Increased radioresistance via G12S K-Ras by compensatory upregulation of MAPK and PI3K pathways in epithelial cancer.
BACKGROUND
Irradiation-induced signaling via the 2 pathways, Raf-MEK-ERK and PI3K-Akt, is known to be closely associated with a limited response to radiotherapy. In the present study we analyzed the relevance of constitutively active K-Ras for postradiogenic pathway stimulation and the option of coordinated inhibition to overcome these rescue mechanisms.


METHODS
We used 2 epithelial tumor cell lines as a model system, one of them harboring a G12S K-Ras mutation. Cells were irradiated and the effect of combined treatment with ionizing radiation and inhibitors on the expression of pERK and pAkt was determined by Western blotting. Additionally, clonogenic assays were performed to functionally analyze survival of the cell lines.


RESULTS
Compared with the nonmutated cells we observed the G12S cell line showing a clearly reduced response to inhibitor treatment under irradiation. In the case of pharmacologic inhibition of 1 of the pathways a compensatory upregulation of the second cascade leading to increased clonogenic survival seems feasible. However, there was a good functional response of this cell line to double inhibition with both compounds represented by minimized colony forming ability. The activation of ERK and Akt after irradiation was confirmed in xenotransplants showing elevated postradiogenic protein levels.


CONCLUSIONS
With our data we confirmed our hypothesis of postradiogenic constitutive activation of the 2 pathways both required for Ras-mediated radioresistance in epithelial cells. If this effect should prove itself as a general mechanism in Ras-mutated tumors, application of specific inhibitors to block both cascades in parallel could contribute to enhance radiosensitivity in these types of cancer.
Department of Otorhinolaryngology, Head and Neck Surgery, University Medical Center of the Johannes Gutenberg University, Mainz, Germany. affolter@hno.klinik.uni-mainz.de
Annette
A
Affolter
Martynas
M
Drigotas
Kai
K
Fruth
Irene
I
Schmidtmann
Christoph
C
Brochhausen
Wolf J
WJ
Mann
Jürgen
J
Brieger
D015153:Blotting, Western; D002294:Carcinoma, Squamous Cell / Q000235:genetics / Q000473:pathology; D045744:Cell Line, Tumor; D004591:Electrophoresis, Polyacrylamide Gel; D004847:Epithelial Cells / Q000528:radiation effects; D011905:Genes, ras / Q000235:genetics*; D006258:Head and Neck Neoplasms / Q000235:genetics / Q000473:pathology; D006801:Humans; D020935:MAP Kinase Signaling System / Q000502:physiology; D020929:Mitogen-Activated Protein Kinase Kinases / Q000235:genetics / Q000528:radiation effects*; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000528:radiation effects*; D016133:Polymerase Chain Reaction / Q000379:methods; D011836:Radiation Tolerance / Q000235:genetics*; D012680:Sensitivity and Specificity; D015398:Signal Transduction / Q000528:radiation effects; D015854:Up-Regulation
D019869:Phosphatidylinositol 3-Kinases; D020929:Mitogen-Activated Protein Kinase Kinases