Journal: Redox Biology
Article Title: Cellular adaptation to xenobiotics: Interplay between xenosensors, reactive oxygen species and FOXO transcription factors
Figure Lengend Snippet: Xenosensor activation by xenobiotics (Xb): simplified schematic representation. ( a,b ) Human pregnane X receptor (PXR) as well as retinoid X receptor (RXR) alpha appear to occur predominantly in the nucleus in unstimulated cultured human cells  ,  , whereas constitutive androstane receptor (CAR) occurs mostly cytosolic  . Upon stimulation by exposure to xenobiotics (i.e. upon ligand binding), both PXR and CAR stimulate the expression of target genes by binding to their respective response elements (PXRE, PBRE) as heterodimers with RXRα. Nuclear translocation of CAR is regulated by its (de)phosphorylation: protein kinase C-dependent phosphorylation induces its cytoplasmic retention  , which is further supported by receptor tyrosine kinase (RTK)-dependent effects. For example, epidermal growth factor (EGF) induces, via activation of ERK, the formation of a CAR homodimer which prevents CAR dephosphorylation and thus helps retain it in the cytoplasm  . CAR ligands bind to the monomer, and by shifting the dimer/monomer equilibrium accordingly support CAR dephosphorylation, followed by nuclear translocation  . Another type of CAR activator, phenobarbital (PB) stimulates CAR indirectly, e.g. by interfering with RTK signaling: both EGF and insulin binding to their cognate receptors are attenuated in the presence of PB  ,  , thus counteracting the CAR inactivation elicited by EGF or insulin. ( c ) Like PXR and CAR, peroxisome proliferator-activated receptors (PPARs) form heterodimers with RXR. RXR/PPAR heterodimers bind to PPRE (peroxisome proliferator response element) sites to stimulate transcription of target genes. Although PPARs were described as predominantly residing in the nucleus, certain stimuli were reported to elicit nuclear exclusion  . Moreover, nucleocytoplasmic shuttling and subcellular localization of PPARs was shown to be affected by PPAR ligands  . ( d ) Aryl hydrocarbon receptor (AhR) is retained in the cytoplasm in a complex containing heat shock protein (hsp) 90 as well as AhR inhibitory protein (AIP) and p23. Ligand binding releases AhR from this complex, followed by its nuclear translocation. AhR forms a heterodimer with AhR nuclear translocator (ARNT) inside the nucleus; the dimer then binds to xenobiotic response elements to stimulate transcription of target genes  . ( e ) Stimulation of Nrf2 transcriptional activity by xenobiotics is through interaction of xenobiotics with Keap-1, releasing Nrf2 from a complex with Keap-1 (see legend to Fig. 5 ). Following nuclear translocation, Nrf2 forms heterodimers, e.g. with small musculoaponeurotic fibrosarcoma (sMaf) proteins, that bind to antioxidant response elements (ARE) (also termed electrophile response element, EpRE) and regulate transcription  .
Article Snippet: Xenobiotics that affect Keap-1/Nrf2 interaction, leading to Nrf2 activation, are usually compounds interacting with thiols, such as (i) electrophiles capable of modifying one of the Keap-1 cysteine residues, (ii) compounds metabolized to such electrophiles, (iii) certain metal(loid) ions or (iv) compounds that cause the cellular generation of reactive oxygen or nitrogen species.
Techniques: Activation Assay, Cell Culture, Ligand Binding Assay, Expressing, Binding Assay, Translocation Assay, De-Phosphorylation Assay, Activity Assay