Adenovirus as a Model for Oncogenesis and 
Control of Gene Expression 
Thomas E. Shenk, Ph.D. — Investigator 
Dr. Shenk is also Elkins Professor of Molecular Biology at Princeton University and Adjunct Professor of 
Biochemistry at the Robert Wood Johnson Medical School, University of Medicine and Dentistry of New 
Jersey. He received his Ph.D. degree in microbiology from Rutgers University for studies with Victor Stollar 
and trained as a postdoctoral fellow with Paul Berg at Stanford University. Before joining the faculty at 
Princeton, he was Assistant Professor of Microbiology at the University of Connecticut Health Center and 
then Professor of Microbiology at the State University of New York School of Medicine at Stony Brook. Dr. 
Shenk counts among his honors the Eli Lilly Award in Microbiology from the American Society for Micro- 
biology and an American Cancer Society Professorship. 
ADENOVIRUSES are widespread in nature. 
Humans are first infected when very young. 
Generally the infection results in cold-like symp- 
toms, and the episode resolves without compli- 
cation. Injected into a rat or hamster, some hu- 
man adenoviruses induce a variety of benign or 
malignant tumors. Adenoviruses are grouped in 
the class of DNA tumor viruses, since they are 
tumorigenic under certain conditions and con- 
tain DNA chromosomes. 
Human adenoviruses can be propagated easily 
in cultured human cells. When human cells are 
infected, the approximately 30 viral genes are 
expressed, the viral chromosome is replicated, 
and individual DNA molecules are packaged into 
virus-coded protein shells to generate virus parti- 
cles. Since viral gene expression is tightly regu- 
lated and occurs at high levels compared with 
that of most cellular genes, adenoviruses are a 
useful probe for the study of gene expression 
control. 
During the past year, much of our effort has 
focused on transcriptional control of viral gene 
expression. The first viral gene to be expressed 
after infection of a cell is the El A gene, which 
encodes a protein that activates expression of ad- 
ditional viral genes at the level of transcription. 
Expression of these viral genes can be induced by 
treatment of cells with cAMP. 
Most viral genes activated by the ElA protein 
contain a small sequence termed a cAMP re- 
sponse element (CRE) within the DNA sequences 
of the transcriptional control region. Cyclic AMP 
is a key player in a regulatory cascade that can 
induce transcription of a variety of cellular genes. 
Increased cAMP levels lead to activation of the 
CRE-binding (CREB) protein, which in turn 
binds to the CRE sequence. 
The ElA gene product and cAMP act in synergy 
to induce transcription of target genes. We have 
not detected a change in the level of CREB activ- 
ity under induced conditions. However, a second 
transcription factor, AP I, is strongly induced. 
AP I binds to a DNA sequence termed a TPA 
response element (TRE). The TRE sequence 
differs from the CRE by only 1 base pair, and AP- 1 
binds almost as well to CRE as to TRE sequences. 
Thus our current hypothesis is that the ElA gene 
product plus cAMP acts to raise the activity level 
of AP I, which binds to both CRE and TRE se- 
quences and thus contributes to the activation of 
viral genes. 
The induction of AP-1 activity occurs at two 
levels. The first is a rapid modification of the fac- 
tor, causing it to display altered physical proper- 
ties. The second level requires active transcrip- 
tion of the products of two proto-oncogenes, 
c-fos and junB. Together these products consti- 
tute one form of AP- 1 . It is likely that the modi- 
fied form of AP- 1 not only induces expression of 
adenovirus genes but also of its own constituents, 
activating an autoregulatory loop. We are now 
working to identify the constituents of the modi- 
fied AP-1 activity and to delineate the role that 
the ElA product plays in the modification 
process. 
The induction of AP-1 activity by ElA plus 
cAMP is transient. A second protein, encoded by 
the adenovirus E4 gene, causes AP-1 activity to 
return to basal levels several hours after treat- 
ment of infected cells with cAMP. Thus the in- 
duction of AP I is tightly regulated in the in- 
fected cell. The E4 protein also causes a 
reduction in the level of phosphorylation of the 
c-fos component of AP- 1 . We are presently work- 
ing to determine whether this change in phos- 
phorylation is functionally related to the reduc- 
tion in AP-1 activity and to elucidate the 
mechanism by which the reduction occurs. 
The ElA protein, in addition to inducing a posi- 
tively acting transcription factor, AP- 1 , can also 
inhibit the activity of a negatively acting factor, 
which we have termed YY- 1 . We first identified 
the binding site for YY-1 in the P5 transcriptional 
control region of adeno-associated virus, a defec- 
tive virus that depends on a variety of adenovirus 
gene products for its replication. The adenovirus 
ElA protein activates expression of the P5 con- 
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