Somatostatin analogs inhibit neonatal retinal neovascularization

Neuroprotective Potential of Pituitary Adenylate Cyclase Activating Polypeptide in Retinal Degenerations of Metabolic Origin

Pituitary adenylate cyclase-activating polypeptide (PACAP1-38) is a highly conserved member of the secretin/glucagon/VIP family. The repressive effect of PACAP1-38 on the apoptotic machinery has been an area of active research conferring a significant neuroprotective potential onto this peptide. A remarkable number of studies suggest its importance in the etiology of neurodegenerative disorders, particularly in relation to retinal metabolic disorders. In our review, we provide short descriptions of various pathological conditions (diabetic retinopathy, excitotoxic retinal injury and ischemic retinal lesion) in which the remedial effect of PACAP has been well demonstrated in various animal models. Of all the pathological conditions, diabetic retinopathy seems to be the most intriguing as it develops in 75% of patients with type 1 and 50% of patients with type 2 diabetes, with concomitant progression to legal blindness in about 5%. Several animal models have been developed in recent years to study retinal degenerations and out of these glaucoma and age-related retina degeneration models bear human recapitulations. PACAP neuroprotection is thought to operate through enhanced cAMP production upon binding to PAC1-R. However, the underlying signaling network that leads to neuroprotection is not fully understood. We observed that (i) PACAP is not equally efficient in the above conditions; (ii) in some cases more than one signaling pathways are activated; (iii) the coupling of PAC1-R and signaling is stage dependent; and (iv) PAC1-R is not the only receptor that must be considered to interpret the effects in our experiments. These observations point to a complex signaling mechanism, that involves alternative routes besides the classical cAMP/protein kinase A pathway to evoke the outstanding neuroprotective action. Consequently, the possible contribution of the other two main receptors (VPAC1-R and VPAC2-R) will also be discussed. Finally, the potential medical use of PACAP in some retinal and ocular disorders will also be reviewed. By taking advantage of, low-cost synthesis technologies today, PACAP may serve as an alternative to the expensive treatment modelities currently available in ocular or retinal conditions.

Growth hormone and ocular dysfunction: Endocrine, paracrine or autocrine etiologies?

The eye is a target site for GH action and growth hormone has been implicated in diabetic retinopathy and other ocular dysfunctions. However, while this could reflect the hypersecretion of pituitary GH, the expression of the GH gene is now known to occur in ocular tissues and it could thus also reflect excess GH production within the eye itself. The possibility that ocular dysfunctions might arise from endocrine, autocrine or paracrine etiologies of GH overexpression is therefore the focus of this brief review.

Therapeutic uses of somatostatin and its analogues: Current view and potential applications

Somatostatin is an endogeneous cyclic tetradecapeptide hormone that exerts multiple biological activities via five ubiquitously distributed receptor subtypes. Classified as a broad inhibitory neuropeptide, somatostatin has anti-secretory, anti-proliferative and anti-angiogenic effects. The clinical use of native somatostatin is limited by a very short half-life (1 to 3min) and the broad spectrum of biological responses. Thus stable, receptor-selective agonists have been developed. The majority of these somatostatin therapeutic agonists bind strongly to two of the five receptor subtypes, although recently an agonist of wider affinity has been introduced. Somatostatin agonists are established in the treatment of acromegaly with recently approved indications in the therapy of neuroendocrine tumours. Potential therapeutic uses for somatostatin analogues include diabetic complications like retinopathy, nephropathy and obesity, due to inhibition of IGF-1, VEGF together with insulin secretion and effects upon the renin-angiotensin-aldosterone system. Wider uses in anti-neoplastic therapy may also be considered and recent studies have further revealed anti-inflammatory and anti-nociceptive effects. This review provides a comprehensive, current view of the biological functions of somatostatin and potential therapeutic uses, informed by the wide range of pharmacological advances reported since the last published review in 2004 by P. Dasgupta. The pharmacology of somatostatin receptors is explained, the current uses of somatostatin agonists are discussed, and the potential future of therapeutic applications is explored.

Pasireotide (SOM230) protects the retina in animal models of ischemia induced retinopathies

The neuropeptide somatostatin and selective analogs for the sst(2/5) receptor subtypes provided neuroprotection against retinal chemical ischemia ex vivo and AMPA [(RS)-α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid hydrobromide] induced retinal toxicity in vivo, when employed in micromolar concentrations (Mastrodimou et al., 2005; Kiagiadaki and Thermos, 2008). The aim of the present study was to investigate the neuroprotective properties of a new metabolically stable agent pasireotide (SOM230) in the above mentioned retinal models of ischemia. Adult Sprague Dawley (250-350 g) rats were employed. For the ex vivo experiments, retinal eye cups were incubated with PBS or the chemical ischemia mixture [iodoacetic acid (5 mM)/sodium cyanide (25 mM)] in the absence or presence of SOM230 (10(-7)-10(-5) M) alone or in the presence of the sst(2) antagonist CYN-154806 (10(-7) or 10(-5) M). In the in vivo model, the animals received intravitreally: PBS (50 mM), AMPA (42 nmol/eye) or AMPA (42 nmol) in combination with SOM230 (10(-7)-10(-5) M). Immunohistochemistry studies using antisera against bNOS, a marker for brain/neuronal NOS containing amacrine cells, protein kinase C (PKC) a marker for rod bipolar cells, and TUNEL studies in conjunction with FACS analysis were employed to examine retinal cell loss and protection. Chemical ischemia led to a loss of bNOS and PKC immunoreactivity which was reversed by SOM230. Partial and full protection of bNOS and PKC immunoreactive neurons, respectively, was observed even at the low concentration of 10(-7) M. The neuroprotective actions of SOM230 (10(-7) or 10(-5) M) were reversed by CYN-154806 (10(-7) or 10(-5) M, respectively). Similarly, SOM230 (10(-7), 10(-6), 10(-5) M) provided neuroprotection in the in vivo model. The dose of 10(-7) M prevented the loss of the bNOS cells and provided almost full protection. These data were substantiated by TUNEL staining and fluorescence-activated cell sorting (FACS) analysis. SOM230 appears very efficacious in its neuroprotective properties in both models of retinal ischemia affording neuroprotection at the concentration or dose of 100 nM. These data suggest that SOM230 might represent a useful pharmacological compound for the treatment of retinal disease.

Octreotide as therapeutic option for congenital idiopathic chylothorax: a case series

BACKGROUND

Octreotide, a somatostatin analogue, is used for the management of patients with refractory chylothorax although its safety and efficacy in neonates have not been evaluated in controlled clinical trials. We present one of the largest case series about the use of octreotide in congenital idiopathic chylothorax.

METHODS

Six cases of congenital chylothorax (CC) were prospectively collected, who were managed with same unit protocol for octreotide. Mean (SD) gestation was 34.5 (±2.2) weeks, and birthweight was 3410 (±840.4) g. All infants required chest drains from day 1 of life, and the mean (SD) duration of insertion was 36.1 (±8.5) days. Octreotide was commenced at a median age of 13.5 days (range 8-22), given for a median duration of 20 days (range 12-27). The starting dose was 0.5-1 μg/kg/h with an increment of 1-2 μg/kg/day to a maximum of 10 μg/kg/day. Resolution of chylothorax was achieved in five patients, being resistant to treatment in the sixth patient. None had adverse effects from octreotide. Full enteral feeds were reached at a mean age of 44 days.

CONCLUSION

Early commencement of octreotide is recommended although further reports to evaluate the safety and efficacy would add to the profile of this medication in the treatment of CC.

Is octreotide treatment useful in patients with congenital chylothorax?

BACKGROUND

Congenital chylothorax (CC) is a severe congenital disorder characterized by the collection of chyle in the pleural space. Recent case reports suggest that the use of octreotide in the treatment of CC may be useful.

OBJECTIVE

To evaluate the effect of octreotide on pleural effusions in CC.

METHODS

Hospital records of 7 patients with a CC, who were admitted to the Neonatal Intensive Care Unit of the Radboud University Nijmegen Medical Centre between 2006 and 2010 and were treated with octreotide, were retrospectively evaluated.

RESULTS

There was no clear and consistent effect of octreotide treatment on pleural effusions. Pleural effusions eventually decreased in all patients after reaching a dose of 5-6 μg/kg/min, but this could also reflect the natural history of CC. Out of 7 patients, 4 were diagnosed with persistent pulmonary hypertension of the newborn. No other known side effects of octreotide were found. There was a mortality rate of 30%.

CONCLUSIONS

No clear and consistent effect of octreotide was identified. Pulmonary hypertension was a common problem in this patient group. It is not clear whether or not this was caused or maintained by octreotide treatment. A randomized controlled trial is needed to investigate the safety and usefulness of octreotide. Until then, clinicians should be careful in using octreotide, especially when persistent pulmonary hypertension is present.

Octreotide for the treatment of chylothorax in neonates

BACKGROUND

Routine care for chylothorax in neonate includes either conservative or surgical approaches. Octreotide, a somatostatin analogue, has been used for the management of patients with refractory chylothorax not responding to conservative management.

OBJECTIVES

To assess the efficacy and safety of octreotide in the treatment of chylothorax in neonates.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) (The Cochrane Library), MEDLINE and EMBASE (to March 7, 2010). We assessed the reference lists of identified trials and abstracts from the annual meetings of the Pediatric Academic Societies published in Pediatric Research (2002 to 2009) without language restrictions.

SELECTION CRITERIA

We planned to include randomised or quasi-randomised controlled trials of octreotide in the treatment of congenital or acquired chylothorax in term or preterm neonates, with any dose, duration or route of administration.

DATA COLLECTION AND ANALYSIS

Data on primary (amount of fluid drainage, respiratory support, mortality) and secondary outcomes (side effects) were planned to be collected and analysed using mean difference, relative risk and risk difference with 95% confidence intervals.

MAIN RESULTS

No randomised controlled trials were identified. Nineteen case reports of 20 neonates with chylothorax in whom octreotide was used either subcutaneously or intravenously were identified. Fourteen case reports described successful use (resolution of chylothorax), four reported failure (no resolution) and one reported equivocal results following use of octreotide. The timing of initiation, dose, duration and frequency of doses varied markedly. Gastrointestinal intolerance and clinical presentations suggestive of necrotizing enterocolitis and transient hypothyroidism were reported as side effects.

AUTHORS' CONCLUSIONS

No practice recommendation can be made based on the evidence identified in this review. A prospective registry of chylothorax patients and a subsequent multicenter randomised controlled trial are needed to assess the safety and efficacy of octreotide in the treatment of chylothorax in neonates.

Neuroendocrine tumors: a critical appraisal of management strategies

OBJECTIVES

This supplement represents the proceedings of a panel of investigators whose goal was to assess the quality of evidence pertaining to current therapeutic approaches in the management of neuroendocrine tumors (NETs). It will examine the differences in opinions between expert faculty and community physicians.

METHODS

The Neuroendocrine Tumors Summit convened in December 2009 to address 6 statements prepared by panel members that reflect important questions in the management of NETs.

RESULTS

Panel members were polled on their acceptance of each statement, and those results were compared with community-based oncologists, gastroenterologists, and endocrinologists.

CONCLUSIONS

More education regarding the treatment of NETs, including pertinent studies, is needed for physicians. Additional multicenter, prospective, placebo-controlled trials are necessary to demonstrate the benefits of somatostatin analogs, including their ability to control symptoms and affect tumor growth, and their synergistic effect with other therapies. New trials should be reported consistently and include time to tumor progression and progression-free survival as primary or secondary end points.

MAP kinase pathway is involved in IGF-1-stimulated proliferation of human retinal pigment epithelial cells (hRPE)

PURPOSE

To investigate the mitogenic activity of insulin-like growth factor-1 (IGF-1) on the proliferation of human retinal pigment epithelial cells (hRPE) and to elucidate the role of vascular endothelial growth factor (VEGF) and MAP kinase (MAPK) in the IGF-1 signaling cascade.

METHODS

Human RPE specimens were obtained from postmortem non-pathological eyes and cultured in vitro through several passages. Cellular proliferation in the presence of increasing concentrations of IGF-1 and IGF-1 + PD98059 (a known MAPK inhibitor) was measured by [(3)H]thymidine incorporation; trypan blue exclusion studies (T) verified cell viability. Under the same experimental conditions, synthesis of VEGF was measured utilizing [(14)C]methionine immunoprecipitation and immunocytochemical methods as well as Western blot analysis.

RESULTS

IGF-1 stimulated hRPE proliferation, as demonstrated by [(3)H]thymidine incorporation. There was also an IGF-1-induced increase in VEGF synthesis as measured quantitatively by [(14)C]methionine-VEGF immunoprecipitation. This was qualitatively confirmed by immunocytochemistry and Western blotting. PD98059 suppressed both IGF-1-induced cell proliferation as well as IGF-1-stimulated VEGF production.

CONCLUSIONS

These studies suggest that IGF-1 is a mitogen for hRPE cells and also stimulates production of the angiogenic factor, VEGF. Additionally, PD98059 inhibits the production of VEGF, suggesting that the MAP kinase pathway is involved in IGF-1-mediated angiogenesis.

Activation of somatostatin receptor (sst 5) protects the rat retina from AMPA-induced neurotoxicity

In a recent study, we employed an in vivo model of retinal excitotoxicity to investigate the neuroprotective effect of somatostatinergic agents. Intravitreal administration of somatostatin and sst(2) selective agonists protected the retina from (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid hydrobromide (AMPA) induced excitotoxicity. The sst(1) and sst(4) selective ligands had no effect (Kiagiadaki and Thermos, 2008). The presence of sst(5) receptors in rat retina was only recently reported (Ke and Zhong, 2007). Synthetic agonists that activate sst(2) receptors also bind with high affinity to the sst(5) subtype. In the present study the putative neuroprotective effects of sst(5) receptor activation were investigated. Adult female and male Sprague-Dawley (250-350g) rats were employed. Groups of animals received intravitreally PBS (50mM) or AMPA (42 nmol/eye) alone or in combination with L-817,818 (sst(5), 10(-5), 10(-4)M). To exclude neuroprotective effects via the activation of sst(2) receptors, L-817,818 (10(-4)M) was coinjected with the sst(2) antagonist CYN-154806 (10(-4)M). Immunohistochemistry (IHC) studies using the anti-retinal marker choline acetyltransferase (ChAT) and TUNEL staining were employed to examine retinal cell loss and protection. IHC and Western blot analysis were also employed to assess whether the sst(5) receptors are viable in the AMPA treated tissue as compared to control retina. sst(5) receptors were not affected by AMPA. L-817,818 protected the retina from the AMPA insult in the dose of 10(-4)M, while CYN-154806 (10(-4)M) had no effect on the sst(5) neuroprotection. TUNEL staining confirmed the AMPA-induced retinal toxicity and the L-817,818 neuroprotection. These results demonstrate for the first time that sst(5) receptors are functional in the retina, and that sst(5) analogs administered intravitreally protect the retina from excitotoxicity. Further studies are essential to ascertain the therapeutic relevance of these results.

Physiology and pathology of somatostatin in the mammalian retina: a current view

In the retina, peptidergic signalling participates in multiple circuits of visual information processing. The neuropeptide somatostatin (SRIF) is localised to amacrine cells and, in some instances, in a subset of ganglion cells. The variegated expression patterns of SRIF receptors (sst(1)-sst(5)) and the variety of signalling mechanisms activated by retinal SRIF suggest that this peptide may exert multiple actions on retinal neurons and on retinal physiology, although our current understanding reflects a rather complicated picture. SRIF, mostly through sst(2), may act as a positive factor in the retina by regulating retinal homeostasis and protecting neurons against damage. In this respect, SRIF analogues seem to constitute a promising therapeutic arsenal to cure different retinal diseases, as for instance, ischemic and diabetic retinopathies. However, further investigations are needed not only to fully understand the functional role of the SRIF system in the retina but also to exploit new chemical space for drug-like molecules.

Novel signals mediating the functions of somatostatin: the emerging role of NO/cGMP

The neuropeptide somatostatin is a cyclic tetradecapeptide, which is widely distributed in the peripheral and central nervous system. It mediates a plethora of physiological actions and functions as a neurotransmitter, neuromodulator or trophic factor. Somatostatin activates six receptor subtypes that are expressed differentially in different tissues and are coupled to diverse signalling pathways. In order to elucidate the functional role of the individual receptor subtypes, many investigations focused on the assignment of each receptor to a particular signalling pathway. Signalling pathways involving enzyme (adenylate cyclase, phospholipases, phosphatases) and ion channel systems in native and recombinant receptor systems have been extensively studied. A one to one situation (receptor/pathway) has yet to be established, thus justifying the diverse actions of somatostatin. Recently, a NO/cGMP pathway has been shown to mediate the functions of somatostatin and its receptors. This review will present the findings that support the emerging role of NO/cGMP as a novel signal in SRIF's actions in retinal physiology and somatotroph release.

Growth hormone in the visual system: comparative endocrinology

Growth hormone (GH) is rarely considered to be involved in ocular development or vision or to be present in the visual system. Basic and clinical studies nevertheless support roles for GH in the ocular function of most vertebrate groups and for its extrapituitary production in ocular tissues. The comparative endocrinology of endocrine, autocrine or paracrine GH in the visual system of vertebrates is the focus of this brief review.

Medical treatment of diabetic retinopathy with somatostatin analogues

Traditional management strategies for retinal neovascularisation accompanying proliferative diabetic retinopathy include photocoagulation laser therapy. The development of preventative pharmacological treatments aimed at replacing or delaying this acute intervention has been an active research area and somatostatin analogues have shown promise in reducing the progression of retinal vascular pathologies. This review summarises the present knowledge on the molecular and cellular mechanisms of neovascularisation, and the rationale for the therapeutic use of somatostatin analogues as well as the results of two key recent clinical trials using octreotide. The potential use of octreotide and other somatostatin analogues in reducing the risk of severe visual impairment in proliferative diabetic retinopathy is discussed and pharmacological treatment regimens are proposed as an additional strategy or a less invasive alternative to laser therapy.

Expression of protein kinase CK2 in astroglial cells of normal and neovascularized retina

We previously documented protein kinase CK2 involvement in retinal neovascularization. Here we describe retinal CK2 expression and combined effects of CK2 inhibitors with the somatostatin analog octreotide in a mouse model of oxygen-induced retinopathy (OIR). CK2 expression in human and rodent retinas with and without retinopathy and in astrocytic and endothelial cultures was examined by immunohistochemistry, Western blotting, and reverse transcriptase-polymerase chain reaction. A combination of CK2 inhibitors, emodin or 4,5,6,7-tetrabromobenzotriazole, with octreotide was injected intraperitoneally from postnatal (P) day P11 to P17 to block mouse OIR. All CK2 subunits (alpha, alpha', beta) were expressed in retina, and a novel CK2alpha splice variant was detected by reverse transcriptase-polymerase chain reaction. CK2 antibodies primarily reacted with retinal astrocytes, and staining was increased around new intraretinal vessels in mouse OIR and rat retinopathy of prematurity, whereas preretinal vessels were negative. Cultured astrocytes showed increased perinuclear CK2 staining compared to endothelial cells. In the OIR model, CK2 mRNA expression increased modestly on P13 but not on P17. Octreotide combined with emodin or 4,5,6,7-tetrabromobenzotriazole blocked mouse retinal neovascularization more efficiently than either compound alone. Based on its retinal localization, CK2 may be considered a new immunohistochemical astrocytic marker, and combination of CK2 inhibitors and octreotide may be a promising future treatment for proliferative retinopathies.

STUDY OF INHIBITORY EFFECTS OF AN ANTIANGIOGENIC SOMATOSTATIN-CAMPTOTHECIN CONJUGATE ON LASER-INDUCED CHOROIDAL NEOVASCULARIZATION IN RATS

PURPOSE

To evaluate the ocular toxicity and efficacy of intravitreal camptothecin-somatostatin conjugate (JF-10-81), a somatostatin type 2 receptor-directed, antiangiogenic compound.

METHODS

Part 1: New Zealand albino rabbits (except for controls) were injected intravitreally with conjugate at various concentrations. Preoperative and postoperative ocular examinations and electroretinography were performed until animals were killed for histology. Part 2: Long-Evans pigmented rats had choroidal neovascularization (CNV) induced by argon laser. One eye per animal was injected with concentration 10M (safe dose), whereas the other eyes were controls and received 30 microL of sterile water at different time intervals after laser application. Fluorescein angiography was performed at various time points to evaluate the lesions and confirm presence of CNV. Animals were euthanized. The eyes were immediately enucleated and prepared for histologic examination.

RESULTS

Part 1: No clinical changes were seen in groups receiving 10(-8)M, 10(-7)M, 10(-6)M, and 10(-5) M of conjugate. Electroretinography showed decreasing b-wave amplitude in groups receiving 10(-4) M and 10(-3) M; cataracts also developed in these eyes. Part 2: Fluorescein angiography revealed that intravitreal injection of somatostatin conjugate JF-10-81 favorably affected the development of CNV when the treatment was performed at least 1 week after the laser application. These results were statistically significant. Histologic analysis results of eyes treated 2 weeks after laser application also showed significant benefit.

CONCLUSIONS

Part 1: Camptothecin-somatostatin conjugate injected intravitreally appeared safe at concentrations of 10(-5)M or less. Part 2: Conjugate JF-10-81 at a concentration of 10(-5)M administered intravitreally 1 to 3 weeks after laser demonstrated statistically significant efficacy in the treatment of choroidal neovascularization.

Effect of somatostatin analogues on chemically induced ischaemia in the rat retina

This study investigated the neuroprotective effect of somatostatin, cortistatin and agonists at somatostatin(2) (sst(2)) receptors in retinal explants subjected to chemical ischaemia. Eyecups of female Sprague-Dawley rats (250-300 g) were immersed in PBS buffer or PBS containing iodoacetic acid (IAA; 0.5, 5, 50, 100 mM) and sodium cyanide (NaCN; 2.5, 25, 250, 500 mM) (chemical ischaemia solution) for 15, 30, 45, 60, 120 min (pilot study). Subsequently, eyecups were incubated with (1) PBS, (2) chemical ischaemia solution (5 mM IAA/25 mM NaCN) or (3) somatostatin, cortistatin, BIM23014 or MK678 (0.1, 1, 10 microM) together with the chemical ischaemia solution for 60 min, followed by a second 60-min incubation in PBS (control and ischaemia groups) or ligands in PBS (neuroprotection groups). The eyecups were subsequently fixed and sectioned for immunohistochemistry. Treatment of the eyecups with IAA/NaCN (5/25 mM) for 60 min abolished choline acetyltransferase (ChAT), tyrosine hydroxylase and brain nitric oxide synthase immunoreactivity in the inner nuclear, inner plexiform and ganglion cell layers. It also abolished protein kinase C immunoreactivity in rod bipolar cells and terminals, but did not damage ganglion cells labelled for microtubule-associated protein-1. TUNEL staining provided evidence of cell death in the ischaemic retina. Cortistatin, BIM23014 and MK678 attenuated the retinal damage caused by the chemical ischaemia in a concentration dependent manner. The ligands afforded approximately 58, 76 and 49% neuroprotection, respectively, of the ChAT immunoreactive cells. These results demonstrate that somatostatin analogues can protect the retina from ischaemic damage. The chemical ischaemia model is presently employed for the elucidation of the mechanisms involved in the neuroprotection.

Somatostatin inhibits IGF-1 mediated induction of VEGF in human retinal pigment epithelial cells

Neovascularization stimulated by IGF-1 mediated induction of vascular endothelial growth factor (VEGF) is one of the leading causes of blindness in humans. It plays a central role in the pathogenesis of proliferative diabetic retinopathy (DR), neovascular glaucoma, exudative age-related macular degeneration (AMD) and retinopathy of prematurity. Neovascularization is a multi-step process that involves complex interactions of a variety of mitogenic factors such as VEGF and IGF-I which are produced locally in the human eye by a variety of cells including retinal pigment epithelial (RPE) cells, retinal capillary pericytes, endothelial cells, Mueller cells and ganglion cells. We hypothesized that somatostatin would inhibit the IGF-1 signal transduction pathway in RPE cells, resulting in decreased VEGF production. We have observed expression of somatostatin receptor protein in retinal pigment epithelial (RPE) cells of the human eye using immunohistochemistry and have confirmed expression of somatostatin receptors in cultured human RPE cells using reverse transcriptase-PCR. IGF-1 induced a dose dependent increase in IGF-1R phosphorylation and in VEGF mRNA levels in cultured human RPE cells. Somatostatin and octreotide, a somatostatin analogue, inhibited IGF-1 receptor (IGF-1R) phosphorylation and decreased VEGF production. Both IGF-1R phosphorylation and accumulation of VEGF mRNA were inhibited by physiological levels of somatostatin and octreotide (1 nM). These results demonstrate somatostatin and octreotide mediated attenuation of both IGF-1R signal transduction and VEGF mRNA accumulation via somatostatin receptor type 2 (sst2). Furthermore, these data suggest a rationale for the use of octreotide as a prophylactic and therapeutic option in disease states that cause ocular neovascularization.

Somatostatin analogues: multiple roles in cellular proliferation, neoplasia, and angiogenesis

Angiogenesis, the development of new blood vessels is a crucial process both for tumor growth and metastatic dissemination. Additionally, dysregulation in angiogenesis has been implicated in the pathogenesis of cardiovascular disease, proliferative retinopathy, diabetic nephropathy, and rheumatoid arthritis (RA). The neuropeptide somatostatin has been shown to be a powerful inhibitor of neovascularization in several experimental models. Furthermore, somatostatin receptors (sst) are expressed on endothelial cells; particularly, sst2 has been found to be uniquely up-regulated during the angiogenic switch, from quiescent to proliferative endothelium. The present manuscript reviews the anti-angiogenic activity of somatostatin and its analogues in neoplastic and nonneoplastic disease. The role of sst subtypes particularly sst2 in mediating its angioinhibitory activity is described. Somatostatin agonists may also exert their anti-angiogenic activity indirectly by inhibition of growth factors like vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and the growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis or through its immunomodulatory effects. However, the therapeutic utility of somatostatin agonists as anti-angiogenic drugs in these diseases remains confusing because of conflicting results from different studies. More basic research, as well as patient-oriented studies, is required to firmly establish the clinical potential of somatostatin agonists in therapeutic angiogenesis. The currently available somatostatin agonists have high affinity of sst2 with lower affinities for sst3 and sst5. The emergence of novel somatostatin agonists especially bispecific analogues (agonists targeting multiple cellular receptors) and conjugates (synthesized by chemically linking somatostatin analogues with other antineoplastic agents) with improved receptor specificity signify a new generation of anti-angiogenics, which may represent novel strategies in the treatment of neovascularization-related diseases.

Angiogenesis in hepatocellular carcinoma: the retrospectives and perspectives

Hepatocellular carcinoma (HCC) is a typical hypervascular tumor. Many angiogenic factors have been studied in HCC, and several anti-angiogenic therapies have been tested in animal models and patients. This paper summarizes the latest findings, especially regarding the clinical significance of endothelial cell markers and angiogenic factors in HCC, and experimental and clinical anti-angiogenesis therapies. Further developments in this area, such as endothelial cell-oriented research and better experimental and clinical designs in the evaluation of anti-angiogenic therapies are discussed.

Functional mapping of somatostatin receptors in the retina: a review

The peptide somatostatin is one of many neuroactive agents that influence retinal physiology. It is synthesized primarily in a subclass of amacrine cells and believed to function as a neurotransmitter, neuromodulator or trophic factor. The cloning of the somatostatin receptors (sst1-5) in the early nineties provided the appropriate tools for the study of ssts in many tissues, including the retina. In this review, emphasis is given to recent studies that have provided significant information on the functional role of somatostatin in retinal circuitry and the retinal pigment epithelium. The important role of somatostatin in retinal disease therapeutics is also discussed.

Octreotide acts as an antitumor angiogenesis compound and suppresses tumor growth in nude mice bearing human hepatocellular carcinoma xenografts

PURPOSE

To investigate the effect of octreotide on angiogenesis induced by human hepatocellular carcinoma (HCC) and to evaluate whether octreotide can suppress tumor growth in nude mice bearing human HCC xenografts through inhibition of angiogenesis.

METHODS

Using MTT assay, invasion assay, migration assay, and Matrigel assay, the effects of octreotide on endothelial cells stimulated by vascular endothelial growth factor (VEGF) were evaluated in vitro. MTT assay was also used to investigate the effects of octreotide on human HCC cells with high (MHCC97-H) and low (MHCC97-L) metastatic potential that were established from the animal model of human HCC LCI-D20 in nude mice. The expression of somatostatin receptor (SSTR) subtypes in human umbilical vein endothelial cells (HUVECs), MHCC97-H, and MHCC97-L cells was detected by RT-PCR analysis. An LCI-D20 corneal micropocket model in nude mice was used to evaluate the effect of octreotide on angiogenesis induced by human HCC in vivo. Male nude mice were subcutaneously implanted with LCI-D20 tumor tissues for the tumor xenograft studies. Microvessel density was analyzed in CD34-stained tumor sections by the immunohistochemical SP method.

RESULTS

In vitro, octreotide inhibited the proliferation, invasion, and differentiation of HUVECs elicited by VEGF. RT-PCR analysis demonstrated that HUVECs expressed the somatostatin receptor subtype SSTR3. In vivo, octreotide was sufficiently potent to suppress nude mice corneal neovascularization induced by tumor tissues from LCI-D20. Systemic administrations of octreotide produced a significant suppression of the growth of LCI-D20. In cell culture, MHCC97-H and MHCC97-L cells were insensitive to octreotide at concentrations that significantly inhibited endothelial cells proliferation. The HCC cells used did not express any known SSTRs. Immunohistochemical studies of tumor tissues revealed decreased microvessel density in octreotide-treated animals as compared with controls.

CONCLUSIONS

The present study demonstrates that the somatostatin analogue octreotide is a potent antitumor angiogenesis compound and the antiproliferative effect of octreotide on tumor growth in nude mice bearing HCC xenografts may be mediated, at least in part, by its suppressive effect on blood vessel supply. The somatostatin analogue octreotide might provide a useful and relatively nontoxic adjuvant therapy in the treatment of HCC.

Inhibition of experimental angiogenesis of cornea by somatostatin

BACKGROUND

This study evaluated the inhibitory activity of somatostatin 14 on the angiogenesis of cornea in vivo.

METHODS

Corneal neovascularization was induced with a pellet containing 90 ng of basic fibroblast growth factor (bFGF) in a rat corneal pocket model. Three kinds of pellets were made containing bFGF plus somatostatin (SST) 0 ng, 20 ng and 200 ng for the control group, group 1 and group 2, respectively. Neovascularization was observed biomicroscopically from day 4 to day 8, and the corneas were then examined for changes in histology. Quantitation of angiogenesis in the cornea was accomplished by caliper and image analysis.

RESULTS

The 200-ng dose of SST showed significant inhibition of both length and area of neovascularization on day 7 (0.62+/-0.11 mm vs 1.29+/-0.16 mm, 0.50+/-0.16 mm2 vs 1.35+/-0.29mm2, group 2 vs control; P<0.05). The 20 ng of somatostatin did not demonstrate any significant inhibition of neovascularization compared with the control group.

CONCLUSION

Our study demonstrated that SST 14 can reduce bFGF-induced corneal angiogenesis. This shows the potential value of somatostatin in the treatment of corneal neovascularization.