Inhaled and intranasal
glucocorticoids (GCs) are the most common and effective drugs for controlling symptoms and airway
inflammation in
respiratory diseases such as
allergic rhinitis, chronic
rhinosinusitis with/without
nasal polyps, and
asthma, and the respiratory epithelium is a primary target of GC anti-inflammatory actions. GC effects are mediated through the GC receptor (GR). In humans, one single GR gene gives rise to two main GR products, namely
GRalpha and
GRbeta, which are subject to translational and posttranslational modifications.
GRalpha is expressed in virtually all human cells and tissues, including respiratory epithelial cells, and - at least in vitro - is downregulated by GC.
GRalpha mediates the anti-inflammatory actions of GC by activating transcription of anti-inflammatory genes through binding of
GRalpha to
glucocorticoid response elements (GRE) located in the promoter region of target genes, repressing transcription of proinflammatory genes through direct interaction between
GRalpha and proinflammatory
transcription factors, such as
AP-1 and
NF-kappaB (transrepression), and also by destabilizing the
mRNA of proinflammatory mediators.
GRbeta acts as a dominant negative inhibitor of
GRalpha-mediated transactivation and transrepression in certain in vitro studies with transfected cells. The
GRbeta message is expressed at low levels in numerous tissues and its
protein is mainly expressed in inflammatory cells, although it has also been detected in airway epithelial cells. Increased
GRbeta expression has been reported in
bronchial asthma and nasal polyposis, and after incubation of cells with certain proinflammatory stimuli. However, the role of
GRbeta in modulating GC sensitivity in vivo has been highly debated and is as yet unclear.