| The most common brain tumors are the astrocytic gliomas, the majority of which are aggressive, incurable cancers. Despite intensive research, chemotherapy for these tumors remains ineffective. A dramatic chemotherapeutic response occurs occasionally, but response is unpredictable. As a result, oncologists must treat all patients to help a few, with attendant toxicity and cost, or deny some a useful therapy to spare the majority unnecessary treatment. Although no molecular marker has been identified to predict chemotherapeutic response, the p53 gene may be of interest. Loss of p53 function is a hallmark of many human cancers. p53 is a key regulator of cellular response to DNA damage. The integrity of p53 could influence the success of cancer chemotherapy. Our laboratory previously observed that neonatal mouse astrocytes lacking the p53 gene are more sensitive to BCNU [1,3-b&barbelow;is(2-c&barbelow;hloroethyl)-1-n&barbelow;itrosou&barbelow;rea] mediated DNA damage than wild-type astrocytes. Given that p53 is thought to play an important role in the behavior and genesis of astrocytic gliomas, I examined whether it might also be an important determinant of chemosensitivity of astrocytic glioma cells. p53 inactivation by expression of the viral oncoprotein E6 sensitized U87MG cells to BCNU. Compared with control U87MG-neo cells with intact p53 function, the clonogenic survival of U87MG-E6 cells exposed to BCNU was reduced significantly. In U87MG-E6 cells, sensitization to BCNU was associated with failure of p21WAF1 induction, transient cell cycle arrest in S phase, accumulation of polyploid cells, and significant cell death. U87MG cells with disrupted p53 function were less able to repair BCNU induced DNA damage and survive this chemotherapeutic insult. I subsequently found that disruption of p53 function also sensitized two other astrocytic glioma cell lines, D54 and A172, to BCNU and temozolomide (TMZ). Similar to observations in U87MG, sensitization to both BCNU and TMZ was associated with failure of p21WAF1 induction, lack of a sustained G2 cell cycle arrest and significant tumor cell death. These similar findings using three astrocytic glioma cell lines and two cytotoxic drugs suggest that enhanced chemosensitivity might be a general property of human astrocytic glioma cells in which p53 was disrupted. However, further evaluation revealed that p53 inactivation rendered D54 cells significantly more resistant to cisplatin (CDDP, c&barbelow;is-d&barbelow;ichlorod&barbelow;iammine p&barbelow;latinum). Together these results suggest that p53 status may influence the chemosensitivity of astrocytic glioma cells in a drug-type specific manner, a finding which may have implications for the selection of drug treatments for patients with astrocytic gliomas. Lastly, I observed that exposure of U87MG cells to PFTalpha (pifithrin-alpha, an abbreviation for "p-fifty three inhibitor"), a small molecule inhibitor of p53, prior to cytotoxic chemotherapy attenuated transcriptional activation of p53 and p21WAF1 sensitizing U87MG cells to BCNU and TMZ. Sensitization was associated with G1 arrest, delayed entry into S phase and decreased repair of DNA damage by BCNU. Our findings suggest that small molecule inhibitors of p53, like PFTalpha, may be useful in clinical oncology for sensitizing certain resistant cancers to cytotoxic chemotherapies. The finding presented in this thesis may help to clarify the relationship between p53 and chemotherapy response in astrocytic gliomas and to develop novel strategies to enhance the efficacy of conventional chemotherapeutic agents used clinically for glioma treatment. |
