Intracellular Signaling Section

Michael A Beaven, B Pharmacy, PhD, Principal Investigator

Mast cells are best known for their role in IgE-dependent (atopic) allergic diseases which include allergic rhinitis, asthma, atopic dermatitis, and anaphylactic reactions (1). Mast cells are uniquely capable of performing this role because they express several hundred thousand high affinity receptors for IgE (FcεR1) and thus respond to IgE-directed antigens. However, mast cells also express the pathogen-recognizing Toll-like receptors (TLRs) which probably account for the ability of mast cells to mount an effective innate immune response to lethal bacterial infections. Activated mast cells release an array of potent inflammatory mediators by several mechanisms that include degranulation with release of preformed mediators, the generation of inflammatory lipids (eicosanoids) from arachidonic acid, and robust production of numerous cytokines (Figure 1). Among the pharmacological options for treatment of mast cell-related inflammatory diseases, the glucocorticoids remain the most effective drugs available but they have undesirable side effects (1).

Our objective is to elucidate the biochemical signaling pathways that mediate the various responses of mast cells to antigen or TLR ligands.  A complementary objective is to understand how therapeutic agents, especially the glucocorticoids, interrupt signaling processes so as to provide a rational basis for design of more selective agents. In fact, the mast cell is a pre-eminent model for such studies. Most, if not all, known signaling processes can be activated through the various receptors that are expressed on mast cells, some of which act in synergy with FcεRI such as adenosine receptors and Kit (2-4). This permits studies of the influence of other physiologic factors on antigen-induced responses.

Past studies highlighted the role of phosphoinositides (5), calcium (6), and
protein kinase C (7) in degranulation and that of MAP kinases in the generation of arachidonic acid (8, 9). More recent studies have focused on the signaling pathways that lead to downstream activation of transcription factors and production of cytokines (4) and on the inhibition of these events by glucocorticoids through induction of inhibitory regulators (10) (Figure 2). Also, we have revisited a perplexing but key enzyme, phospholipase D, with respect to its activation (11) and its role in degranulation (12). Most recent studies have revealed how TLR-ligands act as potent adjuvants to IgE-mediated reactions which may account for the exacerbation of allergic diseases by infection (13).

Figure 1. Mast cells are activated primarily through the IgE receptor (FceRI) by allergens to cause rapid release of granules (stained blue) that contain histamine, potent proteases, and proteoglycans such as heparin. Other responses include rapid production of arachidonic acid-derived prostaglandins and leukotrienes. At later stages, numerous inflammatory cytokines and chemokines are produced as a result of gene transcription. As we have shown, these responses are markedly augmented by pathogenic ligands of Toll-like receptors (TLR) and the growth factor, Kit ligand also known as stem cell factor.

Figure 2. Glucocorticoids act through the glucocorticoid receptor which may remain as a monomer and thereby interact with transcription factors to inhibit transcription of cytokine genes (transrepression) or they can dimerize and thereby interact with glucocorticoid response elements (GRE) to induce transcription of genes (transactivation) for metabolic enzymes and, as we have now shown, inhibitory regulators of signaling processes in mast cells. The latter observation has broadened our understanding of the mechanisms of action of glucocorticoids in allergic disease.

References

1. Beaven, M. A. and T. R. Hundley. 2003. Mast cell related diseases: Genetics, signaling pathways, and novel therapies. In ASignal Transduction and Human Disease@ Eds  T. Finkel and J. S. Gutkind. John Wiley & Sons, Hoboken, NJ. pp307-355
   
   
2. Ali, H., O. H. Choi, K. Yamada, H. M. S. Gonzaga, and M. A. Beaven. 1996. Sustained activation of phospholipase D via adenosine A3 receptors is associated with enhancement of antigen- and Ca2+-ionophore- induced secretion in a rat mast cell line. J. Pharmacol. Exptl. Therap. 276:837-845
   
   
3. Beaven, M. A., and R. A. Baumgartner.  1996.  Downstream signals initiated in mast cells by FcεRI and other receptors.  Curr. Opin. Immunol. 8:766-772
   
   
4. Hundley, T. R., A. M. Gilfillan, C. Tkaczyk, M. V. Andrade, D. D. Metcalfe, and M. A. Beaven. 2004.  Kit and FcεRI mediate unique and convergent signals for release of inflammatory mediators from human mast cells. Blood, 104:2410-2417
   
   
5. Beaven, M. A., J. P. Moore, G. A. Smith, T. R. Hesketh, and J. C. Metcalfe.  1984.  The calcium signal and phosphatidylinositol breakdown in 2H3 cells.  J. Biol. Chem. 259:7137‑7142
   
   
6. 6) Beaven, M. A., J. Rogers, J. P. Moore, T. R. Hesketh, G. A. Smith, and J. C. Metcalfe.  1984.  The mechanism of the calcium signal and correlation with histamine release in 2H3 cells.  J. Biol. Chem. 259:7129‑7136
   
   
7. Ozawa, K., Z. Szallasi, M. G. Kazanietz, P. M. Blumberg, H. Mischak, J. F. Mushinski, and M. A. Beaven.  1993.  Ca2+‑Dependent and Ca2+‑independent isozymes of protein kinase C mediate exocytosis in antigen‑stimulated rat basophilic RBL‑2H3 cells: Reconstitution of secretory responses with Ca2+ and purified isozymes in washed permeabilized cells. J. Biol. Chem. 268:1749‑1756
   
   
8. Hirasawa, N., F. Santini, and M. A. Beaven.  1995.  Activation of the mitogen‑activated protein kinase/cytosolic phospholipase A2 pathway in a rat mast cell line by receptor‑dependent and receptor‑independent stimulants‑Indications of different pathways for release of arachidonic acid and secretory granules.  J. Immunol. 154:5391-5402.
   
   
9. Zhang, C., R. A. Baumgartner, K. Yamada, and M. A. Beaven. 1997. Mitogen
   Activated Protein (MAP) Kinase Regulates Production of TNFa and Release of
   Arachidonic Acid in Mast Cells: Indications of Communication between p38 and p42
   MAP kinases. J. Biol. Chem. 272: 13397-13402
   
   
10. Hiragun, T. Peng, Z., and M. A. Beaven. 2005. Dexamethasone up-regulates the inhibitory adaptor protein Dok-1 and suppresses downstream activation of mitogen-activated protein kinase pathway in antigen-stimulated RBL-2H3 mast cells. Mol. Pharmacol. 67: 598-603
    
    
11. Choi, W. S., T. Hiragun, J. H. Lee, Y. M. Kim, H.-Y. Kim, A. Chahdi, E. Her, J. W. Han, and M. A. Beaven. 2004. Activation of RBL-2H3 mast cells is dependent on tyrosine phosphorylation of phospholipase D2 by Fyn and Fgr. Mol. Cell. Biol. 24: 6980-6992
    
    
12. Choi, W. S., Y. M. Kim, C. Combs, M. A. Frohman, and M. A. Beaven. 2002. Phospholipase D1 and D2 regulate different phases of exocytosis in mast cells. J. Immunol. 168:5682-5689
    
    
13. Qiao, H., Andrade, M. V., Lisboa, F. A., Morgan, K., and Beaven, M. A. 2006. FceRI and toll-like receptors mediate synergistic signals to markedly augment production of inflammatory cytokines in murine cells. Blood 107: 610-618