PHOSPHORYLATION OF SIGNALING PROTEINS IN FACTOR-INDEPENDENT MYELOID-LEUKEMIA CELL-LINES

We investigated the mechanisms of factor-independent growth of four hu man myeloid leukemia cell lines. The autocrine mechanisms were ruled o ut by RT-PCR method examining growth factor mRNA. The immunoblotting m ethod showed that many proteins were tyrosine phosphorylated irrespect ive of the stimulation with growth factors (G-CSF and GM-CSF) in facto r-independent cell lines while the phosphorylation was induced stimula tion dependently in a factor-dependent cell line. MAP kinase was const itutively phosphorylated in factor-independent cell lines. JAK2 protei n was not tyrosine phosphorylated before the stimulation. It was signi ficantly phosphorylated after the stimulation in three factor-independ ent cell lines although the stimulation did not affect their growth. J AK1 protein was not phosphorylated either before or after the stimulat ion. In conclusion, constitutive phosphorylation of signaling proteins seemed to be related to factor-independent growth. MAP kinase was inv olved in the phosphorylation, while JAK1 and JAK2 were not.