Polyamines and Their Role in Human Disease 361 The role of polyamine catabolism in anti-tumour drug response R.A. Casero, Jr* 1 , Y. Wang*, T.M. Stewart*, W. Devereux*, A. Hacker*, Y. Wang*, R. Smith and P.M. Woster† *Sidney Kimmel Comprehensive Cancer Center, Johns Hopkins University, 1650 Orleans Street, Baltimore, MD 21231, U.S.A., and †Wayne State University, Detroit, MI 48202, U.S.A. Abstract Interest in polyamine catabolism has increased since it has been directly associated with the cytotoxic response of multiple tumour types to exposure to specific anti-tumour polyamine analogues. Human polyamine catabolism was considered to be a two-step pathway regulated by the rate-limiting enzyme spermidine/spermine N 1 -acetyltransferase (SSAT) that provides substrate for an acetylpolyamine oxidase (APAO). Further, the super-induction of SSAT by several anti-tumour polyamine analogues has been implicated in the cytotoxic response of specific solid-tumour phenotypes to these agents. This high induction of SSAT has been correlated with cellular response to the anti-tumour polyamine analogues in several systems and considerable progress has been made in understanding the molecular mechanisms that regulate the analogue-induced expression of SSAT. A polyamine response element has been identified and the transacting transcription factors that bind and stimulate transcription of SSAT have been cloned and characterized. The link between SSAT activity and cellular toxicity is thought to be based on the production of H 2 O 2 by the activity of the constitutive APAO that uses the SSAT-produced acetylated polyamines. The high induction of SSAT and the subsequent activity of APAO are linked to the cytotoxic response of some tumour cell types to specific polyamine analogues. However, we have recently cloned a variably spliced human polyamine oxidase (PAOh1) that is inducible by specific polyamine analogues, efficiently uses unacetylated spermine as a substrate, and also produces toxic H 2 O 2 as a product. The results of studies with PAOh1 suggest that it is an additional enzyme in polyamine catabolism that has the potential to significantly contribute to polyamine homoeostasis and drug response. Most importantly, PAOh1 is induced by specific polyamine analogues in a tumour-phenotype-specific manner in cell lines representative of the major forms of solid tumours, including lung, breast, colon and prostate. The sensitivity to these anti- tumour polyamine analogues can be significantly reduced if the tumour cells are co-treated with 250 μM of the polyamine oxidase inhibitor N 1 ,N 4 -bis(2,3-butadienyl)-1,4-butanediamine (MDL 72,527), suggesting that the H 2 O 2 produced by PAOh1 does in fact play a direct role in the observed cytotoxicity. These results strongly implicate PAOh1 as a new target that, in combination with SSAT, may be exploited for thera- peutic advantage. The current understanding of the role and regulation of these two important polyamine catabolic enzymes are discussed. Polyamine metabolism as a therapeutic target The metabolism of polyamines has been pursued as a target for anti-neoplastic therapeutic intervention subsequent to the discovery that polyamines were obligatory factors for growth [1]. Initially, much attention was focused on inhibition of the biosynthetic pathway as a means to inhibit tumour growth, and inhibitors to essentially all of the biosynthetic enzymes have been successfully synthesized and tested [2]. Although much was learned about the requirement of polyamines through the development of inhibitors of polyamine biosynthesis, none of the inhibitors have been Key words: polyamine, polyamine oxidase, spermidine/spermine-N 1 -acetyltransferase. Abbreviations used: APAO, acetylpolyamine oxidase; PAO, polyamine oxidase; SSAT, spermidine/spermine-N 1 -acetyltransferase; PRE, polyamine response element; BENSpm, N 1 ,N 11 - bis(ethyl)norspermine; PMF-1, polyamine-modulated factor-1; Nrf-2, NF-E2-related transcription factor. 1 To whom correspondence should be addressed (e-mail rcasero@jhmi.edu). successful as single agents in the treatment of cancer. 2-Difluoromethylornithine (‘DFMO’), an irreversible inhibi- tor of ornithine decarboxylase (‘ODC’), has been among the most successful of the inhibitors and is currently a front- line agent in the treatment of African sleeping sickness [3–5], and is undergoing extensive testing as a chemo- preventive agent, particularly in gastrointestinal cancers [6–9]. Recently, there has been an increased interest in the role that polyamine catabolism may play in determining tumour cell response to agents that interfere with or alter polyamine metabolism. This interest is a direct result of the development of several different anti-tumour polyamine analogues. These analogues, typified by N 1 ,N 11 -bis(ethyl)norspermine (BENSpm) and N 1 -ethyl-N 11 -[(cyclopropyl)methyl]-4,8- diazaundecane (‘CPENSpm’), decrease intracellular poly- amines by down-regulating polyamine biosynthesis and, in specific instances, significantly up-regulating polyamine catabolism [10–16]. It is this effect, the up-regulation of C  2003 Biochemical Society