Functional interleukin-6 receptor-α is located in tanycytes at the base of the third ventricle.

Interleukin (IL)-6- /- mice develop mature onset obesity, whereas i.c.v. injection of IL-6 decreases obesity in rodents. Moreover, levels of IL-6 in cerebrospinal fluid (CSF) were reported to be inversely correlated with obesity in humans. Tanycytes lining the base of the third ventricle (3V) in the hypothalamus have recently been reported to be of importance for metabolism. In the present study, we investigated whether tanycytes could respond to IL-6 in the CSF. With immunohistochemistry using a well characterised antibody directed against the ligand binding receptor for IL-6, IL-6 receptor α (IL-6Rα), it was found that tanycytes, identified by the two markers, vimentin and dopamine- and cAMP-regulated phosphoprotein of 32 kDa, contained IL-6Rα. There were fewer IL-6Rα on another type of ventricle-lining cells, ependymal cells, as identified by the marker glucose transporter-1. To demonstrate that the immunoreactive IL-6Rα were responsive to IL-6, we injected IL-6 i.c.v. This treatment increased immunoreactive phosphorylated signal transducer and activator of transcription-3 (pSTAT3) in tanycytes after 5 minutes and in cells in the medial part of the arcuate nucleus after 5 and 15 minutes. Intracerebroventricular injection of leptin exerted similar effects. As expected, i.p. injection of leptin also induced pSTAT3 staining in the hypothalamus, whereas i.p. IL-6 injection had little effect on this parameter. Intracerebroventricular or i.p. injection of vehicle only had no effect on pSTAT3-immunoreactivity. In summary, there are functional IL-6Rα on tanycytes at the bottom of the 3V, in agreement with the possibility that ventricular administration of IL-6 decreases obesity in mice via an effect on this cell type.

Female Mice Lacking Estrogen Receptor-α in Hypothalamic Proopiomelanocortin (POMC) Neurons Display Enhanced Estrogenic Response on Cortical Bone Mass.

Estrogens are important regulators of bone mass and their effects are mainly mediated via estrogen receptor (ER)α. Central ERα exerts an inhibitory role on bone mass. ERα is highly expressed in the arcuate (ARC) and the ventromedial (VMN) nuclei in the hypothalamus. To test whether ERα in proopiomelanocortin (POMC) neurons, located in ARC, is involved in the regulation of bone mass, we used mice lacking ERα expression specifically in POMC neurons (POMC-ERα(-/-)). Female POMC-ERα(-/-) and control mice were ovariectomized (OVX) and treated with vehicle or estradiol (0.5 µg/d) for 6 weeks. As expected, estradiol treatment increased the cortical bone thickness in femur, the cortical bone mechanical strength in tibia and the trabecular bone volume fraction in both femur and vertebrae in OVX control mice. Importantly, the estrogenic responses were substantially increased in OVX POMC-ERα(-/-) mice compared with the estrogenic responses in OVX control mice for cortical bone thickness (+126 ± 34%, P < .01) and mechanical strength (+193 ± 38%, P < .01). To test whether ERα in VMN is involved in the regulation of bone mass, ERα was silenced using an adeno-associated viral vector. Silencing of ERα in hypothalamic VMN resulted in unchanged bone mass. In conclusion, mice lacking ERα in POMC neurons display enhanced estrogenic response on cortical bone mass and mechanical strength. We propose that the balance between inhibitory effects of central ERα activity in hypothalamic POMC neurons in ARC and stimulatory peripheral ERα-mediated effects in bone determines cortical bone mass in female mice.

Ghrelin.

BACKGROUND: The gastrointestinal peptide hormone ghrelin was discovered in 1999 as the endogenous ligand of the growth hormone secretagogue receptor. Increasing evidence supports more complicated and nuanced roles for the hormone, which go beyond the regulation of systemic energy metabolism. SCOPE OF REVIEW: In this review, we discuss the diverse biological functions of ghrelin, the regulation of its secretion, and address questions that still remain 15 years after its discovery. MAJOR CONCLUSIONS: In recent years, ghrelin has been found to have a plethora of central and peripheral actions in distinct areas including learning and memory, gut motility and gastric acid secretion, sleep/wake rhythm, reward seeking behavior, taste sensation and glucose metabolism.

Inter-relation between interleukin (IL)-1, IL-6 and body fat regulating circuits of the hypothalamic arcuate nucleus.

Interleukin (IL)-1 and IL-6 are immune modulating cytokines that also affect metabolic function because both IL-1 receptor I deficient (IL-1RI⁻/⁻) and IL-6 deficient (IL-6⁻/⁻) mice develop late-onset obesity and leptin resistance. Both IL-1 and IL-6 appear to target the central nervous system (CNS) to increase energy expenditure. The hypothalamic arcuate nucleus (ARC) is a major relay between the periphery and CNS in body fat regulation (e.g. by being a target of leptin). The present study aimed to investigate the possible mechanisms responsible for the effects exerted by endogenous IL-1 and IL-6 on body fat at the level of the ARC, as well as possible interactions between IL-1 and IL-6. Therefore, we measured the gene expression of neuropeptides of the ARC involved in energy balance in IL-1RI⁻/⁻ and IL-6⁻/⁻ mice. We also investigated the interactions between expression of IL-1 and IL-6 in these mice, and mapped IL-6 receptor α (IL-6Rα) in the ARC. The expression of the obesity promoting peptide neuropeptide Y (NPY), found in the ARC, was increased in IL-1RI⁻/⁻ mice. The expression of NPY and agouti-related peptide (AgRP), known to be co-expressed with NPY in ARC neurones, was increased in cold exposed IL-6⁻/⁻ mice. IL-6Rα immunoreactivity was densely localised in the ARC, especially in the medial part, and was partly found in NPY positive cell bodies and also α-melanocyte-stimulating hormone positive cell bodies. The expression of hypothalamic IL-6 was decreased in IL-1RI⁻/⁻ mice, whereas IL-1ß expression was increased in IL-6⁻/⁻ mice. The results of the present study indicate that depletion of the activity of the fat suppressing cytokines IL-1 and IL-6 in knockout mice can increase the expression of the obesity promoting neuropeptide NPY in the ARC. Depletion of IL-1 activity suppresses IL-6 expression, and IL-6Rα-like immunoreactivity is present in neurones in the medial ARC, including neurones containing NPY. Therefore, IL-6, IL-1 and NPY/AgRP could interact at the level of the hypothalamic ARC in the regulation of body fat.

Interleukin-6 receptor α is co-localised with melanin-concentrating hormone in human and mouse hypothalamus.

Interleukin (IL)-6 deficient mice develop mature-onset obesity. Furthermore, i.c.v. administration of IL-6 increases energy expenditure, suggesting that IL-6 centrally regulates energy homeostasis. To investigate whether it would be possible for IL-6 to directly influence the energy homeostasis via hypothalamic regulation in humans and rodents, we mapped the distribution of the ligand binding IL-6 receptor α (IL-6Rα) in this brain region. In the human hypothalamus, IL-6Rα-immunoreactivity was detected in perikarya and first-order dendrites of neurones. The IL-6Rα-immunoreactive (-IR) neurones were observed posterior to the level of the interventricular foramen. There, IL-6Rα-IR neurones were located in the lateral hypothalamic, perifornical, dorsal and posterior hypothalamic areas, the hypothalamic dorsomedial nucleus and in the zona incerta. In the caudal part of the hypothalamus, the density of the IL-6Rα-IR neurones gradually increased. Double-labelling immunofluorescent studies demonstrated that IL-6Rα immunoreactivity was localised in the same neurones as the orexigenic neuropeptide, melanin-concentrating hormone (MCH). By contrast, IL-6Rα-immunoreactivity was not observed in the orexin B-IR neurones. To determine whether the observed expression of IL-6Rα is evolutionary conserved, we studied the co-localisation of IL-6Rα with MCH and orexin in the mouse hypothalamus, where IL-6Rα-immunoreactivity was present in numerous MCH-IR and orexin-IR neurones. Our data demonstrate that the MCH neurones of the human hypothalamus, as well as the MCH and orexin neurones of the mouse hypothalamus, contain IL-6Rα. This opens up the possibility that IL-6 influences the energy balance through the MCH neurones in humans, and both MCH and orexin neurones in mice.

Interleukin-6-deficient mice develop hepatic inflammation and systemic insulin resistance.

AIMS/HYPOTHESIS: The role of IL-6 in the development of obesity and hepatic insulin resistance is unclear and still the subject of controversy. We aimed to determine whether global deletion of Il6 in mice (Il6 (-/-)) results in standard chow-induced and high-fat diet (HFD)-induced obesity, hepatosteatosis, inflammation and insulin resistance. METHODS: Male, 8-week-old Il6 (-/-) and littermate control mice were fed a standard chow or HFD for 12 weeks and phenotyped accordingly. RESULTS: Il6 (-/-) mice displayed obesity, hepatosteatosis, liver inflammation and insulin resistance when compared with control mice on a standard chow diet. When fed a HFD, the Il6 (-/-) and control mice had marked, equivalent gains in body weight, fat mass and ectopic lipid deposition in the liver relative to chow-fed animals. Despite this normalisation, the greater liver inflammation, damage and insulin resistance observed in chow-fed Il6 (-/-) mice relative to control persisted when both were fed the HFD. Microarray analysis from livers of mice fed a HFD revealed that genes associated with oxidative phosphorylation, the electron transport chain and tricarboxylic acid cycle were uniformly decreased in Il6 (-/-) relative to control mice. This coincided with reduced maximal activity of the mitochondrial enzyme ß-hydroxyacyl-CoA-dehydrogenase and decreased levels of mitochondrial respiratory chain proteins. CONCLUSIONS/INTERPRETATION: Our data suggest that IL-6 deficiency exacerbates HFD-induced hepatic insulin resistance and inflammation, a process that appears to be related to defects in mitochondrial metabolism.

A variant near the interleukin-6 gene is associated with fat mass in Caucasian men.

CONTEXT: Regulation of fat mass appears to be associated with immune functions. Studies of knockout mice show that endogenous interleukin (IL)-6 can suppress mature-onset obesity. OBJECTIVE: To systematically investigate associations of single nucleotide polymorphisms (SNPs) near the IL-6 (IL6) and IL-6 receptor (IL6R) genes with body fat mass, in support for our hypothesis that variants of these genes can be associated with obesity. DESIGN AND STUDY SUBJECTS: The Gothenburg Osteoporosis and Obesity Determinants (GOOD) study is a population-based cross-sectional study of 18- to 20-year-old men (n=1049), from the Gothenburg area (Sweden). Major findings were confirmed in two additional cohorts consisting of elderly men from the Osteoporotic Fractures in Men (MrOS) Sweden (n=2851) and MrOS US (n=5611) multicenter population-based studies. MAIN OUTCOME: The genotype distributions and their association with fat mass in different compartments, measured with dual-energy X-ray absorptiometry. RESULTS: Out of 18 evaluated tag SNPs near the IL6 and IL6R genes, a recently identified SNP rs10242595 G/A (minor allele frequency=29%) 3' of the IL6 gene was negatively associated with the primary outcome total body fat mass (effect size -0.11 standard deviation (s.d.) units per A allele, P=0.02). This negative association with fat mass was also confirmed in the combined MrOS Sweden and MrOS US cohorts (effect size -0.05 s.d. units per A allele, P=0.002). When all three cohorts were combined (n=8927, Caucasian subjects), rs10242595(*)A showed a negative association with total body fat mass (effect size -0.05 s.d. units per A allele, P<0.0002). Furthermore, the rs10242595(*)A was associated with low body mass index (effect size -0.03, P<0.001) and smaller regional fat masses. None of the other SNPs investigated in the GOOD study were reproducibly associated with body fat. CONCLUSIONS: The IL6 gene polymorphism rs10242595(*)A is associated with decreased fat mass in three combined cohorts of 8927 Caucasian men.

Interleukin-6 gene knockout influences energy balance regulating peptides in the hypothalamic paraventricular and supraoptic nuclei.

Interleukin (IL)-6 is a pro-inflammatory cytokine that also affects metabolic function because IL-6 depleted (IL-6(-/-)) mice develop late-onset obesity. IL-6 appears to act in the central nervous system, presumably in the hypothalamus, to increase energy expenditure that appears to involve stimulation of the sympathetic nervous system. In the present study, we explored possible central mechanisms for the effects exerted by IL-6 on body fat. Therefore, we measured the effects of IL-6 depletion in IL-6(-/-) mice on expression of key hypothalamic peptide genes involved in energy balance by the real time polymerase chain reaction. Additionally, co-localisation between such peptides and IL-6 receptor alpha was investigated by immunohistochemistry. IL-6 deficiency decreased the expression of several peptides found in the paraventricular nucleus (PVN), which is a nucleus that has been attributed an adipostatic function. For example, corticotrophin-releasing hormone (CRH), which is reported to stimulate the sympathetic nervous system, was decreased by 40% in older IL-6(-/-) mice. Oxytocin, which is reported to prevent obesity, was also decreased in older IL-6(-/-) animals, as was arginine vasopressin (AVP). The IL-6 receptor alpha was abundantly expressed in the PVN, but also in the supraoptic nucleus, and was shown to be co-expressed to a high extent with CRH, AVP, oxytocin and thyrotrophin-releasing hormone. These data indicate that depletion of endogenous IL-6, a body fat suppressing cytokine, is associated with the decreased expression of CRH and oxytocin (i.e. energy balance regulating peptides) as well as AVP in the PVN. Because IL-6 receptor alpha is co-expressed with CRH, oxytocin and AVP, IL-6 could stimulate the expression of these peptides directly.

Variants of the interleukin-1 receptor antagonist gene are associated with fat mass in men.

CONTEXT: Immune functions seem to have connections to variations in body fat mass. Studies of knockout mice indicate that endogenous interleukin (IL)-1 can suppress mature-onset obesity. OBJECTIVE: To systematically investigate our hypotheses that single-nucleotide polymorphisms (SNPs) and/or haplotypes variants in the IL-1 gene system are associated with fat mass. SUBJECTS: The Gothenburg osteoporosis and obesity determinants (GOOD) study is a population-based cross-sectional study of 18-20 year-old men (n=1068), from Gothenburg, Sweden. Major findings were confirmed in elderly men (n=3014) from the Swedish part of the osteoporotic fractures in men (MrOS) multicenter population-based study. MAIN OUTCOME MEASURE: The genotype distributions and their association with body fat mass in different compartments, measured with dual-energy X-ray absorptiometry (DXA). RESULTS: Out of 15 investigated SNPs in the IL-1 receptor antagonist (IL1RN) gene, a recently identified 3' untranslated region C>T (rs4252041, minor allele frequency=4%) SNP was associated with the primary outcome total fat mass (P=0.003) and regional fat masses, but not with lean body mass or serum IL-1 receptor 1 (IL1RN) levels. This SNP was also associated with body fat when correcting the earlier reported IL1RN+2018 T>C (rs419598) SNP (in linkage disequilibrium with a well-studied variable number tandem repeat of 86 bp). The association between rs4252041 SNP and body fat was confirmed in the older MrOS population (P=0.03). The rs4252041 SNP was part of three haplotypes consisting of five adjacent SNPs that were identified by a sliding window approach. These haplotypes had a highly significant global association with total body fat (P<0.001). None of the other investigated members of the IL-1 gene family displayed any SNPs that have not been described previously to be significantly associated with body fat. CONCLUSIONS: The IL1RN gene, shown to enhance obesity by suppressing IL-1 effects in experimental animals, have not [corrected] previously described gene polymorphisms and haplotypes that are associated with fat, but not lean mass in two populations of men.

A missense mutation in the non-neural G-protein alpha-subunit isoforms modulates susceptibility to obesity.

OBJECTIVE: The Gnas transcription unit located within an imprinting region encodes several proteins, including the G-protein alpha-subunit, Gsalpha, its isoform XLalphas and their variant truncated neural forms GsalphaN1 and XLN1. Gsalpha and GsalphaN1 are expressed predominantly from the maternally derived allele in some tissues, whereas XLalphas and XLN1 are expressed exclusively from the paternally derived allele. The relative contribution of full-length Gsalpha and XLalphas, and truncated forms GsalphaN1 and XLN1 to phenotype is unknown. The edematous-small point mutation (Oed-Sml) in exon 6 of Gnas lies downstream of GsalphaN1 and XLN1, but affects full-length Gsalpha and XLalphas, allowing us to address the role of full-length Gsalpha and XLalphas. The aim of this study was therefore to determine the metabolic phenotypes of Oed and Sml mice, and to correlate phenotypes with affected transcripts. METHODS: Mice were fed standard or high-fat diets and weighed regularly. Fat mass was determined by DEXA analysis. Indirect calorimetry was used to measure metabolic rate. Glucose was measured in tolerance tests and biochemical parameters in fasted plasma samples. Histological analysis of fat and liver was carried out post mortem. RESULTS: Oed mice are obese on either diet and have a reduced metabolic rate. Sml mice are lean and are resistant to a high-fat diet and have an increased metabolic rate. CONCLUSION: Adult Oed and Sml mice have opposite metabolic phenotypes. On maternal inheritance, the obese Oed phenotype can be attributed to non-functional full-length Gsalpha. In contrast, on paternal inheritance, Sml mice were small and resistant to the development of obesity on a high-fat diet, effects that can be attributed to mutant XLalphas. Thus, the neural isoforms, GsalphaN1 and XLN1, do not appear to play a role in these metabolic phenotypes.

Endocrine, liver-derived IGF-I is of importance for spatial learning and memory in old mice.

IGF-I is a neuroprotective hormone, and neurodegenerative disorders, including Alzheimer's disease, have been associated with decreased serum IGF-I concentration. In this study, IGF-I production was inactivated in the liver of adult mice (LI-IGF-I(-/-)), resulting in an approximately 80-85% reduction of circulating IGF-I concentrations. In young (6-month-old) mice there was no difference between the LI-IGF-I(-/-) and the control mice in spatial learning and memory as measured using the Morris water maze test. In old (aged 15 and 18 months) LI-IGF-I(-/-) mice, however, the acquisition of the spatial task was slower than in the controls. Furthermore, impaired spatial working as well as reference memory was observed in the old LI-IGF(-/-) mice. Histochemical analyses revealed an increase in dynorphin and enkephalin immunoreactivities but decreased mRNA levels in the hippocampus of old LI-IGF-I(-/-) mice. These mice also displayed astrocytosis and increased metabotropic glutamate receptor 7a-immunoreactivity. These neurochemical disturbances suggest synaptic dysfunction and early neurodegeneration in old LI-IGF-I(-/-) mice. The decline in serum IGF-I with increasing age may therefore be important for the age-related decline in memory function.

Ghrelin treatment reverses the reduction in weight gain and body fat in gastrectomised mice.

BACKGROUND AND AIMS: The gastric hormone ghrelin has been reported to stimulate food intake, increase weight gain, and cause obesity but its precise physiological role remains unclear. We investigated the long term effects of gastrectomy evoked ghrelin deficiency and of daily ghrelin injections on daily food intake, body weight, fat mass, lean body mass, and bone mass in mice. METHODS: Ghrelin was given by subcutaneous injections (12 nmol/mouse once daily) for eight weeks to young female mice subjected to gastrectomy or sham operation one week previously. RESULTS: Gastrectomy reduced plasma concentrations of total ghrelin (octanoylated and des-octanoylated) and active (octanoylated) ghrelin by approximately 80%. Immediately after injection of ghrelin, the plasma concentration was supraphysiological and was still elevated 16 hours later. Daily food intake was not affected by either gastrectomy or ghrelin treatment. The effect of ghrelin on meal initiation was not studied. At the end point of the study, mean body weight was 15% lower in gastrectomised mice than in sham operated mice (p<0.001); daily ghrelin injections for eight weeks partially prevented this weight loss. In sham operated mice, ghrelin had no effect on body weight. The weight of fat was reduced in gastrectomised mice (-30%; p<0.01). This effect was reversed by ghrelin, enhancing the weight of fat in sham operated mice also (+20%; p<0.05). Gastrectomy reduced lean body mass (-10%; p<0.01) and bone mass (-20%; p<0.001) compared with sham operated mice. Ghrelin replacement prevented the gastrectomy induced decrease in lean body mass but did not affect bone. In sham operated mice, ghrelin affected neither of these two parameters. CONCLUSIONS: Ghrelin replacement partially reversed the gastrectomy induced reduction in body weight, lean body mass, and body fat but not in bone mass. In sham operated mice, ghrelin only increased fat mass. Our results suggest that ghrelin is mainly concerned with the control of fat metabolism and that ghrelin replacement therapy may alleviate the weight loss associated with gastrectomy.

A common polymorphism in the interleukin-6 gene promoter is associated with overweight.

OBJECTIVE: Human body fat mass is to a large extent genetically determined, but little is known about the susceptibility genes for common obesity. Interleukin-6 (IL-6) suppresses body fat mass in rodents, and IL-6 treatment increases energy expenditure in both rodents and humans. The -174 G/C single-nucleotide polymorphism (SNP) in the IL-6 gene promoter is common in many populations, and -174 C-containing promoters have been found to be weaker enhancers of transcription. Moreover, a SNP at position -572 in the IL-6 promoter has recently been reported to affect transcription. The objective was to investigate the association between the IL-6 gene promoter SNPs and obesity. DESIGN: Trans-sectional association study of IL-6 gene promoter SNPs and indices of obesity. SUBJECTS: Two study populations, the larger one consisting of hypertensive individuals (mean age 57 y, 73% males, n=485) and the other consisting of 20 y younger nonobese healthy females (n=74). MEASUREMENTS: Genotyping for the -174 IL-6 G/C and the -572 G/C SNPs, body mass index (BMI), serum leptin levels, serum IL-6 levels, C-reactive protein, fasting blood glucose and various blood lipids. RESULTS: The common -174 C allele (f(C)=0.46), but not any -572 allele, was associated with higher BMI and higher serum leptin levels in both study populations. In the larger population, there were significant odds ratios for the association of CC (2.13) and GC (1.76) genotypes with overweight (BMI>25 kg/m(2)). Moreover, as the C allele was common, it accounted for a significant population-attributable risk of overweight (12%; CI 2-21%), although its average effect was modest in this sample. CONCLUSION: Genetically determined individual differences in production of IL-6 may be relevant for the regulation of body fat mass.

A transgenic model to determine the physiological role of liver-derived insulin-like growth factor I.

Insulin-like growth factor-I (IGF-I) has important growth promoting and metabolic effects and is expressed in virtually every tissue of the body. The highest expression is found in the liver but the physiological role of liver-derived IGF-I is unknown. It has been difficult to separate the endocrine effects of liver-derived IGF-I from the autocrine/paracrine effects of locally produced IGF-I in peripheral tissues. Therefore, we have developed a mouse model with a liver-specific inducible deletion of the IGF-I gene. The liver-IGF-I deficient mouse have dramatically reduced (>80%) serum IGF-I levels, demonstrating that the major part of serum IGF-I is liver-derived. Surprisingly, liver-IGF-I deficient mice demonstrate a normal appendicular skeletal growth up to at least 12 months of age despite the dramatic decrease in circulating IGF-I levels, indicating that liver-derived IGF-I is not required for appendicular skeletal growth. However, the adult axial skeletal growth is clearly reduced in the liver-IGF-I deficient mice. Furthermore, the amount of cortical bone is reduced due to decreased radial growth of the cortical bone while the amount of trabecular bone is unchanged in the liver-IGF-I deficient mice. The decreased levels of circulating IGF-I are associated with increased serum levels of growth hormone (GH), indicating a role for liver-derived IGF-I in the negative feedback regulation of GH secretion. Measurements of factors regulating GH-secretion in the pituitary and in the hypothalamus revealed an increased expression of growth hormone releasing hormone (GHRH) and growth hormone secretagogue (GHS) receptors in the pituitary of liver-IGF-I deficient mice. This in turn results in an increased sensitivity to systemically administered GHRH and GHS, demonstrating that the regulatory action of liver-derived IGF-I on GH secretion is at the pituitary rather than at the hypothalamic level. The liver is an important metabolic organ and liver-IGF-I deficient mice are markedly hyperinsulinemic and yet normoglycemic, consistent with an adequately compensated insulin resistance. Interestingly, liver-IGF-I deficient mice display a reduced age-dependent fat mass accumulation compared with control mice. In conclusion, liver-derived IGF-I is important for carbohydrate- and lipid-metabolism and for the regulation of GH-secretion at the pituitary level. Furthermore, it regulates adult axial skeletal growth and cortical radial growth while it is not required for appendicular skeletal growth.

Metabolic functions of liver-derived (endocrine) insulin-like growth factor I.

Until now it has been difficult to determine the relative importance of locally produced (autocrine/paracrine) versus systemically derived (endocrine) insulin-like growth factor I (IGF-I) in the intact organism. We recently eliminated IGF-I production in the livers of mice using the Cre/loxP recombination system. These mice displayed a reduction in serum IGF-I levels of more than 80%, but demonstrated normal body growth, suggesting that autocrine/paracrine-acting IGF-I, but not endocrine-acting IGF-I, regulates body growth. Long-term metabolic studies of mice in which IGF-I production had been inactivated in the liver, have shown that the mice have decreased fat mass, but increased serum levels of insulin and cholesterol. Despite the marked increase in plasma insulin following glucose administration, the glucose elimination was not altered in these animals. Thus, the mice showed an adequately compensated insulin resistance. In conclusion, liver-derived or endocrine IGF-I is not required for post-natal statural growth, but seems to be of vital importance for normal carbohydrate and lipid metabolism.

Liver-derived IGF-I regulates GH secretion at the pituitary level in mice.

We have reported that liver-specific deletion of IGF-I in mice (LI-IGF-I-/-) results in decreased circulating IGF-I and increased GH levels. In the present study, we determined how elimination of hepatic IGF-I modifies the hypothalamic-pituitary GH axis to enhance GH secretion. The pituitary mRNA levels of GH releasing factor (GHRF) receptor and GH secretagogue (GHS) receptor were increased in LI-IGF-I-/- mice, and in line with this, their GH response to ip injections of GHRF and GHS was increased. Expression of mRNA for pituitary somatostatin receptors, hypothalamic GHRF, somatostatin, and neuropeptide Y was not altered in LI-IGF-I-/- mice, whereas hypothalamic IGF-I expression was increased. Changes in hepatic expression of major urinary protein and the PRL receptor in male LI-IGF-I-/- mice indicated an altered GH release pattern most consistent with enhanced GH trough levels. Liver weight was enhanced in LI-IGF-I-/- mice of both genders. In conclusion, loss of liver-derived IGF-I enhances GH release by increasing expression of pituitary GHRF and GHS receptors. The enhanced GH release in turn affects several liver parameters, in line with the existence of a pituitary-liver axis.

The somatomedin hypothesis revisited in a transgenic model.

Studies of insulin-like growth factor I (IGF-I) gene knockout mice models have clearly shown that IGF-I is necessary for prenatal as well as postnatal body growth in mice. Clinical studies of a patient with an IGF-I gene defect which caused complete absence of IGF-I, verified that it is important for intrauterine and postnatal growth. Recent studies of mice with liver-specific and inducible IGF-I gene knockout indicated that liver-derived IGF-I is not necessary for postnatal body growth, although serum IGF-I levels are decreased by more than 80% in these mice. Therefore, extrahepatic IGF-I is sufficient for maintenance of postnatal body growth in mice. Further investigations are needed to assess whether liver-derived circulating IGF-I is essential for other biological functions.

Liver-derived IGF-I is of importance for normal carbohydrate and lipid metabolism.

IGF-I is important for postnatal body growth and exhibits insulin-like effects on carbohydrate metabolism. The function of liver-derived IGF-I is still not established, although we previously demonstrated that liver-derived IGF-I is not required for postnatal body growth. Mice whose IGF-I gene in the liver was inactivated at 24 days of age were used to investigate the long-term role of liver-derived IGF-I for carbohydrate and lipid metabolism. Serum levels of leptin in these mice were increased by >100% at 3 months of age, whereas the fat mass of the mice was decreased by 25% at 13 months of age. The mice became markedly hyperinsulinemic and yet normoglycemic, indicating an adequately compensated insulin resistance. Furthermore, they had increased serum levels of cholesterol. We conclude that liver-derived IGF-I is of importance for carbohydrate and lipid metabolism.

Retarded liver growth in interleukin-6-deficient and tumor necrosis factor receptor-1-deficient mice.

The liver size in adult mammals is tightly regulated in relation to body weight, but the hormonal control of this is largely unknown. We investigated the roles of interleukin-6 (IL-6) and tumor necrosis factor (TNF) receptor-1 in the regulation of intact liver weight in adult mice. The relative liver wet and dry weights of older adult (5- to 10-month-old) IL-6 knockout (IL-6(-/-)) mice were decreased by 22-28%, and total contents of DNA and protein were decreased compared with those in age-matched wild-type mice. Weights of other visceral organs were unaffected. Older adult (6- to 8-month-old) TNF receptor-1 knockout (TNFR1(-/-)) mice displayed decreased relative liver weight. Treatment with a single injection of IL-6 increased liver wet and dry weights in IL-6(-/-) and wild-type mice, but not TNFR1(-/-) mice. Treatment with TNFalpha enhanced liver weight and DNA synthesis of nonparenchymal liver cells at 24 h in wild-type, but not IL-6(-/-), mice. At 48 h, TNFalpha induced DNA synthesis in nonparenchymal cells and hepatocytes of both wild-type and IL-6(-/-) mice. In conclusion, TNF receptor-1 stimulation and IL-6 production are both necessary for normal liver weight gain in older adult mice. The results of TNFalpha and IL-6 treatment further indicate that the effects of TNF receptor-1 and IL-6 depend on each other for full stimulation of liver growth.

Possible roles of insulin-like growth factor in regulation of physiological and pathophysiological liver growth.

BACKGROUND/AIMS: Almost all circulating insulin-like growth factor-1 (IGF-1) is produced and secreted from the liver. However, the possible role of IGF-1 in local regulation of liver functions including liver growth is unclear. In the present study, we investigated the role of IGF-1 on liver growth in vivo and in hepatic stellate cell function in vitro. RESULTS: Liver-specific knock-out of the IGF-1 gene by use of the cre-loxP system caused enhanced liver growth, possibly reflecting increased growth hormone (GH) secretion due to decreased negative feedback by IGF-1. Studies on cultured rat hepatic stellate cells (HSC) showed that IGF-1 and hepatocyte-conditioned medium (PCcM) time- and dose-dependently increased hepatocyte growth factor (HGF) mRNA and HGF immunoreactivity. IGF-1 and PCcM also enhanced DNA synthesis in the HSC cultures. The PCcM did not contain bioactive IGF-1 and was also able to stimulate proliferation when prepared under serum- and hormone-free conditions. CONCLUSION: In vivo results show that IGF-1 is not essential for normal growth of the intact liver. The in vitro results indicate that both IGF-1 and IGF- 1-independent factor(s) from hepatocytes can stimulate HGF production by HSC. It remains to be investigated whether these effects are of importance for liver regeneration or pathological conditions.