Inhibition of the extracellular signal‐regulated kinase (ERK) pathway and the induction of radioresistance in rat 3Y1 cells

PURPOSE: Activation of the extracellular signal-regulated kinase (ERK) pathway generally results in stimulation of cell growth and confers a survival advantage. However, the potential involvement of this pathway in cellular radiosensitivity remains unclear. The study was designed to examine whether the ERK pathway affects intrinsic radiosensitivity in mammalian cells. MATERIALS AND METHODS: Exponentially growing rat 3Y1 cells were used. A specific inhibitor of mitogen-activated ERK kinase (MEK), PD98059, was used to inhibit the ERK pathway. In addition, kinase-deficient MEK was expressed in cells to inhibit the pathway in a dominant-negative manner. Activation of ERK was visualized by Western blot using an antibody that recognizes the phosphorylated form of ERK. Radiosensitivity was evaluated by a colony-forming assay. RESULTS: 3Y1 cells treated with PD98059 exhibited a significant inhibition of cell proliferation and radiation-induced transient activation of ERK. Unexpectedly, it was found that the inhibitor enhanced clonogenic radioresistance. This effect on radioresistance was confirmed by expression of kinase-deficient MEK. Apoptotic activities following irradiation were significantly inhibited in PD98059-treated cells as determined by caspase-3-like activities. CONCLUSION: Activation of the MEK/ERK pathway increases clonogenic radiosensitivity in rat 3Y1 cells. These findings, together with a variety of other data, suggest that there might be clinical implications in targeting the MEK/ERK pathway in radiotherapy.