Increased cellular hypoxia and reduced proliferation of both normal and leukaemic cells during progression of acute myeloid leukaemia in rats

The microenvironmental changes in the bone marrow, spleen and liver during progression of the transplantable promyelocytic leukaemia in the Brown Norw egian rat (BNML) have been studied. We used flow cytometry to estimate cell ular hypoxia and proliferation based on in vivo pulse-labelling with a mixt ure of 2-nitroimidazole linked to theophylline (NITP) and bromodeoxyuridine (BrdUrd). The leukaemic cells were identified with the RM124 antibody. In rats inoculated with leukaemic cells the fraction of RM124(+) cells was sig nificantly increased from day 20 onwards in the spleen and from day 27 in t he bone marrow and liver, reaching a level of 65-87% in these organs at day 32. At day 32, the NITP+ fraction of RM124(+) cells had increased signific antly in the bone marrow and spleen to 88% and 90%, respectively. The corre sponding fractions of NITP+ normal cells reached 63% and 65%, respectively. From day 13 to day 32, the DNA-synthesizing (BrdUrd(+)) fraction of RM124( +) cells in the bone marrow decreased significantly from 52% to 25%, and of normal cells from about 20% to 6%. In the bone marrow and spleen at day 27 and 32, the S-phase and G(2)/M-phase fractions according to DNA content we re higher for the NITP+ than for the NITP- cells. This could partly be expl ained by an impaired cell cycle progression due to hypoxia. Nevertheless, w e found indications of leukaemic cells that were simultaneously labelled wi th NITP and BrdUrd, in the bone marrow and spleen. These latter findings su ggest that in contrast to normal cells some of the leukaemic cells can prol iferate even during hypoxia, and this subpopulation may consequently renew and expand the leukaemic cell load.