Modulation of growth and radiochemosensitivity of human malignant glioma cells by acidosis

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Abstract
BACKGROUND Glioblastoma commonly is characterized by hypoxia and acidosis and the histologic features of tissue necrosis and neovascularization. Current approaches of adjuvant radiochemotherapy for patients with glioblastoma have only a modest impact on the natural course of this disease. METHODS The authors examined the effects of acidosis on growth and response to irradiation and chemotherapy in cultured human malignant glioma cells. RESULTS The authors found that mild acidosis (pH 7.0) inhibited the growth of cell lines that retained wild type p53 activity but did not inhibit the growth of cell lines that were devoid of p53 function. Transfer of a dominant‐negative p53 gene into p53 wild type cells failed to override the acidosis‐conferred growth arrest, suggesting that loss of p53 activity per se does not mediate escape from acidosis‐induced growth inhibition. Moderate acidosis (pH 6.6) inhibited the growth of all cell lines. Acidosis‐mediated growth arrest was not associated with a specific type of cell cycle arrest, e.g., in G0/G1 or G2/M phase. Acidosis did not result in consistent changes in radiosensitivity; however, it enhanced the cytotoxic effects of lomustine but conferred protection from topotecan, vincristine, teniposide, and cisplatin cytotoxicity. Lomustine exhibited enhanced stability at low pH, providing a putative mechanism for the enhanced cytotoxic effects of lomustine in acidotic conditions. Decreased sensitivity to the other drugs did not result from altered multidrug resistance drug transport activity. CONCLUSIONS Taken together, the current results suggest that tissue acidosis may be an important determinant of glioma cell responses to adjuvant radiochemotherapy. The superior activity of nitrosoureas, such as lomustine, compared with other agents in patients with glioblastoma may result in part from prolonged drug stability in an acidotic microenvironment. Cancer 2002;95:1113–9. © 2002 American Cancer Society. DOI 10.1002/cncr.10767