Jörgen Isgaard

Ghrelin and des-acyl ghrelin inhibit cell death in cardiomyocytes and endothelial cells through ERK1/2 and PI 3-kinase/AKT

Ghrelin is an acyl-peptide gastric hormone acting on the pituitary and hypothalamus to stimulate growth hormone (GH) release, adiposity, and appetite. Ghrelin endocrine activities are entirely dependent on its acylation and are mediated by GH secretagogue (GHS) receptor (GHSR)-1a, a G protein–coupled receptor mostly expressed in the pituitary and hypothalamus, previously identified as the receptor for a group of synthetic molecules featuring GH secretagogue (GHS) activity. Des-acyl ghrelin, which is far more abundant than ghrelin, does not bind GHSR-1a, is devoid of any endocrine activity, and its function is currently unknown. Ghrelin, which is expressed in heart, albeit at a much lower level than in the stomach, also exerts a cardio protective effect through an unknown mechanism, independent of GH release. Here we show that both ghrelin and des-acyl ghrelin inhibit apoptosis of primary adult and H9c2 cardiomyocytes and endothelial cells in vitro through activation of extracellular signal–regulated kinase-1/2 and Akt serine kinases. In addition, ghrelin and des-acyl ghrelin recognize common high affinity binding sites on H9c2 cardiomyocytes, which do not express GHSR-1a. Finally, both MK-0677 and hexarelin, a nonpeptidyl and a peptidyl synthetic GHS, respectively, recognize the common ghrelin and des-acyl ghrelin binding sites, inhibit cell death, and activate MAPK and Akt. These findings provide the first evidence that, independent of its acylation, ghrelin gene product may act as a survival factor directly on the cardiovascular system through binding to a novel, yet to be identified receptor, which is distinct from GHSR-1a.

Aspects of Growth Hormone and Insulin-Like Growth Factor-I Related to Neuroprotection, Regeneration, and Functional Plasticity in the Adult Brain

Apart from regulating somatic growth and metabolic processes, accumulating evidence suggests that the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis is involved in the regulation of brain growth, development, and myelination. In addition, both GH and IGF-I affect cognition and biochemistry in the adult brain. Some of the effects of GH are attributable to circulating IGF-I, while others may be due to IGF-I produced locally within the brain. Some of the shared effects in common to GH and IGF-I may also be explained by cross-talk between the GH and IGF-I transduction pathways, as indicated by recent data from other cell systems. Otherwise, it also seems that GH may act directly without involving IGF-I (either circulating or locally). Plasticity in the central nervous system (CNS) may be viewed as changes in the functional interplay between the major cell types, neurons, astrocytes, and oligodendrocytes. GH and IGF-I affect all three of these cell types in several ways. Apart from the neuroprotective effects of GH and IGF-I posited in different experimental models of CNS injury, IGF-I has been found to increase progenitor cell proliferation and new neurons, oligodendrocytes, and blood vessels in the dentate gyrus of the hippocampus. It appears that the MAPK signaling pathway is required for IGF-I—stimulated proliferation in vitro, whereas the PI3K/Akt or MAPK/Erk signaling pathway appears to mediate antiapoptotic effects. The increase of IGF-I on endothelial cell phenotype may explain the increase in cerebral arteriole density observed after GH treatment. The functional role of GH and IGF-I in the adult brain will be reviewed with reference to neurotransmitters, glucose metabolism, cerebral blood flow, gap junctional communication, dendritic arborization, exercise, enriched environment, depression, learning, memory, and aging.Briefly, these findings suggest that IGF-I functions as a putative regenerative agent in the adult CNS. Hitherto less studied regarding in these aspects, GH may have similar effects, especially as it is the main regulator of IGF-I in vivo. Some of the positive cognitive features of GH treatment are likely attributable to the mechanisms reviewed here.

The Role of Liver-Derived Insulin-Like Growth Factor-I

IGF-I is expressed in virtually every tissue of the body, but with much higher expression in the liver than in any other tissue. Studies using mice with liver-specific IGF-I knockout have demonstrated that liver-derived IGF-I, constituting a major part of circulating IGF-I, is an important endocrine factor involved in a variety of physiological and pathological processes. Detailed studies comparing the impact of liver-derived IGF-I and local bone-derived IGF-I demonstrate that both sources of IGF-I can stimulate longitudinal bone growth. We propose here that liver-derived circulating IGF-I and local bone-derived IGF-I to some extent have overlapping growth-promoting effects and might have the capacity to replace each other (= redundancy) in the maintenance of normal longitudinal bone growth. Importantly, and in contrast to the regulation of longitudinal bone growth, locally derived IGF-I cannot replace (= lack of redundancy) liver-derived IGF-I for the regulation of a large number of other parameters including GH secretion, cortical bone mass, kidney size, prostate size, peripheral vascular resistance, spatial memory, sodium retention, insulin sensitivity, liver size, sexually dimorphic liver functions, and progression of some tumors. It is clear that a major role of liver-derived IGF-I is to regulate GH secretion and that some, but not all, of the phenotypes in the liver-specific IGF-I knockout mice are indirect, mediated via the elevated GH levels. All of the described multiple endocrine effects of liver-derived IGF-I should be considered in the development of possible novel treatment strategies aimed at increasing or reducing endocrine IGF-I activity.

IGF-I neuroprotection in the immature brain after hypoxia-ischemia, involvement of Akt and GSK3β?

Insulin‐like growth factor I (IGF‐I) is a neurotrophic factor that promotes neuronal growth, differentiation and survival. Neuroprotective effects of IGF‐I have previously been shown in adult and juvenile rat models of brain injury. We wanted to investigate the neuroprotective effect of IGF‐I after hypoxia‐ischemia (HI) in 7‐day‐old neonatal rats and the mechanisms of IGF‐I actions in vivo. We also wanted to study effects of HI and/or IGF‐I on the serine/threonine kinases Akt and glycogen synthase kinase 3β (GSK3β) in the phophatidylinositol‐3 kinase (PI3K) pathway. Immediately after HI, phosphorylated Akt (pAkt) and phosphorylated GSK3β (pGSK3β) immunoreactivity was lost in the ipsilateral and reduced in the contralateral hemisphere. After 45 min, pAkt levels were restored to control values, whereas pGSK3β remained low 4 h after HI. Administration of IGF‐I (50 µg i.c.v.) after HI resulted in a 40% reduction in brain damage (loss of microtubule‐associated protein) compared with vehicle‐treated animals. IGF‐I treatment without HI was shown to increase pAkt whereas pGSK3β decreased in the cytosol, but increased in the nuclear fraction. IGF‐I treatment after HI increased pAkt in the cytosol and pGSK3β in both the cytosol and the nuclear fraction in the ipsilateral hemisphere compared with vehicle‐treated rats, concomitant with a reduced caspase‐3‐ and caspase‐9‐like activity. In conclusion, IGF‐I induces activation of Akt during recovery after HI which, in combination with inactivation of GSK3β, may explain the attenuated activation of caspases and reduction of injury in the immature brain.

Endocrine regulation of longitudinal bone growth

Longitudinal bone growth is the result of a strictly regulated endochondral ossification process in the long bones. During this process, a cartilaginous structure is produced by proliferating chondrocytes in the epiphyseal growth plate, and this cartilaginous template then degenerates and is replaced by bone tissue in the direction of the metapbysis (Fig. 1). The greatest height velocity in humans occurs during the first year of life. Thereafter, height velocity gradually declines until puberty, when a growth spurt occurs. Human epiphyseal growth plates are closed after puberty, and this results in cessation of longitudinal bone growth. The regulation of longitudinal bone growth is complex, and several factors, including nutritional, endocrine, paracrine and autocrine ones, are necessary for optimal longitudinal bone growth.

Long‐term cardiovascular effects of growth hormone treatment in GH‐deficient adults Preliminary data in a small group of patients

OBJECTIVE The long‐term cardiovascular effects of GH administration in adults are of major clinical importance, given the increasing use of such treatment. We have evaluated long‐term cardiovascular effects of recombinant human GH (rhGH) substitution in GH deficient men.

Possible Protective Role of Growth Hormone in Hypoxia-Ischemia in Neonatal Rats

Perinatal asphyxia still constitutes a clinical hazard associated with considerable neurologic morbidity. Several growth factors, including insulin-like growth factor-I (IGF-I), have been reported to have a neuroprotective effect in experimental models of hypoxic ischemia (HI). In the present study, we have applied solution hybridization for quantification of the time course for mRNA expression of IGF-I, IGF-I receptor, and growth hormone (GH) receptor after HI in 7-d-old rats. There was a significant increase in IGF-I mRNA in the damaged hemisphere 72 h (1.19 ± 0.28vs 0.48 ± 0.02 amol/µg DNA, p < 0.05) and 14 d (0.61 ± 0.18 vs 0.19 ± 0.05 amol/µg DNA, p < 0.05) after HI. In the contralateral hemisphere, both IGF-I and GH receptor mRNA had increased by 14 d after the insult (0.36 ± 0.042 vs 0.13 ± 0.011, p < 0.05, and 0.31 ± 0.013 vs 0.11 ± 0.004 amol/µg DNA, p < 0.001, respectively). There were no changes in IGF-I receptor mRNA throughout the study period. We have also evaluated the neuroprotective effect of GH after HI in neonatal rats. GH administered s.c. after HI in daily doses of 50 and 100 mg/kg provided a moderate neuroprotection of 20%. These results suggest a role for the GH/IGF-I axis in the neurochemical process leading to HI brain injury.

Effects of unilateral arterial infusion of GH and IGF-I on tibial longitudinal bone growth in hypophysectomized rats.

SummaryWe have studied the effect of local arterial infusion of bacterially produced human growth hormone (hGH), insulinlike growth factor I (IGF-I), or pituitary-derived ovine prolactin (oPRL) on longitudinal bone growth of hypophysectomized rats. The substances were infused during a 14-day period by osmotic mini-pumps through a catheter which was implanted into the femoral artery of one hindlimb. Longitudinal bone growth was measured by the intravital marker tetracycline. Infusion of 1 μg hGH per day stimulated bone growth only of the treated limb and not of the uninfused contralateral limb. Infusion of 10 μg hGH per day also stimulated unilateral longitudinal bone growth, but the uninfused contralateral limb also showed a significant growth response, probably because local administration of GH at this dose caused a significant elevation of GH in the systemic circulation. As a result, the differential growth response between the GH-treated and untreated limbs decreased compared to rats that were infused with 1 μg hGH per day. Unilateral arterial infusion of 5 μg human IGF-I or 10 μg oPRL per day did not produce a significant growth response. The results of the present study confirm the observation by Schlechter and co-workers [9, 16], who demonstrated that unilateral arterial infusion of GH maintained tibial cartilage width following hypophysectomy in the rat. The results of Schlechter and co-workers and the results of the present study show that GHin vivo stimulates epiphyseal cartilage growth directly. However, an increased local production of insulinlike growth factors is probably of importance for the expression of the direct effect of GH on longitudinal bone growth. The present results do not completely rule out the possibility that insulinlike growth factors in the circulation might have the growth plate as a target organ.

Metformin Prevents the Development of Chronic Heart Failure in the SHHF Rat Model

Insulin resistance is a recently identified mechanism involved in the pathophysiology of chronic heart failure (CHF). We investigated the effects of two insulin-sensitizing drugs (metformin and rosiglitazone) in a genetic model of spontaneously hypertensive, insulin-resistant rats (SHHF). Thirty SHHF rats were randomized into three treatment groups as follows: 1) metformin (100 mg/kg per day), 2) rosiglitazone (2 mg/kg per day), and 3) no drug. Ten Sprague-Dawley rats served as normal controls. At the end of the treatment period (12 months), the cardiac phenotype was characterized by histology, echocardiography, and isolated perfused heart studies. Metformin attenuated left ventricular (LV) remodeling, as shown by reduced LV volumes, wall stress, perivascular fibrosis, and cardiac lipid accumulation. Metformin improved both systolic and diastolic indices as well as myocardial mechanical efficiency, as shown by improved ability to convert metabolic energy into mechanical work. Metformin induced a marked activation of AMP-activated protein kinase, endothelial nitric oxide synthase, and vascular endothelial growth factor and reduced tumor necrosis factor-α expression and myocyte apoptosis. Rosiglitazone did not affect LV remodeling, increased perivascular fibrosis, and promoted further cardiac lipid accumulation. In conclusion, long-term treatment with metformin, but not with rosiglitazone, prevents the development of severe CHF in the SHHF model by a wide-spectrum interaction that involves molecular, structural, functional, and metabolic-energetic mechanisms.

Regulation of cartilage growth by growth hormone and insulin-like growth factor I

A number of studies have shown that growth hormone (GH) and insulin-like growth factor-I (IGF-I) have important regulatory roles for skeletal growth. However, it has been a matter of controversy whether GH acts directly on cells in the growth plate or if the growth-promoting effects of GH are mediated by liver-derived (endocrine-acting) IGF-I. With the recognition that GH regulates the production of IGF-I in multiple extra-hepatic tissues, autocrine and paracrine functions of IGF-I have been suggested as important components of GH action. This review focuses on recent developments in our understanding of the cellular mechanisms by which GH promotes longitudinal bone growth and the inter-relationship between GH and IGF-I in the growth plate.