Growth hormone directly inhibits leptin gene expression in visceral fat tissue in fatty Zucker rats

The acromegaly lipodystrophy

Growth hormone (GH) and insulin-like growth factor 1 (IGF-1) are essential to normal growth, metabolism, and body composition, but in acromegaly, excesses of these hormones strikingly alter them. In recent years, the use of modern methodologies to assess body composition in patients with acromegaly has revealed novel aspects of the acromegaly phenotype. In particular, acromegaly presents a unique pattern of body composition changes in the setting of insulin resistance that we propose herein to be considered an acromegaly-specific lipodystrophy. The lipodystrophy, initiated by a distinctive GH-driven adipose tissue dysregulation, features insulin resistance in the setting of reduced visceral adipose tissue (VAT) mass and intra-hepatic lipid (IHL) but with lipid redistribution, resulting in ectopic lipid deposition in muscle. With recovery of the lipodystrophy, adipose tissue mass, especially that of VAT and IHL, rises, but insulin resistance is lessened. Abnormalities of adipose tissue adipokines may play a role in the disordered adipose tissue metabolism and insulin resistance of the lipodystrophy. The orexigenic hormone ghrelin and peptide Agouti-related peptide may also be affected by active acromegaly as well as variably by acromegaly therapies, which may contribute to the lipodystrophy. Understanding the pathophysiology of the lipodystrophy and how acromegaly therapies differentially reverse its features may be important to optimizing the long-term outcome for patients with this disease. This perspective describes evidence in support of this acromegaly lipodystrophy model and its relevance to acromegaly pathophysiology and the treatment of patients with acromegaly.

The Importance of Leptin to Reproduction

A healthy nutritional state is required for all aspects of reproduction and is signaled by the adipokine leptin. Leptin acts in a relatively narrow concentration range: too much or too little will compromise fertility. The leptin signal timing is important to prepubertal development in both sexes. In the brain, leptin acts on ventral premammillary neurons which signal kisspeptin (Kiss1) neurons to stimulate gonadotropin releasing hormone (GnRH) neurons. Suppression of Kiss1 neurons occurs when agouti-related peptide neurons are activated by reduced leptin, because leptin normally suppresses these orexigenic neurons. In the pituitary, leptin stimulates production of GnRH receptors (GnRHRs) and follicle-stimulating hormone at midcycle, by activating pathways that derepress actions of the messenger ribonucleic acid translational regulatory protein Musashi. In females, rising estrogen stimulates a rise in serum leptin, which peaks at midcycle, synchronizing with nocturnal luteinizing hormone pulses. The normal range of serum leptin levels (10-20 ng/mL) along with gonadotropins and growth factors promote ovarian granulosa and theca cell functions and oocyte maturation. In males, the prepubertal rise in leptin promotes testicular development. However, a decline in leptin levels in prepubertal boys reflects inhibition of leptin secretion by rising androgens. In adult males, leptin levels are 10% to 50% of those in females, and high leptin inhibits testicular function. The obesity epidemic has elucidated leptin resistance pathways, with too much leptin in either sex leading to infertility. Under conditions of balanced nutrition, however, the secretion of leptin is timed and regulated within a narrow level range that optimizes its trophic effects.

Somatotropic Axis Dysfunction in Non-Alcoholic Fatty Liver Disease: Beneficial Hepatic and Systemic Effects of Hormone Supplementation

Background: Somatotropic axis dysfunction associated with non-alcoholic fatty liver disease (NAFLD) has potential multisystemic detrimental effects. Here, we analysed the effects of growth hormone (GH) and insulin-like growth factor-1 (IGF-1) supplementation on liver histology, adipokine profile and muscle function in an NAFLD model. Methods: C57BL/6 mice were fed with a high fat diet (HFD) for 12 weeks and were separated into three groups treated for 4 weeks with: (1) High fat diet (HFD) (n = 10); (2) HFD + GH 9 μg/g/d (n = 10); (3) HFD + IGF-1 0.02 µg/g/d (n = 9). A control group fed a chow diet was included (n = 6). Liver histology, liver triglycerides content, serum alanine aminotransferase (ALT) activity, adiponectin and leptin serum levels, in vivo muscle strength, tetanic force and muscle fibre cross-sectional area (CSA) were measured. Results: HFD + GH and HFD + IGF-1 groups showed significantly lower ALT activity compared to HFD (p < 0.01). Liver triglyceride content in HFD + GH was decreased compared to HFD (p < 0.01). Histologic steatosis score was increased in HFD and HFD + GH group (p < 0.01), whereas HFD + IGF-1 presented no difference compared to the chow group (p = 0.3). HFD + GH group presented lower serum leptin and adiponectin levels compared to HFD. GH and IGF-1 supplementation therapy reverted HFD-induced reduction in muscle strength and CSA (sarcopenia). Conclusions: GH and IGF-1 supplementation induced significant improvement in liver steatosis, aminotransferases and sarcopenia in a diet-induced NAFLD model.

Genetic profiling of two phenotypically distinct outbred rats derived from a colony of the Zucker fatty rats maintained at Tokyo Medical University

The Zucker fatty (ZF) rat is an outbred rat and a well-known model of obesity without diabetes, harboring a missense mutation (fatty, abbreviated as fa) in the leptin receptor gene (Lepr). Slc:Zucker (Slc:ZF) outbred rats exhibit obesity while Hos:ZFDM-Leprfa (Hos:ZFDM) outbred rats exhibit obesity and type 2 diabetes. Both outbred rats have been derived from an outbred ZF rat colony maintained at Tokyo Medical University. So far, genetic profiles of these outbred rats remain unknown. Here, we applied a simple genotyping method using Ampdirect reagents and FTA cards (Amp-FTA) in combination with simple sequence length polymorphisms (SSLP) markers to determine genetic profiles of Slc:ZF and Hos:ZFDM rats. Among 27 SSLP marker loci, 24 loci (89%) were fixed for specific allele at each locus in Slc:ZF rats and 26 loci (96%) were fixed in Hos:ZFDM rats, respectively. This indicates the low genetic heterogeneity in both colonies of outbred rats. Nine loci (33%) showed different alleles between the two outbred rats, suggesting considerably different genetic profiles between the two outbred rats in spite of the same origin. Additional analysis using 72 SSLP markers further supported these results and clarified the profiles in detail. This study revealed that genetic profiles of the Slc:ZF and Hos:ZFDM outbred rats are different for about 30% of the SSLP marker loci, which is the underlying basis for the phenotypic difference between the two outbred rats.

Adipokines and cardiovascular risk in Cushing's syndrome

Cushing's syndrome (CS) is associated with increased cardiovascular morbidity and mortality. Recent evidence also suggests that increased cardiovascular risk may persist even after long-term remission of CS. Increased central obesity, a typical feature of CS, is associated with altered production of adipokines, which contributes to the pathogenesis of several metabolic and cardiovascular complications observed in this condition. In vitro and in vivo studies have shown a relationship between cortisol and adipokines in several experimental settings. In patients with either active or 'cured' CS, an increase in leptin and resistin levels as well as the release of pro-inflammatory cytokines, such as tumor necrosis factor-α and interleukin-6, may be associated with increased cardiovascular risk. For other adipokines, including adiponectin, results are inconclusive. Studies are needed to further elucidate the interactions between clinical and subclinical increases in cortisol production and altered adipokine release in CS.

Role of Growth Hormone Deficiency and Treatment in Chronic Kidney Disease

Malnutrition and inflammation are strong predictors of mortality in advanced kidney disease, especially in patients on renal replacement therapy. The complex relationship between kidney disease, uremia, and malnutrition significantly contributes to the increased morbidity and mortality in this patient population potentially through a relative deficiency in growth hormone (GH). With an approximate 26 million Americans currently affected by some stage of chronic kidney disease and a predicted 750,000 people to be on dialysis by 2020, there is a need to develop innovative strategies aimed at reducing the high mortality seen in dialysis patients. We will review evidence on one such intervention with infusion of recombinant GH to improve the nutritional and inflammatory state, thereby expecting to improve the mortality and morbidity in this patient population.

Interleukin 1 receptor antagonist is associated with changes in body composition during physiological GH substitution in patients with adult-onset growth hormone deficiency

OBJECTIVE

We examined the effect of GH substitution on adipose tissue-derived hormones and cytokines and sought to identify predictors for changes in body composition during therapy. Long-standing adult-onset GH deficiency (AO-GHD) is associated with increased body fat mass (FM) which, through production of hormones and inflammatory cytokines from adipose tissue, may contribute to different manifestations of the metabolic syndrome.

DESIGN, PATIENTS AND MEASUREMENTS

Fifty-five patients with AO-GHD (24 women, 31 men, mean age 49 years) were enrolled in a placebo-controlled, double-blind crossover study. GH therapy was individually dosed to obtain an IGF-I concentration within the normal range for age and sex. GH and placebo were administered for 9 months each, separated by a 4-month washout period. Adipose tissue-derived cytokines were measured by enzyme immunoassay.

RESULTS

GH treatment was associated with a significant decrease in IL-1 receptor antagonist (IL-1Ra) compared to placebo, which correlated with declining body FM (truncal and total) after GH substitution. The change in IL-1Ra was the strongest predictor of the variation in BFM in regression models. No changes were observed for leptin, adiponectin, soluble TNF receptor 1 or interleukin (IL)-8.

CONCLUSION

The data indicate a possible unrecognized association between IL-1Ra and changes in body composition during GH substitution and suggest further research on the interaction between the GH-IGF axis and the IL-1 system.

Daylong pituitary hormones in morbid obesity: effects of bariatric surgery

Moderate obesity is known to be associated with multiple endocrine abnormalities. Less information is available on the hormonal status of patients with morbid obesity and on the effects of major weight loss. We studied 16 severely obese (BMI 40.6-69.9 kg/m(2)) nondiabetic patients and 7 nonobese (BMI range 24.6-27.7 kg/m(2)), sex- and age-matched healthy volunteers. During 24 h in a metabolic ward, four meals were administered and hourly blood samples were drawn from a central venous catheter for the measurement of glucose, insulin, leptin, thyrotropic hormone (TSH), growth hormone (GH) and prolactin. Insulin sensitivity was measured by a euglycaemic hyperinsulinaemic clamp. Studies were repeated 6 months after biliopancreatic diversion, a mainly malabsorptive surgical approach, which caused an average weight loss of 35+/-4 kg (or 26+/-2% of initial weight). Compared with controls, patients were hyperinsulinaemic (290+/-31 vs 88+/-4 pmol l(-1), P=0.0002), insulin resistant (23.5+/-2.8 vs 52.9+/-4.9 micromol min(-1) kg(FFM)(-1), P=0.0006) and hyperleptinaemic (52.5+/-5.8 vs 10.9+/-3 ng ml(-1), P=0.0002). Plasma TSH levels were increased throughout the day-night cycle (averaging 2.02+/-0.18 vs 1.09+/-0.19 muU ml(-1) of controls, P=0.01), whereas serum GH levels were suppressed (0.46+/-0.10 vs 3.01+/-1.15, P=0.002). Following surgery, the hyperinsulinaemia and insulin resistance were fully normalized; in concomitance with a major drop in leptin levels (to 14.4+/-2.7 ng ml(-1), P=0.02), TSH decreased and GH increased to near-normal levels. In the whole dataset, mean 24-h leptin levels were directly related to mean 24-h TSH levels after controlling for confounders this relationship was lost only after adjusting for fat mass. We conclude that in morbid obesity leptin is a determinant of changes in pituitary function.

Fasting and glucose effects on pituitary leptin expression: is leptin a local signal for nutrient status?

Leptin, a potent anorexigenic hormone, is found in the anterior pituitary (AP). The aim of this study was to determine whether and how pituitary leptin-bearing cells are regulated by nutritional status. Male rats showed 64% reductions in pituitary leptin mRNA 24 hr after fasting, accompanied by significant (30-50%) reductions in growth hormone (GH), prolactin, and luteinizing hormone (LH), and 70-80% reductions in target cells for gonadotropin-releasing hormone or growth hormone-releasing hormone. There was a 2-fold increase in corticotropes. Subsets (22%) of pituitary cells coexpressed leptin and GH, and <5% coexpressed leptin and LH, prolactin, thyroid-stimulating hormone, or adrenocorticotropic hormone. Fasting resulted in significant (55-75%) losses in cells with leptin proteins or mRNA, and GH or LH. To determine whether restoration of serum glucose could rescue leptin, LH, and GH, additional fasted rats were given 10% glucose water for 24 hr. Restoring serum glucose in fasted rats resulted in pituitary cell populations with normal levels of leptin and GH and LH cells. Similarly, LH and GH cells were restored in vitro after populations from fasted rats were treated for as little as 1 hr in 10-100 pg/ml leptin. These correlative changes in pituitary leptin, LH, and GH, coupled with leptin's rapid restoration of GH and LH in vitro, suggest that pituitary leptin may signal nutritional changes. Collectively, the findings suggest that pituitary leptin expression could be coupled to glucose sensors like glucokinase to facilitate rapid responses by the neuroendocrine system to nutritional cues.

Molecular cloning and characterization of rat brain endothelial cell derived gene-1 (tumor suppressor candidate 5) expressing abundantly in adipose tissues

We report the cloning and expressional analysis of rat brain endothelial cell derived gene-1 (BEC-1), detected as a gene dominantly expressed in rat brain endothelial cells by the use of suppression subtractive hybridization technique. The complementary deoxyribonucleic acid sequence of BEC-1 messenger ribonucleic acid was completely determined with a full length of 3410 bp. The open reading frame within the sequence consisted of 522 bp, and the predicted protein sequence was 173 amino acid residues. BEC-1 gene was thought to be rat tumor suppressor candidate 5 (TUSC5), since BEC-1 had considerable homology with both mouse TUSC5 and human located at 17-p-13 point three 1 (LOST1) categorized as human TUSC5 (identities of 97% and 85%, respectively), which were recently identified as a novel tumor suppressor gene candidate. Expressional analyses for BEC-1 mRNA with real-time PCR and of BEC-1 protein by Western blotting demonstrated that both were dominantly expressed in the adipose tissues of Sprague-Dawley (SD) rats. We analyzed and compared the differential expressions of BEC-1 (TUSC5) mRNA and protein in fat tissues between obese homozygous (fa/fa) and lean wild-type (+/+) Zucker rats. Both expressions in the epididymal white adipose tissue (WAT) were highest, followed by those in the interscapular brown adipose tissue (BAT), subcutaneous, and mesenteric WATs, respectively. Interestingly, both expressions in epididymal WAT of obese Zucker rats were significantly lower than those in lean rats. Although cold exposure at 4 degrees C for 6 h significantly stimulated uncoupling protein-1 (UCP-1) mRNA expression, it significantly inhibited BEC-1 (TUSC5) mRNA expression in the interscapular BAT. These data indicated that rat BEC-1 (TUSC5) was abundantly expressed in adipose tissues, and that it might be involved in their regulation independently of UCP-1.

Leptin and leptin receptor expression in skeletal muscle and adipose tissue in response to in vivo porcine somatotropin treatment

The present study was performed to examine the response of leptin and leptin receptor (Rb) genes to porcine somatotropin (pST) stimuli in finishing pigs. Twelve crossbred barrows (Yorkshire x Landrace) were used in this study. Animals were individually fed a basal diet containing 18% CP, 1.2% lysine, and 3.5 Mcal of DE/kg ad libitum (as-fed basis). At 90 kg, six pigs were treated with daily injections of recombinant pST (10 mg) in sterile bicarbonate buffer, whereas the other six pigs were injected with sterile bicarbonate buffer (controls). With initiation of pST treatment, the quantity of feed offered was 85% of calculated ad libitum intake based on BW and adjusted every 3 d. Diet restriction was designed to correct for the effects of the known inhibition in feed intake because of pST treatment in swine. Animals were maintained on treatment for 2 wk. A blood sample was obtained from each pig on d 14 of treatment, 6 h after pST injection. Tissue samples were collected on d 15, frozen in liquid N2, and stored at -80 degrees C before analysis for mRNA abundance. Total RNA was amplified by reverse transcription (RT) PCR with subsequent quantification of transcripts by capillary electrophoresis with laser-induced fluorescence detection. Samples included outer subcutaneous adipose tissue (OSQ), middle subcutaneous adipose tissue (MSQ), leaf fat (LF), liver, latissimus dorsi (LD), and biceps femoris (BF). Restricted feeding resulted in no change in BW of control pigs, whereas pST treatment increased BW by 6.9 +/- 0.5 kg (P < 0.001). Treatment with pST produced a 12-fold increase in serum ST concentration relative to control pigs (P < 0.002). Serum leptin concentration was increased by 17% in swine treated with pST relative to control pigs (P < 0.011). Leptin mRNA abundance was increased in liver by pST treatment (P < 0.05). Administration of pST decreased leptin Rb (Ob-Rb) mRNA abundance by 27% in liver (P < 0.044) and by 49.5% in OSQ (P < 0.025) relative to controls. The present data suggest that pST does not affect leptin expression independent of dietary intake because the restricted feeding regimen used in the present study precluded detection of major change in leptin gene expression. Changes in Ob-Rb mRNA abundance by pST treatment indicate that ST or the metabolic adaptations to ST have a role in regulating Ob-Rb expression.

The expression of IGF-I and myostatin mRNAs in skeletal muscle of hypophysectomized and underfed rats during postnatal growth

AIM

To determine the roles of myostatin and insulin-like growth factor-I (IGF-I) during postnatal growth, we examined IGF-I and myostatin mRNA expression in the skeletal muscles of hypophysectomized and underfed rats during postnatal growth.

METHODS

Five-week-old rats were divided into four groups: freely fed control, moderately underfed, severely underfed and hypophysectomized. Four weeks later, blood and muscle samples were gathered to determine serum IGF-I, myosin heavy chain (MHC) isoforms, IGF-I Ea, IGF-I Eb and myostatin mRNA.

RESULTS

The weights of soleus, plantaris and masseter muscles were decreased in underfed and hypophysectomized rats. Hypophysectomy resulted in significant increases of type I MHC at the expense of type IIx in plantaris muscle and of neonatal MHC at the expense of types IIx and IIb in masseter muscle. Serum IGF-I was decreased by underfeeding and hypophysectomy. Plantaris muscle IGF-I Ea mRNA in underfed and hypophysectomized rats was significantly lower than in normal controls. Plantaris muscle IGF-I Eb mRNA in underfed rats was significantly lower than in normal controls. Masseter muscle IGF-I Eb mRNA in severely underfed rats was significantly lower than in normal control and hypophysectomized rats. Soleus muscle myostatin mRNA in hypophysectomized rats was significantly higher than in normal and significantly underfed rats. No significant differences in plantaris and masseter muscle myostatin mRNA were observed between groups.

CONCLUSION

Suppressed muscle growth caused by hypophysectomy and underfeeding may be attributed mainly to reduced circulating IGF-I and partially to reduced IGF-I mRNA, rather than to a change in myostatin.

Reproduction and plasma concentrations of leptin, insulin and insulin-like growth factor 1 in growth-hormone-transgenic female sheep before and after artificial insemination

The transgenic sheep used in this study expressed an additional copy of the gene for ovine growth hormone (GH), so they had continuously high plasma concentrations of GH. They were used to test whether the GH transgene affected plasma concentrations of the metabolic hormones leptin, insulin-like growth factor 1 (IGF-1) and insulin, and whether these effects were associated with changes in conception, pregnancy or parturition following artificial insemination. Compared with control animals, the GH-transgenic sheep had higher bodyweight, lower body condition score and less subcutaneous fat (P < 0.05). These sheep also had lower plasma concentrations of leptin, higher plasma concentrations of insulin, and higher plasma concentrations of IGF-1 (P < 0.001). A similar proportion of GH-transgenic and control ewes came into oestrus, but the conception rate to artificial insemination was lower in GH-transgenic ewes than in the controls. Only four live lambs were recovered from 12 GH-transgenic ewes (33%) compared with 38 lambs from 43 controls (88%). This outcome was not associated with any difference in plasma progesterone profile in the period leading up to artificial insemination (Day 0). The GH-transgenic ewes had lower concentrations of FSH at all times measured (Day -19, Day -2 and Day 19). These results indicate that appropriate regulation of GH secretion from pituitary or peripheral tissues is necessary for normal reproduction and normal levels of metabolic hormones. Chronically high concentrations of GH were associated with increased levels of IGF-1 and insulin, and decreased levels of leptin.

Regulation of development and metabolism of adipose tissue by growth hormone and the insulin-like growth factor system

White adipose tissue plays a key role in the regulation of the energy balance of vertebrates. This tissue is also now recognized to secrete a variety of factors such as leptin, which is thought to be involved in the modulation of adipose mass. Unlike other tissues, adipose tissue mass has considerable capacity to expand. The review deals primarily on the regulation of development and metabolism of adipose tissue by growth hormone (GH) and the insulin-like growth factor (IGF) system, with a special focus on the pig. The anti-insulin effects of GH are well-documented in pigs as in other species. In vitro exposure of adipose precursor cells to GH leads to a decrease in differentiation of those cells in pigs, in contrast to data obtained in murine cell lines. In vivo treatment and prolonged in vitro incubation of adipose tissue or isolated adipocytes with GH result in a decrease in glucose transport and lipogenesis, especially at the level of the fatty acid synthase gene, resulting in a reduction of the lipid content and adipose tissue mass. The mechanism by which GH antagonizes insulin stimulation of lipogenesis is still unresolved, as it is not mediated by protein kinase A, protein kinase C and Janus kinase-2 at the signaling level, or upstream stimulatory factor 1 or sterol regulatory element binding protein-1 at the transcriptional level. GH is apparently the main regulator of IGF-I mRNA expression in adipose tissue, however, the effects of IGF-I on this tissue are rather unclear.

Comparing adiposity profiles in three mouse models with altered GH signaling

Three mouse lines with altered growth hormone (GH) signaling were used to study GH's role in adiposity. Dwarf GH receptor knockout mice (GHR -/-) and bovine GH antagonist expressing mice (GHA) had an increased percent body fat with most of the excess fat mass accumulating in the subcutaneous region. Giant bovine GH expressing mice (bGH) had a reduced percent body fat. Only GHA mice consumed significantly more food per body weight. Serum leptin levels were significantly increased in GHA mice and decreased in bGH mice but unchanged in the GHR -/- mice. Interestingly, serum adiponectin levels were significantly increased in the GHR -/- and GHA lines but decreased in bGH mice. These data suggest that suppression or absence of GH action and enhanced GH action indeed have opposite metabolic effects in terms of adiposity. Interestingly, adiponectin levels were positively correlated with previously reported insulin sensitivity of these mice, but also positively correlated with adiposity, which is contrary to findings in other mouse models. Thus, adiponectin levels were negatively correlated with GH function suggesting a role for adiponectin in GH-induced insulin resistance.

The effect of pegvisomant-induced serum IGF-I normalization on serum leptin levels in patients with acromegaly

BACKGROUND

In humans, serum leptin correlates positively with fat mass. GH is lipolytic and patients with active acromegaly have lowered serum leptin compared to age, sex and body mass index (BMI)-matched controls, but a direct influence of GH on serum leptin remains unclear. In patients with acromegaly, total leptin increases following successful pituitary surgery and during somatostatin (SMS) analogue therapy (despite no change in BMI) but whether this represents changes in soluble leptin receptor, bound or free leptin is unclear. Pegvisomant, a GH receptor antagonist capable of normalizing serum IGF-I in over 97% of patients, represents a novel treatment strategy in acromegaly and its effect on leptin has not previously been reported.

PATIENTS

Sixteen patients (nine male (M), seven female (F), median age 52 years, range 27-78 years) with active acromegaly (serum IGF-I at least 30% above the upper limit of an age-related reference range) were studied. Serum IGF-I was normalized in all subjects with pegvisomant [mean duration 7 months (range 3-11 months), median dose 20 mg/day (range 10-40 mg/day)]. A single batch measurement of leptin, bound leptin (BL), soluble leptin receptor (SLR), insulin and glucose were performed on samples from baseline and first occurrence of serum IGF-I normalization.

RESULTS

As in normal subjects, females with acromegaly had higher baseline serum leptin [median M = 6.1 (range 1.6-58.7), F = 25.9 (range 3.19-54.1) ng/ml; P = 0.04], which correlated positively with BMI (R = 0.78, P = 0.0004). Forward step-wise regression analysis demonstrated that BMI and gender accounted for 90% of the variance in mean serum log10 leptin. Pegvisomant-induced serum IGF-I normalization was associated with a rise in serum leptin [8.9 (range 1.6-58.3) to 12.7 (range 2.3-90.8) ng/ml, P < 0.0001]. Although the absolute increase was not significantly different, mean percentage increase was greater in men (M = 66.6 +/- 51%, F = 11.8 +/- 16%, P = 0.017) despite similar serum IGF-I and BMI in male and female subjects at baseline. No change in BL or SLR accompanied serum IGF-I normalization [0.27 (range 0.15-1.26) to 0.27 (range 0.14-1.2) nmol/l, P = 0.27 and 3.2 (range 1.2-6.8) to 2.7 (range 1-7.4) nmol/l, P = 0.5, respectively]. After normalization of serum IGF-I, a correlation between BMI and leptin remained (R = 0.86, P < 0.0001) and together BMI and gender accounted for 87% of the variance in mean log10 serum leptin (P = 0.0002).

CONCLUSION

Pegvisomant-induced serum IGF-I normalization in patients with active acromegaly is associated with a significant increase in total, and by implication, free leptin.

Relationships between plasma concentrations of leptin and other metabolic hormones in GH-transgenic sheep infused with glucose

To study the regulation of leptin secretion in sheep, we infused glucose (0.32 g/h/kg for 12 h) into GH-transgenic animals (n = 8) that have chronically high plasma concentrations of ovine GH and insulin, but low body condition and low plasma leptin concentrations, and compared the responses with those in controls (n = 8). In both groups, the infusion increased plasma concentrations of glucose and insulin within 1 h and maintained high levels throughout the infusion period (P < 0.0001). Compared with controls, GH-transgenics had higher concentrations of insulin, IGF-1, GH (all P < 0.0001) and cortisol (P < 0.05), but lower GH pulse frequency (P < 0.0001). Overall, leptin concentrations were lower in GH-transgenics than in controls (P < 0.01). A postprandial increase in leptin concentrations was observed in both groups, independently of glucose treatment, after which the values remained elevated in animals infused with glucose, but returned to basal levels in those infused with saline, independently of transgene status. In both GH-transgenics and controls, glucose infusion did not affect the concentrations of GH, IGF-1, or cortisol. In conclusion, GH-transgenic and control sheep show similar responses to glucose infusion for leptin and other metabolic hormones, despite differences between them in body condition and basal levels of these hormones. Glucose, insulin, GH, IGF-1 and cortisol are probably not major factors in the acute control of leptin secretion in sheep, although sustained high concentrations of GH and IGF-1 might reduce adipose tissue mass or inhibit leptin gene expression.

Leptin: a review of its peripheral actions and interactions

Following the discovery of leptin in 1994, the scientific and clinical communities have held great hope that manipulation of the leptin axis may lead to the successful treatment of obesity. This hope is not yet dashed; however the role of the leptin axis is now being shown to be ever more complex than was first envisaged. It is now well established that leptin interacts with pathways in the central nervous system and through direct peripheral mechanisms. In this review, we consider the tissues in which leptin is synthesized and the mechanisms which mediate leptin synthesis, the structure of leptin and the knowledge gained from cloning leptin genes in aiding our understanding of the role of leptin in the periphery. The discoveries of expression of leptin receptor isotypes in a wide range of tissues in the body have encouraged investigation of leptin interactions in the periphery. Many of these interactions appear to be direct, however many are also centrally mediated. Discovery of the relative importance of the centrally mediated and peripheral interactions of leptin under different physiological states and the variations between species is beginning to show the complexity of the leptin axis. Leptin appears to have a range of roles as a growth factor in a range of cell types: as be a mediator of energy expenditure; as a permissive factor for puberty; as a signal of metabolic status and modulation between the foetus and the maternal metabolism; and perhaps importantly in all of these interactions, to also interact with other hormonal mediators and regulators of energy status and metabolism such as insulin, glucagon, the insulin-like growth factors, growth hormone and glucocorticoids. Surely, more interactions are yet to be discovered. Leptin appears to act as an endocrine and a paracrine factor and perhaps also as an autocrine factor. Although the complexity of the leptin axis indicates that it is unlikely that effective treatments for obesity will be simply derived, our improving knowledge and understanding of these complex interactions may point the way to the underlying physiology which predisposes some individuals to apparently unregulated weight gain.

In vitro effect of human recombinant leptin and expression of leptin receptors on growth hormone-secreting human pituitary adenomas

OBJECTIVE AND STUDY DESIGN

Leptin is a circulating hormone secreted by adipose tissue and a few other tissues. It has recently been demonstrated that leptin and leptin receptors are expressed in normal and adenomatous pituitary cells. The aim of this study was to investigate the effect of recombinant human leptin on GH release from adenomatous GH-secreting cells in culture. Specimens were obtained from 10 patients with acromegaly who had undergone selective transsphenoidal adenomectomy. Cells (2 x 10(5)/well) preincubated for 24 h with leptin (10(-10)-10(-8) m) or control medium were exposed to GHRH for 2 h. The GH released into the medium was measured before and after GHRH incubation. The expression of leptin receptor isoforms was evaluated by reverse-transcriptase polymerase chain reaction (RT-PCR) in cells obtained from five adenomas.

RESULTS

After the first incubation, there was a slight dose-dependent leptin-induced decrease in GH released into the medium. A significant increase in GH release after GHRH was noted from cells previously exposed to leptin in comparison with cells incubated with medium alone. Expression of leptin receptors was found in two out of five GH-secreting adenomas evaluated.

CONCLUSIONS

Our data confirm that some, but not all, GH-secreting adenomas express leptin receptors. Leptin seems to exert both a slight inhibitory effect on spontaneous GH secretion and a stimulatory effect on GHRH-stimulated GH secretion from GH-secreting adenomatous tissue.

Differential effects of GH replacement on the components of the leptin system in GH-deficient individuals

GH therapy is associated with a reduction in fat mass and an increase in lean mass in subjects with GH deficiency (GHD). Leptin, like GH, plays an important role in the regulation of body composition. GH treatment has been shown to reduce serum leptin; however, the physiological interactions between the leptin system (free leptin, bound leptin, and soluble leptin receptor) and the GH/IGF-I system largely remain unknown. Twenty-five patients with childhood (n = 10) and adult-onset (n = 15) GHD were studied. GH status had previously been determined using an insulin tolerance test and/or an arginine stimulation test. The following parameters were recorded at baseline (V1) and then after 3 months (V2) and 6 months (V3) on GH treatment: fat mass, body mass index (BMI), and waist/hip ratio (WHR); blood samples were taken after an overnight fast for free leptin, bound leptin, soluble leptin receptor, insulin, and IGF-I. At V2 and V3, respectively, a fall in free leptin (P < 0.001 for each), and at V3 a fall in in percent fat mass (P < 0.001) were observed. There were no significant changes in BMI or WHR. Simultaneously, there was a rise in insulin (P = 0.068 and P < 0.001), IGF-I (P < 0.001 and P < 0.001), bound leptin (P = 0.005 and P < 0.001), and soluble leptin receptor (P = 0.61 and P < 0.001). A positive relationship was noted between free leptin and BMI (P < 0.001) and between free leptin and fat mass (P < 0.001), and a negative relationship was found between free leptin and IGF-I (P < 0.001) and, within patient, between free leptin and insulin (P < 0.001). There was no significant correlation between free leptin and WHR. Bound leptin had a positive association with IGF-I (P < 0.001) and insulin (P = 0.002) and a negative relationship with percent fat mass (P = 0.023). Soluble leptin receptor was also positively related to IGF-I (P < 0.001). In conclusion, our data suggest that the reduction in serum leptin with GH treatment, as noted by others, is mediated through a fall in free leptin. The fall in free leptin and in part the rise in bound leptin are most likely through a reduction in percent fat mass. However, the observed changes in free leptin and bound leptin and, more importantly, the rise in soluble leptin receptor, are not explained entirely by modifications in body composition and may be a direct result of GH/IGF-I.

Leptin--much more than a satiety signal

Much attention has focused on the effects of leptin as a central satiety agent. There is now a significant amount of evidence that leptin is active in the periphery. This review focuses on the ability of leptin to modify insulin sensitivity, tissue metabolism, stress responses, and reproductive function. Leptin's effect on several of these systems is mediated via the hypothalamic-pituitary axis. Therefore, although in vitro studies provide evidence for direct effects on specific tissues and metabolic pathways, it is essential to consider the interactions between leptin and other regulatory factors in vivo. Little is known about the regulation of peripheral receptor expression or the production of binding proteins. Both of these factors determine the bioactivity of circulating leptin and have the potential to induce a peripheral resistance to leptin, similar to the central "leptin resistance" observed in obese subjects. Future research will clarify which of the endocrine and metabolic actions of peripheral leptin are of physiological relevance and which should be considered a pharmacological manipulation.

Regulation of metabolism and body fat mass by leptin

The relative stability of body weight over the long term and under a variety of environmental conditions that alter short-term energy intake and expenditure provides strong evidence for the regulation of body energy content. The lipostatic theory of energy balance regulation proposed 40 years ago that circulating factors, generated in proportion to body fat stores, acted as signals to the brain, eliciting changes in energy intake and expenditure. The discovery of leptin and its receptors has now provided a molecular basis for this theory. Leptin functions as much more than an adipocyte-derived signal of lipid stores, however. Although suppression of food intake is an important centrally mediated effect of leptin, considerable evidence indicates that leptin also functions both directly and indirectly, via the brain, to orchestrate complex metabolic changes in a number of organs and tissues, altering nutrient flux to favor energy expenditure over energy storage.

Effect of Sandostatin LAR on serum leptin levels in patients with acromegaly

Serum leptin levels are decreased in patients with acromegaly and rise after GH is normalized by surgical treatment. We have evaluated the effect of Sandostatin LAR on leptin levels in acromegalic patients since there are recent data to suggest that somatostatin, in addition to its GH lowering effect, also reduces serum leptin levels in humans. Nineteen patients with active acromegaly were studied. Eleven patients received monthly injection of Sandostatin LAR and eight patients underwent transsphenoidal surgery. Serum concentrations of leptin, GH, IGF-1 and insulin were measured before and after treatment. Serum leptin concentrations were lower in patients with active acromegaly than controls matched for age, sex and body mass index (BMI) [2.79 microg/l (2.60) vs. 4.41 microg/l (5.07); median (inter-quartile range); P < 0.01]. A positive correlation between serum leptin concentrations and BMI was observed in the controls (r = 0.46, P < 0.05) but not in the acromegalic patients before treatment (r = 0.32, ns). In the group of patients treated with Sandostatin LAR, a marked reduction in GH and IGF-1 was achieved by week 8 and GH and IGF-1 remained suppressed throughout the 6 months of treatment. There was no change in BMI. A significant increase in leptin levels only became evident after 6 months of treatment [2.99 microg/l (2.60) vs. 4.21 microg/l (3.84), P < 0.05]. Leptin levels also significantly increased after transsphenoidal surgery [3.05 microg/l (5.73) vs. 5.19 microg/l (4.93), P < 0.05]. The positive correlation between serum leptin concentrations and BMI was restored in acromegalic patients both after treatment with Sandostatin LAR (r = 0.62, P < 0.05) and after surgery (r = 0.81, P < 0.05). Leptin concentrations were decreased in patients with active acromegaly and lowering GH by either Sandostatin LAR or transsphenoidal surgery led to an increase in leptin concentrations.

Leptin: an essential regulator of lipid metabolism

This paper reviews the general mechanisms by which leptin acts as a regulator of lipid reserves through changes in food intake, energy expenditure and fuel selection, with an emphasis on its direct effects on cellular lipid metabolism. Briefly, when leptin levels increase, food consumption decreases via modulation of hypothalamic neuropeptides. As well, normal decreases in energy expenditures (e.g. with diurnal cycles or reduced caloric intake) do not occur. This is probably caused by an increase in mitochondrial proton leak mediated by leptin via increases in sympathetic nervous system stimulation and thyroid hormone release. The decrease in caloric input coupled with relatively higher energy expenditure, therefore, leads to negative energy balance. Leptin also changes the fuel source from which ATP is generated. Fuel preference switches from carbohydrate (glucose) to lipid (fatty acids). This effect arises through stimulation of triacylglycerol catabolism by leptin. In vitro studies show that leptin is a potent stimulator of lipolysis and fatty acid oxidation in adipocytes and other cell types. Consequently, leptin is also a regulator of cellular triacylglycerol content. Hormonal regulation of leptin, as well as its role in fasting and seasonal weight gain and energy expenditure are also briefly discussed.