Tissue involution and the acute phase response

Effects of mouse and human lipocalin homologues 24p3/lcn2 and neutrophil gelatinase-associated lipocalin on gastrointestinal mucosal integrity and repair

BACKGROUND & AIMS

The lipocalin superfamily, including the mouse and human homologues 24p3/lcn2 and neutrophil gelatinase-associated lipocalin, show great functional diversity including roles in olfaction, transportation, and prostaglandin synthesis in mammals. Their potential role in maintaining gastrointestinal mucosal integrity and repair is, however, unclear.

METHODS

Changes in 24p3/lcn2 expression in the mouse gut in response to various noxious agents were examined using Northern blot, in situ hybridization, and immunohistochemistry. Effects of recombinant 24p3/lcn2 on proliferation ([3H]-thymidine uptake), and restitution (cell-wounding migration) were assessed using human colonic HT29 and HCT116 cells. In addition, the effects of recombinant 24p3/lcn2 on the amount of gastric damage were assessed in rats treated with indomethacin (20 mg/kg) and restraint.

RESULTS

Marked up-regulation of expression of 24p3/lcn2 was seen throughout the gut in response to indomethacin or dextran sodium sulfate treatment. Expression was increased particularly in the surface epithelial cells and infiltrating inflammatory cells. Proliferation and restitution assays in the presence of recombinant wild-type sequence neutrophil gelatinase-associated lipocalin, wild-type cys(98)-24p3/lcn2, and mutant ala98-24p3/lcn2 showed that all 3 peptides caused a 3- to 4-fold increase in promigratory activity (P < .01 vs control) but did not influence proliferation. The administration of wild-type cys98-, or mutant ala98-24p3/lcn2 (25 and 50 microg/kg/h, respectively), given via the subcutaneous route, both caused similar reductions in the rat gastric damage model (60% reduction at highest dose, P < .01 vs control), although oral administration was ineffective.

CONCLUSIONS

24p3/lcn2 facilitates mucosal regeneration by promoting cell migration.

Identification of common transcriptional regulatory elements in interleukin-17 target genes

Interleukin (IL)-17 is the founding member of a novel family of inflammatory cytokines. Although produced by T cells, IL-17 activates genes and signals typical of innate immune mediators such as tumor necrosis factor (TNF)-alpha and IL-1beta. Most IL-17 target genes characterized to date are cytokines or neutrophil-attractive chemokines. Our recent microarray studies identified an acute phase response gene, 24p3/lipocalin 2, as a novel IL-17-induced gene. Here we describe a detailed analysis of the 24p3 promoter. We find that, unlike cytokine or chemokine gene target genes, 24p3 is regulated primarily at the level of transcription rather than mRNA stability and that synergy between IL-17 and TNFalpha occurs at the level of the 24p3 promoter. Two key transcription factor binding sites (TFBS) were identified, corresponding to NF-kappaB and CCAAT/enhancer-binding protein (C/EBP). Deletion of either site eliminated 24p3 promoter activity in response to IL-17. These findings were strikingly similar to the IL-6 promoter, where IL-17-mediated regulation of both NF-kappaB and C/EBP is essential. To determine whether joint use of NF-kappaB and C/EBP is common to all IL-17 target genes, we performed a computational analysis on 18 well documented IL-17 target promoters to assess statistical enrichment of specific TFBSs. Indeed, NF-kappaB and C/EBP sites were over-represented in these genes, as were AP1 and OCT1 sites. Moreover, these promoters fell into three definable subcategories based on TFBS location and usage. Analysis of IL-17 target gene regulation is key for understanding this important host-defense molecule and also contributes to an understanding of upstream signaling mechanisms used by IL-17, either alone or in concert with TNFalpha.

Lipocalin 2-deficient mice exhibit increased sensitivity to Escherichia coli infection but not to ischemia-reperfusion injury

Diverse functions have been reported for lipocalin 2. To investigate these functions in vivo, we generated gene-targeted lipocalin 2-deficient mice (Lcn2-/- mice). In vitro studies have suggested that lipocalin 2 is important for cellular apoptosis induced by IL-3 withdrawal, and for the induction of kidney differentiation during embryogenesis. Analysis of Lcn2-/- mice showed normal cell death upon IL-3 withdrawal and normal kidney development. However, we found that Lcn2-/- mice exhibited an increased susceptibility to bacterial infections, in keeping with the proposed function of lipocalin 2 in iron sequestration. Neutrophils isolated from Lcn2-/- mice showed significantly less bacteriostatic activity compared with WT controls. The bacteriostatic property of the WT neutrophils was abolished by the addition of exogenous iron, indicating that the main function of lipocalin 2 in the antibacterial innate immune response is to limit this essential element. Another important function ascribed to lipocalin 2 has been its protective role against kidney ischemia-reperfusion injury. We analyzed Lcn2-/- mice using a mouse model for severe renal failure and could not detect any significant differences compared with their WT littermates.

Neutrophil gelatinase-associated lipocalin as a survival factor

NGAL (human neutrophil gelatinase-associated lipocalin) and its mouse analogue 24p3 are members of the lipocalin family of small secreted proteins. These proteins are up-regulated in a number of pathological conditions, including cancers, and may function as transporters of essential factors. Although previous publications have suggested that 24p3 has pro-apoptotic functions, other data are more suggestive of a survival function. The current study was designed to determine whether NGAL is pro- or anti-apoptotic. Apoptosis induced in human adenocarcinoma A549 cells by the 5-lipoxygenase-activating-protein inhibitor MK886, or several celecoxib-derived PDK1 (phosphoinositide-dependent kinase 1) inhibitors that are devoid of cyclo-oxygenase-2 inhibitory activity, was accompanied by a dose- and time-dependent increase of NGAL mRNA levels, as was reported previously with 24p3. A similar induction of NGAL mRNA was observed in human breast cancer MCF7 cells treated with MK886, indicating this was not a cell-specific effect. Treatment of A549 cells with up to 150 mug/10(6) cells of purified recombinant NGAL protein had no effect on viability, whereas antisera against the full-length NGAL protein induced apoptosis in these cells. The stable overexpression of NGAL in A549 cells had no effect on proliferation or viability. However, the cell death induced by a PDK1 inhibitor was reduced by 50% in NGAL-overexpressing cells. Decreasing NGAL mRNA and protein expression with siRNA (small interfering RNA) in A549 cells increased the toxicity of a PDK1 inhibitor by approx. 45%. These data indicate that, although the induction of NGAL correlates with apoptosis, this induction represents a survival response. Because NGAL is a secreted protein, it may play an extracellular role in cell defence against toxicants and/or facilitate the survival of the remaining cells.

Exploiting genotypic differences to identify genes important for EAE development

Experimental autoimmune encephalomyelitis (EAE) is an animal model of the human autoimmune disease multiple sclerosis (MS) and is primarily driven by T helper type 1 (Th1) cells. Interleukin (IL)-12 and interferon (IFN)-gamma are important cytokines involved in the differentiation and amplification of Th1 cells, however mice deficient in either IFN-gamma or IL-12 still develop EAE. We have used microarray analysis of EAE-affected CNS tissues in wild-type, IFN-gamma -/- and IL-12 -/- animals to identify genes critical for development of EAE. Over 500 genes were regulated in at least one genotype and over 94 genes were regulated in all three. Of those, 17 were also upregulated in spleen during the disease. We show that a majority of the genes regulated in EAE are also regulated in diseased regions of human MS tissues. The genes in the pool of 94 are more likely to be found regulated in MS patients than the genes regulated in only one or two of the mouse strains suggesting that analyzing gene expression under these multiple genetic conditions may lead to better identification of the genes critical for disease development.

Identification of neutrophil gelatinase-associated lipocalin (NGAL) as a discriminatory marker of the hepatocyte-secreted protein response to IL-1beta: a proteomic analysis

The liver is the major source of proteins used throughout the body for various functions. Upon injury or infection, an acute phase response (APR) is initiated in the liver that is primarily mediated by inflammatory cytokines such as interleukin-1beta (IL-1beta) and interleukin-6. Among others, the APR is characterized by an altered protein synthetic profile. We used two-dimensional gel electrophoresis to study the dynamics of changes in protein synthesis in hepatocytes exposed to these inflammatory cytokines. Protein profiles were quantified using image analysis and further analyzed using multivariate statistical methods. Our results indicate that IL-1beta and IL-6 each induces secreted protein responses with distinct dynamics and dose-dependence. Parallel stimulation by IL-1beta and IL-6 results in a protein pattern indistinguishable from the IL-1beta pattern, indicating a dominant effect of IL-1beta over IL-6 at the doses tested. Multidimensional scaling (MDS) of correlation distances between protein secretion levels revealed two protein pairs that are robustly co-secreted across the various cytokine stimulation conditions, suggesting shared regulatory pathways. Finally, we also used multivariate alternating conditional expectation (MACE) to identify transformation functions that discriminated the cytokine-stimulated and untreated hepatocyte-secreted protein profiles. Our analysis indicates that the expression of neutrophil gelatinase-associated lipocalin (NGAL) was sufficient to discriminate between IL-1beta and IL-6 stimulation. The combination of proteomics and multivariate analysis is expected to provide new information on the cellular regulatory networks involved in generating specific cellular responses.

Expression of SIP24 in the peripartum and postpartum rat uterus

SIP24 is, a 24 kDa superinducible protein, is an acute phase protein that is expressed in several tissues and organs and is identified in the human, mouse, and rat genome as lipocalin 2 (lcn2). This study investigated SIP24 expression by the rat uterus just before delivery and during postpartum involution. The distribution and levels of expression of SIP24 and myeloperoxidase (MPO) were compared by Western blot analysis and immunocytochemistry. The SIP24 level was high on day 22 of pregnancy and on days 1 and 2 postpartum, decreased on day 3 postpartum, and declined to low levels similar to those in nonpregnant rats by day 5 postpartum. SIP24 positive cells were identified by immunohistochemistry in the luminal and glandular epithelium on day 22 of pregnancy and on days 1, 2, 3, and 5 postpartum in addition to nonpregnant rats. Immunostaining was intense on day 22 of pregnancy and on days 1 and 2 postpartum. MPO also was identified in uterine tissues by immunohistochemistry. An evaluation of the SIP24 and MPO distribution and levels of expression in the rat uterus is consistent with the hypothesis that SIP24 modulates the neutrophil population by activities such as inducing their apoptosis.

Cytokines link osteoblasts and inflammation: microarray analysis of interleukin-17- and TNF-alpha-induced genes in bone cells

The novel cytokine interleukin (IL)-17 has been implicated in many infectious and autoimmune settings, especially rheumatoid arthritis. Consistent with its proinflammatory effects on bone, osteoblast cells are highly responsive to IL-17, particularly in combination with other inflammatory cytokines. To better understand the spectrum of activities controlled by IL-17, we globally profiled genes regulated by IL-17 and tumor necrosis factor alpha (TNF-alpha) in the preosteoblast cell line MC3T3-E1. Using Affymetrix microarrays, 80-90 genes were up-regulated, and 19-50 genes were down-regulated with IL-17 and TNF-alpha as compared with TNF-alpha alone. These included proinflammatory chemokines and cytokines, inflammatory genes, transcriptional regulators, bone-remodeling genes, signal transducers, cytoskeletal genes, genes involved in apoptosis, and several unknown or unclassified genes. The CXC family chemokines were most dramatically induced by IL-17 and TNF-alpha, confirming the role of IL-17 as a potent mediator of inflammation and neutrophil recruitment. Several transcription factor-related genes involved in inflammatory gene expression were also enhanced, including molecule possessing ankyrin repeats induced by lipopolysaccharide/inhibitor of kappaBzeta (MAIL/kappaBzeta), CCAAT/enhancer-binding protein delta (C/EBPdelta), and C/EBPbeta. We also identified the acute-phase gene lipocalin-2 (LCN2/24p3) as a novel IL-17 target, which is regulated synergistically by TNF-alpha and IL-17 at the level of its promoter. A similar but not identical pattern of genes was induced by IL-17 and TNF-alpha in ST2 bone marrow stromal cells and murine embryonic fibroblasts. This study provides a profile of genes regulated by IL-17 and TNF-alpha in osteoblasts and suggests that in bone, the major function of IL-17 is to cooperate and/or synergize with other cytokines to amplify inflammation.

IL-22, a Th1 cytokine that targets the pancreas and select other peripheral tissues

IL-22, also termed IL-TIF, is a member of the IL-10 family of cytokines. Its principal source appears to be memory CD4 T cells with a Th1 polarized phenotype. IL-22 induces its signals through a two-component receptor comprised of IL-22R1 and CRF2-4/IL10Rb. Both of these receptor components also participate in separate receptor complexes specific for other IL-10 family cytokines. Because CRF2-4 exhibits ubiquitous expression, the tropism of IL-22 action appears to be dictated by the expression of IL-22R1. IL-22R1 has a highly restricted expression pattern. Its highest expression, by far, is in the acinar cell population of the pancreas. Lower, but still functional, levels of expression are also observed in skin, colon, liver, and kidney. The responses that have been observed to date for IL-22 resemble the "acute phase" type responses elicited by IL-6, suggesting that IL-22 might be appropriately considered as a T cell-derived IL-6-like activity having distinct target cell specificity. The functional role of this system remains unclear, but it is likely that the responses elicited by this cytokine serve to contribute both to acute host defense against pathogens and to safeguard vulnerable target tissues under conditions of stress.

Evolving views of involution

The developmental preparation of the mammary gland for milk production that occurs during pregnancy is followed by an equally dramatic process of involution as the gland returns to its prepregnancy state. Two detailed, microarray-based surveys reported in this issue extend our understanding of the nature and timing of molecular and cellular events in involution that underlie these developmental changes.

Gene expression profiling of mammary gland development reveals putative roles for death receptors and immune mediators in post-lactational regression

INTRODUCTION

In order to gain a better understanding of the molecular processes that underlie apoptosis and tissue regression in mammary gland, we undertook a large-scale analysis of transcriptional changes during the mouse mammary pregnancy cycle, with emphasis on the transition from lactation to involution.

METHOD

Affymetrix microarrays, representing 8618 genes, were used to compare mammary tissue from 12 time points (one virgin, three gestation, three lactation and five involution stages). Six animals were used for each time point. Common patterns of gene expression across all time points were identified and related to biological function.

RESULTS

The majority of significantly induced genes in involution were also differentially regulated at earlier stages in the pregnancy cycle. This included a marked increase in inflammatory mediators during involution and at parturition, which correlated with leukaemia inhibitory factor-Stat3 (signal transducer and activator of signalling-3) signalling. Before involution, expected increases in cell proliferation, biosynthesis and metabolism-related genes were observed. During involution, the first 24 hours after weaning was characterized by a transient increase in expression of components of the death receptor pathways of apoptosis, inflammatory cytokines and acute phase response genes. After 24 hours, regulators of intrinsic apoptosis were induced in conjunction with markers of phagocyte activity, matrix proteases, suppressors of neutrophils and soluble components of specific and innate immunity.

CONCLUSION

We provide a resource of mouse mammary gene expression data for download or online analysis. Here we highlight the sequential induction of distinct apoptosis pathways in involution and the stimulation of immunomodulatory signals, which probably suppress the potentially damaging effects of a cellular inflammatory response while maintaining an appropriate antimicrobial and phagocytic environment.