The precise mechanism whereby granulocytes proliferate when haematopoietic colony stimulating factors (CSFs) are used in neutropenic cancer patients is poorly understood. The purpose of this study was to investigate whether these cytokines bring about leucocyte proliferation by increasing the levels of multiple forms of dihydrofolate reductase (DHFR). Blood samples were collected from 36 cancer patients (25 males and 11 females) with chemotherapy-induced neutropenia. One sample of blood from each patient was obtained before therapy either with CSF, such as granulocyte colony stimulating factor (G-CSF) and granulocyte-macrophage colony stimulating factor (GM-CSF) or with placebo, and another one at the time of resolution of neutropenia. Peripheral blood leucocytes in these blood samples were counted, separated and lysed. From lysates, cytoplasmic samples were prepared and analyzed for active DHFR by a methotrexate-binding assay and for total immunoreactive DHFR by an enzyme linked immunosorbent assay. The increase in total leucocyte count (TLC) was most prominent (P<0.005) in the CSF group and less so (P<0.05) in the placebo group. The mean+/-SD concentration values of active DHFR before and after stimulation with GM-CSF found were to be 0.34+/-0.4 ng/mg protein and 0.99+/-0.82 ng/mg protein, respectively, and in the group treated with G-CSF, 0.24+/-0.32 ng/mg protein and 1.18 +/- 2.4 ng/mg protein, respectively. This increase in active DHFR after stimulation with CSF was statistically significant (P<0.05), Similarly, concentration values of immunoreactive but nonfunctional form of DHFR (IRE) were 110+/-97 ng/mg protein and 605+/-475 ng/mg protein before and after stimulation with GMCSF, and 115+/-165 ng/mg protein and 1054+/-1095 ng/mg protein before and after stimulation with G-CSF. This increase in concentration of IRE after stimulation with GM-CSF or G-CSF was statistically significant (P<0.005). In the control group, there was an increase in the concentration of both active DHFR and IRE after treatment with placebo. However, this was not statistically significant. Resolution of neutropenia was quicker in the groups treated with CSF compared to the control group. Results of this study indicate that colony stimulating factors (G-CSF and GM-CSF) induce white cell proliferation by increasing the levels of multiple forms of DHFR.
