A conditionally-active form of MEK1 results in autocrine transformation of human and mouse hematopoietic cells

The Raf/MEK/MAP kinase cascade plays a critical role in transducing growth signals from activated cell surface receptors, Using Delta MEK1:ER, a conditionally-active form of MEK1, we demonstrate the ability of this dual specificity protein kinase to abrogate the cytokine-dependency of the human rand murine hematopoietic cells lines TF-1, FDC-P1 and FL5.12. Cytokine-independent cells were obtained from TF-1, FDC-P1 and FL5.12 cells at frequencies of 2.5 x 10(-3), 5 x 10(-5) and 10(-7) respectively, indicating that not all cells expressing Delta MEK1:ER were factor-independent. In general, cells that were converted to a cytokine-independent phenotype displayed a higher level of MAP kinase activity in response to Delta MEK1:ER activation than those that remained cytokine-dependent, Delta ME K1:ER-responsive cells could be maintained long-term in the presence of beta-estradiol as well as the estrogen-receptor antagonist 4-Hydroxy-Tamoxifen and the antiestrogen ICI 164383, Removal of hormone led to the rapid cessation of cell growth in a manner similar to that observed when cytokine is withdrawn from the parental cells. Treatment of Delta MEK1:ER-responsive cells with a specific and selective inhibitor, PD98059, prevented growth in response to beta-estradiol. GM-CSF mRNA transcripts were detected in the MEK1-responsive cells indicating that the activated Delta MEK1:ER may induce a pathway leading to autocrine proliferation. Treatment of MEK1-responsive cells with an anti-GM-CSF antibody, but not a control antibody, suppressed cell growth. The cell. lines described here will be useful for elaborating the ability of the MAP kinase pathway to regulate cell proliferation in hematopoietic cells.