Bacterial lipopolysaccharide induces type 2 deiodinase in cultured rat astrocytes

In the brain, 3,5,3'-triiodothyronine, which binds to the thyroid hormone receptor with high affinity, is locally generated from thyroxine by type 2 iodothyronine deiodinase (D2) expressed mainly in astrocytes and tanycytes. We have investigated the effects of bacterial lipopolysaccharide (LPS) on D2 in cultured rat astrocytes. LPS induced D2 activity with a lag-time of 4-8 h and a maximum at 24 h. LPS also promoted D2 mRNA accumulation. Glucocorticoids enhanced both the basal and LPS-stimulated D2 activity and mRNA accumulation. These glucocorticoid effects were blocked by the glucocorticoid receptor antagonist RU486. Our results obtained with different specific signaling pathway inhibitors indicated that D2 induction by LPS required ERK and p38-MAPK signaling pathways. NF-κB inhibitor sulfasalazine blocked the effects of LPS on both D2 activity and mRNA accumulation. Hence, D2 induction by LPS appeared to implicate NF-κB pathway in astrocytes. NF-κB responsiveness of the rat dio2 gene was studied in astrocytes with dio2 5'-flanking region promoter assays. The long form of the dio2 promoter was transactivated by NF-κB. CCAAT/enhancer-binding protein β, which is upregulated by LPS in astrocytes, increased the transcriptional activity of the dio2 promoter in its long or truncated forms containing CCAATs. Our observations, which demonstrate D2 induction by LPS in astrocytes and specify some characteristics of D2 induction mechanism, support the possible implication of brain D2 in adaptative responses to an infectious stress.