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 β-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 18-20 years old men (n=1 049), 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=2 851) and MrOS US (n=5 611) 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 (DXA).

Results

Out of 18 evaluated tag single nucleotide polymorphisms (SNPs) near the IL6 and IL6R genes, a recently identified SNP rs10242595 G/A [minor allele frequency (MAF) = 29%] 3′ of the IL6 gene was negatively associated with the primary outcome total body fat mass (effect size -0.11 standard deviation (SD) units/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 SD units/A allele; P=0.002). When all three cohorts were combined (n= 8 927, Caucasian subjects), rs10242595*A showed a negative association with total body fat mass (effect size -0.05 SD units/A allele, P<0.0002). Furthermore, the rs10242595*A was associated with low body mass index [(BMI, 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 8 927 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.

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.

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.

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.

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.

Effects of growth hormone and its secretagogues on bone

The growth hormone (GH)/insulin-like growth factor-1 axis is not only of importance for linear body growth during childhood, but it is also one of the major determinants of adult bone mass. Studies show that GH treatment increases bone mass in rodents as well as in adult GH-deficient humans, but the effect of GH treatment on bone mass in healthy humans has so far not been impressive. Recently, a new class of GH secretagogues (GHSs) has been developed. In humans, GHS treatment affects biochemical markers of bone turnover and increases growth velocity in selected short children with or without GH deficiency. In rodents, GHS treatment increase bone mineral content, but it has not yet been shown that GHS treatment can affect bone mass in adult humans.

Growth hormone (GH)-independent stimulation of adiposity by GH secretagogues

Growth hormone secretagogues (GHSs) stimulate growth hormone (GH) secretion, which is lipolytic. Here we compared the effects of twice daily s.c. treatment of GH and the GHS, ipamorelin, on body fat in GH-deficient (lit/lit) and in GH-intact (+/lit and +/+) mice. In +/lit and lit/lit mice ipamorelin induced a small (15%) increase in body weight by 2 weeks, that was not further augmented by 9 weeks. GH treatment markedly enhanced body weight in both groups. Ipamorelin also increased fat pad weights relative to body weight in both lit/lit and +/lit mice. Two weeks GHS treatment (ipamorelin or GHRP-6) also increased relative body fat, quantified by in vivo dual energy X-ray absorpiometry (DEXA) in GH-intact mice. GH decreased relative fat mass in lit/lit mice and had no effect in GH-intact mice. Treatment with GHS, but not GH, increased serum leptin and food intake in GH-intact mice. Thus, GHSs increase body fat by GH-independent mechanisms that may include increased feeding.

Normal pharmacologically-induced, but decreased regenerative liver growth in interleukin-6-deficient (IL-6(-/-)) mice

BACKGROUND/AIMS

In the absence of liver damage, rapid liver growth can be induced pharmacologically by so-called primary liver growth promoters. The importance of the acute-phase cytokines interleukin-6 and tumor necrosis factor-alpha for the actions of these compounds is not clear. This study aimed to investigate the importance of IL-6 and TNF-receptor-1 in pharmacologically-induced liver growth.

METHODS

IL-6 knockout (IL-6(-/-)), TNF-receptor-1 knockout (TNFR1(-/-)) and wild-type mice were treated with the peroxisome proliferator nafenopin and the anti-androgen cyproterone acetate (CPA) in one single injection or for 6 days with daily injections, and examined at 24 or 48 h after treatment. In a control experiment, IL-6(-/-) mice were subjected to two-thirds partial hepatectomy.

RESULTS

Nafenopin treatment increased relative liver weight and DNA synthesis similarly in IL-6(-/-), TNFR1(-/-) and wild-type mice. CPA increased liver weight similarly in all groups, but did not increase DNA synthesis. Expression of peroxisome proliferator activated receptor-alpha mRNA was increased in both IL-6(-/-) and wild-type mice by nafenopin treatment, but not by CPA treatment. After hepatectomy DNA synthesis was suppressed in IL-6(-/-) mice compared to wild-type mice.

CONCLUSIONS

Liver growth induced by nafenopin and CPA was not dependent on the presence of IL-6 or TNF receptor-1, whereas liver regeneration was decreased in IL-6(-/-) mice.

Effects of growth hormone and insulinlike growth factor-I on body growth and adult bone metabolism

The anabolic action of growth hormone (GH) on bone is well demonstrated by the short stature and delayed bone maturation in children with GH deficiency and in acromegalic patients with increased cortical bone mass. The body growth is regulated by growth hormone and insulin-like growth factor-I (IGF-I). The classic somatomedin hypothesis of this regulation is that most IGF-I in the blood originates in the liver and that body growth is controlled by the concentration of IGF-I in the blood. We have recently abolished IGF-I production in the livers of mice by using the Cre/loxP recombination system. The mice, in which IGF-I production had been inactivated in the liver, displayed a more than 80% reduction in serum IGF-I. In contrast, they demonstrated a normal postnatal growth, indicating that extrahepatic, autocrine/paracrine-acting IGF-I is the main determinant of postnatal growth. GH is also important for normal adult bone remodeling. Adults with GH deficiency have reduced bone mass, and GH treatment increases bone mass in GH-deficient adults. Future clinical studies will determine whether some patients with decreased bone mass for other reasons will benefit from treatment with GH alone or in combination with other treatments.

The relative importance of endocrine versus autocrine/paracrine insulin-like growth factor-I in the regulation of body growth

Body growth is regulated by growth hormone (GH) and insulin-like growth factor-I (IGF-I). The classical somatomedin hypothesis of this regulation is that most IGF-I in the blood originates in the liver and that body growth is controlled by the concentration of IGF-I in the blood. We have recently abolished IGF-I production in the livers of mice by using the Cre/loxP recombination system. These mice displayed a more than 75% reduction in serum IGF-I associated with increased serum levels of GH. In contrast, they demonstrated a normal postnatal growth, indicating that extrahepatic, autocrine/paracrine-acting IGF-I is the main determinant of postnatal growth. Thus, the "classical" somatomedin hypothesis needs revision. We propose the "dual somatomedin hypothesis" according to which: (1) autocrine/paracrine IGF-I is the main determinant of postnatal body growth and (2) liver-derived, endocrine-acting, IGF-I exerts negative feedback on GH secretion and possibly also exerts other effects on carbohydrate and lipid metabolism.

The GH secretagogues ipamorelin and GH-releasing peptide-6 increase bone mineral content in adult female rats

Growth hormone (GH) is of importance for normal bone remodelling. A recent clinical study demonstrated that MK-677, a member of a class of GH secretagogues (GHSs), increases serum concentrations of biochemical markers of bone formation and bone resorption. The aim of the present study was to investigate whether the GHSs, ipamorelin (IPA) and GH-releasing peptide-6 (GHRP-6), increase bone mineral content (BMC) in young adult female rats. Thirteen-week-old female Sprague-Dawley rats were given IPA (0.5 mg/kg per day; n=7), GHRP-6 (0.5 mg/kg per day; n=8), GH (3.5 mg/kg per day; n=7), or vehicle administered continuously s.c. via osmotic minipumps for 12 weeks. The animals were followed in vivo by dual X-ray absorptiometry (DXA) measurements every 4th week. After the animals were killed, femurs were analysed in vitro by mid-diaphyseal peripheral quantitative computed tomography (pQCT) scans. After this, excised femurs and vertebrae L6 were analysed by the use of Archimedes' principle and by determinations of ash weights. All treatments increased body weight and total tibial and vertebral BMC measured by DXA in vivo compared with vehicle-treated controls. However, total BMC corrected for the increase in body weight (total BMC:body weight ratio) was unaffected. Tibial area bone mineral density (BMD, BMC/area) was increased, but total and vertebral area BMDs were unchanged. The pQCT measurements in vitro revealed that the increase in the cortical BMC was due to an increased cross-sectional bone area, whereas the cortical volumetric BMD was unchanged. Femur and vertebra L6 volumes were increased but no effect was seen on the volumetric BMDs as measured by Archimedes' principle. Ash weight was increased by all treatments, but the mineral concentration was unchanged. We conclude that treatment of adult female rats with the GHSs ipamorelin and GHRP-6 increases BMC as measured by DXA in vivo. The results of in vitro measurements using pQCT and Archimedes' principle, in addition to ash weight determinations, show that the increases in cortical and total BMC were due to an increased growth of the bones with increased bone dimensions, whereas the volumetric BMD was unchanged.

Characterization of hepatocyte-derived mitogenic activity on hepatic stellate cells

BACKGROUND/AIMS

Hepatic stellate cells (HSC) are located in close proximity to hepatocytes in Disse's space. Hepatocyte derived factors have earlier been implicated in the paracrine regulation of HSC proliferation. The aim of the present study was to further characterize this mitogenic activity of the parenchymal cell conditioned medium (PCcM).

METHODS

Primary rat HSC were cultured for 4 days. DNA synthesis was measured by 3H-thymidine incorporation. TGFbeta1 immunoreactivity was quantified by ELISA. PCcM was obtained from hepatocytes cultured in medium without serum or hormones for two days.

RESULTS

Incubation of 4-day-old HSC on plastic surface with PCcM for 2 days increased DNA synthesis, while no effect was seen in HSC cultured on Matrigel. Heat-, acid-, and protease-treatment of PCcM abolished its stimulatory effect. Size fractionations with spin columns indicated that the stimulatory effect was contained in the fractions of a molecular size between 30 and 100 kD. The addition of LY 294002, a phosphatidylinositol 3-kinase (PI3-K) inhibitor, dose-dependently inhibited the PCcM induced increase in DNA synthesis to about 9% of the control values. The specific MAP kinase (MAPK) inhibitor, PD 98059 only suppressed the PCcM induced DNA synthesis to 35% of control cultures at the highest dose (10 microM). DNA content in the cultures was not affected by either blocker. HSC seemed to produce immunoreactive TGFbeta1. However, addition of latency-associated peptide (LAP), a potent TGFbeta1 blocker, stimulated DNA synthesis to a much less extent than PCcM.

CONCLUSIONS

The factor(s) that stimulate DNA synthesis in HSC from hepatocytes are most likely protein(s) with a molecular size between 30-100 kD. These factor(s) rely more on PI3-K than on MAPK for their mitogenic effect and are probably not acting via TGFbeta1 inhibition.

Growth hormone induces CCAAT/enhancer binding protein alpha (C/EBPalpha) in cultured rat hepatocytes

BACKGROUND/AIMS

The transcription factor CCAAT/enhancer binding protein alpha (C/EBPalpha) is a transactivator of several genes in the liver, which are regulated by growth hormone.

METHODS

Growth hormone (100 ng/ml) was added to primary rat hepatocytes cultured on a laminin-rich matrix. C/EBP mRNA and protein levels were measured by RNase protection assay and Western blotting, respectively. DNA binding activity was measured by electrophoretic mobility shift assay (EMSA).

RESULTS

Growth hormone treatment for 6 h to 3 days increased C/EBPalpha mRNA levels. Addition of growth hormone for 24 h and 4 days also enhanced the levels of the 42 and 30 kDa isoforms of immunoreactive C/EBPalpha. EMSA showed that addition of growth hormone for 24 h enhanced the abundance of a protein complex binding to a consensus C/EBP binding DNA oligonucleotide. This protein complex was supershifted by antibodies directed against C/EBPalpha but not against C/EBPbeta. There were no consistent effects on C/EBPbeta mRNA or protein at any timepoint. The growth hormone effect on C/EBPalpha expression was not affected by simultaneous incubation with insulin or glucocorticoids, two hormones that previously have been reported to affect C/EBPs.

CONCLUSIONS

Growth hormone enhances the levels of C/EBPalpha mRNA and protein as well as the DNA binding activity of C/EBPalpha in cultured rat hepatocytes.

Overexpression of the hepatocyte growth factor (HGF) receptor (Met) and presence of a truncated and activated intracellular HGF receptor fragment in locally aggressive/malignant human musculoskeletal tumors

Enhanced hepatocyte growth factor (HGF) receptor (Met) signaling has been suggested to play an important role in the development and progression of various epithelial and nonepithelial tumors. N-terminally truncated forms of the HGF receptor have been shown to be constitutively activated and tumorigenic in animal experiments. In the present study, 102 benign and malignant human musculoskeletal tumors were examined for expression of the HGF receptor by Western blotting and/or immunohistochemistry. A clear predominance of HGF receptor expression was seen in malignant as compared to benign tumors (Western blotting, P < 0.001; immunohistochemistry, P < 0.02). For the first time we show HGF receptor expression in the following four tumor types: dermatofibrosarcoma protuberans, clear cell sarcoma of tendons, malignant primitive neuroectodermal tumor, and benign fibrous histiocytoma. In three cases of sarcoma with high HGF receptor expression by Western blotting, we found indications of a short 85-kd N-terminally truncated HGF receptor that was tyrosine phosphorylated and located in the cytoplasm. Although fragments of this length were seen in 18 of 65 tumors, most were not tyrosine-phosphorylated. Northern blotting revealed only the 7.5-kb full-length HGF receptor transcript, suggesting that the 85-kd fragment is generated by an alternative initiation of translation or by proteolytic cleavage. Southern blotting detected no amplification of the Hgfr/Met gene in the 35 tumors examined, in contrast to our recent report of Hgfr/Met gene amplification in 7, 12-dimethylbenz(a)anthracene (DMBA)-induced rat sarcomas. The present data suggest that the locally aggressive and malignant properties of human mesenchymal tumors maybe related, in part, to high levels of full-length HGF receptors, and in some cases to the occurrence of N-terminally truncated HGF receptors, activated independently of HGF.

Insulin-like growth factor signaling pathways in rat hepatic stellate cells: importance for deoxyribonucleic acid synthesis and hepatocyte growth factor production

It has been shown recently that insulin-like growth factor 1 (IGF-1) increases both DNA synthesis and hepatocyte growth factor (HGF) production in cultured hepatic stellate cells. In this study, we used selective blockers to investigate crucial signaling pathways for these effects of IGF-1 in cultured rat hepatic stellate cells. Both LY 294002 [a phosphatidylinositol 3-kinase (PI3-K) inhibitor], and rapamycin [a blocker of activation of the serine/threonine p70 S6 kinase (p70S6K), a molecule downstream from PI3-K] completely reversed the IGF-1-induced stimulation of DNA synthesis. Mitogen-activated protein kinase (MAPK) inhibition by PD 98059 had a less pronounced suppressory effect, although the used PD 98059 dose was fully effective in inhibiting MAPK phosphorylation. Both LY 294002 and PD 98059 lowered the IGF-1-induced increase of HGF in the medium by about 40%, but LY 294002 was 10 times more potent than PD 98059. Inhibition of p70S6K activation by rapamycin blocked IGF-1-induced DNA synthesis but not the increase in HGF. In conclusion, PI3-K (and, to some extent, MAPK) signaling pathways seem to be important for IGF-1-stimulated DNA synthesis and HGF production. DNA synthesis also seems to be dependent on rapamycin-sensitive activation of the PI3-K effector p70S6K.

Liver-derived insulin-like growth factor I (IGF-I) is the principal source of IGF-I in blood but is not required for postnatal body growth in mice

The body growth of animals is regulated by growth hormone and IGF-I. The classical theory of this regulation is that most IGF-I in the blood originates in the liver and that body growth is controlled by the concentration of IGF-I in the blood. We have abolished IGF-I production in the livers of mice by using the Cre/loxP recombination system. These mice demonstrated complete inactivation of the IGF-I gene in the hepatocytes. Although the liver accounts for less than 5% of body mass, the concentration of IGF-I in the serum was reduced by 75%. This finding confirms that the liver is the principal source of IGF-I in the blood. However, the reduction in serum IGF-I concentration had no discernible effect on postnatal body growth. We conclude that postnatal body growth is preserved despite complete absence of IGF-I production by the hepatocytes.

Treatment of obese subjects with the oral growth hormone secretagogue MK-677 affects serum concentrations of several lipoproteins, but not lipoprotein(a)

Obesity is associated with blunted GH secretion and an unfavorable lipoprotein pattern. The objective of this study was to investigate the effects of treatment with the oral GH secretagogue MK-677 on lipoproteins in otherwise healthy obese males. The study was randomized, double blind, and parallel. Twenty-four obese males, aged 18-50 yr, with body mass index greater than 30 kg/m2 and waist/hip ratio above 0.95 were treated with 25 mg MK-677 (n = 12) or placebo (n = 12) daily for 8 weeks. MK-677 treatment did not significantly change serum lipoprotein(a) [Lp(a)] levels. Serum apolipoprotein A-I and E (apoA-I and apoE) were increased at 2 weeks (P < 0.001 and P < 0.01 vs. placebo, respectively), but were not changed at study end. Serum total cholesterol and low density lipoprotein (LDL) cholesterol (LDL-C) levels were not significantly changed by MK-677 treatment. Serum high density lipoprotein (HDL) cholesterol (HDL-C) was increased at 2 weeks of MK-677 treatment (P < 0.01 vs. placebo), but not at 8 weeks. The LDL-C/HDL-C ratio was reduced after 8 weeks of MK-677 treatment (P < 0.05 vs. placebo). Mean LDL particle diameter was decreased at 2 weeks (P < 0.05 vs. placebo), but was unchanged compared with baseline values at 8 weeks (P = NS vs. placebo). The level of serum triglycerides was increased at 2 (P < 0.05 vs. placebo), but not at 8, weeks. Lipoprotein lipase activity in abdominal and gluteal sc adipose tissue was not affected by active treatment. In conclusion, treatment with the oral GH secretagogue MK-677 affected circulating lipoproteins. The effects on serum apoA-1, apoE, triglycerides, and mean LDL particle diameter were transient. At study end, the LDL-C/HDL-C ratio was decreased. MK-677 treatment did not significantly affect serum Lp(a) concentrations at the present dose and administration protocol.

The growth hormone receptor associates with Jak1, Jak2 and Tyk2 in human liver

Studies in cell lines have shown that Jak2 is the primary tyrosine kinase involved in signal transduction by the growth hormone receptor (GHR). In addition, growth hormone (GH) stimulates tyrosine phosphorylation of Jak1 and Jak3 in certain cell lines, while the effect on Tyk2 has not been analysed. We have investigated the expression of Jak proteins in human liver and analysed their interactions with the GHR. Using Western blot analysis and immunohistochemistry, we demonstrate that Jak1, Jak2, Jak3 and Tyk2 are present in human liver. Immunoprecipitation by an antibody against the GHR (Mab 263) followed by immunoblotting with specific antibodies against Jak proteins showed that Jak1, Jak2 and Tyk2 were associated with the GHR in this tissue. We conclude that the GHR associates with Jak1, Jak2 and Tyk2 in human liver. Although experiments in vitro indicate that Jak2 mediates GH signalling, our results open the possibility that other Jak proteins may influence GHR signalling in human liver.

Amplification and overexpression of the hepatocyte growth factor receptor (HGFR/MET) in rat DMBA sarcomas

In the present study subcutaneous fibrosarcomas were induced by the carcinogen 7,12-dimethylbenz(a)anthracene (DMBA) in rats from F1 generation cross breedings of two different inbred strains. Comparative genomic hybridization (CGH) analysis, which allows detection of DNA sequence copy changes, was applied to one of the tumors and it was found that there were increased copy numbers of sequences at chromosome 4q12-q21 in this tumor. We have previously determined that the loci for the hepatocyte growth factor (Hgf) and hepatocyte growth factor receptor (Hgfr/Met), a protooncogene, are situated in this particular chromosome region. Using probes for the two genes in FISH (fluorescence in situ hybridization) and in Southern blots we found that the Hgfr/Met gene was amplified in five of the 19 sarcomas studied, and that the Hgf gene was coamplified in two of them. Northern and Western blots and tyrosine phosphorylation analysis showed that the HGF receptor was overexpressed and functional in all five tumors, as well as in two additional tumors. In summary, both amplification and overexpression of the Hgfr/Met gene was found in about 25% of DMBA-induced experimental rat sarcomas, and HGF receptor overexpression alone was seen in two additional tumors. Possibly this reflects an involvement in paracrine or autocrine stimulation of growth and invasiveness by HGF. Our finding could provide a rodent model system to increased knowledge about causality and therapy, which may be applicable to the sizeable fraction of human musculoskeletal tumors displaying MET overexpression.

Discrepancy between serum leptin values and total body fat in response to the oral growth hormone secretagogue MK-677

OBJECTIVE

Growth hormone (GH) treatment decreases total body fat while this effect has not yet been documented for the oral GH secretagogue MK-677. In the present study, the effects of MK-677 treatment on serum levels of leptin, thyroid hormones and testosterone were determined.

DESIGN

This was a randomized, double-blind, and parallel study. Twenty-four healthy obese males, 19-49 years of age, with body mass index (BMI) > 30 kg/m2 and a waist:hip ratio > 0.95, were treated with MK-677 (25 mg/day; n = 12) or placebo (n = 12) for 8 weeks.

RESULTS

MK-677 treatment increased serum leptin levels and leptin/body fat ratio at 2 weeks of treatment (P < 0.05 vs. placebo) but no significant change was observed at 8 weeks. An increase in serum free 3, 5, 3'-triiodothyronine (free T3) was not detected until 8 weeks of MK-677 treatment (P < 0.05 vs. placebo). Peak serum thyroid stimulating hormone (TSH) concentration after MK-677 administration was similar to that after placebo administration at initiation of treatment and at 2 weeks. At 8 weeks of MK-677 treatment, mean peak serum TSH concentration was increased (P < 0.05 vs. placebo) although it remained within the normal range. Serum peak values of luteinizing hormone (LH) and follicle stimulating hormone (FSH) were similar after MK-677 and placebo administration. MK-677 treatment reduced serum total testosterone (P < 0.05 vs. placebo) although total testosterone/sex hormone-binding globulin (SHBG) ratio (an index of free testosterone) was not changed.

CONCLUSION

Treatment with the oral GH secretagogue MK-677 transiently increased serum leptin levels and leptin/body fat ratio at 2 weeks of treatment, and increased serum free T3 after 8 weeks. These results indicate that MK-677 treatment is able to affect circulating factors of importance for adipose tissue mass and fuel metabolism.

Hepatocyte-stimulated expression of hepatocyte growth factor (HGF) in cultured rat hepatic stellate cells

BACKGROUND/AIMS

Hepatic stellate cells appear to be the main producers of hepatocyte growth factor of the normal liver. Insulin-like growth factors in doses over 20 ng/ml have been reported to stimulate hepatocyte growth factor production in cultured hepatic stellate cells. The aim of the present study was to investigate whether parenchymal cell conditioned medium had insulin-like growth factor-independent effects on hepatic stellate cells.

METHODS

Primary rat hepatic stellate cells were cultured for 1-7 days. DNA synthesis was measured by 3H-thymidine incorporation. Hepatocyte growth factor and transforming growth factor beta1 immunoreactivity was quantified by ELISA. Hepatocyte growth factor mRNA levels were determined with gel RNase protection assay. Parenchymal cell conditioned medium was obtained from hepatocytes cultured for 2 days in medium without added serum or hormones.

RESULTS

Incubation of 1-7-day-old hepatic stellate cells for 2 days with parenchymal cell conditioned medium enhanced the medium content of hepatocyte growth factor. Parenchymal cell conditioned medium contained less than 5.0 ng/ml immunoreactive insulin-like growth factor-1 as measured by radio immunoassay. Parenchymal cell conditioned medium did not contain any insulin-like growth factor bioactivity measured as phosphorylation of type 1 insulin-like growth factor receptor beta subunit and a protein with a size consistent with that of insulin receptor substrate-1. The stimulatory effect of parenchymal cell conditioned medium on hepatocyte growth factor was time- and dose-dependent. The effects of a high dose of parenchymal cell conditioned medium (dilution 1:2 containing less than 2.5 ng/ml insulin-like growth factor-1) were additive to that of high doses (100 ng/ml) of insulin-like growth factor-1 or des (1-3) insulin-like growth factor-1, an analogue with low affinity to insulin-like growth factor binding proteins. Neither parenchymal cell conditioned medium nor insulin-like growth factor-1 enhanced transforming growth factor beta1 immunoreactivity in the medium. Both parenchymal cell conditioned medium and insulin-like growth factor-1 stimulated DNA synthesis in hepatic stellate cells, confirming previous reports.

CONCLUSIONS

The present results indicate that both insulin-like growth factor-1 and insulin-like growth factor-1-independent factors from hepatocytes can stimulate hepatocyte growth factor production by hepatic stellate cells. Therefore, insulin-like growth factor-1 and other hepatocyte-derived factors may indirectly affect hepatocytes via a paracrine loop.

Treatment with the oral growth hormone secretagogue MK-677 increases markers of bone formation and bone resorption in obese young males

The effect of 2 months of treatment with the oral growth hormone (GH) secretagogue MK-677 on markers of bone metabolism was determined in healthy obese male subjects. This was a randomized, double-blind, parallel, placebo-controlled study. Twenty-four healthy obese males, 19-49 years of age, with body mass index > 30 kg/m2 were treated with MK-677 (25 mg/day; n = 12) or placebo (n = 12) for 8 weeks. MK-677 increased markers of bone formation; a 23% increase in the carboxy-terminal propeptide of type I procollagen levels and a 28% increase in procollagen III peptide levels were seen with as little as 2 weeks of MK-677 treatment (p < 0.01 and p = 0.001 vs. placebo, respectively) while a 15% increase in serum levels of osteocalcin was not detected until 8 weeks of treatment (p < 0.01 vs. placebo). Markers of bone resorption were induced within 2 weeks of treatment with MK-677; serum levels of the carboxy-terminal cross-linked telopeptide of type I collagen were increased 26% at 8 weeks (p = 0.001 vs. placebo), and urine hydroxyproline/creatinine and calcium/creatinine ratios at 8 weeks were increased by 23% (p < 0.05 vs. placebo) and 46% (p < 0.05 vs placebo), respectively, MK-677 increased serum insulin-like growth factor binding protein-5 (IGFBP-5) by 43-44% after 2-8 weeks of treatment (p < 0.01 vs. placebo). Serum IGFBP-4 was increased by 25% after 2 weeks of treatment (p < 0.001 vs. placebo) but no significant change from baseline was observed after 8 weeks of treatment. Plasma interleukin-6 was not significantly changed by active treatment. In conclusion, short-term treatment of healthy obese male volunteers with the GH secretagogue MK-677 increases markers of both bone resorption and formation. Large increases in serum levels of IGF-1 and IGFBP-5 and a transient increase in serum IGFBP-4 were found. Future long-term studies are needed to investigate if prolonged treatment with MK-677 increases bone mass.

Two-month treatment of obese subjects with the oral growth hormone (GH) secretagogue MK-677 increases GH secretion, fat-free mass, and energy expenditure

Obesity is associated with blunted GH secretion, unfavorable body composition, and increased cardiovascular mortality. The objective of this study was to investigate the effects of oral treatment with the GH secretagogue MK-677 on GH secretion and body composition in otherwise healthy obese males. The study was randomized, double blind, parallel, and placebo controlled. Twenty-four obese males, aged 18-50 yr, with body mass indexes greater than 30 kg/m2 and waist/hip ratios greater than 0.95, were treated with MK-677 25 mg (n = 12) or placebo (n = 12) daily for 8 weeks. Serum insulin-like growth factor I (IGF-I) increased approximately 40% with MK-677 treatment (P < 0.001 vs. placebo). Serum IGF-binding protein-3 was also significantly increased (P < or = 0.001 vs. placebo). GH and PRL (peak and area under the curve values) were significantly increased after the initial dose of MK-677. Significant increases, with the exception of peak PRL, persisted at 2 and 8 weeks of treatment. The increases in GH and PRL after the initial dose were significantly greater than the increase seen after multiple doses. Serum and urinary concentrations of cortisol were not increased at 2 and 8 weeks (P = NS, vs. placebo). Fat-free mass increased significantly in the MK-677 treatment group when determined with dual energy x-ray absorptiometry (P < 0.01) or using a four-compartment model (P < 0.05). Total and visceral fat were not significantly changed with active therapy. The basal metabolic rate was significantly increased at 2 weeks of MK-677 treatment (P = 0.01) but not at 8 weeks (P = 0.1). Fasting concentrations of glucose and insulin were unchanged, whereas an oral glucose tolerance test showed impairment of glucose homeostasis at 2 and 8 weeks. We conclude that 2-month treatment with MK-677 in healthy obese males caused a sustained increase in serum levels of GH, IGF-I, and IGF-binding protein-3. The effects on cortisol secretion were transient. Changes in body composition and energy expenditure were of an anabolic nature, with a sustained increase in fat-free mass and a transient increase in basal metabolic rate. Further studies are needed to evaluate whether a higher dose of MK-677 or a more prolonged treatment period can promote a reduction in body fat.

Acromegaly and Cushing's syndrome due to ectopic production of GHRH and ACTH by a thymic carcinoid tumour: in vitro responses to GHRH and GHRP-6

A 50-year-old male presented with diabetes mellitus and Cushing's syndrome associated with a large mediastinal mass. The levels of serum cortisol were high (1500-1800 nmol/l) without diurnal variation. Plasma ACTH levels (200-250 ng/l) and urinary excretion of cortisol were also increased. The levels of these hormones did not change in response to stimulation with corticotrophin releasing hormone (CRH) or suppression with high doses of dexamethasone. The patient had an elevated baseline GH level (7.3 mU/l), and the levels of immunoreactive GH-releasing hormone (GHRH) in eight plasma samples were markedly increased (600-1500 ng/l). Circulating levels of IGF-1, chromogranin A and neuropeptide Y (NPY) were also increased. Computer-assisted tomography and octreotide scintigraphy revealed a large mediastinal tumour and metastases in the left supraclavicular fossa. During treatment with octreotide, the baseline GH level was decreased (to 4.4 mU/l), while the GH pulse height was unchanged. Surgical removal of most of the tumour tissue resulted in a further decrease in the baseline serum GH level to a value (1.6 mU/l) about 20% of that before treatment, while the pulse height and mean GH were affected to a lesser extent. Postoperatively, circulating levels of cortisol and IGF-1 decreased, and the patient exhibited clinical improvement. Histological examination showed a neuroendocrine tumour with characteristics consistent with a foregut carcinoid of thymic origin. Immunoreactive GHRH, ACTH and NPY, but not immunoreactive GH, were detected in 80-90% of the tumour cells and the three peptides appeared to be co-localized. In primary culture, cells from this tumour displayed calcium influx in response to GHRH or GH releasing peptide-6 (GHRP-6), while there were not such responses by cells from another carcinoid not producing GHRH, ACTH or NPY. These results demonstrate a rare case of ectopic production of GHRH, ACTH and NPY, and indicate that the tumour cells were responsive to GHRH and GHRP-6 as well as octreotide.

Insulin-like growth factors stimulate expression of hepatocyte growth factor but not transforming growth factor beta1 in cultured hepatic stellate cells

Hepatic stellate cells (HSC) are located adjacent to hepatocytes and produce hepatocyte growth factor (HGF) in the normal liver, whereas transformed HSC in fibrotic livers produce transforming growth factor beta1 (TGFbeta1), an inhibitor ofhepatocyte proliferation. In addition to the endocrine actions of hepatic insulin-like growth factor-I (IGF-I), it also stimulates the proliferation of HSC. In this study we found that addition of IGF-1 (20-500 ng/ml) for 48 h to 2- to 7-day-old primary cultures of rat HSC resulted in a time- and dose-dependent increase by 50-190% of the concentrations of immunoreactive HGF in the medium. The levels of HGF as well as DNA synthesis measured as thymidine incorporation were also enhanced by IGF-II and des(1-3)IGF-I, which has reduced binding to IGF binding proteins. There was no consistent effect of the IGFs on the levels of immunoreactive TGFbeta1 or on the total DNA content of the cultures. There was no effect of human GH on medium levels of HGF or TGFbeta1, thymidine incorporation, or total DNA content. IGF-I increased the abundance of HGF messenger RNA, as measured by the RNase protection/solution hybridization technique, whereas there was no effect on TGFbeta1 or glyceraldehyde phosphate dehydrogenase messenger RNA. The results suggest that IGFs stimulate the production of HGF but not TGFbeta1 by HSC in vitro.

Changes in expression of CCAAT/enhancer binding protein alpha (C/EBP alpha) and C/EBP beta in rat liver after partial hepatectomy but not after treatment with cyproterone acetate

BACKGROUND/AIMS

The proliferation rate of adult rat liver is normally very low. It is markedly enhanced during compensatory regeneration, e.g. after partial hepatectomy, or after administration of certain growth promoters, e.g. cyproterone acetate. These two types of liver cell proliferation appear to differ, since the expression of immediate early genes is induced during compensatory regeneration but not after cyproterone acetate treatment. The transcription factor C/EBP alpha, which has been associated with hepatocyte differentiation and growth arrest, is suppressed during compensatory regeneration. In contrast, C/EBP beta, associated with acute phase reaction, is increased during regeneration. We have investigated the effects of the liver growth promoter cyproterone acetate on the hepatic expression of C/EBP alpha and C/EBP beta.

METHODS

Adult male rats received either cyproterone acetate treatment or were subjected to partial hepatectomy. Livers were obtained at different time intervals for measurement of C/EBP alpha and C/EBP beta mRNA with solution hybridization/RNAse protection assay, and C/EBP alpha and C/EBP beta content with immunoblotting.

RESULTS

The levels of both C/EBP alpha and C/EBP beta mRNA and the corresponding immunoreactivities were unchanged 2-48 h after injection of cyproterone acetate. The levels of C/EBP alpha mRNA and immunoreactivity were significantly suppressed 10-18 h and 18-26 h after partial hepatectomy, respectively. The levels of C/EBP beta mRNA and immunoreactivity were enhanced during compensatory regeneration 2 h after partial hepatectomy.

CONCLUSIONS

Liver cell proliferation during regeneration, but not in response to cyproterone acetate treatment, is associated with changes in C/EBP alpha and C/EBP beta expression. This further supports the notion that changes in expression of transcription factors during liver growth in vivo are dependent on the growth inducer.

Chromosomal localization of rat hepatocyte growth factor (Hgf) and HGF receptor (Met) and characterization of HGF receptor cDNA

The Met protooncogene encodes the tyrosine kinase receptor for the hepatocyte growth factor (HGF), a potent mitogen for hepatocytes and other epithelial cells produced by mesenchymal cells. Many of the studies on the physiologic and neoplastic growth of the liver, as well as other organs, have been performed in the rat. Therefore, chromosomal mapping of the rat Hgf gene and the gene of its receptor is of particular value. To achieve this, a probe of the coding part of rat HGF cDNA was used to isolate four genomic probes from a lambda phage rat genomic library. These probes were used to map the Hgf gene to Chromosome (Chr) 4q12 by the FISH technique. To obtain a probe for the mapping of the HGF receptor/Met gene, we cloned the complete coding region of the rat HGF receptor mRNA. Complementary DNA (cDNA) was synthesized with reverse transcriptase from total RNA for use as a template for the PCR. The two PCR primers were designed based on human and mouse sequences and were located in the flanking regions of the open reading frame of the HGF receptor mRNA. Amplification resulted in a band of an estimated size of 4.1 kb, which was cloned and sequenced. The nucleotide sequence showed about 93% and 85% homology compared with mouse and human HGF receptor sequences, respectively. A full-length probe of the coding part of the cDNA was used to map the rat HGF receptor/Met gene to Chr 4q21 by the FISH technique. Therefore, the rat Hgf and HGF receptor/Met genes are located relatively close to each other, in a way similar to humans but not mice.

Topical zinc oxide treatment increases endogenous gene expression of insulin-like growth factor-1 in granulation tissue from porcine wounds

Application of zinc oxide has been shown to accelerate the healing of both chronic and acute wounds, but the mechanisms are unknown. We quantified the gene expression (mRNA) for one important growth factor, insulin-like growth factor-1 (IGF-1) in 12 full-thickness wounds in each of three domestic pigs treated with or without topical zinc oxide. We used a RNAase protection/solution hybridisation technique to measure IGF-1 mRNA concentrations, which were 50% higher in the granulation tissue in wounds treated with zinc oxide compared with control wounds on days 3-4 (p < 0.05), but not thereafter (up to postoperative day 11). Topical zinc oxide increased the healing rate of wounds compared to the control group (p < 0.01). The cell composition of the granulation tissue was similar in the two groups. The increased gene expression of IGF-1 may be one mechanism by which topical zinc oxide enhances wound healing.

Increased gene expression of scatter factor-hepatocyte growth factor and basic fibroblast growth factor in granulation tissue in the rat

Scatter factor-hepatocyte growth factor is a protein secreted by fibroblasts which disperses colonies of epithelial cells and keratinocytes in culture. The factor is also a patent mitogen for hepatocytes, synthesized in the liver. Basic fibroblast growth factor, another heparin-binding factor, is most abundant in the brain but also plays a role in wound healing. Using a solution hybridization/RNAase protection assay, we have measured the abundance of messenger RNA for scatter factor-hepatocyte growth factor and basic fibroblast growth factor in granulation tissue obtained from subcutaneously Hunt-Schilling wound cylinders. The levels of scatter factor-hepatocyte growth factor messenger RNA increased after weeks 2 through 4 to a twofold higher level in weeks 5 through 7 after implantation of the cylinders, whereas no changes in basic fibroblast growth factor messenger RNA levels were noticed. At week 3 after implantation of the cylinders, scatter factor-hepatocyte growth factor messenger RNA levels in granulation tissue were more than threefold higher than in skin dermis fibroblasts but markedly lower than in the liver. The abundance of basic fibroblast growth factor messenger RNA was also significantly increased in granulation tissue compared with dermis but, as expected, markedly lower than in the brain. In conclusion, the gene expression of the scatter factor-hepatocyte growth factor, as well as basic fibroblast growth factor, is increased in granulation tissue. Because there was a time-dependent increase in the expression of scatter factor-hepatocyte growth factor, it is hypothesized that scatter factor-hepatocyte growth factor acts as a signal from fully developed granulation tissue to stimulate skin epithelial cells to scatter over the wound.

Growth hormone promotes early initiation of hepatocyte growth factor gene expression in the liver of hypophysectomized rats after partial hepatectomy

GH accelerates hepatic regeneration in the rat. Hepatocyte growth factor (HGF), a potent hepatocyte mitogen in vitro, is considered to be a major regulator of hepatic regeneration. In the present study, the effects of GH and insulin-like growth factor-I (IGF-I) on HGF gene expression in regenerating rat liver was investigated. In hypophysectomized rats treated with GH, hepatic HGF mRNA levels were increased 3 h after partial hepatectomy and reached peak levels after 5 h. In rats with intact pituitaries and in hypophysectomized rats not given GH treatment, HGF mRNA levels in liver were unchanged during the first 5 h following hepatectomy and reached peak levels after 10-18 h. DNA synthesis in the liver of GH-treated rats increased from low levels 10 h after hepatectomy to peak levels after 18 h. In rats without GH treatment the synthesis of DNA was still low 18 h after hepatectomy and was increased after 26 h. Treatment of hypophysectomized rats with IGF-I promoted increases in hepatic HGF mRNA levels and DNA synthesis 3.5 h and 15 h after hepatectomy respectively. HGF mRNA levels were constantly lower after sham-hepatectomy than after partial hepatectomy. In summary, in hypophysectomized rats the responses of hepatic HGF gene expression and DNA synthesis to partial hepatectomy were both accelerated by treatment with GH or IGF-I.

Roles of pulsatility and continuity of growth hormone (GH) administration in the regulation of hepatic GH-receptors, and circulating GH-binding protein and insulin-like growth factor-I

GH secretion in the male rat is characterized by regular GH peaks of high amplitude, and their GH receptors and GH-binding protein (BP) are low. Female rats have a more continuous secretion, and their GH receptors and GH-BP are high. To determine how the pattern of GH delivery may affects the physiological responses to the hormone, hypophysectomized male rats were infused with human GH (1.5 U/kg.day) for 5 days through an implanted iv cannula in either a pulsatile manner (for 5 min every 3 h) or continuously. We then measured free and total (after MgCl2 treatment) somatogenic and lactogenic receptors in hepatic cell membranes, and GH-BP and immunoreactive insulin-like growth factor-I (IGF-I) in serum. The concentrations of occupied, but not free, somatogenic and lactogenic binding sites were higher 5 min after a 5-min infusion of human GH than 90 min after the infusion. Consequently, the total levels (free plus ligand binding) of somatogenic and lactogenic receptors were also higher 5 min after a GH pulse than after 90 min. The levels of circulating GH-BP were higher 90 min than 5 min after a GH pulse, and the serum IGF-I levels did not differ between these time points. The concentrations of free and total somatogenic and free lactogenic receptors in the liver, as well as GH-BP and IGF-I in circulation were higher and body weight gain was lower in rats given continuous GH infusion than after pulsatile infusions. It is concluded that the nature of GH-levels profile has a marked influence on the interrelationship of GH, GH receptors, GH-BP, serum IGF-I and growth, whereby the later is stimulated maximally by pulsatile GH-levels, while the receptor and GH-BP are enhanced maximally by continuous levels. The basis for this discrepancy may lay in the competitive power of GH-BP toward GH receptor binding. A pulsatile GH pattern induced cyclicity in the levels of hepatic cell membrane GH receptors and circulating GH-BP. It is suggested that these factors are regulated in a similar way by the plasma GH pattern in pituitary intact rats.

Gene expression of insulin-like growth factor-I and IGF-I receptor during wound healing in rats

OBJECTIVE

To measure the time course of changes in the concentrations of insulin-like growth factor-I (IGF-I), IGF-I messenger RNA (mRNA), and IGF-I receptor mRNA during wound healing.

DESIGN

Open experimental study.

MATERIAL

40 male Sprague-Dawley rats, each weighing 300 g.

INTERVENTION

Stainless steel wire mesh cylinders implanted in the subcutaneous tissue of the back. Five rats killed at each time point (1.5, 2, 3, 4, 5, 7, 11, and 18 weeks).

MAIN OUTCOME MEASURES

Wet weight and concentrations of IGF-I mRNA, and IGF-I receptor mRNA of granulation tissue, and concentration of IGF-I in wound fluid.

RESULTS

The wet weight of granulation tissue increased significantly between week 1.5 and weeks 4-5, and then decreased. IGF-I mRNA concentration (amol/microgram DNA) increased significantly (threefold) between week 1.5 and weeks 3-5, and decreased between weeks 5 and 7. The concentration of IGF-I receptor RNA remained constant throughout the study, and the concentration of IGF-I in wound fluid remained constant until week 8 and then increased to a higher level at weeks 11-18.

CONCLUSION

There is a transient rise in the gene expression of IGF-I but not of IGF-I receptor mRNA during wound healing.

Growth hormone stimulates granulation tissue formation and insulin-like growth factor-I gene expression in wound chambers in the rat

It has been reported that GH stimulates fibroblast growth and wound healing. In the present study we measured the effect of locally applied GH on insulin-like growth factor (IGF-I) mRNA concentrations and granulation tissue formation in wound cylinders, implanted s.c. Four stainless-steel wiremesh cylinders were implanted s.c. in the back of male rats (280 g). Each cylinder was then injected every day with either 0.014 or 0.14 U human GH, or vehicle only. Ingrown granulation tissue and wound fluid was obtained on day 17 after implantation. The wet weight of granulation tissue was determined and concentrations of IGF-I mRNA in the tissue were measured by solution hybridization/RNAase protection assay. Similar assays were used to measure the levels of IGF-I receptor mRNA and GH receptor mRNA, while the IGF-I concentration in wound fluid and serum was determined by radioimmunoassay (RIA) after acid-ethanol extraction. The concentrations of IGF-I mRNA in ingrown granulation tissue as well as the wet weight of this tissue were significantly higher in the GH-treated cylinders. There was no significant effect of GH on IGF-I receptor mRNA and GH receptor mRNA levels. Consistent with the results of previous studies, wound fluid IGF-I levels were lower than serum IGF-I levels, but no significant difference was found between the GH-treated cylinders and the control cylinders. The results of the present study show that GH stimulates granulation tissue formation and increases the concentration of IGF-I mRNA in the ingrown granulation tissue.

The plasma pattern of growth hormone in conscious rats during late pregnancy

The plasma GH levels of female rats during late pregnancy were determined using an automatic method for repetitive blood sampling from conscious animals. The plasma GH patterns were analysed by a pulse analysis computer program (PULSAR). The mean plasma GH levels were about twofold higher in pregnant females on days 15, 18 and 22 of gestation than in age-matched non-pregnant females. The basal plasma GH levels were also increased, while there was no change in GH pulse amplitude or frequency. The augmentation of GH release was even more pronounced on day 20 of gestation, with a fourfold increase in mean plasma GH levels compared with those in non-pregnant females. This increase reflected an increase in both basal plasma GH levels and GH pulse amplitude, but there was no increase in pulse frequency. In female rats that delivered on day 22 of gestation, the basal and mean plasma GH levels increased during parturition. Pregnant females consistently responded to multiple i.v. infusions of 1 microgram human GH-releasing factor analogue (hGRF(1-29)-NH2) given at 45-min intervals on day 18 of gestation. Both basal and GRF analogue-stimulated plasma GH levels were undetectable after hypophysectomy of pregnant rats. The present study demonstrates an increase in basal plasma GH levels during late pregnancy and a marked increase in both basal plasma GH levels and GH pulse amplitude on day 20 of gestation. Furthermore, hypophysectomy of pregnant rats results in undetectable GH levels, indicating that the high levels of GH during pregnancy are derived from the pituitary.