This study was undertaken to clarify how p53, presumably the best known cell cycle regulator and tumor suppressor, is regulated in transient growth arrest triggered by UV light. The overall cellular response to UV radiation was in many respects different from that shown to ionizing radiation, but some of the findings that at first appeared surprising have later been supported by others.
Unlike ionizing radiation, UV response studied in multiple cell lines was not abrogated by the absence of p53. Even during G1-phase growth arrest, the area believed to be governed by p53, pRB hypophosphorylation seemed to be at least equally important as activation of p53. While UV-induced transactivation of p53 was rapid taking place in all the cell cycle phases, accumulation of p53 seemed to correlate with replication of damaged DNA and, supposedly, the existence of unrepaired DNA lesions.
Regardless of apparently normal p53 status no melanoma cell line displayed normal p53 function. The identical growth arrest pattern in cell lines expressing mutant and wild-type p53 suggests no role for p53 in melanomas, whereas the functional inactivation of wild-type p53 indicates a loss of p53 function in melanomas. UV-triggered p21 induction was found to be regulated at the transcriptional level also in the absence of p53, but dissociation of p21 mRNA and protein expression was detected in several situations, and in some of them a defective p21 response was clearly caused by lack of functional p53.
Dissociation of stabilization and transcriptional activities renders p53 even more interesting, since accumulation may be involved in tasks other than those requiring transcriptional activity. Since these findings thus far apply only to mouse fibroblasts, it would be of particular interest to study the cell cycle phase-specific activation and phosphorylation of N-terminal residues of p53 in normal human fibroblasts. It would be also useful to explore whether induction of ARF, which represents a p53-activating pathway that is not induced by DNA damage, would result in different p53 responses than those detected after UV radiation.
UV radiation provides one model for damaging genetic material, and the knowledge gathered from cells treated with both UV and g-irradiation is indicative of how damaged cells respond to and survive the DNA damage. Cell cycle phase studies like those presented in this thesis may determine in which phases p53 is indispensable and in which phases, and to what extent, its functions in damage response can be replaced by other cell cycle regulators, for example pRB or CKIs.
Along with advances in basic cancer research, the underlying mechanisms of the development or prevention of malignant transformation at the cellular level have become better understood. Instead of today´s often unspecific cancer treatment, novel therapies can be developed in the future by taking advantage of the molecular level differences between normal and transformed cells. For example, the knowledge gained from the interplay between p53 and adenovirus proteins have yielded a clinical trial where promising results have been obtained by infecting carcinoma patients with mutant adenoviruses that cause cytopathic effect only in cells lacking functional p53. These kinds of trials, demanding wide-ranging expertise of many fields, lie at the very heart of medicine reaching from knowledge to action.