Inhibition of the mitogen activated protein kinase pathway potentiates radiation-induced cell killing via cell cycle arrest at the G2/M transition and independently of increased signaling by the JNK/c-Jun pathway.
The ability of low dose ionizing radiation (2 Gy) to modulate the activities of the mitogen activated protein kinase (MAPK) and c-Jun NH2-terminal kinase (JNK1) cascades in human monocytic leukemia (U937/pREP4) cells and in cells over-expressing dominant negative c-Jun (TAM67) (U937/TAM67) was investigated. Radiation exposure caused prolonged ( approximately 1 h) MAPK activations in U937 cells. In contrast, low dose irradiation weakly modulated JNK1 activity in these cells. Inhibition of the MAPK pathway by use of the specific MEK1/2 inhibitor (10 microM PD98059) in both U937/pREP4 and U937/TAM67 cells prior to radiation exposure permitted strong prolonged radiation-induced activations of JNK1. Expression of TAM67 decreased the ability of radiation to cause apoptosis compared to control transfected cells. However, combined MEK1/2 inhibition and radiation exposure in both cell types caused a large decrease in suspension culture growth and a large increase in apoptosis, when compared to either treatment alone. Reduced proliferation after combined irradiation and PD98059 treatment in both cell types correlated with prolonged cell cycle arrest in G2/M phase. Prolonged growth arrest was abolished when MEK1/2 inhibitor was removed 6 h following irradiation, which was associated with a reduction in apoptosis. The ability of MEK1/2 inhibition to cause prolonged G2/M growth arrest was reduced in U937 cells stably transfected with a p21Cip-1/WAF1 antisense construct (U937/p21AS). This data correlated with an enhancement of radiation-induced apoptosis and a reduced ability of MEK1/2 inhibition to potentiate apoptosis. Collectively our data demonstrate that inhibition of MEK1/2 function increases the radiation sensitivity of U937 cells, independently of c-Jun function, and decreases the ability of these cells to recover from the radiation-induced G2/M cell cycle checkpoint arrest. In addition, our data also demonstrate that the ability of MEK1/2 inhibition to potentiate radiation-induced cell death in U937 cells in part requires an ability of cells to express low levels of p21Cip-1/WAF1.
Cartee
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Department of Medicine, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA 23298-0058, USA.
Vrana
J A
JA
Wang
Z
Z
Park
J S
JS
Birrer
M
M
Fisher
P B
PB
Grant
S
S
Dent
P
P
D017209:Apoptosis / Q000502:physiology / Q000528:radiation effects*; D002453:Cell Cycle / Q000502:physiology / Q000528:radiation effects; D016195:G2 Phase / Q000502:physiology / Q000528:radiation effects*; D006801:Humans; D048031:JNK Mitogen-Activated Protein Kinases; D007951:Leukemia, Myeloid / Q000378:metabolism; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism / Q000528:radiation effects*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism / Q000528:radiation effects*; D008938:Mitosis / Q000502:physiology / Q000528:radiation effects*; D020298:U937 Cells / Q000528:radiation effects
D048031:JNK Mitogen-Activated Protein Kinases; D020928:Mitogen-Activated Protein Kinases; D020929:Mitogen-Activated Protein Kinase Kinases