Lipopolysaccharide-induced expression of TNF-alpha gene in the macrophage cell line ANA-1 is regulated at the level of transcription processivity

Abstract

Increasing evidence suggests that regulation of transcription at the level of elongation or processivity may be an important mechanism governing expression of eukaryotic genes. In this study we compared LPS- and IFN-gamma-induced transcription of the TNF-alpha gene in two murine macrophage cell lines, ANA1 and Pu5-1.8. Our data from nuclear run-on analysis indicate that in ANA-1 cells TNF-alpha expression is regulated at the transcriptional level, as previously found in primary macrophages. In contrast, in Pu5-1.8 cells the TNF gene is constitutively transcribed. Using several short probes spanning the TNF gene we find that in ANA-1 cells transcription can be initiated before activation, but such transcripts have low processivity and are prematurely terminated or arrested within the gene. Induction with LPS alone or with LPS plus IFN-gamma results both in increased transcription initiation, and in the increased processivity of these transcripts. In Pu 5-1.8 cells neither type of transcriptional regulation of the TNF gene is observed. Our results indicate that the TNF gene is preactivated in ANA-1 cells, and RNA polymerase is allowed to initiate transcription, but due to the low processivity of the transcripts very little mRNA is formed. After LPS stimulation the TNF gene is maximally activated both by increased initiation and by higher processivity of the transcript, and each of these components of activation do not require a new protein synthesis. Our findings are consistent with a recently proposed model that the same transcriptional activators contribute to both initiation and processivity of transcription. In the case of LPS and LPS+IFN-gamma stimulation of macrophages, inducible members of NF-kappa B/Rel family are likely candidate transcriptional activators.