C/EBPβ regulates multiple IL-1β-induced human astrocyte inflammatory genes
© Fields and Ghorpade licensee BioMed Central Ltd. 2012
Received: 13 February 2012
Accepted: 20 July 2012
Published: 20 July 2012
CCAAT enhancer-binding protein (C/EBP)β regulates gene expression in multiple organ systems and cell types, including astrocytes in the central nervous system (CNS). Inflammatory stimuli, interleukin (IL)-1β, tumor necrosis factor-α, human immunodeficiency virus (HIV)-1 and lipopolysaccharide induce astrocyte C/EBPβ expression. C/EBPβ is detectable in brains of Alzheimer’s disease (AD), Parkinson’s disease (PD) and HIV-1-associated dementia (HAD) patients, yet little is known about how C/EBPβ contributes to astrocyte gene regulation during neuroinflammation.
The expression of 92 human inflammation genes was compared between IL-1β-treated primary human astrocytes and astrocytes transfected with C/EBPβ-specific small interfering (si)RNA prior to IL-1β treatment for 12 h. Transcripts altered by > two-fold compared to control were subjected to one-way analysis of variance and Newman-Keuls post-test for multiple comparisons. Expression of two genes, cyclooxygenase-2 (COX-2) and bradykinin receptor B2 (BDKRB2) was further confirmed in additional human astrocyte donors. Astrocytes were treated with mitogen-activated protein kinase-selective inhibitors, then with IL-1β for 12 or 24 h followed by COX-2 and BDKRB2, expression analyses.
IL-1β altered expression of 29 of 92 human inflammation genes by at least two-fold in primary human astrocytes in 12 h. C/EBPβ knockdown affected expression of 17 out of 29 IL-1β-regulated genes by > 25%. Two genes relevant to neuroinflammation, COX-2 and BDKRB2, were robustly decreased and increased, respectively, in response to C/EBPβ knockdown, and expression was confirmed in two additional donors. COX-2 and BDKRB2 mRNA remained altered in siRNA-transfected astrocytes at 12, 24 or 72 h. Inhibiting p38 kinase (p38K) activation blocked IL-1β-induced astrocyte COX-2 mRNA and protein expression, but not IL-1β-induced astrocyte BDKRB2 expression. Inhibiting extracellular-regulated kinase (ERK)1/2 activation blocked IL-1β-induced BDKRB2 mRNA expression while increasing COX-2 expression.
These data support an essential role for IL-1β in the CNS and identify new C/EBPβ functions in astrocytes. Additionally, this work suggests p38K and ERK1/2 pathways may regulate gene expression in a complementary manner to fine tune the IL-1β-mediated astrocyte inflammatory response. Delineating a role for C/EBPβ and other involved transcription factors in human astrocyte inflammatory response may lead to effective therapies for AD, PD, HAD and other neurological disorders.
KeywordsAstrocyte Interleukin-1β C/EBPβ ERK1/2 p38K
Neuroinflammation is a contributing factor of many central nervous system (CNS) pathologies; yet the details of onset and progression remain enigmatic. Astrocytes, the most numerous cells of the CNS, contribute to homeostasis in the CNS, regulate neural signaling and maintain the blood–brain barrier (BBB) [1–3]. Accordingly, astrocytes respond to inflammatory stimuli by altering gene expression, morphology and function. Activated astrocytes undergo rapid replication, migrate to areas of insult and attempt to mitigate collateral damage by isolating the damaged area [4–6]. Previously, we reported that CCAAT enhancer-binding protein (C/EBP)-β is expressed in the brains of human immunodeficiency virus (HIV)-1-infected patients and contributes to interleukin (IL)-1β-induced tissue inhibitor metalloproteinases (TIMP)-1 expression in astrocytes . In a related study, we explored the signal transduction pathways mediating IL-1β-induced astrocyte C/EBPβ and TIMP-1 expression . We found that a p38 kinase (p38K)-selective inhibitor blocked IL-1β-induced astrocyte C/EBPβ expression, whereas an extracellular regulated kinase (ERK) 1/2-selective inhibitor blocked IL-1β-induced astrocyte TIMP-1 expression. In this report, we explore the role of C/EBPβ in regulating IL-1β-induced astrocyte inflammatory genes and the signal transduction pathways involved.
C/EBPβ is evolutionarily conserved among species and is expressed in multiple organ systems [9–11]. The gene is expressed as a single transcript that can be translated into three isoforms: 42 kilodalton (kDa), 40 kDa and 20 kDa [12, 13]. The two large isoforms, designated liver-activating proteins, are named for their transcriptional activating properties, while the 20-kDa liver-inhibiting protein is named for its inhibitory properties . It is now clear that the isoform-specific divergent roles for C/EBPβ isoforms do not completely explain their function [15, 16]. In the CNS, astrocytes and microglia increase C/EBPβ expression in response to various inflammatory stimuli including IL-1β, lipopolysaccharides, tumor necrosis factor (TNF)-α and HIV-1 [7, 17]. Since the discovery that C/EBPβ regulates IL-6, studies have shown that it regulates nitric oxide synthase (NOS)-2, complementary protein 3 and other important genes [15, 16, 18–21]. These data suggest an important role of this highly conserved transcription factor, but C/EBPβ isoform-specific activity is contextual in regard to the tissue microenvironment and cell type [15, 16, 19]. Given the plethora of cell-type- and ligand-dependent outcomes of C/EBPβ-mediated gene responses, a complete understanding of neuroinflammation warrants elucidating C/EBPβ function in the human astrocyte inflammatory response. Our group found that C/EBPβ is expressed in the brains of HIV-1 patients and contributes to regulation of human astrocyte TIMP-1 . Overall, these data implicate C/EBPβ activity during CNS pathologies, but the extent to which the transcription factor regulates global astrocyte immune responses is unknown.
It is well established that IL-1β mediates neuroinflammation through activation of glial cells and subsequent changes in gene expression . Following IL-1β-mediated activation of glial cells, nuclear C/EBPβ levels increase and affect gene transcription [7, 17]. In this study, we profile the role of C/EBPβ in regulating IL-1β-mediated expression of 92 inflammatory genes in primary human astrocytes. We found that IL-1β altered expression of ~32% (29/92) mRNA transcripts tested. Furthermore, C/EBPβ regulated 59% (17/29) of these genes by increasing or decreasing transcript levels. Because of their role in neuroinflammation, two genes that were affected oppositely by C/EBPβ knockdown, cyclooxygenase (COX)-2 and bradykinin receptor b2 (BDKRB2) were chosen for further studies. ERK1/2 and p38K pathway-selective inhibitors also exhibited opposite effects on IL-1β-induced astrocyte COX-2 and BDKRB2 expression. These data illustrate the complexity of the IL-1β-mediated astrocyte inflammatory response and provide details of the regulatory mechanisms involved.
Isolation, cultivation and activation of human astrocytes
Human astrocytes were isolated from first- and early second-trimester (gestational weeks 12–18) aborted specimens obtained from the Birth Defects Laboratory, University of Washington, Seattle, in full compliance with the ethical guidelines of the NIH, the Universities of Washington and North Texas Health Science Center. The Birth Defects Laboratory is an NIH-funded institution with the mission of disseminating tissues for the advancement of biomedical research. Astrocytes were isolated from specimens as originally described earlier . Activation of astrocytes was achieved by applying IL-1β for various time intervals. IL-1β is a prototypical inflammatory cytokine expressed during HIV-1 CNS infection, making IL-1β an excellent choice for studying human astrocyte function during neuroinflammation . We empirically tested IL-1β dosage and transfection of astrocytes with C/EBPβ small interfering (si)RNA as described in Fields et al. . Accordingly, these data provide relevant implications for astrocyte function in many pathologies involving neuroinflammation.
RNA isolation and TaqMan® human inflammation array and real-time reverse transcription polymerase chain reaction (RT2PCR)
Astrocytes were transfected with nonspecific control (siCON) or C/EBPβ-specific (siC/EBPβ) siRNA by nucleofection and then cultured as adherent monolayers in 75 cm2 flasks at a density of 8 × 106 cells per flask. The following day, medium was exchanged with or without IL-1β (20 ng/ml) for 24 h. Alternatively, astrocytes were cultured in 75 cm2 flasks at a density of 8 × 106 cells per flask for 24 h, and then the medium was exchanged with or without MAPK-selective inhibitors (SB203580, U0126; 20 μM) for 1 h and then with inhibitor plus IL-1β (20 ng/ml) for 24 h. Cells were lysed; total cell extracts were isolated using mammalian protein extraction reagent (Thermo Fisher Scientific, Waltham, MA, USA). Equal amounts of proteins (15 μg/lane) were resolved by 12% sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and subsequently transferred to a polyvinyl difluoride (PVDF) membrane using i-Blot (Life Techologies). The membrane was incubated in anti-C/EBPβ (C-19, Santa Cruz Biotechnology) or anti-COX-2 (no. 4842, Cell Signaling Inc., Boston, MA, USA) at a dilution of 1:200. Blots were then incubated in secondary antibody (1:5,000, Promega Inc., Madison, WI). β-actin (Sigma-Aldrich Inc., St. Louis, MO, USA) was used as a loading control. The Western blot was visualized with supersignal chemiluminescent substrate (Thermo Fisher Scientific), and band intensities were quantified by densitometry analysis (ProteinSimple, Santa Clara, CA, USA).
Monolayers of astrocytes were treated with 2× the final dose of the MAPK-selective inhibitors, SB203580 (p38K-20 μM), SB202190 (p38K-20 μM), U0126 (ERK1/2-20 μM) and PD184352 (ERK1/2-20 μM) for 1 h before adding equal volume of 2× the final dose of IL-1β treatment.
Astrocytes were cultured as adherent monolayers in a 48-well plate at a density of 0.1 × 106 cells per well for 24 h, and then with or without MAPK-selective inhibitors (SB203580, U0126) for 1 h and then inhibitor plus IL-1β (20 ng/ml) for 24 h. Experimental cells were fixed with cold acetone:methanol (1:1) for 30 min at −20 °C. Cells were then blocked in phosphate-buffered saline (PBS) with 2% bovine serum albumin (BSA) and 0.1% triton X-100 for 1 h at room temperature. Cells were incubated in PBS with 2% BSA and 0.1% triton X-100 plus anti-COX-2 antibody at 1:500 and anti-glial fibrillary acidic protein (GFAP) antibody at 1:600 for 8 h at 4 °C. Cells were washed with PBS and then incubated in secondary antibody (1:800) for 1 h at room temperature. Micrographs were taken on a Nikon Eclipse T i . (Nikon Inc., Melville, NY, USA).
Statistical analyses were carried out using GraphPad Prism 5.0 software (GraphPad Software, Inc., La Jolla, CA), with one-way ANOVA and Newman-Keuls post-test for multiple comparisons. Data generated from each assay from the TaqMan® Human Inflammation Array (Life Technologies, C/N: 4414074) were analyzed independently. Significance was set at p < 0.05, and data represent mean values ± SEM. Data presented are representative of a minimum of three independent experiments with two or more independent donors, unless noted, in which case, n represents cumulative data from a specific number of independent human donors (TaqMan® Human Inflammation Array and western blots).
Human astrocyte IL-1β-induced C/EBPβ, directly or indirectly, regulates 17 of 29 selected astrocyte inflammation genes
As previously reported, IL-1β induces astrocyte C/EBPβ expression and localization to nuclei, where the transcription factor regulates gene expression [7, 17]. Astrogliosis is a hallmark of many CNS diseases, yet little is known about how astrocyte C/EBPβ-regulated gene expression may contribute to progression of these pathologies. Here, we used the TaqMan® Human Inflammation Array to evaluate human astrocyte C/EBPβ’s contribution to expression of 92 inflammatory genes in response to IL-1β. Figure 1 shows cumulative data from two independent astrocyte donors. Primary human astrocyte C/EBPβ expression was silenced using siRNA technology, and cells were cultured in the presence of IL-1β for 12 h. As Figure 1 indicates, IL-1β altered mRNA levels of 29 of the 92 genes by two-fold or greater. C/EBPβ knockdown by siRNA affected expression of 17 of the 29 genes by 25% or more. Moreover, our data are supported by previous reports, and we confirmed two targets in additional donors. Data from previous studies support our findings that IL-1β-activated astrocytes express higher levels of NOS-2 and intercellular adhesion molecule (ICAM)-1, and each was down- and upregulated, respectively, in C/EBPβ-deficient astrocytes [25, 26]. Interestingly, only 4 of the 17 IL-1β-induced genes affected by C/EBPβ are downregulated in C/EBPβ-deficient astrocytes; the remaining 13 genes are upregulated. IL-1β induced the expression of astrocyte prostaglandin endoperoxide synthase 2, or COX-2, mRNA by an average of 824 fold, while C/EBPβ knockdown in parallel experiments led to an average of 37% reduction. IL-1β induced the expression of BDKRB2 mRNA by an average of 35 fold; C/EBPβ knockdown further enhanced this increase by an average of 68%. These data suggest that IL-1β-mediated astrocyte C/EBPβ expression functions to activate or inhibit 17 of 29 of the IL-1β-induced human astrocyte inflammation genes.
siRNA knockdown of C/EBPβ affects IL-1β-induced astrocyte COX-2 and BRKRB2 expression
Differential roles of p38K and ERK1/2 signaling pathways in astrocyte COX-2 and BDKRB2 regulation
Astrocytes are multifunctional glial cells that maintain CNS homeostasis, neuronal signaling, BBB and responses to trauma. Neuroinflammation is a contributing factor of many CNS diseases and profoundly affects astrocyte gene expression [1, 27]. Immune-induced changes in astrocyte gene expression are well documented and play an important role in restoring normal CNS function after trauma [1, 28]. Currently, investigators lack a full understanding of how astrocytes contribute to the initiation and control of CNS immune responses. To this end, we sought to characterize the role of C/EBPβ in regulating IL-1β-mediated increases in primary human astrocyte expression of a panel of inflammatory genes. Here, we used an array of 92 human inflammatory genes to assay the effect of IL-1β on expression of these genes in two independent human astrocytes donors. Expression of 29 of the 92 mRNAs was affected by at least two fold, and C/EBPβ knockdown affected expression of 17 of the 29 genes by at least 25%. We confirmed IL-1β-mediated COX-2 and BDKRB2 expression, with and without C/EBPβ knockdown. C/EBPβ knockdown decreased COX-2 mRNA and protein levels, while it increased BDKRB2 mRNA expression. Data from a related study show p38K inhibition blocks IL-1β-mediated astrocyte C/EBPβ expression, whereas ERK1/2 inhibition enhances expression . Accordingly, we found that the IL-1β-mediated increase in COX-2 expression is p38K-dependent, whereas IL-1β-mediated expression of BDKRB2 is ERK1/2-dependent. Interestingly, ERK1/2 pathway inhibition exacerbated IL-1β-mediated COX-2 induction. On the contrary, BDKRB2 induction by IL-1β was robustly diminished with ERK1/2 inhibition. Lastly, data showed that IL-1β signals through the p38K pathway to increase expression of COX-2. Our data show that C/EBPβ, in concert with other factors, may contribute to regulation of many human astrocyte genes during neuroinflammation.
Advances in gene expression technology have facilitated the study of astrocytes in disease processes. Genomic array data of IL-1β-induced astrocyte gene expression studies were compiled and thoroughly reviewed by John et al. (2005); several of our data are corroborated therein. As in this report, multiple arrays have shown IL-1β induces CD40, NOS-2, vascular cell adhesion molecule-1, ICAM-1, TNF and COX-2 [29, 30]. Other studies have utilized more diverse stimuli to study immune-induced astrocyte gene expression, such as interferon, HIV-1 virion particles or viral proteins [29, 30]. Interestingly, HIV-1-treated murine astrocytes increase expression of many of these same genes: CD40, COX-2 and TIMP-1 . Overall, the results of this study corroborate those of past studies while adding critical regulators of neuroinflammation and BBB integrity, such as BDKRB2, to the list of IL-1β-induced human astrocyte genes.
C/EBPβ expression is detectable in immune-activated rodent glia, and it is now clear that this factor is involved in inflammatory processes in tissues throughout the body [17, 31]. In the brain, data suggest that C/EBPβ is a direct downstream target of neural growth factor during neurogenesis . C/EBPβ activates regeneration-associated gene expression following axon injury and provides cerebral protection to excitotoxic injury in mouse brains [33–35]. Our group recently reported that C/EBPβ is detectable in brains of HIV-1-infected patients and the factor contributes to regulating IL-1β-mediated astrocyte TIMP-1 . C/EBPβ regulates IL-1β-mediated human astrocyte C3 expression , but more in-depth studies on human astrocytes are lacking [20, 21]. Here, we found that the majority of IL-1β-induced transcript levels were affected by C/EBPβ knockdown. Studies have shown that all three isoforms can function as repressors of transcription . C/EBPβ binds with C/EBPδ, nuclear factor (NF)κB and activator protein-1 (AP-1) in various cell types to affect gene expression . NFκB is a key factor in IL-1β-induced gene expression; however, varying combinations of transcription factors may determine how transcription is affected at each promoter. Furthermore, posttranslational modifications can affect transcription factor function; ERK1/2-mediated phosphorylation or sumoylation represses C/EBPβ transcription [16, 37]. Therefore, manipulating C/EBPβ expression levels alone may have limited effect. Conversely, overexpression or repression of multiple factors or mutations that alter their posttranslational modifications may provide a route to modify gene expression in a highly specific manner. To this end, researchers must identify the proportion, and derivatives of the various important factors (NFκB, C/EBPβ, C/EBPδ and AP-1), and then manipulate them in a way that results in therapeutic changes in gene expression. Such drug formulations would have the potential to limit side effects while maximizing potency. Here, C/EBPβ knockdown increased expression of 13 of 17 transcripts tested.
As previously reported, NOS-2 and ICAM-1 expression levels were negatively and positively affected by C/EBPβ knockdown, respectively [15, 19]. Of the four mRNA levels decreased in siC/EBPβ-transfected astrocytes, we first chose to focus on COX-2, the most highly induced gene that was affected by C/EBPβ knockdown. COX-2 is an enzyme that converts arachidonic acid to prostaglandin endoperoxide H2. COX-2 enzyme is a key player during inflammation and therapeutic target of non-steroidal anti-inflammatory drugs. C/EBPβ affects COX-2 expression in a cell-dependent manner; supporting the notion that multiple factors and the derivatives thereof ultimately determine gene expression in any given cell type [16, 36]. The regulation of COX-2 via C/EBPβ is thus highly significant both from the perspective of understanding inflammation as well as therapeutic approaches. C/EBPβ, C/EBPδ, NFκB, cyclic adenosine monophosphate response element-binding protein and AP-1 all regulate COX-2 expression in various cell types [15, 38]. C/EBPβ is essential for rodent macrophage biphasic expression of COX-2; however, in A431 cells, all C/EBPβ isoforms repress COX-2 expression, whereas fibroblast COX-2 expression is not C/EBPβ-dependent [16, 36, 39]. These data suggest that the highly inducible COX-2 may be regulated in a cell-specific manner to respond to inflammatory stimuli. In our studies, IL-1β induced robust increases in human astrocyte COX-2 expression; COX-2 was the second most induced gene among all donors tested. Additionally, p38K signaling and C/EBPβ expression may be crucial for astrocyte COX-2 expression during neuroinflammation. In murine macrophages, IL-1β signals through ERK1/2 to increase COX-2 expression . In our studies, an ERK1/2-selective inhibitor increased IL-1β-mediated human astrocyte C/EBPβ and COX-2 expression . MAPK pathway activity is implicated in many disease processes, and therefore, pathway-selective inhibitors are being tested as effective therapies . The current studies illustrate why these pharmacological inhibitors may represent a “blunt-ended” tool when trying to affect expression-specific genes. Blocking a specific pathway or even transcription factor can have profound effects; however, manipulating multiple factors in specific ways may allow fine-tuning of gene expression. These data suggest that the ERK1/2 pathway may activate inhibitors of C/EBPβ and thereby inhibit COX-2 expression. Indeed, ERK1/2 is capable of phosphorylating and thereby repressing C/EBPβ activity .
C/EBPβ knockdown increased IL-1β-induced astrocyte BDKRB1 and BDKRB2 expression as well as that of 11 other genes (Figure 1). We chose to further investigate BDKRB2 expression because of the key role the kallikrein-kinin system plays in the CNS . Most of the biological effects of the kinin system are mediated through BDKRB2 . Interestingly, BDKRB2 signaling mediates prostaglandin release ; collectively, these data suggest that C/EBPβ may play a key role in modulating the inflammatory response by effectively decreasing BDKRB2 signaling (through downregulation of mRNA) while concomitantly increasing COX-2 expression. These data bolster the assertion that transcription factors may function in a context-dependent manner, possibly even within a single cell. Data suggest BDKRB2 signaling may increase BBB permeability [43–46]; an important function during CNS immune responses . Our group is particularly interested in the progression of HAD, and BBB permeability is a contributing factor to HIV-1 entering the CNS [48, 49]. IL-1β induced a robust increase in human astrocyte BDKRB2 mRNA expression, and C/EBPβ knockdown consistently enhanced this effect. Furthermore, ERK1/2 signaling is critical for IL-1β-mediated astrocyte BDKRB2 expression. The p38K-selective inhibitor showed an increase, but no significant effect on BDKRB2 mRNA levels. These signaling data further support complementary regulation of astrocyte COX-2 and BDKRB2 at the level of transcription factors and signal transduction. These data may lead to novel mechanisms to manipulate prostaglandin production and affect inflammation in tissue microenvironments.
JF is a fifth year graduate student; this work, together with a concomitantly submitted manuscript, represents the completion of JF’s dissertation project. AG is JF’s graduate mentor/advisor.
analysis of variance
bovine serum albumin
bradykinin receptor b2
CCAAT enhancer-binding protein
central nervous system
extracellular regulated kinase
glyceraldehyde phosphate dehydrogenase
glial fibrillary acidic protein
human immunodeficiency virus
intercellular adhesion molecule
nuclear factor kappa B
nitric oxide synthase
real-time reverse transcription polymerase chain reaction
sodium dodecyl sulfate polyacrylamide gel electrophoresis
tissue inhibitor metalloproteinases-1
tumor necrosis factor.
This work was supported by 2R01NS048837 from NINDS to A. Ghorpade, and a fellowship from NIA T32 AG020494 and 1F31NS072006-01A1 to J. Fields. We appreciate the assistance of the Laboratory of Developmental Biology for providing us with human brain tissues, also supported by NIH award no. 5R24HD0008836 from the Eunice Kennedy Shriver National Institute of Child Health & Human Development. Lisa Cisneros patiently assisted with the immunocytochemistry, and Lin Tang graciously provided cell cultures.
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