Glucocorticosteroids: current and future directions

Abstract

Glucocorticoids are the most effective anti-inflammatory therapy for asthma yet are relatively ineffective in chronic obstructive pulmonary disease. Glucocorticoids suppress inflammation via several molecular mechanisms. Glucocorticoids suppress the multiple inflammatory genes that are activated in chronic inflammatory diseases, such as asthma, by reversing histone acetylation of activated inflammatory genes through binding of ligand-bound glucocorticoid receptors (GR) to co-activator molecules and recruitment of histone deacetylase-2 to the activated inflammatory gene transcription complex (trans-repression). At higher concentrations of glucocorticoids GR homodimers interact with DNA recognition sites to activate transcription through increased histone acetylation of anti-inflammatory genes and transcription of several genes linked to glucocorticoid side effects (trans-activation). Glucocorticoids also have post-transcriptional effects and decrease stability of some pro-inflammatory mRNA species. Decreased glucocorticoid responsiveness is found in patients with severe asthma and asthmatics who smoke, as well as in all patients with chronic obstructive pulmonary disease. Several molecular mechanisms of glucocorticoid resistance have now been identified which involve post-translational modifications of GR. Histone deacetylase-2 is markedly reduced in activity and expression as a result of oxidative/nitrative stress so that inflammation becomes resistant to the anti-inflammatory actions of glucocorticoids. Dissociated glucocorticoids and selective GR modulators which show improved trans-repression over trans-activation effects have been developed to reduce side effects, but so far it has been difficult to dissociate anti-inflammatory effects from adverse effects. In patients with glucocorticoid resistance alternative anti-inflammatory treatments are being investigated as well as drugs that may reverse the molecular mechanisms of glucocorticoid resistance.

Figure 1

Glucocorticoid activation of anti-inflammatory gene expression. Glucocorticoids bind to cytoplasmic glucocorticoid receptors (GR) which translocate to the nucleus where they bind to glucocorticoid response elements (GRE) in the promoter region of steroid-sensitive genes and also directly or indirectly to co-activator molecules such as CREB-binding protein (CBP), p300/CBP activating factor (pCAF) or steroid receptor coactivator-2 (SRC-2), which have intrinsic histone acetyltransferase (HAT) activity, causing acetylation of lysines on histone H4, which leads to activation of genes encoding anti-inflammatory proteins, such as secretory leukoprotease inhibitor (SLPI), mitogen-activated kinase phosphatase-1 (MKP-1), inhibitor of nuclear factor κB (IκB-α) and glucocorticoid-induced leucine zipper protein (GILZ).

Figure 2

Glucocorticoids regulate gene expression in several ways. Glucocorticoids enter the cell to bind to glucocorticoid receptors (GR) in the cytoplasm that translocate to the nucleus. GR homodimers bind to glucocorticoid-response elements (GRE) in the promoter region of steroid-sensitive genes, which may encode anti-inflammatory proteins. Less commonly, GR homodimers interact with negative GREs to suppress genes. Nuclear GR also interact with co-activator molecules, such as CREB-binding protein (CBP), which is activated by pro-inflammatory transcription factors, such as nuclear factor-κB (NF-κB), thus switching off the inflammatory genes that are activated by these transcription factors. Other abbreviations: CRF, corticotrophin releasing factor; GILZ, glucocorticoid-induced leucine zipper protein; IκB-α, inhibitor of NF-κB; MKP-1, mitogen-activated kinase phosphatase-1; POMC, proopiomelanocortin; SLPI, secretory leukoprotease inhibitor.

Figure 3

Glucocorticoid suppression of activated inflammatory genes. Inflammatory genes are activated by inflammatory stimuli, such as interleukin-1β (IL-1β) or tumour necrosis factor-α (TNF-α), resulting in activation of IKKβ (inhibitor of I-κB kinase-β), which activates the transcription factor nuclear factor κB (NF-κB). A dimer of p50 and p65 NF-κB proteins translocates to the nucleus and binds to specific κB recognition sites and also to co-activators, such as CREB-binding protein (CBP) or p300/CBP-activating factor (pCAF), which have intrinsic histone acetyltransferase (HAT) activity. This results in acetylation of core histone H4, resulting in increased expression of genes encoding multiple inflammatory proteins. Glucocorticoid receptors (GR) after activation by glucocorticoids translocate to the nucleus and bind to co-activators to inhibit HAT activity directly and recruiting histone deacetylase-2 (HDAC2), which reverses histone acetylation leading in suppression of these activated inflammatory genes.

Figure 4

Acetylation of glucocorticoid receptors (GR). Binding of a glucocorticoids to GR results in its acetylation by histone acetyltransferases (HAT), such as CREB-binding protein (CBP), and a dimer of acetylated GR then binds to glucocorticoid response elements (GRE) to activate or suppress genes (such as side effect genes). Deacetylation of GR by histone deacetylase-2 (HDAC2) is necessary for GR to interact with CBP and inhibit nuclear factor-κB (NF-κB) to switch off inflammatory genes. GM-CSF, granulocyte-macrophage colony stimulating factor; IL, interleukin.

Figure 5

Inhibition of p38 mitogen-activated protein (MAP) kinase by glucocorticoids. p38 MAP kinase is activated by inflammatory stresses though activation of MAP kinase kinase (MKK)-3 and -6. p38 phosphorylates (P) MAP kinase-activated protein kinase(MAPKAPK)-2, which plays a role in stabilizing messenger RNA (mRNA) encoding several inflammatory proteins, such as tumour necrosis factor-α (TNF-α), interleukin(IL)-1β, IL-6, IL-8, granulocyte-macrophage colony stimulating factor (GM-CSF) and cyclo-oxygenase(COX)-2. This mRNA is characterized by AU-rich elements (ARE) in the 3'-untranslated region, which make the mRNA unstable and rapidly degraded. ARE-binding proteins (AREBP) stabilize these proteins and may be activated (probably indirectly) by MAPKAPK-2. Corticosteroids induce the expression of MAP kinase phosphatase (MKP)-1, which inhibits p38 and thus prevents the stabilization of multiple inflammatory proteins.