The role of a low pH intracellular compartment in the processing, storage, and secretion of ACTH and endorphin

Peptidylglycine α-amidating monooxygenase as a therapeutic target or biomarker for human diseases

Peptides play a key role in controlling many physiological and neurobiological pathways. Many bioactive peptides require a C-terminal α-amide for full activity. The bifunctional enzyme catalysing α-amidation, peptidylglycine α-amidating monooxygenase (PAM), is the sole enzyme responsible for amidated peptide biosynthesis, from Chlamydomonas reinhardtii to Homo sapiens. Many neuronal and endocrine functions are dependent upon amidated peptides; additional amidated peptides are growth promoters in tumours. The amidation reaction occurs in two steps, glycine α-hydroxylation followed by dealkylation to generate the α-amide product. Currently, most potentially useful inhibitors target the first reaction, which is rate-limiting. PAM is a membrane-bound enzyme that visits the cell surface during peptide secretion. PAM is then used again in the biosynthetic pathway, meaning that cell-impermeable inhibitors or inactivators could have therapeutic value for the treatment of cancer or psychiatric abnormalities. To date, inhibitor design has not fully exploited the structures and mechanistic details of PAM.

Tea tree oil extract causes mitochondrial superoxide production and apoptosis as an anticancer agent, promoting tumor infiltrating neutrophils cytotoxic for breast cancer to induce tumor regression

The antitumor activity of the tea tree oil (TTO) derived product, Melaleuca Alternifolia Concentrate (MAC) was characterized mechanistically at the molecular and cellular level. MAC was analyzed for its anticancer activity against human prostate (LNCaP) and breast (MCF-7) cancer cell lines growing in vitro. MAC (0.02-0.06% v/v) dose-dependently induced the intrinsic (mitochondrial) apoptotic pathway in both the LNCaP and MCF-7 cell lines, involving increased mitochondrial superoxide production, loss of mitochondrial membrane potential (MMP), caspase 3/7 activation, as well as the presence of TUNEL⁺ and cleaved-PARP⁺ cell populations. At concentrations of 0.01-0.04% v/v, MAC caused cell cycle arrest in the G_0/1-phase, as well as autophagy. The in vivo anticancer actions of MAC were examined as a treatment in the FVB/N c-Neu murine model for spontaneously arising breast cancers. Intratumoral MAC injections (1-4% v/v) significantly suppressed tumor progression in a dose-dependent manner and was associated with greater levels of tumor infiltrating neutrophils exhibiting anticancer cytotoxic activity. Induction of breast cancer cell death by MAC via the mitochondrial apoptotic pathway was also replicated occurring in tumors treated in vivo. In conclusion, our data highlights the potential for the Melaleuca-derived MAC product inducing anticancer neutrophil influx, supporting its application as a novel therapeutic agent.

Dual Stimuli-Triggered Nanogels in Response to Temperature and pH Changes for Controlled Drug Release

Poly-N-isopropyl acrylamide (PNIPAM) nanogels have been modified with different acrylic acid (AAc) contents for the efficient control of lower critical solution temperature (LCST). In this study, PNIPAM-co-AAc nanogels nanogels showed two volume phase transitions in comparison with PNIPAM. The transition temperature of PNIPAM nanogels was increased with AAc contents. The controlled drug release performance of PNIPAM-co-AAc nanogels loaded with β-lapachone was attributed to the AAc content ratio and was efficiently triggered in response to temperature and pH. Moreover, a colorimetric cell proliferation assay and direct fluorescence-based live/dead staining were used to confirm the concurrence on drug release profiles. Finally, PNIPAM-co-AAc20 showed a relatively low level of drug release in the range of acidic to neutral pH at body temperature, while maximizing drug release at basic pH. Therefore, we demonstrated that the PNIPAM-based nanogel with the temperature- and pH-responsive features could be a promising nanocarrier for potential intestine-specific drug delivery.

Identification of compound CA-5f as a novel late-stage autophagy inhibitor with potent anti-tumor effect against non-small cell lung cancer

Currently, particular focus is placed on the implication of autophagy in a variety of human diseases, including cancer. Discovery of small-molecule modulators of autophagy as well as their potential use as anti-cancer therapeutic agents would be of great significance. To this end, a series of curcumin analogs previously synthesized in our laboratory were screened. Among these compounds, (3E,5E)-3-(3,4-dimethoxybenzylidene)-5-[(1H-indol-3-yl)methylene]-1-methylpiperidin-4-one (CA-5f) was identified as a potent late-stage macroautophagy/autophagy inhibitor via inhibiting autophagosome-lysosome fusion. We found that CA-5f neither impaired the hydrolytic function nor the quantity of lysosomes. Use of an isobaric tag for relative and absolute quantitation (iTRAQ)-based proteomic screen in combination with bioinformatics analysis suggested that treatment of human umbilical vein endothelial cells (HUVECs) with CA-5f for 1 h suppressed the levels of cytoskeletal proteins and membrane traffic proteins. Subsequent studies showed that CA-5f exhibited strong cytotoxicity against A549 non-small cell lung cancer (NSCLC) cells, but low cytotoxicity to normal human umbilical vein endothelial cells (HUVECs), by increasing mitochondrial-derived reactive oxygen species (ROS) production. Moreover, CA-5f effectively suppressed the growth of A549 lung cancer xenograft as a single agent with an excellent tolerance in vivo. Results from western blot, immunofluorescence, and TdT-mediated dUTP nick end labeling (TUNEL) assays showed that CA-5f inhibited autophagic flux, induced apoptosis, and did not affect the level of CTSB (cathepsin B) and CTSD (cathepsin D) in vivo, which were consistent with the in vitro data. Collectively, these results demonstrated that CA-5f is a novel late-stage autophagy inhibitor with potential clinical application for NSCLC therapy. Abbreviations: 3-MA, 3-methyladenine; ANXA5, annexin A5; ATG, autophagy related; CA-5f, (3E,5E)-3-(3,4-dimethoxybenzylidene)-5-[(1H-indol-3-yl)methylene]-1-methylpiperidin-4-one; CQ, chloroquine; CTSB, cathepsin B; CTSD, cathepsin D; DMSO, dimethyl sulfoxide; DNM2, dynamin 2; EBSS, Earle's balanced salt solution; GFP, green fluorescent protein; HCQ, hydroxyl CQ; HEK293, human embryonic kidney 293; HUVEC, human umbilical vein endothelial cells; LAMP1, lysosomal associated membrane protein 1; LC-MS/MS, liquid chromatography coupled to tandem mass spectrometry; LDH, lactic acid dehydrogenase; LMO7, LIM domain 7; MAP1LC3B/LC3B, microtubule associated protein 1 light chain 3 beta; NAC, N-acetyl cysteine; MYO1E, myosin IE; NSCLC, non-small cell lung cancer; PARP1, poly(ADP-ribose) polymerase 1; PI, propidium iodide; RFP, red fluorescent protein; ROS, reactive oxygen species; SQSTM1, sequestosome 1; TUNEL, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling.

Gambogic acid counteracts mutant p53 stability by inducing autophagy

Mutant p53 (mutp53) proteins are frequently present at higher levels than the wild-type (wt) protein in tumors, and some of them can acquire oncogenic properties. Consistently, knockdown of mutp53 protein in human cancer cell lines leads to reduced cell proliferation and invasion as well as to an increased sensitivity to some anticancer drugs. Therefore, the exploitation of cellular pathways and/or molecules that promote mutp53 degradation may have a therapeutic interest. Recently, autophagy is emerging as an important pathway involved in the stability of mutp53. In this paper, we explored the autophagic potential of gambogic acid (GA), a molecule that stimulates the degradation of mutp53 and increases the sensitivity of cancer cells to chemotherapeutic agents. We demonstrated that GA may induce mutp53 degradation through autophagy in cancer cells expressing the p53-R280K (MDA-MB-231) and the p53-S241F (DLD1) proteins. The inhibition of autophagy with bafilomycin A1 or chloroquine counteracted mutp53 degradation by GA. However, the autophagy induction and mutp53 degradation affected cell survival and proliferation only at low GA concentrations. At higher GA concentrations, when cells undergo massive apoptosis, autophagy is no longer detectable by immuno-fluorescence analysis. We concluded that autophagy is a relevant pathway for mutp53 degradation in cancer cells but it contributes only partially to GA-induced cell death, in a time and dose-dependent manner.

Magnetic ferroferric oxide nanoparticles induce vascular endothelial cell dysfunction and inflammation by disturbing autophagy

Despite the considerable use of magnetic ferroferric oxide nanoparticles (Fe3O4NPs) worldwide, their safety is still an important topic of debate. In the present study, we detected the toxicity and biological behavior of bare-Fe3O4NPs (B-Fe3O4NPs) on human umbilical vascular endothelial cells (HUVECs). Our results showed that B-Fe3O4NPs did not induce cell death within 24h even at concentrations up to 400 μg/ml. The level of nitric oxide (NO) and the activity of endothelial NO synthase (eNOS) were decreased after exposure to B-Fe3O4NPs, whereas the levels of proinflammatory cytokines were elevated. Importantly, B-Fe3O4NPs increased the accumulation of autophagosomes and LC3-II in HUVECs through both autophagy induction and the blockade of autophagy flux. The levels of Beclin 1 and VPS34, but not phosphorylated mTOR, were increased in the B-Fe3O4NP-treated HUVECs. Suppression of autophagy induction or stimulation of autophagy flux, at least partially, attenuated the B-Fe3O4NP-induced HUVEC dysfunction. Additionally, enhanced autophagic activity might be linked to the B-Fe3O4NP-induced production of proinflammatory cytokines. Taken together, these results demonstrated that B-Fe3O4NPs disturb the process of autophagy in HUVECs, and eventually lead to endothelial dysfunction and inflammation.

Molecular effects of the phosphatidylinositol-3-kinase inhibitor NVP-BKM120 on T and B-cell acute lymphoblastic leukaemia

BACKGROUND

Constitutive activation of the PI3K pathway in T cell acute lymphoblastic leukaemia (T-ALL) has been reported and in a mouse model, PI3K activation, together with MYC, cooperates in Burkitt lymphoma (BL) pathogenesis. We investigated the effects of NVP-BKM120, a potent pan-class I PI3K inhibitor, in lymphoblastic leukaemia cell lines.

METHODS

Effects of NVP-BKM120 on cell viability, clonogenicity, apoptosis, cell cycle, cell signalling and autophagy were assessed in vitro on T-ALL (Jurkat and MOLT-4) and BL (Daudi and NAMALWA) cell lines.

RESULTS

NVP-BKM120 treatment decreased cell viability and clonogenic growth in all tested cells. Moreover, the drug arrested cell cycling in association with a decrease in Cyclin B1 protein levels, and increased apoptosis. Immunoblotting analysis of cells treated with the drug revealed decreased phosphorylation, in a dose-dependent manner, of AKT, mTOR, P70S6K and 4EBP1, with stable total protein levels. Additionally, we observed a dose-dependent decrease in BAD phosphorylation, in association with augmented BAX:BCL2 ratio. Quantification of autophagy showed a dose-dependent increase in acidic vesicular organelles in all cells tested.

CONCLUSION

In summary, our present study establishes that NVP-BKM120 presents an effective antitumour activity against T-ALL and BL cell lines.

Apogossypolone inhibits the proliferation of LNCaP cells in vitro and in vivo

The aim of the present study was to investigate the anti-tumor effect of apogossypolone (ApoG2) on human LNCaP cells in vitro and in vivo. Cell viability was evaluated using an MTT assay. Cell autophagy and apoptosis were detected by flow cytometry and using a terminal deoxynucleotidyl transferase dUTP nick end labeling assay, respectively. Morphological autophagy alterations were observed by transmission electron microscopy. The formation of acidic vesicular organelles was assessed by acridine orange staining and fluorescence microscopy. Quantitative polymerase chain reaction (qPCR) was conducted to detect the expression levels of apoptosis-associated protein B-cell lymphoma 2 (Bcl-2) and Bak. The models of transplantation tumors in nude mice were established via subcutaneous injection of LNCaP cells. Growth of LNCaP cells was inhibited by ApoG2 treatment. Flow cytometry demonstrated that ApoG2 induced apoptosis in LNCaP cells. The Bcl-2 expression was decreased while Bak expression was increased. In addition, activation of cysteine aspartate protease (caspase)-3 and -8 was observed and 3-methyladenine (3-MA) enhanced apoptosis of LNCaP cells. Furthermore, nude mice treated with ApoG2 demonstrated a significant decrease in tumor volume and a significant increase in cell viability. Immunohistochemical analysis of tumor tissues demonstrated that ApoG2 enhanced caspase-3, -8, LC-3B and beclin-1 expression and reduced the expression of Bcl-2. ApoG2 was able to effectively suppress the growth of LNCaP cells through the induction of autophagy and apoptosis.

Matrix remodeling stimulates stromal autophagy, "fueling" cancer cell mitochondrial metabolism and metastasis

We have previously demonstrated that loss of stromal caveolin-1 (Cav-1) in cancer-associated fibroblasts is a strong and independent predictor of poor clinical outcome in human breast cancer patients. However, the signaling mechanism(s) by which Cav-1 downregulation leads to this tumor-promoting microenvironment are not well understood. To address this issue, we performed an unbiased comparative proteomic analysis of wild-type (WT) and Cav-1(-/-) null mammary stromal fibroblasts (MSFs). Our results show that plasminogen activator inhibitor type 1 and type 2 (PAI-1 and PAI-2) expression is significantly increased in Cav-1(-/-) MSFs. To establish a direct cause-effect relationship, we next generated immortalized human fibroblast lines stably overexpressing either PAI-1 or PAI-2. Importantly, PAI-1/2(+) fibroblasts promote the growth of MDA-MB-231 tumors (a human breast cancer cell line) in a murine xenograft model, without any increases in angiogenesis. Similarly, PAI-1/2(+) fibroblasts stimulate experimental metastasis of MDA-MB-231 cells using an in vivo lung colonization assay. Further mechanistic studies revealed that fibroblasts overexpressing PAI-1 or PAI-2 display increased autophagy ("self-eating") and are sufficient to induce mitochondrial biogenesis/activity in adjacent cancer cells, in co-culture experiments. In xenografts, PAI-1/2(+) fibroblasts significantly reduce the apoptosis of MDA-MB-231 tumor cells. The current study provides further support for the "Autophagic Tumor Stroma Model of Cancer" and identifies a novel "extracellular matrix"-based signaling mechanism, by which a loss of stromal Cav-1 generates a metastatic phenotype. Thus, the secretion and remodeling of extracellular matrix components (such as PAI-1/2) can directly regulate both (1) autophagy in stromal fibroblasts and (2) epithelial tumor cell mitochondrial metabolism.

Inactivation of ataxia telangiectasia mutated gene can increase intracellular reactive oxygen species levels and alter radiation-induced cell death pathways in human glioma cells

PURPOSE

To investigate the effects of ataxia telangiectasia mutated (ATM)-regulated reactive oxygen species (ROS) and cell death pathways on the response of U87MG glioma cells to ionising radiation (IR) and oxidative stress.

MATERIAL AND METHODS

ATM expression was blocked in U87MG glioma cells using a small interfering RNA (siRNA) technique. Cell survival, sub-lethal damage (SLD), and potential lethal damage (PLD) repair following IR were assessed by clonogenic assay while changes in intracellular ROS, the apoptosis, and autophagy were followed by flow cytometry and Western blotting.

RESULTS

Blocking ATM expression in U87MG cells increased intracellular ROS levels and sensitivity to the cytotoxic effects of IR and oxygen stress; effects that could be partly counteracted by the antioxidant N-acetylcysteine (NAC). Knock down of ATM rendered cells unable to repair sub-lethal or potentially lethal damage and DNA double strand breaks (DSB) after IR exposure; something that NAC could not counteract. ATM did control the pathways a cell used to die following IR and this did seem to be ROS-dependent.

CONCLUSION

ATM is involved in redox control but ROS elevations following ATM knock down seem more involved in the decision as to what cell death pathway is utilised after IR than DSB repair and radiosensitivity.

Wilms' tumor 1 silencing decreases the viability and chemoresistance of glioblastoma cells in vitro: a potential role for IGF-1R de-repression

Wilms' tumor 1 (WT1) is a transcription factor with a multitude of downstream targets that have wide-ranging effects in non-glioma cell lines. Though its expression in glioblastomas is now well-documented, the role of WT1 in these tumors remains poorly defined. We hypothesized that WT1 functions as an oncogene to enhance glioblastoma viability and chemoresistance. WT1's role was examined by studying the effect of WT1 silencing and overexpression on DNA damage, apoptosis and cell viability. Results indicated that WT1 silencing adversely affected glioblastoma viability, at times, in synergy with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and cisplatin. To investigate other mechanisms through which WT1 could affect viability, we measured cell cycle distribution, senescence, and autophagy. WT1 silencing had no effect on these processes. Lastly, we examined WT1 regulation of IGF-1R expression. Counterintuitively, upregulation of IGF-1R was evident after WT1 silencing. In conclusion, WT1 functions as a survival factor in glioblastomas, possibly through inhibition of IGF-1R expression.

Inhibitors of the V0 subunit of the vacuolar H+-ATPase prevent segregation of lysosomal- and secretory-pathway proteins

The vacuolar H(+)-ATPase (V-ATPase) establishes pH gradients along secretory and endocytic pathways. Progressive acidification is essential for proteolytic processing of prohormones and aggregation of soluble content proteins. The V-ATPase V(0) subunit is thought to have a separate role in budding and fusion events. Prolonged treatment of professional secretory cells with selective V-ATPase inhibitors (bafilomycin A1, concanamycin A) was used to investigate its role in secretory-granule biogenesis. As expected, these inhibitors eliminated regulated secretion and blocked prohormone processing. Drug treatment caused the formation of large, mixed organelles, with components of immature granules and lysosomes and some markers of autophagy. Markers of the trans-Golgi network and earlier secretory pathway were unaffected. Ammonium chloride and methylamine treatment blocked acidification to a similar extent as the V-ATPase inhibitors without producing mixed organelles. Newly synthesized granule content proteins appeared in mixed organelles, whereas mature secretory granules were spared. Following concanamycin treatment, selected membrane proteins enter tubulovesicular structures budding into the interior of mixed organelles. shRNA-mediated knockdown of the proteolipid subunit of V(0) also caused vesiculation of immature granules. Thus, V-ATPase has a role in protein sorting in immature granules that is distinct from its role in acidification.

Potentiation of radiation sensitivity in breast tumor cells by the vitamin D3analogue, EB 1089, through promotion of autophagy and interference with proliferative recovery

1,25-Dihydroxyvitamin D(3) and vitamin D(3) analogues, such as EB 1089, potentiate the response to ionizing radiation in breast tumor cells. The current studies address the basis for this interaction by evaluating DNA damage and repair, the effect of interference with reactive oxygen generation, the involvement of p53 and caspase-3, signaling through c-myc, as well as the induction of senescence and multiple modes of cell death. EB 1089 failed to increase the extent of radiation-induced DNA damage or to attenuate the rate of DNA repair. The reactive oxygen scavengers N-acetyl-l-cysteine and reduced glutathione failed to protect the cells from the promotion of cell death by EB 1089 and radiation. Whereas MCF-7 cells expressing caspase-3 showed significant apoptosis with radiation alone as well as with EB 1089 followed by radiation, EB 1089 maintained its ability to confer susceptibility to radiation-induced cell killing, in large part by interference with proliferative recovery. In contrast, in breast tumor cells lacking p53, where radiation promoted extensive apoptosis and the cells failed to recover after radiation treatment, EB 1089 failed to influence the effect of radiation. EB 1089 treatment interfered with radiation-induced suppression of c-myc; however, induction of c-myc did not prevent senescence by radiation alone or radiation-induced cell death promoted by EB 1089. EB 1089 did not increase the extent of micronucleation, indicative of mitotic catastrophe, induced by radiation alone. However, EB 1089 did promote extensive autophagic cell death in the irradiated cells. Taken together, these studies suggest that the effect of EB 1089 treatment on the radiation response is related in part to enhanced promotion of autophagic cell death and in part to interference with the proliferative recovery that occurs with radiation alone in p53 wild-type breast tumor cells.

Not all secretory granules are created equal: Partitioning of soluble content proteins

Secretory granules carrying fluorescent cargo proteins are widely used to study granule biogenesis, maturation, and regulated exocytosis. We fused the soluble secretory protein peptidylglycine alpha-hydroxylating monooxygenase (PHM) to green fluorescent protein (GFP) to study granule formation. When expressed in AtT-20 or GH3 cells, the PHM-GFP fusion protein partitioned from endogenous hormone (adrenocorticotropic hormone, growth hormone) into separate secretory granule pools. Both exogenous and endogenous granule proteins were stored and released in response to secretagogue. Importantly, we found that segregation of content proteins is not an artifact of overexpression nor peculiar to GFP-tagged proteins. Neither luminal acidification nor cholesterol-rich membrane microdomains play essential roles in soluble content protein segregation. Our data suggest that intrinsic biophysical properties of cargo proteins govern their differential sorting, with segregation occurring during the process of granule maturation. Proteins that can self-aggregate are likely to partition into separate granules, which can accommodate only a few thousand copies of any content protein; proteins that lack tertiary structure are more likely to distribute homogeneously into secretory granules. Therefore, a simple "self-aggregation default" theory may explain the little acknowledged, but commonly observed, tendency for both naturally occurring and exogenous content proteins to segregate from each other into distinct secretory granules.

Pathways of myocyte death: implications for development of clinical laboratory biomarkers

The recognition that cardiac myocytes die by multiple mechanisms and thus substantially affect ventricular remodeling in diseased human hearts supports the concept of ongoing myocyte death in the progression of heart failure and constitutes the basis of this review. In addition, based on the pathophysiology of myocardial cell deaths, the present study emphasizes that currently methodologies, although with some inherent limitations, are available to recognize and measure quantitatively the contribution of myocyte cell death to the progression of the pathologic state of the heart. Our own studies show that application of such methodologies including modern microscopy techniques and the use of different molecular and immunohistochemical markers may generate the consensus that myocyte cell death is a quantifiable parameter in the normal and pathological human heart. The present study also demonstrates that myocyte cell death, apoptotic, oncotic or autophagic in nature, has to be regarded as an additional critical variable of the multifactorial events implicated in the alterations of cardiac anatomy and myocardial structure of the diseased human heart.

Alkaline induces metallothionein gene expression and potentiates cell proliferation in Chinese hamster ovary cells

Metallothionein (MT) gene expression is increased in cadmium resistant Chinese hamster ovary cells (CHO Cd(R)) upon medium (regular or serum-free) change during culturing. Among the major components of the medium, NaHCO3 was found to be able to induce MT gene expression in a dose- and time-dependent manner. The same effect was observed with other alkaline solutions, such as HEPES and NaOH. Using MT promoter-luciferase reporter gene constructs, we found that the presence of metal response elements (MREs) in the promoter region is necessary for NaHCO3-induced MT gene transcription. This finding is further supported by the observation that the binding activity between the metal-responsive transcription factor 1 (MTF-1) and the MRE were increased after NaHCO3 treatment. Following NaHCO3 treatment, an increase in cell proliferation was observed in CdR cells but not in the parental CHO K1 cells that do not express MT transcripts due to MT gene methylation. Using synchronized cells, an increase in cell proliferation was observed 9 h after NaHCO3 addition. Notably, proliferation of CHO K1 cells was increased when transfected with an MT gene. The effect of MT on cell growth was affirmed by treating CHO K1 cells with 5-azacytidine (Aza) to demethylate the MT gene. Proliferation increased in Aza-treated CHO K1 cells after NaHCO3 treatment. These results demonstrate that NaHCO3 stimulates MT gene expression and causes an enhancement of cell proliferation in CHO cells.

Drosophila uses two distinct neuropeptide amidating enzymes, dPAL1 and dPAL2

Neuropeptide alpha-amidation is a common C-terminal modification of secretory peptides, frequently required for biological activity. In mammals, amidation is catalyzed by the sequential actions of two enzymes [peptidylglycine-alpha-hydroxylating monooxygenase (PHM) and peptidyl-alpha-hydroxyglycine alpha-amidating lyase (PAL)] that are co-synthesized within a single bifunctional precursor. The Drosophila genome predicts expression of one monofunctional PHM gene and two monofunctional PAL genes. Drosophila PHM encodes an active enzyme that is required for peptide amidation in vivo. Here we initiate studies of the two Drosophila PAL genes. dPAL1 has two predicted transmembrane domains, whereas dPAL2 is predicted to be soluble and secreted. dPAL2 expressed in heterologous cells is secreted readily and co-localized with hormone. In contrast, dPAL1 is secreted poorly, even when expressed with a cleaved signal replacing the predicted transmembrane domains; the majority of dPAL1 stays in the endoplasmic reticulum. Both proteins display PAL enzymatic activity. Compared to the catalytic core of rat PAL, the two Drosophila lyases have higher K(m) values, higher pH optima and similarly broad divalent metal ion requirements. Antibodies to dPAL1 and dPAL2 reveal co-expression in many identified neuroendocrine neurons. Although dPAL1 is broadly expressed, dPAL2 is found in only a limited subset of neurons. dPAL1 expression is highly correlated with the non-amidated peptide proctolin. Tissue immunostaining demonstrates that dPAL1 is largely localized to the cell soma, whereas dPAL2 is distributed throughout neuronal processes.

Aggregation and lack of secretion of most newly synthesized proinsulin in non-beta-cell lines

Myoblasts transfected with HB10D insulin secrete more hormone than those transfected with wild-type insulin, as published previously, indicating that production of wild-type insulin is not efficient in these cells. The ability of non-beta-cells to produce insulin was examined in several cell lines. In clones of neuroendocrine GH(4)C(1) cells stably transfected with proinsulin, two thirds of (35)S-proinsulin was degraded within 3 h of synthesis, whereas (35)S-prolactin was stable. In transiently transfected neuroendocrine AtT20 cells, half of (35)S-proinsulin was degraded within 3 h after synthesis, whereas (35)S-GH was stable. In transiently transfected fibroblast COS cells, (35)S-proinsulin was stable for longer, but less than 10% was secreted 8 h after synthesis. Proinsulin formed a concentrated patch detected by immunofluorescence in transfected cells that did not colocalize with calreticulin or BiP, markers for the endoplasmic reticulum, but did colocalize with membrin, a marker for the cis-medial Golgi complex. Proinsulin formed a Lubrol-insoluble aggregate within 30 min after synthesis in non-beta-cells but not in INS-1E cells, a beta-cell line that normally produces insulin. More than 45% of (35)S-HB10D proinsulin was secreted from COS cells 3 h after synthesis, and this mutant formed less Lubrol-insoluble aggregate in the cells than did wild-type hormone. These results indicate that proinsulin production from these non-beta-cells is not efficient and that proinsulin aggregates in their secretory pathways. Factors in the environment of the secretory pathway of beta-cells may prevent aggregation of proinsulin to allow efficient production.

Acidification and protein traffic

Acidification of some organelles, including the Golgi complex, lysosomes, secretory granules, and synaptic vesicles, is important for many of their biochemical functions. In addition, acidic pH in some compartments is also required for the efficient sorting and trafficking of proteins and lipids along the biosynthetic and endocytic pathways. Despite considerable study, however, our understanding of how pH modulates membrane traffic remains limited. In large part, this is due to the diversity of methods to perturb and monitor pH, as well as to the difficulties in isolating individual transport steps within the complex pathways of membrane traffic. This review summarizes old and recent evidence for the role of acidification at various steps of biosynthetic and endocytic transport in mammalian cells. We describe the mechanisms by which organelle pH is regulated and maintained, as well as how organelle pH is monitored and quantitated. General principles that emerge from these studies as well as future directions of interest are discussed.

Using salivary glands as a tissue target for gene therapeutics

Gene transfer offers a potential way to correct local and systemic protein deficiency disorders by using genes as drugs, so called gene therapeutics. Salivary glands present an interesting target site for gene therapeutic applications. Herein, we review proofs of concept achieved for salivary glands with in vivo animal models. In that context we discuss problems (general and salivary tissue-specific) that limit immediate clinical use for this application of gene transfer. Ongoing efforts, however, suggest that salivary glands may be suitable as gene therapeutic target sites for drug delivery in the near future.

Aggregation of human wild-type and H27A-prolactin in cells and in solution: roles of Zn(2+), Cu(2+), and pH

Aggregation of hormones is an important step in the formation of secretory granules that results in concentration of hormones. In transfected AtT20 cells, but not COS cells, Lubrol-insoluble aggregates of human prolactin (PRL) accumulated within 30 min after synthesis. Aggregation in AtT20 cells was reduced by incubation with 30 microM chloroquine, which neutralizes intracellular compartments, and was slowed by incubation with diethyldithiocarbamate, which chelates Cu(2+) and Zn(2+). H27A-PRL aggregated in AtT20 cells as well as wild-type PRL, indicating that a high affinity Zn(2+)-binding site is not necessary. In solution, purified recombinant human PRL was precipitated by 20 microM Cu(2+) or Zn(2+). In solution without polyethylene glycol there was no precipitation with acidic pH alone, precipitation with Zn(2+) was most effective at neutral pH, and the ratio of Zn(2+) to PRL was greater than 1 in the precipitate. In solution with polyethylene glycol, precipitation occurred with acidic pH, precipitation with Zn(2+) occurred effectively at acidic pH, and the ratio of Zn(2+) to PRL was less than 1. The aggregates obtained in polyethylene glycol are therefore better models for aggregates in cells. Unlike human PRL, aggregation of rat PRL has been shown to occur at neutral pH in cells and in solution, and therefore these two similar proteins form aggregates that are the cores of secretory granules in ways that are not completely identical.

Biogenesis of processing-competent secretory organelles in vitro

Propeptide processing occurs in specific compartments of the secretory pathway, but how these processing-competent organelles are generated from their processing-incompetent precursor compartments is unknown. To dissect the process biochemically, we have developed a novel cell-free system reconstituting the production of processing-competent secretory granules in AtT-20 cells. Using donor membranes containing [(35)S]sulfate labeled pro-opiomelanocortin (POMC)(5) in the trans-Golgi, we can reconstitute cytosol- and ATP-dependent prohormone processing as well as incorporation of processed ACTH into immature secretory granules (ISGs). Under limiting cytosol conditions, both reactions are greatly stimulated by ADP-ribosylation factor 1 (ARF1) but not by the GDP-bound ARF1 T31N mutant. pH studies show that lumenal acidification, most likely due to ARF-mediated sorting of proton pumps and leaks during budding, confers processing competency to the resulting organelle. Surprisingly, comparison of onset of processing and ISG release reveals that they are distinct biochemical processes with different kinetics and separate cytosolic requirements. Moreover, ARF regulates the onset of prohormone processing but not ISG release. Our data suggest a two-step mechanism (onset of processing followed by ISG release) for the production of processing-competent organelles from the trans-Golgi and provide the first system with which these two steps may be individually dissected.

Acquisition of Lubrol insolubility, a common step for growth hormone and prolactin in the secretory pathway of neuroendocrine cells

Rat prolactin in the dense cores of secretory granules of the pituitary gland is a Lubrol-insoluble aggregate. In GH(4)C(1) cells, newly synthesized rat prolactin and growth hormone were soluble, but after 30 min about 40% converted to a Lubrol-insoluble form. Transport from the endoplasmic reticulum is necessary for conversion to Lubrol insolubility, since incubating cells with brefeldin A or at 15 degrees C reduced formation of insoluble rat (35)S-prolactin. Formation of Lubrol-insoluble aggregates has protein and cell specificity; newly synthesized human growth hormone expressed in AtT20 cells underwent a 40% conversion to Lubrol insolubility with time, but albumin did not, and human growth hormone expressed in COS cells underwent less than 10% conversion to Lubrol insolubility. del32-46 growth hormone, a naturally occurring form of growth hormone, and P89L growth hormone underwent conversion, although they were secreted more slowly, indicating that there is some tolerance in structural requirements for aggregation. An intracellular compartment with an acidic pH is not necessary for conversion to Lubrol insolubility, because incubation with chloroquine or bafilomycin slowed, but did not prevent, the conversion. GH(4)C(1) cells treated with estradiol, insulin, and epidermal growth factor accumulate more secretory granules and store more prolactin, but not more growth hormone, than untreated cells; Lubrol-insoluble aggregates of prolactin and growth hormone formed to the same extent in hormone-treated or untreated GH(4)C(1) cells, but prolactin was retained longer in hormone-treated cells. These findings indicate that aggregation alone is not sufficient to cause retention of secretory granule proteins, and there is an additional selective process.

Inhibition of the vacuolar H+-ATPase perturbs the transport, sorting, processing and release of regulated secretory proteins

Vacuolar H+-ATPases (V-ATPases) are multisubunit enzymes that acidify various intracellular organelles, including secretory pathway compartments. We have examined the effects of the specific V-ATPase inhibitor bafilomycin A1 (Baf) on the intracellular transport, sorting, processing and release of a number of neuroendocrine secretory proteins in primary Xenopus intermediate pituitary cells. Ultrastructural examination of Baf-treated intermediate pituitary cells revealed a reduction in the amount of small dense-core secretory granules and the appearance of vacuolar structures in the trans-Golgi area. Pulse-chase incubations in combination with immunoprecipitation analysis showed that in treated cells, the proteolytic processing of the newly synthesized prohormone proopiomelanocortin, prohormone convertase PC2 and secretogranin III (SgIII) was inhibited, and an intracellular accumulation of intact precursor forms and intermediate cleavage products became apparent. Moreover, we found that treated cells secreted considerable amounts of a PC2 processing intermediate and unprocessed SgIII in a constitutive fashion. Collectively, these data indicate that in the secretory pathway, V-ATPases play an important role in creating the microenvironment that is essential for proper transport, sorting, processing and release of regulated secretory proteins.

Chloroquine inhibits arginine vasopressin production in isolated rat inner medullary segments induced cAMP collecting duct

Previous studies showed that acute chloroquine administration increases plasma arginine vasopressin (AVP) concentration in the rat without influencing urine flow rate. The present study was designed to investigate whether chloroquine inhibits the AVP-induced cAMP production that mediates the antidiuretic effects of vasopressin. Single inner medullary collecting duct (IMCD) segments were pre-incubated at 35 degrees C for 10 min followed by 4 min at 37 degrees C with combinations of AVP and/or chloroquine with 1 mM 3-isobutyl-I-methylxanthine (IBMX) and cAMP concentrations were measured by radioimmunoassay. To establish the possible site of interference in cAMP production IMCD segments were incubated in the presence of chloroquine and forskolin. Chloroquine at concentrations ranging from 10(-9) M to 10(-6) M did not affect cAMP production by comparison with control. However, AVP (10(-8) M) and forskolin (10(-6) M) significantly (p < 0.01) increased cAMP accumulation. Chloroquine at all concentrations significantly suppressed the AVP stimulated cAMP production (e.g., chloroquine (10(-8) M) + AVP (10(-8) M) 41 +/- 12 fmol/4 mm (n = 9 tubules) vs. AVP (10(-8) M) alone 82 +/- 9 fmol/4 min/mm (n = 37 tubules). Chloroquine at all concentrations tested did not have any effect an forskolin-induced cAMP production. The data suggest that chloroquine inhibits the AVP induced cAMP production at the level of hormone/receptor complex. This possibly explains the previously reported lack of the normal antidiuretic responses of AVP in rats following chloroquine administration.

Role of endoplasmic reticulum, endosomal-lysosomal compartments, and microtubules in amyloid precursor protein metabolism of human neurons

A wide interest in amyloid precursor protein (APP) metabolism stems from the fact that increased amounts of amyloid beta peptide (Abeta), arising through proteolytic processing of APP, likely play a significant role in Alzheimer's disease. As Alzheimer's disease pathology is limited almost exclusively to the human species, we established human primary neuron cultures to address the possibility of distinctive APP processing in human CNS neurons. In the present study, we investigate the role of organelles and protein trafficking in APP metabolism. Using brefeldin A, we failed to detect APP processing into Abeta in the endoplasmic reticulum. Monensin and the lysomotropic agents, NH4Cl and chloroquine, revealed a bypass pH-dependent secretory pathway in a compartment between the endoplasmic reticulum and the medial Golgi, resulting in the secretion of full-length APP. Colchicine treatment resulting in the loss of neurites inhibited processing of APP through the secretory, but not the endosomal-lysosomal, pathway of APP metabolism. The serine protease inhibitor, leupeptin, indicates a role for lysosomes in APP, Abeta, and APP C-terminal fragment turnover. These results demonstrate that the regulation of APP metabolism in human neurons differs considerably from those reported in rodent CNS primary neuron cultures or continuously dividing cell types.

Induction of integral membrane PAM expression in AtT-20 cells alters the storage and trafficking of POMC and PC1

Peptidylglycine alpha-amidating monooxygenase (PAM) is an essential enzyme that catalyzes the COOH-terminal amidation of many neuroendocrine peptides. The bifunctional PAM protein contains an NH2-terminal monooxygenase (PHM) domain followed by a lyase (PAL) domain and a transmembrane domain. The cytosolic tail of PAM interacts with proteins that can affect cytoskeletal organization. A reverse tetracycline-regulated inducible expression system was used to construct an AtT-20 corticotrope cell line capable of inducible PAM-1 expression. Upon induction, cells displayed a time- and dose-dependent increase in enzyme activity, PAM mRNA, and protein. Induction of increased PAM-1 expression produced graded changes in PAM-1 metabolism. Increased expression of PAM-1 also caused decreased immunofluorescent staining for ACTH, a product of proopiomelanocortin (POMC), and prohormone convertase 1 (PC1) in granules at the tips of processes. Expression of PAM-1 resulted in decreased ACTH and PHM secretion in response to secretagogue stimulation, and decreased cleavage of PC1, POMC, and PAM. Increased expression of a soluble form of PAM did not alter POMC and PC1 localization and metabolism. Using the inducible cell line model, we show that expression of integral membrane PAM alters the organization of the actin cytoskeleton. Altered cytoskeletal organization may then influence the trafficking and cleavage of lumenal proteins and eliminate the ability of AtT-20 cells to secrete ACTH in response to a secretagogue.

Carboxypeptidase E, a peripheral membrane protein implicated in the targeting of hormones to secretory granules, co-aggregates with granule content proteins at acidic pH

Carboxypeptidase E (CPE) is a prohormone-processing enzyme and peripheral membrane protein of endocrine/neuroendocrine secretory granules. CPE has been shown to bind to an amino-terminal peptide of pro-opiomelanocortin (N-POMC) at pH 5.5 and hypothesized to be critically involved in the targeting of hormones such as POMC to the regulated secretory pathway [Cool, D. R., Normant, E., Shen, F., Chen, H. C., Pannell, L., Zhang, Y., and Loh, Y. P. (1997) Cell 88, 73-83]. To further explore the possibility that CPE serves to mediate the association of content proteins with the membrane during granule biogenesis, the binding of CPE to granule content proteins was investigated using an in vitro aggregation assay in which the selective precipitation of granule content proteins is induced by titration of the pH to <6.0. CPE was observed to co-aggregate efficiently with pituitary and chromaffin granule content proteins at concentrations well below those that promote its self-aggregation. In addition, CPE co-precipitated at pH 5.8 with purified prolactin and with insulin, which homophillically self-aggregate yet are structurally distinct from N-POMC. N-POMC when added to the assays did not inhibit the aggregation of CPE with prolactin or insulin, indicating that these interactions do not involve a binding site for N-POMC. The data show that CPE interacts at acidic pH with a variety of different content proteins, resembling in this regard other granule membrane proteins. The results support the idea that co-aggregation of abundant membrane proteins with content proteins is an important general mechanism for the sorting and retention of secretory granule proteins during granule maturation.

Biogenesis of secretory granules in the trans-Golgi network of neuroendocrine and endocrine cells

Secretory granule formation requires selection of soluble and membrane proteins into nascent secretory granules, and exclusion of proteins not required for the function of secretory granules. Both selection and exclusion presumably can occur in the compartment where assembly of the secretory granule begins, the trans most cisternae of the Golgi complex. Current research focused on the initial stages of secretory granule formation includes a search for the 'signals' which may mediate active sorting of components into secretory granules, and the role of aggregation of regulated secretory proteins in sorting. In addition, the temporal sequence of the sorting events in the Golgi, and post-Golgi compartments has gained much attention, as summarized by the alternative but not mutually exclusive 'sorting for entry' vs. 'sorting by retention' models. 'Sorting for entry' which encompasses the most popular models requires selection of cargo and membrane and exclusion of non-secretory granule proteins in the TGN prior to secretory granule formation. 'Sorting by retention' stipulates that protein selection or exclusion may occur after secretory granule formation: secretory granule specific components are retained during maturation of the granule while non-secretory granule molecules are removed in vesicles which bud from maturing secretory granules. Finally, some progress has been made in the identification of cytosolic components involved in the budding of nascent secretory granules from the TGN. This review will focus on the recent data concerning the events in secretory granule formation which occur, in the trans-Golgi network.

Suramin enters and accumulates in low pH intracellular compartments of v-sis-transformed NIH 3T3 cells

Using acridine orange as a reporter compound, we demonstrate that suramin enters and accumulates in low pH intracellular compartments (endosomes, lysosomes, and trans-Golgi complex) of normal and v-sis-transformed NIH 3T3 cells. The concentration of suramin in these acidic compartments is estimated to be > 150 microM, higher than the concentration known to completely inhibit interaction of the platelet-derived growth factor (PDGF) receptor and v-sis gene product. These results support the hypothesis that suramin reverses the transformed phenotype of v-sis-transformed cells by entering the cell via endocytosis and blocking interaction of the v-sis gene product and PDGF receptor in intracellular organelles.

Passive sorting in maturing granules of AtT-20 cells: the entry and exit of salivary amylase and proline-rich protein

Previous studies have suggested that salivary amylase and proline-rich protein are sorted differently when expressed in AtT-20 cells (Castle, A.M., L.E. Stahl, and J.D. Castle. 1992. J. Biol. Chem. 267:13093- 13100; Colomer, V., K. Lal, T.C. Hoops, and M.J. Rindler. 1994.EMBO (Eur. Mol. Biol. Organ.) J. 13:3711- 3719). We now show that both exocrine proteins behave similarly and enter the regulated secretory pathway as judged by immunolocalization and secretagogue- dependent stimulation of secretion. Analysis of stimulated secretion of newly synthesized proline-rich protein, amylase, and endogenous hormones indicates that the exogenous proteins enter the granule pool with about the same efficiency as the endogenous hormones. However, in contrast to the endogenous hormones, proline-rich protein and amylase are progressively removed from the granule pool during the process of granule maturation such that only small portions remain in mature granules where they colocalize with the stored hormones. The exogenous proteins that are not stored are recovered from the incubation medium and are presumed to have undergone constitutive-like secretion. These results point to a level of sorting for regulated secretion after entry of proteins into forming granules and indicate that retention is essential for efficient storage. Consequently, the critical role of putative sorting receptors for regulated secretion may be in retention rather than in granule entry.

Proteolytic processing of pro-opiomelanocortin occurs in acidifying secretory granules of AtT-20 cells

Using antibodies specific for pro-opiomelanocortin (POMC), amidated joining peptide (JP), and the prohormone convertase PC1, we showed immunocytochemically that PC1 in a corticotrophic tumor cell line, AtT-20, was co-localized either with POMC or with amidated JP in secretory granules, and also confirmed that POMC was cleaved mainly in secretory granules. Analysis using DAMP (3- [2,4-dinitroanilino]-3'-amino-N-methyldipropylamine) as the pH probe suggested a correlation between POMC processing and acidic pH in the secretory granules. Bafilomycin A1, a specific inhibitor of vacuolar-type H(+)-ATPase, completely inhibited POMC processing and caused constitutive secretion of the unprocessed precursor. By contrast, chloroquine, a weak base that is known to neutralize acidic organelles, was unable to inhibit POMC processing. Electron microscopic analysis revealed that, in AtT-20 cells treated with bafilomycin A1, the trans-Golgi cisternae were dilated and few secretory granules were present in the cytoplasm. These observations suggest that acidic pH provides a favorable environment for proteolytic processing of POMC by PC1 but is not required, and that integrity of the trans-Golgi network and sorting of POMC into secretory granules are important for POMC processing.

Effect of monensin on secretory granules and basal beta-endorphin secretion in the melanotroph of the rat pituitary

The effect of monensin on the Golgi complex, formation of secretory granules and basal beta-endorphin secretion in cultured melanotrophs from the rat pituitary was studied. Earlier studies on the effect of monensin on regulated secretion have generally showed only minor effects on secretory granules. The initial (within 5 min) effect of monensin on the melanotroph was the appearance of large vacuoles at the trans-side of the thiamine pyrophosphatase-positive trans-most Golgi cisternae. This was associated with a dose-dependent inhibition of the condensation of electron-dense secretory products. After 1 h of treatment with 1 microM monensin the Golgi stack was completely vacuolized. At the same time mature secretory granules were enlarged to severalfold their original size, and after 4 h of treatment secretory granules were no longer observed. Despite the marked effects on granule formation and mature secretory granules monensin did not affect the basal release of beta-endorphin-immunoreactive material during continued incubation for up to 4 h, indicating that basal peptide secretion can bypass the monensin block.

Peptide biosynthetic processing: distinguishing prohormone convertases PC1 and PC2

To determine whether manipulation of time, temperature and intragranular pH could be used to distinguish the actions of two subtilisin-related endoproteases, PC1 and PC2, in peptide biosynthesis, the biosynthetic processing of proneuropeptide Y (proNPY) and proopiomelanocortin (POMC) was examined in pituitary cell lines. AtT-20 cells express PC1 and POMC endogenously; stably transfected AtT-20 lines expressing NPY or PC2 were studied. GH3 cells express PC2 endogenously; NPY-expressing GH3 transfectants were investigated. PC1 mediated rapid processing of NPY and POMC; PC1-dependent cleavages were relatively insensitive to 20 degrees C blockade (which arrests secretory pathway transport at the trans-Golgi network) and do not require an acidic intracellular compartment (as in secretory granules). PC2 mediated much slower processing of proNPY and POMC which was totally blocked at 20 degrees C and required an acidic intracellular compartment. Thus, kinetics, abolition of intracellular pH gradients, and incubation at reduced temperatures can be used to distinguish PC1 and PC2 actions in neuroendocrine cells.

The propeptide of anglerfish preprosomatostatin-I rescues prosomatostatin-II from intracellular degradation

Polypeptide hormones and neuropeptides are initially synthesized as precursors possessing one or several domains that constitute the propeptide. Previous work from our laboratory demonstrated that expression of anglerfish prosomatostatin-I (proSRIF-I) in rat anterior pituitary GH3 cells resulted in efficient and accurate cleavage of the prohormone to generate the mature 14-amino acid peptide, SRIF-I. We also implicated the propeptide in mediating intracellular sorting to the trans Golgi network where proteolytic processing is initiated. In contrast, expression of a second form of the precursor, proSRIF-II in GH3 cells resulted in its intracellular degradation in an acidic, post-trans Golgi network compartment, most probably lysosomes. To further investigate the positive sorting signal present in proSRIF-I, we constructed a chimera comprising the signal peptide and proregion of SRIF-I fused to proSRIF-II and expressed the cDNA in GH3 cells. Here we demonstrate that the propeptide of SRIF-I rescued proSRIF-II from intracellular degradation quantitatively and diverted it to secretory vesicles. Furthermore, the chimera was processed to SRIF-28, an amino-terminally extended form of the hormone that is the physiological cleavage product of proSRIF-II processing in vivo. Most significantly, the SRIF-I propeptide functioned only in cis as part of the fusion protein and not in trans when expressed as a separate polypeptide. These data suggest that the SRIF-I propeptide may possess a sorting signal for sequestration into the secretory pathway rather than functioning as an intramolecular chaperone to promote protein folding.

Cell vacuolization induced by Helicobacter pylori: inhibition by bafilomycins A1, B1, C1 and D

All available bafilomycins (A1, B1, C1 and D) inhibit and revert macroscopic vacuolization induced by Helicobacter pylori cell-free extracts. Bafilomycin A1 displays the highest activity, followed by bafilomycin B1, C1 and D. The different potency of bafilomycins correlates with their ability to inhibit the vacuolar-type ATPase (V-ATPase) and to dissipate the membrane pH gradient of intracellular acidic organelles. These results suggest that bafilomycins should be considered as possible therapeutic agents in the treatment of gastritis.

Posttranslational processing of carboxypeptidase E, a neuropeptide-processing enzyme, in AtT-20 cells and bovine pituitary secretory granules

Carboxypeptidase E (CPE) functions in the posttranslational processing of peptide hormones and neurotransmitters. Like other peptide processing enzymes, CPE is present in secretory granules in soluble and membrane-associated forms that arise from posttranslational processing of a single precursor, "proCPE." To identify the intracellular site of proCPE processing, the biosynthesis and posttranslational processing were investigated in the mouse anterior pituitary-derived cell line, AtT-20. Following a 15-min pulse with [35S]Met, both soluble and membrane-bound forms of CPE were identified, indicating that the posttranslational processing event that generates these forms of CPE occurs in the endoplasmic reticulum or early Golgi apparatus. The relative proportion of soluble and membrane-bound forms of CPE changed when cells were chased for 2 h at 37 degrees C but was unaffected when cells were chased at either 20 or 15 degrees C, suggesting that further processing of membrane forms to the soluble form occurs in a post-Golgi compartment. Treatment of the cells with chloroquine did not alter the relative distribution of soluble and membrane forms, suggesting that an acidic compartment is not required for this processing event. Overexpression of CPE did not influence the distribution of soluble and membrane forms of CPE, indicating that the CPE-processing enzymes are not rate-limiting. To examine directly CPE-processing enzymes, bovine anterior pituitary secretory vesicles were isolated. An enzyme activity that releases the membrane-bound form of CPE was detected in the purified secretory vesicle membranes. This enzyme, which removes the C-terminal region of CPE, is partially inhibited by EDTA and phenylmethylsulfonyl fluoride and is activated by CaCl2. Together, the data indicate that posttranslational processing of CPE occurs in secretory granules and that this activity may be mediated by a prohormone convertase-like enzyme.

Investigation of a possible role of the amino-terminal pro-region of proopiomelanocortin in its processing and targeting to secretory granules

Proopiomelanocortin (POMC) is a polyprotein which is targeted to the regulated secretory pathway of neuroendocrine cells where it undergoes tissue-specific proteolysis to yield peptides such as adrenocorticotropic hormone, beta-lipotropin and beta-endorphin. The pro-region of POMC is 49 amino acid long with two disulfide bonds between cysteine residues 2 and 24 and 8 and 20. These cysteine residues are conserved across the species. The pro-region contains no known hormonal sequence. Sorting to the regulated secretory pathway is thought to involve targeting signals encoded in the structure of secretory proteins. In the present study, we have examined the possibility that the disulfide bridges located in the NH2-terminal portion of the pro-region of POMC are essential for maintaining a determinant involved in the sorting of POMC to the regulated secretory pathway. Using site-directed and deletion mutagenesis of the porcine POMC cDNA, we created mutants in which one or both disulfide bridges were disrupted or in which the first 26 amino acid residues of the pro-region were deleted. Recombinant retroviruses carrying the mutated POMC cDNAs were used to infect Neuro2A cells. Immunofluorescence and immunoelectron microscopy studies performed on infected cells revealed that the unmutated and mutated POMC-immunoreactive peptides were localized in dense-core vesicles at the tips of cellular extensions. Analysis of the POMC-immunoreactive peptides extracted from the infected Neuro2A cells indicated that the mutated precursors in which one disulfide bridge was disrupted (POMC-S2 or POMC-S8) were stored and processed as efficiently as the unmutated POMC. By contrast, the mutated precursor in which both disulfide bridges were disrupted (POMC-S2,8) did not accumulate in intracellular compartments to the same extent as unmutated POMC. Moreover, this mutant was very inefficiently processed and no release could be observed upon stimulation of the cells with K+/Ca2+. These results suggest that POMC-S2,8 entered the regulated secretory pathway less efficiently than the unmutated precursor. However, when both disulfide bridges were removed from the precursor from the precursor by deletion of the first 26 amino acid residues of POMC, the truncated precursor (POMC delta 1-26) behaved as the unmutated POMC. Taken together our results indicate that the NH2-terminal portion of the pro-region including both disulfide bridges can be deleted without affecting the targeting of the molecule to secretory granules. However, when the entire POMC sequence is expressed in Neuro2A cells, the proper folding of the NH2-terminal region might be important for efficient processing and targeting.

Regulation of ACTH secretory pathways in cultured pituitary cells

Although chloroquine, an agent that disrupts regulated protein secretion, has previously been shown to decrease the adrenocorticotropic hormone (ACTH) secretory response to adenosine 3',5'-cyclic monophosphate or corticotropin-releasing factor (CRF) in AtT-20 and rat anterior pituitary cells, respectively, it has no effect on the response to vasopressin. The present study extended experiments with chloroquine to cultured sheep anterior pituitary cells, which have a greater maximum response to vasopressin. Chloroquine (200 microM) had no effect on basal ACTH secretion or on stimulation by vasopressin. In contrast to the rat, the net response to CRF was tripled by chloroquine in ovine cells. The effect of chloroquine on the response to CRF was more effective by coexposure of cells to CRF and chloroquine than by pretreatment with chloroquine. Monensin or vinblastine did not increase the ACTH response to CRF. The results indicate ACTH release in response to vasopressin is chloroquine insensitive in this way, can be dissociated from the mechanism that responds to CRF, and would be consistent with the CRF response mechanism involving pathways that can alter the secretory pool of ACTH. When chloroquine acts to increase the response to CRF, it is likely not to act by stabilizing the CRF-receptor complex.

The influence of glycosylation on the fate of renin expressed in Xenopus oocytes

It has been recently reported that, in Xenopus oocytes injected with the mRNA for human renin, this secretory renal glycoprotein acquires phosphomannosyl residues on its asparagine-linked oligosaccharide chains, remains intracellular and undergoes a proteolytic cleavage which removes the prosegment. To understand the influence of glycosylation on the fate of renin in Xenopus oocytes and whether it is specific for human renin, we have expressed human renin and mouse Ren1 renin, which are glycosylated at two and three selected asparagine residues, respectively, and mouse Ren2 renin, which is not glycosylated, in Xenopus oocytes. The majority of human and Ren1 renins remained intracellular and underwent proteolytic cleavage, whereas mouse Ren2 renin was secreted efficiently. When human and Ren1 renins were expressed in oocytes treated with tunicamycin, both were secreted efficiently. A mutant of human renin, which had amino-acid substitutions at both glycosylation sites, was also secreted efficiently, whereas that mutated at one of the two sites was not. These results indicate that the majority of all of the glycosylated renin molecules remain intracellular and undergo proteolytic cleavage, probably due to the acquisition of phosphomannosyl residues, and the human renin remains intracellular if it is only glycosylated at one of the two sites.

Low temperature blocks exit of pro-opiomelanocortin from the endoplasmic reticulum but not subsequent delivery to the site of prohormone processing

The effect of reduced temperature on the delivery of the prohormone pro-opiomelanocortin (POMC) to the site of prohormone processing was investigated in the mouse anterior pituitary cell line AtT20. At 20 degrees C processing was substantially inhibited and was almost completely arrested at 18 degrees C. Earlier studies with membrane glycoproteins indicated that at these temperatures protein movement was blocked at the level of exit from the Golgi apparatus. In contrast it was found here that the inhibition of processing at reduced temperature was due to the retention of POMC in the endoplasmic reticulum. When POMC was allowed to progress to the Golgi before temperature was reduced, subsequent processing was only slightly retarded by incubation at 18 degrees C. This indicates either that Golgi exit is not inhibited at this temperature, or that the processing apparatus exists in the Golgi. A surprising incidental result was that when held in the endoplasmic reticulum at low temperature POMC is apparently subject to post-translational N-linked glycosylation.

Hypercapnia depresses nociception: endogenous opioids implicated

Hypoventilation produces hypercapnia which can elevate pain thresholds. Hypercapnia is a potent stressor which releases catecholamines and activates the sympathetic nervous system. Some stressors produce analgesia by releasing endogenous opioids. To determine the roles of endogenous opioids and catecholamines in hypercapnic analgesia, we administered CO2 in the inspired gas mixture to conscious rats. CO2 in the range 5-10% elevated tail flick and leg flexion latencies 2- to 3-fold in both intact and spinalised animals. The effects on reflex latencies but not on paCO2 or pHa were blocked by naloxone (2 mg/kg), and were not present in morphine-tolerant animals. The effects were reduced by dexamethasone but were not changed either by adrenalectomy or by systemic guanethidine, propanolol or phentolamine. Hypercapnia delayed the onset of the late phase of behavioural responses to formalin injected into the plantar surface of the hindpaw. We conclude that moderate hypercapnia powerfully depresses flexor withdrawal responses to noxious stimuli, by a mechanism involving release of endogenous opioids but not systemic catecholamines. This effect may account in part for the elevation in pain threshold during hypoventilation.

Chloroquine Decreases Adrenocorticotrophin-Secretory Response to Corticotrophin-Releasing Factor but not to Vasopressin in Rat Pituitary Cells: Further Evidence for Differentially Responsive Subpopulations

Abstract The relationships between adrenocorticotrophin (ACTH) content, basal ACTH secretion and the ACTH-secretory response to corticotrophin-releasing factor (CRF) or vasopressin in cultured rat anterior pituitary cells were examined in studies using chloroquine, an agent that diverts nascent ACTH from a storage pathway to direct release from cells. Chloroquine (200 mUM from t=minus 2 h) significantly elevated the basal ACTH secretory rate (by 0.7 +/- 0.2 ng over 30 min incubation, 1.1 +/- 0.3 ng over 60 min and 2.3 +/- 0.6 ng over 120 min). It also decreased the ACTH-secretory response to CRF (2.9+/-0.5 vs 4.6 +/- 0.5 ng/well over 30 min; 4.3 +/- 0.7 vs 8.0 +/- 1.4 ng/well over 60 min; 12.4+/-1.7 vs 20.2 +/- 3.6 ng/well over 120 min). In marked contrast, the net responses to vasopressin were unaltered (0.9 +/- 0.3 vs 0.9+/-0.2 ng/well over 30 min; 2.2 +/- 0.9 vs 2.1 +/-0.3 ng/well over 60 min; 3.8+/-0.9 vs 3.3 +/- 1.0 ng/well over 120 min). These data further support the association of the ACTH-secretory response to CRF with stored cellular ACTH, and the minimal dependence of the response to vasopressin on stored ACTH.