Conventional chemotherapeutic drugs target proliferating cells, as a result normal proliferating cells are also targeted by these drugs resulting in a number of unwanted side-effects and toxicities. Cancer has been described as a disease of the cell cycle, as many of the genes mutated in the process of oncogenesis are cell cycle control genes, for example p53, Rb, and ATM. Many of these genes are involved in cell cycle checkpoint mechanisms, which respond to cellular insults such as DNA damage, replication errors, and external stress, by delaying cell cycle progression. This delay gives the cell time to assess the damage and undertake repair, or if the damage is to severe then initiation of apoptosis may occur. The loss of checkpoints in cancer cells provides them with a growth advantage, but it is also a weakness as they are no longer be able to respond correctly to cellular insults. Therefore identifying checkpoints that are commonly lost in a wide range of tumour cells and developing chemotherapeutics that selectively target these lost checkpoints may provide a means of producing more potent anti-cancer drugs that have few side effects than current treatments.
To assess the validity of this hypothesis, two different approaches were undertaken. The first involved the detailed analysis of the mode of action of a new class of chemotherapeutic agents known as histone deacetylase inhibitors (HDIs). These compounds have been shown to be selectively cytotoxic towards a range of tumour cell types, while remaining relatively non-toxic toward normal cells. In addition, these drugs are currently being used in phase I and II clinical trials, yet the anti-tumour action of these drugs is unknown.
This HDI-resistance in normal cells correlates with the induction of a specific G2 cell cycle checkpoint, which is defective in tumour cell lines. The consequence of losing this G2 checkpoint is that sensitive cells progress through an aberrant mitosis, resulting in apoptosis. In addition, HDI-treatment of non-proliferating tumour cells also resulted in cell death, indicating the possible use of these compounds for treating tumours with a low mitotic index. However, HDI-treatment directly induced the upregulation of the cyclin dependent kinase inhibitor p21waf1/Cip1 in the majority of sensitive tumour cell lines tested. This upregulation hindered apoptosis increasing resistance to HDI-induced cell death, compared to hypersensitive cell lines which did not upregulated p21.
The second approach was to characterise a checkpoint pathway that is defective in a high proportion of tumour cells, with the hope that new drug targets can be identified. To do this I have utilised previous research from this laboratory which demonstrated that treatment with sub-erythemal doses of ultraviolet radiation (UVR), resulted in an upregulation of the melanoma susceptibility gene product p16INK4A basal and supra-basal layers of the skin. The upregulation of p16 correlated with a p53 independent G2 arrest in these cells, which is also observed in epidermal derived cell lines, while cells that do not contain functional p16 fail to arrest in G2 in response to low-dose UVR. Furthermore the majority of melanoma cell lines have either lost or contain non-functional p16. In addition, p16 is associated with an increased risk of melanoma predisposition, and loss of pi6 expression correlates with more aggressive and invasive melanoma tumours.