Interleukin-1beta modulates endogenous thyroid hormone receptor alpha gene transcription in liver cells

One of the main characteristics of nonthyroidal illness (NTI) is a decrease in serum triiodothyronine, partly caused by a decrease in liver deiodinase type 1 (D1) mRNA and activity. Proinflammatory cytokines have been associated with NTI in view of their capability to decrease D1 and thyroid hormone receptor (TR)beta1 mRNA expression in hepatoma cells. Proinflammatory cytokine induction leads to activation of the inflammatory pathways nuclear factor (NF)kappaB and activator protein (AP)-1. The proinflammatory cytokine interleukin (IL)-1beta decreases thyroid hormone receptor (TR)beta1 mRNA in an NFkappaB-dependent way. The aim of this study was to unravel the effects of IL-1beta on endogenous TRalpha gene expression in an animal model and in a liver cell line. The TRalpha gene product is alternatively spliced in TRalpha1 and TRalpha2, TRalpha2 is capable of inhibiting TRalpha1-induced gene transcription. We showed that both TRalpha1 and TRalpha2 mRNA decreased not only after lipopolysaccharide administration in liver of mice, but also after IL-1beta stimulation of hepatoma cells (HepG2). Using the NFkappaB inhibitor sulfasalazine and the AP-1 inhibitor SP600125, it became clear that the IL-1beta-induced decrease in TRalpha mRNA expression in HepG2 cells can only be abolished by simultaneous inhibition of NFkappaB and AP-1. The IL-1beta-induced TRalpha1 and TRalpha2 mRNA decrease in HepG2 cells is the result of decreased TRalpha gene promoter activity, as evident from actinomycin D experiments. Cycloheximide experiments showed that the decreased promoter activity is independent of de novo protein synthesis and therefore most likely due to posttranslational modifications such as phosphorylation or subcellular relocalization.