Hypothalamo-pituitary-IGF-1 axis in female rats made obese by overfeeding

Early overnutrition sensitizes the growth hormone axis to the impact of diet-induced obesity via sex-divergent mechanisms

In addition to its essential role in the physiological control of longitudinal growth, growth-hormone (GH) is endowed with relevant metabolic functions, including anabolic actions in muscle, lipolysis in adipose-tissue and glycemic modulation. Adult obesity is known to negatively impact GH-axis, thereby promoting a vicious circle that may contribute to the exacerbation of the metabolic complications of overweight. Yet, to what extent early-overnutrition sensitizes the somatotropic-axis to the deleterious effects of obesity remains largely unexplored. Using a rat-model of sequential exposure to obesogenic insults, namely postnatal-overfeeding during lactation and high-fat diet (HFD) after weaning, we evaluated in both sexes the individual and combined impact of these nutritional challenges upon key elements of the somatotropic-axis. While feeding HFD per se had a modest impact on the adult GH-axis, early overnutrition had durable effects on key elements of the somatotropic-system, which were sexually different, with a significant inhibition of pituitary gene expression of GH-releasing hormone-receptor (GHRH-R) and somatostatin receptor-5 (SST5) in males, but an increase in pituitary GHRH-R, SST2, SST5, GH secretagogue-receptor (GHS-R) and ghrelin expression in females. Notably, early-overnutrition sensitized the GH-axis to the deleterious impact of HFD, with a significant suppression of pituitary GH expression in both sexes and lowering of circulating GH levels in females. Yet, despite their similar metabolic perturbations, males and females displayed rather distinct alterations of key somatotropic-regulators/ mediators. Our data document a synergistic effect of postnatal-overnutrition on the detrimental impact of HFD-induced obesity on key elements of the adult GH-axis, which is conducted via mechanisms that are sexually-divergent.

PDK1-FoxO1 pathway in AgRP neurons of arcuate nucleus promotes bone formation via GHRH-GH-IGF1 axis

Objective

In the hypothalamic arcuate nucleus (ARC), orexigenic agouti-related peptide (AgRP) neurons regulate feeding behavior and energy homeostasis, functions connected to bone metabolism. The 3-phosphoinositide-dependent protein kinase-1 (PDK1) serves as a major signaling molecule particularly for leptin and insulin in AgRP neurons. We asked whether PDK1 in AGRP neurons also contributes to bone metabolism.

Methods

We generated AgRP neuron-specific PDK1 knockout (Agrp Pdk1^−/−) mice and those with additional AgRP neuron-specific expression of transactivation-defective FoxO1 (Agrp Pdk1^−/−Δ256Foxo1). Bone metabolism in KO and WT mice was analyzed by quantitative computed tomography (QCT), bone histomorphometry, measurement of plasma biomarkers, and qPCR analysis of peptides.

Results

In Agrp Pdk1^−/− female mice aged 6 weeks, compared with Agrp Cre mice, both stature and femur length were shorter while body weight was unchanged. Cortical bone mineral density (BMD) and cancellous BMD in the femur decreased, and bone formation was delayed. Furthermore, plasma GH and IGF-1 levels were reduced in parallel with decreased mRNA expressions for GH in pituitary and GHRH in ARC. Osteoblast activity was suppressed and osteoclast activity was enhanced. These changes in stature, BMD and GH level were rescued in Agrp Pdk1^−/−Δ256Foxo1 mice, suggesting that the bone abnormalities and impaired GH release were mediated by enhanced Foxo1 due to deletion of PDK1.

Conclusions

This study reveals a novel role of PDK1-Foxo1 pathway of AgRP neurons in controlling bone metabolism primarily via GHRH-GH-IGF-1 axis.

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Agrp neuron-selective Pdk1 knockout mice exhibit short stature, shortened limbs and decreased bone density in both cortical and cancellous bones.

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In Agrp Pdk1 knockout mice, GHRH-GH-IGF1 axis was markedly down-regulated.

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Retarded bone growth and reduced GH in Agrp Pdk1 knockout mice were rescued by additional expression of dominant negative FoxO1 in AgRP neurons.

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Pdk1-FoxO1 signaling in AgRP neurons is linked to regulation of GHRH-GH-IGF1 axis and bone metabolism.

Growth Hormone Dynamics in Healthy Adults Are Related to Age and Sex and Strongly Dependent on Body Mass Index

BACKGROUND

Studies on 24-hour growth hormone (GH) secretion are rare. The influences of sex, age, and adiposity are well recognized but generally derived from specific, selected subject groups, not spanning sexes, many age decades, and a range of body weights.

OBJECTIVE

Our goal was to investigate GH dynamics in a group of 130 healthy adult subjects, both men and women, across 5 age decades as well as a 2.5-fold range of body mass index (BMI) values.

METHODS

GH was measured by a sensitive immunofluorometric assay. Secretion parameters were quantified by automated deconvolution and relative pattern randomness by approximate entropy (ApEn).

RESULTS

The median age was 40 years (range 20-77). The median BMI was 26 (range 18.3-49.8). Pulsatile 24-hour GH secretion was negatively correlated with age (p = 0.002) and BMI (p < 0.0001). Basal GH secretion negatively correlated with BMI (p = 0.003) but not with age. The sex- dependent GH secretion (greater in women) was no longer detectable after 50 years of age. Insulin-like growth factor (IGF)-1 levels were lower in women over 50 years of age compared with men of a similar age. ApEn showed an age-related increase in both sexes and was higher in premenopausal and postmenopausal women than in men of comparable age (p < 0.0001). A single fasting GH measurement is not informative of 24-hour GH secretion.

CONCLUSIONS

BMI dominates the negative regulation of 24-hour GH secretion across 5 decades of age in this up till now largest cohort of healthy adults who underwent 24-hour blood sampling. Sex also impacts GH secretion before the age of 50 years as well as its regularity at all ages. Differences in serum IGF-1 partly depend on the pre- or postmenopausal state. Finally, a single GH measurement is not informative of 24-hour GH secretion.

Insulin resistance impairs cutaneous wound healing in mice

Obesity is associated with significant changes in skin combined with metabolic alterations such as insulin resistance. Our aim was to investigate the effects of insulin resistance induced by a high-fat diet on cutaneous wound healing. Male C57BL/6 mice were fed standard chow (SC group) or high-fat chow (HFC group) for 30 weeks. On day 0 (28th week), an excisional wound was performed. After 14 days (30th week), the mice were euthanized. Starting from the 8th week, the HFC group had a higher body weight. The HFC group became intolerant to glucose, resistant to insulin, and presented increased plasma cholesterol and triglyceride levels. The wound area was greater in the HFC group. The inflammatory infiltrate and the amount of "fibroblast-like" cells increased in superficial regions of the lesions in the HFC group. The collagen fibers were more organized and denser in the SC group. Hydroxyproline levels were lower in the HFC group. The nitric oxide synthase-2-positive cells were higher in the HFC group. Lipid peroxidation and protein carbonyl levels were higher in the HFC group. The expression levels of alpha-smooth muscle actin and transforming growth factor-β were higher in the HFC group. These findings support the hypothesis that insulin resistance leads to delayed cutaneous wound healing.

Male and female rats with severe protein restriction present delayed wound healing

Malnutrition remains a significant problem, not only in developing countries, but also in the developed world. The aim of this study was to investigate the effects of protein restriction on rat excisional cutaneous healing. Male and female rats (12 weeks old) were exposed to different degrees of protein restriction (23%, 12% (slight restriction), and 0% (severe restriction)) for 12 weeks. On week 9, a full-thickness excisional skin lesion was performed, and the lesion area was measured to evaluate wound contraction and re-epithelialization. Euthanasia was performed after 12 weeks, and the lesion and adjacent skin were removed, fixed in formalin, and embedded in paraffin. Sections were stained with hematoxylin-eosin, toluidine blue, picro-mallory, and sirius red, and were immunostained for alpha-smooth muscle actin. Animals (males and females) exposed to severe protein restriction (0% protein) presented impairment of wound contraction. Inflammatory cells were present in higher amounts in the protein-restricted groups than in the 23% group. Extracellular matrix was poorly deposited in the severely restricted group (0%), but only mildly disturbed in the slightly restricted group (12%). Neovascularization was disturbed in both restricted groups. Our study demonstrates that animals exposed to slight protein restriction present disturbed wound healing, but animals exposed to severe protein restriction present impaired wound healing.

Overweight induced by high-fat diet delays rat cutaneous wound healing

Prolonged wound healing is a complication that contributes to morbidity and mortality. Overweight people regularly undergo surgery and trauma, and often develop chronic wounds, but the effects of the adipose tissue excess on cutaneous wound healing are not well understood. This study tested the hypothesis that overweight induced by a high-fat diet impairs rat cutaneous wound healing. Male Wistar rats were fed with either a high-fat or a standard (control) diet. After 15 weeks, an excisional lesion was done and the animals were killed 21 d later. Wound contraction and re-epithelialization, blood pressure, glucose and retroperitoneal fat were evaluated. After killing, lesion and adjacent normal skin were formol-fixed and paraffin-embedded. Inflammatory infiltrate, myofibroblasts, collagen fibres and cellular proliferation were analysed and blood vessels were evaluated using stereological methods. There was no difference in blood pressure and glucose, but retroperitoneal fat increased in the high-fat diet group. Animals fed with the high-fat diet presented delayed wound contraction and re-epithelialization. It was found that 21 d after wounding, overweight induced by a high-fat diet increased the inflammatory infiltrate and delayed myofibroblastic differentiation, collagen deposition, epithelial and connective tissue cell proliferation, and angiogenesis. These findings support the hypothesis that a high-fat diet exerts negative effects on rat cutaneous wound healing, due mainly to the prolongation of the inflammatory phase.

Ghrelin-induced food intake and growth hormone secretion are altered in melanocortin 3 and 4 receptor knockout mice

Ghrelin stimulates food intake in part by activating hypothalamic neuropeptide Y (NPY) neurons/agouti related peptide (AGRP) neurons. We investigated the role of AGRP/melanocortin signaling in ghrelin-induced food intake by studying melanocortin 3 and 4 receptor knockout (MC3R KO and MC4R KO) mice. We also determined whether reduced ghrelin levels and/or an altered sensitivity to the GH-stimulating effects of ghrelin accompany the obesity syndromes of MC3R KO and MC4R KO mice. Compared to wild-type (WT) mice, the effects of ghrelin on food intake were reduced in MC3R KO and MC4R KO mice and circulating ghrelin levels were reduced in female MC4R KO mice. Female MC3R KO and MC4R KO mice exhibited a diminished responsiveness to the GH-releasing effects of ghrelin. Thus, deletion of the MC3R or MC4R results in a decreased sensitivity to ghrelin and verifies the involvement in the melanocortin system in ghrelin-induced food intake.

Effects of long-term intake of edible oils on hypertension and myocardial and aortic remodelling in spontaneously hypertensive rats

BACKGROUND

The nature of dietary lipid intake contributes to blood pressure control.

OBJECTIVE

To test whether different edible lipid compounds are either beneficial or harmful to blood pressure and cardiac and aortic structure.

METHODS

Six groups of 3-month-old male spontaneously hypertensive rats (n = 5) received different edible oils (fish, canola, palm, olive and soybean oils, 1.5 g/kg per day + 1 IU/ml vitamin E) or a placebo (water) by gavage for 13 weeks. Stereology was used to analyse left ventricular cardiomyocytes, intramyocardial vessels, connective tissue, aortic lamellae and tunica media smooth muscle cells.

RESULTS

Fish oil decreased blood pressure, and increases in blood pressure were prevented by both canola and palm oils. The cardiomyocyte and intramyocardial vessel indices were greater in the fish-, canola- and palm-oil groups and smaller in the soybean-oil, olive-oil and control groups; the opposite effects were found in interstitial connective tissue. The number of lamellae was smaller in the fish-oil group but greater in the soybean-oil, canola-oil, and olive-oil groups. Canola oil reduced aortic wall thickness, but palm oil did not. The number of smooth muscle cells was smaller in the groups given fish, canola and olive oils.

CONCLUSION

The most beneficial cardiac and aortic structural effects occurred in the fish-oil group. Both canola oil and palm oil were also effective in reducing blood pressure, favouring myocardial remodelling, although they produced contrasting effects with regard to aorta wall structure. Soybean oil and olive oil had mild effects on myocardial and aortic structure.

Neuroendocrine control of growth hormone secretion

The secretion of growth hormone (GH) is regulated through a complex neuroendocrine control system, especially by the functional interplay of two hypothalamic hypophysiotropic hormones, GH-releasing hormone (GHRH) and somatostatin (SS), exerting stimulatory and inhibitory influences, respectively, on the somatotrope. The two hypothalamic neurohormones are subject to modulation by a host of neurotransmitters, especially the noradrenergic and cholinergic ones and other hypothalamic neuropeptides, and are the final mediators of metabolic, endocrine, neural, and immune influences for the secretion of GH. Since the identification of the GHRH peptide, recombinant DNA procedures have been used to characterize the corresponding cDNA and to clone GHRH receptor isoforms in rodent and human pituitaries. Parallel to research into the effects of SS and its analogs on endocrine and exocrine secretions, investigations into their mechanism of action have led to the discovery of five separate SS receptor genes encoding a family of G protein-coupled SS receptors, which are widely expressed in the pituitary, brain, and the periphery, and to the synthesis of analogs with subtype specificity. Better understanding of the function of GHRH, SS, and their receptors and, hence, of neural regulation of GH secretion in health and disease has been achieved with the discovery of a new class of fairly specific, orally active, small peptides and their congeners, the GH-releasing peptides, acting on specific, ubiquitous seven-transmembrane domain receptors, whose natural ligands are not yet known.