Publications

Mycobacterium tuberculosis is a highly successful human pathogen. A major component of this success is the pathogen's ability to avoid eradication by the innate and adaptive immune responses throughout the course of infection. IFN-gamma, a potent activator of the microbicidal activities of macrophages, is essential for control of M. tuberculosis infection, but is unable to stimulate macrophages to kill M. tuberculosis. We have found that infection of the human monocytic cell line, THP-1, resulted in reduced cellular responses to IFN-gamma, manifested as impaired induction of CD64 surface expression and transcription. This defect in transcription occurred despite normal activation of STAT1 in infected macrophages: there was no decrease in STAT1 tyrosine or serine phosphorylation, nuclear translocation, or binding of a minimal IFN-gamma response sequence. Assays of STAT1 function in M. tuberculosis-treated cells also revealed no defect in activation of a minimal gamma-activated sequence construct or STAT1 recruitment to and binding at the endogenous CD64 promoter. In addition, M. tuberculosis did not affect histone acetylation at the CD64 promoter. The inhibition of transcription was gene selective: while transcription of CD64 and class II transactivator were decreased, certain other IFN-gamma-responsive genes either were unaffected or were increased by M. tuberculosis. These results indicate that M. tuberculosis inhibits the response to IFN-gamma by a mechanism distinct from either suppressor of cytokine signaling-1 inhibition of STAT1 phosphorylation or protein inhibitor of activated STAT interference with DNA binding, and indicate that other mechanisms of inhibition of IFN-gamma responses remain to be discovered.

Although the proximal cytoplasmic signaling events that control the activation of the NF-kappaB transcription factor are understood in considerable detail, the subsequent intranuclear events that regulate the strength and duration of the NF-kappaB-mediated transcriptional response remain poorly defined. Recent studies have revealed that NF-kappaB is subject to reversible acetylation and that this posttranslational modification functions as an intranuclear molecular switch to control NF-kappaB action. In this review, we summarize this new and fascinating mechanism through which the pleiotropic effects of NF-kappaB are regulated within the cells. NF-kappaB is a heterodimer composed of p50 and RelA subunits. Both subunits are acetylated at multiple lysine residues with the p300/CBP acetyltransferases playing a major role in this process in vivo. Further, the acetylation of different lysines regulates different functions of NF-kappaB, including transcriptional activation, DNA binding affinity, IkappaBalpha assembly, and subcellular localization. Acetylated forms RelA are subject to deacetylation by histone deacetylase 3 (HDAC3). This selective action of HDAC3 promotes IkappaBalpha binding and rapid CRM1-dependent nuclear export of the deacetylated NF-kappaB complex, which terminates the NF-kappaB response and replenishes the cytoplasmic pool of latent NF-kappaB/IkappaBalpha complexes. This readies the cell for the next NF-kappaB-inducing stimulus. Thus, reversible acetylation of RelA serves as an important intranuclear regulatory mechanism that further provides for dynamic control of NF-kappaB action.

Prostaglandin E(2) (PGE(2)) and epinephrine act directly on nociceptors to produce mechanical hyperalgesia through protein kinase A (PKA) alone or through a combination of PKA, protein kinase C epsilon (PKCepsilon), and extracellular signal-regulated kinase (ERK), respectively. Disruptors of the cytoskeleton (microfilaments, microtubules, and intermediate filaments) markedly attenuated the hyperalgesia in rat paws caused by injection of epinephrine or its downstream mediators. In contrast, the hyperalgesia induced by PGE(2) or its mediators was not affected by any of the cytoskeletal disruptors. These effects were mimicked in vitro, as measured by enhancement of the tetrodotoxin-resistant sodium current. When PGE(2) hyperalgesia was shifted to dependence on PKCepsilon and ERK as well as PKA, as when the tissue is "primed" by prior treatment with carrageenan, it too became dependent on an intact cytoskeleton. Thus, inflammatory mediator-induced mechanical hyperalgesia was differentially dependent on the cytoskeleton such that cytoskeletal dependence correlated with mediation by PKCepsilon and ERK.