Interleukin-6 stimulates cell proliferation of rat pituitary clonal cell lines in vitro

We investigated the effect of recombinant human IL-6 (rhlL-6) on cell proliferation using the MtT/E rat pituitary tumor cell line, which was recently established by Inoue et al. This cell line expresses the homeodomain protein Pit-1/GHF 1 and does not produce any significant amount of pituitary hormones, but retains its tumorigenicity by back-transplantation into rats, resulting in production of prolactin. MtT/E cells were seeded into Falcon 24-well plates at a density of 2×104 cells/well in a cultured medium, containing 10% horse serum and 2.5% fetal bovine, with test drug. After four-days (12 days for the time-course study) incubations, the cells were counted using a hemocytometer. Incubation for 4 days with rhlL-6 caused concentration-dependent stimulation of MtT/E cell growth and [3H]-thymidine incorporation into MtT/E cells. Addition of 20 ng/ml rhlL-6 to the culture medium stimulated MtT/E cell growth in a time-dependent manner, withdrawal of rhlL-6 from the culture medium reduced MtT/E cell growth, and re-addition of rhlL-6 to the culture medium again stimulated MtT/E cell growth. Among the cytokines tested, granulocyte colony-stimulating factor (rh G-CSF) also showed a slight but significant mitogenic activity on the MtT/E cells. Analysis of 125|-rhlL-6 binding to the MtT/E cells indicated a dissociation constant of 0.953×10−9 mmol/l and the presence of 968 binding sites per cell. These results confirm the previous finding that IL-6 exerts mitogenic activity on pituitary tumor cells and also support the hypothsis that IL-6 may be involved in the control of cell growth and proliferation in the pituitary.