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Interactions between the PI3K and Raf signaling pathways can result in the transformation of hematopoietic cells
J A McCubrey1, J T Lee, L S Steelman
1Department of Microbiology and Immunology, Brody School of Medicine at East Carolina University, Greenville, NC 27858, USA.
Abstract:
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.
Insights
The Raf/MEK/ERK pathway, when activated, can promote oncogenic transformation in hematopoietic cells, especially when combined with anti-apoptotic proteins. This pathway
Area of Science:
- Cell Biology
- Molecular Biology
- Oncology
Background:
- The PI3K/Akt and Raf/MEK/ERK signaling pathways are crucial for cell growth, survival, and gene expression.
- These pathways transmit signals from cell surface receptors to intracellular targets.
- Dysregulation of these pathways is implicated in various cancers.
Purpose of the Study:
- To investigate the roles of PI3K/Akt and Raf/MEK/ERK signaling in abrogating cytokine dependence in hematopoietic cells.
- To determine the oncogenic potential of activated Raf/MEK/ERK signaling.
- To explore the interactions between these pathways and anti-apoptotic proteins in cell transformation.
Main Methods:
- Retroviral expression of activated PI3K, Akt, Raf, and MEK proteins in hematopoietic cell lines (FDC-PI, TF-1, FL5.12).
- Assessment of cytokine independence and transformation frequency.
- Tumorigenicity assays in immunocompromised mice.
- Co-expression studies with anti-apoptotic proteins (Bcl-2, Bcl-XL).
Main Results:
- Activated PI3K or Akt alone did not efficiently abolish cytokine dependence.
- Activated Raf and MEK abrogated cytokine dependence in FDC-PI and TF-1 cells, with transformation efficiency varying by specific oncogene.
- Cytokine-independent cells driven by activated Raf formed tumors in mice, confirming oncogenic effects.
- Overexpression of Bcl-2 enhanced Raf/MEK/ERK-mediated transformation.
- Activated Raf genes did not relieve cytokine dependence in FL5.12 cells, but combinations with PI3K or Akt did, further enhanced by Bcl-2/Bcl-XL.
Conclusions:
- The Raf/MEK/ERK pathway activation can lead to oncogenic transformation of certain hematopoietic cells.
- Interactions between the Raf/MEK/ERK pathway and PI3K/Akt pathway, along with anti-apoptotic proteins, can overcome cytokine dependence and promote transformation.
- These findings highlight the complex interplay of signaling pathways in cancer development and suggest potential therapeutic targets.