Biomedical Engineering Reference
In-Depth Information
The other theory relates to phosphorylation of GR. The effects of
IL-2
=
IL-4 on GR ligand binding and dexamethasone regulation of IL-10
release are reversed by the p38 MAPK inhibitor SB203580 (64). Tumor
necrosis factor-
a
as well as IL-2
=
IL-4 activates p38 MAPK (67) and this
results in serine phosphorylation of GR and reduced dexamethasone repres-
sion of LPS-stimulated granulocyte-macrophage colony-stimulating factor
(GM-CSF) release (64). These data show that p38 MAPK inhibitors may
have the potential to reverse glucocorticoid insensitivity and reestablish
the beneficial effects of glucocorticoids in patients with severe asthma and
COPD. It is unclear whether this is a direct or indirect effect of p38 MAPK
or whether GR phosphorylation alters ligand-binding affinity directly. This
may result from either a change in GR conformation due to association of
distinct cofactors, or partial blocking of the ligand-binding domain, due to
association of GR with nuclear transcriptional modulating proteins. Similar
results have been seen following NO treatment of GR, whereby nitrosyla-
tion of GR at an hsp90 interaction site modifies ligand binding (63). Serine
226 and the sequences immediately surrounding it are highly conserved
suggesting that its phosphorylation may alter or disrupt the protein-protein
interactions regulating GR action.
C. Cross-Talk with Transcription Factors
The major anti-inflammatory effects of corticosteroids are thought to be due
to repression of inflammatory and immune genes. The inhibitory effect of
corticosteroids is due largely to protein-protein-complex interactions
between activated GR and transcription factors, such as NF-
k
Band
AP-1, which mediate the expression of these inflammatory genes (Fig. l).
The interplay between proinflammatory transcription factors and GR
may reflect differing effects on histone acetylation
=
deacetylation (38).
There is an increase in the basal levels of AP-1 DNA binding in the
nuclei from CR patients, although no differences in the sequences of c-fos
and c-jun mRNA are detectable (58). These results suggested that AP-1
was altered in CR patients and that increased levels of AP-1 might inhibit
GR function. In a subsequent study using nuclear run-on, RT-PCR and
Western blotting, a 2-4-fold greater increase in the c-fos transcription and
mRNA and protein expression in PBMCs isolated from CR compared with
corticosteroid-sensitive (CS) asthmatics and normal subjects were demon-
strated (68). When cells were stimulated with PMA, the time- and concen-
tration-dependent induction of c-Fos was greater in the CR group.
Overexpression of c-Fos induced by stimulation of PBMCs derived from
CS subjects PMA for 6 hr, attenuated the ability of these cells to induce
GR-GRE binding after l hr dexamethasone treatment. In these experi-
ments, GR-GRE binding was reduced to levels similar to that seen in
CR subjects. Incubation of PBMCs derived from CR subjects with
