TGF-β1 induction of the adenine nucleotide translocator 1 in astrocytes occurs through Smads and Sp1 transcription factors
© Law et al; licensee BioMed Central Ltd. 2004
Received: 23 July 2003
Accepted: 13 January 2004
Published: 13 January 2004
The adenine nucleotide translocator 1 (Ant1) is an inner mitochondrial membrane protein involved with energy mobilization during oxidative phosphorylation. We recently showed that rodent Ant1 is upregulated by transforming growth factor-beta (TGF-β) in reactive astrocytes following CNS injury. In the present study, we describe the molecular mechanisms by which TGF-β1 regulates Ant1 gene expression in cultured primary rodent astrocytes.
Transcription reporter analysis verified that TGF-β1 regulates transcription of the mouse Ant1 gene, but not the gene encoding the closely related Ant2 isoform. A 69 basepair TGF-β1 responsive element of the Ant1 promoter was also identified. Electrophoretic mobility shift assays demonstrated that astrocyte nuclear proteins bind to this response element and TGF-β1 treatment recruits additional nuclear protein binding to this element. Antibody supershift and promoter deletion analyses demonstrated that Sp1 consensus binding sites in the RE are important for TGF-β1 regulation of Ant1 in astrocytes. Additionally, we demonstrate that Smad 2, 3 and 4 transcription factors are expressed in injured cerebral cortex and in primary astrocyte cultures. TGF-β1 activated Smad transcription factors also contribute to Ant1 regulation since transcription reporter assays in the presence of dominant negative (DN)-Smads 3 and 4 significantly reduced induction of Ant1 by TGF-β1.
The specific regulation of Ant1 by TGF-β1 in astrocytes involves a cooperative interaction of both Smad and Sp1 binding elements located immediately upstream of the transcriptional start site. The first report of expression of Smads 2, 3 and 4 in astrocytes provided here is consistent with a regulation of Ant1 gene expression by these transcription factors in reactive astrocytes. Given the similarity in TGF-β1 regulation of Ant1 with other genes that are thought to promote neuronal survival, this interaction may represent a general mechanism that underlies the neuroprotective effects of TGF-β1.
The secreted signaling molecule TGF-β1 is rapidly and chronically elevated in response to CNS injury. Astroglial cells at the site of injury become reactive and hypertrophy leading to the formation of a glial scar, a barrier for regenerating axons. TGF-β1 stimulates production by reactive astrocytes of glial fibrillary acidic protein (GFAP), the diagnostic marker of reactive astrogliosis as well as extracellular matrix molecules that contribute to the inhibition of axonal regeneration [1–5]. Using an in vivo filter implant model of the glial scar, we have recently shown that the expression of Ant1, a gene involved in energy mobilization, is elevated in reactive astrocytes and that astrocytic Ant1 expression is regulated by TGF-β1 both in vivo and in vitro .
Ant1 is a major mitochondrial inner membrane protein that exchanges mitochondrial ATP for cytosolic ADP and is thereby an important component of oxidative phosphorylation (OXPHOS) energy production. Patients with myocarditis and cardiac myopathy exhibit lower activity of the translocator  as well as altered levels of Ant isoform expression . A familial mutation of ANT1 is associated with autosomal dominant progressive external ophthalmoplegia, characterized by large-scale mitochondrial DNA deletions . In addition, mice deficient in Ant1 demonstrate characteristics of cardiac myopathy, including severe exercise intolerance and mitochondrial proliferation in the heart .
The importance of Ant1 function is further underscored by the highly conserved Ant genes that have been identified in a number of mammalian species, including bovine, rat, human and mouse [10–13] as well as other eukaryotes such as yeast and plants [14–16]. Unlike humans with three ANT isoforms, rodents have two Ant genes that share with each other 78% cDNA and 85% amino acid sequence identity. Interestingly, Ant1 isoforms from different species (e.g. human Ant1 and mouse Ant1) are more closely related than different Ant isoforms within the same species. Rodent Ant1 has an expression pattern similar to human ANT1, with the highest level found in brain, heart and skeletal muscles [17–20]. Rodent Ant2, on the other hand, is readily detected in all tissues except for skeletal muscles whereas human ANT2 is only weakly expressed in most tissues examined [18, 20].
Examination of transcriptional regulation of human ANT genes revealed OXBOX and REBOX response elements (REs), sensitive to oxidative phosphorylation (OXPHOS) activity and redox state, respectively. These promoter elements can regulate transcription of human ANT1 and other OXPHOS genes, including ATP synthase β [21, 22]. Little is known about the regulation of Ant gene expression by factors other than metabolic and redox influences, although estrogen can regulate Ant1 expression in female rat hearts  and may induce Ant2 mRNA expression in the rat hypothalamus . We have recently shown that TGF-β1 upregulates expression of Ant1, but not Ant2 mRNA in glial scars. Despite functional similarity and sequence homology between the mouse isoforms however, Ant2 is neither upregulated in reactive astrocytes following CNS injury, nor in TGF-β1 treated primary astrocytes . In the present study, we have taken advantage of this differential regulation of Ant1 and Ant2 in mouse astrocytes to examine TGF-β1 regulation of Ant1 gene expression in primary astrocyte cultures.
The best characterized mechanism of TGFβ-mediated regulation of gene expression is via the Smad family of transcription factors. Following TGFβ receptor activation, the cytosolic receptor-regulated Smads (R-Smads) 2 and 3 are phosphorylated and associate with the common-partner Smad (co-Smad), Smad 4, to form hetero-oligomers. This complex enters the nucleus to induce gene expression (reviewed by Massagué ). The diverse effects of TGFβ in multiple cell types, suggests that regulation of transcription by this cytokine is likely to occur at multiple levels to provide precise activation of target genes. For instance, Smad activity can be regulated by crosstalk with the MAPK signaling pathway [26, 27] as well as protein kinase C . Smads can also directly bind to promoter DNA response elements or interact with other transcription factors as part of a nuclear complex. A large number of nuclear Smad binding partners have been reported [reviewed by ]. In particular, Smads interact with Sp1 to induce transcription of p21/WAF1/Cip1 [30, 31], integrin β5 subunit , α 2(I) collagen  and plasminogen activator inhibitor-1 (PAI) . Despite the well documented effect of TGF-β1 on gene expression in astrocytes, however, expression of endogenous Smads in reactive astrocytes or other CNS cell types has not been reported.
Our studies were aimed to determine the molecular mechanisms that regulate the astrocytic expression of Ant1, a protein that mobilizes mitochondrial energy. We have identified a TGF-β1 responsive element within the mouse Ant1 promoter. This response element sequence is sufficient to confer TGF-β1 responsivity to the Ant2 gene promoter, which is not normally regulated by this cytokine in astrocytes. Furthermore, the transcription factors Sp1 and Smads 3 and 4 are expressed in the CNS and in cultured astrocytes and appear to account for specific upregulation of the Ant1 gene in reactive astrocytes following CNS injury.
Identification of a TGF-β1 response element within the Ant1 promoter
Localization to this proximal upstream promoter sequence of the Ant1 is consistent with the localization of sequences involved in the TGF-β1 regulation of astrocytic GFAP  and PAI  which appear to be confined to regions 0.8 to 1.8 kb upstream of the transcription start site, respectively. The difference in transcription induction between the full length and the 485 bp reporter constructs may be due to the loss of basal transcription elements residing in the more distal 6.5 kb promoter region. Interestingly however, this upstream 6.5 kb of promoter sequence not only showed a lack of induction in response to treatment with this cytokine, but transcriptional activity from this fragment was reduced by 43% in the presence of TGF-β1 compared to untreated controls (Fig 1b). Together, these results suggest that TGF-β1 regulation of Ant1 expression is complex and that induction of gene transcription by this cytokine is principally localized to the 485 bp of promoter upstream from the transcription start site. Overlapping reporter constructs spanning the distal (Ant1Prom1), medial (Ant1Prom2) and proximal (Ant1Prom3) regions of the TGF-β1-responsive 485 bp sequence were then employed to further localize the Ant1 TGF-β1 RE (Fig 1a). TRAs with these constructs revealed that the TGF-β1 responsivity is localized entirely to the 5' half of Ant1Prom3 (Ant1Prom3-5') (Fig 1b). TGF-β1 induction of Ant1 was confirmed by immunoblot analysis demonstrating elevated expression of endogenous Ant1protein in TGF-β1 treated versus control astrocyte cultures (Fig 1c).
The Ant1 TGF-β1 RE is sufficient for cytokine-mediated transcription induction in astrocytes
Activation of the Ant1 RE by TGF-β1 was further tested in the context of this non-responsive Ant2 promoter. The promoter region corresponding to the TGF-β1 responsive Ant1Prom3-5' was inserted into 2.2 kb Ant2 reporter construct for TRA analysis. Ant1Prom3-5' conferred significant TGF-β1 responsivity (p < 0.05) in the otherwise non-responsive Ant2 isoform promoter (Fig 2c). The decreased magnitude of induction with this construct compared with the Ant1 promoter constructs may result from the distance of the TGF-β1 RE from the point of transcription initiation. Alternatively, there may be inhibitory regulation in the context of the Ant2 promoter sequence. Nevertheless, these results indicate that the Ant1Prom3-5' promoter sequence is both necessary and sufficient for regulation of Ant1 mRNA expression by this cytokine.
TGF-β1 induces binding of Sp1 to the Ant1 TGF-β1 RE
TGF-β1 stimulation of nuclear protein binding to the Ant1 RE was demonstrated with two overlapping oligonucleotides from Ant1Prom3-5' (Oligo 1, -87 to -54; Oligo 2, -64 to -40) (Fig 3b). A shifted band with increasing intensity at one and three hours of TGF-β1 treatment was apparent with Oligo 1, which contains the tandem Sp1-1GC and Sp1-2TCC repeats. Indeed, an Sp1-specific antibody further retarded migration of this oligo, thus confirming the presence of Sp1 in the nuclear protein complex binding to the Ant1 TGF-β1 RE (Fig 3b). In contrast, the same nuclear extracts did not demonstrate TGF-β1 induced protein binding to Oligo 2, which includes the putative SBE and 10 out of 11 bases of Sp1-2TCC (Fig 3a). These results suggest that the Sp1-1GC site may be most important for nuclear protein binding to the Ant1 TGF-β1 RE and that TGF-β1 treatment likely results in recruitment of an Sp1 transcription factor species to this RE. On the other hand, the lack of inducible nuclear proteins binding to the SBE suggests that either Smads do not directly bind to this response element, or that these transcription factors bind via an as yet unidentified SBE sequence.
The consensus sequence in Oligo 2 containing all but the last base of the Sp1-2TCC motif did not demonstrate a TGF-β1 induced protein binding in the EMSA study, suggesting that this Sp1 site does not directly contribute to nuclear protein complex binding to the Ant1 RE in response to TGF-β1. To further explore the relationship between the first two Sp1 sites, EMSA was performed with a truncated version of the Oligo 1 probe (-87 to -60), lacking half of Sp1-2TCC (Oligo 1Δ – Fig 3a). Surprisingly, TGF-β1-induced binding of nuclear protein to this Oligo was not detected (Fig 3c). This result suggests both Sp1-1GC and Sp1-2TCC must be present in order for TGF-β1 induced Sp1 binding to the RE to occur.
Expression of Smads in the in vivoglial scar and primary astrocytes
Immunostaining of R-Smads provides additional support for expression of these transcription factors in the in vivo glial scar and astrocyte cultures (Fig. 4c). Immunohistochemical analysis indicates the presence of cells expressing Smads 2 and/or 3 within the in vivo glial scar, which also exhibits strong GFAP immunoreactivity indicative of reactive astrogliosis. Expression of these TGF-β1 activated R-Smads was also demonstrated in primary astrocyte cultures. These immunocytochemical analyses further reflect the similarities between cultured astrocytes and reactive astrocytes of the in vivo glial scar and validates the use of primary astrocyte cultures to examine TGF-β1 regulation of Ant1. Thus the presence of TGF-β1 activated Smads in astrocytes is supported by RT-PCR, immunoblot and immunohistochemical studies. To our knowledge, these results are the first direct demonstration that these Smads are expressed by reactive astrocytes in response to CNS injury and by cultured primary astrocytes.
Functional role of Smads 3 and 4 in the TGF-β1 induction of Ant1
TGF-β is a pluripotent cytokine with well-described actions in numerous tissues during development. TGF-β exists as three isoforms with TGF-β2 and TGF-β3 predominating during development when TGF-β1 levels are low. Following CNS injury, however, TGF-β1, but not -β2 or -β3, is upregulated where it is thought to serve as a modulator of the inflammatory response and to promote neuronal survival [reviewed by [47, 48]]. This cytokine also directs astrocytic expression of a number of genes including p15 (INK4B) , amyloid precursor protein , GFAP [35, 51, 52] and PAI-1 . The molecular mechanisms whereby TGF-β1 induces transcription in the CNS, however, are not well understood. We recently found that Ant1 is upregulated by TGF-β1 in astrocytes following CNS injury. The biological significance of this TGF-β1 mediated response is evident from in vitro studies demonstrating that glutamate uptake by Ant1 null mutant astrocytes is significantly impaired compared to uptake by wild type astrocytes . Because glutamate levels can rise to toxic levels following CNS injury, regulating the expression of Ant1 may be one way TGF-β1 enhances neuronal survival. In this study, we directly examined the effects of TGF-β1 on Ant1 gene expression in primary astrocytes since this cell culture system has been extensively used as a model of the reactive astrocyte response to CNS injury [35, 39, 51, 53, 54]. As was the case in the in vivo glial scar, primary astrocyte cultures respond to TGF-β1 with increased expression of Ant1, but not of the closely related Ant2 isoform (Fig 1; ).
Transcription of a reporter plasmid construct including the transcription start stie and approximately 500 bp of the Ant1 promoter was induced 1.7-fold by TGF-β1; smaller constructs including the Ant1 TGF-β1 RE were similarly induced. The level of induction we observe for Ant1 is remarkably similar to the extent of TGF-β1 stimulation of alpha 2(I) collagen via the synergistic actions of Sp1 and Smad3 in a primary human mesangial cell culture system . Surprisingly, an expression construct with 6.5 kb of Ant1 promoter upstream of the TGF-β1 responsive 500 bp fragment exhibited reduced expression in the presence of TGF-β1. These results may reflect a transcriptional inhibition of Ant1 expression that is overcome by the positive response of the TGF-β1 RE following CNS injury, thus suggesting a tightly controlled and complex regulation of Ant1 gene expression in reactive astrocytes.
Following CNS injury, TGF-β1 treatment has been reported to promote neuronal survival [47, 48] but the mechanisms of this neuroprotective effect have not been elucidated. Recently, TGF-β1 directed expression of PAI-1 in astrocytes following injury has been shown to protect neurons from glutamate excitotoxicity . Our recent demonstration of the role of Ant1 in astrocytic glutamate uptake  further supports a role for TGF-β1 in promoting neuronal cell survival in the injured CNS. Given that both Smads and Sp1 are involved in the induction of Ant1 and PAI-1  in astrocytes, this partnership of transcription factors may represent a general mechanism for TGF-β1 enhancement of cell survival following CNS injury. In addition to Ant1 and PAI-1, TGF-β1 mediated induction of the extracellular matrix molecule neurocan by astrocytes has also been documented [5, 56]. The neurocan promoter also contains consensus binding sites for Sp1  and for Smads (-103 to -100 and -325 to -322 relative to transcription initiation), again consistent with the possibility that these transcription factors cooperate in TGF-β1 induced neurocan gene expression following CNS injury. TGF-β1 also stimulates the astrocytic expression of GFAP, the hallmark of reactive astrogliosis following CNS injury [58, 59]. Recent studies examining the TGF-β1 regulation of GFAP expression in astrocytes  only identified an NF-1-like response element. Hence even in the reactive astrocyte response to CNS injury, TGF-β1 may direct target gene expression via multiple pathways.
The molecular mechanisms of TGF-β1 directed transcription have been studied extensively in many non-CNS systems with transformed cell lines and many Smad interacting partners identified [reviewed by ]. For example, in cooperation with Sp1, Smads mediate the expression of alpha 2(I) collagen , PAI , β5 integrin , and p21/WAF1/Cip1 . Our results demonstrate that Smads also interact with Sp1 to induce Ant1 gene expression. Interestingly, Smad-Sp1 interactions with all of these genes occur predominantly through Smad3 rather than Smad2; although each of these R-Smads can be activated by TGF-β1. Specific interaction between Smad3 and Sp1 may represent a general mechanism for Smad-Sp1 interaction. The apparent interaction of both Smads 2 and 3 in TGF-β1 induction of p15 (INK4B) however, indicates that this is not an exclusive mechanism .
Although human ANT2 mRNA is weakly expressed in most tissues, this gene is induced in highly proliferative cell types . This observation coincides with the upregulation of mouse Ant1 in reactive astrocytes that may proliferate following CNS injury during the formation of the glial scar. The similarity between these two genes is also evident in the structure of the gene promoters. The human ANT2 gene promoter contains three Sp1 sites flanking the TATA box, which are important in the regulation of human ANT2 mRNA expression . To our surprise, the mouse Ant1 TGF-β1 RE reported here also contains three Sp1 sites near the TATA box. Given the similarity in promoter sequence structure of human ANT2 and mouse Ant1, we hypothesize that the inducibility of both genes may be partly due to the presence of tandem Sp1 repeats. This hypothesis is consistent with reports demonstrating that the TGF-β1 regulated α 2(I) collagen  and PAI-1  gene promoters also contain multiple Sp1 sites. Further, our studies indicate that TGF-β1 specifically regulates the expression of rodent Ant1 but not Ant2. This may be partly explained by a paucity of Sp1 sites in the mouse Ant2 promoter, which contains only one Sp1 element on the non-coding strand at -32. This suggestion is supported by our finding that a chimeric construct generated by addition of the Ant1 TGF-β1 RE containing the three Sp1 sites to the Ant2 promoter transcription reporter construct confers TGF-β1 responsivity. Taken together, these observations suggest that human ANT2 may respond to TGF-β1 following CNS injury, analogous to mouse Ant1.
Increased production of TGF-β1 after CNS injury is thought to regulate the expression of a number of genes involved in tissue repair and neuronal protection. While TGF-β mediated gene expression typically involves the Smad family of transcription factors, the specific molecular mechanisms utilized during the induction of specific genes needs to be characterized. The results of the present study demonstrate that TGF-β1 mediated induction of Ant1 by astrocytes requires the cooperative interaction of both Smad 3 and multiple Sp1 binding sites and that these binding elements are located within 500 bps immediately upstream of the transcriptional start site. This induction is specific since the closely related Ant2 isoform, which does not contain similar binding elements, is not induced by TGF-β1. The similarity in TGF-β1 regulation of Ant1 and other genes including PAI, which has also been implicated in promoting neuronal survival by astrocytes, may represent a general mechanism that underlies the neuroprotective effects of TGF-β1. Thus, the results of the present study provide novel insights into the astrocytic regulation of Ant1 following CNS injury.
Nitrocellulose filters were implanted into the cerebral cortex of adult TMG129 mice (≥30-day-old) as previously described [5, 62, 63]. The filters were left in place for 14 days prior to removal. Filter implants from 10–20 animals were pooled for RNA or protein extractions. Animal care was in accordance with guidelines established by the Institutional Animal Care and Use Committee at Emory University.
Purified populations of neonatal cortical astrocytes were prepared according to previously described methods . The cells were cultured in DMEM-F-12 supplemented with 10% fetal calf serum (FCS), 2 mM glutamine, and 100 IU/ml penicillin-streptomycin, and were maintained at 37°C in 5% CO2. All experiments were performed on primary astrocytes cultured for one to three months.
Total proteins were purified from nitrocellulose filter implants, uninjured cortex and primary astrocyte cultures as described . Transferred nitrocellulose membranes were blocked at 4°C for one hour at room temperature or overnight. Ant1 and Smad 2/3 immunoblots were blocked in KPL milk blocking reagent in PBS. Ant1 proteins were detected by an affinity-purified polyclonal antisera [6, 18]. The Smad2/3 antibody (Santa Cruz, cat. # 6032) and the Ant1 antisera were used at 1:200 dilution in KPL milk blocking buffer and incubated overnight at 4°C. MAPK proteins were detected by a p42/44 MAPK antibody (New England Biolabs). Blots were washed three times with PBS, incubated with the appropriate secondary antibodies diluted in blocking buffer and detected by ECL chemiluminescence (Amersham).
Adult mice were subjected to cortical nitrocellulose filter implantation to generate a glial scar. Fourteen days after implantation, animals were perfused with 4% paraformaldehyde in PBS (pH 7.0) and 10 μm frozen sections of injured cortex containing the filter implant obtained. Sections were washed twice with PBS and blocked with 0.3% H2O2, 3% normal horse serum in PBS at room temperature for 10 minutes, washed again and incubated with blocking buffer containing 3% normal horse serum and 0.04% Triton X-100 in PBS for 1 hour at room temperature. Sections were again subjected to PBS washes and incubated with Smad2/3 or Smad2 goat antibodies (1:100; Santa Cruz) overnight at 4°C. After similar washes, sections were incubated with a biotinlyated anti-goat secondary antibody (1:1000; Jackson ImmunoResearch Laboratories) and developed with 3,3'-diaminobenzidine (Vector). Primary astrocytes were grown on laminin coated glass coverslips, fixed with 4% paraformaldehyde in PBS at room temperature for 10 minutes and processed as above except the anti-Smad2/3 primary antibody was used at 1:200 dilution. Negative staining controls using inappropriate antibodies conducted with either a monoclonal GFAP antibody (1:500; Sigma) followed by the biotinlyated anti-goat secondary antibody, or the Smad goat antibodies together with a biotinlyated anti-mouse secondary antibody did not show any staining (data not shown).
Reverse transcriptase-polymerase chain reaction (RT-PCR)
Sense and anti-sense oligonucleotides for Smads 2, 3 and 4
Annealing temp (°C)
45 (5 cycles)
57 (35 cycles)
Clones containing the promoter regions of Ant1 and Ant2 spanning 7 kb and 5.8 kb respectively were generated previously . Subclones were inserted into the pGL3 firefly luciferase reporter plasmid using standard molecular biology techniques. Dominant-negative Smad expression plasmids were kindly provided by Dr. Rik Derynk (UCSF).
EMSA and supershift assays
EMSA oligonucleotide sequences
Oligo 1 Sense
Oligo 1 Anti-sense
Oligo 2 Sense
Oligo 2 Anti-sense
Oligo 1Δ Sense
Oligo 1Δ Anti-sense
Transfections and luciferase assays
Primary astrocyte cultures were kept in 12-well dishes with 0.5 ml culture medium containing DMEM with 10% fetal calf serum and L-glutamine. Prior to transfections, 0.25 ml of conditioned culture medium was replaced with 0.25 ml of fresh culture medium. DNA cocktails consisting of 1.0 μg of firefly luciferase reporter and 0.5 μg of renilla luciferase plasmid under the control of the thromboxane synthase promoter (pTS-RL; ) were mixed with 3 μl Fugene 6 (Roche) previously diluted in 97 μl of OPTI-MEM (Life Technologies). The DNA-Fugene mix was incubated at room temperature for 15 minutes and added to each well. Cells were incubated at 37°C for 24 hours, rinsed once with 0.5 ml of serum free media and replaced with an equal volume of N2-supplemented serum free media. Transfected cells were incubated for an additional 48 hours in TGF-β1 (10 ng/ml, R&D Technologies). Cells were washed twice with 350 μl of ice-cold PBS, lysed in 100 μl of 1× passive lysis buffer (Promega), and centrifuged at 15,000 g for 20 seconds to pellet debris. Supernatants were transferred to fresh microcentrifuge tubes and stored at -70°C until assayed. Luciferase assays were performed using the Dual Luciferase Reporter kit (Promega) with 10 to 20 μl of lysates according to manufacturer's protocol. Firefly luciferase activities were normalized to renilla luciferase activity and expressed as relative light units (RLUs). Mean RLUs for TGF-β1 treated samples were compared to untreated controls by student's t-test. For dominant negative Smad co-transfection experiments, 0.5 μg of expression plasmid encoding the DN-Smad or empty vector control was added to the transcription reporter construct transfections.
We thank Peter ten Dijke, Carl Heldin and Rik Dernyk for their generous gifts of plasmids. We are also grateful to Anne Marinovic for technical assistance. This work was supported by NIH grants NS41850, HL64017, NS21328, and AG13154 awarded to DCW, and by NIH grant NS35986 and a grant from the Free Radicals in Medicine Core at Emory University to RJM.
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