Programme Leader: Professor Gerald M. Cohen
Summary of Research Interests
Apoptosis is a major form of cell death, which has been implicated in a number of diseases, including certain cancers, autoimmune disorders, neuro-degenerative diseases such as Alzheimer's and Parkinson's disease, as well as ischaemic injury caused by myocardial infarction and stroke. In general, apoptosis is a controlled form of cell death, which is designed to minimise the potentially more damaging effects of inflammation, which accompany necrotic cell death. Two major pathways of apoptosis have been described (Figure 1). Current research in our group focuses on understanding basic mechanisms of apoptosis and then translating these findings to different disease states in particular to certain forms of malignancy, primarily Chronic Lymphocytic Leukaemia (CLL). CLL is the major form of adult leukaemia in the Western world and is often considered as a disease of failed apoptosis resulting in an accumulation of circulating CLL cells. Work on CLL is carried out in collaboration with Professor Martin Dyer and his groups in the MRC Toxicology Unit as well as the Department of Haematology.
1) The intrinsic pathway is initiated after mitochondrial damage and occurs in response to diverse apoptotic stimuli including many toxic chemicals, DNA damaging agents, growth factor withdrawal and irradiation.
2) The extrinsic pathway occurs following stimulation of cell surface death receptors by CD95L, TNF or TRAIL (TNF-related apoptosis-inducing ligand).
Inhibition of BCL2 family members as a treatment for haematological malignancies
BCL2 proteins regulate the intrinsic pathway of apoptosis at the mitochondria. The BCL2 protein family comprises both anti-apoptotic family members (e.g. BCL2 itself) as well as proapoptotic family members (e.g. BAX, BAK and BH3-only proteins). The pro- and antiapoptotic proteins interact in a complex fashion, thereby regulating the release of cytochrome c from mitochondria into cytosol. Antiapoptotic BCL2 proteins are over-expressed in a variety of tumours, and their expression is associated with resistance to chemotherapy. Therefore, several laboratories have developed small molecule inhibitors of BCL2 that are currently under investigation for their potential in cancer therapy. We are mainly working with ABT-737, a BCL2-antagonist developed by Abbott Laboratories. We found that primary cells from the peripheral blood of patients with B-cell malignancies, such as CLL, are particularly sensitive to ABT-737. By studying the underlying mechanism of cell death, we identified a novel paradigm of cell death involving typical apoptotic nuclear morphologies together with rupture of the outer mitochondrial membrane, previously mainly associated with necrotic cell death (see below) (Vogler et al Cell Death Differ., 15, 820, 2008). Interestingly we found that BAX and BAK localize in clusters exactly at the breakpoints in the outer mitochondrial membrane, pointing to a novel function of these proteins in destabilizing the outer mitochondrial membrane. Based partly on these highly promising results in CLL cells, the University Hospitals of Leicester has started a phase I/IIa clinical trial for ABT-263, a related analogue of ABT-737.
A) CLL cells exposed to 10nM ABT-737 for 2 h showed a typical apoptotic nuclear morphology including chromatin condensation and segregation of the nuclei (bar=5µm).
B) Many mitochondria in CLL cells exposed to ABT-737 showed various degrees of matrix swelling and focal reductions in the electron density of the contents. These changes were often associated with extensive breaks in the outer mitochondrial membrane (arrowheads, bar=0.5µm)
Our current work is focused on potential mechanisms of resistance to ABT-737. To this end, we recently found that under conditions mimicking the lymph node environment, CLL cells develop a ~1,000-fold resistance to ABT-737 within 24 hours (Vogler et al Blood, 2009). This resistance is due to NF-kB mediated upregulation of two antiapoptotic BCL2 proteins, namely BCL-XL and BCL2A1. Current investigations in our laboratory aim to identify the molecular function of these proteins, in order to determine how they are able to confer resistance and how that resistance might be overcome therapeutically.
In addition to ABT-737 several small molecule inhibitors of BCL2 have been developed (Vogler et al Cell Death Differ. 16, 360-367, 2009). Our recent studies indicate that not all these inhibitors are anything like as specific as proposed in the literature and many of these other inhibitors have targets other than anti-apoptotic BCL2 family members. One of these inhibitors induced a marked perturbation of the endoplasmic reticulum (ER) and appears to be inducing a novel form of ER stress. This is currently a major focus of study in the group.
We have also recently shown that the BH3-only protein, NOXA, plays a critical role in the induction of proteasome inhibitor induced apoptosis in CLL cells (Baou et al., Haematologica 95, 1510, 2010). In this study we showed for the first time that NOXA is ubiquitinated. We are currently studying the role of this ubiquitination in the degradation and function of NOXA.
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