Purification of histidine decarboxylase from the liver of fetal rats and its immunochemical and immunohistochemical characterization

Immunohistochemical analysis and distribution of epithelial mast cells in the rat larynx and trachea

Mast cells (MCs) in rat airways have been classified into two subtypes: epithelial MCs and connective tissue MCs (CTMCs). However, the immunohistochemical characteristics, cellular morphology, and distribution of epithelial MCs in the upper airways remain unclear. The present study investigated the morphological characteristics and distribution of epithelial MCs using 5-hydroxytryptamine (5-HT) and other immunohistochemical markers in sectioned or whole-mount preparations of the rat larynx and trachea. A double immunofluorescence analysis revealed the colocalization of 5-HT immunoreactivity with c-kit, a stem cell factor receptor commonly used as a MC marker, in both epithelial MCs and CTMCs. Dopa decarboxylase, an enzyme involved in 5-HT synthesis, was detected in both subtypes, suggesting their ability to synthesize and release 5-HT. Tryptase and histidine decarboxylase (a biosynthetic enzyme of histamine), which are well-known mediators of MCs, were exclusive to CTMCs. Epithelial MCs were pleomorphic with long cytoplasmic processes, whereas CTMCs were round and lacked cytoplasmic processes. The density of epithelial MCs was significantly higher in the glottis and cranial part of the trachea than in the epiglottis and other parts of the trachea. The present results showed that the morphology and immunohistochemical characteristics of epithelial MCs were different from those of CTMCs in the rat larynx and trachea, and variform epithelial MCs were predominantly located at the entrance of the upper airways. Epithelial MCs may release 5-HT to regulate innate immune responses by modulating epithelial cell functions at the entrance gate of the upper airways.

Histamine synthesis and transport are coupled in axon terminals via a dual quality control system

Monoamine neurotransmitters generated by de novo synthesis are rapidly transported and stored into synaptic vesicles at axon terminals. This transport is essential both for sustaining synaptic transmission and for limiting the toxic effects of monoamines. Here, synthesis of the monoamine histamine by histidine decarboxylase (HDC) and subsequent loading of histamine into synaptic vesicles are shown to be physically and functionally coupled within Drosophila photoreceptor terminals. This process requires HDC anchoring to synaptic vesicles via interactions with N-ethylmaleimide-sensitive fusion protein 1 (NSF1). Disassociating HDC from synaptic vesicles disrupts visual synaptic transmission and causes somatic accumulation of histamine, which leads to retinal degeneration. We further identified a proteasome degradation system mediated by the E3 ubiquitin ligase, purity of essence (POE), which clears mislocalized HDC from the soma, thus eliminating the cytotoxic effects of histamine. Taken together, our results reveal a dual mechanism for translocation and degradation of HDC that ensures restriction of histamine synthesis to axonal terminals and at the same time rapid loading into synaptic vesicles. This is crucial for sustaining neurotransmission and protecting against cytotoxic monoamines.

A neurotransmitter histamine mediating phototransduction and photopreference in Callosobruchus maculatus

BACKGROUND

The biogenic amine histamine plays a critical role in the phototransduction and photopreference of most insects. Here, we study the function of histamine in Callosobruchus maculatus, a global storage pest.

RESULTS

In our experiment, we initially identified the histidine decarboxylase (hdc) gene through bioinformation analysis. We subsequently investigated effects of hdc and histamine on the photopreference of C. maculatus using a combination of RNA interference (RNAi), electroretinograms (ERG), immunostaining, and photopreference behavior approaches. Our results showed that histamine was required for visual signal transduction of C. maculatus, and increased its photopreference regardless of the wavelength.

CONCLUSION

This is the first study analyzing the molecular characteristics of C. maculatus photopreference, which forms the basis for a molecular mechanism for the effects of histamine on its visual transduction and preference. In practice, better understanding the photopreference patterns contributes to IPM (integrated pest management) for this storage pest. © 2023 Society of Chemical Industry.

Tadr Is an axonal histidine transporter required for visual neurotransmission in Drosophila

Neurotransmitters are generated by de novo synthesis and are essential for sustained, high-frequency synaptic transmission. Histamine, a monoamine neurotransmitter, is synthesized through decarboxylation of histidine by Histidine decarboxylase (Hdc). However, little is known about how histidine is presented to Hdc as a precursor. Here, we identified a specific histidine transporter, TADR (Torn And Diminished Rhabdomeres), which is required for visual transmission in Drosophila. Both TADR and Hdc localized to neuronal terminals, and mutations in tadr reduced levels of histamine, thus disrupting visual synaptic transmission and phototaxis behavior. These results demonstrate that a specific amino acid transporter provides precursors for monoamine neurotransmitters, providing the first genetic evidence that a histidine amino acid transporter plays a critical role in synaptic transmission. These results suggest that TADR-dependent local de novo synthesis of histamine is required for synaptic transmission.

Peroxisome proliferator-activated receptor α agonist-induced histidine decarboxylase gene expression in the rat and mouse liver

By analysis of the data from the Toxicogenomics Database (TG-GATEs), histidine decarboxylase gene (Hdc) was identified as largely and commonly upregulated by three fibrates, clofibrate, fenofibrate, and WY-14,643, which are known to induce hepatocellular hypertrophy and proliferation via stimulation of peroxisome proliferator-activated receptor α (PPARα) in rodents. As histamine has been reported to be involved in the proliferation of liver cells, the present study was conducted to focus on Hdc. Among other genes related to histidine and histamine, the expression of the gene of histamine ammonia lyase (Hal) was exclusively mobilized by the three fibrates. The expression of Hdc, which was usually very low in the liver, was increased with the repeated administration of fibrates, and concomitantly, the constitutive expression of Hal was suppressed. An interpretation is that the formation of urocanic acid from histidine under the normal condition switches to the formation of histamine. The mobilization of gene expression of Hdc and Hal by PPARα agonists could not be reproduced in primary cultured hepatocytes. The Hdc mRNA appeared to be translated to a protein which is processed differently from brain but similarly to gastric mucosa. Surprisingly, the fibrates caused hepatic hypertrophy but no induction of Hdc mRNA at all in mice. These results revealed that the changes in the histidine catabolism by PPARα agonists might be partially, but not directly, involved in the hepatocyte proliferation in rats, and there is a large genetic distance even between rat and mouse.

Cysteine 180 Is a Redox Sensor Modulating the Activity of Human Pyridoxal 5'-Phosphate Histidine Decarboxylase

Histidine decarboxylase is a pyridoxal 5'-phosphate enzyme catalyzing the conversion of histidine to histamine, a bioactive molecule exerting its role in many modulatory processes. The human enzyme is involved in many physiological functions, such as neurotransmission, gastrointestinal track function, cell growth, and differentiation. Here, we studied the functional properties of the human enzyme and, in particular, the effects exerted at the protein level by two cysteine residues: Cys-180 and Cys-418. Surprisingly, the enzyme exists in an equilibrium between a reduced and an oxidized form whose extent depends on the redox state of Cys-180. Moreover, we determined that (i) the two enzymatic redox species exhibit modest structural changes in the coenzyme microenvironment and (ii) the oxidized form is slightly more active and stable than the reduced one. These data are consistent with the model proposed by bioinformatics analyses and molecular dynamics simulations in which the Cys-180 redox state could be responsible for a structural transition affecting the C-terminal domain reorientation leading to active site alterations. Furthermore, the biochemical properties of the purified C180S and C418S variants reveal that C180S behaves like the reduced form of the wild-type enzyme, while C418S is sensitive to reductants like the wild-type enzyme, thus allowing the identification of Cys-180 as the redox sensitive switch. On the other hand, Cys-418 appears to be a residue involved in aggregation propensity. A possible role for Cys-180 as a regulatory switch in response to different cellular redox conditions could be suggested.

Molecular Regulation of Histamine Synthesis

Histamine is a critical mediator of IgE/mast cell-mediated anaphylaxis, a neurotransmitter and a regulator of gastric acid secretion. Histamine is a monoamine synthesized from the amino acid histidine through a reaction catalyzed by the enzyme histidine decarboxylase (HDC), which removes carboxyl group from histidine. Despite the importance of histamine, transcriptional regulation of HDC gene expression in mammals is still poorly understood. In this review, we focus on discussing advances in the understanding of molecular regulation of mammalian histamine synthesis.

Differential processing of mammalian L-histidine decarboxylase enzymes

In the mammalian species studied so far, the L-histidine decarboxylase (HDC) enzyme responsible for histamine biosynthesis has been shown to undergo post-translational processing. The processing is best characterized for the mouse enzyme, where di-asparate DD motifs mediate the production of active ~55 and ~60 kDa isoforms from the ~74 kDa precursor in a caspase-9 dependent manner. The identification of conserved di-aspartate motifs at similar locations in the rat and human HDC protein sequences has led to proposals that these may represent important processing sites in these species also. Here we used transfected Cos7 cells to demonstrate that the rat and human HDC proteins undergo differential processing compared to each other, and found no evidence to suggest that conserved di-aspartate motifs are required absolutely for processing in this cell type. Instead we identified SKD and EEAPD motifs that are important for caspase-6 dependent production of ~54 and ~59 kDa isoforms in the rat and human proteins, respectively. The addition of staurosporine, which is known to pharmacologically activate caspase enzymes, increased processing of the human HDC protein. We propose that caspase-dependent processing is a conserved feature of mammalian HDC enzymes, but that proteolysis may involve different enzymes and occur at diverse sites and sequences.

Structural study reveals that Ser-354 determines substrate specificity on human histidine decarboxylase

Histamine is an important chemical mediator for a wide variety of physiological reactions. L-histidine decarboxylase (HDC) is the primary enzyme responsible for histamine synthesis and produces histamine from histidine in a one-step reaction. In this study, we determined the crystal structure of human HDC (hHDC) complexed with the inhibitor histidine methyl ester. This structure shows the detailed features of the pyridoxal-5'-phosphate inhibitor adduct (external aldimine) at the active site of HDC. Moreover, a comparison of the structures of hHDC and aromatic L-amino acid (L-DOPA) decarboxylase showed that Ser-354 was a key residue for substrate specificity. The S354G mutation at the active site enlarged the size of the hHDC substrate-binding pocket and resulted in a decreased affinity for histidine, but an acquired ability to bind and act on L-DOPA as a substrate. These data provide insight into the molecular basis of substrate recognition among the group II pyridoxal-5'-phosphate-dependent decarboxylases.

Regulatory cross-talk of mouse liver polyamine and methionine metabolic pathways: a systemic approach to its physiopathological consequences

Both polyamines and methionine derivatives are nitrogen compounds directly related to the regulation of gene expression. In silico predictions and experimental evidence suggest a cross-talk between polyamine and methionine metabolism in mammalian tissues. Since liver is the major organ that controls nitrogen metabolism of the whole organism, it is the best tissue to further test this hypothesis in vivo. In this work, we studied the effects of the chronic administration of a methionine-supplemented diet (0.5% Met in drinking water for 5 months) on the liver of mice (designated as MET-mice). Metabolic and proteomic approaches were performed and the data obtained were subjected to biocomputational analysis. Results showed that a supplemental methionine intake can indeed regulate biogenic amine metabolism in an in vivo model by multiple mechanisms including metabolic regulation and specific gene demethylation. Furthermore, putative systemic effects were investigated by molecular and cellular biology methods. Among other results, altered expression levels of multiple inflammation and cell proliferation/death balance markers were found and macrophage activation was observed. Overall, the results presented here will be of interest across a variety of biomedical disciplines, including nutrition, orphan diseases, immunology and oncology.

Histamine synthesis and lessons learned from histidine decarboxylase deficient mice

This chapter summarizes the information about the transcriptional regulation of histidine decarboxylase (HDC), which is the catabolic enzyme of histamine synthesis, and the activity of histamine in vivo as clarified using HDC gene deficient mice (HDC-KO). The research of the regulatory mechanism of histamine synthesis has been focused on transcriptional and posttranslational aspects. The generation ofHDC-KO mice clarified several new pathophysiological functions of histamine. It is now recognized that the activity of histamine is not limited to allergic, peptic and neurological functions as in the old paradigm, but extends to other fields such as cardiology, immunology and infectious diseases. Therefore, this chapter will focus on these newly revealed functions of histamine. For example, histamine was known to be involved in the effector phase of allergic responses, but a role has now been shown in the sensitization phases and in innate immunity. In the allergic bronchial asthma model using HDC-KO mice it was found that histamine positively controls eosinophilia, but not bronchial hypersensitivity. The effect on eosinophils was afterwards shown to be mediated through the activity of the histamine H4 receptor. The recent advances in the understanding of histamine synthesis and the activity of HDC have dramatically expanded our understanding of the scope of histamine function.

Activation of histidine decarboxylase through post-translational cleavage by caspase-9 in a mouse mastocytoma P-815

L-Histidine decarboxylase (HDC) is the rate-limiting enzyme for histamine synthesis in mammals. Although accumulating evidence has indicated the post-translational processing of HDC, it remains unknown what kinds of proteases are involved. We investigated the processing of HDC in a mouse mastocytoma, P-815, using a lentiviral expression system. HDC was expressed as a 74-kDa precursor form, which is cleaved to yield the 55- and 60-kDa forms upon treatment with butyrate. Alanine-scanning mutations revealed that two tandem aspartate residues (Asp(517)-Asp(518), Asp(550)-Asp(551)) are critical for the processing. Treatment with butyrate caused an increase in the enzyme activity of the cells expressing the wild type HDC, but not in the cells expressing the processing-incompetent mutant. An increase in histamine synthesis by butyrate was accompanied by formation of the 55- and 60-kDa form of HDC. In addition, the in vitro translated 74-kDa form of HDC was found to undergo a limited cleavage by purified human caspase-9, whereas the alanine-substituted mutants were not. Processing and enzymatic activation of HDC in P-815 cells was enhanced in the presence of a Zn(2+) chelator, TPEN. Although treatment with butyrate and TPEN drastically augmented the protease activity of caspase-3, and -9, no apoptotic cell death was observed. Both enzymatic activation and processing of HDC were completely suppressed by a pan-caspase inhibitor, partially but significantly by a specific inhibitor for caspase-9, but not by a caspase-3 inhibitor. These results suggest that, in P-815 cells, histamine synthesis is augmented through the post-translational cleavage of HDC, which is mediated by caspase-9.

Dexamethasone suppresses histamine synthesis by repressing both transcription and activity of HDC in allergic rats

BACKGROUND

Histamine synthesized by histidine decarboxylase (HDC) from L-histidine is a major chemical mediator in the development of nasal allergy which is characterized by nasal hypersensitivity. However the regulatory mechanism of histamine synthesis by HDC remains to be elucidated. The objectives of the present study were to examine the changes of histamine content, HDC activity and HDC mRNA expression in the nasal mucosa of allergy model rats sensitized by the exposure to toluene diisocyanate (TDI) and to investigate the effect of dexamethasone on the above mentioned allergic parameters.

METHODS

Rats were sensitized and provocated by TDI and the nasal allergy-like behaviors were scored during a 10 minute period after provocation. Histamine content and HDC activity in the nasal mucosa were determined using fluorometric high performance liquid chromatography. The expression of HDC mRNA in nasal mucosa was determined using real-time quantitative reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

In TDI-sensitized rats, nasal allergy-like behaviors such as sneezing and watery rhinorrhea were induced. Histamine content, HDC activity and HDC mRNA expression in nasal mucosa were also significantly increased after TDI provocation. Pretreatment with dexamethasone significantly suppressed nasal allergy-like behaviors, up-regulation of histamine content, HDC activity and HDC mRNA induced by TDI in TDI-sensitized rats.

CONCLUSIONS

These findings indicate that increased synthesis of histamine through up-regulation of HDC gene expression and HDC activity in nasal mucosa plays an important role in the development of nasal hypersensitivity. Repression of HDC gene expression and HDC activity by dexamethasone may underlie its therapeutic effect in the treatment of allergy.

Differential c-Fos immunoreactivity in arousal-promoting cell groups following systemic administration of caffeine in rats

Despite the widespread use of caffeine, the neuronal mechanisms underlying its stimulatory effects are not completely understood. By using c-Fos immunohistochemistry as a marker of neuronal activation, we recently showed that stimulant doses of caffeine activate arousal-promoting hypothalamic orexin (hypocretin) neurons. In the present study, we investigated whether other key neurons of the arousal system are also activated by caffeine, via dual immunostaining for c-Fos and transmitter markers. Rats were administered three doses of caffeine or saline vehicle during the light phase. Caffeine at 10 and 30 mg/kg, i.p., increased motor activities, including locomotion, compared with after saline or a higher dose, 75 mg/kg. The three doses of caffeine induced distinct dose-related patterns of c-Fos immunoreactivity in several arousal-promoting areas, including orexin neurons and adjacent neurons containing neither orexin nor melanin-concentrating hormone; tuberomammillary histaminergic neurons; locus coeruleus noradrenergic neurons; noncholinergic basal forebrain neurons that do not contain parvalbumin; and nondopaminergic neurons in the ventral tegmental area. At any dose used, caffeine induced little or no c-Fos expression in cholinergic neurons of the basal forebrain and mesopontine tegmentum; dopaminergic neurons of the ventral tegmental area, central gray, and substantia nigra pars compacta; and serotonergic neurons in the dorsal raphe nucleus. Saline controls exhibited only few c-Fos-positive cells in most of the cell groups examined. These results indicate that motor-stimulatory doses of caffeine induce a remarkably restricted pattern of c-Fos expression in the arousal-promoting system and suggest that this specific neuronal activation may be involved in the behavioral arousal by caffeine.

Mammalian histidine decarboxylase: from structure to function

Histamine is a multifunctional biogenic amine with relevant roles in intercellular communication, inflammatory processes and highly prevalent pathologies. Histamine biosynthesis depends on a single decarboxylation step, carried out by a PLP-dependent histidine decarboxylase activity (EC 4.1.1.22), an enzyme that still remains to be fully characterized. Nevertheless, during the last few years, important advances have been made in this field, including the generation and validation of the first three-dimensional model of the enzyme, which allows us to revisit previous results and conclusions. This essay provides a comprehensive review of the current knowledge of the structural and functional characteristics of mammalian histidine decarboxylase.

The C-terminus of rat L-histidine decarboxylase specifically inhibits enzymic activity and disrupts pyridoxal phosphate-dependent interactions with L-histidine substrate analogues

Full-length rat HDC (L-histidine decarboxylase) translated in reticulocyte cell lysate reactions is inactive, whereas C-terminally truncated isoforms are capable of histamine biosynthesis. C-terminal processing of the approximately 74 kDa full-length protein occurs naturally in vivo, with the production of multiple truncated isoforms. The minimal C-terminal truncation required for the acquisition of catalytic competence has yet to be defined, however, and it remains unclear as to why truncation is needed. Here we show that approximately 74 kDa HDC monomers can form dimers, which is the conformation in which the enzyme is thought to be catalytically active. Nevertheless, the resulting dimer is unable to establish pyridoxal phosphate-dependent interactions with an L-histidine substrate analogue. Protein sequences localized to between amino acids 617 and 633 specifically mediate this inhibition. Removing this region or replacing the entire C-terminus with non-HDC protein sequences permitted interactions with the substrate analogue to be re-established. This corresponded exactly with the acquisition of catalytic competence, and the ability to decarboxylate natural L-histidine substrate. These studies suggested that the approximately 74 kDa full-length isoform is deficient in substrate binding, and demonstrated that C-terminally truncated isoforms with molecular masses between approximately 70 kDa and approximately 58 kDa have gradually increasing specific activities. The physiological relevance of our results is discussed in the context of differential expression of HDC isoforms in vivo.

H3Autoreceptors Modulate Histamine Synthesis through Calcium/Calmodulin- and cAMP-Dependent Protein Kinase Pathways

H(3) autoreceptors provide feedback control of neurotransmitter synthesis in histaminergic neurons, but the transduction pathways involved are poorly understood. In rat brain cortical slices, histamine synthesis can be stimulated by depolarization and inhibited by H(3) agonists. We show that histamine synthesis stimulation by depolarization with 30 mM K(+) requires extracellular calcium entry, mostly through N-type channels, and subsequent activation of calcium/calmodulin-dependent protein kinase type II. In vitro, this kinase phosphorylated and activated histidine decarboxylase, the histamine-synthesizing enzyme. Inhibition of depolarization-stimulated histamine synthesis by the histamine H(3) receptor agonist imetit was impaired by preincubation with pertussis toxin and by the presence of a myristoylated peptide (myristoyl-N-QEHAQEPERQYMHIGTMVE-FAYALVGK) blocking the actions of G-protein betagamma subunits. The stimulation of another G(i/o)-coupled receptor, adenosine A(1), also decreased depolarization-stimulated histamine synthesis. In contrast, protein kinase A activation, which is also repressed by H(3) receptors, elicited a depolarization- and calcium/calmodulin-independent stimulation of histamine synthesis. Protein kinase A was able also to phosphorylate and activate histidine decarboxylase in vitro. These results show how depolarization activates histamine synthesis in nerve endings and demonstrate that both pathways modulating neurotransmitter synthesis are controlled by H(3) autoreceptors.

L-histidine decarboxylase as a probe in studies on histamine

Because the Falck-Hillarp formaldehyde fluorescence method, which was superbly applied to identify catecholaminergic and serotonergic neurons, is not applicable to histamine, the first author (T.W.) developed an antibody to L-histidine decarboxylase (HDC) for identification of the histaminergic neuron system in the brain. The anti-HDC antibody was of great use for mapping the location and distribution of this histaminergic neuron system. (S)-alpha-fluoromethylhistidine, a specific and potent irreversible inhibitor of HDC, was also very useful in studies on functions of the neuron system. The activity of HDC is increased by various agents, treatments, and physiological conditions. We found new compounds that increased HDC activity (i.e., tetradecanoylphobol acetate (TPA), other tumor promoters, and staphylococcal enterotoxin A); and using mast cell-deficient mutant (W/W(v)) mice, we obtained evidence that this increase occurred in macrophages. To further characterize the mechanism of increases in HDC activity, the second author (H.O.) cloned human HDC cDNA and a human HDC gene. In studies on the regulation mechanism of the HDC gene, which is expressed only in limited types of cells such as mast cells, enterochromaffin-like cells in the stomach, cells in the tuberomammillary nucleus of the brain, and macrophages, CpG islands in the promoter region of the HDC gene were found to be demethylated in cells expressing the gene, whereas they are methylated in other cells that do not express the HDC gene. In collaboration with many other researchers, we developed HDC knockout mice. The resulting research is producing a lot of interesting findings in our laboratory as well as in others. In summary, HDC has been and will be useful in studies on functions of histamine.

Local changes in the catalytic site of mammalian histidine decarboxylase can affect its global conformation and stability

Mature, active mammalian histidine decarboxylase is a dimeric enzyme of carboxy-truncated monomers (approximately 53 kDa). By using a biocomputational approach, we have generated a three-dimensional model of a recombinant 1/512 fragment of the rat enzyme, which shows kinetic constants similar to those of the mature enzyme purified from rodent tissues. This model, together with previous spectroscopic data, allowed us to postulate that the occupation of the catalytic center by the natural substrate, or by substrate-analogs, would induce remarkable changes in the conformation of the intact holoenzyme. To investigate the proposed conformational changes during catalysis, we have carried out electrophoretic, chromatographic and spectroscopic analyses of purified recombinant rat 1/512 histidine decarboxylase in the presence of the natural substrate or substrate-analogs. Our results suggest that local changes in the catalytic site indeed affect the global conformation and stability of the dimeric protein. These results provide insights for new alternatives to inhibit histamine production efficiently in vivo.

Subcellular distribution of histamine, GABA and galanin in tuberomamillary neurons in vitro

Histamine acts as a neurotransmitter in the brain and regulates e.g. sleep, hibernation, vigilance, and release of several other transmitters. All histaminergic neurons are found in the tuberomamillary nucleus (TM), and send axons to almost all parts of the CNS. Despite the obvious importance of these neurons, their development, transmitter storage, and compartmentalization of cotransmitters are poorly known. Histaminergic neurons from fetal rat hypothalamus were studied in primary explant cultures and analyzed by confocal microscopy. Most histaminergic neurons were oval in shape, but round and triangular ones were also found. The average size of the 212 analyzed neurons was 19.2 microm (length), 12.5 microm (width) and 11.7 microm (thickness). The cells possessed two to five microtubule-associated protein (MAP2) positive processes, putative dendrites, and in general one MAP2-negative thin process, a putative axon. Granular histamine-immunoreactivity was found in the cell bodies, axons, and dendrites. In tuberomamillary neurons, most histamine-containing structures displayed immunoreactivity for vesicular monoamine transporter 2 (VMAT2), indicating that the two markers may coexist in the same structures. Lack of VMAT2 in some histamine-immunoreactive structures indicates that another transporter for histamine may exist. In the same neurons, gamma-aminobutyric acid (GABA)-immunoreactivity was found in structures, distinct from those containing histamine, indicating that the two transmitters may be differentially localized, regulated and released. Galanin-immunoreactivity in the cultured tuberomamillary neurons was partially located in the same structures as VMAT2. The results suggest that histamine and GABA, the two principal transmitters of tuberomamillary neurons, are not costored in the same structures in tuberomamillary neurons.

RNA interference in ticks: a study using histamine binding protein dsRNA in the female tick Amblyomma americanum

RNA interference (RNAi), a gene silencing process, has been recently exploited to determine gene function by degrading specific mRNAs in several eukaryotic organisms. We constructed a double stranded RNA (dsRNA) from a previously cloned putative Amblyomma americanum histamine binding protein (HBP) to test the significance of using this methodology in the assessment of the function and importance of gene products in ectoparasitic ticks. The female salivary glands incubated in vitro with HBP dsRNA had a significantly lower histamine binding ability. In addition, the injection of HBP dsRNA into the unfed females led both to a reduced histamine binding ability in the isolated salivary glands and to an aberrant tick feeding pattern or host response. Molecular data demonstrated less expression of the HBP mRNA in the RNAi group. Taken together, these results suggest that RNAi might be an important tool for assessing the significance of tick salivary gland secreted proteins modulating responses at the tick-host interface.

The production of 53-55-kDa isoforms is not required for rat L-histidine decarboxylase activity

Post-translational processing of the histamine-producing enzyme, L-histidine decarboxylase (HDC), leads to the formation of multiple carboxyl-truncated isoforms. Nevertheless, it has been widely reported that the mature catalytically active dimer is dependent specifically on the production of carboxyl-truncated 53-55-kDa monomers. Here we use transiently transfected COS-7 cells to study the properties of carboxyl-truncated rat HDC isoforms in the 52-58-kDa size range. Amino acid sequences important for the production of a 55-kDa HDC isoform were identified by successive truncations through amino acids 502, 503, and 504. Mutating this sequence in the full-length protein prevented the production of 55-kDa HDC but did not affect enzymatic activity. Further truncations to amino acid 472 generated an inactive 53-kDa HDC isoform that was degraded by the proteasome pathway. These results suggested that processed isoforms, apart from 53-55-kDa ones, contribute toward histamine biosynthesis in vivo. This was confirmed in physiological studies where regulated increases in HDC activity were associated with the expression of isoforms that were greater than 55 kDa in size. We provide evidence to show that regulation of HDC expression can be achieved by the differential production or differential stabilization of multiple enzyme isoforms.

L-histidine decarboxylase decreases its own transcription through downregulation of ERK activity

A poorly defined negative feedback loop decreases transcription of the L-histidine decarboxylase (HDC) gene. To help understand this regulation, we have studied the effect of HDC protein expression on HDC gene transcription in transfected AGS-B cells. Expression of the rat HDC protein inhibited HDC promoter activity in a dose-dependent fashion. The region of the HDC promoter mediating this inhibitory effect corresponded to a previously defined gastrin and extracellular signal-related kinase (ERK)-1 response element. Overexpression of the HDC protein reduced nuclear factor binding in this region. Experiments employing specific histamine receptor agonists indicated that the inhibitory effect was not dependent on histamine production, and studies with the HDC inhibitor alpha-fluoromethylhistidine revealed that inhibition was unrelated to enzyme activity. Instead, an enzymatically inactive region at the amino terminal of the HDC enzyme (residues 1-271) was shown to mediate inhibition. Fluorescent chimeras containing this domain were not targeted to the nucleus, arguing against specific inhibition of the HDC transcription machinery. Instead, we found that overexpression of HDC protein decreased ERK protein levels and ERK activity and that the inhibitory effect of HDC protein could be overcome by overexpression of ERK1. These data suggest a novel feedback-inhibitory role for amino terminal sequences of the HDC protein.

[Studies on histamine with L-histidine decarboxylase, a histamine-forming enzyme, as a probe: from purification to gene knockout]

I have been studying the functions of the histaminergic neuron system in the brain, the location and distribution of which we elucidated with antibody raised against L-histidine decarboxylase (a histamine-forming enzyme) as a marker in 1984. For this purpose, we used two methods employing (1) pharmacological agents like alpha-fluoromethylhistidine, an HDC inhibitor, and agonists and antagonists of H1, H2 and H3 receptors and (2) knockout mice of the HDC- and H1- and H2-receptor genes. In some cases, we used positron emission tomography (PET) of H1 receptors in living human brains. It turned out that histamine neurons are involved in many brain functions, and particularly, histamine is one of the neuron systems to keep awakefulness. Histamine also plays important roles in bioprotection against various noxious or unfavorable stimuli (convulsion, nociception, drug sensitization, ischemic lesions, stress and so on). Finally, I briefly described interesting phenotypes found in peripheral tissues of HDC-KO mice; the most striking finding is that mast cells in HDC-KO mice are fewer in number, smaller in size and less dense in granule density than those of wild type mice, indicating that histamine is related to the proliferation and differentiation of mast cells. In conclusion, histamine is important not only in the central and peripheral systems as studied so far but also may be related to some new functions that are now under investigation in our laboratories.

Expression analysis of the histamine H(3) receptor in developing rat tissues

Endogenous histamine is involved in tissue growth and cell proliferation. In accordance with a putative function of the H(3) receptor in this mitogenic effect, we show that H(3)-receptor mRNAs are expressed together with those of the histamine-synthesizing enzyme in the embryonic liver and adipose tissue, and in various epithelia. Finally, we show that activation of recombinant H(3) receptors enhances MAP kinase activity.

Amino- and carboxy-terminal PEST domains mediate gastrin stabilization of rat L-histidine decarboxylase isoforms

Control of enzymatic function by peptide hormones can occur at a number of different levels and can involve diverse pathways that regulate cleavage, intracellular trafficking, and protein degradation. Gastrin is a peptide hormone that binds to the cholecystokinin B-gastrin receptor and regulates the activity of L-histidine decarboxylase (HDC), the enzyme that produces histamine. Here we show that gastrin can increase the steady-state levels of at least six HDC isoforms without affecting HDC mRNA levels. Pulse-chase experiments indicated that HDC isoforms are rapidly degraded and that gastrin-dependent increases are due to enhanced isoform stability. Deletion analysis identified two PEST domains (PEST1 and PEST2) and an intracellular targeting domain (ER2) which regulate HDC protein expression levels. Experiments with PEST domain fusion proteins demonstrated that PEST1 and PEST2 are strong and portable degradation-promoting elements which are positively regulated by both gastrin stimulation and proteasome inhibition. A chimeric protein containing the PEST domain of ornithine decarboxylase was similarly affected, indicating that gastrin can regulate the stability of other PEST domain-containing proteins and does so independently of antizyme/antizyme inhibitor regulation. At the same time, endoplasmic reticulum localization of a fluorescent chimera containing the ER2 domain of HDC was unaltered by gastrin stimulation. We conclude that gastrin stabilization of HDC isoforms is dependent upon two transferable and sequentially unrelated PEST domains that regulate degradation. These experiments revealed a novel regulatory mechanism by which a peptide hormone such as gastrin can disrupt the degradation function of multiple PEST-domain-containing proteins.

Homology-based molecular modelling of PLP-dependent histidine decarboxylase from Mmorganella morganii

The 3-D structural information is a prerequisite for a rational ligand design. In the absence of experimental data, model building on the basis of a known 3-D structure of a homologous protein is at present the only reliable method to obtain structural information. A homology model building study of the pyridoxal 5'-phosphate (PLP)-dependent histidine decarboxylase from Morganella morganii (HDC-MM) has been carried out based on the crystal structure of the aspartate aminotransferase from Escherichia coli (AAT-EC). The primary sequences of AAT-EC and HDC-MM were aligned by automated alignment procedure. A 3-D model of HDC-MM was constructed by copying the coordinates of the residues from the crystal structure of AAT-EC into the corresponding residues in HDC-MM. After energy-minimization of the resulting 3-D model of HDC-MM, possible active site residues were identified by fitting the substrate (l-histidine) into the proposed active-site. In our model, several residues, which have an important role in the AAT-EC active-site, are located in positions spatially identical to those in AAT-EC structure. The back-bone of the modelled active site pocket is constructed by residues; Gly-92, Gly-93, Thr-93, Ser-115, Asp-200, Ala-202, Ser-229 and Lys-232 together with residues Asn-8, His-119, Thr-171, His-198, Leu-203, His-231, Ser-236 and Ile-238. In the ligand binding site, it appears that the HDC-MM model will position l-histidine (substrate) in the area consisting of the residues; Glu-29, Ser-30, Leu-38, His-231 and Lys-232. The nitrogen atom of the imidazole ring (N2) of the substrate is predicted to interact with the carboxylate group of Ser-30. The alpha-carboxylate of histidine points toward the Lys-232 to have electrostatic interaction with its side chain nitrogen atom (N(Z)). In conclusion, this combination of sequence and 3-D structural homology between AAT-EC and HDC-MM model could provide insight in assigning the probable active site residues.

In vitro study of proteolytic degradation of rat histidine decarboxylase

Mammalian ornithine decarboxylase (ODC) is a very unstable protein which is degraded in an ATP-dependent manner by proteasome 26S, after making contact with the regulatory protein antizyme. PEST regions are sequences described as signals for protein degradation. The C-terminal PEST region of mammalian ODC is essential for its degradation by proteasome 26S. Mammalian histidine decarboxylase (HDC) is also a short-lived protein. The full primary sequence of mammalian HDC contains PEST-regions at both the N- and C-termini. Rat ODC and different truncated and full versions of rat HDC were expressed in vitro. In vitro degradation of rat ODC and rat 1-512 HDC were compared. Like ODC, rat 1-512 HDC is degraded mainly by an ATP-dependent mechanism. However, antizyme has no effect on the degradation of 1-512 HDC. The use of the inhibitors MG-132 and lactacystine significantly inhibited the degradation of 1-512 HDC, suggesting that a ubiquitin-dependent, proteasome 26S proteolytic pathway is involved. Results obtained with the different modifications of rat HDC containing all three PEST regions (full version, 1-656 HDC), only the N-terminal PEST region (1-512 HDC), or no PEST region (69-512 HDC), indicate that the N-terminal (1-69) fragment, but not the C-terminal fragment, determines that the HDC protein is a proteasome substrate in vitro.

Vesicular monoamine transporters in the rat stomach

Cellular distribution of vesicular monoamine transporters (VMATs), known to regulate vesicular storage and release of biogenic amines (i.e., catecholamines, serotonin, histamine, etc.), have been studied in the rat stomach using in situ hybridization histochemistry (ISHH) and immunohistochemical (IHC) techniques. 35S-UTP labeled riboprobes showed that mRNAs of both VMATs are expressed in the gastric mucosa. A combination of ISHH and IHC verified that most of the parietal cells (among other epithelial cells) express mRNA of the peripheral type transporter (VMAT1) while enterochromaffin-like cells (ECL) of the fundic mucosa express mRNA of the central type (VMAT2). In addition, with double fluorescent IHC we detected VMAT1 protein in serotoninergic enterochromaffin cells (EC) of the stomach and in gastrin producing G cells of the antral mucosa. Similarly to the fundus, VMAT2 protein was present in ECL cells and in the enteric plexus. Surprisingly, serotonin- and/or histamine-containing cells in the connective tissue compartments of the stomach (i.e., lamina propria and submucosa), immunoreactive for a mast cell specific antigen, displayed neither VMATI nor VMAT2 immunoreactivity. Distribution of VMATs in the rat stomach support our previous observations on aminergic properties of two important gastrointestinal (GI) epithelial cell populations primarily known for other specific secretory products, i.e. dopaminergic properties of acid producing parietal cells and histaminergic properties of gastrin producing G cells. These data emphasize the existence of a non-neuronal, intrinsic aminergic system in the GI tract.

The mechanism of histamine secretion from gastric enterochromaffin-like cells

Enterochromaffin-like (ECL) cells play a pivotal role in the peripheral regulation of gastric acid secretion as they respond to the functionally important gastrointestinal hormones gastrin and somatostatin and neural mediators such as pituitary adenylate cyclase-activating peptide and galanin. Gastrin is the key stimulus of histamine release from ECL cells in vivo and in vitro. Voltage-gated K(+) and Ca(2+) channels have been detected on isolated ECL cells. Exocytosis of histamine following gastrin stimulation and Ca(2+) entry across the plasma membrane is catalyzed by synaptobrevin and synaptosomal-associated protein of 25 kDa, both characterized as a soluble N-ethylmaleimide-sensitive factor attachment protein receptor protein. Histamine release occurs from different cellular pools: preexisting vacuolar histamine immediately released by Ca(2+) entry or newly synthesized histamine following induction of histidine decarboxylase (HDC) by gastrin stimulation. Histamine is synthesized by cytoplasmic HDC and accumulated in secretory vesicles by proton-histamine countertransport via the vesicular monoamine transporter subtype 2 (VMAT-2). The promoter region of HDC contains Ca(2+)-, cAMP-, and protein kinase C-responsive elements. The gene promoter for VMAT-2, however, lacks TATA boxes but contains regulatory elements for the hormones glucagon and somatostatin. Histamine secretion from ECL cells is thereby under a complex regulation of hormonal signals and can be targeted at several steps during the process of exocytosis.

Histidine decarboxylase in rat stomach ECL cells: relationship between enzyme activity and different molecular forms

Mammalian HDC mRNA encodes a protein with a molecular mass of 74 kDa. The reported molecular mass for the purified HDC subunit is 53-55 kDa. Western blot analysis of extracts of rat gastric mucosa and fetal rat liver has revealed the presence of at least three different forms of HDC immunoreactivity, having molecular masses of about 74, 63 and 53 kDa. There is evidence from previous studies that full length rat HDC is enzymatically inactive and that activation requires C-terminal truncation. In the present study we examined the various immunoreactive HDC forms in rat oxyntic mucosa and their response to treatments known to affect the HDC activity. Freely fed rats and hypergastrinemic rats (treated with gastrin or the proton pump inhibitor omeprazole) had higher oxyntic mucosal HDC activity and HDC mRNA level than fasted or untreated rats. The difference in HDC activity was greater than the difference in HDC mRNA level. Western blot analysis confirmed the existence of the 74, 63 and 53 kDa HDC forms in the oxyntic mucosa. All three forms were more abundant in the oxyntic mucosa of freely fed and hypergastrinemic rats than in the mucosa of fasted or untreated rats. Of the three HDC forms, the 63 kDa form was the predominant one, the 73 kDa form was quantitatively insignificant by comparison and the 53 kDa form was at or below the limit of detection in fasted rats. The activity of HDC was well correlated to the amount of the 63 kDa HDC form. Administration of cycloheximide to hypergastrinemic rats (undergoing omeprazole treatment) resulted in a rapid decline of the HDC activity (estimated half-life 1 h and 50 min). The 63 kDa HDC form disappeared with a rate that corresponded to the decline in HDC activity. The two other HDC forms seemed to have a slower turnover. Our findings suggest that the 63 kDa form is enzymatically active. The results do not allow any conclusion as to the functional activity of the 74 and 53 kDa forms.

Expression of histidine decarboxylase messenger RNA and histamine N-methyltransferase messenger RNA in nasal allergy

BACKGROUND

Histamine is an important chemical mediator in allergic rhinitis. Histamine is synthesized by L-histidine, catalysed by L-histidine decarboxylase (HDC) and metabolized mainly by histamine N-methyltransferase (HMT). The patients with allergic rhinitis may have altered histamine metabolism in the nasal tissue.

OBJECTIVE

The objective was to compare the expression of HDC mRNA and HMT mRNA in nasal mucosa with normal individuals and patients with allergic rhinitis.

METHODS

We extracted RNA from scrapings of inferior turbinate mucosa of 11 patients with allergic rhinitis and from 11 normal subjects and from surgically dissected inferior turbinate mucosa of 11 patients. HDC, HMT and beta-actin mRNA were amplified by reverse transcription-polymerase chain reaction (RT-PCR).

RESULTS

The mean +/- SD of HDC/beta-actin ratios were 0.58 +/- 0.28 for the scrapings of normal subjects, 0.93 +/- 0.38 for the scrapings of the patients with allergic rhinitis and 1.41 +/- 0.26 for the inferior turbinates of the patients with allergic rhinitis. The mean +/- SD of HMT/beta-actin ratios were 1.66 +/- 0.27 for the scrapings of normal subjects, 0.93 +/- 0.20 for the scrapings of the patients with allergic rhinitis and 0.65 +/- 0.28 for the inferior turbinates of the patients with allergic rhinitis. The differences among the three groups were statistically significant.

CONCLUSIONS

Increased HDC mRNA expression and decreased HMT mRNA expression may be playing some role in the hyperresponsiveness of patients with allergic rhinitis.

Membrane targeting and binding of the 74-kDa form of mouse L-histidine decarboxylase via its carboxyl-terminal sequence

The role of the C-terminal region of the 74-kDa form of L-histidine decarboxylase (HDC) in the targeting to the endoplasmic reticulum (ER) was investigated in COS-7 cells. The deletion of a 10-kDa segment (residues 578-662) of the C-terminal end of HDC, especially a 20 amino acid sequence (residues 588-607), abrogated the targeting to the ER. The C-terminal 10-kDa portion is sufficient to target the green fluorescent protein (GFP) to the ER. The 74-kDa form of HDC synthesized in an in vitro translation system post-translationally associated with the heterogeneous canine microsomal membranes. These results suggest that the C-terminal 10-kDa portion of HDC contains a signal necessary for HDC to be targeted to the ER membrane.

Multiple forms of rat stomach histidine decarboxylase may reflect posttranslational activation of the enzyme

Histidine decarboxylase (HDC) catalyzes the formation of histamine, which takes part in a variety of physiological processes including gastric acid secretion, neurotransmission and inflammation. While purified rat HDC is a homodimer of approximately 54 kDa subunits, molecular cloning of mammalian HDC has revealed that HDC mRNA encodes a 74 kDa protein. This discrepancy in molecular mass may be due to a posttranslational processing of the primary translated product of rat HDC mRNA. In the present study we demonstrate that full-length rat HDC expressed in Escherichia coli or in an in vitro transcription/translation system is enzymatically inactive, while expression of a C-terminus truncated HDC (reducing the molecular mass to 54 kDa) gave rise to a protein with high enzyme activity in the same expression systems. COS-7 cells expressing truncated HDC displayed high HDC activity, whereas COS-7 cells expressing full-length HDC displayed low activity. Western blot analysis of fetal rat liver and oxyntic mucosa of gastrin-stimulated rats revealed the presence of both full-length HDC (approximately 73 kDa) and a approximately 53 kDa subunit form in addition to an intermediate form of about 63 kDa. The results are in line with the view that rat HDC may be produced as an enzymatically inactive proenzyme which is processed to give rise to the active enzyme.

Histidine decarboxylase gene in the mouse uterus is regulated by progesterone and correlates with uterine differentiation for blastocyst implantation

Cell-cell interactions between the blastocyst trophectoderm and uterine luminal epithelium are essential to the process of implantation. The factors that participate in these interactions or their mechanism of actions are poorly understood. Histamine has long been suspected as one of the factors that is involved in implantation. Histamine is formed from L-histidine by histidine decarboxylase (HDC). We examined the expression and regulation of HDC gene in the mouse uterus during early pregnancy and under steroid hormonal stimulation. Northern blot hybridization detected a 2.6-kb transcript of HDC messenger RNA (mRNA) in uterine poly(A)+ RNA samples. Maximum uterine accumulation of HDC mRNA occurred on days 3 and 4 of pregnancy, followed by marked declines on later days (days 5-8). In ovariectomized mice, uterine mRNA levels were up-regulated by an injection of progesterone (P4) by 6 h, and the levels were maintained through 24 h. In contrast, an injection of estradiol-17beta neither stimulated nor antagonized P4-induced HDC mRNA accumulation. P4-induced up-regulation was considerably abrogated by pretreatment with RU-486, a P4 receptor antagonist, suggesting involvement of P4 receptor. In situ hybridization detected HDC mRNA specifically in uterine epithelial cells but not in other cell types. Again, high epithelial accumulation occurred on day 4 of pregnancy. With the progression of implantation (days 5-8), HDC mRNA levels declined in the luminal epithelium surrounding the implanting blastocysts, as compared with that away from the blastocysts. Immunoreactive histamine and HDC were colocalized with HDC mRNA. Western blotting detected a 54-kDa protein in epithelial cell extracts, which also exhibited HDC activity. Expression of HDC in epithelial cells, preceding implantation on day 4, at lower levels after initiation of implantation on day 5, and its regulation by P4 suggest that this gene plays an important role in implantation.

Recombinant expression of rat histidine decarboxylase: generation of antibodies useful for western blot analysis

Histidine decarboxylase catalyses the formation of histamine, an important biological messenger. In spite of the essential biological functions exerted by histamine the knowledge about the mechanisms involved in the regulation of histidine decarboxylase is rather limited. This is most likely due to the limited supply of suitable tools, including highly specific antibodies. In the present study we describe the production and characterisation of specific antisera against rat histidine decarboxylase using recombinant protein synthesised in a bacterial expression system. The antisera were shown to effectively immunoprecipitate histidine decarboxylase activity in extracts of fetal rat liver as well as to detect the histidine decarboxylase protein by Western blot analysis of COS-7 cells expressing recombinant rat histidine decarboxylase. The results demonstrate the successful production of highly specific antisera to histidine decarboxylase which may become valuable tools in future studies of the structure and function of this enzyme.

Intracellular localization of the 74- and 53-kDa forms of L-histidine decarboxylase in a rat basophilic/mast cell line, RBL-2H3

To clarify the process of post-translational modification of L-histidine decarboxylase (HDC), we investigated the conversion of the 74-kDa form of HDC into the 53-kDa form in specialized organella of a rat basophilic/mast cell line (RBL-2H3). With treatment of streptolysin-O, RBL-2H3 cells released approximately 40% of HDC activity accompanied by over 90% of lactate dehydrogenase activity. Only the 74-kDa form of HDC was detected in the leaked fraction by SDS-polyacrylamide gel electrophoresis. The 74-kDa form in the homogenate of pulse-labeled cells was recovered in both the supernatant and particulate fractions, while the 53-kDa form was detected only in the particulate fraction containing marker proteins of microsomes, Golgi, and lysosomal granules. Confocal microscopic observation using double staining immunofluorescence with anti-GST fusion HDC antiserum showed that most of the HDC coexists with protein-disulfide isomerase, a typical marker of the luminal space of the ER. With treatment of digitonin, RBL-2H3 cells released only 74-kDa HDC. Trypsin digestion of digitonin-permeabilized cells resulted in the disappearance of the 74-kDa form but not the 53-kDa form. From these results, it is assumed that the 74-kDa form of HDC, synthesized in the cytosol, is translocated into the lumen of the ER, where it is converted to the 53-kDa form.

Degradation of the 74 kDa form of L-histidine decarboxylase via the ubiquitin-proteasome pathway in a rat basophilic/mast cell line (RBL-2H3)

L-Histidine decarboxylase (HDC) is a dimer consisting of two identical 53 kDa subunits. On the other hand, the size of HDC deduced from its cDNA sequence is around 74 kDa, indicating that the translated 74 kDa form of HDC is subjected to post-translational processing to generate the 53 kDa form. However, modification of the translated 74 kDa form of HDC in histamine-forming cells is unknown. Here we demonstrate that the 74 kDa form is translated in rat basophilic leukemia cells, followed by conversion to the 53 kDa form, and that the 74 kDa form is a short half-life protein because of the degradation mediated by the ubiquitin-proteasome pathway. Degradation of the 74 kDa form was stimulated in the presence of an ATP-generating system, accompanied by ubiquitination, and inhibited by specific proteasome inhibitors such as ZL3H and lactacystin. A significant amount of proteasome activity was detected in RBL-2H3 cells.

Short-term inhibitory effect of somatostatin on gastric histamine synthesis

In this study we investigated the short-term effect of somatostatin on histamine synthesis in a cell population isolated from rabbit gastric mucosa and enriched in enterochromaffin-like cells. Somatostatin inhibited basal and gastrin-stimulated histamine synthesis through a dual mechanism involving a decrease in the affinity of histidine decarboxylase (HDC) for its substrate (L-histidine) and a reduction in the number of functional HDC molecules. H-89 (an inhibitor of cAMP-dependent protein kinase) mimicked somatostatin-induced reduction of HDC affinity, which, on the contrary, was selectively reversed by pertussis toxin (PTX). Furthermore, forskolin was shown to reverse the inhibitory effect of H-89 and to prevent the somatostatin-induced reduction in HDC affinity for L-histidine. Thus, the somatostatin-induced reduction in affinity seems to involve a PTX-sensitive G protein and an inhibition of the cAMP-dependent pathway. On the other hand, the somatostatin-induced decrease in the number of functional HDC molecules seems to be PTX insensitive and independent from a modulation of the cAMP pathway, and does not seem to involve a significant change in HDC messenger RNA expression or a regulation of protein kinase C. The exact nature of this second mechanism will need further studies to be elucidated.

Comparative studies of human recombinant 74- and 54-kDa L-histidine decarboxylases

We have expressed and characterized human recombinant 74-kDa (rHDC74) and 54-kDa (rHDC54) L-histidine decarboxylases (HDCs) in Sf9 cells. By immunoblot analysis, rHDC74 and rHDC54 were shown to be localized predominantly in the particulate and soluble fractions, respectively. rHDC74 exhibited histamine-synthesizing activity equivalent to that of rHDC54. The existence of 74- and 54-kDa HDCs was also confirmed in the particulate and supernatant fractions of the cell lysate, respectively, from the human basophilic leukemia cell line KU-812-F. The ratio of HDC activity to immunoreactivity was similar for the two forms of the enzyme. The specific activity of purified rHDC54 (1.12 mumol/mg/min) was comparable to those of HDCs from other mammalian tissues or cells. The purified rHDC54 was eluted as a monomer form from a Superdex-200 column; the molecular mass of the enzyme was approximately 54 kDa on SDS-polyacrylamide gel electrophoresis without 2-mercaptoethanol. The HDC activity of rHDC54 significantly decreased on dialysis against buffer without pyridoxal 5'-phosphate; addition of pyridoxal 5'-phosphate to the dialysate readily increased in the enzyme activity to the original activity. Taken together, these results suggest that human HDC functions as both 74- and 54-kDa forms having equivalent HDC activity, which are localized in the particulate and soluble fractions, respectively, and that the latter form exhibits its activity as a monomer form.

Processing and activation of recombinant mouse mastocytoma histidine decarboxylase in the particulate fraction of Sf9 cells by porcine pancreatic elastase

Mature 53 kDa histidine decarboxylase (HDC) peptide is produced from a precursor 74 kDa peptide. The mechanism of specific cleavage by processing enzyme is unknown. Using the recombinant mouse 74 kDa HDC, we found that porcine pancreatic elastase specifically converted the inactive 74 kDa HDC to its active form of 53 kDa HDC.

Ca(2+)-activated outward-rectifier K+ channels and histamine release by rat gastric enterochromaffin-like cells

Gastric enterochromaffin-like (ECL) cells were isolated from rat gastric fundic mucosa by Percoll density-gradient centrifugation and counter-flow elutriation. About 67% of cells in the purified cell suspension were ECL cells, which were reacted with anti-histidine decarboxylase antibody. A23187, a calcium ionophore, at 0.1-10 microM induced histamine release from ECL cell-rich suspension, indicating that the Ca2+ pathway is involved in the mechanism of histamine release from the ECL cells. A23187 at 5 microM significantly increased outward-rectifier cationic current in 62% of cells in the ECL cell-rich factions. A23187-sensitive cells showed acridine orange uptake. In single-channel recordings, a Ca(2+)-dependent outward-rectifier K+ channel of large conductance (146 +/- 22 picosiemens) was found in the cell that showed acridine orange uptake. The channel opened in a voltage-dependent manner at 0.1 microM of intracellular free Ca2+ concentration. These results may suggest that opening of the Ca(2+)-activated K+ channel is one of the steps involved in the mechanism of histamine release in ECL cells.

Mobilization of gastric histamine during repeated administration of a proton potassium adenosine triphosphatase inhibitor in intact and antrectomized rats

Intact and antrectomized female rats were treated with the potent proton pump inhibitor, E3810 (daily 40 mg/kg weight, s.c.) for 4 weeks. Plasma gastrin concentration and urinary excretion of N-terminal big gastrin increased until day 14 and persisted at a high level in intact rats treated with E3810, but did not increase in antrectomized rats. Urinary excretion of histamine increased progressively and reached 7 times the control value following 4 weeks of treatment with E3810 in intact rats, but not in antrectomized rats. At the termination of the treatment, the endocrine cell density in the oxyntic mucosa of intact rats had increased by 85% with increased histamine content and elevated histidine decarboxylase activity, while antrectomized rats showed a low histamine level and low histidine decarboxylase activity. Administration of gastrin-17 I (10 micrograms/kg weight, sc) itself caused a significant increase in urinary excretion of histamine, which was inhibited by the specific gastrin receptor antagonist, L-365,260. These results suggests that the massive urinary excretion of histamine caused by the treatment with E3810 reflects gastrin-induced mobilization of gastric histamine and that neither E3810 itself nor E3810-induced luminal pH elevation has direct effects on mobilization of oxyntic mucosal histamine.

Inhibition of gastrin-stimulated enterochromaffin-like cell proliferation and mucosal histamine production in the rat stomach by the somatostatin analogue octreotide

The effect of octreotide, a potent and long-acting analogue of somatostatin, on gastrin-stimulated proliferation and function of enterochromaffin-like (ECL) cells were examined in rats. Animals were divided into four groups and each group was continuously infused with saline, octreotide alone (40 micrograms/kg per day), gastrin alone (60 nmol/kg per day), or octreotide (40 micrograms/kg per day) plus gastrin (60 nmol/kg per day) respectively for 9 days via osmotic minipumps. Gastrin induced the increase of the bromodeoxyuridine labeling index and density of oxyntic mucosal ECL cells as well as oxyntic mucosal histidine decarboxylase activity. Octreotide completely abolished the gastrin-induced increases in the labeling index and density of ECL cells and oxyntic mucosal histidine decarboxylase activity. These results indicate that octreotide inhibits gastrin-stimulated proliferation of ECL cells and histamine production by these cells.