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

The PI3K/Akt and Raf/MEK/ERK signal transduction cascades are pivotal in transmitting signals from membrane receptors to downstream targets that regulate apoptosis, gene expression, and cell growth. The abilities of activated PI3K, Akt, Raf, and MEK proteins to abrogate the cytokine dependence of three different hematopoietic cell lines were determined. Activated PI3K or Akt expression by themselves did not efficiently annul cytokine dependence. Raf and MEK could abrogate the cytokine dependence of murine FDC-PI and human TF-1 cells; however, the frequency of transformation was dependent on the particular oncogene examined, as more factor-independent cells were isolated after infection with activated retroviruses encoding A-Raf or Raf-1 than were with MEK1 or B-Raf. Cytokine-independent deltaRaf-1-infected cells formed tumors on injection into immunocompromised mice, whereas cytokine-dependent cell lines did not, demonstrating the oncogenic effects of activation of the Raf/MEK/ERK pathway. Overexpression of the antiapoptotic Bcl-2 protein synergized with activation of the Raf/MEK/ERK cascade and increased the efficiency of transformation of FDC-PI and TF-1 cells. In contrast to the results observed with FDC-P1 and TF-I cells, the activated Raf genes did not relieve the cytokine dependence of murine FL5.12 cells. The abilities of the Raf and PI3K pathways to interact and annul the cytokine dependence of FL5.12 cells were determined. The combination of Raf and either PI3K or Akt expression relieved cytokine dependence of some FL5.12 cells, and the efficiency of transformation could be enhanced further by Bcl-2 or Bcl-XL overexpression. Thus, the antiapoptotic PI3K/Akt and Bcl-2/Bcl-XL proteins can interact with the growth-promoting Raf/MEK/ERK pathway and annul the cytokine dependence of certain hematopoietic cells.