Cell cycle distribution of hypoxia and progression of hypoxic tumour cells in vivo

Hypoxia was assessed in three murine tumour models in vivo by measuring the incorporation of 7-(4'-(2-nitroimidazole-1-yl)butyl)-theophylline (NITP), an immunologically identifiable hypoxia marker that binds bioreductively to cells under low-oxygen conditions. Proliferating cells were labelled in the same tumours by administering the thymidine analogue bromodeoxyuridine (BrdUrd). The relative hypoxia in each cell cycle phase of cells isolated from tumours was assessed by addition of propidium iodide with analysis by flow cytometry. There was no relationship between tumour volume and hypoxia in either the anaplastic sarcoma SaF or the poorly differentiated carcinoma CaNT and only a slight negative correlation in moderately well-differentiated carcinoma Rh. The G(1)/G(0) phase contained the greatest number of aneuploid hypoxic cells (aneuploid hypoxia ranging from less than 1% up to 40%, 38% and 71% in SaF, CaNT and Rh respectively), although there were significant amounts of hypoxia present in S- and G(2)/M phases for all three tumours examined. However, the highest proportion of hypoxia occurred in the G(2)/M phase, in which up to 60% of the cells were hypoxic. Simultaneous measurement of hypoxia, proliferation and DNA content using a novel triple-staining flow cytometry method showed that hypoxic cells could actively participate in the cell cycle. In addition, the cell cycle distribution of NITP and BrdUrd labelling showed that hypoxic cells could progress through the cell cycle, although their rate of progression was slower than that of better oxygenated cells.