Nuclear export of retinoid X receptor alpha in response to interleukin-1beta-mediated cell signaling: roles for JNK and SER260.
As the obligate heterodimer partner to class II nuclear receptors, the retinoid X receptor alpha (RXRalpha) plays a vital physiological role in the regulation of multiple hepatic functions, including bile formation, intermediary metabolism, and endobiotic/xenobiotic detoxification. Many RXRalpha-regulated genes are themselves suppressed in inflamed liver via unknown mechanisms, which constitute a substantial component of the negative hepatic acute phase response. In this study we show that RXRalpha, generally considered a stable nuclear resident protein, undergoes rapid nuclear export in response to signals initiated by the pro-inflammatory cytokine interleukin-1beta (IL-1beta), a central activator of the acute phase response. Within 30 min of exposure to IL-1beta, nuclear levels of RXRalpha are markedly suppressed in human liver-derived HepG2 cells, temporally coinciding with its appearance in the cytoplasm. The nuclear residence of RXRalpha is maintained by inhibiting c-jun N-terminal kinase (JNK, curcumin or SP600125) or CRM-1-mediated nuclear export (Leptomycin B). Pretreatment with the proteasome inhibitor MG132 blocks IL-1beta-mediated reductions in nuclear RXRalpha levels while increasing accumulation in the cytoplasm. Mutational studies identify one residue, serine 260, a JNK phosphoacceptor site whose phosphorylation status had an unknown role in RXRalpha function, as critical for IL-1beta-mediated nuclear export of transfected human RXRalpha-green fluorescent fusion constructs. These findings indicate that inflammation-mediated cell signaling leads to rapid and profound reductions in nuclear RXRalpha levels, via a multistep, JNK-dependent mechanism involving Ser260, nuclear export, and proteasomal degradation. Thus, inflammation-meditated cell signaling targets RXRalpha for nuclear export and degradation; a potential mechanism that explains the broad suppression of RXRalpha-dependent gene expression in the inflamed liver.
Zimmerman
Tracy L
TL
Texas Children's Liver Center, Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030, USA.
Thevananther
Sundararajah
S
Ghose
Romi
R
Burns
Alan R
AR
Karpen
Saul J
SJ
D021581:Active Transport, Cell Nucleus; D019943:Amino Acid Substitution; D001483:Base Sequence; D001665:Binding Sites; D002460:Cell Line; D004247:DNA / Q000235:genetics; D006801:Humans; D066298:In Vitro Techniques; D007375:Interleukin-1 / Q000494:pharmacology*; D048031:JNK Mitogen-Activated Protein Kinases / Q000378:metabolism*; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D011993:Recombinant Fusion Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D047490:Retinoid X Receptor alpha / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012694:Serine / Q000737:chemistry; D015398:Signal Transduction / Q000187:drug effects; D014162:Transfection
D007375:Interleukin-1; D011993:Recombinant Fusion Proteins; D047490:Retinoid X Receptor alpha; D012694:Serine; D004247:DNA; D048031:JNK Mitogen-Activated Protein Kinases