Stimulation of clonal growth of human colorectal tumor cells by IL-3 and GM-CSF. Modulation of 5-FU cytotoxicity by GM-CSF.

We studied the influence of recombinant human (rh) interleukin-3 (IL-3) and rh granulocyte-macrophage colony-stimulating factor (GM-CSF) on the clonal growth of a human colorectal adenocarcinoma cell line in a methylcellulose assay for colony growth of solid tumor cell lines (HTCAMC) and a capillary modification of a human tumor cloning assay in agar (HTCAcap). Both growth factors stimulated the clonal growth of this cell line in a dose-dependent fashion. Neutralizing the monoclonal antibody abolished the effect of rhGM-CSF. There was an inverse correlation between the spontaneous plating efficacy (PE) of the cells and their susceptibility to the stimulation by the growth factors. From day 4 to 7 we found conditions in which clusters and colonies occurred preferentially in the growth factor-stimulated cultures. Single colonies taken from these cultures grew rapidly into macroscopically visible tumors in liquid cultures and had a high secondary PE (PE2) in the HTCAcap, both presenting an argument against a differentiating effect of the growth factors on this tumor cell line. Furthermore, we were able to define conditions in which rhGM-CSF significantly increased the cytotoxicity of 5-fluorouracil (5-FU). However, this effect was dependent on spontaneous PE of the cells, degree of stimulation by the factor, degree of cytotoxicity of 5-FU in the controls, as well as the therapeutic regimen. Since there were only narrow margins for a beneficial effect of rhGM-CSF in this setting when absolute numbers of surviving colonies were counted, it remains doubtful whether this approach will be exploitable in the clinical situation.