Insulin-like growth factor-1 and growth hormone (GH) levels in canine cerebrospinal fluid are unaffected by GH or GH secretagogue (MK-0677) administration

From Belly to Brain: Targeting the Ghrelin Receptor in Appetite and Food Intake Regulation

Ghrelin is the only known peripherally-derived orexigenic hormone, increasing appetite and subsequent food intake. The ghrelinergic system has therefore received considerable attention as a therapeutic target to reduce appetite in obesity as well as to stimulate food intake in conditions of anorexia, malnutrition and cachexia. As the therapeutic potential of targeting this hormone becomes clearer, it is apparent that its pleiotropic actions span both the central nervous system and peripheral organs. Despite a wealth of research, a therapeutic compound specifically targeting the ghrelin system for appetite modulation remains elusive although some promising effects on metabolic function are emerging. This is due to many factors, ranging from the complexity of the ghrelin receptor (Growth Hormone Secretagogue Receptor, GHSR-1a) internalisation and heterodimerization, to biased ligand interactions and compensatory neuroendocrine outputs. Not least is the ubiquitous expression of the GHSR-1a, which makes it impossible to modulate centrally-mediated appetite regulation without encroaching on the various peripheral functions attributable to ghrelin. It is becoming clear that ghrelin’s central signalling is critical for its effects on appetite, body weight regulation and incentive salience of food. Improving the ability of ghrelin ligands to penetrate the blood brain barrier would enhance central delivery to GHSR-1a expressing brain regions, particularly within the mesolimbic reward circuitry.

Upregulation of insulin-like growth factor binding protein 3 in astrocytes of transgenic mice that express Borna disease virus phosphoprotein

In a previous study, we demonstrated that transgenic mice that express Borna disease virus (BDV) phosphoprotein (P) in astrocytes show striking neurobehavioral abnormalities resembling those in BDV-infected animals. To understand the molecular disturbances induced by the expression of P in astrocytes, we performed microarray analysis with cultured astroglial cells transiently expressing P. We showed that expression of insulin-like growth factor binding protein 3 mRNA increases not only in P-expressing cultured cells but also in astrocytes from the cerebella of P transgenic mice (P-Tg). Furthermore, we demonstrated that insulin-like growth factor signaling is disturbed in the P-Tg cerebellum, a factor that might be involved in the increased vulnerability of Purkinje cell neurons in the brain.

Increased intraocular insulin-like growth factor-I triggers blood-retinal barrier breakdown

Blood-retinal barrier (BRB) breakdown is a key event in diabetic retinopathy and other ocular disorders that leads to increased retinal vascular permeability. This causes edema and tissue damage resulting in visual impairment. Insulin-like growth factor-I (IGF-I) is involved in these processes, although the relative contribution of increased systemic versus intraocular IGF-I remains controversial. Here, to elucidate the role of this factor in BRB breakdown, transgenic mice with either local or systemic elevations of IGF-I have been examined. High intraocular IGF-I, resulting from overexpression of IGF-I in the retina, increased IGF-I receptor content and signaling and led to accumulation of vascular endothelial growth factor. This was parallel to up-regulation of vascular Intercellular adhesion molecule I and retinal infiltration by bone marrow-derived microglial cells. These alterations resulted in increased vessel paracellular permeability to both low and high molecular weight compounds in IGF-I-overexpressing retinas and agreed with the loss of vascular tight junction integrity observed by electron microscopy and the altered junctional protein content. In contrast, mice with chronically elevated serum IGF-I did not show alterations in the retinal vasculature structure and permeability, indicating that circulating IGF-I cannot initiate BRB breakdown. Consistent with a key role of IGF-I signaling in retinal diseases, a strong up-regulation of the IGF-I receptor in human retinas with marked gliosis was also observed. Thus, this study demonstrates that intraocular IGF-I, but not systemic IGF-I, is sufficient to trigger processes leading to BRB breakdown and increased retinal vascular permeability. Therefore, therapeutic interventions designed to counteract local IGF-I effects may prove successful to prevent BRB disruption.

Insulin-like growth factor-I as a promoting factor for cerebellar Purkinje cell development

In the mammalian CNS, the peptide hormone insulin-like growth factor-I (IGF-I) is synthesized in a certain subset of neurons and, it has been suggested, serves as a local neurotrophic factor. A postnatal increase in the expression of IGF-I and the type-1 IGF receptors (IGFR1) in the cerebellar cortex and its related brain regions indicates that developing cerebellar Purkinje cells (PC) may be an important target of IGF-I. However, little is known about how IGF-I influences PC development. Here we addressed this question, using a reduced environment of cerebellar neuron culture derived from perinatal mice. IGF-I exogenously applied at a physiological concentration (10 nm) greatly promoted the dendritic growth and survival of the PCs. By contrast, IGF-I only slightly promoted the somatic growth and little affected the maturation of the electrophysiological excitability of the PCs. The closely related hormone insulin had weaker promoting effects than did IGF-I. IGF-I appeared to at least bind to IGFR1 and to up-regulate the signalling pathways involving the phosphoinositide 3-kinase (PI3-K), mitogen-activated protein kinase (MAPK), p38 kinase (p38K), and an unknown signalling molecule(s). These signalling pathways may be coupled to the individual aspects of PC development in different manners and this may explain the difference in effects of IGF-I among these aspects. These findings suggest that IGF-I serves as a promoting factor for PC development, particularly postnatal survival and dendritic growth.

Growth hormone in the nervous system: autocrine or paracrine roles in retinal function?

Growth hormone (GH) is primarily produced in the pituitary gland, although GH gene expression also occurs in the central and autonomic nervous systems. GH-immunoreactive proteins are abundant in the brain, spinal cord, and peripheral nerves. The appearance of GH in these tissues occurs prior to the ontogenic differentiation of the pituitary gland and prior to the presence of GH in systemic circulation. Neural GH is also present in neonates, juveniles, and adults and is independent of changes in pituitary GH secretion. Neural GH is therefore likely to have local roles in neural development or neural function, especially as GH receptors (GHRs) are widespread in the nervous system. In recent studies, GH mRNA and GH immunoreactive proteins have been identified in the neural retina of embryonic chicks. GH immunoreactivity is present in the optic cup of chick embryos at embryonic day (ED) 3 of the 21-d incubation period. It is widespread in the neural retina by ED 7 but also present in the nonpigmented retina, choroid, sclera, and cornea. This immunoreactivity is associated with proteins in the neural retina comparable in size with those in the adult pituitary gland, although it is primarily associated with 15-16 kDa moieties rather than with the full-length molecule of approximately 22 kDa. These small GH moieties may reflect proteolytic fragments of "monomer" GH and (or) the presence of different GH gene transcripts, since full-length and truncated GH cDNAs are present in retinal tissue extracts. The GH immunoreactivity in the retina persists throughout embryonic development but is not present in juvenile birds (after 6 weeks of age). This immunoreactivity is also associated with the presence of GH receptor (GHR) immunoreactivity and GHR mRNA in ocular tissues of chick embryos. The retina is thus an extrapituitary site of GH gene expression during early development and is probably an autocrine or paracrine site of GH action. The marked ontogenic pattern of GH immunoreactivity in the retina suggests hitherto unsuspected roles for GH in neurogenesis or ocular development.

Growth hormone (GH) action in the brain: neural expression of a GH-response gene

The presence of growth hormone (GH) binding sites and GH-receptor (GHR)-immunoreactive proteins in the brain suggests it is a target site for GH action. This could, however, reflect the presence of GH-binding proteins (GHBP) that are not linked to intracellular signal-transduction mechanisms, rather than authentic receptors. The possibility that GH has actions in the brain therefore has been examined by determining an intracellular mediator of GH action. The mechanism of GH action involves the induction of a number of specific GH-response genes. In chickens, a novel GH-responsive gene (GHRG-1) has been identified as an intracellular marker of GH action, since this gene is not expressed in GH-resistant dwarfs with dysfunctional GHRs and in normal chickens it is upregulated by exogenous GH. In normal chickens GHRG-1 mRNA is also abundant and widespread in the brain. In the cerebellum it is specifically localized in the cerebellar folia. It is present in most cells in the granular layers of the gray matter but is present in only a small number of scattered cells in the molecular layer and white matter. Intense labeling for GHRG-1 mRNA is also present in the large Purkinje cells and their dendrites at the interface between the molecular and granular layers. Labeling is also seen in the interneuronal basket cells projecting onto the Purkinje cells. In the mid-brain, cells in the ocular nerve complex and the tractus isthmo-opticus were strongly stained for GHRG-1 mRNA, with less intense staining in the central gray. In the hypothalamus, numerous small cells in periventricular locations and ependymal cells lining the III ventricle also label for GHRG-1 mRNA. These results clearly show, for the first time, the expression of a GH-responsive gene in neural tissues. Moreover, as GH- and GHR-immunoreactivity previously has been shown to be present in the same tissues expressing GHRG-1, it is possible that GH acts as an autocrine or paracrine within the CNS.

Transcriptional regulation of iodothyronine deiodinases during embryonic development

A single dose of chicken growth hormone (cGH) or dexamethasone acutely increases circulating T(3) levels in 18-day-old chicken embryos through a reduction of hepatic type III iodothyronine deiodinase (D3). The data in the present study suggest that this decrease in D3 is induced by a direct downregulation of hepatic D3 gene transcription. The lack of effect of cGH or dexamethasone on brain and kidney D3 activity, furthermore suggests that both hormones affect peripheral thyroid hormone metabolism in a tissue specific manner. Dexamethasone administration also results in an increase in brain type II iodothyronine deiodinase (D2) activity and mRNA levels that is also regulated at a transcriptional level. In contrast, however, cGH has no effect on brain D2 activity, thereby suggesting that either GH cannot pass through the blood-brain barrier in chicken or that cGH and dexamethasone regulate thyroid hormone deiodination by different mechanisms. In addition, the very short half-life of D2 and D3 (t(1/2)<1 h) in comparison with the longer half life of type I iodothyronine deiodinase (D1, t(1/2)>8 h), allows for D2 and D3 to play a more prominent role in the acute regulation of peripheral thyroid hormone metabolism than D1.

IGF-I mRNA and signaling in the diabetic retina

IGF-I promotes the survival of multiple cell types by activating the IGF-I receptor (IGF-IR), which signals downstream to a serine/threonine kinase termed Akt. Because in diabetes vascular and neural cells of the retina undergo accelerated apoptosis, we examined IGF-I synthesis and signaling in the human and rat diabetic retina. In retinas obtained postmortem from six donors aged 64 +/- 8 years with a diabetes duration of 7 +/- 5 years, IGF-I mRNA levels were threefold lower than in the retinas of six age-matched nondiabetic donors (P = 0.005). In the retinas of rats with 2 months' duration of streptozotocin-induced diabetes, IGF-I mRNA levels were similar to those of control rats, but after 5 months of diabetes they failed to increase to the levels recorded in age-matched controls (P < 0.02). Retinal IGF-I expression was not altered by hypophysectomy, proving to be growth-hormone independent. IGF-IR levels were modestly increased in the human diabetic retinas (P = 0.02 vs. nondiabetic retinas) and were unchanged in the diabetic rats. Phosphorylation of the IGF-IR could be measured only in the rat retina, and was not decreased in the diabetic rats (94 +/- 18% of control values). In the same diabetic rats, phosphorylation of Akt was 123 +/- 21% of control values. There was not yet evidence of increased apoptosis of retinal microvascular cells after 5 months of streptozotocin-induced diabetes. Hence, in the retina of diabetic rats, as in the retina of diabetic human donors, IGF-I mRNA levels are substantially lower than in age-matched nondiabetic controls, whereas IGF-IR activation and signaling are not affected, at least for some time. This finding suggests that in the diabetic retina, the activation of the IGF-IR is modulated by influences that compensate for, or are compensated by, decreased IGF-I synthesis.

Pituitary and extrapituitary growth hormone: Pit-1 dependence?

Growth hormone (GH) is primarily produced in pituitary somatotrophs. The synthesis of this hormone is thought to be dependent upon a pituitary-specific transcription factor (Pit-1). However, many extrapituitary tissues are now known to express GH genes. The extrapituitary production of GH may therefore indicate an extrapituitary distribution of the Pit-1 gene. The extrapituitary production of GH may, alternatively, indicate that GH expression occurs independently of Pit-1 in extrapituitary tissues. These possibilities are considered in this brief review.Key words: growth hormone, pituitary, pituitary transcription factor 1.