Functional characterization of histamine receptor subtypes in a human bronchial epithelial cell line.

Histamine is a well-known mediator eliciting a broad range of responses in different cell types. Four different subtypes of G protein-coupled histamine receptors (H1-H4) have been cloned and pharmacologically characterized. However, involvement of the different histamine receptor subtypes in immunomodulatory functions of bronchial epithelium has only been investigated marginally. The expression and function of histamine receptor subtypes on the human bronchial epithelial cell line BEAS-2B was analyzed by PCR, intracellular Ca++ -measurements and ELISA. We show mRNA expression of the histamine receptor subtypes H1, H2, and H3, but not H4 in the human bronchial epithelial cell line BEAS-2B. Using intracellular Ca++ -measurements, we demonstrated functional expression of the H1 and H3 receptors. To characterize the biological properties of histamine in airway epithelial biology, we also investigated its effects on cytokine secretion by BEAS-2B cells. Thereby, we were able to show up-regulation of the proinflammatory mediators IL-6 and CXCL8/ IL-8 via activation of the H1, H2 and H3 receptor subtypes. The Th1 cytokines CXCL9/MIG and CXCL10/IP-10 and the chemokine CCL5/RANTES were regulated in a distinct manner: Whereas histamine inhibited the IFN-gamma/TNF-alpha-induced secretion of MIG via the histamine receptor subtypes H1, H2, and H3, the histamine-induced suppression of RANTES was due to activation of the H2 and H3 receptors, while reduction of cytokine-triggered IP-10 secretion was mediated only by triggering the H2 receptor. In summary our data provide evidence that histamine released during allergic lung diseases exerts regulatory influence on airway epithelial cells.

The serotoninergic receptors of human dendritic cells: identification and coupling to cytokine release.

The neurotransmitter 5-hydroxytryptamine (5-HT), commonly known as serotonin, is stored at peripheral sites in mast cells and released from this peripheral source upon IgE cross-linking. In this study, we investigated the expression of serotoninergic receptors (5-HTR), the signaling pathway, and biological activity of 5-HT on human dendritic cells (DC), showing that immature and mature DC expressed mRNA for different serotoninergic receptors. Thereby, the mRNA of 5-HTR(1B), 5-HTR(1E), 5-HTR(2A), 5-HTR(2B), one splicing variant of the 5-HTR(3), 5-HTR(4), and 5-HTR(7) receptors were detected. Immature DC preferentially expressed mRNA for the heptahelical 5-HTR(1B), 5-HTR(1E), and 5-HTR(2B) receptors, while mature DC mostly expressed 5-HTR(4) and 5-HTR(7). The mRNA expression level of the ligand-gated cation channel 5-HTR(3) and the heptahelical 5-HTR(2A) did not significantly change during maturation. Isotype-selective receptor agonists allowed us to show that 5-HT stimulated 5-HTR(3)-dependent Ca(2+) influx in immature and mature DC. Moreover, we revealed that 5-HTR(1) and 5-HTR(2) receptor stimulation induced intracellular Ca(2+) mobilization via G(i/o) proteins in immature, but not mature, DC. Activation of 5-HTR(4) and 5-HTR(7) induced cAMP elevation in mature DC. Functional studies indicated that activation of 5-HTR(4) and 5-HTR(7) enhanced the release of the cytokines IL-1beta and IL-8, while reducing the secretion of IL-12 and TNF-alpha in mature DC. In summary, our study shows that 5-HT stimulated, in a maturation-dependent manner, different signaling pathways in DC. These data point to a role for 5-HT in regulating the immune response at peripheral sites.