General and Comparative Endocrinology 153 (2007) 124–131

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Growth hormone in the visual system: Comparative endocrinology

Steve Harvey ¤, Brent T. Martin, Marie-Laure Baudet, Phil Davis, Yves Sauve, Esmond J. Sanders

Department of Physiology, University of Alberta, Edmonton, Alta., Canada T6G 2H7

Received 19 September 2006; revised 21 November 2006; accepted 25 December 2006Available online 11 January 2007

Abstract

Growth hormone (GH) is rarely considered to be involved in ocular development or vision or to be present in the visual system. Basicand clinical studies nevertheless support roles for GH in the ocular function of most vertebrate groups and for its extrapituitary produc-tion in ocular tissues. The comparative endocrinology of endocrine, autocrine or paracrine GH in the visual system of vertebrates is thefocus of this brief review.© 2007 Elsevier Inc. All rights reserved.

Keywords: Growth hormone; Visual system; Retina; Ganglion cells; Vision

1. Introduction

Traditionally, growth hormone (GH) has been viewed asan endocrine, produced and released from pituitarysomatotrophs into systemic circulation, with actions at dis-tant target sites. This view has, however, been challenged bythe realization that GH gene expression also occurs innumerous extrapituitary sites (e.g. in the nervous system,Harvey and Hull, 2003; in the immune system, Clark, 1997;in the reproductive system, Hull and Harvey, 2000; in therespiratory system, Beyea et al., 2005; in the digestive sys-tem, Tresguerres et al., 1999; in the integumentary system,Palmetshofer et al., 1995), in which it is thought to act as anautocrine or paracrine growth factor, either directly or viaits local induction of insulin-like growth factors (IGFs)(Sanders and Harvey, 2004). This possibility is supportedby the recent discovery of GH in the visual system of verte-brates and of hitherto unsuspected roles of GH in ocularfunction.

* Corresponding author. Fax: +1 780 492 3956.E-mail address: steve.harvey@ualberta.ca (S. Harvey).

0016-6480/$ - see front matter © 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.ygcen.2006.12.024

2. GH in the visual system of Wsh

Recent evidence indicates GH/IGF involvement in thepersistent growth-associated neurogenesis of the teleost ret-ina, in which GH receptors (GHRs) are expressed (Ottesonand Hitchcock, 2003). The teleost retina is thus a target sitefor GH action. Indeed, exogenous GH stimulates themitotic activity of retinal progenitors within the circumfer-ential germinal zone of the retina and in cells of the rod-photoreceptor lineage, including the stem cells within theinner nuclear layer (Otteson et al., 2002). As this action issimilar to that induced by IGF-1 (Otteson and Hitchcock,2003) and as GH stimulates IGF-1 expression in the retina(Otteson et al., 2002), this action may be direct or IGF-1mediated. As this action is induced by intraperitoneal injec-tions of GH, it is thought to reXect changes in pituitary GHsecretion, as are ontogenetic changes in retinal size.

3. GH in the visual system of amphibia

In frogs, hypophysectomy arrests cell proliferation in thelens epithelium (Wainwright et al., 1978). This is thought toreXect the loss of pituitary GH, since lens epithelial cell pro-liferation can be restored by exogenous GH (Wainwrightet al., 1978; Rothstein et al., 1980) and/or by IGF-1

S. Harvey et al. / General and Comparative Endocrinology 153 (2007) 124–131 125

(Rothstein et al., 1980; Redden and Wilson-Dziedzic, 1983).As this action of hypophysectomy and GH replacementdoes not occur in altricial rodents such as rats (Klein et al.,1989), it may reXect diVerences between the vertebrategroups in the accessibility of intraocular target sites to GHin systemic circulation and the permeability of the blood–ocular barriers.

4. GH in the visual system of birds

Precocial avian species, like chickens, have open, well-developed eyes at hatch. As pituitary somatotrophs do notdiVerentiate until the last trimester of the 21 day incubationperiod and as GH is not present in systemic circulationuntil at least embryonic day (ED) 17 (Harvey et al., 1998),most ocular development of the chick embryo occurs in theabsence of pituitary GH. Ocular development in the chickembryos is, however, unlikely to be GH-independent, asGH is present throughout the visual system.

4.1. GH localization

Immunocytochemistry, using three diVerent antibodiesraised against pituitary GH, has demonstrated the presenceof GH-like proteins in the eyes of early chick embryos priorto the development of the pituitary gland. Indeed, GHimmunoreactivity is abundantly present in the optic cup byED2 (Harvey et al., 2001). By ED7 this immunoreactivity ispresent throughout the neural retina and in the pigmentedretinal epithelium (RPE), choroid, sclera, and vitreous(Harvey et al., 2001, 2003, 2004; Baudet et al., 2003) and inthe cornea and lens epithelium (Fig. 1). It is particularlyintense in the retinal ganglion cells (RGCs) (Sanders et al.,2003) and their axons that form fascicles in the optic Wbrelayer (OFL) (Harvey et al., 2003a; Baudet et al., 2007). TheGH immunoreactivity in these Wbres has been mapped intothe optic nerve head, the optic nerve and the optic chiasmat the back of the eye, through which they decussate toform the optic tract (Harvey et al., 2003a). GH immunore-activity is also present in the optic tectum (Harvey et al.,2003a), the primary retinal projection centre of the brain,even before (at ED5–ED6) synaptogenesis occurs with theoptic tract (Mey and Thanos, 1992). In the vitreous, GHimmunoreactivity is at concentrations comparable withthose in adult pituitary tissue and likely reXects its secretionfrom retinal tissue (Baudet et al., 2003; Harvey et al.,2003b). The immunoreactivity in these ocular tissues ismostly associated with a submonomer protein of 15 kDa(Baudet et al., 2003), which is partly bound to a 45 kDa pro-teoglycan, opticin, in vitreous and with which it is colocal-ized in RGCs (Sanders et al., 2003). Another submonomerprotein (16 kDa) with GH immunoreactivity is also presentin ocular tissues, but it is not secreted from retinal explantsin vitro nor present in vitreous humor (Baudet et al., 2003,2007; Sanders et al., 2003). This protein is not, however, thesmall chicken GH (scGH) variant (with a predicted size of16.5 kDa), derived from the novel truncated transcript dis-

covered by Takeuchi et al. (2001) in the chick eye, since thisprotein has no cross-reactivity with antibodies raisedagainst pituitary (25 kDa) GH (Baudet et al., 2007). SpeciWcscGH immunoreactivity in the chick eye is mostly associ-ated with a protein of 31 kDa, indicating that scGH is nor-mally dimerized or tightly bound to another protein(Baudet et al., 2007). Immunoreactivity for scGH is presentin most ocular tissues, particularly in the corneal epitheliumand lens epithelium, but it is not in the optic nerve head,nor in most samples of vitreous (Harvey et al., 2006a; Bau-det et al., 2007). While scGH is present in retinal ganglioncells, like full-length GH immunoreactivity, it is (in markedcontrast), not present in their axons that comprise the OFL(Fig. 2). This diVerential localization suggests tissue speciWcexpression or translation of GH mRNA in the visual sys-tem of the embryonic chicks and/or diVerential traYckingof the translated proteins.

The subcellular localization of GH immunoreactivity incells in the RGC layer of ED7 chicks is in both cytoplasmicand nuclear compartments (Baudet et al., 2003; Sanderset al., 2003), as is scGH immunoreactivity (Fig. 3). Indeed,scGH staining is particularly prominent within a nuclearsubcompartment that is not labeled by a speciWc marker forRGC nuclei nor by a marker for the nucleolus. The identityof this compartment is currently unknown. The presence ofGH in the cytoplasm is expected, after translation of itsmRNA on ribosomes but nuclear GH immunoreactivity issurprising. Nuclear GH immunoreactivity has, however,now been demonstrated in many extrapituitary tissues (e.g.Harvey et al., 2000) and may reXect the nuclear localizationof most GHRs (e.g. Lobie et al., 1991, 1994; Lincoln et al.,1997; Mertani et al., 2003). A nuclear localization has alsobeen seen for many cytokines and growth factors and isconsistent with the putative roles of these factors as intrac-rines (Samlheiser, 1996).

4.2. GH expression

The presence of immunoreactivity for pituitary GH inthe chick visual system reXects the presence of mRNA inextracts of the retina and whole eye with 100% sequenceidentity to full-length pituitary GH mRNA (Baudet et al.,2003). The localization of this transcript is, moreover, iden-tical to that of GH-immunoreactivity, except it is not pres-ent in the OFL of the neural retina. It is therefore surprisingthat monomer (25 kDa) GH is not the most abundant GHmoiety in ocular tissues, as in the pituitary gland. Thisapparent paradox is, however, likely to reXect the rapidproteolysis of monomer GH in retinal tissue (Fig. 4).

4.3. Changes during ontogeny

Between ED7 and ED18, GH immunoreactivity is lost inthe OFL of embryonic chicks (unpublished observations),coincident with increased GH staining in the RGC layerand with the appearance of GH in the inner plexiform layer(IPL) and inner nuclear layer (INL). This pattern may be

126 S. Harvey et al. / General and Comparative Endocrinology 153 (2007) 124–131

related to temporal variations in the diVerentiation andstratiWcation of the retina (Mey and Thanos, 1992) andwith developmental waves of naturally occurring RGCdeath, that peak on ED8 and ED12 (Sanders et al., 2005).The traYcking and/or release of GH within the neural ret-ina may also change during ontogeny. The total amount ofGH in the neural retina (determined by Western blotting)does not, however, appear to diVer during embryogenesisand remains at the same level for at least 12 days afterhatch (Baudet et al., 2003). GH immunoreactivity is, never-theless, not detectable in the neural retina of slaughter-house fowl at 42 days of age, although 15 kDa GH remainsat high concentrations in the vitreous of these birds (Baudetet al., 2003). This result therefore suggests the vitreous mayact as a reservoir for the storage and/or protection of ocular

Fig. 1. BrightWeld microscopy of growth hormone (GH) receptor (GHR)-immunoreactivity (A) and GH-immunoreactivity (B) in the lens epithe-lium (long arrow) and corneal epithelium (short arrow) in eyes fromembryonic day (ED) 7 embryonic chicks. Whole eyes were dissected, Wxedin Carnoy’s Wxative, dehydrated in alcohol, embedded in paraYn wax and6 �m sections were collected onto charged slides, as previously described(Baudet et al., 2003). The sections were then rehydrated, blocked in 1%H2O2 (in 50%methanol) for 30 min and then in 10% normal goat serum(NGS) for 1 h and incubated overnight (at 4 °C) with primary antibodies,raised in rabbit, against chicken pituitary GH (�SH-1, at a Wnal dilution of1:1000) or raised in mouse against a synthetic portion (CH17) of theextracellular domain of the chicken GHR (�CH17, at a Wnal dilution of1:1000). After incubation and washing, the tissue sections were incubatedfor 1 h in a biotinylated goat anti-rabbit IgG or goat anti-mouse IgG (at1:500) and for 1 h in ABC reagent. Staining was visualized usingdiaminobenzidine tetrahydrochloride as a chromogenic substrate. Repre-sentative of at least six similar embryos.

Fig. 2. Confocal microscopy of immunoreactivity for full-length chickenpituitary growth hormone (cGH) (A) and for the truncated GH variant(scGH) (B) in the retinal ganglion cell layer (GCL) of the neural retina inthe eyes of embryonic day (ED) 7 chick embryos. Note the intense cGHlabeling of the optic Wbre layer (OFL), which is devoid of scGH staining.Presumptive ganglion cells below the OFL have intense scGH immunore-activity and relatively weak cGH immunoreactivity. In both cases thestaining in the GCL is in cytoplasmic (arrows) and nuclear (arrowheads)compartments of the cells. Representative of at least four similar embryos.The asterisk indicates fasicles in the OFL. After dehydration and blockingwith 10% NGS, sections were incubated overnight at 4 °C with rabbitantisera (at 1:1000) against chicken pituitary GH or with antisera raisedagainst a synthetic N-terminus (20 amino acids) of scGH, as detailed else-where (Baudet et al., 2003, 2007). The sections were then incubated for 1 hat room temperature in a goat anti-rabbit IgG conjugated to Alexa Fluor488 (F(ab�)2 fragment, 2 mg/ml), at a dilution of 1:500. The labeled sec-tions were examined using a LSM 510 confocal microscope equipped withappropriate lasers.

Fig. 3. Confocal microscopy of scGH-immunoreactivity (A, red) in pre-sumptive ganglion cells in the retinal ganglion cell layer (GCL) in the eyesof embryonic day (ED) 7 chick embryos, in comparison with immunore-activity for a nucleolar (NCL) marker (Syto RNASelect, MolecularProbes, Invitrogen, at 500 nM) (B, green). Note the nuclear labeling in (A,arrowheads) and (B, arrows) are in separate compartments, since they donot overlap when the images are merged (C). Methodological details asdescribed in Fig. 2. (For interpretation of the references to color in thisWgure legend, the reader is referred to the web version of this paper.)

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GH in chicks and that the actions of retinal GH are age-related.

4.4. GH actions

The possibility that GH acts within the visual system ofembryonic chicks is supported by the presence of mRNA inthe neural retina of ED7 embryos that codes for the full-length (extracellular, transmembrane and intracellulardomains) GHR (Harvey et al., 2003b; Baudet et al., 2004).This transcript and the immunoreactivity for the extracellu-lar domain of the GHR are both, like GH and GH mRNA,found in the neural retina, and OFL, in which they are colo-calized (Figs. 5 and 6). GH signaling through the retinalGHR is indicated by the Wve-fold induction of IGF-1 inexplants of the neural retina when incubated with exoge-nous chicken GH. The expression of a serine protease, GH-response gene (GHRG)-1 in the optic tectum of ED7 chicks(unpublished observations) also provides evidence ofendogenous GH signaling in the visual system, sinceGHRG-1 is a speciWc marker of GH action in chickens(Agarwal et al., 1995; Radecki et al., 1997). It is thereforerelevant that GHRG-1 expression has also been observedin the midbrain of adult hens in the nucleus nervi oculomo-torii and tractus isthmo-opticus, further supporting a rolefor GH in vision (Harvey et al., 2002).

In the neural retina, the survival of retinal cells of ED6and ED8 explants in vitro is markedly increased in responseto GH stimulation (Sanders et al., 2005), as is the survivalof ED8 immunopuriWed RGCs (Sanders et al., 2006). TheseWndings indicate that GH is neuroprotective against devel-opmental and spontaneous (caused by the severing of theoptic nerve) cell death. Moreover, as the immunoneutrali-zation of retinal GH promotes the apoptosis of retinal cells

Fig. 4. Western blotting of growth hormone (GH)-immunoreactivitybefore and 15 and 60 min after 10 �g chicken pituitary GH (obtained fromDr. A. Parlow, National Hormone and Peptide Program, Riverside, CA)was incubated with a phosphate-buVered saline (PBS) retinal extract(200 �l) (made from the neural retinas of 8 embryonic day (ED) 7 chickembryos that were sonicated in 1 ml PBS). After incubation the mediawere centrifuged and the supernatants stored at ¡80 °C until analysis. Anequivalent amount of the GH preparation was loaded in each lane. Meth-odological details for Western blotting, as detailed by Baudet et al. (2003).

in vitro and in vivo (Sanders et al., 2005), this neuroprotec-tive mechanism likely reXects a physiologically importantautocrine or paracrine action of GH during neurogenesis.This action has been shown to be mediated through a sup-pression of caspase-3 expression (Harvey et al., 2006a,b)and caspase-3 activity (Sanders et al., 2006), by a suppres-sion of Apoptosis-Inducing Factor (AIF) expression(Harvey et al., 2006b) and by an increase in Akt phosphory-lation (Sanders et al., 2006).

5. GH in the visual system of mammals

5.1. Rats

The possibility that the eye is a GH target site is sug-gested by the Wnding of GHR immunoreactivity in the rateye during the last trimester of gestation, particularly in theRGC layer (Garcia-Aragon et al., 1992; Lobie et al., 1993).The possibility that GH-signaling in the eye might reXectautocrine or paracrine pathways is suggested by the recentdiscovery of GH in rat ocular tissues.

Harvey et al. (2006c) Wrst demonstrated that the GHgene was expressed in mRNA extracted from the eyes ofrats and the neural retinas of newborn and neonatal rats. Itis not, however, present in whole-eye extracts from fetalrats younger than ED17, unlike the early ontogeny of GHexpression in chick eyes. Using in situ hybridization, theGH transcript is found to be abundant throughout the neu-ral retina, primarily within the RGC layer, in large roundedcells with nascent axons, morphologically resemblingRGCs. Translation of this transcript is conWrmed by thepresence of a single band of GH immunoreactivity in reti-nal extracts from perinatal rats, with a molecular size(22 kDa) identical of that in the pituitary gland. As in thechick embryo, GH-immunoreactivity is widespread in therat neural retina, but is initially most intense in the RGCand OFL layers. This immunoreactivity also appears to bein both cytoplasmic and nuclear compartments of the cellsin the RGC layer. The number of GH-immunoreactive cellsin the RGC layer clearly declines during aging (as in thechick) but these cells remain detectable, even in adults(unlike adult chickens). Immunoreactivity for GH is alsoclearly seen in the non-pigmented RPE of albino rats(Harvey et al., 2006c).

In subsequent studies, Modanlou et al. (2006) found GHimmunoreactivity, measurable by ELISA (Enzyme-LinkedImmunoSorbent Assay), in retinal homogenates from new-born and neonatal rats. These authors also found GH to bemeasurable in the vitreous, in which the concentration atbirth is comparable with that in serum, although <10% ofthat in serum during the Wrst, second and third weeks ofweaning. The GH concentration in the vitreous does, how-ever, decline during weaning, in parallel with the serum GHconcentration and in contrast to the unchanged GH con-centration in retinal homogenates. Serum and vitreous GHconcentrations are also correlated in another study by thesame group, in which rat pups were treated with Ibuprofen

128 S. Harvey et al. / General and Comparative Endocrinology 153 (2007) 124–131

Fig. 5. BrightWeld microscopy for growth hormone (GH)- and GH receptor (GHR)-immunoreactivity in the neural retina (NR) and optic Wbre layer(OFL) in the eyes of embryonic day (ED) 7 chick embryos. Note the similar distribution of GH and GHR staining. Methodological details as in Fig. 1.

Fig. 6. Confocal microscopy of chicken growth hormone (GH, green) and GH receptor (GHR, red)-immunoreactivity in the neural retina of embryonicday (ED)7 chick embryos. SpeciWc staining is seen in and just below the optic Wbre layer (OFL). The yellow-orange coloration from the overlay of theimages shows, at higher magniWcation, the colocalization of GH and GHR in the OFL and in the cytoplasm of the underlying cells. Methodologicaldetails as in Figs. 1 and 2. (For interpretation of the references to color in this Wgure legend, the reader is referred to the web version of this paper.)

Fig. 7. (A) In situ hybridization for growth hormone (GH) mRNA in the retina of a newborn albino mouse. MagniWcation: 200£. A higher magniWcationof the boxed area is shown in (B). Note the intense hybridization of the Dig-labeled riboprobe to the large, rounded cells (arrows) in the ganglion cell layer(GCL) and the widespread hybridization in the neural retina (NR). Dig-labeling is also seen in the retinal epithelium (RPE), although the choroid (Ch) is

devoid of labeling. Methodological details as outlined by Harvey et al. (2006c).

S. Harvey et al. / General and Comparative Endocrinology 153 (2007) 124–131 129

during weaning (Beharry et al., 2006). A blood ocular bar-rier would thus appear to impair the entry of circulatingGH into the eye after birth, although this barrier is incom-plete or leaky perinatally and during weaning.

5.2. Mice

In recent studies, GH immunoreactivity and GH mRNAhave been detected in the eyes of newborn and adult mice.As in rats, GH mRNA is abundantly present in the neuralretina of newborn mice, particularly in large rounded cellsin the RGC layer (Fig. 7). GH expression is also intense inthe stratiWed retina of adult mice (Fig. 8), in which GHmRNA is still present in the RGC layer, but less abundantthan in the newborn (Fig. 7). Instead, in the adult mouse,GH expression is intense in the INL and ONL. A few scat-tered cells with GH mRNA are also present in the (unpig-mented) retinal epithelium of adult albino mice (Fig. 8).

Immunohistochemical staining of the adult mouse neu-ral retina also shows the presence of GH in the cytoplasmof presumptive ganglion cells in the narrow RGC layer andin the overlying OFL (Fig. 9). GH immunoreactivity is alsoseen in cells in the IPL, INL and the OPL (Fig. 8). GHimmunoreactivity is, conversely, sparsely seen in the ONL,despite the abundant presence of GH mRNA (Fig. 10). Theexpression, translation and sequestering of GH within theneural retina therefore appears to be region speciWc.

Fig. 8. (A) In situ hybridization for growth hormone (GH) mRNA in theretina of an adult albino mouse. MagniWcation: 200£. Higher magniWca-tions of the boxed areas are shown in (B) and (C). Note slight hybridiza-tion to the Dig-labelled riboprobe in the narrow ganglion cell layer(GCL), and the intense labeling of the inner nuclear layer (INL) and theouter nuclear layer (ONL) and the absence of hybridization in the innerplexiform layer (IPL), outer plexiform layer (OPL) and outer photorecep-tor layer (PR). In (C), hybridization is also shown in retinal epithelial cells(arrows). The absence of staining in control sections treated with a senseprobe is shown for comparison (D). Methodological details as outlined byHarvey et al. (2006c).

Fig. 9. (A and C) Confocal microscopy of growth hormone (GH) immu-noreactivity in the retina of an adult albino mouse. The bar is 20 �m. Notethe presence of GH immunoreactivity (green) in the ganglion cell layer(GCL) and in the inner plexiform layer (IPL), the inner nuclear layer(INL) and the outer plexiform layer (OPL). The outer nuclear layer(ONL) is devoid of GH staining (A). A higher magniWcation of the boxedarea in (A) is shown in (B). Note labeling of the cytoplasm in the large,rounded cells in the ganglion cell layer (arrows). A higher magniWcation ofthe boxed area in (C) is shown in (D). Note the labeling of cytoplasm inthe large cells in the inner nuclear layer (arrowhead). Methodologicaldetails as outlined by Harvey et al. (2006c). (For interpretation of thereferences to color in this Wgure legend, the reader is referred to the webversion of this paper.)

Fig. 10. A comparison of growth hormone (GH) (A) and GH mRNA (B)distribution in the retina of an adult albino mouse. Note that GH and GHmRNA are similarly localized in the ganglion cell layer (GCL) and innernuclear layer (INL), whereas GH but not GH mRNA is present in theinner plexiform layer (IPL) and outer plexiform layer (OPL), while GHmRNA but not GH is present in the outer nuclear layer (ONL).

130 S. Harvey et al. / General and Comparative Endocrinology 153 (2007) 124–131

A role for GH in mouse vision has also been suggestedby Hattori and Fukuchi (1974), who found that intraperito-neal ovine GH decreased the amplitude of the b-wave inelectroretinogram (ERG) recordings of the mouse eye andshortened the culminating time of the oscillatory potentials.Although this may be a direct action of GH in the eye, Hat-tori and Fukuchi (1974) suggested it was indirect, mediatedby actions of GH on glucose mobilization that suppressedthe entry of glucose into ocular tissue, since this action ofGH was not seen in diabetic mice with glucose resistance.

5.3. Bovines

When injected into the cilioretinal artery of the bovineeye, radio-iodinated human GH is unable to pass throughretinal endothelial cells or enter into the retina for at least5 h afterwards (James and Cotlier, 1983). A blood-ocularbarrier excluding GH from systemic circulation is thereforefunctional in this mammalian species.

5.4. Humans

In recent studies, immunoreactivity for monomer(22 kDa) GH has been detected in the vitreous of someelderly patients with ocular disease (with diabetic retinopa-thy, retinal detachment, vitreous hemorrhage or epiretinalmembrane) (Harvey et al., 2006d). This suggests the blood–ocular barrier is leaky in these patients or that GH is pro-duced within human retinal tissues, as in adult rats andmice. The presence of GH in the vitreous suggests it hasroles in retinal function. This possibility is supported byoptic nerve and optic disc dysfunctions in patients with GHdeWciency (Mutz et al., 1984; Willnow et al., 1996), indicat-ing neural sites of GH action within the eye. In the humaneye, retinal blood vessels have also been thought to betarget sites of GH action (Frystyk, 2005), since GH isangiogenic and directly stimulates the proliferation ofendothelial cells from the human retinal microvasculaturein vitro (Rymaszewski et al., 1991). The increased GH secre-tion in diabetes mellitus is thus thought to be a causal fac-tor in the induction of diabetic retinopathy (Merimme,1990; Alzaid et al., 1994), especially as drugs that block GHsecretion (Higgins et al., 2002; Grant and Caballero, 2002)or GH action (Okada and Kopchick, 2001; Kopchick andOkada, 2001) are therapeutically eVective in this disease.Retinopathy is similarly a problem associated with GHreplacement therapy (Koller et al., 2000) and may disap-pear after the discontinuation of therapy (Hanson et al.,2000). This correlation is also seen in GH-deWcient patients,which have reduced retinal neovascularization (Hellstromet al., 1999).

6. Summary

Although GH has rarely been thought to be involved inocular function, basic and clinical studies support this pos-sibility. Indeed, the presence of GH within the visual system

has been demonstrated in most vertebrate groups, in whichGH may have roles in ocular function, particularly duringdevelopment. Functional studies are, however, required tocharacterize these endocrine and/or autocrine or paracrineroles.

Acknowledgments

This work was supported by the Natural Science andEngineering Research Council (NSERC) of Canada. BrentT. Martin and Marie-Laure Baudet are in receipt of stu-dentships from NSERC and the Alberta Heritage Founda-tion for Medical Research, respectively. The help of ZahraHassanali in some of the unpublished data is gratefullyacknowledged.

References

Agarwal, S.K., Cogburn, L.A., Burnside, J., 1995. Comparison of geneexpression in normal and growth hormone receptor-deWcient dwarfchickens reveals a novel growth hormone regulated gene. Biochem.Biophys. Res. Commun. 206, 153–160.

Alzaid, A.A., Dinneen, S.F., Melton 3rd, L.J., Rizza, R.A., 1994. The role ofgrowth hormone in the development of diabetic retinopathy. DiabetesCare 17, 531–534.

Baudet, M.L., Martin, B., Hassanali, Z., Parker, E., Sanders, E.J., Harvey,S., 2007. Expression, translation and localization of a novel, smallgrowth hormone variant. Endocrinology 148, 103–115.

Baudet, M.L., Rattray, D., Sanders, E.J., Harvey, S., 2004. Growth hor-mone: a survival agent in the developing chick retina. GH & IGF Res.14, 105.

Baudet, M.L., Sanders, E.J., Harvey, S., 2003. Retinal growth hormone inthe chick embryo. Endocrinology 144, 5459–5468.

Beharry, K.D., Modanlou, H.D., Hasan, J., Gharraee, Z., Abad-Santos, P.,Sills, J.H., Jan, A., Nageotte, S., Aranda, J.V., 2006. Comparative eVectsof early postnatal ibuprofen and indomethacin on VEGF, IGF-I, andGH during rat ocular development. Invest. Ophthalmol. Vis. Sci. 47,3036–3043.

Beyea, J.A., Olson, D.M., Harvey, S., 2005. Growth hormone expression inthe perinatal and postnatal rat lung. Dev. Dyn. 232, 1037–1046.

Clark, R., 1997. The somatogenic hormones and insulin-like growth fac-tor-1: stimulators of lymphopoiesis and immune function. Endocr.Rev. 18, 157–159.

Frystyk, J., 2005. The growth hormone hypothesis—2005 revision. Horm.Metab. Res. 37, 44–48.

Garcia-Aragon, J., Lobie, P.E., Muscat, G.E., Gobius, K.S., Norstedt, G.,Waters, M.J., 1992. Prenatal expression of the growth hormone (GH)receptor/binding protein in the rat: a role for GH in embryonic andfetal development? Development 114, 869–876.

Grant, M.B., Caballero, S., 2002. Somatostatin analogues as drug therapiesfor retinopathies. Drugs Today 38, 783–791.

Hanson, R., Koller, E.A., Malozowski, S., 2000. Full remission of growthhormone (GH)-induced retinopathy after GH treatment discontinua-tion: long-term follow-up. J. Clin. Endocrinol. Metab. 85, 2627.

Harvey, S., Johnson, C.D., Sharma, P., Sanders, E.J., Hull, K.L., 1998.Growth hormone: a paracrine growth factor in embryonic develop-ment? Comp. Biochem. Physiol. C 119, 305–315.

Harvey, S., Johnson, C.D., Sanders, E.J., 2000. Extra-pituitary growth hor-mone in peripheral tissues of early chick embryos. J. Endocrinol. 166,489–502.

Harvey, S., Johnson, C.D., Sanders, E.J., 2001. Growth hormone in neuraltissues of the chick embryo. J. Endocrinol. 169, 487–498.

Harvey, S., Lavelin, I., Pines, M., 2002. Growth hormone (GH) action inthe brain: neural expression of a GH-response gene. J. Mol. Neurosci.18, 89–95.

S. Harvey et al. / General and Comparative Endocrinology 153 (2007) 124–131 131

Harvey, S., Baudet, M.-L., Sanders, E.J., 2003a. Growth hormone in thevisual system of embryonic chicks. Neuropeptides 37, 178.

Harvey, S., Kakebeeke, M., Murphy, A.E., Sanders, E.J., 2003b. Growthhormone in the nervous system: autocrine or paracrine roles in retinalfunction? Can. J. Physiol. Pharmacol. 81, 371–384.

Harvey, S., Hull, K., 2003. Neural growth hormone: an update. J. Mol.Neurosci. 20, 1–14.

Harvey, S., Kakebeeke, M., Sanders, E.J., 2004. Growth hormone localiza-tion in the neural retina and retinal pigmented epithelium of embry-onic chicks. J. Mol. Neurosci. 22, 139–145.

Harvey, S., Baudet, M.L., Sanders, E.J., 2006a. Evidence for the expressionand translation of a novel growth hormone transcript in central andperipheral tissues of chick embryos. Neuropeptides 40, 137.

Harvey, S., Baudet, M., Sanders, E.J., 2006b. Growth hormone and cellsurvival in the neural retina: caspase dependence and independence.NeuroReport 17, 1715–1718.

Harvey, S., Baudet, M.L., Sanders, E.J., 2006c. Retinal growth hormone inperinatal and adult rats. J. Mol. Neurosci. 28, 257–264.

Harvey, S., Baudet, M.L., Sanders, E.J., 2006d. Growth hormone in thevisual system: comparative endocrinology. In: Proceedings of the 23rdConference of European Comparative Endocrinologists, Manchester,England.

Hattori, M., Fukuchi, S., 1974. Studies on ERG of KK mice especially onoscillatory potentials. 3. InXuence of ovine growth hormone. NipponGanka Gakkai Zasshi 78, 705–714.

Higgins, R.D., Yan, Y., Schrier, B.K., 2002. Somatostatin analogs inhibitneonatal retinal neovascularization. Exp. Eye Res. 74, 553–559.

Hellstrom, A., Svensson, E., Carlsson, B., Niklasson, A., Albertsson-Wik-land, K., 1999. Reduced retinal vascularization in children with growthhormone deWciency. J. Clin. Endocrinol. Metab. 84, 795–798.

Hull, K.L., Harvey, S., 2000. Growth hormone: a reproductive endocrine-paracrine regulator? Rev. Reprod. 5, 175–182.

James, C.R., Cotlier, E., 1983. Fate of insulin in the retina: an autoradio-graphic study. Br. J. Ophthalmol. 67, 80–88.

Klein, M., Moore, B., Rothstein, H., Hayden, J., Gordon, S., Holsclaw, D.,Sobrin, J., 1989. A comparison of growth regulation of mammalianwith amphibian lens epithelium. Lens Eye Toxic. Res. 6, 675–686.

Koller, E.A., Green, L., Gertner, J.M., Bost, M., Malozowski, S.N., 2000.Retinal changes mimicking diabetic retinopathy in two nondiabetic,growth hormone-treated patients. J. Clin. Endocrinol. Metab. 83,2380–2383.

Kopchick, J.J., Okada, S., 2001. Growth hormone receptor antagonists:discovery and potential uses. Growth Horm. IGF Res. 11, S103–S109.

Lincoln, D.T., Sinowatz, F., Gabius, S., Gabius, H.J., Temmim, L., Baker,H., Mathew, T.C., Waters, M.J., 1997. Subpopulations of stromal cellsfrom long-term human bone marrow cultures: ontogeny of progenitorcells and expression of growth hormone receptors. Anat. Histol.Embryol. 26, 11–28.

Lobie, P.E., Barnard, R., Waters, M.J., 1991. The nuclear growth hormonereceptor binding protein. Antigenic and physicochemical characteriza-tion. J. Biol. Chem. 266, 22645–22652.

Lobie, P.E., Garcia-Aragon, J., Lincoln, D.T., Barnard, R., Wilcox, J.N,Waters, M.J., 1993. Localization and ontogeny of growth hormonereceptor gene expression in the central nervous system. Develop. BrainRes. 74, 225–233.

Lobie, P.E., Wood, T.J., Chen, C.M., Waters, M.J., Norstedt, G., 1994.Nuclear translocation and anchorage of the growth hormone receptor.J. Biol. Chem. 269, 31735–31746.

Merimme, T.J., 1990. Diabetic retinopathy. A synthesis of perspective. N.Engl. J. Med. 322, 978–983.

Mertani, H.C., Raccurt, M., Abbate, A., Kindblom, J., Tornell, J., Billest-rup, N., Usson, Y., Morel, G., Lobie, P.E., 2003. Nuclear translocationand retention of growth hormone. Endocrinology 144, 3182–3195.

Mey, J., Thanos, S., 1992. Development of the visual system of the chick-areview. J. Hirnforsch. 33, 673–702.

Modanlou, H.D., Gharraee, Z., Hasan, J., Waltzman, J., Nageotte, S.,Beharry, K.D., 2006. Ontogeny of VEGF, IGF-I, and GH in neonatalrat serum, vitreous Xuid, and retina from birth to weaning. Invest.Ophthalmol. Vis. Sci. 47, 738–744.

Mutz, I., Millner, M., Borkenstein, M., 1984. Optic nerve hypoplasia andgrowth hormone deWciency: de Morsier’s syndrome. Wien. Klin.Wochenschr. 96, 432–435.

Okada, S., Kopchick, J.J., 2001. Biological eVects of growth hormone andits antagonist. Trends Mol. Med. 7, 126–132.

Otteson, D.C., Cirenza, P.F., Hitchcock, P.F., 2002. Persistent neurogenesisin the teleost retina: evidence for regulation by the growth-hormone/insulin-like growth factor-I axis. Mech. Dev. 117, 137–149.

Otteson, D.C., Hitchcock, P.F., 2003. Stem cells in the teleost retina: persistentneurogenesis and injury-induced regeneration. Vision Res. 4, 927–936.

Palmetshofer, A., Zechner, D., Luger, T.A., Barta, A., 1995. Splicing vari-ants of the human growth hormone mRNA: detection in pituitary,mononuclear cells and dermal Wbroblasts. Mol. Cell. Endocrinol. 113,225–234.

Radecki, S.V., McCann-Levorse, L., Agarwal, S.K., Burnside, J., Proud-man, J.A., Scanes, C.G., 1997. Chronic administration of growth hor-mone (GH) to adult chickens exerts marked eVects on circulatingconcentrations of insulin-like growth factor-I (IGF-I), IGF bindingproteins, hepatic GH regulated gene I, and hepatic GH receptormRNA. Endocrine 6, 117–124.

Redden, J.R., Wilson-Dziedzic, D., 1983. Insulin growth factor and epider-mal growth factor trigger mitosis in lenses cultured in a serum-freemedium. Invest. Ophthalmol. Vis. Sci. 24, 409–416.

Rothstein, H., Van Wyk, J.J., Hayden, J.H., Gordon, S.R., Weinsieder, A.,1980. Somatomedin C: restoration in vivo of cycle traverse in G0/G1blocked cells of hypophysectomized animals. Science 208, 410–412.

Rymaszewski, Z., Cohen, R.M., Chomczynski, P., 1991. Human growthhormone stimulates proliferation of human retinal microvascularendothelial cells in vitro. Proc. Natl. Acad. Sci. USA 88, 617–621.

Sanders, E.J., Walter, M.A., Parker, E., Aramburo, C., Harvey, S., 2003.Opticin binds retinal growth hormone in the embryonic vitreous.Invest. Ophthalmol. Vis. Sci. 44, 5404–5409.

Sanders, E.J., Harvey, S., 2004. Growth hormone as an early embryonicgrowth and diVerentiation factor. Anat. Embryol. 209, 1–9.

Sanders, E.J., Parker, E., Aramburo, C., Harvey, S., 2005. Retinal growthhormone is an anti-apoptotic factor in embryonic retinal ganglion celldiVerentiation. Exp. Eye Res. 81, 551–560.

Sanders, E.J., Parker, E., Harvey, S., 2006. Retinal ganglion cell survival indevelopment: mechanisms of retinal growth hormone action. Exp. EyeRes. 83, 1205–1214.

Samlheiser, N.R., 1996. Proteins in unexpected locations. Mol. Biol. Cell 7,1003–1014.

Takeuchi, S., Haneda, M., Teshigawara, K., Takahashi, S., 2001. IdentiWca-tion of a novel GH isoform: a possible link between GH and melano-cortin systems in the developing chicken eye. Endocrinology 142,5158–5166.

Tresguerres, J.A., Ariznavarreta, C., Granados, B., Costoya, J.A., Perez-Romero, A., Salame, F., Hermanussen, M., 1999. Salivary gland is capableof synthesis under GHRH stimulation. J. Endocrinol. 160, 217–222.

Wainwright, N., Hayden, J., Rothstein, H., 1978. Total disappearance ofcell proliferation in the lens of a hypophysectomized animal: in vivoand in vitro maintenance of inhibition with reversal by pituitaryfactors. Cytobios 23, 79–92.

Willnow, S., Kiess, W., Butenandt, O., Dorr, H.G., Enders, A., Strasser-Vogel, B., Egger, J., Schwarz, H.P., 1996. Endocrine disorders in septo-optic dysplasia (De Morsier Syndrome)—evaluation and follow up of18 patients. Eur. J. Pediatr. 155, 179–184.