Induction of transcription factor interferon regulatory factor-1 by interferon-gamma (IFN gamma) and tumor necrosis factor-alpha (TNF alpha) in FRTL-5 cells

While it is well known that interferon-gamma (IFN gamma) and tumor necrosis factor-alpha (TNF alpha) play a role in the regulation of thyroid growth a nd differentiated functions, the cellular and molecular mechanisms involved in mediating the effects of IFN gamma and TNF alpha on thyroid function ar e unknown. In the present study, we used FRTL-5 rat thyroid cells to examin e the effects of IFN gamma and TNF alpha on gene expression of transcriptio n factor interferon regulatory factor-1 (IRF-1), which is involved in media ting the effects of these cytokines in a number of cell types. Northern blo t analysis of FRTL-5 mRNA showed a single IRF-1 mRNA at 2.2 Kb. In quiescen t FRTL-5 cells, IRF-1 mRNA levels were low but detectable by Northern analy sis. Incubation of FRTL-5 cells with IFN gamma or TNF alpha resulted in a d ose- and time-dependent increase in IRF-1 mRNA levels. We have shown that T NF-alpha and IFN-gamma act synergistically to block the TSH-induced increas e in type I 5'-deiodinase (5'D-I) activity and 5'D-I gene expression in FRT L-5 rat thyroid cells. Incubation of FRTL-5 cells with IFN gamma and TNF al pha in combination, however, did not synergistically increase IRF-1 mRNA le vels. Electrophoretic mobility shift assay (EMSA) revealed that IFN gamma i nduced the formation of a single complex to a IFN gamma activation site (GA S) probe in a dose dependent manner. Several lines of evidence suggest that TNF alpha activates transcription factor nuclear factor-kappa B (NF kappa B) through activation of protein kinase C (PKC) or the hydrolysis of sphing omyelin to ceramide in a number of cell types. Here we demonstrate that hyd rolysis of sphingomyelin to ceramide by sphingomyelinase (SMase), but not a ctivation of PKC by 12-O-tetradecanoylphorbol 13-acetate (TPA), was involve d in the activation of NF kappa B in FRTL-5 cells. Similarly, hydrolysis of sphingomyelin to ceramide, but not activation of PKC, resulted in an incre ased in IRF-1 mRNA levels in FRTL-5 cells. The present data demonstrate tha t IFN gamma and TNF alpha increase IRF-1 mRNA levels in FRTL-5 cells throug h activation of GAS and NF kappa B binding proteins, respectively. Thus, ou r results suggest that upregulation of IRF-1 may play a role in mediating t he effects of IFN gamma and TNF alpha on thyroid function. Our results also suggest that the induction of IRF-1 mRNA by IFN gamma and TNF alpha is not the cellular mechanism involved in the synergistic effect of these cytokin es on thyroid function. J. Cell. Biochem. 74:211-219, 1999. (C) 1999 Wiley- Liss, Inc.