The C/EBP bZIP domain can mediate lipopolysaccharide induction of the proinflammatory cytokines interleukin-6 and monocyte chemoattractant protein-1

C/EBP alpha, beta, and delta are all expressed by bone marrow-derived macro phages, Ectopic expression of any of these transcription factors is suffici ent to confer lipopolysaccharide (LPS)-inducible expression of interleukin- 6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1) to a B lymphoblast cell line, which normally lacks C/EBP factors and does not display LPS indu ction of proinflammatory cytokines. Thus, the activities of C/EBP alpha, be ta, and delta are redundant in regard to expression of IL-6 and MCP-1. Surp risingly, the bZIP region of C/EBP beta, which lacks any previously describ ed activation domains, can also confer LPS-inducible expression of IL-6 and MCP-1 in stable transfectants, Transient transfections reveal that the bZI P regions of C/EBP beta, C/EBP delta, and, to a lesser extent, C/EBP alpha can activate the IL-6 promoter and augment its induction by LPS. Furthermor e, the transdominant inhibitor, LIP, can activate expression from the IL-6 promoter. The ability of the C/EBP beta bZIP region to activate the IL-6 pr omoter in transient transfections is completely dependent upon an intact NF -kappa B-binding site, supporting a model where the bZIP protein primarily functions to augment the activity of NF-kappa B. Replacement of the leucine zipper of C/EBP beta with that of GCN4 yields a chimeric protein that can dimerize and specifically bind to a C/EBP consensus sequence, but shows a m arkedly reduced ability to activate IL-6 and MCP-1 expression. These result s implicate the leucine zipper domain in some function other than dimerizat ion with known C/EBP family members, and suggest that C/EBP redundancy in r egulating cytokine expression may result from their highly related bZIP reg ions.