Peter Barnes and colleagues show that corticosteroids have a potent inhibitory effect on GATA-3 via two interacting mechanisms that suppress Th2 cytokine expression. This novel mechanism of corticosteroid action may help explain the efficacy of corticosteroids in allergic diseases.
GATA-3 plays a critical role in regulating the expression of the cytokines interleukin (IL)-4, IL-5, and IL-13 from T helper-2 (Th2) cells and therefore is a key mediator of allergic diseases. Corticosteroids are highly effective in suppressing allergic inflammation, but their effects on GATA-3 are unknown. We investigated the effect of the corticosteroid fluticasone propionate on GATA-3 regulation in human T-lymphocytes in vitro and in vivo.
Methods and Findings
In a T lymphocyte cell line (HuT-78) and peripheral blood mononuclear cells stimulated by anti-CD3 and anti-CD28 in vitro we demonstrated that fluticasone inhibits nuclear translocation of GATA-3 and expression of Th2 cytokines via a mechanism independent of nuclear factor-κB and is due, in part, to competition between GATA-3 and the ligand-activated glucocorticoid receptor for nuclear transport through the nuclear importer importin-α. In addition, fluticasone induces the expression of mitogen-activated protein kinase (MAPK) phosphatase-1 (MKP-1), the endogenous inhibitor of p38 MAPK, which is necessary for GATA-3 nuclear translocation. These inhibitory effects of fluticasone are rapid, potent, and prolonged. We also demonstrated that inhaled fluticasone inhibits GATA-3 nuclear translocation in peripheral blood lymphocytes of patients with asthma in vivo.
Corticosteroids have a potent inhibitory effect on GATA-3 via two interacting mechanisms that potently suppress Th2 cytokine expression. This novel mechanism of action of corticosteroids may account for the striking clinical efficacy of corticosteroids in the treatment of allergic diseases.
Please see later in the article for Editors' Summary
The immune system protects the human body from viruses, bacteria, parasites, and fungi. When one of these foreign invaders enters the body, immune system cells called T lymphocytes recognize specific molecules on the invader's surface and release chemical messengers (cytokines) that recruit and activate other types of immune cell, which then attack the invader. Sometimes, however, the immune system responds to a normally harmless material (for example, house-dust mites or grass pollen; scientists call these materials allergens) and triggers an allergic disease such as asthma or hay fever. Contact with an allergen activates a type of T lymphocyte called a T helper-2 (Th2) cell that subsequently makes (expresses) three cytokines called interleukin-4 (IL-4), IL-5, and IL-13. These cytokines ultimately cause inflammation (swelling) of the part of the body exposed to the allergen. Corticosteroids, which suppress the expression of cytokines by Th2 cells, are often used to treat inflammation in allergic diseases. Other treatments for these common conditions—about 50 million people in the US have an allergic disease—include minimizing exposure to allergens and diminishing the response of the immune system to allergens by using various immunotherapies.
Why Was This Study Done?
Scientists know that corticosteroids reduce allergic inflammation by binding to proteins in immune system cells called glucocorticoid receptors. After binding to a corticosteroid, these receptors move into the nucleus of the cell (the part of the cell that contains its genes), where they suppress the expression of certain proinflammatory genes. However, it is still not known how corticosteroids inhibit the expression of Th2 cytokines. A key regulator of the expression of these cytokines and of allergic inflammation is a transcription factor called GATA-3. Transcription factors are proteins that control the expression of other proteins by binding to specific sequences in the genes that encode them. In this study, the researchers try to discover more about how corticosteroids reduce allergic inflammation by investigating the effects of the corticosteroid fluticasone on the regulation of GATA-3 activity in T lymphocytes.
What Did the Researchers Do and Find?
Transcription factors have to move into the nucleus of cells (so-called nuclear translocation) to control the expression of their target genes, so the researchers first asked whether fluticasone affects the cellular localization of GATA-3. Fluticasone treatment of activated T lymphocytes growing in dishes, they report, inhibited the nuclear translocation of GATA-3 and reduced Th2 cytokine expression. Other experiments showed that the inhibition of GATA-3 nuclear translocation was partly caused by competition between the glucocorticoid receptor bound to fluticasone and GATA-3 for binding to importin-α, a protein that is required for nuclear import. However, fluticasone also prevented the nuclear translocation of GATA-3 in a second way. Before GATA-3 can bind to importin-α, phosphate groups have to be added to specific sites in GATA-3. This “phosphorylation” requires an enzyme called p38 MAP kinase, and the researchers found that fluticasone treatment of activated T lymphocytes induced the expression of MAP kinase phophatase-1, a p38 MAP kinase inhibitor. Finally, when the researchers treated seven patients with mild asthma with inhaled fluticasone, they found that fluticasone also inhibited GATA-3 nuclear translocation in the lymphocytes circulating in the patients' blood.
What Do These Findings Mean?
These findings, obtained both in the laboratory and in patients, suggest that corticosteroids inhibit the expression of Th2 cytokines and thus reduce allergic inflammation through two interacting mechanisms. They suggest that corticosteroids prevent the nuclear translocation of GATA-3, a key regulator of Th2 cytokine expression, by competing with GATA-3 for binding to importin-α and by preventing the phosphorylation of GATA-3, a modification that allows GATA-3 to bind to importin-α. This dual mechanism of corticosteroid action may help to explain why these drugs are so effective in the treatment of allergic diseases, although further experiments are needed to show that the lymphocytes resident at sites of allergic inflammation respond to corticosteroids in the same way as lymphocytes in the blood. Finally, these findings suggest that the interaction between phosphorylated GATA-3 and importin-α might be a potential target for new treatments for allergic diseases.
Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1000076.
The US National Institute of Allergy and Infectious Diseases provides information on allergic diseases and a simple description of the immune system
The UK National Health Service Choices service provides information about allergies
Links to other information about allergies are available from MedlinePlus (in English and Spanish)