Roles of the RAF/MEK/ERK and PI3K/PTEN/AKT pathways in malignant transformation and drug resistance

Author(s): McCubrey JA, Steelman LS, Abrams SL, Lee JT, Chang F, et al.


The Ras/Raf/MEK/ERK and PI3K/PTEN/AKT signaling cascades play critical roles in the transmission of signals from growth factor receptors to regulate gene expression and prevent apoptosis. Components of these pathways are mutated or aberrantly expressed in human cancer (e.g., Ras, B-Raf, PI3K, PTEN, Akt). Also, mutations occur at genes encoding upstream receptors (e.g., EGFR and Flt-3) and chimeric chromosomal translocations (e.g., BCR-ABL) which transmit their signals through these cascades. These pathways interact with each other to regulate growth and in some cases tumorigenesis. For example, in some cells, PTEN mutation may contribute to suppression of the Raf/MEK/ERK cascade due to the ability of elevated activated Akt levels to phosphorylate and inactivate Raf-1. We have investigated the genetic structures and functional roles of these two signaling pathways in the malignant transformation and drug resistance of hematopoietic, breast and prostate cancer cells. Although both of these pathways are commonly thought to have anti-apoptotic and drug resistance effects on cells, they display different cell-lineage-specific effects. Induced Raf expression can abrogate the cytokine dependence of certain hematopoietic cell lines (FDC-P1 and TF-1), a trait associated with tumorigenesis. In contrast, expression of activated PI3K or Akt does not abrogate the cytokine dependence of these hematopoietic cell lines, but does have positive effects on cell survival. However, activated PI3K and Akt can synergize with activated Raf to abrogate the cytokine dependence of another hematopoietic cell line (FL5.12) which is not transformed by activated Raf expression by itself. Activated Raf and Akt also confer a drug-resistant phenotype to these cells. Raf is more associated with proliferation and the prevention of apoptosis while Akt is more associated with the long-term clonogenicity. In breast cancer cells, activated Raf conferred resistance to the chemotherapeutic drugs doxorubicin and paclitaxel. Raf induced the expression of the drug pump Mdr-1 (a.k.a., Pgp) and the Bcl-2 anti-apoptotic protein. Raf did not appear to induce drug resistance by altering p53/p21Cip-1 expression, whose expression is often linked to regulation of cell cycle progression and drug resistance. Deregulation of the PI3K/PTEN/Akt pathway was associated with resistance to doxorubicin and 4-hydroxyl tamoxifen, a chemotherapeutic drug and estrogen receptor antagonist used in breast cancer therapy. In contrast to the drug-resistant breast cancer cells obtained after overexpression of activated Raf, cells expressing activated Akt displayed altered (decreased) levels of p53/p21Cip-1. Deregulated expression of the central phosphatase in the PI3K/PTEN/Akt pathway led to breast cancer drug resistance. Introduction of mutated forms of PTEN, which lacked lipid phosphatase activity, increased the resistance of the MCF-7 cells to doxorubicin, suggesting that these lipid phosphatase deficient PTEN mutants acted as dominant negative mutants to suppress wild-type PTEN activity. Finally, the PI3K/PTEN/Akt pathway appears to be more prominently involved in prostate cancer drug resistance than the Raf/MEK/ERK pathway. Some advanced prostate cancer cells express elevated levels of activated Akt which may suppress Raf activation. Introduction of activated forms of Akt increased the drug resistance of advanced prostate cancer cells. In contrast, introduction of activated forms of Raf did not increase the drug resistance of the prostate cancer cells. In contrast to the results observed in hematopoietic cells, Raf may normally promote differentiation in prostate cells which is suppressed in advanced prostate cancer due to increased expression of activated Akt arising from PTEN mutation. Thus in advanced prostate cancer it may be advantageous to induce Raf expression to promote differentiation, while in hematopoietic cancers it may be beneficial to inhibit Raf/MEK/ERK-induced proliferation. These signaling and anti-apoptotic pathways can have different effects on growth, prevention of apoptosis and induction of drug resistance in cells of various lineages which may be due to the expression of lineage-specific factors.

Similar Articles

Current management of small cell lung cancer

Author(s): Neal JW, Gubens MA, Wakelee HA

Mechanisms of resistance to cisplatin

Author(s): Kartalou M, Essigmann JM

Expression of clusterin in human pancreatic cancer

Author(s): Xie MJ, Motoo Y, Su SB, Mouri H, Ohtsubo K, et al.

Clusterin interacts with Paclitaxel and confer Paclitaxel resistance in ovarian cancer

Author(s): Park DC, Yeo SG, Wilson MR, Yerbury JJ, Kwong J, et al.

Regulation of chemosensitivity and migration by clusterin in non-small cell lung cancer cells

Author(s): Cheng CY, Cherng SH, Wu WJ, Yang TY, Huang XY, et al.

Clusterin as a therapeutic target for radiation sensitization in a lung cancer model

Author(s): Cao C, Shinohara ET, Li H, Niermann KJ, Kim KW, et al.

Deguelin, A PI3K/AKT inhibitor, enhances chemosensitivity of leukaemia cells with an active PI3K/AKT pathway

Author(s): Bortul R, Tazzari PL, Billi AM, Tabellini G, Mantovani I, et al.

Modulation of PI3K/Akt pathway by E1a mediates sensitivity to cisplatin

Author(s): Guinea Viniegra J, Hernández Losa J, Sánchez-Arévalo VJ, ParadaCobo C, FernándezSoria VM, et al.

Roles of the Raf/MEK/ERK pathway in cell growth, malignant transformation and drug resistance

Author(s): McCubrey JA, Steelman LS, Chappell WH, Abrams SL, Wong EW, et al.

Raf kinase as a target for anticancer therapeutics

Author(s): Sridhar SS, Hedley D, Siu LL

Mechanisms controlling sensitivity to platinum complexes: role of p53 and DNA mismatch repair

Author(s): Manic S, Gatti L, Carenini N, Fumagalli G, Zunino F, et al.

Clusterin regulates drug-resistance in melanoma cells

Author(s): Hoeller C, Pratscher B, Thallinger C, Winter D, Fink D, et al.

The Raf/MEK/ERK pathway: new concepts of activation

Author(s): Peyssonnaux C, Eychène A