Accumulation of spermidine/spermine N1-acetyltransferase and alternatively spliced mRNAs as a delayed response of HeLa S3 cells following Xray irradiation

Purpose: A key enzyme of polyamine catabolism, spermidine/spermine N1-acetyltransferase (SSAT), is responsive to antiproliferative agents. The role of SSAT in cellular responses to X-ray irradiation was examined. Materials and methods: Exponentially growing HeLa S3 cells were irradiated by X-rays, and mRNA levels for SSAT were measured as a function of postirradiation time through Northern hybridization. Reverse transcription-polymerase chain reaction (RT-PCR) was used to detect alternatively spliced SSAT mRNAs. The intracellular polyamine content was measured by the o-phthalaldehyde method and the enzymatic activity of SSAT by the increased amount of acetylated spermidine after incubation. Results: Not only SSAT mRNA, but also an alternatively spliced mRNA accumulated at the initial stage of growth inhibition after the first or second replication of irradiated cells. The maximum fold increase relative to the level of non-irradiated cells was 3.0-3.5 for both transcripts after 5-Gy irradiation. On the other hand, the mRNA of ornithine decarboxylase, a key enzyme of polyamine synthesis, was little influenced by X-ray treatment. Enzymatic activity of SSAT and the acetylspermidine level were elevated after X-ray irradiation. Conclusions: Activation of SSAT and the induction of alternatively spliced mRNA of the SSAT gene play an important role in regulating growth inhibition and cell death after X-ray irradiation.