Obesity and insulin resistance in human growth hormone transgenic rats

Hypothalamic growth hormone receptor (GHR) controls hepatic glucose production in nutrient-sensing leptin receptor (LepRb) expressing neurons

Objective

The GH/IGF-1 axis has important roles in growth and metabolism. GH and GH receptor (GHR) are active in the central nervous system (CNS) and are crucial in regulating several aspects of metabolism. In the hypothalamus, there is a high abundance of GH-responsive cells, but the role of GH signaling in hypothalamic neurons is unknown. Previous work has demonstrated that the Ghr gene is highly expressed in LepRb neurons. Given that leptin is a key regulator of energy balance by acting on leptin receptor (LepRb)-expressing neurons, we tested the hypothesis that LepRb neurons represent an important site for GHR signaling to control body homeostasis.

Methods

To determine the importance of GHR signaling in LepRb neurons, we utilized Cre/loxP technology to ablate GHR expression in LepRb neurons (Lepr^EYFPΔGHR). The mice were generated by crossing the Lepr^cre on the cre-inducible ROSA26-EYFP mice to GHR^L/L mice. Parameters of body composition and glucose homeostasis were evaluated.

Results

Our results demonstrate that the sites with GHR and LepRb co-expression include ARH, DMH, and LHA neurons. Leptin action was not altered in Lepr^EYFPΔGHR mice; however, GH-induced pStat5-IR in LepRb neurons was significantly reduced in these mice. Serum IGF-1 and GH levels were unaltered, and we found no evidence that GHR signaling regulates food intake and body weight in LepRb neurons. In contrast, diminished GHR signaling in LepRb neurons impaired hepatic insulin sensitivity and peripheral lipid metabolism. This was paralleled with a failure to suppress expression of the gluconeogenic genes and impaired hepatic insulin signaling in Lepr^EYFPΔGHR mice.

Conclusion

These findings suggest the existence of GHR-leptin neurocircuitry that plays an important role in the GHR-mediated regulation of glucose metabolism irrespective of feeding.

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GHR and LepRb are co-localized in the ARH, DMH and LHA neurons.

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GHR signaling does not regulate food intake and body weight in LepRb neurons.

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Diminished GHR signaling in LepRb neurons impairs hepatic glucose production.

Liuwei Dihuang, a traditional Chinese herbal formula, suppresses chronic inflammation and oxidative stress in obese rats

OBJECTIVE

To investigate the anti-inflammatory, anti-oxidative stress, and adipokine-ameliorating effects of Liuwei Dihuang (LWDH), a traditional Chinese herbal formula, in obese rats.

METHODS

After 2 weeks of acclimation with free access to regular rodent chow and water, obese-prone-caesarean-derived (OP-CD) rats were fed a modified AIN-93G diet containing 60% energy from fat. Treatment was performed twice daily by gavage feeding with 500, 1 500, or 3 500 mg/kg body weight LWDH suspended in water (n=12 rats per group). Twelve obese-resistant-CD (OR-CD) rats were fed the atherogenic diet and gavaged with water, and served as the normal control. Blood biomarkers of inflammation, oxidative stress and adiponectin were measured post-sacrifice and used to determine the treatment effect of LWDH and assess the suitability of OR/OP-CD rats for studying these parameters.

RESULTS

After 9 weeks of treatment, LWDH lowered serum C-reactive protein (CRP) and tumour necrosis factor-α (TNF-α) levels. Serum interleukin-6 (IL-6) levels showed a tendency towards reduction, but were not significantly different from the OP-CD control. Liver superoxide dismutase (SOD) activity was increased in response to all three doses of LWDH, while the levels of reduced (GSH) and oxidized glutathione (GSSG) and thiobarbituric acid reactive substances (TBARS) were unchanged. Serum adiponectin levels were increased in response to oral administration of LWDH at the dose of either 500 or 1 500 mg/kg body weight. In addition, comparisons between OR-CD and OP-CD rats revealed differential, and for some biomarkers, conflicting characteristics of high-fat diet-fed OP-CD rats in reference to obese human subjects in terms of inflammatory and oxidative stress biomarkers and circulating adiponectin levels.

CONCLUSION

The results show, for the first time, the anti-inflammatory, anti-oxidative stress and adiponectin-ameliorating effects of LWDH in obese rats. The suitability of the OR/OP-CD rat model as a research tool to study inflammation, oxidative stress, and adipokine production requires further investigation.

Reciprocal occupancy of BCL6 and STAT5 on Growth Hormone target genes: contrasting transcriptional outcomes and promoter-specific roles of p300 and HDAC3

Expression of the Growth Hormone (GH)-stimulated gene Socs2 (Suppressor of Cytokine Signaling 2) is mediated by the transcription activator STAT5 (Signal Transducer and Activator of Transcription 5) and the transcription repressor BCL6 (B-Cell Lymphoma 6). ChIP-Sequencing identified Cish (Cytokine-Inducible SH2-containing protein) and Bcl6 as having similar patterns of reciprocal occupancy by BCL6 and STAT5 in response to GH, though GH stimulates Cish and inhibits Bcl6 expression. The co-activator p300 occupied Socs2, Cish and Bcl6 promoters, and enhanced STAT5-mediated activation of Socs2 and Cish. In contrast, on Bcl6, p300 functioned as a repressor and inhibited in conjunction with STAT5 or BCL6. The co-repressor HDAC3 (Histone deacetylase 3) inhibited the Socs2, Cish and Bcl6 promoters in the presence of STAT5. Thus transcriptional outcomes on GH-regulated genes occupied by BCL6 and STAT5 are determined in a promoter-specific fashion by co-regulatory proteins which mediate the distinction between activating and repressive transcription factors.

Free access to running wheels abolishes hyperphagia in human growth hormone transgenic rats

Obesity is a major health problem, and increased food intake and decreased physical activity are considered as two major factors causing obesity. Previous studies show that voluntary exercise in a running wheel decreases not only body weight but also food intake of rats. We previously produced human growth hormone transgenic (TG) rats, which are characterized by severe hyperphagia and obesity. To gain more insight into the effects on physical activity to food consumption and obesity, we examined whether voluntary running wheel exercise causes inhibition of hyperphagia and alteration of body composition in TG rats. Free access to running wheels completely abolished hyperphagia in TG rats, and this effect persisted for many weeks as far as the running wheel is accessible. Unexpectedly, though the running distances of TG rats were significantly less than those of wild type rats, it was sufficient to normalize their food consumption. This raises the possibility that rearing environment, which enables them to access to a running wheel freely, rather than the amounts of physical exercises is more important for the maintenance of proper food intake.

Growth hormone (GH) differentially regulates NF-kB activity in preadipocytes and macrophages: implications for GH's role in adipose tissue homeostasis in obesity

Adipose tissue remodeling in obesity involves macrophage infiltration and chronic inflammation. NF-kB-mediated chronic inflammation of the adipose tissue is directly implicated in obesity-associated insulin resistance. We have investigated the effect of growth hormone (GH) on NF-kB activity in preadipocytes (3T3-F442A) and macrophages (J774A.1). Our studies indicate that whereas GH increases NF-kB activity in preadipocytes, it decreases NF-kB activity in macrophages. This differential response of NF-kB activity to GH correlates with the GH-dependent expression of a cadre of NF-kB-activated cytokines in these two cell types. Activation of NF-kB by GH in preadipocytes heightens inflammatory response by stimulating production of multiple cytokines including TNF-α, IL-6, and MCP-1, the mediators of both local and systemic insulin resistance and chemokines that recruit macrophages. Our studies also suggest differential regulation of miR132 and SIRT1 expression as a mechanism underlying the observed variance in GH-dependent NF-kB activity and altered cytokine profile in preadipocytes and macrophages. These findings further our understanding of the complex actions of GH on adipocytes and insulin sensitivity.

Metformin inhibits growth hormone-mediated hepatic PDK4 gene expression through induction of orphan nuclear receptor small heterodimer partner

Growth hormone (GH) is a counter-regulatory hormone that plays an important role in preventing hypoglycemia during fasting. Because inhibition of the pyruvate dehydrogenase complex (PDC) by pyruvate dehydrogenase kinase 4 (PDK4) conserves substrates for gluconeogenesis, we tested whether GH increases PDK4 expression in liver by a signaling pathway sensitive to inhibition by metformin. The effects of GH and metformin were determined in the liver of wild-type, small heterodimer partner (SHP)-, PDK4-, and signal transducer and activator of transcription 5 (STAT5)-null mice. Administration of GH in vivo increased PDK4 expression via a pathway dependent on STAT5 phosphorylation. Metformin inhibited the induction of PDK4 expression by GH via a pathway dependent on AMP-activated protein kinase (AMPK) and SHP induction. The increase in PDK4 expression and PDC phosphorylation by GH was reduced in STAT5-null mice. Metformin decreased GH-mediated induction of PDK4 expression and metabolites in wild-type but not in SHP-null mice. In primary hepatocytes, dominant-negative mutant-AMPK and SHP knockdown prevented the inhibitory effect of metformin on GH-stimulated PDK4 expression. SHP directly inhibited STAT5 association on the PDK4 gene promoter. Metformin inhibits GH-induced PDK4 expression and metabolites via an AMPK-SHP-dependent pathway. The metformin-AMPK-SHP network may provide a novel therapeutic approach for the treatment of hepatic metabolic disorders induced by the GH-mediated pathway.

A comparative expression analysis of gene transcripts in brain tissue of non-transgenic and GH-transgenic zebrafish (Danio rerio) using a DDRT-PCR approach

The presence of higher level of exogenous growth hormone (GH) in transgenic animals could lead to several physiological alterations. A GH transgenic zebrafish (Danio rerio) line was compared to nontransgenic (NT) samples of the species through a DDRT-PCR approach, with the goal of identifying candidate differentially expressed transcripts in brain tissues that could be involved in GH overexpression. Densitometric analyses of two selected amplification products, p300 and ADCY2, pointed to a significant lower gene expression in the transgenic zebrafish (104.02 ± 57.71; 224.10 ± 91.73) when compared to NT samples (249.75 ± 30.08; 342.95 ± 65.19). The present data indicate that p300 and ADCY2 are involved in a regulation system for GH when high circulating levels of this hormone are found in zebrafishes.

Transgenic mice expressing the human growth hormone gene provide a model system to study human growth hormone synthesis and secretion in non-tumor-derived pituitary cells: differential effects of dexamethasone and thyroid hormone

Growth hormone (GH) is regulated by pituitary and hypothalamic factors as well as peripheral endocrine factors including glucocorticoids and thyroid hormone. Studies on human GH are limited largely to the assessment of plasma levels in endocrine disorders. Thus, insight into the regulation of synthesis versus secretion has come mainly from studies done on non-human GH and/or pituitary tumor cells. However, primate and non-primate GH gene loci have differences in their structure and, by extension, regulation. We generated transgenic (171hGH/CS-TG) mice containing the intact hGH1 gene and locus control region, including sequences required for integration-independent and preferential pituitary expression. Here, we show hGH co-localizes with mouse (m) GH in somatotrophs in situ and in primary pituitary cells. Dexamethasone treatment increased hGH and mGH, as well as GH releasing hormone (GHRH) receptor RNA levels, and hGH release was stimulated by GHRH treatment. By contrast, triiodothyronine decreased or had no effect on hGH and mGH production, respectively, and the negative effect on hGH was also seen in the presence of dexamethasone. Thus, 171hGH/CS-TG mouse pituitary cultures represent a model system to investigate hormonal control of hGH synthesis and secretion.

Osteoclast motility: putting the brakes on bone resorption

As the skeleton ages, the balanced formation and resorption of normal bone remodeling is lost, and bone loss predominates. The osteoclast is the specialized cell that is responsible for bone resorption. It is a highly polarized cell that must adhere to the bone surface and migrate along it while resorbing, and cytoskeletal reorganization is critical. Podosomes, highly dynamic actin structures, mediate osteoclast motility. Resorbing osteoclasts form a related actin complex, the sealing zone, which provides the boundary for the resorptive microenvironment. Similar to podosomes, the sealing zone rearranges itself to allow continuous resorption while the cell is moving. The major adhesive protein controlling the cytoskeleton is αvβ3 integrin, which collaborates with the growth factor M-CSF and the ITAM receptor DAP12. In this review, we discuss the signaling complexes assembled by these molecules at the membrane, and their downstream mediators that control OC motility and function via the cytoskeleton.

Skeletal muscle growth defect in human growth hormone transgenic rat is accompanied by phenotypic changes in progenitor cells

Growth hormone (GH) is known to have a pivotal role in the maintenance of skeletal muscle mass. Sarcopenia, the loss of skeletal muscle mass, is a common phenomenon in aging, and it is widely accepted that sarcopenia is largely attributed to age-related decline in GH secretion. In the present study, we tested if human growth hormone transgenic rats (GH-TG rats) whose plasma GH levels are maintained relatively low could be an appropriate model for sarcopenia. Analyses of GH-TG rats revealed that they exhibit skeletal muscle growth defect as well as atrophy of myofibers. The number of myofibers in tibialis anterior muscle was comparable to that of WT rats, while the proportion of type I slow myofibers in tibialis anterior muscle was increased in GH-TG rats after 5 months. Neither increased expression of ubiquitin ligases, MuRF1 and MAFbx, nor indication of apoptotic cell death was observed. Notably, myogenic differentiation potential of skeletal muscle progenitor cells in GH-TG rats was lower than WT rats, and this was accompanied by increased adipogenic potential. These results indicate that GH-TG rats could be a useful model to elucidate the mechanism of sarcopenia induced by reduced GH action and raised the possibility that decreased GH action may cause an alteration of differentiation potential of skeletal muscle progenitor cells.

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.

Hyperphagia-mediated obesity in transgenic mice misexpressing the RNA-editing enzyme ADAR2

ADAR2 is a double-stranded RNA-specific adenosine deaminase involved in the editing of mammalian RNAs by the site-specific conversion of adenosine to inosine. To examine the physiologic consequences resulting from ADAR2 misexpression, we have generated mutant mice expressing either wild-type or deaminase-deficient ADAR2 transgenes under the control of the human cytomegalovirus promoter. Transgenic mice expressing either wild-type or inactive ADAR2 isoforms demonstrated adult onset obesity characterized by hyperglycemia, hyperleptinemia, and increased adiposity. Paired feeding analysis revealed that mutant mice on caloric restriction had a growth rate and body composition indistinguishable from wild-type littermates, indicating that the observed obesity predominantly results from hyperphagia rather than a metabolic derangement. The observation that expression of catalytically inactive ADAR2 also is capable of producing an obese phenotype in mutant animals suggests that ADAR2 may possess additional biological activities beyond those required for the site-selective deamination of adenosine or may interfere with the actions of other double-stranded RNA-specific binding proteins in the cell.

Short-term application of low-dose growth hormone in surgical patients: Effects on nitrogen balance and blood glucose

AIM: To investigate the effectiveness and safety of recombinant human growth hormone (rhGH) in postoperative patients.

METHODS: A total of 48 consecutive patients undergoing abdominal operations were randomized to receive either subcutaneous rhGH (0.15 IU/kg) or placebo (menstruum) injections daily for 7 d after surgery. The two groups had similar nutritional intake. Blood samples for serum fibronectin, albumin, prealbumin, transferrin and the total lymphocyte count, as well as glucose levels were collected to study the rhGH effect. Basal laboratory evaluation, and nutritional status were estimated on d 1 before as baseline and d 3 and 10 after operation using standard laboratory techniques. Nitrogen balance was measured from d 3 to 9 after operation.

RESULTS: The cumulative nitrogen balance was significantly improved in rhGH group compared with the placebo group (11.37 ± 16.82 vs -9.11 ± 17.52, P = 0.0003). Serum fibronectin was also significantly higher in the rhGH group than in the placebo group (104.77 ± 19.94 vs 93.03 ± 16.03, P < 0.05), whereas changes in serum albumin, prealbumin, transferrin and total lymphocyte counts were not statistically significant. Mean blood glucose levels were significantly higher in the rhGH group from d 3 to 6 after operation.

CONCLUSION: If blood glucose can be controlled, low-dose growth hormone together with hypocaloric nutrition is effective on improving positive nitrogen balance and protein conservation and safe is in postoperative patients.

Clinical evidence of growth hormone for patients undergoing abdominal surgery: Meta-analysis of randomized controlled trials

AIM: To assess the effectiveness and safety of perioperative growth hormone (GH) in patients undergoing abdominal surgery.

METHODS: We searched the following electronic databases: MEDLINE, EMBASE, the Cochrane Controlled Trials Register, Chinese Bio-medicine Database. The search was undertaken in February 2003. No language restrictions were applied. Randomized controlled trials (RCT) comparing GH with placebo in patients undergoing abdominal surgery were extracted and evaluated. Methodological quality was evaluated using the Jadad scale.

RESULTS: Eighteen trials involving 646 patients were included. The combined results showed that GH had a positive effect on improving postoperative nitrogen balance (standardized mean difference [SMD] = 3.37, 95%CI [2.46, 4.27], P < 0.00001), and decreasing the length of hospital stay (weighted mean difference [WMD] = -2.07, 95%CI [-3.03, -1.11], P = 0.00002), and reducing the duration of postoperative fatigue syndrome (SMD = -1.83, 95%CI [-2.37, -1.30], P < 0.00001), but it could increase blood glucose levels (WMD = 0.91, 95%CI [0.56, 1.25], P < 0.00001).

CONCLUSION: GH for patients undergoing abdominal surgery is effective and safe, if blood glucose can be controlled well. Further trials are required with a sufficient size to account for clinical heterogeneity and to measure other important outcomes such as infection, morbidity, mortality, fluid retention, immunomodulatory effects, and tumor recurrence.

Unaltered TNF-alpha production by macrophages and monocytes in diet-induced obesity in the rat

Background

Recent findings have established an association between obesity and immune dysfunction. However, most of the studies investigating the effects of obesity on immune function have been carried out in genetically obese rodent models. Since human obesity is mostly due to intake of a high fat diet and decreased energy expenditure, we asked whether immunological defects also occur in diet-induced obesity. Specifically, we focused on the function of monocytes and macrophages, as these cells are thought to be involved in the low-grade inflammation present in obesity.

Methods

Male Sprague-Dawley rats were fed a high-fat or a standard chow diet for either 2 or 10 weeks. At the end of the intervention period animals were anaesthetised, blood collected for determination of plasma mediator concentrations and lipopolysaccharide (LPS) stimulated production of TNF-α by monocytes. LPS stimulated production of TNF-α in alveolar macrophages was also determined.

Results

High-fat feeding for either 2 or 10 weeks resulted in significant increases in fat mass and serum leptin. Although increased serum leptin has previously been linked to modulation of innate immunity, we found no significant difference in the LPS stimulated production of TNF-α by either blood monocytes or alveolar macrophages between the dietary groups. Furthermore, we failed to find a significant increase in circulating TNF-α concentrations in obese animals, as reported for genetically obese animals.

Conclusion

Our data suggest that defects in innate immune function observed in genetically obese animals are not mimicked by dietary obesity, and may more likely reflect the gross abnormality in leptin function of these models. Further work is required delineate the effects of dietary obesity on inflammatory state and immune function.

Relationship between growth hormone (GH) pulses in the peripheral circulation and GH-releasing hormone and somatostatin profiles in the cerebrospinal fluid of goats

Growth hormone (GH) is secreted in a pulsatile manner, but the underlying mechanisms of GH pulse generation remain to be resolved. In the present study, we investigated the relationship between GH pulses in the peripheral circulation and GH-releasing hormone (GHRH) and somatostatin (SRIF) profiles in the cerebrospinal fluid (CSF) of male goats. The effects of an intracerebroventricular (icv) injection of neuropeptide Y (NPY), galanin and ghrelin were also analyzed. Blood and CSF samples were collected every 15 min for 8 hr from the jugular vein and third ventricle, respectively. GH pulsatility in the goat was found to consist of distinct large pulses of 5 hr periodicity and small pulses of 1 hr periodicity. GHRH and SRIF in the CSF fluctuated in a pulsatile manner with 1 hr periodicity, and most of the descending phase of SRIF pulses were associated with the initiation of GH pulses. Icv injections of NPY, galanin and ghrelin stimulated GHRH release without affecting SRIF release. In addition, NPY suppressed, and galanin and ghrelin induced large GH pulses, although ghrelin was much more effective than galanin. These results suggest that an hourly fall in SRIF is involved in generating intrinsic circhoral rhythm of GH pulsatility. The mechanisms underlying the generation of large GH pulses of 5 hr periodicity remain unknown, while direct action of NPY and/or ghrelin on the pituitary might be involved.

Growth hormone overexpression in the central nervous system results in hyperphagia-induced obesity associated with insulin resistance and dyslipidemia

It is well known that peripherally administered growth hormone (GH) results in decreased body fat mass. However, GH-deficient patients increase their food intake when substituted with GH, suggesting that GH also has an appetite stimulating effect. Transgenic mice with an overexpression of bovine GH in the central nervous system (CNS) were created to investigate the role of GH in CNS. This study shows that overexpression of GH in the CNS differentiates the effect of GH on body fat mass from that on appetite. The transgenic mice were not GH-deficient but were obese and showed increased food intake as well as increased hypothalamic expression of agouti-related protein and neuropeptide Y. GH also had an acute effect on food intake following intracerebroventricular injection of C57BL/6 mice. The transgenic mice were severely hyperinsulinemic and showed a marked hyperplasia of the islets of Langerhans. In addition, the transgenic mice displayed alterations in serum lipid and lipoprotein levels and hepatic gene expression. In conclusion, GH overexpression in the CNS results in hyperphagia-induced obesity indicating a dual effect of GH with a central stimulation of appetite and a peripheral lipolytic effect.

Analyses of hind leg skeletons in human growth hormone transgenic rats

Growth hormone (GH) is essential in the development and growth of the skeleton and for the maintenance of bone mass and density, and its secretion is known to decline with aging. We have previously produced transgenic rats with low circulating GH that represent several age-associated phenotypes such as obesity, insulin-resistance and leptin-resistance. In the present study, the cross-sectional area, bone mineral density, and strength indexes of the hind leg skeletons of the transgenic rats were examined by an X-ray computed tomography scanning. The mean cross-sectional area of the transgenic rats showed no increase after 2 months old up to 8 months old and the strength indexes were significantly lower than their non-transgenic siblings at all ages examined. The trabecular bone mineral density in the transgenic rats drastically decreased at 8 months old, while the cortical bone mineral density was comparable to the non-transgenic rats, suggesting the onset of osteoporosis at this period. The results obtained in this study indicate that the transgenic rats could be useful model to gain insight into the complex mechanism leading to osteoporosis with aging.

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.

Regulation of adipocytokines and insulin resistance

It has long been known that obesity and insulin resistance are linked. Recently, it has been shown that adipocytes secrete several proteins including tumour necrosis factor-alpha, interleukin-6, resistin, and adiponectin. Since several of these so-called adipocytokines influence insulin sensitivity and glucose metabolism profoundly, they might provide a molecular link between increased adiposity and impaired insulin sensitivity. Thiazolidinediones which decrease insulin resistance and are used in the treatment of Type 2 diabetes seem to mediate part of their insulin-sensitising effects via modulation of adipocytokine expression. Furthermore, hormones such as beta-adrenergic agonists, insulin, glucocorticoids, and growth hormone might impair insulin sensitivity at least in part via up-regulation or down-regulation of adipocytokine synthesis. We summarise the current knowledge on how major adipocyte-secreted proteins are regulated by hormones and drugs influencing insulin sensitivity and discuss its implications for insulin resistance and obesity.

The role of the growth hormone-insulin-like growth factor axis in glucose homeostasis

Homeostatic mechanisms normally maintain the plasma glucose concentration within narrow limits despite major fluctuations in supply and demand. There is increasing evidence that the growth hormone (GH)-insulin-like growth factor (IGF) axis may play an important role in glucose metabolism. GH has potent effects on intermediary metabolism, some of which antagonize the actions of insulin. In contrast, IGF-I has insulin-like actions, which are, in the case of glucose metabolism, opposite to those of GH. There is often deranged glucose metabolism in situations where GH is deficient or in excess. The clinical administration of GH or IGF-I results in altered glucose metabolism and changes in insulin resistance. Despite these observations, the precise role of GH and IGF-I and their interactions with insulin in controlling normal glucose homeostasis are unknown. In diabetes, GH secretion is abnormally increased as a result of reduced portal insulin resulting in impaired hepatic IGF-I generation. Evidence suggests that this may contribute to the development of diabetic microvascular complications. IGF-I 'replacement' in diabetes is under investigation and new methods of delivering IGF-I as a complex with IGFBP-3 offer exciting new prospects.

Growth hormone-induced alterations in the insulin-signaling system

Growth hormone (GH) counteracts insulin action on lipid and glucose metabolism. However, the sequence of molecular events leading to these changes is poorly understood. Insulin action is initiated by binding of the hormone to its cell surface receptor (IR). This event activates the intrinsic tyrosine kinase activity residing in the beta-subunit of the IR and leads to autophosphorylation of the cytoplasmic portion of the beta-subunit and further activation of its tyrosine kinase towards several intermediate proteins, including the family of IR substrates (IRS) and the Shc proteins. When tyrosine phosphorylated, these cellular substrates connect the IR with several downstream signaling molecules. One of them is the enzyme phosphatidylinositol (PI) 3-kinase. The insulin antagonistic action of GH is not a consequence of a direct interaction with the IR. Instead, long-term exposure to GH is, in general, associated with hyperinsulinemia, which leads to a reduction of IR levels and an impairment of its tyrosine kinase activity. The signals of GH and insulin may converge at post-receptor levels. The signaling pathway leading to activation of PI 3-kinase appears to be an important site of convergence between the signals of these two hormones and seems to be mediated principally by IRS-1. Rodent models of chronic GH excess have been useful tools to investigate the mechanism by which GH induces insulin resistance. Decreased IR, IRS-1, and IRS-2 tyrosyl phosphorylation in response to insulin was found in skeletal muscle, whereas a chronic activation of the IRS-PI 3-kinase pathway was found in liver. The induction of the expression of proteins that inhibit IR signaling such as suppressors of cytokine signaling (SOCS)-1 and -6 may also be involved in this alteration. Interestingly, the modulation of insulin signaling and action observed in states of GH excess, deficiency, or resistance seems to be relevant to the changes in longevity associated with those states.

Glomerulosclerosis in mice transgenic for human insulin-like growth factor-binding protein-1

BACKGROUND

The growth hormone (GH)/insulin-like growth factor (IGF) system is thought to participate in the glomerulosclerosis process. Because IGF-binding proteins (IGFBPs) modulate IGF actions and hence GH secretion, this study assessed whether mice transgenic for human IGFBP-1 have altered susceptibility to glomerulosclerosis.

METHODS

A line of transgenic mice that express human IGFBP-1 mRNA in the liver under the control of the alpha1-antitrypsin promoter has been obtained, and morphological changes in the kidney tissue were assessed. Glomerulosclerosis was identified using light microscopy, light microscopic morphometry, and electron microscopy. Extracellular matrix components were analyzed by immunohistochemistry.

RESULTS

There was a marked increase in mesangial extracellular matrix area in homozygous transgenic mice at three months of age as compared with heterozygous transgenic mice and nontransgenic littermates. These changes were not associated with alterations in glomerular volume or cellularity. The expansion of extracellular matrix area was related to a marked increase in laminin and type IV collagen and to the appearance of type I collagen.

CONCLUSIONS

These observations indicate that the enhanced expression of IGFBP-1 may result in the development of glomerulosclerosis without glomerular hypertrophy. The changes are potentially related to a decrease in IGF-I availability and/or to an IGF-I-independent role of IGFBP-1.

Stimulation of lipolysis but not of leptin release by growth hormone is abolished in adipose tissue from Stat5a and b knockout mice

The present studies examined the effects of growth hormone (GH) on lipolysis and leptin release by adipose tissue from mice incubated for 24 h in primary culture. In adipose tissue from control mice GH enhanced lipolysis without affecting leptin release. The lipolytic action of GH was unaffected in adipose tissue from Stat5b-/- male mice but leptin release was enhanced by GH in fat from Stat5b-/- mice. In adipose tissue from Stat5ab-/- female mice no significant lipolytic action of GH was seen but leptin release was enhanced by GH. An insulin-like effect of GH on glucose conversion to lactate was also seen in mice deficient in Stat5ab-/-. These results suggest that the lipolytic action of GH involves the Stat5 proteins while the insulin-like effects of GH on glucose metabolism and leptin release involve different mechanisms.