of the Max - MDC

Clinical and Molecular
Oncology
Peter Daniel
Cell death and cell cycle deregulation in cancer and resistance to anticancer therapy. Virtually all medical anticancer
therapies rely on the induction of cell cycle arrest or cell death in the malignant cells. Consequently, the
analysis of such genetic events allows for the identification of patients at risk for an insufficient response to treatment
with chemotherapeutic drugs or ionising irradiation, and poor survival. Such analyses provide a rational basis
for a molecular understanding of the response to anticancer therapies and the clinical use of cancer therapeutics.
The aim of the group is, therefore, to define genetic defects in cancer that result in aggressive disease, poor prognosis,
and resistance to clinical cancer therapy. To this end, we have established an extensive genotyping program
in solid tumors and leukemias. Recent pharmacogenomic data obtained from these screenings depict that defects in
central regulatory genes, e.g. of the p53 pathway, do not result in global resistance to therapy but may be overcome
by adequate therapeutic modalities. Functional consequences of such cell death and cell cycle defects are analysed
in vitro, often by the use of adenoviral gene transfer for complementation of disrupted genes. In addition, these
systems are exploited to gain insights into novel aspects of cell cycle and cell death regulation and their intricate
interactions.
Understanding resistance to anticancer therapy
Anticancer therapies, i.e. chemotherapy and ionising irradiation,
activate nuclear stress responses to induce cell cycle
arrest and DNA repair. When repair fails, the same stress
responses trigger cellular senescence or death and demise
of the affected cell. The molecular basis of these events has
been studied extensively during recent years and comprehensive
models are now established for large parts of these
signaling events. We have investigated the consequences of
genetic defects in genes acting as effectors or inducers of
p53 that trigger apoptosis and cell cycle arrest programs
upon genotoxic stress. In this context, we recently
described selective loss of multiple BH3-only proteins, proapoptotic
homologs of the Bcl-2 family, including Nbk and
Bim in renal carcinoma. This is a unifying feature of renal
carcinoma and appears to be linked to the impressive clinical
resistance of this tumor entity to anticancer therapy.
Regulation of cell death by pro-apoptotic Bcl-2
family members
Apoptosis is mediated through at least three major pathways
that are regulated by (1) the death receptors, (2) the
mitochondria, and (3) the endoplasmic reticulum (ER). In
most cells, these pathways are controlled by the Bcl-2 family
of proteins that can be divided into antiapoptotic and
proapoptotic members. Although the overall amino acid
sequence homology between the family members is relatively
low, they contain highly conserved domains, referred to
as Bcl-2 homology domains (BH1 to BH4) that are essential
for homo- and heterocomplex formation as well as for their
cell death inducing capacity. Structural and functional
analyses revealed that the proapoptotic homologs can be
subdivided into the Bax subfamily and the growing BH3-
only subfamily. BH3-only proteins link upstream signals
from different cellular or functional compartments to the
mitochondrial apoptosis pathway (Figure 1). Puma, Noxa,
Hrk, and Nbk (Bik) are induced by p53 and mediate cell
death originating from the nucleus, e.g. upon DNA damage.
Nbk localizes to the ER and activates Bax (but not Bak) indirectly,
through a so far undefined ER-initiated death pathway.
The aim of our work is to gain structural and functional
insights into how these subfamilies promote or inhibit cell
death signals and how these properties may be utilized for
development of apoptosis-promoting cancer therapies. Our
studies therefore deal with questions such as how cell cycle
stress responses including anticancer therapies and oncogene
deregulation feed into the mitochondrial death pathway.
We recently established that Nbk stabilizes the antiapoptotic
multidomain protein Mcl-1 that acts as an
endogenous inhibitor of Bak. This fully explains the entirely
Bax dependent induction of apoptosis by Nbk. Ongoing
work addresses the transcriptional control of Nbk expression
and its functional involvement in the regulation of cell
death following ER stress responses.
130 Cancer Research