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J. Virol., Dec 1996, 8590-8605, Vol 70, No. 12
YJ Chan, CJ Chiou, Q Huang and GS Hayward
Cytomegalovirus (CMV) infection is nonpermissive or persistent in many
lymphoid and myeloid cell types but can be activated in differentiated
macrophages. We have shown elsewhere that both the major immediate- early
gene (MIE) and lytic cycle infectious progeny virus expression can be
induced in otherwise nonpermissive monocyte-like U-937 cell cultures
infected with either human CMV (HCMV) or simian CMV (SCMV) by treatment
with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). Two
multicopy basal enhancer motifs within the SCMV MIE enhancer, namely, 11
copies of the 16-bp cyclic AMP response element (CRE) and 3 copies of novel
17-bp serum response factor (SRF) binding sites referred to as the SNE
(SRF/NFkappaB-like element), as well as four classical NFkappaB sites
within the HCMV version, contribute to TPA responsiveness in transient
assays in monocyte and T-cell types. The SCMV SNE sites contain potential
overlapping core recognition binding motifs for SRF, Rel/NFkappaB, ETS, and
YY1 class transcription factors but fail to respond to either serum or
tumor necrosis factor alpha. Therefore, to evaluate the mechanism of TPA
responsiveness of the SNE motifs and of a related 16-bp SEE (SRF/ETS
element) motif found in the HCMV and chimpanzee CMV MIE enhancers, we have
examined the functional responses and protein binding properties of
multimerized wild-type and mutant elements added upstream to the SCMV MIE
or simian virus 40 minimal promoter regions in the U-937, K-562, HL-60,
THP-1, and Jurkat cell lines. Unlike classical NFkappaB sites, neither the
SNE nor the SEE motif responded to phosphatase inhibition by okadaic acid.
However, the TPA responsiveness of both CMV elements proved to involve
synergistic interactions between the core SRF binding site (CCATATATGG) and
the adjacent inverted ETS binding motifs (TTCC), which correlated directly
with formation of a bound tripartite complex containing both the cellular
SRF and ELK-1 proteins. This protein complex was more abundant in U-937,
K-562, and HeLa cell extracts than in Raji, HF, BALB/c 3T3, or HL-60 cells,
but the binding activity was altered only twofold after TPA treatment. A
40-fold stimulation of chloramphenicol acetyltransferase activity mediated
by four tandem repeats of the SNE could be induced within 2 h (and up to
250-fold within 6 h) after addition of TPA in DNA-transfected U-937 cells,
indicating that the stimulation appeared likely to be a true protein kinase
C-mediated signal transduction event rather than a differentiation
response. Slight differences in the sequence of the core SRF binding site
compared with that of the classical c-Fos promoter serum response element,
together with differences in the spacing between the SRF and ETS motifs,
appear to account for the inability of the SCMV SNEs to respond to serum
induction.
Copyright © 1996, American Society for Microbiology
Synergistic interactions between overlapping binding sites for the serum response factor and ELK-1 proteins mediate both basal enhancement and phorbol ester responsiveness of primate cytomegalovirus major immediate-early promoters in monocyte and T-lymphocyte cell types
Department of Pharmacology, Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, USA.
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