Secretin activation of chromogranin A gene transcription. Identification of the signaling pathways in cis and in trans

Endoplasmic reticulum stress impedes regulated secretion by governing key exocytotic and granulogenic molecular switches

Dense core vesicles (DCVs) and synaptic vesicles are specialised secretory vesicles in neurons and neuroendocrine cells, and abnormal release of their cargo is associated with various pathophysiologies. Endoplasmic reticulum (ER) stress and inter-organellar communication are also associated with disease biology. To investigate the functional status of regulated exocytosis arising from the crosstalk of a stressed ER and DCVs, ER stress was modelled in PC12 neuroendocrine cells using thapsigargin. DCV exocytosis was severely compromised in ER-stressed PC12 cells and was reversed to varying magnitudes by ER stress attenuators. Experiments with tunicamycin, an independent ER stressor, yielded similar results. Concurrently, ER stress also caused impaired DCV exocytosis in insulin-secreting INS-1 cells. Molecular analysis revealed blunted SNAP25 expression, potentially attributed to augmented levels of ATF4, an inhibitor of CREB that binds to the CREB-binding site. The effects of loss of function of ATF4 in ER-stressed cells substantiated this attribution. Our studies revealed severe defects in DCV exocytosis in ER-stressed cells for the first time, mediated by reduced levels of key exocytotic and granulogenic switches regulated via the eIF2α (EIF2A)-ATF4 axis.

Chromogranin A and its derived peptides: potential regulators of cholesterol homeostasis

Chromogranin A (CHGA), a member of the granin family of proteins, has been an attractive therapeutic target and candidate biomarker for several cardiovascular, neurological, and inflammatory disorders. The prominence of CHGA stems from the pleiotropic roles of several bioactive peptides (e.g., catestatin, pancreastatin, vasostatins) generated by its proteolytic cleavage and by their wide anatomical distribution. These peptides are emerging as novel modulators of cardiometabolic diseases that are often linked to high blood cholesterol levels. However, their impact on cholesterol homeostasis is poorly understood. The dynamic nature of cholesterol and its multitudinous roles in almost every aspect of normal body function makes it an integral component of metabolic physiology. A tightly regulated coordination of cholesterol homeostasis is imperative for proper functioning of cellular and metabolic processes. The deregulation of cholesterol levels can result in several pathophysiological states. Although studies till date suggest regulatory roles for CHGA and its derived peptides on cholesterol levels, the mechanisms by which this is achieved still remain unclear. This review aims to aggregate and consolidate the available evidence linking CHGA with cholesterol homeostasis in health and disease. In addition, we also look at common molecular regulatory factors (viz., transcription factors and microRNAs) which could govern the expression of CHGA and genes involved in cholesterol homeostasis under basal and pathological conditions. In order to gain further insights into the pathways mediating cholesterol regulation by CHGA/its derived peptides, a few prospective signaling pathways are explored, which could act as primers for future studies.

MicroRNA 27a Is a Key Modulator of Cholesterol Biosynthesis

Hypercholesterolemia is a strong predictor of cardiovascular diseases. The 3-hydroxy-3-methylglutaryl coenzyme A reductase gene (Hmgcr) coding for the rate-limiting enzyme in the cholesterol biosynthesis pathway is a crucial regulator of plasma cholesterol levels. However, the posttranscriptional regulation of Hmgcr remains poorly understood. The main objective of this study was to explore the role of microRNAs (miRNAs) in the regulation of Hmgcr expression. Systematic in silico predictions and experimental analyses reveal that miRNA 27a (miR-27a) specifically interacts with the Hmgcr 3' untranslated region in murine and human hepatocytes. Moreover, our data show that Hmgcr expression is inversely correlated with miR-27a levels in various cultured cell lines and in human and rodent tissues. Actinomycin D chase assays and relevant experiments demonstrate that miR-27a regulates Hmgcr by translational attenuation followed by mRNA degradation. Early growth response 1 (Egr1) regulates miR-27a expression under basal and cholesterol-modulated conditions. miR-27a augmentation via tail vein injection of miR-27a mimic in high-cholesterol-diet-fed Apoe ^-/- mice shows downregulation of hepatic Hmgcr and plasma cholesterol levels. Pathway and gene expression analyses show that miR-27a also targets several other genes (apart from Hmgcr) in the cholesterol biosynthesis pathway. Taken together, miR-27a emerges as a key regulator of cholesterol biosynthesis and has therapeutic potential for the clinical management of hypercholesterolemia.

Cyclic AMP-dependent protein kinase A and EPAC mediate VIP and secretin stimulation of PAK4 and activation of Na+,K+-ATPase in pancreatic acinar cells

Rat pancreatic acinar cells possess only the p21-activated kinase (PAKs), PAK4 of the group II PAK, and it is activated by gastrointestinal hormones/neurotransmitters stimulating PLC and by a number of growth factors. However, little is known generally of cAMP agents causing PAK4 activation, and there are no studies with gastrointestinal hormones/neurotransmitters activating cAMP cascades. In the present study, we examined the ability of VIP and secretin, which stimulate cAMP generation in pancreatic acini, to stimulate PAK4 activation, the signaling cascades involved, and their possible role in activating sodium-potassium adenosine triphosphatase (Na⁺,K⁺-ATPase). PAK4 activation was compared with activation of the well-established cAMP target, cyclic AMP response element binding protein (CREB). Secretin-stimulated PAK4 activation was inhibited by KT-5720 and PKA Type II inhibitor (PKI), protein kinase A (PKA) inhibitors, whereas VIP activation was inhibited by ESI-09 and HJC0197, exchange protein directly activated by cAMP (EPAC) inhibitors. In contrast, both VIP/secretin-stimulated phosphorylation of CREB (pCREB) via EPAC activation; however, it was inhibited by the p44/42 inhibitor PD98059 and the p38 inhibitor SB202190. The specific EPAC agonist 8-CPT-2- O-Me-cAMP as well 8-Br-cAMP and forskolin stimulated PAK4 activation. Secretin/VIP activation of Na⁺,K⁺-ATPase, was inhibited by PAK4 inhibitors (PF-3758309, LCH-7749944). These results demonstrate PAK4 is activated in pancreatic acini by stimulation of both VIP-/secretin-preferring receptors, as is CREB. However, they differ in their signaling cascades. Furthermore, PAK4 activation is needed for Na⁺,K⁺ATPase activation, which mediates pancreatic fluid secretion. These results, coupled with recent studies reporting PAKs are involved in both pancreatitis/pancreatic cancer growth/enzyme secretion, show that PAK4, similar to PAK2, likely plays an important role in both pancreatic physiological/pathological responses. NEW & NOTEWORTHY Pancreatic acini possess only the group II p21-activated kinase, PAK4, which is activated by PLC-stimulating agents/growth factors and is important in enzyme-secretion/growth/pancreatitis. Little information exists on cAMP-activating agents stimulating group II PAKs. We studied ability/effect of cyclic AMP-stimulating agents [vasoactive intestinal polypeptide (VIP), secretin] on PAK4 activity in rat pancreatic-acini. Both VIP/secretin activated PAK4/CREB, but the cAMP signaling cascades differed for EPAC, MAPK, and PKA pathways. Both hormones require PAK4 activation to stimulate sodium-potassium adenosine triphosphatase activity. This study shows PAK4 plays an important role in VIP-/secretin-stimulated pancreatic fluid secretion and suggests it plays important roles in pancreatic acinar physiological/pathophysiological responses mediated by cAMP-activating agents.

Functional promoter polymorphisms direct the expression of cystathionine gamma-lyase gene in mouse models of essential hypertension

Despite the well-known role of cystathionine γ-lyase (Cth) in cardiovascular pathophysiology, transcriptional regulation of Cth remains incompletely understood. Sequencing of the Cth promoter region in mouse models of genetic/essential hypertension (viz. Blood Pressure High [BPH], Blood Pressure Low [BPL] and Blood Pressure Normal [BPN] mice) identified several genetic variations. Transient transfections of BPH/BPL-Cth promoter-reporter plasmids into various cell types revealed higher promoter activity of BPL-Cth than that of BPH-Cth. Corroboratively, endogenous Cth mRNA levels in kidney and liver tissues were also elevated in BPL mice. Computational analysis of the polymorphic Cth promoter region predicted differential binding affinity of c-Rel, HOXA3 and IRF1 with BPL/BPH-Cth promoter domains. Over-expression of c-Rel/HOXA3/IRF1 modulated BPL/BPH-Cth promoter activities in a consistent manner. Gel shift assays using BPH/BPL-Cth-promoter oligonucleotides with/without binding sites for c-Rel/HOXA3/IRF1 displayed formation of specific complexes with c-Rel/HOXA3/IRF1; addition of antibodies to reaction mixtures resulted in supershifts/inhibition of Cth promoter-transcription factor complexes. Furthermore, chromatin immunoprecipitation (ChIP) assays proved differential binding of c-Rel, HOXA3 and IRF1 with the polymorphic promoter region of BPL/BPH-Cth. Tumor necrosis factor-α (TNF-α) reduced the activities of BPL/BPH-Cth promoters to different extents that were further declined by ectopic expression of IRF1; on the other hand, siRNA-mediated down-regulation of IRF1 rescued the TNF-α-mediated suppression of the BPL/BPH-Cth promoter activities. In corroboration, ChIP analysis revealed enhanced binding of IRF1 with BPH/BPL-Cth promoter following TNF-α treatment. BPL/BPH-Cth promoter activity was diminished upon exposure of hepatocytes and cardiomyoblasts to ischemia-like pathological condition due to reduced binding of c-Rel with BPL/BPH-Cth-promoter. Taken together, this study reveals the molecular basis for the differential expression of Cth in mouse models of essential hypertension under basal and pathophysiological conditions.

Molecular mechanism of monoamine oxidase A gene regulation under inflammation and ischemia-like conditions: key roles of the transcription factors GATA2, Sp1 and TBP

Monoamine oxidase A (MAOA) plays important roles in the pathogenesis of several neurological and cardiovascular disorders. The mechanism of transcriptional regulation of MAOA under basal and pathological conditions, however, remains incompletely understood. Here, we report systematic identification and characterization of cis elements and transcription factors that govern the expression of MAOA gene. Extensive computational analysis of MAOA promoter, followed by 5'-promoter deletion/reporter assays, revealed that the -71/-40 bp domain was sufficient for its basal transcription. Gel-shift and chromatin immunoprecipitation assays provided evidence of interactions of the transcription factors GATA-binding protein 2 (GATA2), Sp1 and TATA-binding protein (TBP) with this proximal promoter region. Consistently, over-expression of GATA2, Sp1 and TBP augmented MAOA promoter activity in a coordinated manner. In corroboration, siRNA-mediated down-regulation of GATA2/Sp1/TBP repressed the endogenous MAOA expression as well as transfected MAOA promoter activity. Tumor necrosis factor-α and forskolin activated MAOA transcription that was reversed by Sp1 siRNA; in support, tumor necrosis factor-α- and forskolin-induced activities were enhanced by ectopic over-expression of Sp1. On the other hand, MAOA transcription was diminished upon exposure of neuroblasts or cardiac myoblasts to ischemia-like conditions because of reduced binding of GATA2/Sp1/TBP with MAOA promoter. In conclusion, this study revealed previously unknown roles of GATA2, Sp1 and TBP in modulating MAOA expression under basal as well as pathophysiological conditions such as inflammation and ischemia, thus providing new insights into the molecular basis of aberrant MAOA expression in neuronal/cardiovascular disease states. Dysregulation of monoamine oxidase A (MAOA) have been implicated in several behavioral and neuronal disease states. Here, we identified three crucial transcription factors (GATA2, Sp1 and TBP) that regulate MAOA gene expression in a coordinated manner. Aberrant MAOA expression under pathophysiological conditions including inflammation and ischemia is mediated by altered binding of GATA2/Sp1/TBP with MAOA proximal promoter. Thus, these findings provide new insights into pathogenesis of several common diseases. GATA2, GATA-binding protein 2; Sp1, specificity protein 1; TBP, TATA-binding protein.

Transcriptional regulation of the novel monoamine oxidase renalase: Crucial roles of transcription factors Sp1, STAT3, and ZBP89

Renalase, a novel monoamine oxidase, is emerging as an important regulator of cardiovascular, metabolic, and renal diseases. However, the mechanism of transcriptional regulation of this enzyme remains largely unknown. We undertook a systematic analysis of the renalase gene to identify regulatory promoter elements and transcription factors. Computational analysis coupled with transfection of human renalase promoter/luciferase reporter plasmids (5'-promoter-deletion constructs) into various cell types (HEK-293, IMR32, and HepG2) identified two crucial promoter domains at base pairs -485 to -399 and -252 to -150. Electrophoretic mobility shift assays using renalase promoter oligonucleotides with and without potential binding sites for transcription factors Sp1, STAT3, and ZBP89 displayed formation of specific complexes with HEK-293 nuclear proteins. Consistently, overexpression of Sp1, STAT3, and ZBP89 augmented renalase promoter activity; additionally, siRNA-mediated downregulation of Sp1, STAT3, and ZBP89 reduced the level of endogenous renalase transcription as well as the transfected renalase promoter activity. In addition, chromatin immunoprecipitation assays showed in vivo interactions of these transcription factors with renalase promoter. Interestingly, renalase promoter activity was augmented by nicotine and catecholamines; while Sp1 and STAT3 synergistically activated the nicotine-induced effect, Sp1 appeared to enhance epinephrine-evoked renalase transcription. Moreover, renalase transcript levels in mouse models of human essential hypertension were concomitantly associated with endogenous STAT3 and ZBP89 levels, suggesting crucial roles for these transcription factors in regulating renalase gene expression in cardiovascular pathological conditions.

Secretin receptor promotes the proliferation of endocrine tumor cells via the PI3K/AKT pathway

The secretin receptor (SR), a G protein-coupled receptor, mediates the effects of the gastrointestinal hormone secretin on digestion and water homeostasis. Recently, high SR expression has been observed in pancreatic ductal adenocarcinomas, cholangiocellular carcinomas, gastrinomas, and bronchopulmonary carcinoid tumors. Receptor overexpression associates with enhanced secretin-mediated signaling, but whether this molecule plays an independent role in tumorigenesis is currently unknown. We recently discovered that pheochromocytomas developing in rats affected by the MENX (multiple endocrine neoplasia-like) syndrome express at very high-level Sctr, encoding SR. We here report that SR are also highly abundant on the membranes of rat adrenal and extraadrenal pheochromocytoma, starting from early stages of tumor development, and are functional. PC12 cells, the best characterized in vitro pheochromocytoma model, also express Sctr at high level. Thus, we used them as model to study the role of SR in neoplastic transformation. Small interfering RNA-mediated knockdown of Sctr decreases PC12 cells proliferation and increases p27 levels. The proproliferative effect of SR in PC12 cells is mediated, in part, by the phosphatidylinositol 3 kinase (PI3K)/serine-threonine protein kinase (AKT) pathway. Transfection of Sctr in Y1 adrenocortical carcinoma cells, expressing low endogenous levels of Sctr, stimulates cell proliferation also, in part, via the PI3K/AKT signaling cascade. Because of the link between SR and PI3K/AKT signaling, tumor cells expressing high levels of the receptor (MENX-associated primary pheochromocytoma and NCI-H727 human bronchopulmonary carcinoid cells) respond well and in a SR-dependent manner to PI3K inhibitors, such as NVP-BEZ235. The association between SR levels and response to PI3K inhibition might open new avenues for the treatment of tumors overexpressing this receptor.

Catecholamine biosynthesis and secretion: physiological and pharmacological effects of secretin

Pituitary adenylyl cyclase activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) augment the biosynthesis of tyrosine hydroxylase (TH). We tested whether secretin belonging to the glucagon/PACAP/VIP superfamily would increase transcription of the tyrosine hydroxylase (Th) gene and modulate catecholamine secretion. Secretin activated transcription of the endogenous Th gene and its transfected promoter (EC(50) ∼4.6 nM) in pheochromocytoma (PC12) cells. This was abolished by pre-treatment with a secretin receptor (SCTR) antagonist and by inhibition of protein kinase A (PKA), mitogen-activated protein kinase, or CREB (cAMP response element-binding protein). In agreement, secretin increased PKA activity and induced phosphorylation of CREB and binding to Th CRE, suggesting secretin signaling to transcription via a PKA-CREB pathway. Secretin stimulated catecholamine secretion (EC(50) ∼3.5 μM) from PC12 cells, but this was inhibited by pre-treatment with VIP-preferring receptor (VPAC1)/PACAP-preferring receptor (PAC1) antagonists. Secretin-evoked secretion occurred without extracellular Ca(2+) and was abolished by intracellular Ca(2+) chelation. Secretin augmented phospholipase C (PLC) activity and increased inositol-1,4,5-triphosphate (IP(3)) levels in PC12 cells; PLC-β inhibition blocked secretin-induced catecholamine secretion, indicating the participation of intracellular Ca(2+) from a phospholipase pathway in secretion. Like PACAP, secretin evoked long-lasting catecholamine secretion, even after only a transient exposure. Thus, transcription is triggered by nanomolar concentrations of the peptide through SCTR, with signaling along the cAMP-PKA and extracellular-signal-regulated kinase 1/2 pathways and through CREB. By contrast, secretion is triggered only by micromolar concentrations of peptide through PAC1/VPAC receptors and by utilizing a PLC/intracellular Ca(2+) pathway.

Functional promoter polymorphisms govern differential expression of HMG-CoA reductase gene in mouse models of essential hypertension

3-Hydroxy-3-methylglutaryl-coenzyme A [HMG-CoA] reductase gene (Hmgcr) is a susceptibility gene for essential hypertension. Sequencing of the Hmgcr locus in genetically hypertensive BPH (blood pressure high), genetically hypotensive BPL (blood pressure low) and genetically normotensive BPN (blood pressure normal) mice yielded a number of single nucleotide polymorphisms (SNPs). BPH/BPL/BPN Hmgcr promoter-luciferase reporter constructs were generated and transfected into liver HepG2, ovarian CHO, kidney HEK-293 and neuronal N2A cells for functional characterization of the promoter SNPs. The BPH-Hmgcr promoter showed significantly less activity than the BPL-Hmgcr promoter under basal as well as nicotine/cholesterol-treated conditions. This finding was consistent with lower endogenous Hmgcr expression in liver and lower plasma cholesterol in BPH mice. Transfection experiments using 5′-promoter deletion constructs (strategically made to assess the functional significance of each promoter SNP) and computational analysis predicted lower binding affinities of transcription factors c-Fos, n-Myc and Max with the BPH-promoter as compared to the BPL-promoter. Corroboratively, the BPH promoter-luciferase reporter construct co-transfected with expression plasmids of these transcription factors displayed less pronounced augmentation of luciferase activity than the BPL construct, particularly at lower amounts of transcription factor plasmids. Electrophoretic mobility shift assays also showed diminished interactions of the BPH promoter with HepG2 nuclear proteins. Taken together, this study provides mechanistic basis for the differential Hmgcr expression in these mouse models of human essential hypertension and have implications for better understanding the role of this gene in regulation of blood pressure.

Gastrin transactivates the chromogranin A gene through MEK-1/ERK- and PKC-dependent phosphorylation of Sp1 and CREB

Our previous work revealed that gastrin regulates chromogranin A (CgA) transcription through enhanced binding of Sp1, CREB and Egr-1 to a proximal gastrin-responsive promoter element (Gas-RE). Here, we provide a detailed characterization of the signalling pathways transmitting the effect of gastrin on the CgA promoter. Gastrin treatment of gastric AGS-B cells potently stimulated MEK-1 as well as MAP kinases ERK-1/-2, JNK and p38 in a time-dependent manner. Interruption of ERK-1/-2/MEK-1 pathways abolished the transactivating effect of gastrin, whereas blockade of JNK or p38 activity was without effect. Functional promoter analysis revealed that the minimal element CgA-85/-64 was sufficient and necessary to confer MEK-1/ERK responsiveness. Analysis of proximal signalling pathways showed that activation of the MEK-1/ERK-1/2 module by gastrin does not require Ras, PI3-kinase or intracellular calcium signals, but depends on activation of kinases of the PKC family. This report demonstrates that a pathway comprising PKCs>Raf-1>MEK-1>ERK-1/-2 mediates the effect of gastrin on the CgA promoter, and strongly suggests that enhanced phosphorylation of Sp1 and CREB is crucial for CgA transactivation through the G protein-coupled CCK-B/gastrin receptor.

Molecular basis of neuroendocrine cell type-specific expression of the chromogranin B gene: Crucial role of the transcription factors CREB, AP-2, Egr-1 and Sp1

The molecular basis of neuroendocrine-specific expression of chromogranin B gene (Chgb) has remained elusive. Utilizing wild-type and mutant Chgb promoter/luciferase reporter constructs, this study established a crucial role for the cAMP response element (CRE) box at -102/-95 bp in endocrine [rat pheochromocytoma (chromaffin) cell line (PC12) and rat pituitary somatotrope cell line (GC)] and neuronal [rat dorsal root ganglion/mouse neuroblastoma hybrid cell line (F-11), cortical and hippocampal primary neurons] cells. Additionally, G/C-rich domains at -134/-127, -125/-117 and -115/-110 bp played especially important roles for endocrine-specific expression of the Chgb gene. Co-transfection of expression plasmids for CREB, activator protein-2 (transcription factor) (AP-2), early growth response protein (transcription factor) (Egr-1) or specificity protein 1 (transcription factor) (Sp1) with the Chgb promoter constructs trans-activated expression of the Chgb gene. Nuclear extracts from either PC12 or F-11 cells formed specific complexes with the Chgb (-110/-87 bp) (CRE) oligonucleotide, which were either supershifted or disrupted by anti-CREB antibodies. In addition PC12 nuclear extracts also formed a specific complex with a Chgb (-140/-104-bp) oligonucleotide containing three G/C-rich regions, which was dose-dependently disrupted by anti-AP-2, anti-Egr-1 or anti-Sp1 antibodies; indeed, any one of these three antibodies completely abolished the complex, suggesting that all three factors bind the region simultaneously, at least in vitro. Chromatin immunoprecipitation assays documented the binding of the transcription factors CREB, AP-2, Egr-1 and Sp1 to the chromosomal Chgb gene promoter in vivo in PC12 cells within the context of chromatin. We conclude that the neuroendocrine-specific expression of Chgb is mediated by the CRE and G/C boxes in cis and the transcription factors CREB, AP-2, Egr-1 and Sp1 in trans.

The chromogranin A fragment catestatin: specificity, potency and mechanism to inhibit exocytotic secretion of multiple catecholamine storage vesicle co-transmitters

BACKGROUND

Secretory granules of chromaffin cells and neurons co-store and release, by exocytosis, the acidic soluble protein chromogranin A (human, CHGA; rodent, Chga) along with catecholamines, neuropeptides and adenosine triphosphate (ATP). CHGA serves as a pro-protein and upon proteolytic cleavage it generates active peptides, including catestatin (human CHGA352-372), first discovered in adrenal medullary chromaffin granules. Studies in our laboratory demonstrated that catestatin acts at the nicotinic acetylcholine receptor to inhibit catecholamine secretion. However, the specificity of catestatin to exert nicotinic-cholinergic antagonism among its co-transmitters is not clearly known, nor is the potential effect of catestatin on multiple vesicle co-transmitters understood.

AIM

Here we probed the specificity of catestatin's actions among its co-transmitters: catecholamines, ATP, and neuropeptide Y (NPY).

METHODS

We studied the effects of each transmitter on exocytotic secretion of its co-transmitters from PC12 chromaffin cells, stimulating secretion by triggering physiological pathways at multiple sites.

RESULTS

We observed that, among chromaffin granule co-transmitters, only catestatin and NPY inhibited catecholamine release induced by nicotinic-cholinergic stimulation; catestatin was more than tenfold more potent than NPY in this setting. We also stimulated norepinephrine secretion by other chromaffin cell agonists: catestatin blocked norepinephrine release induced by nicotine, but not by other agents (such as membrane depolarization) acting at later stages in the secretory pathway, nor by agents acting on other receptor classes. By contrast, NPY acted less specifically, blocking norepinephrine release triggered by either nicotine or membrane depolarization. Catestatin inhibited nicotinic-cholinergic co-release of all classes of chromaffin granule co-transmitters: catecholamines, chromogranins, neuropeptides, and ATP. Naturally occurring variants of human catestatin (Gly364Ser and Pro370Leu) exhibited parallel changes in potency to inhibit secretion of catecholamines and ATP.

CONCLUSION

We conclude that, among the chromaffin granule co-transmitters, catestatin acts as the most specific and potent inhibitor of physiological pathway (nicotinic-cholinergic) stimulated secretion. Furthermore, catestatin generally inhibits nicotinically triggered exocytotic release of multiple co-transmitters from chromaffin granules. The results have physiological and pharmacological implications for co-transmission in the sympathochromaffin system.

Dense-core secretory granule biogenesis

The dense-core secretory granule is a key organelle for secretion of hormones and neuropeptides in endocrine cells and neurons, in response to stimulation. Cholesterol and granins are critical for the assembly of these organelles at the trans-Golgi network, and their biogenesis is regulated quantitatively by posttranscriptional and posttranslational mechanisms.

What turns CREB on?

The transactivation domain of the cAMP response element-binding protein (CREB) consists of two major domains. The glutamine-rich Q2 domain, which interacts with the general transcription factor TAFII130/135, is sufficient for the recruitment of a functional RNA polymerase II complex and allows basal transcriptional activity. The kinase-inducible domain, however, mediates signal-induced activation of CREB-mediated transcription. It is generally believed that recruitment of the coactivators CREB-binding protein (CBP) and p300 after signal-induced phosphorylation of this domain at serine-133 strongly enhances CREB-dependent transcription. Transcriptional activity of CREB can also be potentiated by phosphoserine-133-independent mechanisms, and not all stimuli that provoke phosphorylation of serine-133 stimulate CREB-dependent transcription. This review presents an overview of the diversity of stimuli that induce CREB phosphorylation at Ser-133, focuses on phosphoserine-133-dependent and -independent mechanisms that affect CREB-mediated transcription, and discusses different models that may explain the discrepancy between CREB Ser-133 phosphorylation and activation of CREB-mediated transcription.

The catecholamine release-inhibitory "catestatin" fragment of chromogranin a: naturally occurring human variants with different potencies for multiple chromaffin cell nicotinic cholinergic responses

The catestatin fragment of chromogranin A is an endogenous inhibitor of nicotinic cholinergic transmission, functioning in negative feedback control of catecholamine secretion. We explored naturally occurring polymorphisms in the amino acid sequence of catestatin. Three human variants were identified: Gly364Ser, Pro370Leu, and Arg374Gln. Variants were tested for ability to inhibit four nicotinic processes. The rank order of potency for inhibition of catecholamine secretion was Pro370Leu > wild type > Gly364Ser > Arg374Gln. Decrease in potency was paralleled by decline in Hill slope, suggesting that negative cooperativity at ascending dose might underlie loss of potency. Several lines of evidence indicated that each variant acted as a nicotinic antagonist: potency to inhibit secretion paralleled inhibition of agonist-triggered (22)Na(+) uptake (r = 0.986); variants inhibited secretion with similar potency when triggered by several nicotinic agonists, though not by agents using other secretory pathways or bypassing the nicotinic receptor; and blockade of secretion was noncompetitive with agonist. Variants also inhibited desensitization of secretion after prior agonist exposure and stimulation of secretory protein biosynthesis by agonist. Rank order of variant inhibitory potency for all four nicotinic processes was identical (Pro370Leu > wild type > Gly364Ser > Arg374Gln), suggesting mediation by similar combinations of receptor alpha/beta subunits and that crucial catestatin residues are likely to be identical across the four processes. When catestatin variants were mixed in likely heterozygotic (1:1 M ratio) combinations, the inhibitory curve was left-shifted onto that of the more potent variant in the combination, suggesting phenotypic dominance. The results have quantitative implications for interindividual variations in human nicotinic signaling.

Association of ABCA2 expression with determinants of Alzheimer's disease

With the use of a novel method for detecting differential gene expression, alterations in functional gene clusters related to transport or oxidative stress response and beta-amyloid (Abeta) peptide metabolism were identified in a HEK293 cell line engineered to overexpress the human ATP binding cassette transporter ABCA2. These included fatty acid binding protein, phospholipid binding protein, phospholipid synthesis protein, transporter cofactors, seladin-1, Abeta precursor protein (APP), vimentin, and low-density lipoprotein receptor-related protein. ABCA2 was highly expressed in neuroblastoma cells and colocalized with Abeta and APP. Additionally, increased APP protein levels were detected within ABCA2/APP double-transfected cells, and increased Abeta was detected in the media of ABCA2-transfected cells relative to controls. The transporter was abundant in the temporal and frontal regions of both normal and Alzheimer's disease (AD) brain but was detected at lower concentrations in the parietal, occipital, and cerebellar regions. The ABCA2 transfected cell line expressed resistance to a free radical initiator, confirming involvement in protection against reactive oxygen species and suggesting a further possible link to AD.