Pro-apoptotic effect of high concentrations of histamine on human neutrophils

Modulation of Dendritic Cell Apoptosis and CD8+ Cytotoxicity by Histamine: Role of Protein Kinase C

Dendritic cells (DC) are able to present extracellular antigens associated with the molecules of the major histocompatibility complex class I. In a previous work, we demonstrated that the histamine (HIS), acting through H1/H4 receptors, increases the cross-presentation of soluble ovalbumin by murine DC and can enhance the recruitment of specific CD8⁺ T lymphocytes during the development of chronic inflammatory responses. Here, we studied in more depth the mechanisms underlying this enhancement. We showed that the cytotoxicity of specific CD8⁺ lymphocytes is increased in HIS-treated DC and it is lost by inhibition of vacuolar-ATPase that prevents endosome acidification. It is known that HIS acts through G protein-coupled receptors. The H1/H4 receptors are associated with a G_q subunit, which involves PKC signaling, a pathway related to the apoptotic process. Interestingly, we demonstrated for the first time that HIS prevents DC apoptosis induced by heat shock through the inhibition of caspase-3, a mechanism dependent on PKC activation, since it is reversed by its inhibition. By contrast, cytolytic activity of T lymphocytes induced by HIS-stimulated DC was independent of PKC pathway.

Mutations in the histamine N-methyltransferase gene, HNMT, are associated with nonsyndromic autosomal recessive intellectual disability

Histamine (HA) acts as a neurotransmitter in the brain, which participates in the regulation of many biological processes including inflammation, gastric acid secretion and neuromodulation. The enzyme histamine N-methyltransferase (HNMT) inactivates HA by transferring a methyl group from S-adenosyl-l-methionine to HA, and is the only well-known pathway for termination of neurotransmission actions of HA in mammalian central nervous system. We performed autozygosity mapping followed by targeted exome sequencing and identified two homozygous HNMT alterations, p.Gly60Asp and p.Leu208Pro, in patients affected with nonsyndromic autosomal recessive intellectual disability from two unrelated consanguineous families of Turkish and Kurdish ancestry, respectively. We verified the complete absence of a functional HNMT in patients using in vitro toxicology assay. Using mutant and wild-type DNA constructs as well as in silico protein modeling, we confirmed that p.Gly60Asp disrupts the enzymatic activity of the protein, and that p.Leu208Pro results in reduced protein stability, resulting in decreased HA inactivation. Our results highlight the importance of inclusion of HNMT for genetic testing of individuals presenting with intellectual disability.

Perivascular mast cells promote atherogenesis and induce plaque destabilization in apolipoprotein E-deficient mice

BACKGROUND

Mast cells are major effector cells in allergy and host defense responses. Their increased number and state of activation in perivascular tissue during atherosclerosis may point to a role in cardiovascular disorders. In the present study, we investigated the contribution of perivascular mast cells to atherogenesis and plaque stability in apolipoprotein E-deficient mice.

METHODS AND RESULTS

We show here that episodes of systemic mast cell activation during plaque progression in mice leads to robust plaque expansion. Targeted activation of perivascular mast cells in advanced plaques sharply increases the incidence of intraplaque hemorrhage, macrophage apoptosis, vascular leakage, and CXCR2/VLA-4-mediated recruitment of leukocytes to the plaque. Importantly, treatment with the mast cell stabilizer cromolyn does prevent all the adverse phenomena elicited by mast cell activation.

CONCLUSIONS

This is the first study to demonstrate that mast cells play a crucial role in plaque progression and destabilization in vivo. We propose that mast cell stabilization could be a new therapeutic approach to the prevention of acute coronary syndromes.

Histamine reverses IL-5-afforded human eosinophil survival by inducing apoptosis: pharmacological evidence for a novel mechanism of action of histamine

Eosinophils are essential inflammatory cells in the pathogenesis of asthma and atopic conditions. Histamine, released from mast cells and basophils in response to allergen exposure, is a critical mediator in the allergic response. Histamine exerts its effects via four unequivocally characterized histamine receptors, H(1-4). Several functions of eosinophils have previously been shown to be stimulated by histamine. However, its effects on eosinophil apoptosis are unknown. The aim of the present study was to resolve the effects of histamine on constitutive apoptosis of human eosinophils and on the survival-enhancing action of interleukin (IL)-5. Additional experiments were conducted to elucidate the histamine receptor(s) involved in any response seen and the associated signal transduction cascade. Human isolated peripheral blood eosinophils were cultured in the absence or presence of histamine, IL-5 and receptor antagonists/agonists or mediator inhibitors/analogues. Apoptosis was assessed by measuring the relative DNA content of propidium iodide (PI)-stained cells and the effects were confirmed by morphological analysis with bright field microscopy. Caspase activities were assessed by using commercial Caspase-Glo 3/7, 8 and 9 luminescence assays. Histamine (10-100 microM) partially reversed IL-5-induced human eosinophil survival by enhancing apoptosis as assessed by measuring the relative DNA content of PI-stained cells. This effect was not mediated through any of the known histamine receptors or through non-specific activation of 5-hydroxytryptamine receptors or alpha-adrenoceptors. Moreover, the reversal of IL-5-inhibited eosinophil apoptosis by histamine seemed not to utilize the conventional intracellular second-messenger pathways including cyclic AMP, protein kinase A or phospholipase C. Inhibition of caspase 6 and caspases 1, 10 or 12 reversed the effects of histamine but also inhibited apoptosis in general. In conclusion, the data presented herein indicate that histamine induces human eosinophil apoptosis in the presence of a survival-prolonging cytokine by a mechanism that does not apparently involve the activation of any of the currently known histamine receptor subtypes. The possibility exists that another, as yet unidentified, histamine receptor may exist in human eosinophils that regulates survival, although the participation of histamine receptor-independent mechanisms cannot be excluded.

Ethanol induces apoptotic death of developing beta-endorphin neurons via suppression of cyclic adenosine monophosphate production and activation of transforming growth factor-beta1-linked apoptotic signaling

The mechanism by which ethanol induces beta-endorphin (beta-EP) neuronal death during the developmental period was determined using fetal rat hypothalamic cells in primary cultures. The addition of ethanol to hypothalamic cell cultures stimulated apoptotic cell death of beta-EP neurons by increasing caspase-3 activity. Ethanol lowered the levels of adenylyl cyclase (AC)7 mRNA, AC8 mRNA, and/or cAMP in hypothalamic cells, whereas a cAMP analog blocked the apoptotic action of ethanol on beta-EP neurons. The AC inhibitor dideoxyadenosine (DDA) increased cell apoptosis and reduced the number of beta-EP neurons, and it potentiated the apoptotic action of ethanol on these neurons. beta-EP neurons in hypothalamic cultures showed immunoreactivity to transforming growth factor-beta1 (TGF-beta1) protein. Ethanol and DDA increased TGF-beta1 production and/or release from hypothalamic cells. A cAMP analog blocked the activation by ethanol of TGF-beta1 in these cells. TGF-beta1 increased apoptosis of beta-EP neurons, but it did not potentiate the action of ethanol or DDA actions on these neurons. TGF-beta1 neutralizing antibody blocked the apoptotic action of ethanol on beta-EP neurons. Determination of TGF-beta1-controlled cell apoptosis regulatory gene levels in hypothalamic cell cultures and in isolated beta-EP neurons indicated that ethanol, TGF-beta1, and DDA similarly alter the expression of these genes in these cells. These data suggest that ethanol increases beta-EP neuronal death during the developmental period by cellular mechanisms involving, at least partly, the suppression of cAMP production and activation of TGF-beta1-linked apoptotic signaling.