A conditionally-active form of MEK1 results in autocrine transformation ofhuman 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 condi tionally-active form of MEK1, we demonstrate the ability of this dual speci ficity protein kinase to abrogate the cytokine-dependency of the human rand murine hematopoietic cells lines TF-1, FDC-P1 and FL5.12. Cytokine-indepen dent 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 al l 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 cou ld 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Removal of hormone led to the rapid cessation of cell growth in a m anner similar to that observed when cytokine is withdrawn from the parental cells. Treatment of Delta MEK1:ER-responsive cells with a specific and sel ective inhibitor, PD98059, prevented growth in response to beta-estradiol. GM-CSF mRNA transcripts were detected in the MEK1-responsive cells indicati ng that the activated Delta MEK1:ER may induce a pathway leading to autocri ne proliferation. Treatment of MEK1-responsive cells with an anti-GM-CSF an tibody, but not a control antibody, suppressed cell growth. The cell. lines described here will be useful for elaborating the ability of the MAP kinas e pathway to regulate cell proliferation in hematopoietic cells.