INHIBITION OF THE MITOGEN-ACTIVATED PROTEIN (MAP) KINASE CASCADE POTENTIATES CELL-KILLING BY LOW-DOSE IONIZING-RADIATION IN A431 HUMAN SQUAMOUS CARCINOMA-CELLS

The molecular mechanism(s) by which tumor cells survive after exposure to ionizing radiation are not fully understood. Exposure of A431 cell s to low doses of radiation (1Gy) caused prolonged activations of the mitogen activated protein (MAP) kinase and stress activated protein (S AP) kinase pathways, and induced p21(Cip-1/WAF1) via a MAP kinase depe ndent mechanism. In contrast, higher doses of radiation (6 Gy) caused a much weaker activation of the MAP kinase cascade, but a similar degr ee of SAP kinase cascade activation. In the presence of MAP kinase blo ckade by the specific MEK1 inhibitor (PD98059) the basal activity of t he SAP kinase pathway was enhanced twofold, and the ability of a 1 Gy radiation exposure to activate the SAP kinase pathway was increased si milar to sixfold 60 min after irradiation, In the presence of MAP kina se blockade by PD98059 the ability of a single 1Gy exposure to cause d ouble stranded DNA breaks (TUNEL assay) was enhanced at least threefol d over the following 24-48h. The increase in DNA damage within 48 h wa s also mirrored by a similar decrease in A431 cell growth as judged by MTT assays over the next 4-8 days following radiation exposure. This report demonstrates that the MAP kinase cascade is a key cytoprotectiv e pathway in A431 human squamous carcinoma cells which is activated in response to clinically used doses of ionizng radiation, Inhibition of this pathway potentiates the ability of low dose radiation exposure t o induce cell death im vitro.