Evaluation of the role of Ap1-like proteins in the enhanced apolipoprotein E gene transcription accompanying phorbol ester induced macrophage differentiation

Expression of Human Endogenous Retrovirus Group K (HERV-K) HML-2 Correlates with Immune Activation of Macrophages and Type I Interferon Response

Human endogenous retroviruses (HERVs) comprise about 8.3% of the human genome and are capable of producing RNA molecules that can be sensed by pattern recognition receptors, leading to the activation of innate immune response pathways. The HERV-K (HML-2) subgroup is the youngest HERV clade with the highest degree of coding competence. Its expression is associated with inflammation-related diseases. However, the precise HML-2 loci, stimuli, and signaling pathways involved in these associations are not well understood or defined. To elucidate HML-2 expression on a locus-specific level, we used the retroelement sequencing tools TEcount and Telescope to analyze publicly available transcriptome sequencing (RNA-seq) and chromatin immunoprecipitation (ChIP) sequencing data sets of macrophages treated with a wide range of agonists. We found that macrophage polarization significantly correlates with modulation of the expression of specific HML-2 proviral loci. Further analysis demonstrated that the provirus HERV-K102, located in an intergenic region of locus 1q22, constituted the majority of the HML-2 derived transcripts following pro-inflammatory (M1) polarization and was upregulated explicitly in response to interferon gamma (IFN-γ) signaling. We found that signal transducer and activator of transcription 1 and interferon regulatory factor 1 interact with a solo long terminal repeat (LTR) located upstream of HERV-K102, termed LTR12F, following IFN-γ signaling. Using reporter constructs, we demonstrated that LTR12F is critical for HERV-K102 upregulation by IFN-γ. In THP1-derived macrophages, knockdown of HML-2 or knockout of MAVS, an adaptor of RNA-sensing pathways, significantly downregulated genes containing interferon-stimulated response elements (ISREs) in their promoters, suggesting an intermediate role of HERV-K102 in the switch from IFN-γ signaling to the activation of type I interferon expression and, therefore, in a positive feedback loop to enhance pro-inflammatory signaling. IMPORTANCE The human endogenous retrovirus group K subgroup, HML-2, is known to be elevated in a long list of inflammation-associated diseases. However, a clear mechanism for HML-2 upregulation in response to inflammation has not been defined. In this study, we identify a provirus of the HML-2 subgroup, HERV-K102, which is significantly upregulated and constitutes the majority of the HML-2 derived transcripts in response to pro-inflammatory activation of macrophages. Moreover, we identify the mechanism of HERV-K102 upregulation and demonstrate that HML-2 expression enhances interferon-stimulated response element activation. We also demonstrate that this provirus is elevated in vivo and correlates with interferon gamma signaling activity in cutaneous leishmaniasis patients. This study provides key insights into the HML-2 subgroup and suggests that it may participate in enhancing pro-inflammatory signaling in macrophages and probably other immune cells.

Response of human macrophages to gamma radiation is mediated via expression of endogenous retroviruses

Ionizing radiation-induced tissue damage recruits monocytes into the exposed area where they are differentiated to macrophages. These implement phagocytic removal of dying cells and elicit an acute inflammatory response, but can also facilitate tumorigenesis due to production of anti-inflammatory cytokines. Using primary human monocyte-derived macrophages (MDMs) and the THP1 monocytic cell line, we demonstrate that gamma radiation triggers monocyte differentiation toward the macrophage phenotype with increased expression of type I interferons (IFN-I) and both pro- and anti-inflammatory macrophage activation markers. We found that these changes correlate with significantly upregulated expression of 622 retroelements from various groups, particularly of several clades of human endogenous retroviruses (HERVs). Elevated transcription was detected in both sense and antisense directions in the HERV subgroups tested, including the most genetically homogeneous clade HML-2. The level of antisense transcription was three- to five-fold higher than of the sense strand levels. Using a proximity ligation assay and immunoprecipitation followed by RNA quantification, we identified an increased amount of the dsRNA receptors MDA-5 and TLR3 bound to an equivalent number of copies of sense and antisense chains of HERVK HML-2 RNA. This binding triggered MAVS-associated signaling pathways resulting in increased expression of IFN-I and inflammation related genes that enhanced the cumulative inflammatory effect of radiation-induced senescence. HML-2 knockdown was accompanied with reduced expression and secretion of IFNα, pro-inflammatory (IL-1β, IL-6, CCL2, CCL3, CCL8, and CCL20) and anti-inflammatory (IL10) modulators in irradiated monocytes and MDMs. Taken together, our data indicate that radiation stress-induced HERV expression enhances the IFN-I and cytokine response and results in increased levels of pro-inflammatory modulators along with expression of anti-inflammatory factors associated with the macrophage tumorigenic phenotype.

Ionizing radiation is a powerful stressogenic factor that induces massive cell damage. The signals released from radiation-damaged tissues recruit the monocytes, which are differentiated into macrophages that remove dying cells via phagocytosis and facilitate inflammation but can also contribute to tumorigenesis through anti-inflammatory and regenerative activities. The mechanism of this dual response of macrophages to irradiation is not fully understood. Using primary human macrophages and a monocytic cell line, we demonstrated that gamma radiation doses activate expression of various human endogenous retroviruses (HERVs). At the molecular level, we have shown that increased numbers of sense and antisense transcripts of tested HERV subgroups bind to double-stranded RNA receptors inducing the expression of type I interferons, multiple pro-inflammatory and some anti-inflammatory factors. At the phenotypic level, polarized macrophages exhibit a potent inflammatory response along with potentially tumorigenic characteristics. Our data suggest that endogenous retroviruses represent an important contributor of the macrophage-mediated inflammation in response to radiation-induced stress but may also indirectly influence tumorigenesis via biased macrophage polarization.

Virus-producing cells determine the host protein profiles of HIV-1 virion cores

Background

Upon HIV entry into target cells, viral cores are released and rearranged into reverse transcription complexes (RTCs), which support reverse transcription and also protect and transport viral cDNA to the site of integration. RTCs are composed of viral and cellular proteins that originate from both target and producer cells, the latter entering the target cell within the viral core. However, the proteome of HIV-1 viral cores in the context of the type of producer cells has not yet been characterized.

Results

We examined the proteomic profiles of the cores purified from HIV-1 NL4-3 virions assembled in Sup-T1 cells (T lymphocytes), PMA and vitamin D₃ activated THP1 (model of macrophages, mMΦ), and non-activated THP1 cells (model of monocytes, mMN) and assessed potential involvement of identified proteins in the early stages of infection using gene ontology information and data from genome-wide screens on proteins important for HIV-1 replication. We identified 202 cellular proteins incorporated in the viral cores (T cells: 125, mMΦ: 110, mMN: 90) with the overlap between these sets limited to 42 proteins. The groups of RNA binding (29), DNA binding (17), cytoskeleton (15), cytoskeleton regulation (21), chaperone (18), vesicular trafficking-associated (12) and ubiquitin-proteasome pathway-associated proteins (9) were most numerous. Cores of the virions from SupT1 cells contained twice as many RNA binding proteins as cores of THP1-derived virus, whereas cores of virions from mMΦ and mMN were enriched in components of cytoskeleton and vesicular transport machinery, most probably due to differences in virion assembly pathways between these cells. Spectra of chaperones, cytoskeletal proteins and ubiquitin-proteasome pathway components were similar between viral cores from different cell types, whereas DNA-binding and especially RNA-binding proteins were highly diverse. Western blot analysis showed that within the group of overlapping proteins, the level of incorporation of some RNA binding (RHA and HELIC2) and DNA binding proteins (MCM5 and Ku80) in the viral cores from T cells was higher than in the cores from both mMΦ and mMN and did not correlate with the abundance of these proteins in virus producing cells.

Conclusions

Profiles of host proteins packaged in the cores of HIV-1 virions depend on the type of virus producing cell. The pool of proteins present in the cores of all virions is likely to contain factors important for viral functions. Incorporation ratio of certain RNA- and DNA-binding proteins suggests their more efficient, non-random packaging into virions in T cells than in mMΦ and mMN.

Macrophage-specific up-regulation of apolipoprotein E gene expression by STAT1 is achieved via long range genomic interactions

In atherogenesis, macrophage-derived apolipoprotein E (apoE) has an athero-protective role by a mechanism that is not fully understood. We investigated the regulatory mechanisms involved in the modulation of apoE expression in macrophages. The experiments showed that the promoters of all genes of the apoE/apoCI/apoCIV/apoCII gene cluster are enhanced by multienhancer 2 (ME.2), a regulatory region that is located 15.9 kb downstream of the apoE gene. ME.2 interacts with the apoE promoter in a macrophage-specific manner. Transient transfections in RAW 264.7 macrophages showed that the activity of ME.2 was strongly decreased by deletion of either 87 bp from the 5' end or 131 bp from the 3' end. We determined that the minimal fragment of this promoter that can be activated by ME.2 is the proximal -100/+73 region. The analysis of the deletion mutants of ME.2 revealed the importance of the 5' end of ME.2 in apoE promoter transactivation. Chromatin conformational capture assays demonstrated that both ME.2 and ME.1 physically interacted with the apoE promoter in macrophages. Our data showed that phorbol 12-myristate 13-acetate-induced differentiation of macrophages is accompanied by a robust induction of apoE and STAT1 expression. In macrophages (but not in hepatocytes), STAT1 up-regulated apoE gene expression via ME.2. The STAT1 binding site was located in the 174-182 region of ME.2. In conclusion, the specificity of the interactions between the two multienhancers (ME.1 and ME.2) and the apoE promoter indicates that these distal regulatory elements play an important role in the modulation of apoE gene expression in a cell-specific manner.

A novel role for c-Jun N-terminal kinase and phosphoinositide 3-kinase in the liver X receptor-mediated induction of macrophage gene expression

Liver X receptors (LXRs) are ligand-dependent transcription factors that are activated by metabolites of cholesterol, oxysterols, and a number of synthetic agonists. LXRs play potent anti-atherogenic roles in part by stimulating the efflux of cholesterol from macrophage foam cells. The LXR-induced expression of ATP-binding cassette transporter (ABC)-A1 and Apolipoprotein E (ApoE) in macrophages is essential for the stimulation of cholesterol efflux and the prevention of atherosclerotic development. Unfortunately, the signaling pathways underlying such regulation are poorly understood and were therefore investigated in human macrophages. The expression of ApoE and ABCA1 induced by synthetic or natural LXR ligands [TO901317, GW3965, and 22-(R)-hydroxycholesterol (22-(R)-HC), respectively] was attenuated by inhibitors of c-Jun N-terminal kinase (JNK) (curcumin and SP600125) and phosphoinositide 3-kinase (PI3K) (LY294002). Similar results were obtained with ABCG1 and LXR-α, two other LXR target genes. LXR agonists activated several components of the JNK pathway (SEK1, JNK and c-Jun) along with AKT, a downstream target for PI3K. In addition, dominant negative mutants of JNK and PI3K pathways inhibited the LXR-agonists-induced activity of the ABCA1 and LXR-α gene promoters in transfected cells. LXR agonists also induced the binding of activator protein-1 (AP-1), a key transcription factor family regulated by JNK, to recognition sequences present in the regulatory regions of the ApoE and ABCA1 genes. These studies reveal a novel role for JNK and PI3K/AKT signaling in the LXR-regulated expression in macrophages of several key genes implicated in atherosclerosis.

IL-33 reduces macrophage foam cell formation

The development of atherosclerosis, a chronic inflammatory disease characterized by the formation of arterial fibrotic plaques, has been shown to be reduced by IL-33 in vivo. However, whether IL-33 can directly affect macrophage foam cell formation, a key feature of atherosclerotic plaques, has not been determined. In this study, we investigated whether IL-33 reduces macrophage foam cell accumulation in vivo and if IL-33 reduces their formation in vitro using THP-1 and primary human monocyte-derived macrophages. In Apolipoprotein E(-/-) mice fed on a high fat diet, IL-33 treatment significantly reduced the accumulation of macrophage-derived foam cells in atherosclerotic plaques. IL-33 also reduced macrophage foam cell formation in vitro by decreasing acetylated and oxidized low-density lipoprotein uptake, reducing intracellular total and esterified cholesterol content and enhancing cholesterol efflux. These changes were associated with IL-33-mediated reduction in the expression of genes involved in modified low-density lipoprotein uptake, such as CD36, and simultaneous increase in genes involved in cholesterol efflux, including Apolipoprotein E, thereby providing a mechanism for such an action for this cytokine. IL-33 also decreased the expression of key genes implicated in cholesterol esterification and triglyceride storage, including Acyl-CoA:cholesterol acyltransferase 1 and Adipocyte differentiation-related protein. Furthermore, using bone marrow-derived macrophages from ST2(-/-) mice, we demonstrate that the IL-33 receptor, ST2, is integral to the action of IL-33 on macrophage foam cell formation. In conclusion, IL-33 has a protective role in atherosclerosis by reducing macrophage foam cell formation suggesting that IL-33 maybe a potential therapeutic agent against atherosclerosis.

Transforming growth factor-beta-induced expression of the apolipoprotein E gene requires c-Jun N-terminal kinase, p38 kinase, and casein kinase 2

OBJECTIVE

The cytokine transforming growth factor-beta (TGF-beta) and apolipoprotein E (apoE) play potent antiatherogenic roles. Despite such importance, the mechanisms underlying the regulation of apoE expression by TGF-beta have not been characterized and were therefore investigated.

METHODS AND RESULTS

Using THP-1 cell line as a model system, with key findings confirmed in primary cultures, we show that TGF-beta induces the expression of apoE, and this is prevented by pharmacological inhibitors of c-Jun N-terminal kinase (JNK), p38 kinase, and casein kinase 2 (CK2). In support for an important role for these pathways, TGF-beta activates JNK, p38 kinase, and CK2, and dominant-negative (DN) forms of these proteins inhibit the cytokine-induced apoE expression. TGF-beta also increases the phosphorylation and expression of c-Jun, a downstream target for JNK action and a component of activator protein-1 (AP-1), and DN c-Jun inhibits the induction of apoE expression in response to the cytokine. AP-1 DNA binding was also induced by TGF-beta, and the action of p38 kinase, JNK, and CK2 converged on the activation of c-Jun/AP-1.

CONCLUSIONS

These studies reveal a novel role for JNK, p38 kinase, CK2, and c-Jun/AP-1 in the TGF-beta-induced expression of apoE.

The key role of apolipoprotein E in atherosclerosis

Apolipoprotein E is a multifunctional protein that is synthesized by the liver and several peripheral tissues and cell types, including macrophages. The protein is involved in the efficient hepatic uptake of lipoprotein particles, stimulation of cholesterol efflux from macrophage foam cells in the atherosclerotic lesion, and the regulation of immune and inflammatory responses. Apolipoprotein E deficiency in mice leads to the development of atherosclerosis and re-expression of the protein reduces the extent of the disease. This review presents evidence for the potent anti-atherogenic action of apolipoprotein E and describes our current understanding of its multiple functions and regulation by factors implicated in the pathogenesis of cardiovascular disease.

Duplicated downstream enhancers control expression of the human apolipoprotein E gene in macrophages and adipose tissue

Two distal enhancers that specify apolipoprotein (apo) E gene expression in isolated macrophages and adipose tissue were identified in transgenic mice that were generated with constructs of the human apoE/C-I/C-I'/C-IV/C-II gene cluster. One of these enhancers, multienhancer 1, consists of a 620-nucleotide sequence located 3.3 kilobases (kb) downstream of the apoE gene. The second enhancer, multienhancer 2, is a 619-nucleotide sequence located 15.9 kb downstream of the apoE gene and 5.9 kb downstream of the apoC-I gene. The two enhancers are 95% identical in sequence, and they are likely to have arisen as a consequence of the gene duplication event that yielded the apoC-I gene and the apoC-I' pseudogene. Both enhancer sequences appear to have equivalent activity in directing apoE gene expression in peritoneal macrophages and in adipocytes, suggesting that their activity in specific cell types may be determined by common regulatory elements.

Apolipoprotein E gene promoter polymorphisms in Alzheimer's disease

Alzheimer's disease, the most frequent form of senile dementia, presents in the vast majority of cases as a multifactorial trait, where a series of genetic and environmental risk factors converge. The increasing body of data, both epidemiological and functional, is strengthening the evidence that apolipoprotein E (APOE, gene; apoE, protein) is a true susceptibility factor for the onset of the common form of Alzheimer's disease. The E4 isoform of apoE remains to date as the main genetic risk factor for the disease, although the mechanisms responsible for this association are not well understood. It is also clear that apoE4 is not necessary or sufficient to cause the disease, indicating that other risk and protecting factors exist. ApoE is upregulated in response to nervous system injury, suggesting that it could have a neuroprotective role; on the other hand, there is evidence indicating that apoE is neurotoxic when present at high levels. Thus, apoE levels seem to be relevant for the functionality of the protein. The APOE proximal promoter hosts numerous regulatory elements, raising the possibility that polymorphisms in this region could produce variation in apoE levels by altering APOE transcriptional activity, which could finally result in AD susceptibility. We will review here the current evidence on the relationship between APOE proximal promoter polymorphisms, APOE gene transcriptional activity and apoE protein levels, and risk for AD.

Protein kinase C-alpha and -beta play antagonistic roles in the differentiation process of THP-1 cells

The roles of protein kinase C (PKC) isoenzymes in the differentiation process of THP-1 cells are investigated. Inhibition of PKC by RO 31-8220 reduces the phagocytosis of latex particles and the release of superoxide, prostaglandin E(2) (PGE(2)), and tumour necrosis factor (TNF)-alpha. The proliferation of THP-1 cells is slightly enhanced by RO 31-8220. Stable transfection of THP-1 cells with asPKC-alpha, and incubation of THP-1 cells with antisense (as) PKC-alpha oligodeoxynucleotides reduces PKC-alpha levels and PKC activity. asPKC-alpha-transfected THP-1 cells show a decreased phagocytosis and a decreased release of superoxide, PGE(2) and TNF-alpha. The proliferation of asPKC-alpha-transfected THP-1 cells is enhanced. Stable transfection of THP-1 cells with asPKC-beta, and incubation of THP-1 cells with asPKC-beta oligodeoxynucleotides, reduces PKC-beta levels and PKC activity. asPKC-beta-transfected THP-1 cells show a decreased phagocytosis, a decreased TNF-alpha release, and a decreased proliferation. However, no difference is measured in the release of superoxide and PGE(2). These results suggest that: (1) PKC-alpha but not PKC-beta is involved in the release of superoxide and PGE(2); (2) TNF-alpha release and the phagocytosis of latex particles are mediated by PKC-alpha, PKC-beta, and other PKC isoenzymes; and (3) PKC-alpha and PKC-beta play antagonistic roles in the differentiation process of THP-1 cells. PKC-alpha promotes the differentiation process of THP-1 cells, PKC-beta retards the differentiation of THP-1 cells into macrophage-like cells.

Transcriptional regulation of apolipoprotein A-I expression in Hep G2 cells by phorbol ester

The regulation of apolipoprotein A-I (apo A-I) gene expression by 12-O-tetradecanoylphorbol 13-acetate (TPA) was investigated in the human hepatoma cell line Hep G2. TPA treatment decreased apo A-I mRNA levels in a time-dependent manner, by up to 50% versus control cells within 24 h. Nuclear run-on transcription assays demonstrated a transcriptional effect of TPA. Using transfection analysis with a plasmid construct containing the -1378/+11 apo A-I promoter fused to the secreted placental alkaline phosphatase (SPAP) reporter gene, we showed that the SPAP activity was decreased to 50% when Hep G2 cells were incubated in the presence of TPA. The inhibitory effect of TPA was still maintained when fragment -253 to -4 of apo A-I promoter was linked to the CAT reporter gene. These data indicate that transcriptional modulation of apolipoprotein A-I gene expression following phorbol ester treatment is transduced by gene elements located between -253 and -4 of the apo A-I promoter.

Tumor necrosis factor-alpha modulates monocyte/macrophage apoprotein E gene expression

apo E has been shown to modulate cholesterol balance in arterial wall cells. Production of apo E by macrophages in atherosclerotic plaques could thereby influence the development of the plaque lesion. Cytokines, including TNF alpha, have been identified in human lesions, therefore, we undertook a series of studies to evaluate the effect of TNF alpha on monocyte/macrophage apo E production. The addition of TNF alpha to freshly isolated human monocytes led to a four- to fivefold increase of apo E mRNA abundance. The addition of TNF alpha to fully differentiated macrophages either had no effect or modestly inhibited apo E mRNA expression. THP1 human monocytic cells also responded to TNF alpha in a phenotype-specific manner. Treatment of these cells with TNF alpha produced a dose- and time-dependent increase in apo E mRNA. This increase was reflected in apo E synthesis and was associated with inhibition of DNA synthesis, and with induction of c-fos and ICAM-1 gene expression. Cell-permanent analogues of ceramide did not reproduce TNF alpha effect on apo E, but antagonists of protein kinase C did inhibit its effect. TNF alpha induction of apo E mRNA abundance was associated with stimulation of apo E promoter-dependent gene transcription. In summary, TNF alpha stimulates apo E gene transcription, mRNA abundance, and protein synthesis in the monocyte/macrophage in a phenotype-specific manner. Such regulation could significantly modify the amount of apo E present in vessel wall lesions.