Programmed cell death or apoptosis is of fundamental importance to cancer as it both limits tumorigenesis and is also triggered by many cancer chemotherapeutics. Importantly, cancer cells often acquire mutations that compromise the apoptotic process, allowing these cells both to escape normal growth constraints and to become resistant to many anti-cancer drugs, resulting in the emergence of drug-resistant malignancies. Thus discovering how apoptosis is regulated and why it fails in cancer is central both to understanding cancer progression and developing new therapies to counter chemo-resistant cancers.

Biochemical and genetic studies in a range of model systems have identified the key components of the apoptotic machinery. One gene family that contributes to the commitment to apoptosis and execution of the death program encodes a family of cysteine proteases called caspases. Caspases are expressed in cells as inactive zymogens and are activated at the onset of apoptosis. In many cases (but not all) this is sufficient to kill the cell. Different types of apoptotic signals initiate apoptosis by activating different ?initiator? caspases. These ?initiator? caspases then activate a common set of ?effector? caspases by proteolysis. Ultimately, it is these effector caspases that produce the apoptotic phenotype by cleaving a wide range of intracellular substrates.

Caspases and Differentiation

The caspases comprise a family of cysteine proteases and some of the family are responsible for causing apoptosis.  Of the apoptotic caspases, caspases-3,-6 and -7 are classed as ?effectors? and cleave a wide range of intracellular proteins and responsible for causing many of the changes typical of apoptosis.   Effectors are expressed as zymogens and are activated when cleaved by ?initiator? caspases such as -2, -8 and -9.  Initiators are also expressed as zymogens but, in contrast to effectors, are activated by their recruitment into large multi-protein complexes.  Different apoptotic signals activate different initiators, but all the initiators activate a common set of effectors.  Thus, during apoptosis very different pro-apoptotic signals are all integrated and amplified through a proteolytic cascade of initiator and effector caspase activity.  Implicit in this model is that the activity of initiator and effector caspases is synonymous with apoptosis and that once effectors are active an irreversible choice has been made.

Not all caspases are involved in apoptosis; caspases-1, -4 and -5 are involved in inflammatory responses where they are important for the proteolytic maturation of cytokines.  The specificity of caspases for a particular process is defined by the selective cleavage of substrate; inflammatory caspases preferentially bind and cleave cytokines, while apoptotic caspases cleave a different set of proteins.  It is therefore remarkable that the apoptotic effector caspase, caspase-3 is not only required for the differentiation of muscle progenitor cells into myofibers but apparently is sufficient for this differentiation.

To date there is no explanation that reconciles the apoptotic activity of caspase-3 with its role in differentiation.  The explanation of how a cell containing active caspase-3 retains the ability to choose a fate other than apoptosis is likely to be critical for our understanding of how cells make life-and-death decisions.  It may also have important implications beyond muscle biology; mutations that allow cells to avoid apoptosis contribute to tumourigenesis and uncovering a mechanism through which cells survive despite the presence of active caspase-3 may reveal new pathways important in cancer cell biology. Figure. Myoblast differentiation into myotube; cell fusion and expression of myosin heavy chain detected by immunofluorescence.  A caspase inhibitor compromises differentiation.

Putative Serine proteases and apoptosis

Many chemotherapeutics exert their anti-cancer activity through the induction of programmed cell death or apoptosis. Much recent research into the mechanisms of apoptosis has identified new therapeutic targets within the apoptotic machinery and is driving the development of novel chemotherapeutics. Proteases, most notably the caspase cysteine proteases, are critical in the induction of apoptosis. However, roles for serine proteases have also been suggested based, in part, on the ability of protease inhibitors like N-a-tosyl-L-phenylalanine chloromethyl ketone (TPCK) to block apoptosis. Despite these data, the relevant target for TPCK has not been identified. Identifying this target is the first step in assessing whether it represents a valid target for cancer chemotherapy. The research aims to identify TPCK targets by a biochemical approach and then to use a genetic approach to test the roles of these candidates in apoptosis.

MicroRNAs, apoptosis and cancer

Breast cancers are phenotypically diverse, presumably reflecting a spectrum of distinct molecular defects in processes controlling cell proliferation and survival. This complexity makes breast cancer progression hard to predict and treatment difficult to manage. Currently, clinicians rely heavily on the status of two receptors, the estrogen receptor and HER2/neu receptor for clinical decisions relating to prognosis and treatment. These markers, while enormously valuable, do not adequately define different types of breast cancers or predict their sensitivity to therapy.