Two active copies of the X-linked gene spermidine/spermine N¹- acetyltransferase (SSAT) in a female lung cancer cell line are associated with an increase in sensitivity to an antitumor polyamine analogue

The expression of the polyamine catabolic enzyme, spermidine/spermine N¹-acetyltransferase (SSAT), has been associated with tumor sensitivity to antitumor polyamine analogues. In the sensitive cell types the level of SSAT is greatly induced by these agents. Although SSAT expression is regulated at many levels, the initial regulation of this X-linked gene occurs at the level of transcription. Because most previous work in human cell lines has been performed in cells of male origin and because the SSAT gene is located near the pseudoautosomal region of the X chromosome, we investigated the possibility that both copies of SSAT could be expressed in normal and tumor cells in women. DNA methyl-sensitive restriction enzyme analysis of DNA from normal peripheral lymphocytes suggested that like most X-linked genes, only one copy of SSAT is actively transcribed. However, in an examination of four representative human lung tumor cell lines derived from women, two were found to have a methylation pattern identical to male-derived cells, suggesting a reactivation of the normally inactive allele or loss of the inactive allele. Microsatellite repeat polymorphism analysis indicated that one of the lines, a female carcinoid line, NCI H727, had reactivated the previously inactive copy, thus providing H727 with two active alleles, whereas a small cell lung cancer line, H889, appears to have lost the inactive allele. Most importantly, the H727 line expressed high amounts of SSAT mRNA and protein in response to treatment with the polyamine analogue, N¹,N¹²- bis(ethyl)spermine, a compound known to increase SSAT transcription in sensitive cell types. H727 was also the only female line that responded to treatment in a cytotoxic manner. These data suggest that both copies of the SSAT allele may be expressed and that the inappropriate expression of the second copy can lead to an increase in tumor sensitivity to polyamine analogues that induce SSAT.