Synergistic effects of cyclic AMP and Ca2+ ionophore A23187 on de novo synthesis of histidine decarboxylase in mastocytoma P-815 cells

Expression of Histidine Decarboxylase and Its Roles in Inflammation

Histamine is a well-known mediator of inflammation that is released from mast cells and basophils. To date, many studies using histamine receptor antagonists have shown that histamine acts through four types of receptors: H1, H2, H3, and H4. Thus, histamine plays more roles in various diseases than had been predicted. However, our knowledge about histamine-producing cells and the molecular mechanisms underlying histamine production at inflammatory sites is still incomplete. The histamine producing enzyme, histidine decarboxylase (HDC), is commonly induced at inflammatory sites during the late and chronic phases of both allergic and non-allergic inflammation. Thus, histamine levels in tissues are maintained at effective concentrations for hours, enabling the regulation of various functions through the production of cytokines/chemokines/growth factors. Understanding the regulation of histamine production will allow the development of a new strategy of using histamine antagonists to treat inflammatory diseases.

Cytosolic phospholipase A(2)α and eicosanoids regulate expression of genes in macrophages involved in host defense and inflammation

The role of Group IVA cytosolic phospholipase A₂ (cPLA₂α) activation in regulating macrophage transcriptional responses to Candida albicans infection was investigated. cPLA₂α releases arachidonic acid for the production of eicosanoids. In mouse resident peritoneal macrophages, prostacyclin, prostaglandin E₂ and leukotriene C₄ were produced within minutes of C. albicans addition before cyclooxygenase 2 expression. The production of TNFα was lower in C. albicans-stimulated cPLA₂α^+/+ than cPLA₂α^-/- macrophages due to an autocrine effect of prostaglandins that increased cAMP to a greater extent in cPLA₂α^+/+ than cPLA₂α^-/- macrophages. For global insight, differential gene expression in C. albicans-stimulated cPLA₂α^+/+ and cPLA₂α^-/- macrophages (3 h) was compared by microarray. cPLA₂α^+/+ macrophages expressed 86 genes at lower levels and 181 genes at higher levels than cPLA₂α^-/- macrophages (≥2-fold, p<0.05). Several pro-inflammatory genes were expressed at lower levels (Tnfα, Cx3cl1, Cd40, Ccl5, Csf1, Edn1, CxCr7, Irf1, Irf4, Akna, Ifnγ, several IFNγ-inducible GTPases). Genes that dampen inflammation (Socs3, Il10, Crem, Stat3, Thbd, Thbs1, Abca1) and genes involved in host defense (Gja1, Csf3, Trem1, Hdc) were expressed at higher levels in cPLA₂α^+/+ macrophages. Representative genes expressed lower in cPLA₂α^+/+ macrophages (Tnfα, Csf1) were increased by treatment with a prostacyclin receptor antagonist and protein kinase A inhibitor, whereas genes expressed at higher levels (Crem, Nr4a2, Il10, Csf3) were suppressed. The results suggest that C. albicans stimulates an autocrine loop in macrophages involving cPLA₂α, cyclooxygenase 1-derived prostaglandins and increased cAMP that globally effects expression of genes involved in host defense and inflammation.

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.

Crucial role of histamine for regulation of gastric acid secretion ascertained by histidine decarboxylase-knockout mice

Histidine decarboxylase (HDC) represents the sole enzyme that produces histamine in the body. The present work investigated the role of endogenous histamine in carbachol- and gastrin-induced gastric acid secretion with HDC-knockout (HDC-/-) mice. Acid secretion was measured in either mice subjected to acute fistula production under urethane anesthesia or conscious mice that had previously undergone pylorus ligation. In wild-type mice, carbachol and gastrin significantly stimulated acid secretion, increasing gastric mucosal histamine. In contrast, in HDC-/- mice, carbachol and gastrin had little impact when either delivered alone or together. Nonetheless, the two agents achieved a synergistic effect when delivered together with exogenous histamine, stimulating acid secretion in HDC-/- mice. Such synergism was abolished by the histamine H2-receptor antagonist famotidine. cAMP involvement in acid secretion was also examined with theophylline, a phosphodiesterase inhibitor, and forskolin, an adenylate cyclase activator. In wild-type mice, theophylline significantly increased acid secretion, enhancing carbachol- and gastrin-stimulated acid secretion. In contrast, in HDC-/- mice, theophylline failed to exert an effect on basal acid secretion, as well as carbachol- and gastrin-stimulated acid secretion. Although forskolin interacted with carbachol, allowing acid secretion in HDC-/- mice, similar results were not achieved with gastrin. Such results suggest that 1) histamine is essential for carbachol- and gastrin-stimulated gastric acid secretion in mice; and 2) histamine-induced cAMP production contributes to the in vivo response to carbachol or gastrin.

Release of non-mast cell histamine from rat aorta

We previously reported that A23187 induces release of histamine from bovine intrapulmonary vein and provided pharmacological evidence against an involvement of mast cells as the source of histamine. This study was conducted to test more definitively the hypothesis that histamine is released from non-mast cell sources in blood vessels. The effects of A23187 on release of histamine were determined using rat aorta which does not contain mast cells. Aortic rings were mounted for recording of isometric tension, and following exposure to A23187 or vehicle, histamine in the bathing media was measured using enzyme immunoassay. A23187 (100 nmol/l - 10 micromol/l) induced concentration-related release of histamine from rings with endothelium. The accumulation of histamine in the bathing media induced by 10 microM A23187 reached plateau at 60 min (6.2 +/- 1.1 pmol/mg) and was markedly and significantly higher than vehicle control (0.4 +/- 0.1 pmol/mg, p < 0.05). Destruction of endothelium significantly inhibited A23187-induced histamine release (5.5 +/- 1.5 pmol/mg with endothelium, 1.1 +/- 0.3 pmol/mg without endothelium, p < 0.05). The results demonstrate that A23187 induces release of histamine from rat aorta which does not contain mast cells and that the release of histamine is largely dependent on the presence of endothelium.

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.

Pharmacological analysis of the inflammatory exudate-induced histamine production in bone marrow cells

The inflammatory exudate at the post-anaphylaxis phase of allergic inflammation in rats has an ability to enhance histamine production by bone marrow cells. To analyze the mechanism of the inflammatory exudate-induced histamine production pharmacologically, the effects of several drugs were examined in cultures of bone marrow cells. Incubation of the bone marrow cells in the presence of the inflammatory exudate that had been centrifuged and dialyzed against Hanks' balanced salt solution increased histidine decarboxylase activity in the cells and histamine concentration in the conditioned medium. The induction of histamine production by the inflammatory exudate was inhibited by actinomycin D (0.01-1 microM), an inhibitor of RNA synthesis, and cycloheximide (0.1-10 microM), a protein synthesis inhibitor. The protein kinase C inhibitors staurosporine (2-20 nM), K-252a (6-200 nM), and H-7 (10.3-103 microM) also inhibited the inflammatory exudate-induced histamine production in a concentration-dependent manner. The tyrosine kinase inhibitor genistein (3.7-37 microM) also inhibited the inflammatory exudate-induced histamine production, but the protein kinase A inhibitor H-89 (0.2 microM), and the adenylate cyclase activator forskolin (0.1 microM) showed no effect. These findings suggest that histamine production induced by the inflammatory exudate is mediated by the de novo synthesis of histidine decarboxylase and by the activation of protein kinase C and tyrosine kinase.

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.