CCAAT/enhancer binding protein alpha interacts with ZTA and mediates ZTA-induced p21(CIP-1) accumulation and G(1) cell cycle arrest during the Epstein-Barr virus lytic cycle.
Cellular CCAAT/enhancer binding protein alpha (C/EBPalpha) promotes cellular differentiation and has antimitotic activities involving cell cycle arrest at G(1)/S through stabilization of p21(CIP-1)/WAF1 and through transcriptional activation of the p21 promoter. The Epstein-Barr virus lytic-cycle transactivator protein ZTA is known to arrest the host cell cycle at G(1)/S via a p53-independent p21 pathway, but the detailed molecular mechanisms involved have not been defined. To further evaluate the role of ZTA in cell cycle arrest, we constructed a recombinant adenovirus vector expressing ZTA (Ad-ZTA), whose level of expression at a low multiplicity of infection in normal human diploid fibroblast (HF) cells was lower than or equal to the physiological level seen in Akata cells lytically induced by EBV (EBV-Akata cells). Fluorescence-activated cell sorting analysis of HF cells infected with Ad-ZTA confirmed that G(1)/S cell cycle arrest occurred in the majority of ZTA-positive cells, but not with an adenovirus vector expressing green fluorescent protein. Double-label immunofluorescence assays (IFA) performed with Ad-ZTA-infected HF cells revealed that only ZTA-positive cells induced the expression of both endogenous C/EBPalpha and p21 and blocked the progression into S phase, as detected by a lack of incorporation of bromodeoxyuridine. The stimulation of endogenous ZTA protein expression either through treatment with tetradecanoyl phorbol acetate in D98/HR1 cells or through B-cell receptor cross-linking with anti-immunoglobulin G antibody in EBV-Akata cells also coincided with the induction of both C/EBPalpha and p21 and their mRNAs, as assayed by Northern blot, Western blot, and IFA experiments. Mechanistically, the ZTA protein proved to directly interact with C/EBPalpha by coimmunoprecipitation in EBV-Akata cells and with DNA-bound C/EBPalpha in electrophoretic mobility shift assay experiments, and the in vitro interaction domain encompassed the basic leucine zipper domain of ZTA. ZTA also specifically protected C/EBPalpha from degradation in a protein stability assay with a non-EBV-induced Akata cell proteasome extract. Furthermore, both C/EBPalpha and ZTA were found to specifically associate with the C/EBPalpha promoter in chromatin immunoprecipitation assays, but the interaction with ZTA appeared to be mediated by C/EBPalpha because it was abolished by clearing with anti-C/EBPalpha antibody. ZTA did not bind to or activate the C/EBPalpha promoter directly but cooperatively enhanced the positive autoregulation of the C/EBPalpha promoter by cotransfected C/EBPalpha in transient luciferase reporter gene assays with Vero and HeLa cells as well as with DG75 B lymphocytes. Similarly, ZTA alone had little effect on the p21 promoter in transient reporter gene assays, but in the presence of cotransfected C/EBPalpha, ZTA enhanced the level of C/EBPalpha activation. This effect proved to require a previously unrecognized region in the proximal p21 promoter that contains three high-affinity C/EBPalpha binding sites. Finally, in C/EBPalpha-deficient mouse embryonic fibroblasts (MEF), Ad-ZTA was unable to induce either p21 or G(1) arrest, whereas it was able to induce both in wild-type MEF. Overall, we conclude that C/EBPalpha is essential for at least one pathway of ZTA-induced G(1) arrest during EBV lytic-cycle DNA replication and that this process involves a physical piggyback interaction between ZTA and C/EBPalpha leading to greatly enhanced C/EBPalpha and p21 levels through both transcriptional and posttranslational mechanisms.
Wu
Frederick Y
FY
Molecular Virology Laboratories, Department of Pharmacology and Molecular Sciences, School of Medicine, The Johns Hopkins University, Baltimore, Maryland 21231-1000, USA.
Chen
Honglin
H
Wang
Shizhen Emily
SE
ApRhys
Collette M J
CM
Liao
Gangling
G
Fujimuro
Masahiro
M
Farrell
Christopher J
CJ
Huang
Jian
J
Hayward
S Diane
SD
Hayward
Gary S
GS
D000256:Adenoviridae / Q000235:genetics; D022763:CCAAT-Enhancer-Binding Protein-alpha / Q000378:metabolism*; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000378:metabolism*; D024202:Electrophoretic Mobility Shift Assay; D019084:Fluorescent Antibody Technique, Indirect; D016193:G1 Phase; D017930:Genes, Reporter; D005822:Genetic Vectors; D004854:Herpesvirus 4, Human / Q000502:physiology*; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D016196:S Phase; D015534:Trans-Activators / Q000378:metabolism*; D014764:Viral Proteins
C063676:BZLF1 protein, Herpesvirus 4, Human; D022763:CCAAT-Enhancer-Binding Protein-alpha; D050759:Cyclin-Dependent Kinase Inhibitor p21; D016213:Cyclins; D004268:DNA-Binding Proteins; D015534:Trans-Activators; D014764:Viral Proteins