Study On The Function And Regulation Of Human Activating Transcription Factor 5 (ATF5)

ATF5,a member of ATF/CREB family of b-ZIP transcription factors,is involved in a number of processes,including neuronal differentiation, cell growth and apoptosis.ATF5 could functionally and physically interact with a variety of proteins including cyclin D3,GABAβreceptor, HTLV-1 viral protein Tax,E2 ubiquitin-conjugating enzyme cdc34,PRL-1 and DISC1.Accumulated data from our laboratory and other investigators indicated that ATF5 functions as an apoptosis-related protein in response to a number of environment factors and DNA damage.Interference with the function or expression of ATF5 in glioma cells leaded to their death in vitro and in vivo,indicating that ATF5 might be an attractive target for therapeutic intervention in glioblastoma.Consistent with this,ATF5 was widely expressed in carcinomas and interference with its function caused apoptotic cell death of neoplastic breast cell lines.Furthermore,ATF5 functioned as an anti-apoptotic role in an interleukin 3(IL-3)-dependent cell line.In our study,we show that the ectopic expression of ATF5 promotes cisplatin-induced apoptosis and up-regulates cyclin D3 expression in HeLa cells.Moreover,interference of cyclin D3 expression by transfection with cyclin D3 RNAi could protect cells from ATF5-induced apoptosis,suggesting that cyclin D3 is an essential target gene of pro-apoptosis protein ATFS.Although the progresses in ATF5 interacting partners,function and its relation to some diseases have been made in recent years,the precise molecular mechanisms of ATF5 protein regulation during apoptosis are largely unknown.Previously,Debanda et al.showed that ATF5 interacted with E2 ubiquitin-conjugating enzyme cdc34.Cdc34 and RAD6 had been reported to be important components of the ubiquitin-proteasome system in the nucleus which was responsible for the degradation of some transcriptional factors.We provided evidence that ATF5 was degraded via the ubiquitin-proteasome pathway through its N-terminal ubiquitinylation. And,cisplatin increased ATF5 protein expression by preventing its proteasome-mediated degradation.Furthermore,we reported for the first time that cisplatin-reduced ubiqutin-dependent degradation of ATF5 might be associated with its promoting the nucleus-to-cytoplasm translocation of cdc34 and inhibiting the interaction between ATF5 and cdc34.These data indicated that a down-regulation of proteasome-mediated degradation of ATF5 via N-terminal ubiquitinylation might contribute to cisplatin-induced apoptosis,which provided a new mechanism of cisplatin-induced apoptosis.To increase the knowledge about ATF5 transcriptional regulation,we identified the transcription start site of the ATF5 gene,cloned its 5'-flanking region and identified the region -105 to +3 relative to the transcription start site as that having promoter activity.This region contained potential binding sites for several transcription factors, including EBF1,Sp1 and E2F1.Mutation of EBFl-binding site between nucleotide -16 to -11 obviously impaired its promoter activity.EBF1 belongs to a bHLH transcription factor family and contributes to neuronal differentiation.Its overexpression significantly induced the activity of ATF5 promoter in an EBF1-binding site independent manner.Thus,our studies not only provided molecular basis of ATF5 transcriptional regulation,but also identified ATF5 as a target gene of EBF1 transcription factor.In summary,ATF5 play critical roles in apoptosis,differentiation and development.However,the mechanisms involving the function of ATF5 should been clearly investigated.