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Secretory Protein Trafficking, Granule Biogenesis, and Cancer in Neuroendocrine Cells

Y. Peng Loh, PhD, Head, Section on Cellular Neurobiology
Niamh X. Cawley, PhD, Staff Scientist
Hong Lou, MD, Senior Research Assistant
Yong Cheng, PhD, Postdoctoral Fellow
Saravana Murthy, PhD, Postdoctoral Fellow
Prabhuanand Selvaraj, PhD, Postdoctoral Fellow
Erwan Thouennon, PhD, Postdoctoral Fellow
Nigel Birch, PhD, Special Volunteer
Xioyan Qin, MD, Special Volunteer
Vipula Kolli, PhD, Special Volunteer
Alexander Chen, BS, Postbaccalaureate Fellow
Evan Dupart, BS, Postbaccalaureate Fellow

We study the cell biology of endocrine and neuroendocrine cells. Our focus is three-fold: (i) to investigate the mechanisms of biosynthesis and intracellular trafficking of peptide hormones and neuropeptides and their processing enzymes; (ii) to uncover mechanisms involved in the regulation of dense-core secretory granule biogenesis, transport, and exocytosis; and (iii) to determine the physiological and pathological roles of the prohormone-processing enzyme carboxypeptidase E (CPE). Our work led to the discovery of novel molecular mechanisms of protein trafficking to the regulated secretory pathway and identified players and mechanisms that control secretory granule biogenesis and transport in endocrine and neuroendocrine cells. We also discovered new roles of carboxypeptidase E gene in neuroprotection, dendritic pruning, and cancer. Using cell lines, primary cell cultures, and mouse models, our studies have provided a better understanding of diseases related to defects in hormone and neuropeptide targeting, synaptic transmission, neurodegeneration, memory, learning, diabetes, obesity, and metastatic disease.

Mechanism of sorting and vesicle transport of pro-neuropeptides, neurotrophins, and their processing enzymes to the regulated secretory pathway for secretion

The intracellular sorting of pro-neuropeptides and neurotrophins to the regulated secretory pathway (RSP) is essential for processing, storage, and release of active proteins and peptides in the neuroendocrine cell. We investigated the sorting of pro-opiomelanocortin (POMC, pro-ACTH/endorphin), proinsulin, and brain-derived neurotrophic factor (BDNF) to the RSP. Our studies showed that these pro-proteins undergo homotypic oligomerization as they traverse the cell from the site of synthesis in the endoplasmic reticulum (ER) to the trans-Golgi network (TGN). In the TGN, the pro-proteins are sorted into dense-core granules of the RSP for processing by prohormone convertases and CPE and then secreted. We showed that the sorting of prohormones to the RSP occurs by a receptor-mediated mechanism. Site-direct mutagenesis studies identified a 3-D consensus sorting motif consisting of two acidic residues found in POMC, proinsulin, and BDNF. We identified an RSP sorting receptor that is specific for the sorting signal of these proproteins as the transmembrane form of CPE (1).

We investigated the role of membrane CPE and secretogranin III as sorting receptors for targeting POMC to the RSP. We showed that 50% of newly synthesized POMC in primary cultures of pituitary anterior lobe cells from CPE knockout (KO) mouse was degraded, suggesting that, in the absence of efficient sorting to the granules of the RSP owing to the lack of CPE, POMC was targeted for degradation. However, some remaining POMC was sorted into the RSP. A candidate for a compensatory sorting receptor is secretogranin III (SgIII), which has been shown to bind to POMC. SgIII, a member of the granins found in neuroendocrine cells, is involved in trafficking of chromogranin A (CgA) to the RSP. Using RNA interference (siRNA) to knock down SgIII or CPE expression in pituitary AtT20 cells, we demonstrated in both cases that POMC secretion via the constitutive secretory pathway was elevated. However, increased constitutive secretion of CgA occured only in the SgIII knock-down cells. In double CPE–SgIII knock-down cells, elevated constitutive secretion of POMC and stimulated secretion of ACTH were perturbed. Thus CPE mediates trafficking of POMC to the RSP, and SgIII may play a compensatory role for CPE in POMC sorting to the RSP.

Transport of vesicles containing hormone or BDNF to the plasma membrane for activity-dependent secretion is critical for endocrine function and synaptic plasticity. We showed that the cytoplasmic tail of a transmembrane form of CPE in hormone- or BDNF–containing dense-core secretory vesicles plays an important role in their transport to the vesicles' release site. In hippocampal neurons, primary pituitary, and AtT20 cells, overexpression of the CPE tail inhibited the movement of BDNF– and POMC/CPE–containing vesicles to the processes, respectively. The CPE tail interacts with the microtubule-based motors dynactin and KIF1A/KIF3A to effect anterograde vesicle movement to the plasma membrane for secretion (1). We also showed that the CPE tail interacts with the C-terminus of gamma-adducin, a component of the cytoskeleton that binds to and stabilizes F-actin. Overexpression of the C-terminal 38 amino acids of gamma-adducin inhibited the exit of POMC from the Golgi complex in AtT-20 cells. Thus, the CPE cytoplasmic tail plays a novel role in vesicle budding from the Golgi by interacting with gamma-adducin/F-Actin (2); CPE subsequently anchors POMC/ACTH and BDNF vesicles to the microtubule-based motor system for transport along the processes to the plasma membrane for activity-dependent secretion in endocrine cells and neurons.

Role of CPE in neuroprotection, stress, and neurodevelopment

CPE knockout (KO) mice exhibit nervous system deficiencies. Morris water maze and object-preference tests indicate defects in learning and memory, and forced swim tests indicate depression. We showed that in 6- to 14-week-old CPE–KO mice, dendritic pruning was poor in cortical and hippocampal neurons, which could affect synaptogenesis. Electrophysiological measurements revealed a defect in the generation of long-term potentiation (LTP) in hippocampal slices of these mice. A major cause for this defect was the loss of neurons in the CA3 region of the hippocampus of CPE–KO animals observed at 4 weeks of age and older. The neurons, which are normally enriched in CPE, were normal at 3 weeks of age just before the animals were weaned. Interestingly, when weaning was delayed for a week, the degeneration was not observed until postnatal week 5 in the CPE–KO mice. Moreover, the anti-epileptic drug carbamezapine given i.p. at 2 weeks of age prevented the degeneration. The results suggest that the degeneration is correlated with possibly epileptic-like neuronal firing during the stress of weaning and that CPE is important for the survival of CA3 neurons during that period. Indeed, we showed that, when CPE was overexpressed or applied externally to cultured hippocampal neurons, they were protected from apoptosis after inducing oxidative stress with hydrogen peroxide. Thus, CPE is a novel neuroprotective trophic factor for hippocampal neurons.

We showed that during and after mild chronic restraint stress (CRS) for 1h/day for seven days—another stress paradigm—CPE mRNA and protein levels, as well as Bcl2, an anti-apoptotic protein, were significantly elevated in the hippocampus. In situ hybridization studies indicated especially strong elevation of CPE mRNA expression in the CA3 region and no gross neuronal cell death after CRS. Studies on primary hippocampal neurons in culture demonstrated elevated CPE and Bcl2 expression after treatment with the synthetic glucocorticoid dexamethasone, and the regulation was mediated by glucocorticoid binding to glucocorticoid-regulatory element (GRE) sites on the promoter of the cpe gene. The findings suggest that, during CRS, when glucocorticoid is released, CPE and Bcl2 expression are co-ordinately up-regulated to mediate neuroprotection of hippocampal neurons.

We also examined the temporal-spacial expression of CPE in mouse embryos to determine the enzyme's possible role in embryonic development. We found CPE mRNA expression as early as day E5.5, rising each day, peaking at E8.5, and falling slightly at E9.5 prior to expression of the endocrine system. CPE mRNA expression declined sharply at E 10.5–11.5 to below E5.5 levels and then rose sharply at E12.5, in parallel with the development of the endocrine system, and continued to increase into adulthood. In situ hybridization studies indicate that CPE is expressed primarily in the forebrain and somites in mouse embryos. Using neurospheres to study proliferation and differentiation, we found that CPE has a negative effect on cell proliferation and on the formation of neurospheres.

Serpinin, a chromogranin A–derived peptide, regulates secretory granule biogenisis, cell survival, and cardiac function.

Our previous studies in pituitary AtT-20 cells provided evidence for an autocrine mechanism for up-regulating large dense-core vesicle (LDCV) biogenesis to replenish LDCVs following stimulated exocytosis of the vesicles. The autocrine signal was identified as serpinin, a novel 26 amino-acid, CgA–derived peptide cleaved from the C-terminal of CgA. Serpinin is released in an activity-dependent manner from LDCVs. Secreted serpinin then activates adenyl cyclase, to raise cAMP levels, and protein kinase A in the cell. This leads to translocation of the transcription factor sp1 from the cytoplasm into the nucleus and enhanced transcription of a protease inhibitor, protease nexin 1 (PN-1), which then inhibits granule protein degradation in the Golgi complex. Stabilization of the granule proteins increases their levels in the Golgi, resulting in significantly enhanced LDCV formation. We also identified a modified form of serpinin, pyroglutamate-serpinin (pGlu-serpinin), in the secretion medium of pituitary AtT20 cells and in rat heart tissue. pGlu-serpinin is synthesized and stored in secretory granules and secreted in an activity-dependent manner from AtT20 cells. pGlu-serpinin immunostaining has been observed in nerve terminals of neurites in mouse brain and found to exhibit neuroprotective activity against oxidative stress in AtT20 cells and against low K⁺–induced apoptosis in rat cortical neurons. Recently, in collaboration with Bruno Tota, we found that pGlu-serpinin has positive inotropic activity in cardiac function, with no change in blood pressure and heart rate. pGlu-serpinin acts through a β1-adrenergic receptor/adenylate cyclase/cAMP/PKA pathway. pGlu-serpinin and other CgA–derived cardioactive peptides emerge as novel β-adrenergic inotropic and lusitropic modulators. Together, they can play a key role in the myocardium's orchestration of its complex response to sympathochromaffin stimulation (3).

A splice isoform of carboxypeptidase E is a tumor inducer and biomarker for predicting future metastasis.

Despite the numerous biomarkers reported, few are useful for predicting metastasis. In our recent studies (4), we discovered a novel splice isoform of CPE (CPE-deltaN) that is elevated in metastatic hepatocellular, colon, breast, head and neck carcinoma. CPE-deltaN is translocated from the cytoplasm to the nucleus of metastatic cancer cells. Overexpression of CPE-deltaN in hepatocellular carcinoma (HCC) cells promoted their proliferation and migration. SiRNA knockdown of CPE-deltaN expression in highly metastatic HCC cells inhibited their growth and metastasis in nude mice. CPE-deltaN promoted migration by up-regulating expression of the metastasis gene Nedd9, through interaction with histone deacetylase (HDAC) 1/2. The enhanced invasive phenotype of HCC cells stably transfected with CPE-delta-N was suppressed when Nedd9 was silenced by si-RNA. Microarray studies of HCC cells overexpressing CPE-deltaN showed elevated expression of 27 genes associated with metastasis including Nedd9, claudin 2 (cldn2), matrix metallopeptidase 1 (mmp1), and inositol 1,4,5-trisphosphate 3-kinase A (itpka), while 30 genes associated with tumor suppressor function, such as insulin-like growth factor binding protein 5 and 3 (igfbp5 and igfbp3) were down-regulated. In a recent study, we showed that CPE-deltaN can activate the canonical wnt pathway, resulting in increased levels of beta-catenin, which functions with T-cell factor/lymphoid enhancer factor in the nucleus to activate expression of Wnt target genes (5). It is well known that such a mechanism could lead to colorectal cancer progression. We also showed that wild-type full-length CPE secreted by neuroendocrine tumors and glioblastomas negatively regulates the canonical wnt pathway (5), leading perhaps to an anti-metastastic effect reported in another study.

In retrospective clinical studies of 180 patients with HCC, CPE-deltaN mRNA quantification in primary HCC(T) versus surrounding normal tissue HCC(N) established a T/N cut off level above which future metastasis within 2 years could be predicted with high sensitivity and specificity and independently of cancer stage (4). A prospective study on 120 stage I and stage II HCC patients is now in progress to further evaluate CPE-deltaN as a biomarker for predicting future metastasis. In a prospective clinical study on 42 patients with pheochromocytoma/paragangliomas, we were able to predict, with high accuracy from the mRNA copy numbers of CPE-deltaN in the resected tumors, those patients who would develop future metastasis, although they were diagnosed with benign tumors at the time of surgery. Additionally, in an ongoing prospective study of papillary thyroid cancer, CPE-delta N was found to be an excellent biomarker for diagnosis of metastasis and for identifying patients who have high or low risk of recurrence. Continued followup of these patients will substantiate our prediction. In a retrospective study on colorectal cancer, a ratio greater than 2 of CPE-deltaN levels in tumor (T) to that of normal tissue (N) accurately diagnosed metastatic disease. Thus, CPE-deltaN is a novel tumor inducer and a powerful prognostic marker for predicting future metastasis in various cancer types and appears to be superior to histopathological diagnosis.

Publications

Cawley NX, Wetsel WC, Murthy SR, Park JJ, Pacak K, Loh YP. New roles of carboxypeptidase E in endocrine and neural function and cancer. Endocr Rev 2012;33:216-253.
Lou H, Park JJ, Phillips A, Loh YP. gamma-Adducin promotes process outgrowth and secretory protein exit from the Golgi apparatus. J Mol Neurosci 2012;E-pub ahead of print.
Tota B, Gentile S, Pasqua T, Bassino E, Koshimizu H, Cawley NX, Cerra MC, Loh YP, Angelone T. The novel chromogranin A-derived serpinin and pyroglutaminated serpinin peptides are positive cardiac beta-adrenergic-like inotropes. FASEB J 2012;26:2888-2898.
Lee TK, Murthy SRK, Cawley NX, Dhanvantari S, Hewitt SM, Lou H, Lau T, Ma S, Huynh T, Wesley RA, Ng IO, Pacak K, Poon RT, Loh YP. An N-terminal truncated carboxypeptidase E splice isoform induces tumor growth and is a biomarker for predicting future metastasis in human cancers. J Clin Invest 2011;121:880-892.
Skalka N, Caspi M, Caspi E, Loh YP, Rosin-Arbesfeld R. Carboxypeptidase E: a negative regulator of the canonical Wnt signaling pathway. Oncogene 2012;E-pub ahead of print.

Collaborators

Soyhun Ahn, PhD, Program on Genomics of Differentiation, NICHD, Bethesda, MD
Bruce J. Baum, DMD, Gene Therapy and Therapeutics Branch, NIDCR, Bethesda, MD
Angelo Corti, PhD, San Raffaele Scientific Institute, Milano, Italy
Shiu-Feng Huang, MD, PhD, National Health Research Institutes, Taiwan
Jacqueline Jonklaas, MD, Georgetown University Medical Center, Washington, DC
Beata Lecka-Czernik, PhD, University of Toledo, Toledo, OH
Karel Pacak, MD, PhD, Program in Reproductive and Adult Endocrinology, NICHD, Bethesda, MD
Joshua J. Park, PhD, University of Toledo, Toledo, OH
Rina Rosin-Arbesfeld, PhD, Sakler Faculty of Medicine, Tel Aviv University, Israel
Bruno Tota, MD, Università della Calabria, Cosenza, Italy
William Wetsel, PhD, Duke University, Durham, NC
Tulin Yanik, PhD, Middle East Technical University, Ankara, Turkey

Contact

For more information, email lohp@mail.nih.gov or visit scn.nichd.nih.gov.

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