Activation of mammalian innate and acquired immune responses must be tightly regulated by elaborate mechanisms to control their onset and termination. MicroRNAs have been implicated as negative regulators controlling diverse biological processes at the level of posttranscriptional repression. Expression profiling of 200 microRNAs in human monocytes revealed that several of them (miR-146a͞b, miR-132, and miR-155) are endotoxin-responsive genes. Analysis of miR-146a and miR-146b gene expression unveiled a pattern of induction in response to a variety of microbial components and proinflammatory cytokines. By means of promoter analysis, miR-146a was found to be a NF-B-dependent gene. Importantly, miR-146a͞b were predicted to base-pair with sequences in the 3 UTRs of the TNF receptor-associated factor 6 and IL-1 receptor-associated kinase 1 genes, and we found that these UTRs inhibit expression of a linked reporter gene. These genes encode two key adapter molecules downstream of Toll-like and cytokine receptors. Thus, we propose a role for miR-146 in control of Toll-like receptor and cytokine signaling through a negative feedback regulation loop involving down-regulation of IL-1 receptor-associated kinase 1 and TNF receptor-associated factor 6 protein levels.immunity ͉ microRNA ͉ LPS ͉ Toll-like receptor

do not readily transform cells upon transfection. An exception is v-Rel which, however, can acutely transform *Tularik Inc. only avian cells in culture. The transforming potential Two Corporate Drive of a RelA splice variant called p65⌬ in rodent cells re-South San Francisco, California 94080 CA 94080 mains a matter of debate (Narajanan et al., 1992; Grimm Department of Biology and Baeuerle, 1994). T. Gilmore (Boston University, Bos- † Massachusetts Institute of Technology ton) reviewed the properties of v-Rel that contribute to 77 Massachusetts Ave.

Studies on mice deficient in nuclear factor kappa B (NF-kappaB) subunits have shown that this transcription factor is important for lymphocyte responses to antigens and cytokine-inducible gene expression. In particular, the RelA (p65) subunit is required for induction of tumor necrosis factor-alpha (TNF-alpha)-dependent genes. Treatment of RelA-deficient (RelA-/-) mouse fibroblasts and macrophages with TNF-alpha resulted in a significant reduction in viability, whereas RelA+/+ cells were unaffected. Cytotoxicity to both cell types was mediated by TNF receptor 1. Reintroduction of RelA into RelA-/- fibroblasts resulted in enhanced survival, demonstrating that the presence of RelA is required for protection from TNF-alpha. These results have implications for the treatment of inflammatory and proliferative diseases.

Nuclear localization of the transcriptional activator NF-kappaB (nuclear factor kappaB) is controlled in mammalian cells by three isoforms of NF-kappaB inhibitor protein: IkappaBalpha, -beta, and - epsilon. Based on simplifying reductions of the IkappaB-NF-kappaB signaling module in knockout cell lines, we present a computational model that describes the temporal control of NF-kappaB activation by the coordinated degradation and synthesis of IkappaB proteins. The model demonstrates that IkappaBalpha is responsible for strong negative feedback that allows for a fast turn-off of the NF-kappaB response, whereas IkappaBbeta and - epsilon function to reduce the system's oscillatory potential and stabilize NF-kappaB responses during longer stimulations. Bimodal signal-processing characteristics with respect to stimulus duration are revealed by the model and are shown to generate specificity in gene expression.

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