Polyamines and cancer: implications for chemotherapy and chemoprevention

Abstract

Polyamines are small organic cations that are essential for normal cell growth and development in eukaryotes. Under normal physiological conditions, intracellular polyamine concentrations are tightly regulated through a dynamic network of biosynthetic and catabolic enzymes, and a poorly characterised transport system. This precise regulation ensures that the intracellular concentration of polyamines is maintained within strictly controlled limits. It has frequently been observed that the metabolism of, and the requirement for, polyamines in tumours is frequently dysregulated. Elevated levels of polyamines have been associated with breast, colon, lung, prostate and skin cancers, and altered levels of rate-limiting enzymes in both biosynthesis and catabolism have been observed. Based on these observations and the absolute requirement for polyamines in tumour growth, the polyamine pathway is a rational target for chemoprevention and chemotherapeutics. Here we describe the recent advances made in the polyamine field and focus on the roles of polyamines and polyamine metabolism in neoplasia through a discussion of the current animal models for the polyamine pathway, chemotherapeutic strategies that target the polyamine pathway, chemotherapeutic clinical trials for polyamine pathway-specific drugs and ongoing clinical trials targeting polyamine biosynthesis.

Figure 1. The polyamine pathway

(a) Schematic of putrescine and the higher polyamines spermidine and spermine. (b) The amino acid ornithine is a product of the urea cycle. Ornithine is converted to the diamine putrescine by the enzyme ornithine decarboxylase (ODC). Putrescine is then converted to the higher polyamines spermidine and spermine via spermidine synthase and spermine synthase, respectively. The decarboxylation of S-adenosylmethionine (SAM) by S-adenosylmethionine decarboxylase (AdoMetDC) produces decarboxylated SAM (dcSAM), which acts as the propyl amine donor for the formation of spermidine and spermine via the spermidine and spermine synthases. Spermidine/spermine N¹-acetyltransferase (SSAT) is a propylamine acetyltransferase that converts spermine and spermidine to N¹-acetylspermine and N¹-acetylspermidine, respectively. The acetylated polyamines can be either exported out of the cell via an undetermined transport system, or act as substrates for the polyamine oxidase (APAO). APAO catalyses the conversion of N¹-acetylspermine to spermidine and N¹-acetylspermidine to putrescine. Spermine oxidase (SMO) oxidizes non-acetylated spermine to form spermidine. Putrescine, spermidine, and spermine can also be imported into the cell via a poorly understood transport mechanism. All polyamine pathway enzymes are in blue.

Figure 2. Theoretical models of the polyamine transport system

(a) This model proposes that polyamines (PA) are transported into cells via a yet to be identified transporter that is powered by membrane potential. Accumulated polyamines localize to polyamine-sequestering vesicles. This process is dependent on a vacuolar ATPase pH gradient and proton exchange. Yellow transporters represent vesicular polyamine exporters and brown transporters represent vesicular polyamine antiporters. A plasma membrane polyamine permease is shown in blue. (b) This model describes a mechanism by which heparan sulphate and glypican 1 (GPC1) work together to transport spermine (Spm). Spermine binds to heparan sulphate groups in GPC1 on the cell surface and is subsequently internalized (the GPC1 cell surface receptor is depicted in orange). Inside the cell, the spermine is freed by a NO-oxidation-mediated process through nitric oxide synthase-2 (NOS2). (c) This mechanism hypothesizes a caveolin-1-dependent internalization of polyamines that are bound to an unknown polyamine membrane receptor. In this model, only putrescine (PUT) is released from the vesicles through a SLC3A2 exporter. A NOS2-dependent reaction destabilizes the spermine-receptor complex in this model. This figure was adapted from Poulin et al. (Ref. 50).

Figure 3. Representative agents targeting the polyamines, polyamine metabolism, and polyamine catabolism

(a) 2-difluoromethylornithine (DFMO) is an enzyme inhibitor for ODC. It causes a decrease in intracellular putrescine and spermidine pools. (b) Methylglyoxal bis(guanylhydrazone) (MGBG) is an enzyme inhibitor of AdoMetDC. It causes a decrease in spermidine and spermine levels while increasing the level of putrescine. It competes with the natural polyamines for uptake and is a mitochondrial toxin. (c) 4-amidoinoindan-1-one-2’-amidinhydrazone (SAM486A) is an enzyme inhibitor of AdoMetDC that has lower mitochondrial toxicity than MGBG. It decreases the levels of spermidine and spermine while increasing the level of putrescine. (d) N,N¹-bis(2,3-butadienyl)-1,4-butanediamine (MDL 72,527) is an active site inhibitor of APAO that also inhibits SMO. (e) N¹,N¹¹-bis(ethyl)norspermine (BENSpm) is a symmetrically substituted spermine analogue. It induces SSAT and SMO and causes the depletion of putrescine, spermidine, and spermine. (f) CGC-11047 is a conformationally restricted polyamine analogue. It induces both SSAT and SMO and also decreases the levels of putrescine, spermidine, and spermine. (g) CGC-11093 is a conformationally restricted polyamine analogue but, unlike CGC-11047, it does not induce SSAT or SMO. (h) PENSpm is a non-symmetrically substituted polyamine analogue. It causes an induction of SSAT and cell type-specific cytotoxicity. (i) CGC-11144 is an oligoamine that has a high affinity for DNA. It competes with the natural polyamines for uptake and inhibits SMO activity and LSD1 activity. (j) Polyaminohydroxamic acids (PAHAs) incorporate features of the polyamines spermidine and spermine and the hydroxamic acid moiety of the HDAC inhibitors SAHA and Trichostatin A. (k) Polyaminobenzamides (PABAs) incorporate the benzamide moiety of MS-275 as well as features of spermidine and spermine. (l) Polyaminobiguanides are trypanocidal inhibitors and have recently been determined to inhibit LSD1 activity as well as cause the re-expression of some aberrantly silenced genes. (m) Polyamino(bis)guanidines are trypanocidal inhibitors and have recently been determined to inhibit LSD1 activity in cancer cells as well as cause the re-expression of aberrantly silenced genes. (n) Amidoximes were synthesized based on the success of certain polyaminobiguanide and polyamino(bis)guanidine compounds. Some from this series of compounds exhibited LSD1 inhibition as well as caused the re-expression of some aberrantly silenced genes.