A Distinct Element Involved in Lipopolysaccharide Activation of the Tumor Necrosis Factor-α Promoter in Monocytes

Abstract

To delineate the functional role of the tumor necrosis factor-α (TNF-α) activator protein-1 (AP-1)/cAMP-responsive element (CRE)-like binding element (TAC), we transfected the TNF-α promoter lacking TAC into THP-1 monocytic cells and stimulated with lipopolysaccharide (LPS). Chloramphenicol acetyltransferase (CAT) activity was reduced by 22-fold, suggesting that TAC plays a role in LPS induction of the TNF-α promoter. Exposure to LPS resulted in the maximum release of soluble TNF-α by 2 h. Electrophoretic mobility shift assays (EMSA) using the TAC element as a probe showed a unique pattern for LPS-activated cells: the disappearance of the upper band of a doublet seen in untreated and all-trans retinoic acid (ATRA)-treated cells. Supershift analysis identified c-Jun and activating transcription factor-2 (ATF-2) as components of the LPS-stimulated binding complex. Jun N-terminal kinase (JNK), a known phosphorylator of c-Jun and ATF2, increased in activity in LPS-stimulated monocytes. ATRA, on the contrary, did not activate JNK activity up to 72 h. Nuclear extracts from LPS-stimulated cells showed an increase in phosphorylated c-Jun by immunoblotting. Likewise, phosphorylated c-Jun bound to the TAC element, suggesting that c-Jun is activated by JNK to transactivate the TNF-α promoter in LPS-treated monocytes. Thus, phosphorylated c-Jun and ATF-2 play a role in activating the TAC element of the TNF-α promoter.