Repression of peroxisome proliferator-activated receptor gamma by mucosal ribotoxic insult-activated CCAAT/enhancer-binding protein homologous protein

CCAAT/enhancer-binding protein homologous protein (CHOP) is a crucial stress-responsive factor in various mucosal injuries, including cellular translational stress conditions. In this study, chemical ribosome-inactivating stresses were assessed for their effects on stress-inducible CHOP expression and its association with epithelial inflammatory cytokine production. Several representative ribotoxic agents (deoxynivalenol, anisomycin, and 15-acetyldeoxynivalenol) enhanced CHOP expression and its nuclear translocation in human intestinal epithelial cells. Moreover, CHOP was a strong positive regulator of IL-8 production, but CHOP-mediated IL-8 production was inversely associated with expression of the mucosal regulatory factor peroxisome proliferator-activated receptor γ (PPARγ). Based on our recent report that PPARγ is a negative regulator of mRNA stability of IL-8, PPARγ was linked to a notable mRNA stabilizing protein, HuR, since ribotoxin-induced IL-8 mRNA is stabilized by HuR protein. Expression of exogenous PPARγ suppressed ribotoxin-triggered cytoplasmic translocation of HuR. In contrast, PPARγ-regulating CHOP was a positive modulator of HuR protein export from nuclei. Taken together, the results indicate that ribotoxin-induced CHOP protein is positively associated with production of proinflammatory cytokine IL-8, but it downregulates PPARγ action, subsequently allowing the cytosolic translocation of HuR protein and stabilization of IL-8 mRNA in gut epithelial cells. CHOP and PPARγ may represent critical mechanistic links between ribotoxic stress and proinflammatory cytokine production, and they may have a broader functional significance with regard to gastrointestinal stresses by toxic mucosal insults.     

Dynamic regulation of cyclooxygenase-2 promoter activity by isoforms of CCAAT/enhancer-binding proteins.

To elucidate the mechanism by which isoforms of CCAAT/enhancer-binding proteins regulate cyclooxygenase-2 expression, we determined by a novel technique binding of six isoforms of this transactivator to two sequence-specific CCAAT/enhancer-binding protein (-132/-125) and cyclic AMP (-59/-53) regulatory elements in human foreskin fibroblasts treated with phorbol 12-myristate 13-acetate for 4 h. The delta isoform bound to these two elements at basal state, which was displaced by full-length as well as two truncated beta isoforms, a 41-kDa liver-enriched activating protein and a 16-kDa liver-enriched inhibitory protein, after phorbol ester stimulation. Kinetic analysis shows time-dependent changes in beta and delta binding that were concordant with time-dependent increase in cyclooxygenase-2 induction. Overexpression of the 16-kDa beta isoform blocked the promoter activity and protein level induced by phorbol ester. Paradoxically, it increased binding of beta isoforms to the sequence-specific promoter DNA but suppressed cyclooxygenase-2 promoter activation by p300 cotransfection. These findings provide new insight into the regulation of cyclooxygenase-2 promoter by an interplay between two opposite beta isoforms and p300 co-activator.     

Modulation of airway remodeling and airway inflammation by peroxisome proliferator-activated receptor gamma in a murine model of toluene diisocyanate-induced asthma

Toluene diisocyanate (TDI) is a leading cause of occupational asthma. Although considerable controversy remains regarding its pathogenesis, TDI-induced asthma is an inflammatory disease of the airways characterized by airway remodeling. Peroxisome proliferator-activated receptor gamma (PPARgamma) has been shown to play a critical role in the control of airway inflammatory responses. However, no data are available on the role of PPARgamma in TDI-induced asthma. We have used a mouse model for TDI-induced asthma to determine the effect of PPARgamma agonist, rosiglitazone, or pioglitazone, and PPARgamma on TDI-induced bronchial inflammation and airway remodeling. This study with the TDI-induced model of asthma revealed the following typical pathophysiological features: increased numbers of inflammatory cells of the airways, airway hyperresponsiveness, increased levels of Th2 cytokines (IL-4, IL-5, and IL-13), adhesion molecules (ICAM-1 and VCAM-1), chemokines (RANTES and eotaxin), TGF-beta1, and NF-kappaB in nuclear protein extracts. In addition, the mice exposed to TDI developed features of airway remodeling, including thickening of the peribronchial smooth muscle layer, subepithelial collagen deposition, and increased airway mucus production. Administration of PPARgamma agonists or adenovirus carrying PPARgamma2 cDNA reduced the pathophysiological symptoms of asthma and decreased the increased levels of Th2 cytokines, adhesion molecules, chemokines, TGF-beta1, and NF-kappaB in nuclear protein extracts after TDI inhalation. In addition, inhibition of NF-kappaB activation decreased the increased levels of Th2 cytokines, adhesion molecules, chemokines, and TGF-beta1 after TDI inhalation. These findings demonstrate a protective role of PPARgamma in the pathogenesis of the TDI-induced asthma phenotype.     

Dendritic cell immunogenicity is regulated by peroxisome proliferator-activated receptor gamma

Dendritic cells (DC) are the most potent APCs known that play a key role for the initiation of immune responses. Ag presentation to T lymphocytes is likely a constitutive function of DC that continues during the steady state. This raises the question of which mechanism(s) determines whether the final outcome of Ag presentation will be induction of immunity or of tolerance. In this regard, the mechanisms controlling DC immunogenicity still remain largely uncharacterized. In this paper we report that the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-gamma), which has anti-inflammatory properties, redirects DC toward a less stimulatory mode. We show that activation of PPAR-gamma during DC differentiation profoundly affects the expression of costimulatory molecules and of the DC hallmarker CD1a. PPAR-gamma activation in DC resulted in a reduced capacity to activate lymphocyte proliferation and to prime Ag-specific CTL responses. This effect might depend on the decreased expression of costimulatory molecules and on the impaired cytokine secretion, but not on increased IL-10 production, because this was reduced by PPAR-gamma activators. Moreover, activation of PPAR-gamma in DC inhibited the expression of EBI1 ligand chemokine and CCR7, both playing a pivotal role for DC migration to the lymph nodes. These effects were accompanied by down-regulation of LPS-induced nuclear localized RelB protein, which was shown to be important for DC differentiation and function. Our results suggest a novel regulatory pathway for DC function that could contribute to the regulated balance between immunity induction and self-tolerance maintenance.