DNA-dependent protein kinase in normal and malignant cells : with special reference to anti-tumour agent sensitivity

Abstract

The DNA-dependent protein kinase (DNA-PK) is a DNA double strand break (DSB) repair enzyme, essential for the cellular response to DNA damaging agents. DNA-PK is also of importance for lymphoid development, since it includes two processes, V(D)J recombination and lg switch recombination, during which DSBs are naturally occurring events.

We observed that DNA-PK is differently expressed in normal as well as malignant haematological cells of different degrees of maturation. CD34 positive cells and cells from patients with acute lymphocytic leukaemia (ALL) display high levels of DNA-PK, whereas mature Blymphocytes and chronic lymphocytic leukaemia (CLL) cells show low levels of DNA-PK Following antigen presentation of normal lymphocytes, the expression of DNA-PK is increased. In parallel with this, multiple myeloma cells were found to display high frequency of DNA-PK positive cells.

In contrast to CLL cells from untreated patients, CLL cells from previously treated patients show high levels of DNA-PK protein expression, suggesting a role for DNA-PK in acquired drug resistance. In CLL cells we found a correlation between DNA-PK and etoposide sensitivity. This is to be expected, since etoposide induce DNA DSBs. Unexpectedly, DNA-PK activity was found to correlate with vincristine sensitivity in both CLL and acute myeloid leukaemia (AML) cells, indicating that DNA- PK may be involved in the response to anti-tumour drugs others than DNA DSB inducers.

The tumour cell line, M059J, deficient in DNA-PK activity, is known to be sensitive to DNA DSB inducing agents, e.g. X-ray (low linear energy transfer, LET). These cells are also more prone to undergo X-ray induced apoptosis, compared with the DNA-PK proficient cells, M059K. Ionising radiation with high ionisation density (high LET) were found to induce apoptosis to similar extent in M059J and M059K cells, however, the course of events seems to be different between the two cell lines. Our interpretation is that the detected difference is related to giant cell formation and "mitotic catastrophe". While M059K cells show an expected G2 accumulation and release from the G2 block, M059J cells accumulate in both S and G2/M, followed by apoptosis.

In view of the increased sensitivity to DNA damaging agents in cells deficient in DNA-PK, it is tempting to suggest that DNA-PK is a suitable target for sensitisation of cells resistant to anti-cancer agents. DNA^ is inhibited by trifluoperazine (TFP) in vitro, by direct inhibition, and in vivo, by induction of apoptosis. The findings presented in this thesis indicate that the role of DNA-PK in predicting therapy results in various tumours should be further explored, as well as the possibility of using DNA-PK as a target in tumour therapy.