The obese growth hormone (GH)-deficient dwarf rat: body fat responses to patterned delivery of GH and insulin-like growth factor-I

Review: Insulin-like growth factor-related proteins and diabetic complications

The insulin-like growth factor system and, in particular, insulin-like growth factor-I (IGF-I) and insulin-like growth factor binding protein-1 (IGFBP-1) are dysregulated in type 1 and type 2 diabetes. Serum IGF-I levels are low in both forms of diabetes, and this may be in part genetically determined. It is possible that the reduced serum levels of IGF-I are involved in the development of microvascular and macrovascular complications. Fasting serum IGFBP-1 levels are usually low in early type 2 diabetic patients with insulin resistance and hyperinsulinaemia but may be raised in patients with particularly poor glycaemic control and severe beta-cell failure. Treatment with IGF-I/binding protein complexes has been shown to improve glycaemic control in conjunction with insulin and may in future have a place in the treatment of diabetes, potentially to prevent diabetic complications. Serum IGFBP-1 determination may have utility in the assessment of cardiovascular risk and as an indicator for insulin resistance.

Growth Hormone and Insulin-like Growth Factor 1: New Endocrine Therapies in Cardiology?

The hormones growth hormone (GH) and insulin-like growth factor 1 (IGF-1) play a dominant role in whole body growth and metabolism. This is reflected in the use of human GH (hGH) in GH-deficient children to stimulate growth and in GH-deficient adults to reduce visceral fat mass. Recent data suggest that hGH may improve cardiac function in patients with heart failure, so there is current interest in methods to raise GH-IGF levels, including the testing of agents that release GH from the pituitary, administering IGF-1, and most recently, long-acting formulations of hGH. It is hoped that this ongoing integration of cardiology and endocrinology will uncover the pathophysiology of some cardiovascular diseases and yield new treatments based on the hormones of the GH axis. (Trends Cardiovasc Med 1997;7:264-268). © 1997, Elsevier Science Inc.

Sustained low-dose growth hormone therapy optimizes bioactive insulin-like growth factor-I level and may enhance CD4 T-cell number in HIV infection

High-dose recombinant human growth hormone (rhGH) (2-6 mg/day) regimes may facilitate T-cell restoration in patients infected with human immunodeficiency virus (HIV) on highly active antiretroviral therapy (HAART). However, high-dose rhGH regimens increase insulin-like growth factor-I (IGF-I) to supra-physiological levels associated with severe side effects. The present study investigated whether lower doses of rhGH may improve T-cell restoration in patients infected with HIV following an expedient response of total and bioactive (i.e., free) IGF-I. A previous 16-week pilot-study included six HIV-infected patients on stable HAART to receive rhGH 0.7 mg/day, which increased total (+117%, P < 0.01) and free (+155%, P < 0.01) IGF-I levels. The study was extended to examine whether continuous use of low-dose rhGH (0.7 mg/day until week 60; 0.4 mg/day from week 60 to week 140) would maintain expedient IGF-I levels and improve CD4 T-cell response. Total and free IGF-I increased at week 36 (+97%, P < 0.01 and +125%, P < 0.01, respectively) and week 60 (+77%, P = 0.01 and +125%, P < 0.01) compared to baseline levels (161 +/- 15 and 0.75 +/- 0.11 microg/L). CD4 T-cell number increased at week 36 (+15%, P < 0.05) and week 60 (+31%, P = 0.01) compared to baseline levels (456 +/- 55 cells/microL). Following rhGH dose reduction, total IGF-I and CD4 T-cell number remained increased at week 88 (+44%, P = 0.01 and +33%, P < 0.01) and week 140 (+46%, P = 0.07 and +36%, P = 0.02) compared to baseline levels. These data support the notion that low-dose rhGH regimens may increase expediently total and bioactive IGF-I and improve T-cell restoration in patients infected with HIV on HAART.

Effects of pulsatile secretion of growth hormone (GH) on fat deposition in human GH transgenic rats

Growth hormone (GH) is an endocrine regulator of glucose and lipid metabolism as well as body growth. GH levels are decreased and a unique pulsatile secretory pattern becomes obvious after puberty particularly in males. Coincidentally with this, males tend to deposit body fat. Experimental and clinical evidence has accumulated that obesity is associated with a decrease in GH levels. A strain of transgenic rats has been generated with severe obesity but normal nose-to-tail length, which has low circulating GH levels without pulsatility (human growth hormone (hGH) transgenic rats). The present review mainly focuses on recent and current work analysing the relationship between the occurrence of obesity and low GH levels and/or the absence of GH pulsatility in this transgenic animal model. This model has elevated blood glucose, non-esterified fatty acid, insulin and leptin levels associated with hyperphagia, suggesting that these rats also carry insulin- and leptin-resistant characteristics. hGH transgenic rats were subjected to a pair-feeding treatment to normalize food intake and chronic GH replacement to normalize GH levels. While the pair-feeding for 8 weeks successfully suppressed body-weight gain, the fat pad : body weight ratio remained very similar to freely-eating control hGH transgenic rats, which indicates the hyperphagia is not the sole contributor to the excess fat accumulation in this model. However, continuous elevation of peripheral hGH levels (approximately 2-fold) for 8 weeks by means of a slow-release vehicle resulted in a significant decrease in the fat mass : body weight ratios by 30 %. This GH treatment altered neither food intake nor body-weight gain. Thus, two characteristic phenotypes observed in the hGH transgenic rats, hyperphagia and obesity, seem to be closely related to GH levels and GH secretory pattern. This relationship might be working in the regulation of changes in seasonal body composition in wild animals.

Adiposity profile in the dwarf rat: an unusually lean model of profound growth hormone deficiency

This study describes the previously uncharacterized ontogeny and regulation of truncal adipose reserves in the profoundly GH-deficient dwarf (dw/dw) rat. We show that, despite normal proportionate food intake, dw/dw rats develop abdominal leanness and hypoleptinemia (circulating leptin halved in dw/dw males, P < 0.05) during puberty. This contrasts with the hyperleptinemia seen in moderately GH-deficient Tgr rats (circulating leptin doubled at 6 wk of age, P < 0.05) and in GH receptor-binding protein (GHR/BP)-null mice (circulating leptin doubled; P < 0.05). This lean/hypoleptinemic phenotype was not completely normalized by GH treatment, but dw/dw rats developed abdominal obesity in response to neonatal MSG treatment or maintenance on a high-fat diet. Unlike Tgr rats, dw/dw rats did not become obese with age; plasma leptin levels and fat pad weights became similar to those in wild-type rats. In contrast with truncal leanness, tibial marrow adiposity was normal in male and doubled in female dwarves (P < 0.01), this increase being attributable to increased adipocyte number (P < 0.01). Neonatal MSG treatment and high-fat feeding elevated marrow adiposity in dw/dw rats by inducing adipocyte enlargement (P < 0.05). These results demonstrate that, despite lipolytic influence of GH, severe GH deficiency in dw/dw rats is accompanied by a paradoxical leanness. This lean/hypoleptinemic phenotype is not solely attributable to reduced GH signaling and does not appear to result from a reduction in nutrient intake or the ability of dw/dw adipocytes to accumulate lipid. Disruption of preadipocyte differentiation or adipocyte proliferation in the dw/dw rat may lead to the development of this unusually lean/hypoleptinemic phenotype.

Absence of ghrelin protects against early-onset obesity

The gut peptide ghrelin, the endogenous ligand for the growth hormone secretagogue receptor, has been implicated not only in the regulation of pituitary growth hormone (GH) secretion but in a number of endocrine and nonendocrine functions, including appetitive behavior and carbohydrate substrate utilization. Nevertheless, recent genetic studies have failed to show any significant defects in GH levels, food intake, or body weight in adult ghrelin-deficient (Ghrl-/-) mice. Here we demonstrate that male Ghrl-/- mice are protected from the rapid weight gain induced by early exposure to a high-fat diet 3 weeks after weaning (6 weeks of age). This reduced weight gain was associated with decreased adiposity and increased energy expenditure and locomotor activity as the animals aged. Despite the absence of ghrelin, these Ghrl-/- mice showed a paradoxical preservation of the GH/IGF-1 axis, similar to that reported in lean compared with obese humans. These findings suggest an important role for endogenous ghrelin in the metabolic adaptation to nutrient availability.

Prospects for the Development of Long-Acting Formulations of Human Somatropin

In healthy humans, growth hormone (GH) is secreted in distinct pulses with an underlying nyctohemeral pattern. Current forms of somatropin replacement are unable to closely mimic such a release pattern, but are still able to exert the beneficial action of GH. A limited number of short-term studies in rodents and humans suggest that longitudinal growth may be superior when somatropin is given with a pulsatile mode of administration, whereas hepatic insulin-like growth factor-I generation and beneficial changes in body composition appear to be equal or even enhanced with continuous somatropin administration.Recent developments in drug delivery technology have allowed the use of slow-release preparations of somatropin in humans. The most successful technology so far has been the encapsulation of somatropin molecules in poly(D,L-lactic-co-glycolic acid) biodegradable microspheres. Pharmacokinetic and pharmacodynamic data have been published on two such preparations; Nutropin Depot((R)) and hGH-Biosphere((R)). The latter has a superior release profile, but outcomes data from multicenter trials in both children and adults have been presented for the former: catch-up growth was observed in children, although to a lesser degree than historic comparative data obtained with the use of daily somatropin injections and the effects on metabolic derangements in GH-deficient patients appeared similar to those observed with daily injections. Improved sustained-release somatropin preparations will need further study of their long-term efficacy, but, if successful, will be highly attractive in terms of patient compliance and convenience.

Thermoregulation of transgenic growth hormone mice

Transgenic growth hormone (TG) mice (Mus musculus L., 1758) obtain enhanced growth via compensatory feeding at intermediate sizes and via higher growth efficiency. The latter involves diverting resources from other functions such as locomotion and wakefulness. Thermogenesis is a major expense for small mammals, so we explored whether TG mice express a trade-off between growth and thermoregulation. TG mice are hypothermic and cannot maintain their body temperature under cold stress. TG mice showed initial enlargement of brown adipose tissue and subsequent age-related decreases not seen in controls. Some TG mice became torpid after fasting durations not known to affect other mice. On a high-calorie diet, TG mice had higher body temperatures even though controls did not. Our background strain developed obesity on a high-protein and high-fat diet, and on a diet supplemented with carbohydrates, whereas TG mice never developed obesity. White adipose tissue deposits of TG females were relatively larger, but those of TG males were relatively smaller, than those of controls fed standard food. We also found significant effects of the three experimental diets, as well as gender, age, body mass, ambient temperature, and behavioural activity, on rectal temperatures of TG mice and controls in a large breeding colony. Thermogenesis of TG mice fed standard food appears energetically constrained, likely contributing to enhanced growth efficiency.

Visceral obesity without insulin resistance in late-onset obesity rats

We describe a line of transgenic rats in which the males develop a unique autosomal dominant, late-onset obesity (LOB) phenotype. LOB males gradually accumulate fat specifically in visceral, but not peripheral, fat depots despite a normal intake of a low fat diet. LOB females normally develop only mild obesity with advanced age. However, the phenotype can be induced rapidly in young females by ovariectomy and prevented by estrogen replacement. LOB males are highly sensitive to dietary fat. Young, nonobese LOB males gain more weight on a 30% fat diet and lose more weight when treated with the lipase inhibitor, Orlistat, than their nontransgenic littermates. Remarkably, despite severe visceral obesity, LOB rats have normal fasting blood glucose, insulin, and corticosterone; show normal or increased insulin sensitivity in glucose and insulin tolerance tests; have increased plasma adiponectin levels; and display a heightened response to treatment with rosiglitazone. Their visceral adiposity reflects a specific increase in visceral adipocyte number, not size. Analysis of the transgene in LOB rats revealed a deletion in the gene encoding the S26 subunit of the mitochondrial ribosome that results in the production of a truncated protein, which we show to be imported into mitochondria. However, the transgene integrant is complex, so whether this is the sole molecular disruption underlying this phenotype remains to be established. Nevertheless, LOB rats provide a valuable new model of late-onset, male-preponderant, visceral-specific obesity, clearly dissociated from insulin resistance.

Comparison of methods of analysis of body composition in hypophysectomized rats treated with rat growth hormone

The present study compared estimates of body composition derived from dual-emission X-ray absorptiometry (DEXA) and from chemical analyses. The primary aim was to compare the two methods because growth hormone (GH) may cause fluid retention, and DEXA does not distinguish water from lean mass. Hypophysectomized rats were fed ad libitum and were treated with continuous infusions of rat GH in doses of 0, 10, 30, and 100 microg/day for 14 days. By chemical analysis, a decrease in percentage fat from 12.9% in the control group to 11.3%, 11.0%, and 10.2% in the low, medium, and high dose groups was observed (P < 0.0001). The fat percentages were about 3-4% higher by DEXA, but showed the same decline (P < 0.03). Lean mass increased from 74.4% in the control group to 75.8%, 78.0%, and 78.6% in the treatment groups (P < 0.001). A significant increase in the wet weight of the quadriceps muscle, but no difference in dry weight was observed in all four treatment groups, indicating that the increase in muscle weight was exclusively caused by water. This accumulation of water was reflected in the total water content of the carcasses, which increased from 62.0% in the control group to 64.9%, 66.1%, and 66.8% in the GH groups (P < 0.0001). The protein content decreased from 19.8% in the control group to 19.4%, 19.1%, and 18.9% in the GH groups (P < 0.001). Regardless of the decrease in protein, the GH treated groups contained more water in relation to protein as the g water/g protein ratio was increased by 13% from 3.14 in the control group to 3.55 in the group treated with the highest GH dose (P < 0.0001). Also, a close relationship between feed intake and body weight were found, together with increases in epiphyseal growth plate width, insulin-like growth factor I (IGF-I), and insulin-like growth factor binding protein 3 (IGFBP-3). In conclusion, the study shows that estimation of lean mass by DEXA should be carefully evaluated when used in connection with treatment of drugs that cause water retention.

Expressions of leptin and insulin-like growth factor-I are highly correlated and region-specific in adipose tissue of growing rats

OBJECTIVE

Anatomically distinct adipose tissue regions differ in their predominant modality of growth (i.e., cellular hypertrophy vs. hyperplasia). We examined site-specific patterns of expression of two genes whose products, leptin and insulin-like growth factor-I (IGF-I), could be involved in mediating differential growth and metabolism of white adipose tissue. We also related these patterns of expression to measures of adipose depot cellularity.

RESEARCH METHODS AND PROCEDURES

Male Wistar rats were fed ad libitum and studied from ages 7 weeks to approximately 12 months. Terminal measures of body weights; weights, composition, and cellularity of four white adipose depots; circulating leptin and IGF-I; and adipose depot-specific expression levels of leptin and IGF-I were measured in subsets of rats at 7, 12, 22, 42, and 46 weeks of age.

RESULTS

Both leptin and IGF-I mRNAs are quantitatively expressed in a depot-specific manner, in the following order: retroperitoneal approximately equals epididymal > mesenteric > subcutaneous inguinal. Furthermore, there is a marked correlation between the expressions of these hormones in the various regions of adipose tissue of rats during the first year of life. The mechanisms that underlie the parallel expressions of leptin and IGF-I appear to be related to fat-cell volume.

DISCUSSION

Because both leptin and IGF-I have been implicated in the regulation of energy homeostasis and are both expressed in adipose tissue, the depot-specific linkage between the two genes suggests interaction at the autocrine level. This interaction may have an important role in determining functional properties particular to individual adipose depots.

Stimulation of the 150-kilodalton insulin-like growth factor-binding protein-3 ternary complex by continuous and pulsatile patterns of growth hormone (GH) administration in GH-deficient patients

In the circulation insulin-like growth factor I (IGF-I), IGF-binding protein 3 (IGFBP-3), and the acid-labile subunit (ALS) form a 150-kDa ternary complex that is of importance for the regulation of IGF-I bioactivity. GH administration is known to increase each of the single components of the ternary complex, and in GH-deficient rats formation of the 150-kDa complex is induced more by continuous than by pulsatile GH patterns. The aim of the present studies was to study the effects of the GH administration pattern on the formation of the 150-kDa ternary complex in humans. A fixed total GH dose (2 IU/m2-24 h) was administered iv randomly as 1) continuous infusion or 2) eight bolus injections to five GH-deficient patients over a period of 24 h. GH administration significantly increased serum IGF-I and IGFBP-3 levels and the IGF-I/IGFBP-3 ratio. IGF-I levels increased most pronouncedly after continuous administration (P < 0.01). Serum ALS levels increased significantly (both P < 0.005) from 94+/-21 to 180+/-29 (infusion) and from 85+/-17 to 155+/-17 nmol/L (pulses). Employment of neutral size exclusion chromatography enabled separation of IGFBP-3 in ternary complex and noncomplex-bound fractions. IGFBP-3 in the ternary complex increased significantly after GH administration [by 44% (P = 0.048) during infusion and by 62% (P = 0.004) during bolus]. The noncomplex-associated IGFBP-3 fraction, however, did not increase significantly after GH administration (P = NS). Finally, formation of the ternary complex was unaffected by the pattern of GH delivery. In conclusion, short-term GH administration increased all components of the 150-kDa ternary complex. Higher levels of IGF-I after constant GH exposure could indicate an increased bound fraction. However, the GH pattern did not influence the induction of the ternary complex itself. Continuous and intermittent GH patterns may be clinically equally effective during long-term GH therapy, as judged by levels of the components of the ternary complex.

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.

Short-term changes in serum leptin levels provide a strong metabolic marker for the growth response to growth hormone treatment in children. Swedish Study Group for Growth Hormone Treatment

The growth response to GH treatment varies between children. Besides regulating longitudinal growth, GH exerts important metabolic effects, including lipolysis. In this study we examined whether GH-induced changes in serum levels of the adipose tissue-derived hormone leptin can be used as a marker for the long term growth response to GH treatment in short prepubertal children. The study group consisted of 150 children (21 girls and 129 boys), who were 3-15 yr of age at the start of GH treatment and had a maximum GH secretory capacity ranging from very low to high. They were treated with GH (0.1 IU/kg x day) and followed for at least 1 yr. The first year mean increase in height SD score was 0.79 (SD, 0.34), with a broad range (0.08-2.27). Serum leptin concentrations were significantly reduced after 1, 3, and 12 months of GH treatment compared with levels at the start of treatment. The growth response correlated with the serum leptin concentration at the start of treatment (r = 0.49; P < 0.0001) and with the change in serum leptin concentration after both 1 month (r = -0.41; P < 0.01) and 3 months (r = -0.60; P < 0.0001) of treatment. When multiple stepwise regression analysis was applied to the auxological and biochemical variables that correlated (P < 0.10) with the first year growth response to GH treatment, the 3-month change in serum leptin concentration was the single most important variable for explaining the variance in individual growth responses. We conclude that leptin levels at the start of GH treatment as well as short term changes in leptin levels in response to GH treatment are valuable markers of the long term growth response.

Plasma growth hormone secretion is impaired in obesity-prone rats before onset of diet-induced obesity

Sprague-Dawley rats, which become obese (obesity prone) when fed a moderately high-fat (MHF; 32.5% of kcal as fat) diet, have decreased growth hormone (GH) concentrations compared with obesity-resistant rats fed the same diet. To determine whether plasma GH concentrations are different in obesity-prone rats compared with obesity-resistant rats before diet-induced obesity occurs, total integrated GH concentrations were determined in male Sprague-Dawley rats before exposure to the MHF diet. After initial blood sampling, rats were fed an MHF diet for 15 wk, over which time the animals were separated into two discrete populations based on body weight gain. Analysis of GH in episodic blood samples showed that the obesity-prone group had a GH secretion deficit before the onset of obesity (115.2 +/- 12.9 ng . ml-1 . 200 min-1) compared with obesity-resistant rats (237.2 +/- 47.1 ng . ml-1 . 200 min-1). The GH concentration difference was due to a decrease in mean GH peak height in rats that later became obese (34.8 ng/ml) compared with rats that remained lean (74.2 ng/ml). The results suggest that GH secretion impairment exists before dietary challenge or onset of obesity and may contribute to the susceptibility to obesity observed in these animals.