Somatostatin analogues in the treatment of breast and prostate cancer

Newly developed somatostatin analogues may be useful agents in the treatment of breast and prostate cancer. Potential mechanisms of antitumor action include suppression of circulating levels of trophic hormones and growth factors as well as direct effects at the tumor level, potentially involving autocrine/paracrine mechanisms. Pilot clinical trials conducted in heavily pretreated women with advanced breast cancer indicate that SMS 201–995 (Sandostatin®) has minimal toxicity and moderately suppresses stimulated growth hormone secretion and basal somatomedin-C level. Somatostatin analogues have also been found to retard the growth of experimental prostate cancer, particularly when used in combination with LHRH analogues. The therapeutic efficacy of these compounds used alone or in combination with other agents in the treatment of breast and prostate cancer remains to be established in larger clinical trials involving less heavily pretreated patients.     

Pituitary somatostatin receptors. Characterization by binding with a nondegradable peptide analogue

Somatostatin receptors in the rat pituitary gland were characterized by binding analysis with a radioiodinated high affinity somatostatin analogue, 125I-Tyr1[D-Trp8]somatostatin. Receptor binding of this derivative reached equilibrium at 30 min and was maintained at a plateau for at least 60 min. Two L-Trp8- labeled somatostatin analogues. 125I-Tyr1- and [125I-Tyr11]somatostatin, displayed less stable and lower specific uptake and higher nonspecific binding. In contrast to the rapid degradation of the L-Trp8 ligands during binding assay, 125I-Tyr1]D-Trp8]somatostatin retained more than 80% of its binding activity after 90 min of incubation with pituitary particles. Pituitary particles bound 125I-Tyr1]D-Tyr8]somatostatin with high affinity (Ka = 8.6 +/- 1.2 X 10(9) M-1) and capacity of 54.4 +/- 2.6 fmol/mg. These binding sites showed specificity for the native peptide and its active analogues, and other peptide hormones, including angiotensin II, thyrotropin-releasing hormone, vasopressin, oxytocin, substance P, and gonadotropin-releasing hormone, did not inhibit tracer binding. A good correlation was observed between the binding affinities of several somatostatin analogues and their potencies as inhibitors of growth hormone release in rat pituitary cells. These findings emphasize the physiological importance of the pituitary somatostatin receptor in mediating the inhibitory action of the peptide on growth hormone release. The use of Tyr1[d-Trp8]somatostatin as a labeled ligand permits accurate determinations of the binding affinity and concentration of receptors for somatostatin in the normal pituitary gland and provides a basis for further studies of somatostatin receptor regulation and receptor-mediated cellular effects of the tetradecapeptide.     

Inhibition of protein kinase CK2 suppresses angiogenesis and hematopoietic stem cell recruitment to retinal neovascularization sites

Ubiquitous protein kinase CK2 participates in a variety of key cellular functions. We have explored CK2 involvement in angiogenesis. As shown previously, CK2 inhibition reduced endothelial cell proliferation, survival and migration, tube formation, and secondary sprouting on Matrigel. Intraperitoneally administered CK2 inhibitors significantly reduced preretinal neovascularization in a mouse model of proliferative retinopathy. In this model, CK2 inhibitors had an additive effect with somatostatin analog, octreotide, resulting in marked dose reduction for the drug to achieve the same effect. CK2 inhibitors may thus emerge as potent future drugs aimed at inhibiting pathological angiogenesis. Immunostaining of the retina revealed predominant CK2 expression in astrocytes. In human diabetic retinas, mRNA levels of all CK2 subunits decreased, consistent with increased apoptosis. Importantly, a specific CK2 inhibitor prevented recruitment of bone marrow-derived hematopoietic stem cells to areas of retinal neovascularization. This may provide a novel mechanism of action of CK2 inhibitors on newly forming vessels.     

New approaches to the therapy of various tumors based on peptide analogues.

The discovery of hypothalamic hormones was briefly reviewed. The development of new hormonal methods for the therapy of various cancers based on analogues of hypothalmic hormones is then presented. My group isolated luteininzing hormone-releasing hormone (LH-RH), also known as Gn-RH, from pig hypothalmi, elucidated its amino acid sequence, and synthesized it in 1971. The interest in medical applications of LH-RH led to the synthesis of LH-RH analogues by various groups. LH-RH agonists substituted in positions 6 or 10 including Decapeptyl, Leuprolide and Zoladex are much more active than LH-RH and on continuous administration produce inhibition of pituitary and gonads. Chronic administration of LH-RH agonists is being utilized for the treatment of prostate and breast cancer. Octapeptide analogues of somatostatin have various applications in Oncology. In 1980 we developed a new endocrine therapy for advanced prostate cancer based on agonists of LH-RH, which is now preferred by 70-90% of prostate cancer patients for primary treatment. LH-RH antagonists such as Cetrorelix can be used for therapy of BPH. On the basis of the presence of specific receptors for hypothalamic peptides on human cancers, we developed targeted cytotoxic analogues of LH-RH, somatostatin, and bombesin/GRP linked to doxorubicin or 2-pyrrolinodoxorubicin. These analogues inhibit the growth of experimental human prostate, breast, ovarian and endometrial cancer, renal cell carcinoma, pancreatic, colorectal and gastric cancers, small cell lung carcinoma (SCLC) and non-SCLC, brain tumors, melanomas, and lymphomas. Cytotoxic LH-RH analogues are now in clinical trials. Recently we demonstrated that growth hormone-releasing hormone (GH-RH) also serves as an autocrine growth factor in many cancers. Antagonistic analogues of GH-RH synthesized in our laboratory inhibit the growth of diverse tumors. The discovery of LH-RH and somatostatin has led to clinical use of their analogues in the field of cancer treatment and GH-RH antagonists also show a great promise.     

Peptidase inactivation of hypothalamic releasing hormones.

With the structural characterization of the hypothalamic hormones, luteinizing hormone-releasing hormone (LH-RH), thyrotrophin-releasing (TRH), melanocyte-stimulating hormone release-inhibiting hormine (MIH), and growth hormone release-inhibiting hormone, (GH-RIH or somatostatin), it has been possible to investigate their enzymic inactivation by peptidases which are present at various sites in the body. Enzymes may play an important part in the control of polypeptide hormone levels and the peptidases acting on these four hypothalamic hormones may regulate the amount of TRH, LH-RH, MIH and somatostatin released from the hypothalamus, or their action at the level of the pituitary and their removal from the circulation. By studying the peptidase enzymes, further information may be obtained on the physiological mechanisms controlling the secretion and actions of hypothalamic hormones, as well as on the design of analogues with increased or competitive activity.     

Structure-activity studies with somatostatin: the role of tryptophan in position 8

The gastric acid and pepsin inhibitory activities of 21 analogues of somatostatin, the majority modified at position 8, were determined in conscious cats in order to examine the importance of Trp8 for the activity of somatostatin. Pepsin secretion stimulated by pentagastrin was 5 times more sensitive, compared with the acid secretion, to inhibition by somatostatin. All the analogues showed similar differential sensitivity, indicating a similar specificity of somatostatin receptors involved in the inhibition of these two secretions. Halogenated-Trp8 analogues of somatostatin were only equipotent or slightly more active than somatostatin against gastric secretion in the cat, whilst these analogues are up to 30 times more potent against growth hormone release in the rat, indicating a different specificity of the two groups of receptors. Studies with the position 8 modified analogues suggest that the electron density of the aromatic nucleus of Trp8 may be relatively unimportant in determining the gastric inhibitory activity, whilst it can be concluded that the role of Trp8 in somatostatin depends to a large extent on the indole NH group. The precise role of Trp8 in somatostatin could be an involvement in the binding of somatostatin to its receptors, or involvement in forming the biologically active conformation of somatostatin.     

Growth hormone stimulates hypothalamic somatostatin.

Hypothalamic somatostatin concentration is increased in normal rats treated with growth hormone. Somatostatin is reduced in the median eminence of hypophysectomized rats but restored to normal following growth hormone administration. These results suggest that growth hormone exerts a positive feedback effect on hypothalamic somatostatin mechanism by which it may regulate its own secretion.     

Stimulation by somatostatin of dephosphorylation of membrane proteins in pancreatic cancer MIA PaCa-2 cell line

A membrane receptor and a cytosolic receptor for somatostatin were found in a human undifferentiated pancreatic cancer cell line (MIA PaCa-2). Binding of somatostatin to this membrane receptor activates dephosphorylation of a phosphotyrosyl-membrane protein whose phosphorylation was promoted by epidermal growth factor (EGF). Vanadate, a purported inhibitor of dephosphorylation, interferes with the action of somatostatin. These findings suggest a possible biochemical mechanism by which somatostatin may inhibit the growth of human pancreatic cancers.     

Mechanism of worsening diabetic retinopathy with rapid lowering of blood glucose: the synergistic hypothesis

Insulin treatment has been associated with a paradoxical worsening of diabetes retinopathy since many years in European cohorts. Recently, this issue has been stressed by some studies conducted in other parts of the world. However, the mechanism underlying such evolution is not well understood. An osmotic theory has been evocated but failed to explain the clinical features of the disease. Considering recent findings from basic and clinical research, we discuss the possibility of a synergistic hypothesis based on the simultaneous action of insulin and vascular endothelial growth factor on eye blood vessels. We postulate that exogenous insulin could act synergistically with the vascular endothelial growth factor expressed by ischemic retina so as to trigger vascular proliferation and the worsening of diabetes retinopathy.     

TT232, A Novel Signal Transduction Inhibitory Compound in the Therapy of Cancer and Inflammatory Diseases

TT-232 is a structural analogue of somatostatin exhibiting strong and selective growth-inhibitory effects, inhibition of neurogenic inflammation, as well as general anti-inflammatory and analgesic potential without the wide-ranging endocrine side effects of the parent hormone and its “traditional” analogues. The anti-inflammatory action of TT-232 is mediated through the SSTR4 receptor, and its antitumor activity is mediated through the SSTR1 receptor and by the tumor-specific isoform of pyruvate kinase. Its mechanism of action is in line with a new era of molecular medicine called signal transduction therapy, where “false” intracellular or intercellular communication is inhibited or corrected without interfering with basic cell functions and machinery. TT232 has passed phase I clinical trials without toxicity and significant side effects, and phase II studies are running for oncological and anti-inflammatory indications, respectively. This compound has the perspective to become the first drug in molecularly targeted therapy of inflammation where a combined effect of anti-inflammatory, analgesic, and neurogenic inflammation-inhibiting activity can be achieved.     

TT232, a novel signal transduction inhibitory compound in the therapy of cancer and inflammatory diseases

TT-232 is a structural analogue of somatostatin exhibiting strong and selective growth-inhibitory effects, inhibition of neurogenic inflammation, as well as general anti-inflammatory and analgesic potential without the wide-ranging endocrine side effects of the parent hormone and its "traditional" analogues. The anti-inflammatory action of TT-232 is mediated through the SSTR4 receptor, and its antitumor activity is mediated through the SSTR1 receptor and by the tumor-specific isoform of pyruvate kinase. Its mechanism of action is in line with a new era of molecular medicine called signal transduction therapy, where "false" intracellular or intercellular communication is inhibited or corrected without interfering with basic cell functions and machinery. TT232 has passed phase I clinical trials without toxicity and significant side effects, and phase II studies are running for oncological and anti-inflammatory indications, respectively. This compound has the perspective to become the first drug in molecularly targeted therapy of inflammation where a combined effect of anti-inflammatory, analgesic, and neurogenic inflammation-inhibiting activity can be achieved.     

New medical approaches in pituitary adenomas

Recently, the medical approach to patients with secreting and clinically non-functioning pituitary adenomas has received great impulse thanks to the availability of new, selective and long-lasting compounds with dopaminergic activity, such as cabergoline, and of somatostatin analogues provided in slow-release formulations, such as lanreotide and octreotide long acting release (LAR). In particular, the use of cabergoline has induced control of hyperprolactinaemia and tumour shrinkage in the great majority of patients with micro- and macroprolactinomas. Cabergoline treatment restores fertility both in women and men, and partially improves osteoporosis, one of the major complications of hyperprolactinaemia. In acromegaly, disease control (growth hormone [GH] <2.5-1.0 microg/l as a fasting or glucose-suppressed value, respectively, together with age-normalised insulin-like growth factor [IGF]-I) is achievable in more than half of patients receiving treatment with lanreotide or octreotide-LAR. Improvement in cardiomyopathy, sleep apnoea and arthropathy has been reported during GH/IGF-I suppression after pharmacotherapy. A synthetic GH analogue, B2036-PEG, that antagonises endogenous GH binding to its receptor-binding sites and a GH-releasing hormone antagonist that blocks the effect of this releasing factor on the hypothalamus and pituitary are presently under investigation in acromegaly. Preliminary studies have clearly demonstrated the effectiveness of the GH receptor antagonist in suppressing IGF-I levels in acromegalic patients previously unresponsive to somatostatin analogues. Beneficial effects of subcutaneous octreotide and lanreotide have also been reported in adenomas secreting thyroid-stimulating hormone, while the results of treatment with dopamine agonists or somatostatin analogues remain disappointing in patients with clinically non-functioning adenomas. In these patients the possibility of visualising in vivo the expression of D(2) receptors using specific radiotracers such as (123)I-methoxybenzamide has allowed selection of patients likely to respond to cabergoline. Scant effects of pharmacotherapy have also been reported in patients with adenomas secreting adrenocorticotropic hormone. However, some preliminary data suggest a potential use of cabergoline in combination with ketoconazole, or alone, in selected cases of Cushing's disease or Nelson's syndrome.     

The honey-bee “dance language” hypothesis and the foundations of biology and behavior

We have carried out a detailed analysis of the region of the hormone, which can be called an “active site”, that is essential for receptor binding and/or agonist activity, on the basis of structure-activity data of peptide hormones published in the literature. We find that one or more aromatic residues, often in a cluster, is present at the active sites of insulin, glucagon, adrenocorticotropin, gastrin, endorphins and angiotensin. Recognition of the functional importance of aromatic residues, combined with sequence comparisons and secondary structural predictions, enable us to identif active sites of nerve growth factor, somatostatin, calcitonin, parathyroid hormone and luteinizing hormone releasing hormone. Aromatic residues also appear to be essential for the function of nonpeptide hormones that act at the plasma membrane, and opiates. The apparently ubiquitous presence of aromatic groups at the active site of peptide hormones may facilitate understanding of the mechanism of action of hormones and provide insights into the design of hormone analogues.     

Hyperoxia induces retinal vascular endothelial cell apoptosis through formation of peroxynitrite

Hyperoxia exposure induces capillary endothelial cell apoptosis in the developing retina, leading to vaso-obliteration followed by proliferative retinopathy. Previous in vivo studies have shown that endothelial nitric oxide synthase (NOS3) and peroxynitrite are important mediators of the vaso-obliteration. Now we have investigated the relationship between hyperoxia, NOS3, peroxynitrite, and endothelial cell apoptosis by in vitro experiments using bovine retinal endothelial cells (BREC). We found that BREC exposed to 40% oxygen (hyperoxia) for 48 h underwent apoptosis associated with activation of caspase-3 and cleavage of the caspase substrate poly(ADP-ribose) polymerase. Hyperoxia-induced apoptosis was associated with increased formation of nitric oxide, peroxynitrite, and superoxide anion and was blocked by treatment with uric acid, nitro-L-arginine methyl ester, or superoxide dismutase. Analyses of the phosphatidylinositol 3-kinase/Akt kinase survival pathway in cells directly treated with peroxynitrite revealed inhibition of VEGF- and basic FGF-induced activation of Akt kinase. These results suggest that hyperoxia-induced formation of peroxynitrite induces BREC apoptosis by crippling key survival pathways and that blocking peroxynitrite formation prevents apoptosis. These data may have important clinical implications for infants at risk of retinopathy of prematurity. oxygen-induced retinopathy; vaso-obliteration; superoxide; nitric oxide     

Regulation of insulin and glucagon secretion from rat pancreatic islets in vitro by somatostatin analogues

Somatostatin is an inhibitor of hormone secretion through specific receptors (sst1-5). The aim of this study was to investigate the putative regulatory role of somatostatin analogues on the secretion of insulin and glucagon by rat pancreatic islets. After 48 h exposure only the non-selective agonists (somatostatin, octreotide and SOM-230) inhibited insulin accumulation. The inhibition of insulin secretion was accompanied by increased islet insulin contents. None of the analogues showed a consistent effect on the glucagon accumulation in the medium after 48 h. Since we observed a difference in the regulatory effect between the non-selective and selective analogues, combinations of selective analogues were studied. Combination of sst2+sst5 agonists inhibited the medium insulin accumulation, while combination of sst1+sst2 analogues caused a decrease in glucagon accumulation. After removal of somatostatin a rebound effect with increased insulin secretion were observed. This effect was reversed after 6 h. For SOM-230 insulin secretion continued to be suppressed even after the analogue was removed and returned to control values after 3 h. As for glucagon secretion there was an initial decline after culture with octreotide, while the other substances failed to induce any changes. In summary, non-selective somatostatin analogues or combinations of receptor selective analogues may cause inhibition of hormone secretion from rat pancreatic islets. For insulin and glucagon, combinations of sst2+sst5 and sst1+sst2, respectively may exert this effects. Thus, our data suggest that more than one sst must be involved to down-regulate islet glucagon and insulin secretion.     

Somatostatin: a historical perspective

Following the discovery and biochemical characterization of natural somatostatin its action profile has been thoroughly investigated. Although the name somatostatin was coined in virtue of its growth hormone release-inhibiting properties, a number of central and peripheral endocrine and paracrine actions have been ascribed to this peptide. Its inhibitory effect on a series of pituitary and gastrointestinal hormones has characterized somatostatin as a classical brain-gut hormone. Circulating and tissue levels of somatostatin and its possible physiological role are analyzed and clinical implications are drawn.     

Somatostatin receptor 1 (SSTR1)-mediated inhibition of cell proliferation correlates with the activation of the MAP kinase cascade: role of the phosphotyrosine phosphatase SHP-2.

The mitogen activated protein (MAP) kinase cascade represents one of the major regulator of cell growth by hormones and growth factors. However, although the activation of this intracellular pathway has been often regarded as mediator of cell proliferation, in many cell types the increase in MAP kinase (also called extra-cellular signal regulated kinase: ERK) activity may result in cell growth arrest, depending on the length or the intensity of the stimulation. In this review we examine recent data concerning the effects of somatostatin on the MAP kinase cascade through one of its major receptor subtype, the somatostatin receptor 1 (SSTR1), stably expressed in CHO-K1 cells. Somatostatin inhibits the proliferative effects of basic FGF (bFGF) in CHO-SSTR1 cell line. However, in these cells, somatostatin robustly activates the MAP kinase and augments bFGF-induced stimulation of ERK. We show that the activation of ERK via SSTR1 is mediated by the betagamma subunit of a pertussis toxin-sensitive G-protein and requires both the small G protein Ras and the serine/threonine kinase Raf-1. Moreover the phosphatidyl inositol-3kinase and the cytosolic tyrosine kinase c-src participate in the signal transduction regulated by SSTRI to activate ERK, as well as it is involved the protein tyrosine phosphatase (PTP) SHP-2. Previous studies have suggested that somatostatin-stimulated PTP activity mediates the growth inhibitory actions of somatostatin, in CHO-SSTR1 cells. Thus, the activation of SHP-2 by SSTR1 may mediate the antiproliferative activity of somatostatin. SHP-2 may. in turn, regulate the activity of kinases upstream of ERK that require tyrosine dephosphorylation to be activated, such as c-src. Finally, the synergism between somatostatin and bFGF in the activation of ERK results in an increased expression of the cyclin-dependent kinase inhibitor p21cip/WAF1 as molecular effector of the antiproliferative activity of somatostatin.     

Octreotide reduces vitreous hemorrhage and loss of visual acuity risk in patients with high-risk proliferative diabetic retinopathy.

OBJECTIVE: Growth hormone and insulin-like growth factor-I have been implicated as strong promoters of proliferative diabetic retinopathy. We studied reduction of bleeding and preservation of visual acuity by treatment with the long-acting somatostatin analogue, octreotide, in diabetic patients at an advanced stage of proliferative diabetic retinopathy. RESEARCH DESIGN AND METHODS: Randomized trial in a University hospital setting. Reading ophthalmologists were masked for octreotide use, diabetologists were aware of that treatment. Nine patients received 100 microg tid octreotide (verum) subcutaneously for a maximum of 36 months. Nine diabetics served as controls, no placebo treatment was used. Episodes of vitreous hemorrhages were counted, measurement of visual acuity, estimation of neovascularization by stereoscopic fundus photography and fluorescein angiography were carried out. RESULTS: After 3 years of treatment, the incidence of vitreous hemorrhages and the need for vitreoretinal surgery was significantly lower (log rank test p = 0.002) in the octreotide-treated patients. Visual acuity was preserved and significantly better in the octreotide treated group compared to controls (p = 0.05). CONCLUSIONS: In diabetics with high-risk proliferative retinopathy after full scatter laser coagulation, octreotide reduced the number of vitreous hemorrhages, preserving visual acuity.     

Pigment epithelium-derived factor (PEDF)-induced apoptosis and inhibition of vascular endothelial growth factor (VEGF) expression in MG63 human osteosarcoma cells

Pigment epithelium-derived factor (PEDF) has been shown to be the most potent inhibitor of angiogenesis in the mammalian eye, thus suggesting that loss of PEDF is involved in angiogenic eye diseases such as proliferative diabetic retinopathy. Angiogenesis is required for tumor growth and progression as well. We, along with others, have recently found that PEDF could inhibit growth of melanoma and hepatocellular carcinoma in nude mice through its anti-angiogenic effects on tumor endothelial cells. However, the possibility of the direct effect of PEDF on tumor cells has remained. In this study, we investigated the effects of PEDF on growth and vascular endothelial growth factor (VEGF) expression in MG63 human cultured osteosarcoma cells. PEDF decreased viable cell number as well as DNA synthesis in MG63 cells in a dose-dependent manner. Furthermore, PEDF was found to increase caspase-3/7 activity and to subsequently induce apoptotic cell death in MG63 cells. PEDF also inhibited VEGF expression in MG63 cells at both mRNA and protein levels. Our present study provides novel beneficial aspects of PEDF on osteosarcoma cells; one is induction of apoptotic cell death of tumor cells, and the other is the suppression of VEGF expression, which would lead to inhibition of tumor angiogenesis. PEDF therefore might be a promising therapeutic agent for treatment of patients with osteosarcoma.