EXPRESSION OF IMMEDIATE-EARLY GENES AFTER TREATMENT OF HUMAN ASTROCYTOMA-CELLS WITH RADIATION AND TAXOL

Purpose: The promising chemotherapeutic agent, taxol, has been shown t o sensitize the G18 line of human astrocytoma cells to ionizing radiat ion. The present studies were performed to identify specific changes i n gene expression associated with this altered sensitivity. Methods an d Materials: The radioresistant, grade 3 human astrocytoma cell line, G18, was exposed for varying periods of time to treatment with taxol, tetradecanoyl phorbol acetate (TPA), serum, isoproterenol, dibutyryl c yclic adenosine monophosphate, or ionizing radiation alone or in combi nation with taxol pretreatment. Ribonucleic acid samples from the cell s were monitored for the expression of a group of immediate early gene s (IEGs), including c-fos, c-jun, TIS1, TIS7, TIS8, TIS11 and TIS21, b y northern blot hybridization analysis. Results: Transient immediate e arly gene induction was observed after treatment of G18 cells with tet radecanoyl phorbol acetate, serum, isoproterenol, or ionizing radiatio n, but not after treatment with taxol. Of the seven immediate early ge nes analyzed, all but TIS7 were found to be inducible by one or more o f the treatments. Only TIS8 (also known as egr-1 or zif268) was signif icantly inducible by radiation, and this transient induction was decre ased by at least four-fold by pretreatment for 24 hr with a dose of ta xol that was previously shown to block 96.5% of the cells in G2/M and enhance radiosensitivity. Conclusion: The products of immediate early genes, which are induced transiently in cells in response to a variety of treatments, including growth factors, neurotransmitters, and irrad iation with UV light or X rays, are thought to initiate a cascade of g enetic responses to alterations in cellular environment. The present r esults demonstrate a dramatic attenuation in one immediate early gene response in association with a treatment that enhances radiosensitivit y in a refractory human brain tumor line.