Polyamines in cancer: integrating organismal metabolism and antitumour immunity

Abstract

The natural mammalian polyamines putrescine, spermidine and spermine are essential for both normal and neoplastic cell function and replication. Dysregulation of metabolism of polyamines and their requirements is common in many cancers. Both clinical and experimental depletion of polyamines have demonstrated their metabolism to be a rational target for therapy; however, the mechanisms through which polyamines can establish a tumour-permissive microenvironment are only now emerging. Recent data indicate that polyamines can play a major role in regulating the antitumour immune response, thus likely contributing to the existence of immunologically 'cold' tumours that do not respond to immune checkpoint blockade. Additionally, the interplay between the microbiota and associated tissues creates a tumour microenvironment in which polyamine metabolism, content and function can all be dramatically altered on the basis of microbiota composition, dietary polyamine availability and tissue response to its surrounding microenvironment. The goal of this Perspective is to introduce the reader to the many ways in which polyamines, polyamine metabolism, the microbiota and the diet interconnect to establish a tumour microenvironment that facilitates the initiation and progression of cancer. It also details ways in which polyamine metabolism and function can be successfully targeted for therapeutic benefit, including specifically enhancing the antitumour immune response.

Fig. 1 |. Oncogenic regulation of polyamine metabolism and uptake and sources of extracellular polyamines in the TME.

The elevated intracellular polyamine pools required of cancer cells are maintained by oncogenes, including MYC, KRAS and BRAF, through increasing biosynthesis and uptake and decreasing catabolism. a | Putrescine is synthesized by ornithine decarboxylase (ODC), a rate-limiting enzyme inhibited by α-difluoromethylornithine (DFMO). S-Adenosylmethionine decarboxylase (AdoMetDC) produces the aminopropyl group necessary for spermidine synthase (SRM) and spermine synthase (SRS) activities. Spermidine/spermine N¹-acetyltransferase (SSAT) acetylates the N¹ position of spermidine or spermine, allowing either export or oxidative back-conversion by peroxisomal acetylpolyamine oxidase (PAOX). Alternatively, spermine can be directly catabolized to spermidine by spermine oxidase (SMOX). By-products of PAOX and SMOX activity, including H₂O₂, 3-aminopropanal (3-AP) and 3-acetamidopropanal (3-AAP), can result in oxidative stress^,. SSAT and SMOX are induced by polyamine analogues such as N¹,N¹¹-bis(ethyl) norspermine (BENSpm). Polyamine uptake can be blocked by polyamine transport inhibitors (PTIs). b | Extracellular polyamines originate from the diet, microbiota, and sloughed or damaged cells. Most luminal polyamines passively diffuse into the circulation through the proximal portion of the small intestine, while some are actively transported into intestinal epithelial cells (IECs), where they may be interconverted via the polyamine metabolic enzymes or excreted as acetylated polyamines. Active import occurs at both apical and basolateral IEC membranes via the polyamine transport system (PTS)^,. A decreasing expression gradient of antizyme (OAZ), a regulator of both ODC activity and polyamine transport, exists in enterocytes along the crypt–villus axis and correlates with an inverse gradient in ODC activity, suggesting a similar gradient of polyamine uptake. Polyamines and their metabolites entering the circulation can be used by cells throughout the body, thereby affecting tumour microenvironments (TMEs) at distant sites. L-Orn, L-ornithine; dcAdoMet, decarboxylated S-adenosylmethionine; put, putrescine; spd, spermidine; spm, spermine.

Fig. 2 |. Influence of polyamines and their modulation on immune cells in the TME.

Tumour cells maintain elevated polyamine levels through uptake of extracellular polyamines and arginine. Arginine is converted to ornithine by arginase 1 (ARG1) and results in upregulation of ornithine decarboxylase (ODC) and polyamine biosynthesis. The increased intracellular polyamine pool promotes proliferation and survival of tumour cells. Macrophage polarization is mediated by arginine metabolism: conversion of arginine into nitric oxide by nitric oxide synthase (NOS) promotes a proimmune, antitumour M1 phenotype. M1 macrophages release IL-1β and TNF to promote the proliferation and survival of T cells. The cytokines IL-4 and IL-10 released by tumour cells promote M2 polarization by upregulation of ARG1 (REF. ). M2 macrophages lack the ability of M1 macrophages to make nitric oxide and alternatively use upregulated ARG1 to convert arginine into ornithine^,,. M2 macrophages therefore compete with effector T cells for the arginine and glutamine required for T cell function while also producing the immunosuppressive cytokines IL-4 and IL-16 (REFS^76,85,186). Arginine and polyamines in the tumour microenvironment (TME) can be taken up by dendritic cells to increase intracellular polyamine content. This induces indoleamine 2,3-dioxygenase 1 (IDO1) expression and contributes to an immunosuppressive phenotype. IDO1 metabolizes tryptophan (Trp), the metabolites of which inhibit receptor activation and increase apoptosis in T cells and natural killer (NK) cells. Increased polyamine content activates STAT3 in myeloid-derived suppressor cells (MDSCs) and promotes their survival. MDSCs produce nitric oxide and extreme levels of reactive oxygen species (ROS), leading to disruption of the interaction between the T cell receptor (TCR) and major histocompatibility complex-peptide complex and reducing the success of antigen presentation for the effector function of T cells. MDSCs also export polyamines to provide dendritic cells with additional polyamines to exacerbate IDO1 expression. MHCII, major histocompatibility complex class II; l-Orn, l-ornithine.

Fig. 3 |. Hypoxic and chronic infection/inflammatory microenvironments promote carcinogenic polyamine metabolism.

a | Hypoxic conditions stimulate both polyamine uptake and ornithine decarboxylase (ODC)-mediated polyamine biosynthesis, dramatically increasing tumour cell putrescine and spermidine levels. Extracellular spermine augments the hypoxia-initiated reduction in CD44 cell adhesion molecule expression, facilitating tumour cell migration, invasion and metastases. The polyamine catabolic enzyme spermidine/spermine N¹-acetyltransferase (SSAT) regulates the degradation of the master transcription factor hypoxia-inducible factor 1α (HIF1α) under aerobic conditions by stabilizing its interaction with RACK1 (REF. ). HIF1α also directly stimulates the transcription of spermine oxidase (SMOX), a nuclear and cytosolic enzyme capable of generating DNA-damaging reactive oxygen species. Acrolein originating from the SMOX reaction may facilitate cell migration by producing the pro-inflammatory chemokine CXC motif ligand 1 (CXCL1), which is recognized by CXCR2-expressing tumour-associated neutrophils, myeloid-derived suppressor cells and tumour cells. SMOX is negatively regulated by miR-124 (REF. ), expression of which is reduced in hypoxic tissues and is negatively correlated with a hypoxic gene signature^,. b | Exposure to chronic infection and inflammation induces changes in epithelial cell polyamine metabolism, particularly through inducing SMOX and its production of reactive oxygen species, resulting in DNA damage and epigenetic changes leading to neoplasia. Enhanced methylation of SMOX-targeting miR-124 genes is observed in patients at heightened risk of Helicobacter pylori-associated gastric cancer development. Immune and epithelial cell production of inflammatory cytokines in response to infection further stimulates polyamine metabolism. Extracellular polyamines may provide anti-inflammatory effects but at the potential risk of creating an immunosuppressive microenvironment conducive to selective outgrowth of transformed cells. 3-AP, 3-aminopropanal; MRE, microRNA-recognition element; ORF, open reading frame; l-Orn, l-ornithine.