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

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.

Molecular biology of histidine decarboxylase and prostaglandin receptors

Histamine and prostaglandins (PGs) play a variety of physiological roles as autacoids, which function in the vicinity of their sources and maintain local homeostasis in the body. They stimulate target cells by acting on their specific receptors, which are coupled to trimeric G proteins. For the precise understanding of the physiological roles of histamine and PGs, it is necessary to clarify the molecular mechanisms involved in their synthesis as well as their receptor-mediated responses. We cloned the cDNAs for mouse L-histidine decarboxylase (HDC) and 6 mouse prostanoid receptors (4 PGE(2) receptors, PGF receptor, and PGI receptor). We then characterized the expression patterns and functions of these genes. Furthermore, we established gene-targeted mouse strains for HDC and PG receptors to explore the novel pathophysiological roles of histamine and PGs. We have here summarized our research, which should contribute to progress in the molecular biology of HDC and PG receptors.

Inhibitory and structural studies of novel coenzyme-substrate analogs of human histidine decarboxylase

Histamine, a biogenic amine with important biological functions, is produced from histidine by histidine decarboxylase (HDC), a pyridoxal 5'-phosphate-dependent enzyme. HDC is thus a potential target to attenuate histamine production in certain pathological states. Targeting mammalian HDC with novel inhibitors and elucidating the structural basis of their specificity for HDC are challenging tasks, because the three-dimensional structure of mammalian HDC is still unknown. In the present study, we designed, synthesized, and tested potentially membrane-permeable pyridoxyl-substrate conjugates as inhibitors for human (h) HDC and modeled an active site of hHDC, which is compatible with the experimental data. The most potent inhibitory compound among nine tested structural variants was the pyridoxyl-histidine methyl ester conjugate (PHME), indicating that the binding site of hHDC does not tolerate groups other than the imidazole side chain of histidine. PHME inhibited 60% of the fraction of 12-O-tetradecanoylphorbol-13-acetate-induced newly synthesized HDC in human HMC-1 cells at 200 microM and was also inhibitory in cell extracts. The proposed model of hHDC, containing phosphopyridoxyl-histidine in the active site, revealed the binding specificity of HDC toward its substrate and the structure-activity relationship of the designed and investigated compounds.

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.

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.

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.

[Physiological function mediated by histamine synthesis]

Histamine is involved in a variety of physiologic responses, such as inflammation, type I allergy, gastric acid secretion, and neurotransmission. Previous studies have focused on specific receptors for histamine and histamine release through degranulation, and the regulation of histamine synthesis and its physiologic roles remain to be clarified. We have studied histidine decarboxylase (HDC), the rate-limiting enzyme for mammalian histamine synthesis. Immunocytochemical approaches with an anti-HDC antibody revealed that histamine synthesis occurs in two distinct compartments of mast cells, cytosol and granules, and is regulated by the posttranslational processing of HDC. We also found that histamine synthesis in mast cells is markedly induced by IgE even in the absence of antigens, which may be relevant to enhanced responses of mast cells under allergic conditions. We then developed HDC-deficient mice by gene targeting to investigate the physiologic roles of histamine. We not only confirmed that histamine is essential for type I allergy and stimulates gastric acid secretion, but also found that histamine may regulate the proliferation and differentiation of mast cells. Furthermore, in HDC-deficient mice histamine produced by infiltrated neutrophils can suppress the production of antitumoral cytokines, such as interferon-gamma and tumor necrosis factor-alpha through H2 receptors in the tumor tissues. In this review, we describe recent topics in histamine research, including our results focusing on histamine synthesis and its physiologic roles.

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.

[Regulation of histamine production in macrophages]

Stimulating cells of the mouse macrophage-like cell line RAW 264.7 with the Ca(2+)-ATPase inhibitor thapsigargin increased histamine production. Thapsigargin increased the levels of histidine decarboxylase (HDC) mRNA at 4 h and the expression of 74-kDa HDC protein at 8 h. PD98059, a specific inhibitor of MEK-1 which phosphorylates p44/p42 MAP kinase, strongly suppressed the thapsigargin-induced histamine production, the increase in HDC mRNA level and 74-kDa HDC protein expression. In contrast, SB203580, an inhibitor of p38 MAP kinase, showed only a partial inhibition of histamine production. TPA and LPS also induced histamine production in RAW 264.7 cells, and the histamine production induced by TPA or LPS was also inhibited by PD98059, but the effect of SB203580 was partial. The synthetic glucocorticoid dexamethasone inhibited thapsigargin-induced histamine production, 74-kDa HDC protein expression and the activation of p44/p42 MAP kinases. In conclusion, the increase in histamine production in macrophages stimulated with inflammatory stimulants is due to the increased expression of 74-kDa HDC, which is positively regulated by activated p44/p42 MAP kinases. Dexamethasone inhibits thapsigargin-induced HDC protein expression and histamine production by inhibiting the MAP kinase activation.

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.

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.

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.