High specific induction of spermidine/spermine N1-acetyltransferase in a human large cell lung carcinoma

Abstract

The cytotoxic response of the human large cell lung carcinoma line NCI H157 to exposure to the polyamine analogue N1 N12-bis(ethyl)spermine (BESpm) is preceded by an extremely high induction of spermidine/spermine N1-acetyltransferase (SSAT). The human enzyme has now been purified greater than 300-fold to apparent homogeneity and found to cross-react with antisera raised against rat liver SSAT. Although other acetylases are capable of acetylating polyamines using acetyl-CoA as the acetyl donor, the greater than 600-fold induction within 24 h was found to be specifically SSAT, since essentially all activity was precipitable by the specific antisera. The human enzyme appears to be similar to the rat enzyme in subunit size under reducing conditions (approximately 20 kDa), substrate specificity and kinetic parameters. Preliminary results using actinomycin D and cycloheximide suggested that the unusually high induction by N1 N12-bis(ethyl)spermine in the human lung cancer line result from new mRNA and protein synthesis. This hypothesis is further substantiated here by ‘in vitro’ translation experiments comparing poly(A) mRNA from control and treated cells. The large cell lung carcinoma line NCI H157 represents a useful system to produce large amounts of the SSAT protein and to study the molecular events responsible for the induction and control of this important polyamine-metabolic enzyme. By using this rich source of SSAT protein, a partial amino acid sequence was determined by N-terminal sequencing of endoproteinase Lys-C digestion fragments. Further, this system should be useful in determining whether there is an association between the unusually high induction of the acetylase and the observed cytotoxicity in the NCI H157 cells.