Radiation-induced DNA damage in tumors and normal tissues: V. Influence of pH and nutrient depletion on the formation of DNA-protein crosslinks in irradiated partially and fully hypoxic tumor cells

It has been demonstrated that the yield of radiation-induced DNA strand breaks and DNA-protein crosslinks (DPCs) appears to depend predominantly on the oxygen concentration in the microenvironment around the DNA of mammalian cells (Radiat. Res, 142, 163-168, 1995), Consequently, these DNA lesions have the potential to be used to detect hypoxic cells or estimate the hypoxic fraction of solid tumors. Although it has been demonstrated that physiological factors (e.g. pH, temperature, nutrient depletion, etc.) have no influence on the induction and repair of both DNA strand breaks and DPCs in irradiated oxygenated cells (Radiat. Res. 140, 321-326, 1994), there are no data to suggest how these physiological factors influence the induction and repair of DNA strand breaks and DPCs in irradiated partially hypoxic or fully hypoxic cells. In the present study, the influence of pH (6.6-7.3) and nutrient depletion on the formation and/or repair of radiation-induced DNA strand breaks and DPCs in partially hypoxic (O-2 concentration in the gas phase of 2.0-8.0%) or fully hypoxic (O-2 concentration in the gas phase of 0.3-0.4%) populations of exponential (day 3) and plateau-phase (day 5) 9L rat brain tumor cells in culture was determined. Each population of cells was irradiated at 37 degrees C with 15 Gy and trypsinized at 4 degrees C after one half-time of strand break repair at 37 degrees C, and the DNA damage was measured using our alkaline elution technique, with or without proteinase K (PK) in the lysis solution. An analysis of the DNA-protein crosslink factor as a function of the oxygen concentration and a statistical comparison of the single-strand scission factor measured at eluted fraction 7 (SSSF7), with or without PK in the lysis solution, indicate that pH and nutrient depletion do not significantly influence the formation and/or repair of DNA strand breaks and DPCs in irradiated partially hypoxic or fully hypoxic 9L cells, The data also demonstrate that the DNA lesions produced in irradiated partially hypoxic cells will cause the hypoxic fraction or fractional hypoxic volume to be overestimated by this DNA damage assay, not underestimated. However, this potential overestimation of the hypoxic fraction or fractional hypoxic volume should not limit the usefulness of this DNA damage assay In most laboratory studies, because it will take the contribution of a large cohort of partially hypoxic cells to overcome the dominance of the signal from a relatively small cohort of fully hypoxic cells. (C) 1999 by Radiation Research Society.