Granulocyte-colony stimulating factor induced apoptosis in radiation-induced murine leukemia cell line

Cytokines regulate proliferation and differentiation of hematopoietic progenitor cells. Recently it has been clarified that physiological cell death, apoptosis plays important role of hematopoiesis. So we evaluated the effects of granulocyte-colony stimulating factor (G-CSF) on leukemic cells, especially focused on apoptosis. Intravenous inoculation of radiation-induced murine leukemia cell line, C2M-A5 into the parent C3H mice resulted in the development of myeloid leukemia. However, the leukemic death of the mice was completely suppressed by the daily subcutaneous injection of recombinant human (rh)G-CSF from the next day (Bessho M. et al., Leuk Res 1989:13:1001-1007). In the in vitro study using C2M-A5 cells, we found that apoptosis appears on the cells at 48 hours after addition of G-CSF in culture. The cells in this stage lost the leukemogenicity to C3H mice (Bessho M. et al., Leukemia 8:1185-1190:1994). To clarify the mechanism of the induction of apoptosis by G-CSF we studied cell cycle and molecular changes in C2M-A5 cells cultured in medium with or without rhG-CSF by means of using the flowcytometry and Northern and Western blot analyses. After addition of rh G-CSF to culture, C2M-A5 cells removed to S phase, next arrested at G0/G1 phase on and after 24 hours, and 48 hours later, apoptosis was observed. Overexpression of mRNAs for c-myc (3-24 hours later) and for p53 (6-24 hours later), were observed in the cell cultured in rhG-CSF administered medium with a concomitant down-expression of bcl-2 mRNA (from 6 hours later). Tyrosine-phosphorylated protein (17 kd) appeared at 48 hours after administration of rhG-CSF to cell culture. This protein was suggested for specific apoptosis induction by rhG-CSF. These results are summarized as follows. (1) rhG-CSF induced apoptosis to C2M-A5 and deprived its leukemogenicity to mice. (2) Induction of apoptosis was associated with cell cycle and correlated to the changes of the expression rates of c-myc,p53, and bcl-2. (3) Tyrosine kinase may play an important role in apoptosis induction to C2M-A5 by rhG-CSF.