Interactions between the PI3K and Raf signaling pathways can result in thetransformation of hematopoietic cells

The P13K/Akt and Raf/MEK/ERK signal transduction cascades are pivotal in tr ansmitting signals from membrane receptors to downstream targets that regul ate apoptosis, gene expression, and cell growth. The abilities of activated P13K, Akt, Raf, and MEK proteins to abrogate the cytokine dependence of th ree different hematopoietic cell lines were determined. Activated P13K or A kt expression by themselves did not efficiently annul cytokine dependence. Raf and MEK could abrogate the cytokine dependence of murine FDC-P1 and hum an TF-1 cells. however, the frequency of transformation was dependent on th e particular oncogene examined, as more factor-independent cells were isola ted after infection with activated retroviruses encoding A-Raf or Raf-1 tha n were with MEK1 or B-Raf. Cytokine-independent Delta Raf-1-infected cells formed tumors on injection into immunocompromised mice, whereas cytokine-de pendent cell lines did not, demonstrating the oncogenic effects of activati on of the Raf/MEK/ERK pathway. Overexpression of the antiapoptotic Bcl-2 pr otein synergized with activation of the Raf/MEK/ERK cascade and increased t he efficiency of transformation of FDC-P1 and TF-1 cells. In contrast to th e results observed with FDC-P1 and TF-1 cells, the activated Raf genes did not relieve the cytokine dependence of marine FL5.12 cells. The abilities o f the Raf and P13K pathways to interact and annul the cytokine dependence o f FL5.12 cells were determined. The combination of Raf and either P13K or A kt expression relieved cytokine dependence of some FL5.12 cells. and the ef ficiency of transformation could be enhanced further by Bcl-2 or Bcl-X-L ov erexpression. Thus, the antiapoptotic P13K/Akt and Bcl-2/Bcl-X-L proteins c an interact with the growth-promoting Raf/MEK/ERK pathway and annul the cyt okine dependence of certain hematopoietic cells.