TGF-ß/NF1/Smad4-mediated suppression of ANT2 contributes to oxidative stress in cellular senescence.

Oxidative stress and persistent activation of DNA damage response (DDR) are causally involved in the development of cellular senescence, a phenomenon implicated in fundamental (patho)physiological processes such as aging, fetal development and tumorigenesis. Here, we report that adenine nucleotide translocase-2 (ANT2) is consistently down-regulated in all three major forms of cellular senescence: replicative, oncogene-induced and drug-induced, in both normal and cancerous human cells. We previously reported formation of novel NF1/Smad transcription repressor complexes in growth-arrested fibroblasts. Here we show that such complexes form in senescent cells. Mechanistically, binding of the NF1/Smad complexes to the NF1-dependent repressor elements in the ANT2 gene promoter repressed ANT2 expression. Etoposide-induced formation of these complexes and repression of ANT2 were relatively late events co-incident with production and secretion of, and dependent on, TGF-ß. siRNA-mediated knock-down of ANT2 in proliferating cells resulted in increased levels of reactive oxygen species (ROS) and activation of the DDR. Knock-down of ANT2, together with etoposide treatment, further intensified ROS production and DNA damage signaling, leading to enhanced apoptosis. Together, our data show that TGF-ß-mediated suppression of ANT2 through NF1/Smad4 complexes contributes to oxidative stress and DNA damage during induction of cellular senescence.

Sustained induction of cytochrome P4501A1 in human hepatoma cells by co-exposure to benzo[a]pyrene and 7H-dibenzo[c,g]carbazole underlies the synergistic effects on DNA adduct formation.

To gain a deeper insight into the potential interactions between individual aromatic hydrocarbons in a mixture, several benzo[a]pyrene (B[a]P) and 7H-dibenzo[c,g]carbazole (DBC) binary mixtures were studied. The biological activity of the binary mixtures was investigated in the HepG2 and WB-F344 liver cell lines and the Chinese hamster V79 cell line that stably expresses the human cytochrome P4501A1 (hCYP1A1). In the V79 cells, binary mixtures, in contrast to individual carcinogens, caused a significant decrease in the levels of micronuclei, DNA adducts and gene mutations, but not in cell survival. Similarly, a lower frequency of micronuclei and levels of DNA adducts were found in rat liver WB-F344 cells treated with a binary mixture, regardless of the exposure time. The observed antagonism between B[a]P and DBC may be due to an inhibition of Cyp1a1 expression because cells exposed to B[a]P:DBC showed a decrease in Cyp1a1 mRNA levels. In human liver HepG2 cells exposed to binary mixtures for 2h, a reduction in micronuclei frequency was also found. However, after a 24h treatment, synergism between B[a]P and DBC was determined based on DNA adduct formation. Accordingly, the up-regulation of CYP1A1 expression was detected in HepG2 cells exposed to B[a]P:DBC. Our results show significant differences in the response of human and rat cells to B[a]P:DBC mixtures and stress the need to use multiple experimental systems when evaluating the potential risk of environmental pollutants. Our data also indicate that an increased expression of CYP1A1 results in a synergistic effect of B[a]P and DBC in human cells. As humans are exposed to a plethora of noxious chemicals, our results have important implications for human carcinogenesis.

TGF-ß signals the formation of a unique NF1/Smad4-dependent transcription repressor-complex in human diploid fibroblasts.

We earlier reported the formation of a unique nuclear NF1/Smad complex in serum-restricted fibroblasts that acts as an NF1-dependent repressor of the human adenine nucleotide translocase-2 gene (ANT2) [K. Luciakova, G. Kollarovic, P. Barath, B.D. Nelson, Growth-dependent repression of human adenine nucleotide translocator-2 (ANT2) transcription: evidence for the participation of Smad and Sp family proteins in the NF1-dependent repressor complex, Biochem. J. 412 (2008) 123-130]. In the present study, we show that TGF-ß, like serum-restriction: (a) induces the formation of NF1/Smad repressor complexes, (b) increases binding of the complexes to the repressor elements (Go elements) in the ANT2 promoter, and (c) inhibits ANT2 expression. Repression of ANT2 by TGF-ß is eliminated by mutating the NF1 binding sites in the Go repressor elements. All of the above responses to TGF-ß are prevented by inhibitors of TGF-ß RI and MAPK p38. These inhibitors also prevent NF1/Smad4 repressor complex formation and repression of ANT2 expression in serum-restricted cells, suggesting that similar signaling pathways are initiated by TGF-ß and serum-restriction. The present finding that NF1/Smad4 repressor complexes are formed through TGF-ß signaling pathways suggests a new, but much broader, role for these complexes in the initiation or maintenance of the growth-inhibited state.

Expression and purification of recombinant NFI proteins for functional analysis.

Nuclear factor I (NFI) is a transcription factor playing wide role in signal transduction pathways and developmental processes in higher eukaryotes. In order to produce recombinant NFI proteins for functional and structural studies, full length cDNAs of individual isoforms were subcloned into pETM30 vector and expressed in Escherichia coli. Although the fusion proteins containing both glutathione S-transferase (GST) and His6 tags at the N-terminus could be overexpressed in detectable amounts, they were found mainly, if not exclusively, in insoluble form. Purification yield was improved by modification of cell disruption procedure and by the use of detergent Tween 20. The final purification strategy represents a triple-helix affinity chromatography consisting of prepurification of bacterial lysate on Heparin-Sepharose with subsequent immobilized metal affinity and glutathione affinity chromatography. Heparin chromatography was crucial for obtaining active NFI proteins, whereas the other steps significantly improved the purity of isolated proteins. As demonstrated by EMSA and DNase I protection assay, the recombinant proteins were able to recognize their cognate DNA sequences.

Growth-dependent repression of human adenine nucleotide translocator-2 (ANT2) transcription: evidence for the participation of Smad and Sp family proteins in the NF1-dependent repressor complex.

NF1 (nuclear factor 1) binds to two upstream elements of the human ANT2 (adenine nucleotide translocator-2) promoter and actively represses expression of the gene in growth-arrested diploid skin fibroblasts [Luciakova, Barath, Poliakova, Persson and Nelson (2003) J. Biol. Chem. 278, 30624-30633]. ChIP (chromatin immunoprecipitation) and co-immunoprecipitation analyses of nuclear extracts from growth-arrested and growth-activated diploid cells demonstrate that NF1, when acting as a repressor, is part of a multimeric complex that also includes Smad and Sp-family proteins. This complex appears to be anchored to both the upstream NF1-repressor elements and the proximal promoter, Sp1-dependent activation elements in growth-arrested cells. In growth-activated cells, the repressor complex dissociates and NF1 leaves the promoter. As revealed by co-immunoprecipitation experiments, NF1-Smad4-Sp3 complexes are present in nuclear extracts only from growth-inhibited cells, suggesting that the growth-state-dependent formation of these complexes is not an ANT2 promoter-specific event. Consistent with the role of Smad proteins in the repression complex, TGF-beta (transforming growth factor-beta) can fully repress ANT2 transcription in normally growing fibroblasts. Finally, pull-down experiments of in vitro transcribed/translated NF1 isoforms by GST (glutathione transferase)-Smad and GST-Smad MH fusion proteins indicate direct physical interactions between members of the two families. These findings suggest a possible functional relationship between the NF1 and Smad proteins that has not been previously observed.

Identification of NF1 as a silencer protein of the human adenine nucleotide translocase-2 gene.

The human adenine nucleotide translocase-2 (ANT2) promoter contains a silencer region that confers partial repression on the heterologous herpes simplex virus thymidine kinase (HSVtk) promoter [Barath, P., Albert-Fournier, B., Luciakova, K., Nelson, B.D. (1999) J. Biol. Chem.274, 3378-3384]. Two sequences in the silencer (Site-2 and Site-3) are protected in the DNase I assay in vitro, and one of these is a repeated GTCCTG element previously shown to act as the active repressor element. We have now purified the DNA binding protein, and identified it using MALDI-TOF MS as a 33-kDa member of the nuclear factor 1 (NF1) family of transcription factors. NF1 purified from rat liver and HeLa cell nuclei bind to both silencer Site-2 and Site-3, resulting in a DNase I footprint identical to that obtained with purified recombinant NF1. Furthermore, transient transfection experiments with reporter constructs containing mutated silencer Site-2 and/or Site-3 show that both sites contribute to repression of the HSVtk promoter. Finally, chromatin immunoprecipitation analysis reveals that NF1 is bound to both elements on the endogenous HeLa cell ANT2 promoter. Our data support the belief that NF1 acts as a repressor when bound to silencing Site-2 and Site-3 of the ANT2 gene.

Repression of the human adenine nucleotide translocase-2 gene in growth-arrested human diploid cells: the role of nuclear factor-1.

Adenine nucleotide translocase-2 (ANT2) catalyzes the exchange of ATP for ADP across the mitochondrial membrane, thus playing an important role in maintaining the cytosolic phosphorylation potential required for cell growth. Expression of ANT2 is activated by growth stimulation of quiescent cells and is down-regulated when cells become growth-arrested. In this study, we address the mechanism of growth arrest repression. Using a combination of transfection, in vivo dimethyl sulfate mapping, and in vitro DNase I mapping experiments, we identified two protein-binding elements (Go-1 and Go-2) that are responsible for growth arrest of ANT2 expression in human diploid fibroblasts. Proteins that bound the Go elements were purified and identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry as members of the NF1 family of transcription factors. Chromatin immunoprecipitation analysis showed that NF1 was bound to both Go-1 and Go-2 in quiescent human diploid cells in vivo, but not in the same cells stimulated to growth by serum. NF1 binding correlated with the disappearance of ANT2 transcripts in quiescent cells. Furthermore, overexpression of NF1-A, -C, and -X in NIH3T3 cells repressed expression of an ANT2-driven reporter gene construct. Two additional putative repressor elements in the ANT2 promoter, an Sp1 element juxtaposed to the transcription start site and a silencer centered at nucleotide -332, did not appear to contribute to growth arrest repression. Thus, enhanced binding of NF1 is a key step in the growth arrest repression of ANT2 transcription. To our knowledge, this is the first report showing a role for NF1 in growth arrest.