcome and quality of life of TBI patients, an adequate replacement therapy is of paramount importance.

Skeletal muscle cytokines: regulation by pathogen-associated molecules and catabolic hormones

PURPOSE OF REVIEW

This review will update clinicians and basic scientists who study the molecular mechanisms of muscle wasting associated with infection, trauma, cancer cachexia, and AIDS. A special emphasis is placed on recent studies that examine the interaction of insulin-like growth factor 1 and proinflammatory cytokines as positive and negative regulators of muscle mass.

RECENT FINDINGS

Potential mediators of the wasting syndromes include catabolic hormones, such as glucocorticoids, as well as the inflammatory cytokines tumour necrosis factor, IL-1, and IL-6. Cytokines may function either systemically or locally within muscle per se. Lipopolysaccharide and other pathogen-associated molecules stimulate cytokine expression in muscle. The failure to clear pathogen-associated molecules or the introduction of muscle damage may initiate a protracted activation of enzymes and transcription factors that orchestrate a genetic programme that ultimately produces muscle wasting.

SUMMARY

This review highlights recent advances in our understanding of the expression of the afferent and efferent limbs of the innate immune system in skeletal muscle. A special emphasis is placed on the recognition of pathogen-associated molecules by skeletal muscle cells and how these molecules regulate the expression of inflammatory cytokines and other muscle genes to result in muscle wasting, and when sustained, the erosion of lean body mass.

Activity of the growth hormone/insulin-like growth factor-I axis in critically ill children

Critical illness has an important impact on the human endocrine system. Very few studies have been performed to elucidate the alterations of the GH/IGF-I axis in acutely ill children. The aim of this study was to investigate several parameters of this axis in children with trauma (TRA) and sepsis (SEP) requiring admission to the pediatric intensive care unit (PICU). A total of 16 children, ten with TRA and six with SEP (age 1-10 years) as well as 18 healthy children (CS) of similar age and gender were included in the study. Two children, one with TRA and one with SEP, died. Serum IGF-I and -II, IGFBP-1 and -3, and GH levels were measured on days 1, 3 and 7 after admission. GH levels were higher in the patients than in CS (p = 0.04), with no difference between TRA and SEP, and were elevated during PICU stay (p = 0.05). Serum IGF-I, -II and IGFBP-3 were lower in the patients than in CS (p = 0.03, 0.02 and 0.001, respectively) with a tendency to increase up to day 7. Finally, IGFBP-1 levels were similar in the patients and CS. These findings indicate that critically ill children are characterized by low levels of IGF-I and -II as well as IGFBP-3 accompanied by elevated levels of GH, probably reflecting the development of peripheral GH resistance. No significant differences were found between the different catabolic conditions, sepsis and trauma.

Neurohormonal factors in the development of catabolic/anabolic imbalance and cachexia

Mechanisms that lead to cachexia are still poorly understood. The neurohormonal changes that occur in severe disease states may cause an imbalance between protein synthesis and degradation at the cellular level, followed by muscle wasting. Here, we review actions of angiotensin II, TNF-alpha, corticosteroids, insulin-like growth factor-I (IGF-I), and the IGF binding proteins, factors that may each contribute to the metabolic imbalance. The complex endocrine, autocrine and intracellular interactions between these factors will be described with examples from patient, rat and cell culture studies. Moreover, some of the data supporting that each of these hormones may directly affect cellular protein degradation mechanisms will be reviewed. Knowledge on these regulatory mechanisms will facilitate the development of new pharmaceutical strategies to treat cachexia.

Regulation of IGF-I mRNA and signal transducers and activators of transcription-3 and -5 (Stat-3 and -5) by GH in C2C12 myoblasts

GH and IGF-I are critical hormones for the regulation of longitudinal growth and the maintenance of lean body mass in humans. The regulation of IGF-I expression by GH in hepatocytes is well documented; however less is known about the regulation of IGF-I in peripheral tissues such as muscle. We have examined the regulation of IGF-I mRNA by GH and IGF-I in C2C12 myoblasts. GH stimulated the accumulation of IGF-I mRNA dose- and time-dependently. An elevation of IGF-I mRNA was observed with GH doses as low as 0.75 ng/ml and after exposure to GH for as little as 1 h, and the increase required ongoing transcription and translation. GH applied in a pulsatile fashion for 10 min followed by an 8-h interpulse interval increased IGF-I mRNA to a greater extent than continuous exposure. GH stimulated tyrosine phosphorylation of the GH receptor, signal transducer and activator of transcription-3 (Stat3), and Stat5. Stat5 was resistant to additional phosphorylation if cells were given a GH pulse within 2 h of a previous GH exposure. The refractory period lasted for 4 h, and cells could be maximally stimulated again after 6 h. Stat3 phosphorylation was also enhanced in cells that were allowed to recover from a previous application of GH. The tyrosine kinase inhibitors, genistein, PP1, and AG-490, and the MAPK kinase inhibitor, PD98059, did not block Stat3 or Stat5 phosphorylation. In contrast, WHI-P154, a Janus kinase-3 inhibitor, dose-dependently prevented Stat3, but not Stat5, phosphorylation. GH-inducible nuclear transport of Stat3 was likewise inhibited by WHI-P154. Most importantly, GH-dependent IGF-I mRNA expression was inhibited by WHI-P154. In contrast, IGF-I mRNA expression was inhibited by IGF-I peptide, and the effect of IGF-I was dominant over that of GH. IGF-I mRNA was regulated by both PI3K and MAPK signal transduction pathways, but IGF-I peptide signaled predominantly through a wortmannin-sensitive pathway to down-regulate its own mRNA. Our data suggest that Janus kinases (Jak2 or Jak3) and their downstream targets (Stat3 and Stat5) may play important roles in the expression of IGF-I mRNA and the myoblast response to GH. In addition, C2C12 cells appear to be a good model system to examine GH regulation of Janus kinase/Stat signaling and the regulation of IGF-I mRNA.

Rapamycin impairs recovery from acute renal failure: role of cell-cycle arrest and apoptosis of tubular cells

The immunosuppressive effect of rapamycin is mediated by inhibition of interleukin-2-stimulated T cell proliferation. We report for the first time that rapamycin also inhibits growth factor-induced proliferation of cultured mouse proximal tubular (MPT; IC(50) ~1 ng/ml) cells and promotes apoptosis of these cells by impairing the survival effects of the same growth factors. On the basis of these in vitro data, we tested the hypothesis that rapamycin would impair recovery of renal function after ischemic acute renal failure induced in vivo by renal artery occlusion (RAO). Rats given daily injections of rapamycin or vehicle were subjected to RAO or sham surgery. Rapamycin had no effect on the glomerular filtration rate (GFR) of sham-operated animals. In rats subjected to RAO, GFR fell to comparable levels 1 day later in vehicle- and rapamycin-treated rats (0.25 +/- 0.08 and 0.12 +/- 0.05 ml. min(-1). 300 g(-1), respectively) (P = not significant). In vehicle-treated rats subjected to RAO, GFR increased to 0.61 +/- 0.08 ml. min(-1). 300 g(-1) on day 3 (P < 0.02 vs. day 1) and then rose further to 0.99 +/- 0.09 ml. min(-1). 300 g(-1) on day 4 (P < 0.02 vs. day 3). By contrast, GFR did not improve in rapamycin-treated rats subjected to RAO over the same time period. Rapamycin also increased apoptosis of tubular cells while markedly reducing their proliferative response after RAO. Furthermore, rapamycin inhibited activation of 70-kDa S6 protein kinase (p70(S6k)) in cultured MPT cells as well as in the renal tissue of rats subjected to RAO. We conclude that rapamycin severely impairs the recovery of renal function after ischemia-reperfusion injury. This effect appears to be due to the combined effects of increased tubular cell loss (via apoptosis) and profound inhibition of the regenerative response of tubular cells. These effects are likely mediated by inhibition of p70(S6k).

Growth hormone/insulin-like growth factors axis in children undergoing cardiac surgery

OBJECTIVES

To describe the sequential changes in the growth hormone (GH)/insulin-like growth factors axis and their relationship with nitrogen balance in children following cardiac surgery.

DESIGN

Prospective, descriptive study.

SETTING

Pediatric intensive care unit of a university hospital.

PATIENTS

Twenty three postoperative cardiac surgical patients after bypass.

INTERVENTIONS

Blood and urine samples were taken on days 1, 2, and 7 of pediatric intensive care unit admission. An intraanesthesia, presurgery sample was also obtained.

MEASUREMENTS AND MAIN RESULTS

Serum concentrations of insulin, insulin growth factor-I (IGF-I), insulin growth factor binding proteins 1 and 3 (IGFBP-1 and IGFBP-3), growth hormone binding protein (GHBP), and urinary concentrations of GH and free cortisol (UFC) were measured on days 1, 2, and 7 of the study period. C-reactive protein and prealbumin, were also measured in blood samples as conventional markers of inflammatory or nutritional status. Pediatric Risk of Mortality II score and UFC were used as indicators of acute stress. The nitrogen balance and urinary nitrogen urea excretion were used as markers of catabolic state. Urinary concentrations of GH were high from days 1 to 7. Plasma concentrations of IGF-I and GHBP were low and remained low throughout the study period. IGFBP-3 levels were below normal but without reaching statistical significance. The IGFBP-1 levels were initially high but descended progressively toward normal values. Urinary nitrogen urea production was persistently elevated and was associated with a negative nitrogen balance. No relationship was found between nitrogen balance and IGF-I, prealbumin, or C-reactive protein.

CONCLUSIONS

A GH-resistant state is observed in postoperative children following cardiac surgery. Stress response is characterized by an elevation of growth hormone secretion that is not followed by the corresponding increment in IGF-I and IGFBP-3 concentrations. These hormonal changes may be permissive for the catabolic state of these patients. IGF-I and IGFBP-1 and -3 are not related to either nitrogen balance or urinary nitrogen urea excretion.

Comparison of micronutrients in patients having had panproctocolectomy and either ileal pouch anal anastomosis or Brooke ileostomy for chronic ulcerative colitis (UC)

OBJECTIVE

Patients having panproctocolectomy undergo major metabolic changes. A recent study suggested that patients who have had a panproctocolectomy and ileal pouch-anal anastomosis (IPAA) may be trace element-deficient, while other recent evidence has suggested a gradual decrease in vitamin B12. This study was undertaken to compare patients who had a panproctocolectomy for UC in combination with either an IPAA or a Brooke ileostomy (BI), and to determine whether the type of surgery post-proctocolectomy influences the absorption of trace elements, as well as comparing the levels after both operations with the normal population values.

PATIENTS AND METHODS

One hundred randomly selected patients who had had a panproctocolectomy for UC (50 IPAA, 50 BI) were invited to take part in the study by letter. The patients who consented had blood taken for haemoglobin, serum iron, ferritin, serum folate, red cell folate, vitamin B12, insulin-like growth factor-1 (IGF-1), albumin, and the trace elements copper, magnesium, manganese, selenium, and zinc.

RESULTS

Of the 100 patients, 46 consented to participate in the study (23 IPAA, 23 BI). The age of the BI group was significantly higher than those of the IPAA group (mean age IPAA 44 years, BI 52 years, P < 0.05). There was no significant difference between the two groups with respect to time since operation, mean levels of haemoglobin, iron indices, albumin, serum and red cell folate, vitamin B12, or any of the trace elements examined. Plasma IGF-1 was higher in the IPAA group, but this was no longer significant when adjusted for age.

CONCLUSION

No difference was found in trace element status in patients who had had a panproctocolectomy for UC with either an IPAA or BI. Furthermore, no difference existed between these two groups.

Role of the suppressor of cytokine signaling-3 in mediating the inhibitory effects of interleukin-1beta on the growth hormone-dependent transcription of the acid-labile subunit gene in liver cells

During catabolic diseases such as sepsis, inflammation, and infection, a state of growth hormone (GH) resistance develops in liver. This has been attributed in part to increased production of the proinflammatory cytokine interleukin-1beta (IL-1beta). To determine how IL-1beta induces GH resistance, we studied the acid-labile subunit (ALS) gene whose hepatic transcription is increased by GH via the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway. IL-1beta reduced the ability of GH to stimulate ALS mRNA in rat primary hepatocytes and ALS promoter activity in H4-II-E rat hepatoma cells. This inhibition was dependent on ALSGAS1, an element resembling a gamma-interferon activated sequence that mediates the transcriptional effects of GH. Inhibition by IL-1beta was also associated with a reduction of GH-dependent binding of STAT5 to this element after chronic (8 and 24 h), but not after acute treatment (15 min). Because these results indicated that the inhibition by IL-1beta was indirect, expression of the recently discovered suppressors of cytokine action (SOCS) was examined in liver cells. IL-1beta did not alter the expression of SOCS1, SOCS2, and CIS, indicating that they are not involved. In contrast, IL-1beta increased SOCS3 mRNA by 8-fold after 24 h of treatment, whereas GH had no effect. Forced expression of SOCS3 was just as effective as IL-1beta in reducing the GH induction of ALS promoter activity in H4-II-E rat hepatoma cells. Similar results were observed in primary rat hepatocytes. We conclude that the induction of SOCS3 by IL-1beta contributes to the development of GH resistance in liver, and represents a mechanism by which cytokines such as IL-1beta cross-talk with cytokines using the JAK-STAT pathway.

Differential effects of insulin-like growth factor I (IGF-I) and IGF-binding protein-1 on protein metabolism in human skeletal muscle cells

Insulin-like growth factor-binding protein-1 (BP-1) is a multifunctional protein that binds IGF-I in solution and integrins on the cell surface. BP-1 is overexpressed during catabolic illnesses, and the protein accumulates in skeletal muscle. To define a potential physiological role for BP-1 in regulating muscle protein balance, we have examined the effect of IGF-I and BP-1 on protein synthesis and degradation in human skeletal muscle cells. IGF-I-stimulated protein synthesis by 20%, and this was completely inhibited by either phosphorylated or nonphosphorylated BP-1. Half-maximal inhibition of protein synthesis occurred at a molar ratio of BP-1 to IGF-I of 1.5:1. BP-1 failed to form a complex with a truncated form of IGF-I (desIGF-I), and consequently, BP-1 failed to inhibit the ability of desIGF-I to stimulate protein synthesis. IGF-I and BP-1 dose-dependently inhibited protein degradation individually, and both BP-1 phosphovariants failed to block the ability of IGF-I to do the same. Blocking integrin receptor occupancy with the integrin antagonist echistatin blunted the ability of BP-1 to inhibit protein degradation, but had no significant effect on IGF-I-mediated changes in protein synthesis or degradation. The extracellular matrix protein vitronectin also inhibited protein degradation, but vitronectin receptor antibodies failed to block BP-1 action. In contrast, antibodies to the beta1 integrin subunit blocked BP-1-mediated inhibition of protein degradation. Rapamycin inhibited IGF-I-dependent protein synthesis, but not the ability of IGF-I to inhibit proteolysis. In contrast, rapamycin completely blocked the ability of BP-1 to inhibit proteolysis. Our results demonstrate that BP-1 inhibits IGF-I-mediated protein synthesis by binding to IGF-I. BP-1, acting independently of IGF-I, inhibits protein degradation. The IGF-independent response occurs via beta1 integrin binding and stimulation of a rapamycin-sensitive signal transduction pathway.

Anabolic hormones in critically ill patients

Critical illness and severe injury induce a number of neuroendocrine changes that initially enhance survival. However, for skeletal muscle, the hormonal environment is such that accelerated catabolism predominates. The prevailing hypercortisolemia is exacerbated by a resistance to or decreased production of anabolic hormones, or both. The resulting loss of lean body mass compromises the patient's ability to handle acute complications and impairs efforts towards rehabilitation. For these reasons, efforts have been focused upon the restoration of the anabolic influence. Recent studies continue to demonstrate that the amelioration of muscle catabolism is possible with hormonal management. Emerging technologies and conservative administration regimens hold promise for the development of the clinical ability to restore anabolic influence, while minimizing undesirable side effects.