Quiescent S-phase cells as indicators of extreme physiological conditions in human tumor xenografts

Purpose: During the last 20 years, evidence has been accumulating for the e xistence in animal and human tumors of quiescent S-phase cells, i.e. cells with an S-phase DNA content that do not actively synthesize DNA. In cell cu lture studies, quiescent S-phase cells have been observed under physiologic al conditions typical for poorly vascularized regions of tumors such as red uced pH, hypoxia, and glucose deprivation. Therefore, we studied the possib le correlation between the frequency of quiescent S-phase cells and the oxy genation status as determined polarographically in a number of human tumor xenografts. Methods and Materials: Five human tumor xenografts on nude mice were used. Oxygenation was measured polarographically with an Eppendorf pO(2)-Histogra ph in 24 to 30 individual tumors for each entity. Mice were injected intrap eritoneally with 1 mg/30 g bodyweight bromodeoxyuridine (BrdU), tumors were excised 30 min later and prepared into a single-cell suspension. After imm unofluorescence staining with an antibody against BrdU and staining of the DNA with propidium iodide, cells were measured in a FACScan flow cytometer and the frequency of cells in the S-phase compartment that did not incorpor ate BrdU was determined. Results: In most cases, the frequency of measurements of an oxygen partial pressure < 5 mm Hg in the tumor tissue increased with tumor volume. Likewis e, the frequency of quiescent S-phase cells was generally higher in larger tumors;; Taking all five tumor entities together, there was a highly signif icant correlation between tumor oxygenation and the occurrence of quiescent S-phase cells. Conclusions: Our data confirm earlier findings that inactive S-phase cells do exist in vivo. Because their frequency seems to be dependent (directly o r indirectly) on the degree of oxygenation and has been shown to increase n ot only with hypoxia, but also with reduced pH and glucose deprivation in v itro, the frequency of inactive S-phase cells may be considered a summary i ndicator for extreme physiological conditions in tumors. (C) 1999 Elsevier Science Inc.