Hawkins - Cell death regulation in cancer and viral infection
Apoptosis is an evolutionarily conserved, tightly regulated process that destroys surplus and dangerous cells. Excessive apoptosis has been linked with degenerative diseases, while inappropriate cellular survival can promote cancer, infection and autoimmune disease. Apoptotic pathways defects can also limit the efficacy of anti-cancer therapies. Chemotherapy kills tumour cells by damaging DNA, which triggers apoptosis, so blocks in apoptotic pathways can render tumour cells unresponsive to anti-cancer treatments.
Drugs that directly engage apoptotic pathways may bypass these blocks, killing chemotherapy-resistant tumour cells. In addition, because direct apoptosis-inducing drugs do not need to damage DNA to kill cancer cells, they may pose a lower risk than chemotherapy of provoking the development of subsequent therapy-related malignancies in cancer survivors. Our goal is to understand apoptotic regulation, in normal cells, cancerous cells and virally-infected cells, and to exploit this knowledge to explore better and safer therapies for cancer and viral diseases.
Minimising the risk of therapy-related cancers
Around a fifth of cancer survivors will subsequently develop new independent tumours, many of which are caused by the therapies used to eliminate their original cancer. Chemotherapy and radiotherapy work by inducing DNA damage, preferentially but not exclusively in cancerous cells. Cells respond to this DNA damage by triggering apoptosis, which hopefully eliminates the cancer. Unfortunately, non-cancerous cells can also sustain DNA damage during treatment with chemotherapy or radiotherapy. When these mutated cells survive, they can form subsequent malignancies in people successfully treated for their original cancer.
A recent focus of cancer research has been the development of drugs that directly engage apoptosis pathways, rather than provoking DNA damage to indirectly induce tumour cell death. Some of these new drugs have exhibited robust anti-cancer activity in animal experiments and clinical trials. Because direct apoptosis inducers do not need to damage DNA to kill tumour cells, we hypothesised that they may provoke fewer mutations in surviving cells, so may be less likely than current therapies to cause subsequent cancers.
Encouragingly, we found that anti-cancer agents that target Bcl-2 or IAP proteins failed to provoke mutations in surviving cells, in contrast to chemotherapy drugs that were highly mutagenic. These data provide hope that cancer survivors treated with antagonists of Bcl-2 or IAP proteins would have a lower risk of developing therapy-related cancers than those treated with chemotherapy or radiotherapy. Counter-intuitively, death receptor agonists were highly mutagenic. We are exploring the mechanism underlying this activity and the type of mutations these drugs provoke.
IAP antagonists for osteosarcoma
Although most cancers tend to occur in older people, the bone cancer osteosarcoma typically arises in adolescents and young adults. Unfortunately, cure rates for this cancer have improved little in the last four decades. Currently, only 59% of Australians survive more than five years after being diagnosed with osteosarcoma, and only around a quarter of patients with metastatic disease survive for five years or more. The majority of osteosarcoma survivors suffer from on-going treatment-related disease and disability. Some of the more severe side effects include the physical and psychological impacts of amputation, cardiac damage caused by the chemotherapy doxorubicin, and therapy-related second cancers provoked by DNA damaging chemotherapy.
We are investigating the potential for direct apoptosis inducers to treat osteosarcoma more safely and effectively. We have shown that drugs targeting Bcl-2 or IAP proteins are non-mutagenic. This implies that, if these drugs could eliminate osteosarcomas, they may spare survivors the risk of developing therapy-related cancers later in life. In collaboration with Carl Walkley (St Vincent's Institute) we have found that osteosarcoma cells respond to IAP antagonists in vitro. In ongoing work, we are dissecting the molecular pathways by which IAP antagonists kill osteosarcoma cells, and assessing the efficacy of these drugs in vivo.
Probing metazoan apoptotic regulators using yeast
Our group characterises cell death pathways, and their dysregulation in cancer and following viral infection. Some projects involve identification and characterisation of apoptosis pathway components, and definition of their mechanisms of action. In addition to standard protein biochemistry and cell biology techniques, these projects also exploit a suite of yeast-based systems we developed for identifying and characterising proteins and drugs that modulate apoptotic signalling. Active forms of the major apoptotic effectors – caspases, Bax and Bak – are all lethal to the budding yeast Saccharomyces cerevisiae.
We have exploited this lethality to reconstitute mammalian, insect and nematode apoptotic pathways in yeast. These reconstituted pathways have allowed us to investigate the specificity and activity of endogenous and viral proteins regulate apoptosis, and we have identified apoptotic inhibitors using functional yeast-based screens.
We have also recently adapted this system to define the specificity of drugs that modulate apoptotic signalling by antagonising Bcl-2 relatives or caspases. Using a second yeast-based system, caspase activity can be monitored in yeast via cleavage-dependent liberation of a transcription factor from the plasma membrane, enabling it to activate the lacZ reporter gene. We have employed this system to define the specificity of mammalian and nematode caspases.