Distinct Mechanisms Regulate IL1B and TNF Gene Expression

Abstract

We have previously reported that the immediate-early (IE) induction of the genes coding for Interleukin 1β (IL1B) and TNFα (TNF) are regulated by distinct mechanisms. Like many poised IE genes prior to induction, TNF exhibits pre-bound TBP and paused RNA Pol II. In contrast, poised IL1B is devoid of these complexes, revealing only a lineage-specifying factor (Spi-1) and prominent promoter nucleosome depletion. In the case of TNF, LPS treatment releases the paused Pol II for elongation, while for IL1B the activation event results in de novo TBP and Pol II recruitment and elongation. Additionally, once cells are treated with LPS, TNF is rapidly and transiently induced, followed by complete shutdown and is refractory to re-stimulation. For IL1B, a similar rapid-vigorous transient induction is subsequently followed by a lower level of continuous expression, which can be re-induced with LPS. In order to understand the mechanisms involved in these distinct modes of regulation, experiments have been aimed at comparing kinetic RNA expression patterns in the presence of various inducers and inhibitors of transcription. We have observed that in contrast to LPS, phorbol 12-myristate 13-acetate (PMA) generates only the lower level of delayed continuous expression for IL1B, with only a minute effect on TNF. Furthermore, the prolyl hydroxylase activator α-ketoglutarate (α-KG) completely abrogates both the PMA and late/continuous LPS responses, while not affecting TNF. α-KG is a known inhibitor of the HIF-1α transcription factor, which has been previously reported to be important for IL1B gene expression. Our results, suggest that HIF-1α may play a late, rather than immediate-early, role for continuous expression of IL1B.