Macrophage growth arrest by cyclic AMP defines a distinct checkpoint in the mid-G1 stage of the cell cycle and overrides constitutive c-myc expression

Understanding the Biology of Self-Renewing Macrophages

Macrophages reside in specific territories in organs, where they contribute to the development, homeostasis, and repair of tissues. Recent work has shown that the size of tissue macrophage populations has an impact on tissue functions and is determined by the balance between replenishment and elimination. Macrophage replenishment is mainly due to self-renewal of macrophages, with a secondary contribution from blood monocytes. Self-renewal is a recently discovered trait of macrophages, which can have a major impact on their physiological functions and hence on the wellbeing of the organism. In this review, I discuss our current understanding of the developmental origin of self-renewing macrophages and the mechanisms used to maintain a physiologically stable macrophage pool.

The Ubiquitin Proteome of Toxoplasma gondii Reveals Roles for Protein Ubiquitination in Cell-Cycle Transitions

Protein ubiquitination plays key roles in protein turnover, cellular signaling, and intracellular transport. The genome of Toxoplasma gondii encodes ubiquitination machinery, but the roles of this posttranslational modification (PTM) are unknown. To examine the prevalence and function of ubiquitination in T. gondii, we mapped the ubiquitin proteome of tachyzoites. Over 500 ubiquitin-modified proteins, with almost 1,000 sites, were identified on proteins with diverse localizations and functions. Enrichment analysis demonstrated that 35% of ubiquitinated proteins are cell-cycle regulated. Unexpectedly, most classic cell-cycle regulators conserved in T. gondii were not detected in the ubiquitinome. Furthermore, many ubiquitinated proteins localize to the cytoskeleton and inner membrane complex, a structure beneath the plasma membrane facilitating division and host invasion. Comparing the ubiquitinome with other PTM proteomes reveals waves of PTM enrichment during the cell cycle. Thus, T. gondii PTMs are implicated as critical regulators of cell division and cell-cycle transitions.

PI3K p110γ deletion attenuates murine atherosclerosis by reducing macrophage proliferation but not polarization or apoptosis in lesions

Atherosclerosis is an inflammatory disease regulated by infiltrating monocytes and T cells, among other cell types. Macrophage recruitment to atherosclerotic lesions is controlled by monocyte infiltration into plaques. Once in the lesion, macrophage proliferation in situ, apoptosis, and differentiation to an inflammatory (M1) or anti-inflammatory phenotype (M2) are involved in progression to advanced atherosclerotic lesions. We studied the role of phosphoinositol-3-kinase (PI3K) p110γ in the regulation of in situ apoptosis, macrophage proliferation and polarization towards M1 or M2 phenotypes in atherosclerotic lesions. We analyzed atherosclerosis development in LDLR^−/−p110γ^+/− and LDLR^−/−p110γ^−/− mice, and performed expression and functional assays in tissues and primary cells from these and from p110γ^+/− and p110γ^−/− mice. Lack of p110γ in LDLR^−/− mice reduces the atherosclerosis burden. Atherosclerotic lesions in fat-fed LDLR^−/−p110γ^−/− mice were smaller than in LDLR^−/−p110γ^+/− controls, which coincided with decreased macrophage proliferation in LDLR^−/−p110γ^−/− mouse lesions. This proliferation defect was also observed in p110γ^−/− bone marrow-derived macrophages (BMM) stimulated with macrophage colony-stimulating factor (M-CSF), and was associated with higher intracellular cyclic adenosine monophosphate (cAMP) levels. In contrast, T cell proliferation was unaffected in LDLR^−/−p110γ^−/− mice. Moreover, p110γ deficiency did not affect macrophage polarization towards the M1 or M2 phenotypes or apoptosis in atherosclerotic plaques, or polarization in cultured BMM. Our results suggest that higher cAMP levels and the ensuing inhibition of macrophage proliferation contribute to atheroprotection in LDLR^−/− mice lacking p110γ. Nonetheless, p110γ deletion does not appear to be involved in apoptosis, in macrophage polarization or in T cell proliferation.

TLS-ERG leukemia fusion protein deregulates cyclin-dependent kinase 1 and blocks terminal differentiation of myeloid progenitor cells

TLS-ERG fusion protein is derived from the t(16;21) translocation found in human myeloid leukemia. Here, we show that retroviral transduction of TLS-ERG confers a growth advantage to L-G myeloid progenitor cells and blocks terminal differentiation. We found that the level of cyclin-dependent kinase 1 (Cdk1) protein was significantly decreased in controls but unchanged in TLS-ERG-expressing cells after granulocyte colony-stimulating factor treatment or interleukin-3 withdrawal. Injection of TLS-ERG-expressing L-G cells induced rapid development of a leukemia-like disease in syngeneic mice. Through site-directed mutagenesis, we showed that transformation and deregulation of Cdk1 by TLS-ERG require an intact ets DNA-binding domain within the fusion protein. Interestingly, treatment of TLS-ERG-expressing L-G cells with 5-aza-2'-deoxycytidine (Decitabine) or trichostatin A resulted in down-regulation of Cdk1 and induction of terminal differentiation. To investigate whether Cdk1 deregulation is indeed responsible for transformation by TLS-ERG, we constructed lentiviral vectors for delivery of Cdk1 mutants and small interfering RNA (siRNA). Both dominant-negative inhibition and siRNA knockdown of Cdk1 were able to restore the ability of TLS-ERG-expressing L-G cells to undergo terminal differentiation. In addition, siRNA knockdown of Cdk1 in YNH-1 cells derived from a t(16;21) acute myelogenous leukemia patient also resulted in terminal differentiation. As restoration of terminal myeloid differentiation to TLS-ERG cells is dependent on cell cycle arrest, our findings suggest an important role for Cdk1 in cellular transformation and may be useful in the search for new treatments of TLS-ERG-associated myeloid leukemia.

Bacillus anthracis edema toxin activates nuclear glycogen synthase kinase 3beta

Bacillus anthracis edema toxin (ET) generates high levels of cyclic AMP and impacts a complex network of signaling pathways in targeted cells. In the current study, we sought to identify kinase signaling pathways modulated by ET to better understand how this toxin alters cell physiology. Using a panel of small-molecule inhibitors of mammalian kinases, we found that inhibitors of glycogen synthase kinase 3 beta (GSK-3beta) protected cells from ET-induced changes in the cell cycle. GSK-3beta inhibitors prevented declines in cellular levels of cyclin D1 and c-Jun following treatment of macrophages with ET. Strikingly, cell fractionation experiments and confocal immunofluorescence microscopy revealed that ET activates a compartmentalized pool of GSK-3beta residing in the nuclei, but not in the cytoplasm, of macrophages. To investigate the outcome of this event, we examined the cellular location and activation state of beta-catenin, a critical substrate of GSK-3beta, and found that the protein was inactivated within the nucleus following intoxication with ET. To determine if ET could overcome the effects of stimuli that inactivate GSK-3beta, we examined the impact of the toxin on the Wnt signaling pathway. The results of these experiments revealed that by targeting GSK-3beta residing in the nucleus, ET circumvents the upstream cytoplasmic inactivation of GSK-3beta, which occurs following exposure to Wnt-3A. These findings suggest ET arrests the cell cycle by a mechanism involving activation of GSK-3beta residing in the nucleus, and by using this novel mechanism of intoxication, ET avoids cellular systems that would otherwise reverse the effects of the toxin.

Prostaglandin E2 mediates growth arrest in NFS-60 cells by down-regulating interleukin-6 receptor expression

Interleukin-6 (IL-6), a potent myeloid mitogen, and the immunosuppressive prostanoid prostaglandin E2 (PGE2) are elevated following thermal injury and sepsis. We have previously demonstrated that bone marrow myeloid commitment shifts toward monocytopoiesis and away from granulocytopoiesis during thermal injury and sepsis and that PGE2 plays a central role in this alteration. Here we investigated whether PGE2 can modulate IL-6-stimulated growth in the promyelocytic cell line, NFS-60, by down-regulating IL-6 receptor (IL-6r) expression. Exposure of NFS-60 cells to PGE2 suppressed IL-6-stimulated proliferation as well as IL-6r expression. Receptor down-regulation is functionally significant since IL-6-induced signal transduction through activators of transcription (STAT)-3 is also decreased. Down-regulation of IL-6r correlated with the ability of PGE2 to arrest cells in the G0/G1 phase of the cell cycle. PGE2 appears to signal through EP2 receptors. Butaprost (EP2 agonist) but not sulprostone (EP3 agonist) inhibited IL-6-stimulated proliferation. In addition, an EP2 antagonist (AH6809) alleviated the anti-proliferative effects of PGE2. NFS-60 cells express predominantly EP2 and EP4 receptors. While PGE2 down-regulated both the IL-6r protein and mRNA expression, it had no influence on EP2 or EP4 mRNA expression. The present study demonstrates that PGE2 is a potent down-regulator of IL-6r expression and thus may provide a mechanistic explanation for the granulocytopenia seen in thermal injury and sepsis.

A requirement for cyclin D3-cyclin-dependent kinase (cdk)-4 assembly in the cyclic adenosine monophosphate-dependent proliferation of thyrocytes

In different systems, cyclic adenosine monophosphate (cAMP) either blocks or promotes cell cycle progression in mid to late G1 phase. Dog thyroid epithelial cells in primary culture constitute a model of positive control of DNA synthesis initiation and G0-S prereplicative phase progression by cAMP as a second messenger for thyrotropin (TSH). The cAMP-dependent mitogenic pathway is unique as it is independent of mitogen-activated protein kinase activation and differs from growth factor-dependent pathways at the level of the expression of several protooncogenes/transcription factors. This study examined the involvement of D-type G1 cyclins and their associated cyclin-dependent kinase (cdk4) in the cAMP-dependent G1 phase progression of dog thyroid cells. Unlike epidermal growth factor (EGF)+serum and other cAMP-independent mitogens, TSH did not induce the accumulation of cyclins D1 and D2 and partially inhibited the basal expression of the most abundant cyclin D3. However, TSH stimulation enhanced the nuclear detection of cyclin D3. This effect correlated with G1 and S phase progression. It was found to reflect both the unmasking of an epitope of cyclin D3 close to its domain of interaction with cdk4, and the nuclear translocation of cyclin D3. TSH and EGF+serum also induced a previously undescribed nuclear translocation of cdk4, the assembly of precipitable cyclin D3-cdk4 complexes, and the Rb kinase activity of these complexes. Previously, cdk4 activity was found to be required in the cAMP-dependent mitogenic pathway of dog thyrocytes, as in growth factor pathways. Here, microinjections of a cyclin D3 antibody showed that cyclin D3 is essential in the TSH/ cAMP-dependent mitogenesis, but not in the pathway of growth factors that induce cyclins D1 and D2. The present study (a) provides the first example in a normal cell of a stimulation of G1 phase progression occurring independently of an enhanced accumulation of cyclins D, (b) identifies the activation of cyclin D3 and cdk4 through their enhanced assembly and/or nuclear translocation, as first convergence steps of the parallel cAMP-dependent and growth factor mitogenic pathways, and (c) strongly suggests that this new mechanism is essential in the cAMP-dependent mitogenesis, which provides the first direct demonstration of the requirement for cyclin D3 in a G1 phase progression.

Indomethacin suppresses the anti-proliferative effects of transforming growth factor-beta isoforms on fibroblast cell cultures

The transforming growth factor-beta (TGFbeta) family of mediators consists of five closely related isoforms, of which three are present in mammals. TGFbeta1 has been shown to exert a biphasic effect on the proliferation of several cell types, including fibroblasts, with stimulation at low concentrations and inhibition at higher concentrations. The stimulatory effects are well characterized, but the mechanisms by which TGFbeta1 inhibits cell proliferation are incompletely understood. In the present study we have compared the effects of all three mammalian TGFbeta isoforms on human lung fibroblast proliferation, and have elucidated the role of the TGFbeta-induced synthesis of prostaglandin E2 (PGE2) in mediating their actions. All three isoforms stimulated fibroblast proliferation with maximal effects at 5 pg/ml (0.2 pM) and an order of potency of TGFbeta3 > TGFbeta2 > TGFbeta1. At higher concentrations, proliferation declined, and at 40 pg/ml and above all isoforms inhibited fibroblast proliferation. Again TGFbeta3 was the most potent, but there were no significant differences between the inhibitory effects of TGFbeta1 and TGFbeta2. Addition of indomethacin, an inhibitor of PGE2 synthesis, did not alter the proliferative activity of any of the TGFbeta isoforms, but completely overcame their inhibitory effects, restoring the stimulatory actions observed at lower TGFbeta concentrations. All TGFbeta isoforms stimulated PGE2 synthesis; TGFbeta3 was approximately twice as potent as TGFbeta1 and TGFbeta2, each of which had similar effects. These data suggest that the inhibition of fibroblast proliferation at higher concentrations of TGFbeta isoforms may be mediated by autocrine stimulation of PGE2 synthesis.

Phosphatidylcholine hydrolysis and c-myc expression are in collaborating mitogenic pathways activated by colony-stimulating factor 1

Stimulation of diglyceride production via phospholipase C (PLC) hydrolysis of phosphatidylcholine was an early event in the mitogenic action of colony-stimulating factor 1 (CSF-1) in the murine macrophage cell line BAC1.2F5 and was followed by a second phase of diglyceride production that persisted throughout the G1 phase of the cell cycle. Addition of phosphatidylcholine-specific PLC (PC-PLC) from Bacillus cereus to the medium of quiescent cells raised the intracellular diglyceride concentration and stimulated [3H]thymidine incorporation, although PC-PLC did not support continuous proliferation. PC-PLC treatment did not induce tyrosine phosphorylation or turnover of the CSF-1 receptor. The major protein kinase C (PKC) isotype in BAC1.2F5 cells was PKC-delta. Diglyceride production from PC-PLC did not target PKC-delta, since unlike phorbol esters, PC-PLC treatment neither decreased the electrophoretic mobility of PKC-delta nor increased the amount of GTP bound to Ras, and PC-PLC was mitogenically active in BAC1.2F5 cells in which PKC-delta was downregulated by prolonged treatment with phorbol ester. PC-PLC mimicked CSF-1 action by elevating c-fos and junB mRNAs to 40% of the level induced by CSF-1; however, PC-PLC induced c-myc mRNA to only 5% of the level in CSF-1-stimulated cells. PC-PLC addition to CSF-1-dependent BAC1.2F5 clones that constitutively express c-myc increased [3H]thymidine incorporation to 86% of the level evoked by CSF-1 and supported slow growth in the absence of CSF-1. Therefore, PC-PLC is a component of a signal transduction pathway leading to transcription of c-fos and junB that collaborates with c-myc and is independent of PKC-delta and Ras activation.

Phosphatidylcholine hydrolysis and c-myc expression are in collaborating mitogenic pathways activated by colony-stimulating factor 1

Stimulation of diglyceride production via phospholipase C (PLC) hydrolysis of phosphatidylcholine was an early event in the mitogenic action of colony-stimulating factor 1 (CSF-1) in the murine macrophage cell line BAC1.2F5 and was followed by a second phase of diglyceride production that persisted throughout the G1 phase of the cell cycle. Addition of phosphatidylcholine-specific PLC (PC-PLC) from Bacillus cereus to the medium of quiescent cells raised the intracellular diglyceride concentration and stimulated [3H]thymidine incorporation, although PC-PLC did not support continuous proliferation. PC-PLC treatment did not induce tyrosine phosphorylation or turnover of the CSF-1 receptor. The major protein kinase C (PKC) isotype in BAC1.2F5 cells was PKC-delta. Diglyceride production from PC-PLC did not target PKC-delta, since unlike phorbol esters, PC-PLC treatment neither decreased the electrophoretic mobility of PKC-delta nor increased the amount of GTP bound to Ras, and PC-PLC was mitogenically active in BAC1.2F5 cells in which PKC-delta was downregulated by prolonged treatment with phorbol ester. PC-PLC mimicked CSF-1 action by elevating c-fos and junB mRNAs to 40% of the level induced by CSF-1; however, PC-PLC induced c-myc mRNA to only 5% of the level in CSF-1-stimulated cells. PC-PLC addition to CSF-1-dependent BAC1.2F5 clones that constitutively express c-myc increased [3H]thymidine incorporation to 86% of the level evoked by CSF-1 and supported slow growth in the absence of CSF-1. Therefore, PC-PLC is a component of a signal transduction pathway leading to transcription of c-fos and junB that collaborates with c-myc and is independent of PKC-delta and Ras activation.

Growth-regulated expression of D-type cyclin genes in human diploid fibroblasts

The human CCND1 cyclin D1/PRAD1 gene was previously identified by a genetic screen for G1 cyclin function in Saccharomyces cerevisiae and also was identified as the putative BCL1 oncogene. However, its role in human cell proliferation is not known. To determine if expression of human D-type cyclin genes correlates with the state of cell growth, we examined the level of mRNAs for CCND1 and a related gene, CCND3, in normal human diploid fibroblasts (HDF). The levels of both mRNAs decrease upon serum depletion or at high cell densities. Following stimulation of quiescent fibroblasts with serum, the mRNA levels increase gradually to a peak at about 12 hr, prior to the onset of S phase. Induction of cyclin gene expression by serum is reduced concomitantly with the decline in FOS induction in aging HDFs, suggesting a possible relationship to the decrease in the proliferative response to mitogens during cellular senescence. Cycloheximide partially blocks the induction of CCND1 and CCND3 gene expression by serum, suggesting that both de novo protein synthesis-dependent and -independent pathways contribute to induction. Treatment of HDFs with defined growth factors suggests a correlation between CCND mRNA induction and DNA synthesis. However, induction of these genes is not sufficient for the transition from quiescence through G1 into S phase.