Autocrine signals control CCAAT enhancer binding protein β expression, localization, and activity in macrophages

The transcription factor CCAAT/enhancer binding protein beta (C/EBP beta, or NF-IL6) is expressed in macrophages, where it participates in lipopolysaccharide (LPS)-mediated induction of proinflammatory cytokine genes such as interleukin-6 (IL-6) and IL-1 beta. We have identified activities in conditioned medium from a macrophage tumor cell line that regulates the expression, localization, and transcriptional activity of C/EBP beta. One factor was shown to be tumor necrosis factor-alpha (TNF-alpha), which increased C/EBP beta expression by a posttranscriptional mechanism. A second activity, designated autocrine macrophage factor (AMF), elicited a change in C/EBP beta localization from a punctate nuclear staining pattern to diffuse nuclear distribution. The punctate form of C/EBP beta correlated with increased susceptibility of this protein to cleavage by an endogenous protease during nuclear extract preparation. Conditioned medium stimulated the ability of C/EBP beta to transactivate a reporter gene and activated the expression of two cytokine genes that are putative targets of C/EBP beta. These observations suggest that diffuse distribution of C/EBP beta in the nucleus corresponds to an activated form of this protein. AMF activity could not be mimicked by an extensive set of recombinant cytokines and growth factors and therefore may represent a novel extracellular factor. This is a US government work. There are no restrictions on its use.