Atopic dermatitis (AD) is characterized by epidermal tight junction (TJ) defects and a propensity for Staphylococcus aureus (S. aureus) skin infections. S. aureus is sensed by many pattern recognition receptors including toll-like receptor (TLR) 2. We hypothesized that an effective innate immune response will include skin barrier repair and that this response is impaired in AD subjects. S. aureus-derived peptidoglycan (PGN) and synthetic TLR2 agonists enhanced TJ barrier and increased expression of TJ proteins, CLDN1, CLDN23, occludin and ZO-1 in primary human keratinocytes. A TLR2 agonist enhanced skin barrier recovery in human epidermis wounded by tape-stripping. Tlr2−/− mice had a delayed and incomplete barrier recovery following tape-stripping. AD subjects had reduced epidermal TLR2 expression as compared to nonatopic (NA) subjects, which inversely correlated (r= 0.654, P= 0.0004) with transepidermal water loss (TEWL). These observations indicate that TLR2 activation enhances skin barrier in murine and human skin and is an important part of a wound repair response. Reduced epidermal TLR2 expression observed in AD patients may play a role in their incompetent skin barrier.
Inflammation-associated lung injury is a major cause of morbidity and mortality for patients in intensive care units. Although the cellular and molecular events that initiate lung inflammation are now well understood, the mechanisms that promote its resolution remain poorly defined. In this issue of the JCI, D’Alessio et al. show in a mouse model that recovery from acute lung injury is not simply a passive process, but involves Tregs in an active resolution program (see the related article beginning on page 2898).
Negative regulation of innate immune responses is essential in order to prevent excess inflammation and tissue injury and promote homeostasis. Lysophosphatidic acid (LPA) is a pleiotropic lipid that regulates cell growth, migration and activation, and is constitutively produced at low levels in tissues and in serum. Extracellular LPA binds to specific G-protein coupled receptors, the function of which in regulating innate or adaptive immune responses remains poorly understood. Of the classical LPA receptors belonging to the Edg family, lpa2 (edg4) is expressed by dendritic cells (DC) and other innate immune cells. Here we show that DC from lpa2−/− mice are hyperactive compared to their wild-type counterparts, and are also less susceptible to inhibition by different LPA species. In transient transfection assays, we found that lpa2-overexpression inhibits NF-κB-driven gene transcription. Using an adoptive transfer approach, we found that allergen-pulsed lpa2−/− DC induced substantially more lung inflammation than wild-type DC after inhaled allergen challenge. Finally, lpa2−/− mice develop greater allergen-driven lung inflammation than their wild-type counterparts in models of allergic asthma involving both systemic and mucosal sensitization. Taken together, these findings identify LPA acting via lpa2 as a novel negative regulatory pathway that inhibits dendritic cell activation and allergic airway inflammation.
Disruption of the epithelial barrier might be a risk factor for allergen sensitization and asthma. Viral respiratory tract infections are strongly associated with asthma exacerbation, but the effects of respiratory viruses on airway epithelial barrier function are not well understood. Many viruses generate double-stranded RNA, which can lead to airway inflammation and initiate an antiviral immune response.
We investigated the effects of the synthetic double-stranded RNA polyinosinic:polycytidylic acid (polyI:C) on the structure and function of the airway epithelial barrier in vitro.
16HBE14o- human bronchial epithelial cells and primary airway epithelial cells at an air-liquid interface were grown to confluence on Transwell inserts and exposed to polyI:C. We studied epithelial barrier function by measuring transepithelial electrical resistance and paracellular flux of fluorescent markers and structure of epithelial apical junctions by means of immunofluorescence microscopy.
PolyI:C induced a profound decrease in transepithelial electrical resistance and increase in paracellular permeability. Immunofluorescence microscopy revealed markedly reduced junctional localization of zonula occludens-1, occludin, E-cadherin, β-catenin, and disorganization of junction-associated actin filaments. PolyI:C induced protein kinase D (PKD) phosphorylation, and a PKD antagonist attenuated polyI:C-induced disassembly of apical junctions and barrier dysfunction.
PolyI:C has a powerful and previously unsuspected disruptive effect on the airway epithelial barrier. PolyI:C-dependent barrier disruption is mediated by disassembly of epithelial apical junctions, which is dependent on PKD signaling. These findings suggest a new mechanism potentially underlying the associations between viral respiratory tract infections, airway inflammation, and allergen sensitization.
Asthma; polyI:C; Toll-like receptor 3; epithelial permeability; protein kinase C; tight junctions; adherens junctions
atopic dermatitis; eczema herpeticum; claudin-1
Rationale: The differentiation of fibroblasts into myofibroblasts is a cardinal feature of idiopathic pulmonary fibrosis (IPF). The transcription factor Yin Yang 1 (YY1) plays a role in the proliferation and differentiation of diverse cell types, but its role in fibrotic lung diseases is not known.
Objectives: To elucidate the mechanism by which YY1 regulates fibroblast differentiation and lung fibrosis.
Methods: Lung fibroblasts were cultured with transforming growth factor (TGF)-β or tumor necrosis factor-α. Nuclear factor (NF)-κB, YY1, and α-smooth muscle actin (SMA) were determined in protein, mRNA, and promoter reporter level. Lung fibroblasts and lung fibrosis were assessed in a partial YY1-deficient mouse and a YY1f/f conditional knockout mouse after being exposed to silica or bleomycin.
Measurements and Main Results: TGF-β and tumor necrosis factor-α up-regulated YY1 expression in lung fibroblasts. TGF-β–induced YY1 expression was dramatically decreased by an inhibitor of NF-κB, which blocked I-κB degradation. YY1 is significantly overexpressed in both human IPF and murine models of lung fibrosis, including in the aggregated pulmonary fibroblasts of fibrotic foci. Furthermore, the mechanism of fibrogenesis is that YY1 can up-regulate α-SMA expression in pulmonary fibroblasts. YY1-deficient (YY1+/−) mice were significantly protected from lung fibrosis, which was associated with attenuated α-SMA and collagen expression. Finally, decreasing YY1 expression through instilled adenovirus-cre in floxed-YY1f/f mice reduced lung fibrosis.
Conclusions: YY1 is overexpressed in fibroblasts in both human IPF and murine models in a NF-κB–dependent manner, and YY1 regulates fibrogenesis at least in part by increasing α-SMA and collagen expression. Decreasing YY1 expression may provide a new therapeutic strategy for pulmonary fibrosis.
nuclear factor-κB; α-smooth muscle actin; idiopathic pulmonary fibrosis
Cytokine expression patterns of T cells can be regulated by pre-commitment to stable effector phenotypes, further modification of moderately stable phenotypes, and quantitative changes in cytokine production in response to acute signals. We showed previously that the epidermal growth factor family member Amphiregulin is expressed by T cell receptor-activated mouse CD4 T cells, particularly Th2 cells, and helps eliminate helminth infection. Here we report a detailed analysis of the regulation of Amphiregulin expression by human T cell subsets. Signaling through the T cell receptor induced Amphiregulin expression by most or all T cell subsets in human peripheral blood, including naive and memory CD4 and CD8 T cells, Th1 and Th2 in vitro T cell lines, and subsets of memory CD4 T cells expressing several different chemokine receptors and cytokines. In these different T cell types, Amphiregulin synthesis was inhibited by an antagonist of protein kinase A, a downstream component of the cAMP signaling pathway, and enhanced by ligands that increased cAMP or directly activated protein kinase A. Prostaglandin E2 and adenosine, natural ligands that stimulate adenylyl cyclase activity, also enhanced Amphiregulin synthesis while reducing synthesis of most other cytokines. Thus, in contrast to mouse T cells, Amphiregulin synthesis by human T cells is regulated more by acute signals than pre-commitment of T cells to a particular cytokine pattern. This may be appropriate for a cytokine more involved in repair than attack functions during most inflammatory responses.
Atopic dermatitis (AD) is characterized by dry skin and a hyperreactive immune response to allergens, two cardinal features that are caused in part by epidermal barrier defects. Tight junctions (TJ) reside immediately below the stratum corneum and regulate the selective permeability of the paracellular pathway.
We evaluated the expression/function of the TJ protein, claudin-1 in epithelium from AD and nonatopic (NA) subjects and screened two American populations for SNPs in CLDN1.
Expression profiles of nonlesional epithelium from extrinsic AD, NA and psoriasis subjects were generated using Illumina’s BeadChips. Dysregulated intercellular proteins were validated by tissue staining and qPCR. Bioelectric properties of epithelium were measured in Ussing chambers. Functional relevance of claudin-1 was assessed using a knockdown approach in primary human keratinocytes (PHK). Twenty seven haplotype-tagging SNPs in CLDN1 were screened in two independent AD populations.
We observed strikingly reduced expression of the TJ proteins claudin-1 and -23 only in AD, which were validated at the mRNA and protein levels. Claudin-1 expression inversely correlated with Th2 biomarkers. We observed a remarkable impairment of the bioelectric barrier function in AD epidermis. In vitro, we confirmed that silencing claudin-1 expression in human keratinocytes diminishes TJ function while enhancing keratinocyte proliferation. Finally, CLDN1 haplotype-tagging single nucleotide polymorphisms revealed associations with AD in two North American populations.
Taken together, these data suggest that an impaired epidermal TJ is a novel feature of skin barrier dysfunction and immune dysregulation observed in AD, and that CLDN1 may be a new susceptibility gene in this disease.
atopic dermatitis; claudin-1; tight junctions
The biological effects of acute particulate air pollution exposure in host innate immunity remain obscure and have relied largely on in vitro models. We hypothesized that single acute exposure to ambient or engineered particulate matter (PM) in the absence of other secondary stimuli would activate lung dendritic cells (DC) in vivo and provide information on the early immunological events of PM exposure and DC activation in a mouse model naïve to prior PM exposure. Activation of purified lung DC was studied following oropharyngeal instillation of ambient particulate matter (APM). We compared the effects of APM exposure with that of diesel-enriched PM (DEP), carbon black particles (CBP) and silver nanoparticles (AgP). We found that PM species induced variable cellular infiltration in the lungs and only APM exposure induced eosinophilic infiltration. Both APM and DEP activated pulmonary DC and promoted a Th2-type cytokine response from naïve CD4+ T cells ex vivo. Cultures of primary peribronchial lymph node cells from mice exposed to APM and DEP also displayed a Th2-type immune response ex vivo. We conclude that exposure of the lower airway to various PM species induces differential immunological responses and immunomodulation of DC subsets. Environmental APM and DEP activated DC in vivo and provoked a Th2 response ex vivo. By contrast, CBP and AgP induced altered lung tissue barrier integrity but failed to stimulate CD4+ T cells as effectively. Our work suggests that respirable pollutants activate the innate immune response with enhanced DC activation, pulmonary inflammation and Th2-immune responsiveness.
Innate immunity; Allergic immunity; Dendritic cell; Lung; Inflammation; Immunotoxicology; Toxicology; Particulate matter; Nanoparticles
Amphiregulin, a member of the Epidermal Growth Factor family, is expressed by activated mouse Th2 cells. Amphiregulin produced by mouse hematopoietic cells contributes to the elimination of a nematode infection by a Type 2 effector response.
To identify the human peripheral blood cell population expressing amphiregulin.
Amphiregulin-expressing cells were identified by flow cytometry of cell surface markers and histological staining. Histamine and amphiregulin in supernatants were measured by enzyme immunoassay. Quantitative real-time PCR was used to measure mRNA expression.
Stimulation of human peripheral blood mononuclear cells by anti-CD3 + anti-CD28 antibodies induced expression of amphiregulin mRNA and protein by a non-T cell population. The amphiregulin-producing cells were basophils, as judged by morphology and expression of CD203c and CD123 (IL-3 receptor alpha chain). Activated mouse basophils also produced amphiregulin. Amphiregulin expression by basophils in response to anti-TCR stimulation required IL-3 produced by T cells, and IL-3 alone induced high levels of amphiregulin expression by purified basophils. Amphiregulin was expressed at much higher levels when human basophils were stimulated by IL-3 than by IgE cross-linking, whereas the opposite was true for IL-4 expression and histamine release. Heparin-binding Epidermal Growth Factor-like growth factor was also expressed by IL-3-stimulated human basophils. PBMC from asthmatic human subjects contained significantly higher numbers of basophils able to produce amphiregulin, compared to allergic or non-allergic controls.
IL-3 can induce basophils to express high levels of amphiregulin, which may contribute to tissue remodeling during type 2 immune responses such as asthma.
Basophils; amphiregulin; IL-3
Oxidative stress plays an important role in immune regulation and dendritic cell (DC) maturation. Recent studies indicate that allergens, including ragweed extract (RWE), possess prooxidant activities, but how RWE interacts with DCs is not well understood. Nuclear erythroid 2 p45-related factor 2 (Nrf2) is a key transcription factor that regulates constitutive and coordinated induction of a battery of antioxidant genes. We hypothesized that RWE would activate DCs and that this response would be augmented in the absence of Nrf2. We generated bone marrow–derived DCs (BM-DCs) and isolated lung DCs from Nrf2+/+ and Nrf2−/− mice and studied the effects of RWE on DCs in vitro. Under resting conditions, Nrf2−/− BM-DCs exhibited constitutively greater levels of inflammatory cytokines and costimulatory molecules than Nrf2+/+ BM-DCs. Exposure to RWE impaired endocytic activity, significantly induced oxidative stress, and enhanced the expression of CD80, CD86, and MHCII in Nrf2−/− BM-DCs when compared with Nrf2+/+ BM-DC, in association with reduced expression of Nrf2-regulated antioxidant genes. RWE significantly induced the secretion of inflammatory cytokines IL-6 and TNF-α in BM-DCs and lung DCs from Nrf2−/− mice than Nrf2+/+ mice and significantly inhibited the secretion of IL-12 in Nrf2+/+ BM-DCs and IL-18 in Nrf2+/+ and Nrf2−/− BM-DCs. The stimulatory effects of RWE on DC activation were inhibited to varying degrees by the antioxidant N-acetyl cysteine. Our findings indicate that a defect in Nrf2-mediated signaling mechanisms alters the response of DCs to a common environmental allergen, which may contribute to the susceptibility to allergic diseases.
Nrf2; dendritic cells; ragweed extract; antioxidant genes; oxidative stress
Oxidative stress is important in dendritic cell (DC) activation. Environmental particulate matter (PM) directs pro-oxidant activities that may alter DC function. Nuclear erythroid 2 p45-related factor 2 (Nrf2) is a redox-sensitive transcription factor that regulates expression of antioxidant and detoxification genes. Oxidative stress and defective antioxidant responses may contribute to the exacerbations of asthma. We hypothesized that PM would impart differential responses by Nrf2 wild-type DCs as compared with Nrf2−/− DCs. We found that the deletion of Nrf2 affected important constitutive functions of both bone marrow-derived and highly purified myeloid lung DCs such as the secretion of inflammatory cytokines and their ability to take up exogenous Ag. Stimulation of Nrf2−/− DCs with PM augmented oxidative stress and cytokine production as compared with resting or Nrf2+/+ DCs. This was associated with the enhanced induction of Nrf2-regulated antioxidant genes. In contrast to Nrf2+/+ DCs, coincubation of Nrf2−/− DCs with PM and the antioxidant N-acetyl cysteine attenuated PM-induced up-regulation of CD80 and CD86. Our studies indicate a previously underappreciated role of Nrf2 in innate immunity and suggest that deficiency in Nrf2-dependent pathways may be involved in susceptibility to the adverse health effects of air pollution in part by promoting Th2 cytokine responses in the absence of functional Nrf2. Moreover, our studies have uncovered a hierarchal response to oxidative stress in terms of costimulatory molecule expression and cytokine secretion in DCs and suggest an important role of heightened oxidative stress in proallergic Th2-mediated immune responses orchestrated by DCs.
Immune responses can be compartmentalized into innate versus adaptive components. This relatively recent dichotomy positioned the innate immune system at the interface between the host and the external environment and provided a new conceptual framework with which to view allergic diseases, including asthma. Airway epithelial cells and dendritic cells are key components of the innate immune system in the nose and lung and are now known to be intimately involved in allergen recognition and in modulating allergic immune responses. Here we review current thinking about how these two key cell types sense and respond to inhaled allergens, and emphasize how an understanding of “allergic innate immunity” can translate into new thinking about mechanisms of allergen sensitization and potentially lead to new therapeutic targets.
Innate immunity; Allergy; Asthma; Dendritic cells
Dendritic cells (DC) are potent professional antigen-presenting cells that drive primary immune responses to infections or other agonists perceived as ‘dangerous’. Muc1 is the only cell surface mucin or MUC gene product that is expressed in DC. Unlike other members of this glycoprotein family, Muc1 possesses a unique cytosolic region capable of signal transduction and attenuating toll-like receptor (TLR) activation. The expression and function of Muc1 has been intensively investigated on epithelial and tumor cells, but relatively little is known about its function on DC. We hypothesized that Muc1 would influence in vitro generation and primary DC activation in response to the TLR4 and TLR5 ligands lipopolysaccharide and flagellin. Compared with Muc1+/+ DC, we found that Muc1−/− DC were constitutively activated, as determined by higher expression of co-stimulatory molecules (CD40, CD80 and CD86), greater secretion of immunoregulatory cytokines (TNF-α and VEGF), and better stimulation of allogeneic naïve CD4+ T cell proliferation. After activation by either LPS or flagellin and co-culture with allogeneic CD4+ T cells, Muc1−/− DC also induced greater secretion of TNF-α and IFN-γ compared to similarly activated Muc1+/+ DC. Taken together, our results indicate that deletion of Muc1 promotes a heightened functional response of DC in response to TLR4 and TLR5 signaling pathways, and suggests a previously under-appreciated role for Muc1 in regulating innate immune responses of DC.
Inflammation; Dendritic cells; Muc1; Toll-like receptor; Innate immunity; Immunomodulation; Host defence; Cytokines
Secondary lymphoid organs and peripheral tissues are characterized by hypoxic microenvironments, both in the steady state and during inflammation. Although hypoxia regulates T-cell metabolism and survival, very little is known about whether or how hypoxia influences T-cell activation. We stimulated mouse CD4+ T cells in vitro with antibodies directed against the T-cell receptor (CD3) and CD28 under normoxic (20% O2) and hypoxic (1% O2) conditions. Here we report that stimulation under hypoxic conditions augments the secretion of effector CD4+ T-cell cytokines, especially IFN-γ. The enhancing effects of hypoxia on IFN-γ secretion were independent of mouse strain, and were also unaffected using CD4+ T cells from mice lacking one copy of the gene encoding hypoxia-inducible factor-1α. Using T cells from IFN-γ receptor–deficient mice and promoter reporter studies in transiently transfected Jurkat T cells, we found that the enhancing effects of hypoxia on IFN-γ expression were not due to effects on IFN-γ consumption or proximal promoter activity. In contrast, deletion of the transcription factor, nuclear erythroid 2 p45–related factor 2 attenuated the enhancing effect of hypoxia on IFN-γ secretion and other cytokines. We conclude that hypoxia is a previously underappreciated modulator of effector cytokine secretion in CD4+ T cells.
hypoxia; gene regulation; CD4+ T cells; effector cytokine; IFN-γ
There is growing evidence that hypoxia-inducible transcription factors are involved in the pathophysiology of asthma. Hypoxia-inducible factor-1α (HIF-1α) in particular controls the expression of many hypoxia regulated genes, but whether HIF-1α directly contributes to allergen-driven immune responses is not known.
Partially HIF-1α-deficient mice (HIF-1α+/−) or wild-type littermate controls were used in all experiments. Spleen CD4+ T cells were stimulated with anti-CD3 plus anti-CD28 antibodies and cytokine secretion was measured in vitro. Mice were sensitized by intraperitoneal injection of ovalbumin (Ova) plus alum, and then challenged by intranasal Ova followed by bronchoalveolar lavage (BAL) and isolation of spleen cells. BAL cells were counted and the differential determined using cytospin, and splenocytes were incubated with Ova to measure recall cytokine production.
Interferon-γ secretion was significantly higher in anti-CD3 plus anti-CD28 stimulated CD4+ T cells obtained from HIF-1α+/− mice compared to wild-type controls. HIF-1α+/− mice were protected from lung eosinophilia 72 h after allergen challenge, in association with enhanced secretion of interferon-γ in recall responses of splenocytes.
HIF-1α contributes to allergic immune responses and lung eosinophilia in a mouse model of asthma.
Gene regulation; Transcription Factors; Hypoxia; Asthma mouse model
Epidemiologic studies have associated exposure to airborne particulate matter (PM) with exacerbations of asthma. It is unknown how different sources of PM affect innate immunity. We sought to determine how car- and diesel exhaust–derived PM affects dendritic cell (DC) activation. DC development was modeled using CD34+ hematopoietic progenitors. Airborne PM was collected from exhaust plenums of Fort McHenry Tunnel providing car-enriched particles (CEP) and diesel-enriched particles (DEP). DC were stimulated for 48 hours with CEP, DEP, CD40-ligand, or lipopolysaccharide. DC activation was assessed by flow cytometry, enzyme-linked immunosorbent assay, and standard culture techniques. DEP increased uptake of fluorescein isothiocyanate–dextran (a model antigen) by DC. Diesel particles enhanced cell-surface expression of co-stimulatory molecules (e.g., CD40 [P < 0.01] and MHC class II [P < 0.01]). By contrast, CEP poorly affected antigen uptake and expression of cell surface molecules, and did not greatly affect cytokine secretion by DC. However, DEP increased production of TNF, IL-6, and IFN-γ (P < 0.01), IL-12 (P < 0.05), and vascular endothelial growth factor (P < 0.001). In co-stimulation assays of PM-exposed DC and alloreactive CD4+ T cells, both CEP and DEP directed a Th2-like pattern of cytokine production (e.g., enhanced IL-13 and IL-18 and suppressed IFN-γ production). CD4+ T cells were not functionally activated on exposure to either DEP or CEP. Car- and diesel-enriched particles exert a differential effect on DC activation. Our data support the hypothesis that DEP (and to a lesser extent CEP) regulate important functional aspects of human DC, supporting an adjuvant role for this material.
asthma; allergy; innate immunity; Toll-like receptors; pollution
In many subjects who are genetically susceptible to asthma, exposure to environmental stimuli may exacerbate their condition. However, it is unknown how the expression and function of a family of pattern-recognition receptors called toll-like receptors (TLR) are affected by exposure to particulate pollution. TLRs serve a critical function in alerting the immune system of tissue damage or infection—the so-called “danger signals”. We are interested in the role that TLRs play in directing appropriate responses by innate immunity, particularly dendritic cells (DC), after exposing them to particulate pollution. Dendritic cells serve a pivotal role in directing host immunity. Thus, we hypothesized that alterations in TLR expression could be further explored as potential biomarkers of effect related to DC exposure to particulate pollution. We show some preliminary data that indicates that inhaled particulate pollution acts directly on DC by down-regulating TLR expression and altering the activation state of DC. While further studies are warranted, we suggest that alterations in TLR2 and TLR4 expression should be explored as potential biomarkers of DC exposure to environmental particulate pollution.
Toll-Like Receptors; Inflammation; Dendritic Cell; Pollution; Particulate Matter
Oxidative stress has been postulated to play an important role in the pathogenesis of asthma; although a defect in antioxidant responses has been speculated to exacerbate asthma severity, this has been difficult to demonstrate with certainty. Nuclear erythroid 2 p45-related factor 2 (Nrf2) is a redox-sensitive basic leucine zipper transcription factor that is involved in the transcriptional regulation of many antioxidant genes. We show that disruption of the Nrf2 gene leads to severe allergen-driven airway inflammation and hyperresponsiveness in mice. Enhanced asthmatic response as a result of ovalbumin sensitization and challenge in Nrf2-disrupted mice was associated with more pronounced mucus cell hyperplasia and infiltration of eosinophils into the lungs than seen in wild-type littermates. Nrf2 disruption resulted in an increased expression of the T helper type 2 cytokines interleukin (IL)-4 and IL-13 in bronchoalveolar lavage fluid and in splenocytes after allergen challenge. The enhanced severity of the asthmatic response from disruption of the Nrf2 pathway was a result of a lowered antioxidant status of the lungs caused by lower basal expression, as well as marked attenuation, of the transcriptional induction of multiple antioxidant genes. Our studies suggest that the responsiveness of Nrf2-directed antioxidant pathways may act as a major determinant of susceptibility to allergen-mediated asthma.
While transport of RNA-binding protein HuR from nucleus to cytoplasm is emerging as a key regulatory step for HuR function, the mechanisms underlying this process remain poorly understood. Here, we report that the AMP-activated kinase (AMPK), an enzyme involved in responding to metabolic stresses, potently regulates the levels of cytoplasmic HuR. Inhibition of AMPK, accomplished either through cell treatment or by adenovirus infection to express dominant-negative AMPK, was found to increase the level of HuR in the cytoplasm and to enhance the binding of HuR to p21, cyclin B1, and cyclin A mRNA transcripts and elevate their expression and half-lives. Conversely, AMPK activation, achieved by means including infection to express constitutively active AMPK, resulted in reduced cytoplasmic HuR; decreased levels and half-lives of mRNAs encoding p21, cyclin A, and cyclin B1; and diminished HuR association with the corresponding transcripts. We therefore propose a novel function for AMPK as a regulator of cytoplasmic HuR levels, which in turn influences the mRNA-stabilizing function of HuR and the expression of HuR target transcripts.