Polyamines and Kynurenines at the Intersection of Immune Modulation

Polyamines (i.e., putrescine, spermidine, and spermine) are bioactive polycations derived from the decarboxylation of ʟ-ornithine catalyzed by ornithine decarboxylase (ODC).Emerging data suggest that spermidine exerts anti-inflammatory/immunoregulatory effects; it can protect mice from experimental autoimmune encephalomyelitis and imiquimod-induced psoriasis.Kynurenines are carboxylated products derived from the oxidative cleavage of the essential amino acid tryptophan, mediated by indoleamine 2,3-dioxygenase 1 (IDO1).Kynurenines are endowed with several immunoregulatory effects via the activation of the arylhydrocarbon receptor (AhR).Polyamines and kynurenines are emerging players in immunoregulatory networks involving AhR that could be considered in immunotherapy approaches to treat certain autoimmune diseases and cancers. Polyamines (i.e., putrescine, spermidine, and spermine) are bioactive polycations capable of binding nucleic acids and proteins and modulating signaling pathways. Polyamine functions have been studied most extensively in tumors, where they can promote cell transformation and proliferation. Recently, spermidine was found to exert protective effects in an experimental model of multiple sclerosis (MS) and to confer immunoregulatory properties on dendritic cells (DCs), via the indoleamine 2,3-dioxygenase 1 (IDO1) enzyme. IDO1 converts l-tryptophan into metabolites, collectively known as kynurenines, endowed with several immunoregulatory effects via activation of the arylhydrocarbon receptor (AhR). Because AhR activation increases polyamine production, the emerging scenario has identified polyamines and kynurenines as actors of an immunoregulatory circuitry with potential implications for immunotherapy in autoimmune diseases and cancer. Polyamines (i.e., putrescine, spermidine, and spermine) are bioactive polycations capable of binding nucleic acids and proteins and modulating signaling pathways. Polyamine functions have been studied most extensively in tumors, where they can promote cell transformation and proliferation. Recently, spermidine was found to exert protective effects in an experimental model of multiple sclerosis (MS) and to confer immunoregulatory properties on dendritic cells (DCs), via the indoleamine 2,3-dioxygenase 1 (IDO1) enzyme. IDO1 converts l-tryptophan into metabolites, collectively known as kynurenines, endowed with several immunoregulatory effects via activation of the arylhydrocarbon receptor (AhR). Because AhR activation increases polyamine production, the emerging scenario has identified polyamines and kynurenines as actors of an immunoregulatory circuitry with potential implications for immunotherapy in autoimmune diseases and cancer. Amino acid metabolism generates polyamines and kynurenines that exert multiple functions necessary for cell life. In mammals, three polyamines are synthesized, namely, putrescine, spermidine, and spermine. When their metabolism is dysregulated, several biological processes are altered, including cell proliferation and differentiation, signal transduction, and protein synthesis [1.Pegg A.E. Functions of polyamines in mammals.J. Biol. Chem. 2016; 291: 14904-14912Abstract Full Text Full Text PDF PubMed Scopus (171) Google Scholar,2.Pendeville H. et al.The ornithine decarboxylase gene is essential for cell survival during early murine development.Mol. Cell. Biol. 2001; 21: 6549-6558Crossref PubMed Scopus (0) Google Scholar]. Such a pleiotropic nature relies on the ability of the polyamines to bind negatively charged macromolecules (i.e., nucleic acids, proteins, and phospholipids) [3.Watanabe S. et al.Estimation of polyamine binding to macromolecules and ATP in bovine lymphocytes and rat liver.J. Biol. Chem. 1991; 266: 20803-20809Abstract Full Text PDF PubMed Google Scholar]. Polyamines can enhance the efficiency and fine-tune the translation of a specific set of genes belonging to the so-called 'polyamine modulon' (see Glossary) [4.Matsufuji S. et al.Autoregulatory frameshifting in decoding mammalian ornithine decarboxylase antizyme.Cell. 1995; 80: 51-60Abstract Full Text PDF PubMed Scopus (383) Google Scholar], which includes those encoding the eukaryotic translation factor 5A (eIF5A; in mammals) and antizyme 1 (AZ1), a key molecule involved in the autoregulatory circuitry of polyamine metabolism in eukaryotic cells, including mouse and human. Moreover, spermidine, via transformation into hypusine, promotes hypusination and, thus, activation of eIF5A [5.Mandal A. et al.Genome-wide analyses and functional classification of proline repeat-rich proteins: potential role of eIF5A in eukaryotic evolution.PLoS One. 2014; 9e111800Crossref PubMed Scopus (35) Google Scholar,6.Park M.H. et al.Functional significance of eIF5A and its hypusine modification in eukaryotes.Amino Acids. 2010; 38: 491-500Crossref PubMed Scopus (199) Google Scholar]. In colon tumors, the RAS-RAF-MEK-ERK signaling pathway has been associated with increased polyamine transport and, therefore, altered polyamine metabolism [7.Casero Jr., R.A. et al.Polyamine metabolism and cancer: treatments, challenges and opportunities.Nat. Rev. Cancer. 2018; 18: 681-695Crossref PubMed Scopus (108) Google Scholar]. Hence, it is not surprising that efforts are being made to develop strategies that can limit the intracellular polyamine content, although with disappointing results so far [7.Casero Jr., R.A. et al.Polyamine metabolism and cancer: treatments, challenges and opportunities.Nat. Rev. Cancer. 2018; 18: 681-695Crossref PubMed Scopus (108) Google Scholar, 8.Geck R.C. et al.Inhibition of the polyamine synthesis enzyme ornithine decarboxylase sensitizes triple-negative breast cancer cells to cytotoxic chemotherapy.J. Biol. Chem. 2020; 295: 6263-6277Abstract Full Text Full Text PDF PubMed Scopus (4) Google Scholar, 9.Routhu N.K. et al.Polymeric prodrugs targeting polyamine metabolism inhibit Zika virus replication.Mol. Pharm. 2018; 15: 4284-4295Crossref PubMed Scopus (3) Google Scholar, 10.Reigada C. et al.Targeting polyamine transport in Trypanosoma cruzi.Eur. J. Med. Chem. 2018; 147: 1-6Crossref PubMed Scopus (7) Google Scholar]. Conversely, polyamines, particularly spermidine, can have a protective role in mouse experimental models of autoimmune diseases, including multiple sclerosis (MS) and psoriasis [11.Li G. et al.Spermidine suppresses inflammatory DC function by activating the FOXO3 pathway and counteracts autoimmunity.iScience. 2020; 23: 100807Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar,12.Yang Q. et al.Spermidine alleviates experimental autoimmune encephalomyelitis through inducing inhibitory macrophages.Cell Death Differ. 2016; 23: 1850-1861Crossref PubMed Scopus (37) Google Scholar] and their use has been proposed for therapeutically treating age-related disorders, such as cardiac hypertrophy and memory impairment [13.Eisenberg T. et al.Cardioprotection and lifespan extension by the natural polyamine spermidine.Nat. Med. 2016; 22: 1428-1438Crossref PubMed Scopus (313) Google Scholar,14.Gupta V.K. et al.Spermidine suppresses age-associated memory impairment by preventing adverse increase of presynaptic active zone size and release.PLoS Biol. 2016; 14e1002563Crossref PubMed Scopus (41) Google Scholar], or as a caloric restriction mimetic to slow aging [15.Madeo F. et al.Spermidine in health and disease.Science. 2018; 359eaan2788Crossref PubMed Scopus (180) Google Scholar, 16.Madeo F. et al.Spermidine: a physiological autophagy inducer acting as an anti-aging vitamin in humans?.Autophagy. 2019; 15: 165-168Crossref PubMed Scopus (22) Google Scholar, 17.Sacitharan P.K. et al.Spermidine restores dysregulated autophagy and polyamine synthesis in aged and osteoarthritic chondrocytes via EP300.Exp. Mol. Med. 2018; 50: 123Crossref PubMed Scopus (12) Google Scholar]. l-Tryptophan (Trp) is the rarest essential amino acid found in food. Apart from protein synthesis, Trp represents the precursor for an array of biologically active metabolites, mainly produced by the serotonin and kynurenine pathways [18.Grohmann U. Bronte V. Control of immune response by amino acid metabolism.Immunol. Rev. 2010; 236: 243-264Crossref PubMed Scopus (147) Google Scholar]. The kynurenine pathway (KP), which accounts for the catabolism of ~99% of ingested Trp, was originally identified with a role in the biogenesis of NAD+, an essential cofactor for redox reactions occurring in mitochondria. However, over the years, accumulating evidence has attributed several functions to kynurenines (compounds produced upstream of NAD+ along the KP), including the modulation of immune and neural responses, as well as of vascular tone, and tumor proliferation in mice and humans [19.Stone T.W. Darlington L.G. The kynurenine pathway as a therapeutic target in cognitive and neurodegenerative disorders.Br. J. Pharmacol. 2013; 169: 1211-1227Crossref PubMed Scopus (112) Google Scholar,20.Kolodziej L.R. et al.Kynurenine metabolism in health and disease.Amino Acids. 2011; 41: 1173-1183Crossref PubMed Scopus (42) Google Scholar]. Moreover, kynurenines have been shown to trigger apoptosis in several cell types, including neurons [21.Okuda S. et al.3-Hydroxykynurenine, an endogenous oxidative stress generator, causes neuronal cell death with apoptotic features and region selectivity.J. Neurochem. 1998; 70: 299-307Crossref PubMed Google Scholar], endothelial cells [22.Wang Y. et al.Kynurenine is an endothelium-derived relaxing factor produced during inflammation.Nat. Med. 2010; 16: 279-285Crossref PubMed Scopus (250) Google Scholar], and activated human CD3+ T lymphocytes [23.Terness P. et al.Inhibition of allogeneic T cell proliferation by indoleamine 2,3-dioxygenase-expressing dendritic cells: mediation of suppression by tryptophan metabolites.J. Exp. Med. 2002; 196: 447-457Crossref PubMed Scopus (706) Google Scholar]. Apoptosis in T lymphocytes appears to be caused by inhibition of the PDK1 kinase and consequent inhibition of the proinflammatory NF-κB pathway in a mouse experimental model of asthma [24.Hayashi T. et al.3-Hydroxyanthranilic acid inhibits PDK1 activation and suppresses experimental asthma by inducing T cell apoptosis.Proc. Natl. Acad. Sci. U. S. A. 2007; 104: 18619-18624Crossref PubMed Scopus (108) Google Scholar]. Kynurenines also induce apoptosis in human natural killer (NK) cells via reactive oxygen species (ROS) [25.Song H. et al.L-kynurenine-induced apoptosis in human NK cells is mediated by reactive oxygen species.Int. Immunopharmacol. 2011; 11: 932-938Crossref PubMed Scopus (59) Google Scholar]. However, the most important effect of kynurenines appears to be immune regulation, mainly mediated by the aryl hydrocarbon receptor (AhR [26.Mezrich J.D. et al.An interaction between kynurenine and the aryl hydrocarbon receptor can generate regulatory T cells.J. Immunol. 2010; 185: 3190-3198Crossref PubMed Scopus (698) Google Scholar, 27.Nguyen N.T. et al.Aryl hydrocarbon receptor negatively regulates dendritic cell immunogenicity via a kynurenine-dependent mechanism.Proc. Natl. Acad. Sci. U. S. A. 2010; 107: 19961-19966Crossref PubMed Scopus (381) Google Scholar, 28.Opitz C.A. et al.An endogenous tumour-promoting ligand of the human aryl hydrocarbon receptor.Nature. 2011; 478: 197-203Crossref PubMed Scopus (934) Google Scholar], a ligand-activated transcription factor with a major role in health and disease in both mouse and human [29.Rothhammer V. Quintana F.J. The aryl hydrocarbon receptor: an environmental sensor integrating immune responses in health and disease.Nat. Rev. Immunol. 2019; 19: 184-197Crossref PubMed Scopus (109) Google Scholar]. Although addressed separately, polyamines and kynurenines have been the subject of recent reviews [16.Madeo F. et al.Spermidine: a physiological autophagy inducer acting as an anti-aging vitamin in humans?.Autophagy. 2019; 15: 165-168Crossref PubMed Scopus (22) Google Scholar,30.Cervenka I. et al.Kynurenines: tryptophan's metabolites in exercise, inflammation, and mental health.Science. 2017; 357eaaf9794Crossref PubMed Scopus (212) Google Scholar]. Here, we provide evidence that the two structurally distinct sets of molecules may be part of a unique immunomodulatory circuitry requiring AhR activation and, thus, may be amenable for pharmacological interventions. The first step of polyamine biosynthesis comprises the decarboxylation of ʟ-ornithine (Orn) [i.e., an intermediate generated by the arginase 1 (ARG1) enzyme from ʟ-arginine (Arg)] (Figure 1). The conversion of Orn into putrescine is catalyzed by the rate-limiting enzyme ornithine decarboxylase 1 (ODC1), a pyridoxal phosphate (PLP)-depending enzyme the expression of which is induced in response to many stimuli, such as hormones, oncogenes, growth factors, and polyamines themselves. ODC1 activity is controlled at the transcriptional, translational, and post-translational levels [31.Kahana C. The antizyme family for regulating polyamines.J. Biol. Chem. 2018; 293: 18730-18735Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. In fact, ODC1 is subjected to a unique mechanism of ubiquitin-independent proteasomal degradation, which implicates the antizyme 1 (AZ1) and ODC antizyme inhibitor (AZIN1) proteins (Box 1) [31.Kahana C. The antizyme family for regulating polyamines.J. Biol. Chem. 2018; 293: 18730-18735Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. The next step of polyamine biosynthesis is catalyzed by spermidine synthase (SPDS), giving rise to spermidine, which is converted into spermine by spermine synthase (SPMS) (Figure 1).Box 1Controlled Proteolysis as a Common Fate of ODC1, IDO1, and AhR in MammalsDuring homeostasis, the production of polyamines and kynurenines is tightly regulated. In addition to transcriptional and translational regulations, ODC1 (the rate-limiting enzyme in polyamine production) is subjected to a unique mechanism of ubiquitin-independent proteasomal degradation [31.Kahana C. The antizyme family for regulating polyamines.J. Biol. Chem. 2018; 293: 18730-18735Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar,105.Pegg A.E. Regulation of ornithine decarboxylase.J. Biol. Chem. 2006; 281: 14529-14532Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. This mechanism includes binding of AZ1 to ODC1 subunits, preventing ODC1 dimerization and targeting ODC1 for proteolysis by the 26S proteasome without ubiquitination (Figure I) [109.Murakami Y. et al.Ornithine decarboxylase is degraded by the 26S proteasome without ubiquitination.Nature. 1992; 360: 597-599Crossref PubMed Scopus (644) Google Scholar]. Ubiquitin-independent proteasomal degradation applies to a low number of proteins and may represent a mechanism reminiscent of the archetypal mode of targeting that occurs in prokaryotes [110.Verma R. Deshaies R.J. A proteasome howdunit: the case of the missing signal.Cell. 2000; 101: 341-344Abstract Full Text Full Text PDF PubMed Google Scholar]. AZ1 itself is regulated by AZIN1, a protein highly homologous to ODC1 but lacking Orn-decarboxylating activity.IDO1, the rate-limiting enzyme of the KP, is also subjected to regulatory proteolysis, although this occurs in a 'canonical', ubiquitin-dependent fashion [62.Orabona C. et al.SOCS3 drives proteasomal degradation of indoleamine 2,3-dioxygenase (IDO) and antagonizes IDO-dependent tolerogenesis.Proc. Natl. Acad. Sci. U. S. A. 2008; 105: 20828-20833Crossref PubMed Scopus (127) Google Scholar,88.Pallotta M.T. et al.Indoleamine 2,3-dioxygenase is a signaling protein in long-term tolerance by dendritic cells.Nat. Immunol. 2011; 12: 870-878Crossref PubMed Scopus (393) Google Scholar]. In mouse DCs, in the presence of IL-6, IDO1 is phosphorylated by kinases of the Src family at two immunoreceptor tyrosine-based inhibitory motifs (ITIMs) present in the small, noncatalytic domain of the enzyme. In turn, the phosphorylated IDO1 ITIMs become docking sites for the suppressor of cytokine signaling 3 (SOCS3), containing a SOCS box domain that participates in the formation of an E3 ubiquitin ligase complex [111.Orr S.J. et al.CD33 responses are blocked by SOCS3 through accelerated proteasomal-mediated turnover.Blood. 2007; 109: 1061-1068Crossref PubMed Scopus (65) Google Scholar]. The importance of the ubiquitin-dependent proteolysis of IDO1 is sustained by the presence of accelerated proteasomal degradation of the enzyme in nonobese (NOD) mice, an experimental model of human autoimmune type 1 diabetes (T1D), which can be counteracted by treatment with bortezomib (an inhibitor of proteasomal degradation used in multiple myeloma) thus conferring immunosuppressive properties on NOD DCs in blocking DTH responses in vivo and reducing diabetes incidence in NOD mice [112.Mondanelli G. et al.The proteasome inhibitor bortezomib controls indoleamine 2,3-dioxygenase 1 breakdown and restores immune regulation in autoimmune diabetes.Front. Immunol. 2017; 8: 428Crossref PubMed Scopus (13) Google Scholar]. Moreover, in children with T1D, deficiency of IDO1 expression in blood cells can be ascribed to accelerated proteasomal degradation, which can be reduced by either proteasomal inhibitors or tocilizumab, an IL-6 receptor blocker [113.Orabona C. et al.Deficiency of immunoregulatory indoleamine 2,3-dioxygenase 1 in juvenile diabetes.JCI Insight. 2018; 3e96244Crossref PubMed Scopus (20) Google Scholar].Besides being itself the target of ubiquitination and proteasomal degradation [114.Ma Q. Baldwin K.T. 2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced degradation of aryl hydrocarbon receptor (AhR) by the ubiquitin-proteasome pathway. Role of the transcription activation and DNA binding of AhR.J. Biol. Chem. 2000; 275: 8432-8438Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar], AhR has a ligand-dependent, intrinsic E3 ubiquitin ligase function that targets proteins for proteasomal degradation [115.Shinde R. McGaha T.L. The aryl hydrocarbon receptor: connecting immunity to the microenvironment.Trends Immunol. 2018; 39: 1005-1020Abstract Full Text Full Text PDF PubMed Scopus (37) Google Scholar]. The AhR nuclear translocator (ARNT), essential for the nuclear translocation and transcriptional activity of AhR, is not involved in the E3 ubiquitin ligase function. Rather, in the presence of the repressor of AhR function (AhRR), which competes with ARNT for AhR binding, the nuclear translocation of AhR is blocked and the E3 ubiquitin ligase function of the receptor is switched on [116.Luecke-Johansson S. et al.A molecular mechanism to switch the aryl hydrocarbon receptor from a transcription factor to an E3 ubiquitin ligase.Mol. Cell. Biol. 2017; 37e00630-16Crossref PubMed Scopus (12) Google Scholar]. Although the conditions favoring the expression of ARNT versus AhR are still unclear, AhR will impair IDO1 expression via direct proteasomal degradation of the enzyme [78.Pallotta M.T. et al.AhR-mediated, non-genomic modulation of IDO1 function.Front. Immunol. 2014; 5: 497Crossref PubMed Scopus (22) Google Scholar] or Smad4 [117.Lee C.C. et al.Ligand independent aryl hydrocarbon receptor inhibits lung cancer cell invasion by degradation of Smad4.Cancer Lett. 2016; 376: 211-217Crossref PubMed Google Scholar], a protein involved in the signaling pathway of TGF-β [88.Pallotta M.T. et al.Indoleamine 2,3-dioxygenase is a signaling protein in long-term tolerance by dendritic cells.Nat. Immunol. 2011; 12: 870-878Crossref PubMed Scopus (393) Google Scholar], an IDO1 inducer [61.Belladonna M.L. et al.Cutting edge: autocrine TGF-beta sustains default tolerogenesis by IDO-competent dendritic cells.J. Immunol. 2008; 181: 5194-5198Crossref PubMed Google Scholar].Figure IMechanisms of Proteasomal Degradation of ODC1, AhR, and IDO1.Show full captionAbbreviations: Ahr, arylhydrocarbon receptor; Ahrr, Ahr repressor; ARNT, AhR nuclear translocator; AZ1, antizyme 1; AZIn1, ODC antizyme inhibitor; IDO1, indoleamine 2,3-dioxygenase 1; ODC1, ornithine decarboxylase 1; SOCS3, suppressor of cytokine signaling 3.View Large Image Figure ViewerDownload Hi-res image Download (PPT) During homeostasis, the production of polyamines and kynurenines is tightly regulated. In addition to transcriptional and translational regulations, ODC1 (the rate-limiting enzyme in polyamine production) is subjected to a unique mechanism of ubiquitin-independent proteasomal degradation [31.Kahana C. The antizyme family for regulating polyamines.J. Biol. Chem. 2018; 293: 18730-18735Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar,105.Pegg A.E. Regulation of ornithine decarboxylase.J. Biol. Chem. 2006; 281: 14529-14532Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar]. This mechanism includes binding of AZ1 to ODC1 subunits, preventing ODC1 dimerization and targeting ODC1 for proteolysis by the 26S proteasome without ubiquitination (Figure I) [109.Murakami Y. et al.Ornithine decarboxylase is degraded by the 26S proteasome without ubiquitination.Nature. 1992; 360: 597-599Crossref PubMed Scopus (644) Google Scholar]. Ubiquitin-independent proteasomal degradation applies to a low number of proteins and may represent a mechanism reminiscent of the archetypal mode of targeting that occurs in prokaryotes [110.Verma R. Deshaies R.J. A proteasome howdunit: the case of the missing signal.Cell. 2000; 101: 341-344Abstract Full Text Full Text PDF PubMed Google Scholar]. AZ1 itself is regulated by AZIN1, a protein highly homologous to ODC1 but lacking Orn-decarboxylating activity. IDO1, the rate-limiting enzyme of the KP, is also subjected to regulatory proteolysis, although this occurs in a 'canonical', ubiquitin-dependent fashion [62.Orabona C. et al.SOCS3 drives proteasomal degradation of indoleamine 2,3-dioxygenase (IDO) and antagonizes IDO-dependent tolerogenesis.Proc. Natl. Acad. Sci. U. S. A. 2008; 105: 20828-20833Crossref PubMed Scopus (127) Google Scholar,88.Pallotta M.T. et al.Indoleamine 2,3-dioxygenase is a signaling protein in long-term tolerance by dendritic cells.Nat. Immunol. 2011; 12: 870-878Crossref PubMed Scopus (393) Google Scholar]. In mouse DCs, in the presence of IL-6, IDO1 is phosphorylated by kinases of the Src family at two immunoreceptor tyrosine-based inhibitory motifs (ITIMs) present in the small, noncatalytic domain of the enzyme. In turn, the phosphorylated IDO1 ITIMs become docking sites for the suppressor of cytokine signaling 3 (SOCS3), containing a SOCS box domain that participates in the formation of an E3 ubiquitin ligase complex [111.Orr S.J. et al.CD33 responses are blocked by SOCS3 through accelerated proteasomal-mediated turnover.Blood. 2007; 109: 1061-1068Crossref PubMed Scopus (65) Google Scholar]. The importance of the ubiquitin-dependent proteolysis of IDO1 is sustained by the presence of accelerated proteasomal degradation of the enzyme in nonobese (NOD) mice, an experimental model of human autoimmune type 1 diabetes (T1D), which can be counteracted by treatment with bortezomib (an inhibitor of proteasomal degradation used in multiple myeloma) thus conferring immunosuppressive properties on NOD DCs in blocking DTH responses in vivo and reducing diabetes incidence in NOD mice [112.Mondanelli G. et al.The proteasome inhibitor bortezomib controls indoleamine 2,3-dioxygenase 1 breakdown and restores immune regulation in autoimmune diabetes.Front. Immunol. 2017; 8: 428Crossref PubMed Scopus (13) Google Scholar]. Moreover, in children with T1D, deficiency of IDO1 expression in blood cells can be ascribed to accelerated proteasomal degradation, which can be reduced by either proteasomal inhibitors or tocilizumab, an IL-6 receptor blocker [113.Orabona C. et al.Deficiency of immunoregulatory indoleamine 2,3-dioxygenase 1 in juvenile diabetes.JCI Insight. 2018; 3e96244Crossref PubMed Scopus (20) Google Scholar]. Besides being itself the target of ubiquitination and proteasomal degradation [114.Ma Q. Baldwin K.T. 2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced degradation of aryl hydrocarbon receptor (AhR) by the ubiquitin-proteasome pathway. Role of the transcription activation and DNA binding of AhR.J. Biol. Chem. 2000; 275: 8432-8438Abstract Full Text Full Text PDF PubMed Scopus (0) Google Scholar], AhR has a ligand-dependent, intrinsic E3 ubiquitin ligase function that targets proteins for proteasomal degradation [115.Shinde R. McGaha T.L. The aryl hydrocarbon receptor: connecting immunity to the microenvironment.Trends Immunol. 2018; 39: 1005-1020Abstract Full Text Full Text PDF PubMed Scopus (37) Google Scholar]. The AhR nuclear translocator (ARNT), essential for the nuclear translocation and transcriptional activity of AhR, is not involved in the E3 ubiquitin ligase function. Rather, in the presence of the repressor of AhR function (AhRR), which competes with ARNT for AhR binding, the nuclear translocation of AhR is blocked and the E3 ubiquitin ligase function of the receptor is switched on [116.Luecke-Johansson S. et al.A molecular mechanism to switch the aryl hydrocarbon receptor from a transcription factor to an E3 ubiquitin ligase.Mol. Cell. Biol. 2017; 37e00630-16Crossref PubMed Scopus (12) Google Scholar]. Although the conditions favoring the expression of ARNT versus AhR are still unclear, AhR will impair IDO1 expression via direct proteasomal degradation of the enzyme [78.Pallotta M.T. et al.AhR-mediated, non-genomic modulation of IDO1 function.Front. Immunol. 2014; 5: 497Crossref PubMed Scopus (22) Google Scholar] or Smad4 [117.Lee C.C. et al.Ligand independent aryl hydrocarbon receptor inhibits lung cancer cell invasion by degradation of Smad4.Cancer Lett. 2016; 376: 211-217Crossref PubMed Google Scholar], a protein involved in the signaling pathway of TGF-β [88.Pallotta M.T. et al.Indoleamine 2,3-dioxygenase is a signaling protein in long-term tolerance by dendritic cells.Nat. Immunol. 2011; 12: 870-878Crossref PubMed Scopus (393) Google Scholar], an IDO1 inducer [61.Belladonna M.L. et al.Cutting edge: autocrine TGF-beta sustains default tolerogenesis by IDO-competent dendritic cells.J. Immunol. 2008; 181: 5194-5198Crossref PubMed Google Scholar]. Over the past few decades, polyamine catabolism, which involves the back-conversion of the higher polyamines to putrescine, has gained increased importance because it might greatly influence the total cellular amount of each polyamine [32.Casero R.A. Pegg A.E. Polyamine catabolism and disease.Biochem. J. 2009; 421: 323-338Crossref PubMed Scopus (242) Google Scholar]. Both spermine and spermidine are reconverted into spermidine or putrescine, respectively, by two consecutive reactions, namely, an acetylation reaction catalyzed by spermidine/spermine N-1-acetyltransferase (SSAT), and an oxidation reaction mediated by acetylpolyamine oxidase (APAO) (Figure 1) [32.Casero R.A. Pegg A.E. Polyamine catabolism and disease.Biochem. J. 2009; 421: 323-338Crossref PubMed Scopus (242) Google Scholar]. Polyamine content regulates SSAT activity at multiple levels, including transcription, mRNA processing, mRNA translation, and protein degradation [32.Casero R.A. Pegg A.E. Polyamine catabolism and disease.Biochem. J. 2009; 421: 323-338Crossref PubMed Scopus (242) Google Scholar]. Moreover, SSAT expression can be induced by a variety of stimuli, including toxins, hormones, cytokines, natural products such as resveratrol, stress pathways, and ischemia-reperfusion injury in rats [33.Pegg A.E. Spermidine/spermine-N(1)-acetyltransferase: a key metabolic regulator.Am. J. Physiol. Endocrinol. Metab. 2008; 294: E995-E1010Crossref PubMed Scopus (190) Google Scholar]. In contrast to SSAT, APAO is constitutively expressed in human normal tissues and its activity is limited by the availability of acetylated intermediates generated by SSAT [33.Pegg A.E. Spermidine/spermine-N(1)-acetyltransferase: a key metabolic regulator.Am. J. Physiol. Endocrinol. Metab. 2008; 294: E995-E1010Crossref PubMed Scopus (190) Google Scholar]. Moreover, spermine can be reconverted into spermidine via direct oxidation catalyzed by spermine oxidase (SMOX) (Figure 1) [34.Cervelli M. et al.Spermine oxidase: ten years after.Amino Acids. 2012; 42: 441-450Crossref PubMed Scopus (53) Google Scholar]. Given that polyamine catabolism can generate toxic ROS through the activity of both APAO and SMOX oxidases [32.Casero R.A. Pegg A.E. Polyamine catabolism and disease.Biochem. J. 2009; 421: 323-338Crossref PubMed Scopus (242) Google Scholar], a fine regulation of polyamine metabolism is mandatory, not only for balancing polyamine amounts, but also to prevent cellular oxidative stress and damage [35.Wang Y. et al.Cloning and characterization of a human polyamine oxidase that is inducible by polyamine analogue exposure.Cancer Res. 2001; 61: 5370-5373PubMed Google Scholar]. Due to their cationic nature, polyamines require transport systems to cross cell membranes. The solute carrier family (SLC), the organic cation transporter (OCT), and the cationic amino acid transporter (CAT) are the proposed carriers responsible for polyamine flux [36.Igarashi K. Kashiwagi K. Characteristics of cellular polyamine transport in prokaryotes and eukaryotes.Plant Physiol. Biochem. 2010; 48: 506-512Crossref PubMed Scopus (104) Google Scholar]. SLC family members can also provide polyamine secretion, by exchanging putrescine and acetylated spermidine with extracellular Arg [37.Uemura T. et al.Identification and characterization of a diamine exporter in colon epithelial cells.J. Biol. Chem. 2008; 283: 26428-26435Crossref PubMed Scopus (64) Google Scholar]. A peculiar subtype of SLC, SLC18B1, mediates the vesicular storage and subsequent exocytosis of spermine and spermidine in astrocyte cultures obtained from embryonic rat, and in mouse bone marrow-derived mast cells,