Proliferation-independent induction of macrophage cyclin D2, and repression of cyclin D1, by lipopolysaccharide

Cyclin J-CDK complexes limit innate immune responses by reducing proinflammatory changes in macrophage metabolism

Toll-like receptor (TLR) stimulation induces glycolysis and the production of mitochondrial reactive oxygen species (ROS), both of which are critical for inflammatory responses in macrophages. Here, we demonstrated that cyclin J, a TLR-inducible member of the cyclin family, reduced cytokine production in macrophages by coordinately controlling glycolysis and mitochondrial functions. Cyclin J interacted with cyclin-dependent kinases (CDKs), which increased the phosphorylation of a subset of CDK substrates, including the transcription factor FoxK1 and the GTPase Drp1. Cyclin J-dependent phosphorylation of FoxK1 decreased the transcription of glycolytic genes and Hif-1α activation, whereas hyperactivation of Drp1 by cyclin J-dependent phosphorylation promoted mitochondrial fragmentation and impaired the production of mitochondrial ROS. In mice, cyclin J in macrophages limited the growth of tumor xenografts and protected against LPS-induced shock but increased the susceptibility to bacterial infection. Collectively, our findings indicate that cyclin J-CDK signaling promotes antitumor immunity and the resolution of inflammation by opposing the metabolic changes that drive inflammatory responses in macrophages.

Rapid Alterations in Perirenal Adipose Tissue Transcriptomic Networks with Cessation of Voluntary Running

In maturing rats, the growth of abdominal fat is attenuated by voluntary wheel running. After the cessation of running by wheel locking, a rapid increase in adipose tissue growth to a size that is similar to rats that have never run (i.e. catch-up growth) has been previously reported by our lab. In contrast, diet-induced increases in adiposity have a slower onset with relatively delayed transcriptomic responses. The purpose of the present study was to identify molecular pathways associated with the rapid increase in adipose tissue after ending 6 wks of voluntary running at the time of puberty. Age-matched, male Wistar rats were given access to running wheels from 4 to 10 weeks of age. From the 10^th to 11^th week of age, one group of rats had continued wheel access, while the other group had one week of wheel locking. Perirenal adipose tissue was extracted, RNA sequencing was performed, and bioinformatics analyses were executed using Ingenuity Pathway Analysis (IPA). IPA was chosen to assist in the understanding of complex ‘omics data by integrating data into networks and pathways. Wheel locked rats gained significantly more fat mass and significantly increased body fat percentage between weeks 10–11 despite having decreased food intake, as compared to rats with continued wheel access. IPA identified 646 known transcripts differentially expressed (p < 0.05) between continued wheel access and wheel locking. In wheel locked rats, IPA revealed enrichment of transcripts for the following functions: extracellular matrix, macrophage infiltration, immunity, and pro-inflammatory. These findings suggest that increases in visceral adipose tissue that accompanies the cessation of pubertal physical activity are associated with the alteration of multiple pathways, some of which may potentiate the development of pubertal obesity and obesity-associated systemic low-grade inflammation that occurs later in life.

p75NTR enhances PC12 cell tumor growth by a non-receptor mechanism involving downregulation of cyclin D2

p75NTR is a member of the tumor necrosis superfamily of proteins which is variably associated with induction of apoptosis and proliferation. Cyclin D2 is one of the mediators of cellular progression through G1 phase of the cell cycle. The present study demonstrates the inverse relationship between expression of cyclin D2 and expression of p75NTR in PC12 cells. Induction of p75NTR expression in p75NTR-negative PC12 cells results in downregulation of cyclin D2; suppression of p75NTR expression with siRNA in native PC12 cells results in upregulation of cyclin D2. The effects of p75NTR on cyclin D2 expression are mimicked in p75NTR-negative cells by transfection with the intracellular domain of p75NTR. Cyclin-D2-positive PC12 cell cultures grow more slowly than cyclin-D2-negative cultures, and induction of expression of cyclin D2 slows the culture growth rate of cyclin-D2-negative cells. Finally, subcutaneous murine xenografts of cyclin-D2-negative, p75NTR-positive PC12 cells more frequently and more rapidly produce tumors than the analogous xenografts of cyclin-D2-positive, p75NTR-negative cells. These results suggest that p75NTR suppresses cyclin D2 expression in PC12 cells by a mechanism distinct from its function as a nerve growth factor receptor and that cyclin D2 expression decreases cell culture and xenografted tumor growth.

Schlafen-1 causes a cell cycle arrest by inhibiting induction of cyclin D1

Schlafen-1 (Slfn-1), the prototypic member of the Schlafen family of proteins, was described as an inducer of growth arrest in T-lymphocytes and causes a cell cycle arrest in NIH3T3 fibroblasts prior to the G1/S transition. How Slfn-1 exerts its effects on the cell cycle is not currently known. We report that synchronized murine fibroblasts expressing Slfn-1 do not exit G1 when stimulated with fetal calf serum, platelet-derived growth factor BB (PDGF-BB) or epidermal growth factor (EGF). The induction of cyclin D1 by these stimuli was blocked in the presence of Slfn-1 as were all downstream cell cycle processes. Overexpression of cyclin D1 in growth-arrested, Slfn-1-expressing cells induced an increase in cell growth consistent with this protein being the biological target of Slfn-1. Activation of the mitogen-activated protein kinase pathway by EGF or phorbol 12-myristate 13-acetate was unaffected by Slfn-1 expression. PDGF signaling was, however, almost completely blocked. This was due to a lack of PDGF receptor expression in Slfn-1-expressing cells consistent with Slfn-1 blocking the cell cycle in G1 where PDGF receptor expression is normally down-regulated. Finally, overexpression of Slfn-1 inhibited the activation of the cyclin D1 promoter. Slfn-1 therefore causes a cell cycle arrest during G1 by inhibiting induction of cyclin D1 by mitogens.

Macrophages exposed continuously to lipopolysaccharide and other agonists that act via toll-like receptors exhibit a sustained and additive activation state

BACKGROUND

Macrophages sense microorganisms through activation of members of the Toll-like receptor family, which initiate signals linked to transcription of many inflammation associated genes. In this paper we examine whether the signal from Toll-like receptors [TLRs] is sustained for as long as the ligand is present, and whether responses to different TLR agonists are additive.

RESULTS

RAW264 macrophage cells were doubly-transfected with reporter genes in which the IL-12p40, ELAM or IL-6 promoter controls firefly luciferase, and the human IL-1beta promoter drives renilla luciferase. The resultant stable lines provide robust assays of macrophage activation by TLR stimuli including LPS [TLR4], lipopeptide [TLR2], and bacterial DNA [TLR9], with each promoter demonstrating its own intrinsic characteristics. With each of the promoters, luciferase activity was induced over an 8 hr period, and thereafter reached a new steady state. Elevated expression required the continued presence of agonist. Sustained responses to different classes of agonist were perfectly additive. This pattern was confirmed by measuring inducible cytokine production in the same cells. While homodimerization of TLR4 mediates responses to LPS, TLR2 appears to require heterodimerization with another receptor such as TLR6. Transient expression of constitutively active forms of TLR4 or TLR2 plus TLR6 stimulated IL-12 promoter activity. The effect of LPS, a TLR4 agonist, was additive with that of TLR2/6 but not TLR4, whilst that of lipopeptide, a TLR2 agonist, was additive with TLR4 but not TLR2/6. Actions of bacterial DNA were additive with either TLR4 or TLR2/6.

CONCLUSIONS

These findings indicate that maximal activation by any one TLR pathway does not preclude further activation by another, suggesting that common downstream regulatory components are not limiting. Upon exposure to a TLR agonist, macrophages enter a state of sustained activation in which they continuously sense the presence of a microbial challenge.

IFN-gamma inhibits human airway smooth muscle cell proliferation by modulating the E2F-1/Rb pathway

Elucidating the factors that inhibit the increase in airway smooth muscle (ASM) mass may be of therapeutic benefit in asthma. Here, we investigated whether interferon-gamma (IFN-gamma), a potent inducer of growth arrest in various cell types, regulates mitogen-induced ASM cell proliferation. IFN-gamma (1-100 U/ml) was found to markedly decrease both DNA synthesis and ASM cell number induced by the mitogens epidermal growth factor (EGF) and thrombin. Interestingly, IFN-gamma had no effect on mitogen-induced activation of three major mitogenic signaling pathways, phosphatidylinositol 3-kinase, p70(S6k), or mitogen-activated protein kinases. Mitogen-induced expression of cell cycle regulator cyclin D1 was increased by IFN-gamma, whereas no effect was observed on degradation of p27(Kip1). Expression array analysis of 23 cell cycle-related genes showed that IFN-gamma inhibited EGF-induced increases in E2F-1 expression, whereas induction of c-myc, cyclin D2, Egr-1, and mdm2 were unaffected. Induction of E2F-1 protein and Rb hyperphosphorylation after mitogen stimulation was also suppressed by IFN-gamma. In addition, IFN-gamma decreased activation of cdk2 and expression of cyclin E, upstream signaling molecules responsible for Rb hyperphosphorylation in the late G1 phase. IFN-gamma also increased levels of IFI 16 protein, whose mouse homolog p202 has been associated with growth inhibition. Together, our data indicate that IFN-gamma is an effective inhibitor of ASM cell proliferation by blocking transition from G1-to-S phase by acting at two different levels: modulation of cdk2/cyclin E activation and inhibition of E2F-1 gene expression.

Overexpression of cyclin D2 is associated with increased in vivo invasiveness of human squamous carcinoma cells

Overexpression of cyclin D2 was studied in 10 human squamous cell carcinoma lines, to establish whether this gene plays a role in tumor progression. We found that those cell lines that overexpressed cyclin D2 (CCND2) had the most invasive in vivo behavior. The invasive ability of the cell lines was determined by evaluating the penetration of carcinoma cells into the tracheal wall in an in vivo assay with de-epithelialized tracheas transplanted into the subcutaneous tissue of nude mice. From five cell lines that exhibited low invasive ability, we selected two that had very little CCND2 expression (SCC9 and SCC15), to evaluate whether CCND2 gene transfer would increase the invasive behavior. After confirming the successful transfer of CCND2 by Northern, Western, and kinase-activity assays, we assessed the in vivo invasive behavior of the CCND2-transfected cells and their respective vector alone-transfected controls. The cell lines containing the transferred CCND2 gene had a significantly higher invasive ability than respective controls. This was accompanied by a moderate increase in gelatinase activity. In addition, the in vitro proliferative abilities, under normal culture conditions, of the parental CCND2-transfected and vector alone-transfected cells were found to be similar, as was the in vivo labeling index of Ki-67 in the tracheal transplants. These results indicated that the overexpression of CCND2 in squamous cell carcinoma lines modulates cell proliferation after induced quiescence and also has a powerful enhancing effect on in vivo aggressive growth behavior.

Cooperation of multiple signaling pathways in CD40-regulated gene expression in B lymphocytes

CD40/CD40L interaction is essential for multiple biological events in T dependent humoral immune responses, including B cell survival and proliferation, germinal center and memory B cell formation, and antibody isotype switching and affinity maturation. By using high-density microarrays, we examined gene expression in primary mouse B lymphocytes after multiple time points of CD40L stimulation. In addition to genes involved in cell survival and growth, which are also induced by other mitogens such as lipopolysaccharide, CD40L specifically activated genes involved in germinal center formation and T cell costimulatory molecules that facilitate T dependent humoral immunity. Next, by examining the roles of individual CD40-activated signal transduction pathways, we dissected the overall CD40-mediated response into genes independently regulated by the individual pathways or collectively by all pathways. We also found that gene down-regulation is a significant part of the overall response and that the p38 pathway plays an important role in this process, whereas the NF-kappa B pathway is important for the up-regulation of primary response genes. Our finding of overlapping independent control of gene expression modules by different pathways suggests, in principle, that distinct biological behaviors that depend on distinct gene expression subsets can be manipulated by targeting specific signaling pathways.

Lipopolysaccharide-induced cell cycle arrest in macrophages occurs independently of nitric oxide synthase II induction

Lipopolysaccharide (LPS, a Gram-negative bacterium cell wall component) is a potent macrophage activator that inhibits macrophage proliferation and stimulates production of nitric oxide (NO) via NO synthase II (NOSII). We investigated whether NO mediates the LPS-stimulated cell cycle arrest in mouse bone marrow-derived macrophages (BMM). The addition of the NO donor DETA NONOate (200 microM) inhibited BMM proliferation by approx. 80%. However, despite NO being an antimitogen, LPS was as potent at inhibiting proliferation in BMM derived from NOSII-/- mice as from wild-type mice. Consistent with these findings, LPS-induced cell cycle arrest in normal BMM was not reversed by the addition of the NOSII inhibitor S-methylisothiourea. Moreover, in both normal and NOSII-/- BMM, LPS inhibited the expression of cyclin D1, a protein that is essential for proliferation in many cell types. Despite inhibiting proliferation DETA NONOate had no effect on cyclin D1 expression. Our data indicate that while both LPS and NO inhibit BMM proliferation, LPS inhibition of BMM proliferation can occur independently of NOSII induction.

Osmotic stress regulates the stability of cyclin D1 in a p38SAPK2-dependent manner

We report here that different cell stresses regulate the stability of cyclin D1 protein. Exposition of Granta 519 cells to osmotic shock, oxidative stress, and arsenite induced the post-transcriptional down-regulation of cyclin D1. In the case of osmotic shock, this effect was completely reversed by the addition of p38(SAPK2)-specific inhibitors (SB203580 or SB220025), indicating that this effect is dependent on p38(SAPK2) activity. Moreover, the use of proteasome inhibitors prevented this down-regulation. Thus, osmotic shock induces proteasomal degradation of cyclin D1 protein by a p38(SAPK2)-dependent pathway. The effect of p38(SAPK2) on cyclin D1 stability might be mediated by direct phosphorylation at specific sites. We found that p38(SAPK2) phosphorylates cyclin D1 in vitro at Thr(286) and that this phosphorylation triggers the ubiquitination of cyclin D1. These results link for the first time a stress-induced MAP kinase pathway to cyclin D1 protein stability, and they will help to understand the molecular mechanisms by which stress transduction pathways regulate the cell cycle machinery and take control over cell proliferation.

Salmonella typhimurium infection and lipopolysaccharide stimulation induce similar changes in macrophage gene expression

Changes in macrophage phenotype induced during infection result from the recognition of bacterial products as well as the action of bacterial virulence factors. We used the unprecedented opportunity provided by gene arrays to simultaneously study the expression of hundreds of genes during Salmonella typhimurium infection of macrophages and to assess the contribution of the bacterial virulence factor, LPS, in initiating the host responses to Salmonella. We found that S. typhimurium infection caused significant changes in the expression of numerous genes encoding chemokines, cell surface receptors, signaling molecules, and transcriptional activators at 4 h postinfection of the RAW 264.7 murine macrophage cell line. Our results revealed changes in the expression of several genes that had not been previously implicated in the host responses to S. typhimurium infection, as well as changes in the expression of several genes previously shown to be regulated by S. typhimurium infection. An overlapping spectrum of genes was expressed in response to virulent S. typhimurium and purified S. typhimurium LPS, reinforcing the major role of this surface molecule in stimulating the early response of macrophages to bacterial infection. The macrophage gene expression profile was further altered by activation with IFN-gamma, indicating that host cell responses depend on the activation state of the cell.

Type I interferons mediate the lipopolysaccharide induction of macrophage cyclin D2

Lipopolysaccharide (LPS) is a powerful macrophage-activating agent and antimitogen. We recently showed that LPS unexpectedly induces cyclin D2 in macrophages. Since LPS stimulates macrophages to produce autocrine-acting cytokines, we examined whether LPS induction of cyclin D2 was mediated by one such type of cytokine, type I interferons (IFN). We report that bone marrow-derived macrophages (BMM) lacking a component of the type I interferon receptor (IFNAR-1) do not express cyclin D2 mRNA or protein in response to LPS stimulation (0.01-1 microg/ml for 7-30 h). Consistent with this result, addition of anti-IFN-alpha/beta neutralizing antibodies reduced levels of LPS-stimulated cyclin D2 in normal BMM. Furthermore, IFN-alpha alone induced cyclin D2 mRNA and protein in normal BMM. Thus, we have identified a new role for type I IFN in macrophages, namely, as essential mediators of LPS-stimulated cyclin D2 expression.

Bacterial/CpG DNA Down-Modulates Colony Stimulating Factor-1 Receptor Surface Expression on Murine Bone Marrow-Derived Macrophages with Concomitant Growth Arrest and Factor-Independent Survival

Unmethylated CpG motifs within bacterial DNA constitute a pathogen-associated molecular pattern recognized by the innate immune system. Many of the immunomodulatory functions of bacterial DNA can be ascribed to the ability to activate macrophages and dendritic cells. Here we show stimulatory DNA, like LPS, caused growth arrest of murine bone marrow-derived macrophages proliferating in CSF-1. Stimulatory DNA caused selective down-modulation of CSF-1 receptor surface expression. Flow cytometric analysis of CSF-1-deprived bone marrow-derived macrophages revealed that in contrast to the synchronous reduction of CSF-1 receptor upon CSF-1 addition, activating DNA (both bacterial DNA and CpG-containing oligonucleotide) caused rapid removal of receptor from individual cells leading to a bimodal distribution of surface expression at intermediate times or submaximal doses of stimulus. Despite causing growth arrest, both stimulatory DNA and LPS promoted factor-independent survival of bone marrow-derived macrophages, which was associated with phosphorylation of the mitogen-activated protein kinase family members, extracellular-regulated kinase 1 and 2. CSF-1 receptor down-modulation may polarize the professional APC compartment to the more immunostimulatory dendritic cell-like phenotype by suppressing terminal macrophage differentiation mediated by CSF-1.