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FRANKLIN G.
BERGER, PhD
Director of The Center for Colon Cancer Research
George H. Bunch, Sr. Professor
Department of Biological Sciences
DEGREES
Ph.D., 1974, Purdue University
RESEARCH INTEREST
Molecular Genetics of Cancer
Anti-metabolite resistance in
colon tumor cells. Resistance to anti-neoplastic
agents is a major impediment to the chemotherapy of cancer.
The high genetic plasticity of neoplastic cells generally
leads to rapid emergence of cells that are resistant to the
cytotoxic effects of chemotherapeutic drugs. We have been
examining agents that inhibit the S-phase enzyme thymidylate
synthase (TS). This enzyme catalyzes the formation of dTMP
from dUMP, and is indispensible for DNA synthesis during
cell proliferation. Treatment of cells with fluoropyrimidine
analogs (e.g., 5-fluorouracil and 5-fluoro-2'- deoxyuridine),
as well as folate analogs (e.g., tomudex, AG337, and
BW1843), leads to potent inhibition of TS, depletion of dTTP
pools, and cell death. Thus, these compounds have been
useful in the treatment of gastointestinal and head and neck
cancers. Extensive studies have shown that both the
structure and the concentration of TS are important factors
governing cellular response to TS inhibitors. Early studies
established that TS gene amplification, leading to mRNA and
enzyme overproduction, is an important mechanism of
5-fluoro-2'-deoxyuridine (FdUrd)-resistance in human cells.
More recently, we have characterized several structural
variants of TS in human colon tumor cell lines. These
variants confer changes in TS expression and function
through alterations in the enzyme's stability as well as
through its interactions with ligands.? Numerous
investigations have shown that cellular concentrations of TS
undergo about a 2-4-fold induction following treatment with
TS inhibitors. In vitro studies have led to the proposal
that this induction is due to ligand-mediated relief of the
translational repression brought on by binding of TS to its
own mRNA. We have tested several predictions of this
autoregulatory translation model, and find that in contrast
to expectations, TS ligands do not cause a change in the
extent of ribosome binding to TS mRNA. Furthermore,
mutations that abolish the ability of TS mRNA to bind the
enzyme have no effect on the induction. Finally, enzyme
turnover measurements show that the induction is associated
with an increase in the stability of the TS polypeptide.?
Transfection studies with human cell lines have shown that
covalent, inhibitory ternary complex formation among the
enzyme and its ligands is not required for the ligand-mediated
stabilization.
Recent work has established that the
degradation of TS is carried out by the 26S proteasome.
However, the enzyme is not ubiquitinylated, nor is the
ubiquitinylation pathway required for its degradation. Thus,
TS is targeted to the proteasome in a ubiquitin-independent
manner. Mutagenesis studies indicate that the N-terminal
region of the TS polypeptide, particular the first three
residues, directs the enzyme to the proteasome. Current
efforts are focused on the mechanism by which these residues
control the susceptibility of TS to intracellular
degradation.
The expression of nuclear factor kappa B
(NF-kB), which is a transcription factor that regulates the
expression of a multitude of genes involved in cell growth,
differentiation, and apoptosis, is up-regulated in many
types of cancer. We have demonstrated that reduced NF-kB
levels leads to resistance to apoptosis by TS inhibitors in
a human colon tumor cell line. Thus, NF-kB is required for
maximal drug response. Indeed, this also appears to be the
case for apoptosis induced by the death ligands TNF-alpha
and TRAIL. These findings are being followed up in cultured
cells and in animal models.
Animal models of drug
response and toxicity. Much of our current
work involves use of animal models to examine the role of TS
as a chemotherapeutic target in vivo. As noted above,
numerous studies of cultured cell lines have indicated that
the structure and concentration of TS are important
determinants of response to both fluoropyrimidine and folate-based
TS inhibitors. A fundamental question remains: How much of
the information obtained from /in vitro/ studies of cultured
cell lines is applicable to naturally existing tumors in an
intact animal host? We are examining the impact of
alterations in TS structure and expression on tumor response
to TS inhibitors in animal models of intestinal cancer. For
example, recent work with MIN mice, which carry a mutant /Apc/
gene and are predisposed to intestinal tumorigenesis, shows
that tumor development in these animals is reduced by 70-80%
following systemic treatment with 5-fluorouracil, a powerful
TS inhibitor that has clinical utility in the control of
colorectal cancer. Folate analogs, such as raltitrexed,
appear to stimulate tumorigenesis in the MIN mouse, while
the combination of 5-fluorouracil and raltitrexed elicits a
powerful (i.e., >90%) inhibition of tumor development. The
molecular and biochemical basis of the synergism between
these two drugs, both of which are TS inhibitors, are under
investigation.
Tumors are infiltrated by a heterogeneous population of
non-neoplastic, host-derived cells that make up the stroma.
Most cells of the tumor stroma are products of hematopoiesis.
Though their function within tumors is not fully defined,
considerable evidence points to the idea that they regulate
tumor growth and progression, as well as response to
therapeutic agents. We have proposed that these infiltrating
stromal cells regulate tumor response to therapeutic agents.
Using bone marrow transplantation, we have generated
chimeric MIN mice in which the neoplastic cells and stromal
cells within tumors are distinct with regard to sensitivity
to TS inhibitors. Preliminary evidence indicates that the
drug response of tumors in these animals reflects the
chemosensitivity of the exogenously introduced stromal
cells. This important finding could dramatically change the
way we think about targeting of anti-cancer drugs.
Inflammation and cancer.
In collaboration with Dr. Heinz Baumann of Roswell Park
Cancer Institute, we are conducting a study of the role of
inflammation in tumorigenesis. By taking advantage of animal
models such as the MIN mouse, we are addressing the
hypothesis that inflammation supports tumor cell
proliferation by providing growth-promoting molecules
(cytokines, growth factors, etc.) and by assisting in
remodeling of the tumor microenvironment. Focus is on the
physiological role(s) of several inflammation-related
proteins, including haptoglobin, alpha-1-acid glycoprotein,
serine proteinase inhibitors, interleukin-6, and matrix
metalloproteinases. We have shown that genetic misregulation
of these proteins results in alteration in intestinal
tumorigenesis. The mechanisms underlying these effects are
under investigation.
CONTACT INFORMATION
Center for Colon Cancer Research
University of South Carolina
Columbia, SC 29208
E-mail
Office: (803) 777-1171 | Lab:
(803) 777-4398 | CCCR Office: (803)
777-1231
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