G protein coupled receptor signaled apoptosis is associated with activation of a cation insensitive acidic endonuclease and intracellular acidification

The Molecular Effects of Dietary Acid Load on Metabolic Disease (The Cellular PasaDoble: The Fast-Paced Dance of pH Regulation)

Metabolic diseases are becoming more common and more severe in populations adhering to western lifestyle. Since metabolic conditions are highly diet and lifestyle dependent, it is suggested that certain diets are the cause for a wide range of metabolic dysfunctions. Oxidative stress, excess calcium excretion, inflammation, and metabolic acidosis are common features in the origins of most metabolic disease. These primary manifestations of "metabolic syndrome" can lead to insulin resistance, diabetes, obesity, and hypertension. Further complications of the conditions involve kidney disease, cardiovascular disease, osteoporosis, and cancers. Dietary analysis shows that a modern "Western-style" diet may facilitate a disruption in pH homeostasis and drive disease progression through high consumption of exogenous acids. Because so many physiological and cellular functions rely on acid-base reactions and pH equilibrium, prolonged exposure of the body to more acids than can effectively be buffered, by chronic adherence to poor diet, may result in metabolic stress followed by disease. This review addresses relevant molecular pathways in mammalian cells discovered to be sensitive to acid - base equilibria, their cellular effects, and how they can cascade into an organism-level manifestation of Metabolic Syndromes. We will also discuss potential ways to help mitigate this digestive disruption of pH and metabolic homeostasis through dietary change.

pH-Induced Folding of the Caspase-Cleaved Par-4 Tumor Suppressor: Evidence of Structure Outside of the Coiled Coil Domain

Prostate apoptosis response-4 (Par-4) is a 38 kDa largely intrinsically disordered tumor suppressor protein that functions in cancer cell apoptosis. Par-4 down-regulation is often observed in cancer while up-regulation is characteristic of neurodegenerative conditions such as Alzheimer’s disease. Cleavage of Par-4 by caspase-3 activates tumor suppression via formation of an approximately 25 kDa fragment (cl-Par-4) that enters the nucleus and inhibits Bcl-2 and NF-ƙB, which function in pro-survival pathways. Here, we have investigated the structure of cl-Par-4 using biophysical techniques including circular dichroism (CD) spectroscopy, dynamic light scattering (DLS), and intrinsic tyrosine fluorescence. The results demonstrate pH-dependent folding of cl-Par-4, with high disorder and aggregation at neutral pH, but a largely folded, non-aggregated conformation at acidic pH.

Focal photodynamic intracellular acidification as a cancer therapeutic

Cancer cells utilize an array of proton transporters to regulate intra- and extracellular pH to thrive in hypoxic conditions, and to increase tumor growth and metastasis. Efforts to target many of the transporters involved in cancer cell pH regulation have yielded promising results, however, many productive attempts to disrupt pH regulation appear to be non-specific to cancer cells, and more effective in some cancer cells than others. Following a review of the status of photodynamic cancer therapy, a novel light-activated process is presented which creates very focal, rapid, and significant decreases in only intracellular pH (pHi), leading to cell death. The light-activation of the H⁺ carrier, nitrobenzaldehyde, has been effective at initiating pH-induced apoptosis in non-cancerous and numerous cancerous cell lines in vitro, to include breast, prostate, and pancreatic cancers. Also, this intracellular acidification technique caused significant reductions in tumor growth rate and enhanced survival in mice bearing triple negative breast cancer tumors. The efficacy of an NBA-upconverting nanoparticle to kill breast cancer cells in vitro is described, as well as a discussion of the potential intracellular mechanisms underlying the pH-induced apoptosis.

Upregulated expression of SSTR1 is involved in neuronal apoptosis and is coupled to the reduction of bcl-2 following intracerebral hemorrhage in adult rats

Somatostatins are peptide hormones that regulate diverse cellular processes, such as neurotransmission, cell proliferation, apoptosis, and endocrine signaling as well as inhibiting the release of many hormones and other secretory proteins. SSTR1 is a member of the superfamily of somatostatin receptors possessing seven-transmembrane segments. Aberrant expression of SSTR1 has been implicated in several human diseases, including pseudotumor cerebri, and oncogenic osteomalacia. In this study, we investigated a potential role of SSTR1 in the regulation of neuronal apoptosis in the course of intracerebral hemorrhage (ICH). A rat ICH model in the caudate putamen was established and subjected to behavioral tests. Western blot and immunohistochemistry indicated a remarkable up-regulation of SSTR1 expression surrounding the hematoma after ICH. Double-labeled immunofluorescence showed that SSTR1 was mostly co-localized with neurons, and was rarely distributed in activated astrocytes and microglia. Additionally, SSTR1 co-localized with active-caspase-3 and bcl-2 around the hematoma. The expression of active-caspase-3 was parallel with that of SSTR1 in a time-dependent manner. In addition, SSTR1 knockdown specifically resulted in reduced neuronal apoptosis in PC12 cells. All our findings suggested that up-regulated SSTR1 contributed to neuronal apoptosis after ICH, which was accompanied with reduced expression of bcl-2.

Somatostatin analogs regress endometriotic implants in rats by decreasing implant levels of vascular endothelial growth factor and matrix metaloproteinase 9

OBJECTIVE

To examine the effect of somatostatin analogs on surgically induced endometriosis in rat models.

STUDY DESIGN

Endometrial tissue was implanted onto the abdominal peritoneum of 26 rats that were randomized into 3 groups. The rats in group 1(n = 9) were subcutaneously administered with 0.02 mg/kg/d of octreotide (a short-acting analog)for 28 days . The rats in group 2 (n = 8) were subcutaneously injected with 20 mg/kg of a single dose of a long-acting analogue lanreotide The rats in group 3 were given no medication and served as controls (n = 9).

RESULTS

Mean volume and histologic score of implants in groups 1 (P < .01 and P < .05, respectively) and 2 (P < .01and P < .05, respectively) were significantly lower than that in group 3. There were significant reductions in vascular endothelial growth factor (VEGF) and matrix metalloproteinase 9 (MMP-9) immunoreactivities in group 1 (0.67 ± 0.50 and 1.22 ± 0.44, respectively; both P < .01) and group 2 (0.71 ± 0.48 and 0.86 ± 0.69, respectively; both P < .01) when compared with the control group (1.78 ± 0.83 and 2.11 ± 0.78, respectively).

CONCLUSION

Somatostatin analogs has regressed significantly the size of the endometriotic implants and caused atrophy of these lesions in rats by decreasing explant levels of VEGF and MMP-9.

Octreotide induces caspase activation and apoptosis in human hepatoma HepG2 cells

AIM: To investigate the role of octreotide on cellular proliferation and apoptosis of human hepatoma (HepG2) cells.

METHODS: We studied cellular proliferation, apoptosis and the possible internal caspase-mediated apoptosis pathway involved, after treatment of HepG2 carcinoma cells with octreotide in comparison with the apoptosis caused by tumor necrosis factor-α (TNF-α). Activities of caspase-3, caspase-9, caspase-8 and caspase-2 were studied, while apoptosis was investigated through detection of DNA fragmentation and through identification of apoptotic cells with the annexin-V/propidium iodide flow cytometric method.

RESULTS: After an initial increase in HepG2 cellular proliferation, a significant inhibition was observed with 10^-8 mol/L octreotide, while TNF-α dose-dependently decreased proliferation. Early and late apoptosis was significantly increased with both substances. Octreotide significantly increased caspase-3, caspase-8 and caspase-2 activity. TNF-α significantly increased only caspase-2. Cellular proliferation was decreased after treatment with octreotide or TNF-α alone but, in contrast to TNF-α, octreotide decreased proliferation only at concentrations of 10^-8 mol/L, while lower concentrations increased proliferation.

CONCLUSION: Our findings are suggestive of caspase-mediated signaling pathways of octreotide antitumor activity in HepG2 cells, and indicate that measurements of serum octreotide levels may be important, at least in clinical trials, to verify optimal therapeutic drug concentrations.

Somatostatin receptor-3 mediated intracellular signaling and apoptosis is regulated by its cytoplasmic terminal

In the present study, we describe the role of cytoplasmic terminal (C-tail) domain in regulating coupling to adenylyl cyclase, signaling, and apoptosis in human embryonic kidney (HEK-293) cells transfected with wild type (wt)-hSSTR3 and C-tail deleted mutants. Cells transfected with wt-hSSTR3 and C-tail mutants show comparable membrane expression; however, display decreased expression in presence of agonist. wt-hSSTR3 exists as preformed homodimer at cell surface in basal conditions and decreases in response to agonist. Cells expressing C-tail mutants also show evidence of homodimerization with the same intensity as wt-hSSTR3. The agonist-dependent inhibition of cyclic adenosine monophosphate (cAMP) was lost in cells expressing C-tail mutants. Agonist treatment in cells expressing wt-hSSTR3 resulted in inhibition of cell proliferation, increased expression of PARP-1, and TUNEL positivity in proliferating cell nuclear antigen (PCNA)-positive cells. The agonist mediated increase in membrane expression of protein tyrosine phosphatase (PTP) seen with wt-hSSTR3 was diminished in C-tail mutants, which was accompanied with the loss of receptor's ability to induce apoptosis. Taken together, our data provide new insights into C-tail-dependent regulation of cell signaling and apoptosis by hSSTR3.

Antiproliferative effect of somatostatin analogs in gastroenteropancreatic neuroendocrine tumors

Somatostatin analogs were initially developed for the control of hormonal syndromes associated with neuroendocrine tumors (NETs). In recent years, accumulating data has supported their role as antiproliferative agents, capable of stabilizing tumor growth in patients with metastatic neuroendocrine malignancies, including carcinoid and pancreatic endocrine tumors. A phase III, randomized, placebo-controlled trial has now demonstrated that octreotide long-acting repeatable (LAR) 30 mg can significantly prolong time to tumor progression among patients with metastatic midgut NETs regardless of functional status, chromogranin A level or age. In addition to significantly lengthening time to tumor progression in the overall study population, subset analysis suggests that patients with low tumor burden are most likely to experience disease stabilization with octreotide LAR 30 mg, supporting the early use of octreotide LAR in patients with metastatic disease. Further research efforts are underway to evaluate the use of somatostatin analogs as antiproliferative agents in other types of gastroenteropancreatic-NETs. Ongoing studies are also evaluating novel somatostatin analogs and somatostatin analogs in combination with other anti-tumor therapies.

Developmental changes in frequency of the ciliary somatostatin receptor 3 protein

Primary cilia extend from the surface of most vertebrate cells and display several signaling molecules, including the somatostatin receptor 3 (SSTR3), enabling cilia to play essential roles as chemical, osmotic and mechanical sensors. The SSTR3 is widely distributed in the adult rat brain, and also influences cell proliferation and apoptosis. To establish whether the SSTR3 is positioned to influence these developmental processes, we examined, using immunohistochemistry, the embryonic and postnatal development of SSTR3 expression in the rat hippocampal formation, and its association with newly born and mature neurons in adult rats. Elongated SSTR3-immunoreactive (-ir) cilia first appeared in the hippocampal formation CA3 region of postnatal day (P) 0 animals, and their density increased to high levels by P2, remained at high levels through to P30, but were at low levels in 5-month old rats. A similar developmental pattern was observed in the CA1 region, where SSTR3-ir ciliated structures were first detected on P2. In contrast, density levels in the granular cell layer of the dentate gyrus were very high by P30, and remained elevated in adult rats. SSTR3-ir cilia did not colocalize with neuroblasts in the hippocampal formation or olfactory bulb, but appeared to be localized to more mature cells in these regions. A few SSTR3-ir neurons were also observed in the hippocampal formation. These findings support the hypothesis that the ciliary SSTR3 is well positioned to influence the cell cycle and apoptotic processes during postnatal development, and in neurogenic regions of the adult rat brain.

Somatostatin receptors 1, 2, and 5 cooperate in the somatostatin inhibition of C6 glioma cell proliferation in vitro via a phosphotyrosine phosphatase-eta-dependent inhibition of extracellularly regulated kinase-1/2

Somatostatin inhibits cell proliferation through the activation of five receptors (SSTR1-5) expressed in normal and cancer cells. We analyzed the role of individual SSTRs in the antiproliferative activity of somatostatin in C6 rat glioma cells. Somatostatin dose-dependently inhibited C6 proliferation, an effect mimicked, with different efficacy or potency, by BIM-23745, BIM-23120, BIM-23206 (agonists for SSTR1, -2, and -5) and octreotide. The activation of SSTR3 was ineffective, although all SSTRs are functionally active, as demonstrated by the inhibition of cAMP production. All SSTRs induced cytostatic effects through the activation of the phosphotyrosine phosphatase PTPeta and the inhibition of ERK1/2. For possible synergism between SSTR subtypes, we tested the effects of the combined treatment with two agonists (SSTR1+2 or SSTR2+5) or bifunctional compounds. The simultaneous activation of SSTR1 and SSTR2 slightly increased the efficacy of the individual compounds with an IC50 in between the single receptor activation. SSTR2+5 activation displayed a pattern of response superimposable to that of the SSTR5 agonist alone (low potency and higher efficacy, as compared with BIM-23120). The simultaneous activation of SSTR1, -2, and -5 resulted in a response similar to somatostatin. In conclusion, the cytostatic effects of somatostatin in C6 cells are mediated by the SSTR1, -2, and -5 through the same intracellular pathway: activation of PTPeta and inhibition of ERK1/2 activity. Somatostatin is more effective than the individual agonists. The combined activation of SSTR1 and -2 shows a partial synergism as far as antiproliferative activity, whereas SSTR2 and -5 activation results in a response resembling the SSTR5 effects.

Somatostatin/somatostatin receptor signalling: phosphotyrosine phosphatases

Activation of phosphotyrosine phosphatases (PTPs) by somatostatin receptor (SSTR) represents one of the main intracellular mechanisms involved in the antiproliferative effect of somatostatin (SST) and analogues. Since their molecular cloning, the role of PTPs is emerging as a major regulator of different cell functions including cell proliferation, differentiation, cell to cell interactions, cell matrix adhesion and cell migration. It was demonstrated that PTPs possess high substrate specificity and their activity is tightly regulated. Importantly, different G protein-coupled receptors transduce their biological activities through PTPs. PTPs were identified as down-stream effectors of SSTRs to transduce antiproliferative signals, and so far, three family members (SHP-1, SHP-2 and DEP-1/PTPeta) have been identified as selective SSTR intracellular effectors. Here, the molecular mechanisms leading SSTRs to regulate PTP activity are discussed, focusing on recent data showing a close interplay between PTPs and tyrosine kinases to transduce tumoral cell growth arrest following SST analogs administration.

Serotonin 5-HT1A receptor stimulates c-Jun N-terminal kinase and induces apoptosis in Chinese hamster ovary fibroblasts

The 5-HT1A receptor is a prototypical member of the large and diverse serotonin receptor family. One key role of this receptor is to stimulate cell proliferation and differentiation via the extracellular signal regulated protein kinase (ERK) mitogen activated protein (MAP) kinase. There are few reports on the ability of the 5-HT1A receptor to modulate other MAP kinases such as c-Jun N-terminal kinase (JNK), which is activated by various extracellular stimuli, resulting in cell growth, differentiation, and programmed cell death. We report here for the first time that the 5-HT1A receptor stimulates JNK. JNK stimulation was Pertussis toxin-sensitive and was mediated by Rho family low molecular weight GTPases. The 5-HT1A receptor also increased apoptosis, which was mimicked by the MEK inhibitor PD98059, and blocked by the JNK inhibitor SP600125. These results suggest that the 5-HT1A receptor stimulates both ERK-dependent anti-apoptotic pathways and JNK-dependent pro-apoptotic pathways in CHO cells.

Acid DNases and their interest among apoptotic endonucleases

Apoptosis is characterized by cell shrinkage, nuclear condensation and internucleosomal DNA cleavage. Besides the central role of caspases and other proteases, cell death triggers DNA degradation so that DNases have an active role in apoptotic cell death. The best-characterized apoptotic DNase is CAD, a neutral Mg-dependent endonuclease. Its activity is regulated by its inhibitor, ICAD, which is cleaved by caspases. Other neutral DNases have been shown to cleave nuclear DNA in apoptotic conditions: endonuclease G, GADD. In cells, the cytosolic pH is maintained to 7.2, mostly due to the activity of the Na(+)/H(+) exchanger. In many apoptotic conditions, a decrease of the intracellular pH has been shown. This decrease may activate different acid DNases, mostly when pH decreases below 6.5. Three acidic DNases II are so far known: DNase II alpha, DNase II beta and L-DNase II, a DNase II, derived from the serpin LEI (Leukocyte Elastase Inhibitor). Their activation during cell death is discussed in this review.

Rationale for the use of somatostatin analogs as antitumor agents

BACKGROUND

There is a need for novel antitumor agents that demonstrate efficacy in currently refractory tumors without adding to the toxicity of therapy. The somatostatin analogs, which have demonstrated antineoplastic activities in experimental tumor models, and good tolerability and safety profiles are attractive candidates.

MATERIALS AND METHODS

Data from preclinical studies provide evidence for direct and indirect mechanisms by which somatostatin analogs exert antitumor effects.

RESULTS

Direct antitumor activities, mediated through somatostatin receptors (sst(1)-sst(5)) expressed in tumor cells, include blockade of autocrine/paracrine growth-promoting hormone and growth factor production, inhibition of growth factor-mediated mitogenic signals and induction of apoptosis. Indirect antitumor effects include inhibition of growth-promoting hormone and growth factor secretion, and antiangiogenic actions. Many human tumors express more than one somatostatin receptor subtype, with sst(2) being predominant. Somatostatin analogs such as octreotide and lanreotide, which present a high affinity for sst(2), are in current clinical use to alleviate symptoms in patients with endocrine tumors, and radiolabeled somatostatin analogs have been developed for diagnosis and radiotherapy.

CONCLUSIONS

While the rationale exists for the use of somatostatin analogs as antitumor agents, studies are ongoing to identify analogs with activity across the range of receptor subtypes to maximize the potential of such treatment.

Relationship between somatostatin receptor subtype expression and clinicopathology, Ki-67, Bcl-2 and p53 in colorectal cancer

AIM: To study the SSTR1, 2, 3, 4, 5 expression and their relationships with clinico-pathological factors, cell proliferation, Bcl-2 and p53 expression in colorectal cancer cells.

METHODS: Immunohistochemical staining of five SSTR subtypes, Ki-67, Bcl-2 and p53 was performed by the standard streptavidin-peroxidase (SP) technique for the paraffin sections of 127 colorectal cancers. and expression of five SSTR subtypes in 40 specimens of normal colorectal mucosae was detected with the same method.

RESULTS: Positive staining for five SSTR subtypes was observed in colorectal cancer cells and normal colorectal mucosae. SSTR1 was the most predominant subtype in both colorectal cancer and normal colorectal mucosa, and the second was SSTR5 or SSTR2. As compared with normal colorectal mucosa, SSTR4 was more frequently expressed in colorectal cancer cells (2.5 % vs 18.9 %, P  <  0.05); the expression of SSTR2, 4, 5 in moderately to well differentiated colorectal adenocarcinoma was significantly higher than that in poorly differentiated ones (P  <  0.05), the SSTR1 expression in colorectal cancer with positive lymph node metastasis was significantly higher than that with negative lymph node metastasis (72.2 % and 54.5 %，P <  0.05). In addition, in the ulcerative type of colorectal cancer, SSTR2 expression was obviously decreased (P   <  0.05); the correlation did not reach a statistical significance between the five SSTR subtypes expression and Dukes’stages (P  >  0.05), but the frequency of SSTR1 expression increased with Dukes’ stage, while SSTR3 and SSTR5 expression decreased with Dukes’ stage. Moreover, there was no correlation between expression of the five SSTR subtypes and other clinicopathological factors such as age, sex, tumor site, tumor depth, distant metastasis. The proliferative indexes in colorectal cancer cells with negative expression of SSTR2 and SSTR3 were significantly higher than that with positive expression (P < 0.05). The Bcl-2 expression in colorectal cancer cells with positive expression of SSTR1, 2, 3, 5 was significantly lower than that with negative expression (P < 0.05). There was no correlation between five SSTR subtypes and p53 expression.

CONCLUSION: The most predominant SSTR subtype is SSTR1, and the second is SSTR2 or SSTR5. Five SSTR subtypes play different roles in the development of colorectal cancer. SSTR2 and SSTR3 can inhibit the proliferation and promote apoptosis of tumor cells.

Non-genomic immunosuppressive actions of progesterone inhibits PHA-induced alkalinization and activation in T cells

Progesterone is an endogenous immunomodulator, and can suppress T-cell activation during pregnancy. When analyzed under a genome time scale, the classic steroid receptor pathway does not have any effect on ion fluxes. Therefore, the aim of this study was to investigate whether the non-genomic effects on ion fluxes by progesterone could immunosuppress phytohemagglutinin (PHA)-induced human peripheral T-cell activation. The new findings indicated that, first, only progesterone stimulated both [Ca2+]i elevation and pHi decrease; in contrast, estradiol or testosterone stimulated [Ca2+]i elevation and hydrocortisone or dexamethasone stimulated pHi decrease. Secondly, the [Ca2+]i increase by progesterone was dependent on Ca2+ influx, and the acidification was blocked by Na+/H+ exchange (NHE) inhibitor, 3-methylsulphonyl-4-piperidinobenzoyl, guanidine hydrochloride (HOE-694) but not by 5-(N,N-dimethyl)-amiloride (DMA). Thirdly, progesterone blocked phorbol 12-myristate 13-acetate (PMA) or PHA-induced alkalinization, but PHA did not prevent progesterone-induced acidification. Fourthly, progesterone did not induce T-cell proliferation; however, co-stimulation progesterone with PHA was able to suppress PHA-induced IL-2 or IL-4 secretion and proliferation. When progesterone was applied 72 h after PHA stimulation, progesterone could suppress PHA-induced T-cell proliferation. Finally, immobilization of progesterone by conjugation to a large carrier molecule (BSA) also stimulated a rapid [Ca2+]i elevation, pHi decrease, and suppressed PHA-induced proliferation. These results suggested that the non-genomic effects of progesterone, especially acidification, are exerted via plasma membrane sites and suppress the genomic responses to PHA. Progesterone might act directly through membrane specific nonclassical steroid receptors to cause immunomodulation and suppression of T-cell activation during pregnancy.

[Phosphotyrosine phosphatase SHP-1, somatostatin and prostate cancer]

We review the mechanisms involved in prostatic growth based on androgens and product of neuroendocrine secretion, with special reference to the role of somatostatin (SS) in the inhibition of neoplastic growth. Our contributions in the field confirm the antiproliferative effect of SS on the prostate is mediated by phosphotyrosine phosphatase SHP-1, that is present in human prostate. This enzyme plays a role in the control of prostatic cell proliferation and in the progression of prostate cancer. Besides, we consider its presence may determine the therapeutic potential of SS in the control of prostate cancer.

Antineoplastic effects of octreotide on human gallbladder cancer cells in vitro

AIM: To investigate whether octreotide can inhibit the growth of human gallbladder cancer cells in vitro and to elucidate the antineoplastic mechanism of octreotide in gallbladder cancer.

METHODS: A human gallbladder cancer cell line, GBC-SD, was cultured in vitro. The antiproliferative effects of octreotide were examined by means of an MTT assay and a colony forming ability assay. Morphological variation was investigated under scanning electron microscopy and transmission electron microscopy. Cell cycle analysis and apoptosis rate was evaluated by flow cytometry (FCM) after staining by propidium iodide. DNA fragmentation was assayed by agarose gel electrophoresis. Immunohistochemical staining was performed to evaluate the expressions of mutant-type p53 and bcl-2.

RESULTS: The growth curve and colony forming ability assay showed significant inhibition of octreotide to the proliferation of GBC-SD cells in culture in a time- and dose-dependent manner. After exposure to octreotide, GBC-SD cells showed typically apoptotic characteristics, including morphological changes of chromatin condensation, vacuolar degeneration, nucleus fragmentation and apoptotic body formation. In FCM profile apoptotic cells showed increased sub-G₁ peaks in the octreotide group, significantly higher than the control group (P = 0.013). There was also an augmentation in the cell proportion of G₀/G₁ phase (P = 0.015), while the proportion of S phase and G₂/M phase remained unchanged (P = 0.057 and P = 0.280, respectively). DNA agarose gel electrophoresis displayed a ladder after exposure to 1 000 nmol/L octreotide. After being treated with octreotide, the expressions of both mutant-type p53 and bcl-2 decreased considering the percentage of positive cells (P < 0.05).

CONCLUSION: Octreotide has a negative action to the proliferation of GBC-SD cells, and the mechanism may be related to cytostatic and cytotoxic effects. The reduction of mutant-type p53 and bcl-2 expressions may be associated with the apoptosis induced by octreotide.

Glucagon-like peptides: regulators of cell proliferation, differentiation, and apoptosis

Peptide hormones are secreted from endocrine cells and neurons and exert their actions through activation of G protein-coupled receptors to regulate a diverse number of physiological systems including control of energy homeostasis, gastrointestinal motility, neuroendocrine circuits, and hormone secretion. The glucagon-like peptides, GLP-1 and GLP-2 are prototype peptide hormones released from gut endocrine cells in response to nutrient ingestion that regulate not only energy absorption and disposal, but also cell proliferation and survival. GLP-1 expands islet mass by stimulating pancreatic beta-cell proliferation and induction of islet neogenesis. GLP-1 also promotes cell differentiation, from exocrine cells or immature islet progenitors, toward a more differentiated beta-cell phenotype. GLP-2 stimulates cell proliferation in the gastrointestinal mucosa, leading to expansion of the normal mucosal epithelium, or attenuation of intestinal injury in experimental models of intestinal disease. Both GLP-1 and GLP-2 exert antiapoptotic actions in vivo, resulting in preservation of beta-cell mass and gut epithelium, respectively. Furthermore, GLP-1 and GLP-2 promote direct resistance to apoptosis in cells expressing GLP-1 or GLP-2 receptors. Moreover, an increasing number of structurally related peptide hormones and neuropeptides exert cytoprotective effects through G protein-coupled receptor activation in diverse cell types. Hence, peptide hormones, as exemplified by GLP-1 and GLP-2, may prove to be useful adjunctive tools for enhancement of cell differentiation, tissue regeneration, and cytoprotection for the treatment of human disease.

Somatostatin analogs and radiopeptides in cancer therapy

Since the discovery of somatostatin (sst) in 1973, numerous chemical and biological studies have been carried out to develop sst analogs with enhanced resistance to proteases and prolonged activity. Three highly potent sst analogs-octreotide, lanreotide, and vapreotide-are now available in the clinic, and demonstrate efficacy in the treatment of tumors of the pituitary and the gastroenteropancreatic tract. The most striking effect is the control of hormone hypersecretion associated with these tumors. Available data on growth suppression in patients indicate a limited antiproliferative action, tumor shrinkage is observed in 10-20% patients, and tumor stabilization in about half of the patients for duration of 8-16 months. Eventually, however, all patients escape from sst analog therapy with regard to both hormone hypersecretion and tumor growth, the only exception being observed in acromegalic patients who do not experience tachyphylaxis even after more than 10 years of daily octreotide injection. The mechanism underlying the escape phenomenon is not yet clarified. Regarding the molecular mechanisms involved in sst antineoplastic activity, both indirect and direct effects via specific somatostatin receptors (SSTRs) expressed in the target cells have be described. Direct action may result from blockade of mitogenic growth signal or induction of apoptosis following interaction with SSTRs. Indirect effects may be the result of reduced or inhibited secretion of growth-promoting hormones and growth factors that stimulate the growth of various types of cancer; also, inhibition of angiogenesis or influence on the immune system are important factors. Five SSTR subtypes have been identified so far, which are variably expressed in a variety of tumors such as gastroenteropancreatic (GEP) tumors, pituitary tumors, and carcinoid tumors. Although all five SSTR subtypes are linked to adenylate cyclase, they are now known to affect multiple other cellular signaling systems and hence they differentially participate in the regulation of the various cellular processes. The finding of several laboratories that SSTR-expressing tumors frequently contain two or more SSTR subtypes, and the recent discovery that SSTR subtypes might form homo/heterodimers to create a novel receptor with different functional characteristics, expand the array of selective SSTR activation pathways and subsequent intracellular signaling cascades. This may lead to improved clinical protocols that take into account possible synergistic interactions between the SSTR subtypes present on the same cancer cell. Radiolabeled sst analogs, such as [(111)In]-[diethylenetriamine pentaacetic acid (DTPA)-D-Phe(1)]-octreotide (OcreoScan), have proved to be very useful for tumor scintigraphy and internal radiotherapy of SSTR overexpressing tumors. The recent introduction of the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) considerably improved the stability of the radioconjugates, making possible the incorporation of a variety of radionuclides, such as (90)Y for receptor-mediated radionuclide therapy or (68)Ga for positron emission tomography (PET). Another promising area is the development of sst conjugates incorporating cytotoxic anticancer drugs.

Blocking of acidosis-mediated apoptosis by a reduction of lactate dehydrogenase activity through antisense mRNA expression

Lactic acid produced from the cells is a potential cause of extra- and intracellular acidification. Due to scarce technical tools, lactic acid that leads to acidification could not be reduced and direct evidence of the relationship between metabolic lactate and apoptosis has not yet been elucidated. In this study, we designed a cellular pH regulation system in CHO cells by a reduction of lactate dehydrogenase (LDH) activity through LDH antisense mRNA expression. This inhibited lactate production and, therefore, acidification of the cytosol. Under HCO3(-)-buffered growth conditions, both the parent CHO cells and the engineered CHO cells maintained their extracellular pH and intracellular pH fairly well. However, upon acidification of the cytosol, only the parent CHO cells underwent apoptosis under HCO3(-)-free conditions. In fact, we observed a number of apoptosis-related events only in control cells, including mitochondrial dysfunction, cytochrome c release, and an increase in caspase-3 enzymatic activity.

The biology and physiology of somatostatin receptors

The biology and therapeutic application of somatostatin and its receptors are reviewed. The focus is on recent literature and patents, especially with regard to the specific function of each somatostatin receptor subtype. Detailed mechanisms mediating the effects of somatostatin and its analogues remain to be elucidated. Nevertheless, progress is being made towards a clear picture of the cellular signalling and physiological changes regulated by somatostatin and its receptors.

Response of alkalinization or acidification by phytohemagglutinin is dependent on the activity of protein kinase C in human peripheral T Cells

The increase of intracellular free calcium concentration ([Ca(2+)](i)) and protein kinase C (PKC) activity are two major early mitogenic signals to initiate proliferation of human T cells. However, a rapid change in intracellular pH (pH(i)), acidification or alkalinization during the activation, is also associated after these two signals. The aim of this study was to define whether the change in pH(i) is affected by calcium and protein kinase C (PKC), in phytohemagglutinin (PHA)-stimulated T cells. T cells were isolated from human peripheral blood. The [Ca(2+)](i) and the pH(i) were measured using, respectively, the fluorescent dyes, Fura-2, and BCECF. In addition, down-regulation of PKC activity by PMA (1 microM, 18 h) was confirmed in these cells using a protein kinase assay. The results indicated that, (1) alkalinization was induced by PHA or PMA in T cells; the results of alkalinization was PKC-dependent and Ca(2+)-independent, (2) in PKC down-regulated T cells, PHA induced acidification; this effect was enhanced by pre-treating the cells with the Na(+)/H(+) exchange inhibitor, 5-(N,N-dimethyl)-amiloride, (DMA, 10 microM, 20 min), (3) the acidification was dependent on the Ca(2+) influx and blocked by removal of extracellular calcium or the addition of the inorganic channel blocker, Ni(2+), and (4) Thapsigargin (TG), a Ca(2+)-ATPase inhibitor, confirmed that acidification by the Ca(2+) influx occurred in T cells in which PKC was not down-regulated. These findings indicate two mechanisms, alkalinization by PKC and acidification by Ca(2+) influx, exist in regulating pH(i) in T cells. This is the first report that PHA stimulates the acidification by Ca(2+) influx but not alkalinization in T cells after down-regulation of PKC. In conclusion, the activity of PKC in T cells determines the response in alkalinization or acidification by PHA.

Effects of tyrosine kinase inhibitors on cell death induced by sodium fluoride and pertussis toxin in the pancreatic beta-cell line, RINm5F

1. Sodium fluoride causes apoptosis of pancreatic beta-cells and this response is enhanced by pre-treatment with pertussis toxin. In the present study, tyrosine kinase inhibitors were used to investigate the mechanisms of action of NaF and pertussis toxin in the beta-cell line, RINm5F. 2. Exposure of RINm5F cells to low concentrations of genistein or tyrphostin A25 resulted in significant inhibition of cell death induced by 5 mM NaF. Higher concentrations (>25 microM) were cytotoxic in the absence of NaF but, paradoxically, the combination of genistein and NaF induced less cell death than when each agent was used alone. 3. The increase in cell death induced by 100 microM genistein was markedly inhibited by ciprofloxacin, a drug which binds to topoisomerase II. Etoposide (which inhibits topoisomerase II but has no effect on tyrosine kinase activity) also caused an increase in RINm5F cell death. Neither etoposide nor ciprofloxacin altered the response to 5 mM NaF. 4. Pertussis toxin markedly enhanced the extent of RINm5F cell death induced by NaF and this effect was completely prevented by 25 microM genistein. The inhibition caused by genistein was not affected by ciprofloxacin but was reproduced by a structurally dissimilar tyrosine kinase inhibitor, herbimycin A. 5. The results demonstrate that RINm5F beta-cells express a pertussis toxin sensitive pathway that is anti-apoptotic. The activity of this pathway is most evident in cells exposed to pro-apoptotic stimuli where the effects of pertussis toxin can be blocked by inhibitors of tyrosine kinase enzymes. A genistein-sensitive tyrosine kinase does not appear to be involved in RINm5F cell survival under basal conditions.

Antibody-regulated neurotoxic function of cell-surface beta-amyloid precursor protein

APP is a transmembrane precursor of beta-amyloid, and its mutations cause early-onset familial Alzheimer's disease. We report a toxic function of normal wild-type APP (wtAPP). Treatment of neuronal F11 cells, immortalized embryonic day 13 neurons, overexpressing wtAPP with anti-APP antibodies caused death. Death was not induced by antibody in parental F11 cells. Death by antibody occurred through cell-surface APP, not through secreted APP, in a pertussis toxin-sensitive manner and was typical apoptosis, not observed in primary astrocytes or glioma cells overexpressing wtAPP, but observed in primary cortical neurons. Cell-surface APP thus performs a toxic function as an extracellularly controllable regulator of neuronal death. This study provides a novel insight into the normal and pathological functions of cell-surface wtAPP.

Multiple mechanisms underlie neurotoxicity by different types of Alzheimer's disease mutations of amyloid precursor protein

We examined a neuronal cell system in which single-cell expression of either familial Alzheimer's disease (FAD) gene V642I-APP or K595N/M596L-APP (NL-APP) in an inducible plasmid was controlled without affecting transfection efficiency. This system revealed that (i) low expression of both mutants exerted toxicity sensitive to both Ac-DEVD-CHO (DEVD) and glutathione ethyl ester (GEE), whereas wild-type APP (wtAPP) only at higher expression levels caused GEE/DEVD-resistant death to lesser degrees; (ii) toxicity by the V642I mutation was entirely GEE/DEVD sensitive; and (iii) toxicity by higher expression of NL-APP was GEE/DEVD resistant. The GEE/DEVD-sensitive death was sensitive to pertussis toxin and was due to G(o)-interacting His(657)-Lys(676) domain. The GEE/DEVD-resistant death was due to C-terminal Met(677)-Asn(695). APP mutants lacking either domain unraveled elaborate intracellular cross-talk between these domains. E618Q-APP, responsible for non-AD type of a human disease, only exerted GEE/DEVD-resistant death at higher expression. Therefore, (i) different FAD mutations in APP cause neuronal cell death through different cytoplasmic domains via different sets of mechanisms; (ii) expression levels of FAD genes are critical in activating specific death mechanisms; and (iii) toxicity by low expression of both mutants most likely reflects the pathogenetic mechanism of FAD.

V642I APP-inducible neuronal cells: a model system for investigating Alzheimer's disorders

APP is a precursor of beta amyloid deposited in Alzheimer's disease (AD). Although genetic studies established that mutations in APP cause familial AD (FAD), the mechanism for neuronal death by FAD mutants has not been well understood. We established neuronal cells (F11/EcR/V642I cells) in which V642I APP was inducibly expressed by ecdysone. Treatment with ecdysone, but not vehicle, killed most cells within a few days, with rounding, shrinkage, and detachment as well as nuclear fragmentation. Death was suppressed by Ac-DEVD-CHO and pertussis toxin. Electron microscopic analysis revealed that apoptosis occurred in ecdysone-treated cells. V642I-APP-induced death was suppressed by the anti-AD factors estrogen and apoE2. These data demonstrate not only that expression of this FAD gene causes neuronal apoptosis, but that F11/EcR/V642I cells, the first neuronal cells with inducible FAD gene expression, provide a useful model system in investigating AD disorders.

Signal transduction of somatostatin receptors negatively controlling cell proliferation

Somatostatin acts as an inhibitory peptide of various secretory and proliferative responses. Its effects are mediated by a family of G-protein-coupled receptors (sst1-5) that can couple to diverse signal transduction pathways such as inhibition of adenylate cyclase and guanylate cyclase, modulation of ionic conductance channels, and protein dephosphorylation. The five receptors bind the natural peptide with high affinity but only sst2, sst5 and sst3 bind the short synthetic analogues. Somatostatin negatively regulates the growth of various normal and tumour cells. This effect is mediated indirectly through inhibition of secretion of growth-promoting factors, angiogenesis and modulation of the immune system. Somatostatin can also act directly through sst receptors present on target cells. The five receptors are expressed in various normal and tumour cells, the expression of each receptor being receptor subtype and cell type specific. According to the receptor subtypes, distinct signal transduction pathways are involved in the antiproliferative action of somatostatin. Sst1, 4 and 5 modulate the MAP kinase pathway and induce G1 cell cycle arrest. Sst3 and sst2 promote apoptosis by p53-dependent and -independent mechanisms, respectively.

DNases and apoptosis

Here we review the different apoptotic DNases. From a functional point of view, DNases implicated in apoptosis may be classified into three groups: the Ca2+/Mg2+endonucleases, the Mg2+-endonucleases, and the cation-independent endonucleases. The first group includes DNase I which has no specificity for the linker region, DNase gamma which has some homology with DNase I, and other DNases which cleave DNA in the linker region. Both DNase I and DNase gamma have been cloned. The other nucleases of this category have dispersed molecular weights. Their sequences are unknown and it is difficult to determine their role(s) in apoptosis. It seems that different pathways are present and that these nucleases may be activated either by caspases or serine proteases. The caspase 3 activated DNase (CAD, CPAN, or DFF40) belongs to the Mg2+-dependent endonucleases. DNase II belongs to the third group of acid endonucleases or cation-independent DNases. We have shown the involvement of DNase II in lens cell differentiation. Recently, the molecular structure of two different enzymes has been elucidated, one of which has a signal peptide and appears to be secreted. The other, called L-DNase II, is an intracellular protein having two enzymatic activities; in its native form, it is an anti-protease, and after posttranslational modification, it becomes a nuclease.Key words: endonucleases, apoptosis, caspases, serine proteases.

Caspase-8-mediated intracellular acidification precedes mitochondrial dysfunction in somatostatin-induced apoptosis

Activation of initiator and effector caspases, mitochondrial changes involving a reduction in its membrane potential and release of cytochrome c (cyt c) into the cytosol, are characteristic features of apoptosis. These changes are associated with cell acidification in some models of apoptosis. The hierarchical relationship between these events has, however, not been deciphered. We have shown that somatostatin (SST), acting via the Src homology 2 bearing tyrosine phosphatase SHP-1, exerts cytotoxic action in MCF-7 cells, and triggers cell acidification and apoptosis. We investigated the temporal sequence of apoptotic events linking caspase activation, acidification, and mitochondrial dysfunction in this system and report here that (i) SHP-1-mediated caspase-8 activation is required for SST-induced decrease in pH(i). (ii) Effector caspases are induced only when there is concomitant acidification. (iii) Decrease in pH(i) is necessary to induce reduction in mitochondrial membrane potential, cyt c release and caspase-9 activation and (iv) depletion of ATP ablates SST-induced cyt c release and caspase-9 activation, but not its ability to induce effector caspases and apoptosis. These data reveal that SHP-1-/caspase-8-mediated acidification occurs at a site other than the mitochondrion and that SST-induced apoptosis is not dependent on disruption of mitochondrial function and caspase-9 activation.

Regulation of acidification and apoptosis by SHP-1 and Bcl-2

Recruitment of the SH2 domain containing cytoplasmic protein-tyrosine phosphatase SHP-1 to the membrane by somatostatin (SST) is an early event in its antiproliferative signaling that induces intracellular acidification-dependent apoptosis in breast cancer cells. Fas ligation also induces acidification-dependent apoptosis in a manner requiring the presence of SHP-1 at the membrane. Moreover, we have recently reported that SHP-1 is required not only for acidification, but also for apoptotic events that follow acidification (Thangaraju, M., Sharma, K., Liu, D., Shen, S. H., and Srikant, C. B. (1999) Cancer Res. 59, 1649-1654). Here we show that ectopically expressed SHP-1 was predominantly membrane-associated and amplified the cytotoxic signaling initiated upon SST receptor activation and Fas ligation. The catalytically inactive mutant of SHP-1 (SHP-1C455S) abolished the ability of the SST agonists to signal apoptosis by preventing the recruitment of wild type SHP-1 to the membrane. Overexpression of the anti-apoptotic protein Bcl-2 in MCF-7 cells inhibited SST-induced apoptosis upstream of acidification by inhibiting p53-dependent induction of Bax as well as by raising the resting pH(i) and attenuating SST-induced decrease in pH(i). By contrast, Bcl-2 failed to prevent apoptosis triggered by direct acidification. These data demonstrate that (i) membrane-associated SHP-1 is required for receptor-mediated cytotoxic signaling that causes intracellular acidification and apoptosis, and (ii) Bcl-2 acts distal to SHP-1 and p53 to prevent SST-induced acidification but cannot inhibit the apoptotic events that ensue intracellular acidification.

Somatostatin and its receptor family

Somatostatin (SST), a regulatory peptide, is produced by neuroendocrine, inflammatory, and immune cells in response to ions, nutrients, neuropeptides, neurotransmitters, thyroid and steroid hormones, growth factors, and cytokines. The peptide is released in large amounts from storage pools of secretory cells, or in small amounts from activated immune and inflammatory cells, and acts as an endogenous inhibitory regulator of the secretory and proliferative responses of target cells that are widely distributed in the brain and periphery. These actions are mediated by a family of seven transmembrane (TM) domain G-protein-coupled receptors that comprise five distinct subtypes (termed SSTR1-5) that are endoded by separate genes segregated on different chromosomes. The five receptor subtypes bind the natural SST peptides, SST-14 and SST-28, with low nanomolar affinity. Short synthetic octapeptide and hexapeptide analogs bind well to only three of the subtypes, 2, 3, and 5. Selective nonpeptide agonists with nanomolar affinity have been developed for four of the subtypes (SSTR1, 2, 3, and 4) and putative peptide antagonists for SSTR2 and SSTR5 have been identified. The ligand binding domain for SST ligands is made up of residues in TMs III-VII with a potential contribution by the second extracellular loop. SSTRs are widely expressed in many tissues, frequently as multiple subtypes that coexist in the same cell. The five receptors share common signaling pathways such as the inhibition of adenylyl cyclase, activation of phosphotyrosine phosphatase (PTP), and modulation of mitogen-activated protein kinase (MAPK) through G-protein-dependent mechanisms. Some of the subtypes are also coupled to inward rectifying K(+) channels (SSTR2, 3, 4, 5), to voltage-dependent Ca(2+) channels (SSTR1, 2), a Na(+)/H(+) exchanger (SSTR1), AMPA/kainate glutamate channels (SSTR1, 2), phospholipase C (SSTR2, 5), and phospholipase A(2) (SSTR4). SSTRs block cell secretion by inhibiting intracellular cAMP and Ca(2+) and by a receptor-linked distal effect on exocytosis. Four of the receptors (SSTR1, 2, 4, and 5) induce cell cycle arrest via PTP-dependent modulation of MAPK, associated with induction of the retinoblastoma tumor suppressor protein and p21. In contrast, SSTR3 uniquely triggers PTP-dependent apoptosis accompanied by activation of p53 and the pro-apoptotic protein Bax. SSTR1, 2, 3, and 5 display acute desensitization of adenylyl cyclase coupling. Four of the subtypes (SSTR2, 3, 4, and 5) undergo rapid agonist-dependent endocytosis. SSTR1 fails to be internalized but is instead upregulated at the membrane in response to continued agonist exposure. Among the wide spectrum of SST effects, several biological responses have been identified that display absolute or relative subtype selectivity. These include GH secretion (SSTR2 and 5), insulin secretion (SSTR5), glucagon secretion (SSTR2), and immune responses (SSTR2).

Induction of wild-type p53, Bax, and acidic endonuclease during somatostatin-signaled apoptosis in MCF-7 human breast cancer cells

Somatostatin (SST) analogs inhibit tumor cell growth by exerting direct anti-proliferative effects with cytostatic (growth arrest) or cytotoxic (apoptosis) consequences. The SST analog SMS 201-995 (octreotide, OCT) inhibits growth of MCF-7 human breast adenocarcinoma cells, which express multiple SSTRs. Its action has been reported to result in either apoptosis or growth arrest, but the underlying mechanisms have not been elucidated in this tumor cell model. Here, we report that OCT elicits cytotoxic response in these cells, leading to apoptosis, which is associated with a rapid, time-dependent induction of wild-type p53 and an increase in Bax. There was no G1 cell-cycle arrest in these cells during OCT treatment as suggested by the decrease in G1/S ratio and the lack of induction of pRb and p21. Additionally, we demonstrate that OCT-induced DNA fragmentation in this cell line is due to selective activation of a cation-insensitive acidic endonuclease. Our data provide a rationale for utilizing SST analogs to treat SSTR-positive breast cancer cells expressing wild-type p53.

Induction of wild-type p53, Bax, and acidic endonuclease during somatostatin-signaled apoptosis in MCF-7 human breast cancer cells

Somatostatin (SST) analogs inhibit tumor cell growth by exerting direct anti-proliferative effects with cytostatic (growth arrest) or cytotoxic (apoptosis) consequences. The SST analog SMS 201-995 (octreotide, OCT) inhibits growth of MCF-7 human breast adenocarcinoma cells, which express multiple SSTRs. Its action has been reported to result in either apoptosis or growth arrest, but the underlying mechanisms have not been elucidated in this tumor cell model. Here, we report that OCT elicits cytotoxic response in these cells, leading to apoptosis, which is associated with a rapid, time-dependent induction of wild-type p53 and an increase in Bax. There was no G1 cell-cycle arrest in these cells during OCT treatment as suggested by the decrease in G1/S ratio and the lack of induction of pRb and p21. Additionally, we demonstrate that OCT-induced DNA fragmentation in this cell line is due to selective activation of a cation-insensitive acidic endonuclease. Our data provide a rationale for utilizing SST analogs to treat SSTR-positive breast cancer cells expressing wild-type p53.