DIFFERENTIAL TRANSCRIPTION OF THE HUMAN SPERMIDINE SPERMINE N-1-ACETYLTRANSFERASE (SSAT) GENE IN HUMAN LUNG-CARCINOMA CELLS/

The expression of spermidine/spermine N-1-acetyltransferase (SSAT), th e rate-limiting enzyme in the catabolism of polyamines, is highly regu lated by a number of factors including the natural polyamines and thei r analogues. The phenotype-specific cytotoxicity that occurs in respon se to a class of polyamine analogues, the diethylpolyamines, is associ ated with a phenotype-specific superinduction of SSAT in human non-sma ll-cell lung carcinomas, whereas in non-responding cell types, includi ng the small-cell lung carcinomas, the superinduction of SSAT does not occur. In this study, we have investigated the molecular basis of thi s phenotype-specific SSAT induction in human lung carcinoma cells in r esponse to N-1,N-12-diethylspermine (BESpm). To facilitate the study o f transcriptional regulation, we have cloned and characterized 11 kb o f the human SSAT locus, including 3500 bp of the 5' promoter region. N uclear run-on transcription studies suggest that the initial induction of SSAT results from an increase in the rate of gene transcription. R esults from Northern blot analysis and ribonuclease protection assays indicate a differential expression of SSAT mRNA between the analogue-r esponsive H157 and non-responsive H82 cells. There is no detectable SS AT mRNA in H82 cells, even after a 24-h analogue treatment, whereas SS AT mRNA in H157 cells was detectable by Northern blot analysis and inc reased more than 100-fold following drug exposure. Furthermore, nuclea r run-on transcription assays do not detect any active transcription o f SSAT gene in either treated or untreated H82 cells. These results in dicate that at least one component of the phenotype-specific induction of SSAT appears to be due to differences in transcriptional regulatio n of the gene. In addition, mapping of DNase I-hypersensitive sites of the SSAT gene suggest that the cell type-specific promoter/enhancer u tilization may control the expression of the SSAT gene in differential ly sensitive cell types in vivo.