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The two principal rationales for applying hyperthermia in cancer therapy are that: (a) the S phase, which is relatively radioresistant, is the most sensitive phase to hyperthermia, and can be selectively radiosensitized by combining hyperthermia with x-irradiation; the cycling tumor cells in S phase which would normally survive an x-ray dose could thus be killed by subjecting these cells to hyperthermia; and (b) the relatively radioresistant hypoxic cells in the tumor may be selectively destroyed by combinations of hyperthermia and x-irradiation. Both of these rationales have been mentioned as reasons for using high LET irradiation in cancer therapy; therefore where such irradiation may be of use, hyperthermia may also be advantageous.

The extracellular pH of tumor

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The multidrug resistance (mdrl) gene product, P-glycoprotein, is responsible for the ATP-dependent extrusion of a variety of compounds, including chemotherapeutic drugs, from cells. The data presented here show that cells with increased levels of the P-glycoprotein release ATP to the medium in proportion to the concentration of the protein in their plasma membrane. Furthermore, measurements of whole-cell and single-channel currents with patch-clamp electrodes indicate that the P-glycoprotein serves as an ATPconducting channel in the plasma membrane. These findings suggest an unusual role for the P-glycoprotein.Multidrug resistance in tumor cells is mediated by increased expression of the product of the mdrl gene (MDRI, now PGYJ, in human gene nomenclature), the P-glycoprotein (1-5). Its primary structure (6, 7) is similar to that of various transporters of ions, amino acids, peptides, or proteins in bacterial, yeast, and animal cells, which share ATP-binding motifs (8). The P-glycoprotein also resembles structurally the mitochondrial ATP/ADP exchanger (9, 10).The hydrolysis of ATP apparently provides the energy for drug extrusion from the cell (11-19). However, the mechanism of the transport process is not known for any of the transporters of the ATP-binding cassette (ABC) superfamily, including one reconstituted in artificial vesicles (20).In this paper we-show that P-glycoprotein in plasma membranes serves as a channel for steady ATP release from the cell. The possible physiological significance ofthis export will be briefly discussed. MATERIALS AND METHODSCell Lines. Chinese hamster ovary (CHO) cells, designated AUX B1, and P-glycoprotein-overexpressing drug-resistant cells, CHRC5 and CHRB30, were provided by Victor Ling (Ontario Cancer Institute, Toronto) (21). The human lung tumor cell line SW-1573 mutants that express the P-glycoprotein in different amounts were obtained by culturing the cells with adriamycin at 5 or 7 ,ug/ml, yielding SW10 or SWadria,.Drug-sensitive CHO cells (LR73) (25) transfected with multiple copies of murine mdrl cDNA (EX4N), or transfected with the murine mdrl cDNA in which the codons for lysines 432 and 1074 had been replaced by those for arginine residues to modify the two nucleotide-binding sites (88-8), were obtained from Philippe Gros (McGill University, Montreal).Immunofluorescence. P-glycoprotein concentration on cell surfaces was measured by staining with fluorescent mouse monoclonal antibodies directed to surface epitopes of the protein, followed by analysis on a FACScan (Becton Dickinson) using LYSYS software. The hamster cell lines were stained with antibody 4A6B3/10 to the hamster P-glycoprotein (26), and the human cell lines were stained with antibody 4E3 to the human P-glycoprotein (R.J.A., unpublished results).ATP measurements. (i) HPLC. Nucleotides, in the Hanks buffer bathing the cells, were separated by HPLC (Waters) and quantitated as described by Fellenz and Gerweck (27).(ii) Luminometry. After stable baseline measurements were obtained from 1 ml of Hanks...

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Normalization of tumor vasculature is an emerging strategy to improve cytotoxic therapies. Here we show that eliminating nitric oxide (NO) production from tumor cells via neuronal NO synthase silencing or inhibition establishes perivascular gradients of NO in human glioma xenografts in mice and normalizes the tumor vasculature, resulting in improved tumor oxygenation and response to radiation treatment. Creation of perivascular NO gradients may be an effective strategy for normalizing abnormal vasculature.

Substantial evidence suggests that the radiosensitivity of the tumor cells is the primary determinant of tumor response to radiation. More recent studies suggest that tumor stroma radiosensitivity is the principle determinant of response. To assess the relationship between intrinsic tumor cell radiosensitivity and tumor response, we altered the intrinsic radiosensitivity of a cloned tumor cell line and analyzed the effect of this alteration on tumor response. A cloned tumor cell line derived from DNA double-strand break repair-deficient severe combined immunodeficient mice was transfected with the double-strand break repair gene DNA-PKcs. The intrinsic radiosensitivity of the transfected tumor line was decreased by a factor of f1.5. The isogenic lines were used to initiate tumors in NCr-nu/nu mice. When transplanted in the same strain of mice and exposed to the same dose of radiation, the isogenic tumors may be expected to exhibit a similar response to radiation if radiation damage to host stroma is the principle determinant of response. This was not observed. Over the dose range of 20 Gy in four 5-Gy fractions to a single dose of 30 Gy, the 1.5-fold increase in intrinsic tumor cell radioresistance conferred by the introduction of DNA-PKcs caused a 1.5-fold decrease in tumor growth delay. The results show that the intrinsic radiosensitivity of tumor cells is a major determinant of tumor response to radiation. (Cancer Res 2006; 66(17): 8352-5)