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Atherosclerosis (AS), a major pathologic consequence of obesity, is the main etiological factor of cardiovascular disease (CVD), which is the most common cause of death in the western world. A systemic chronic low grade immune-mediated inflammation (scLGI) is substantially implicated in AS and its consequences. In particular, pro-inflammatory cytokines play a major role, with Th1-type cytokine interferon-γ (IFN-γ) being a key mediator. Among various other molecular and cellular effects, IFN- γ activates the enzyme indoleamine 2,3-dioxygenase (IDO) in monocyte-derived macrophages, dendritic, and other cells, which, in turn, decreases serum levels of the essential amino acid tryptophan (TRP). Thus, people with CVD often have increased serum kynurenine to tryptophan ratios (KYN/TRP), a result of an increased TRP breakdown. Importantly, increased KYN/TRP is associated with a higher likelihood of fatal cardiovascular events. A scLGI with increased production of the proinflammatory adipokine leptin, in combination with IFN-γ and interleukin-6 (IL-6), represents another central link between obesity, AS, and CVD. Leptin has also been shown to contribute to Th1-type immunity shifting, with abdominal fat being thus a direct contributor to KYN/TRP ratio. However, TRP is not only an important source for protein production but also for the generation of one of the most important neurotransmitters, 5-hydroxytryptamine (serotonin), by the tetrahydrobiopterin-dependent TRP 5-hydroxylase. In prolonged states of scLGI, availability of free serum TRP is strongly diminished, affecting serotonin synthesis, particularly in the brain. Additionally, accumulation of neurotoxic KYN metabolites such as quinolinic acid produced by microglia, can contribute to the development of depression via NMDA glutamatergic stimulation. Depression had been reported to be associated with CVD endpoints, but it most likely represents only a secondary loop connecting excess adipose tissue, scLGI and cardiovascular morbidity and mortality. Accelerated catabolism of TRP is further involved in the pathogenesis of the anemia of scLGI. The pro-inflammatory cytokine IFN-γ suppresses growth and differentiation of erythroid progenitor cells, and the depletion of TRP limits protein synthesis and thus hemoglobin production, and, through reduction in oxygen supply, may contribute to ischemic vascular disease. In this review we discuss the impact of TRP breakdown and the related complex mechanisms on the prognosis and individual course of CVD. Measurement of TRP, KYN concentrations, and calculation of the KYN/TRYP ratio will contribute to a better understanding of the interplay between inflammation, metabolic syndrome, mood disturbance, and anemia, all previously described as significant predictors of an unfavorable outcome in patients with CVD. The review leads to a novel framework for successful therapeutic modification of several cardinal pathophysiological processes leading to adverse cardiovascular outcome.
L-tryptophan (TRP) is an essential amino acid for the biosynthesis of proteins. The following products of TRP are involved in other important biologic processes: (i) 5-hydroxytryptamine, serotonin, formed by TRP 5-hydroxylase (T5H) following decarboxylation, (ii) kynurenine, formed by TRP 2,3-dioxygenase (TDO); and indoleamine 2,3-dioxygenase (IDO) (Fig. 1).
Tryptophan 2,3-dioxygenase and IDO initiate the first step in the catabolism of TRP. This is called the “kynurenine-pathway” leading to nicotinic acid, the vitamin niacin, and nicotinamide adenine dinucleotides, as end products . Tryptophan 2,3-dioxygenase originates predominantly in the liver and can be induced by a variety of molecules (e.g. cortisol). Indoleamine 2,3-dioxygenase is expressed by various cells, and its main inducer is the Th1-type cytokine IFN-γ [2–5]. Both, TDO and IDO catalyze the first step in TRP breakdown, leading to the formation of N-formylkynurenine which is subsequently deformylated to kynurenine (KYN). Interferon-γ is the most potent in vitro and in vivo stimulator for IDO. Tryptophan 2,3-dioxygenasebreakdown was detected in antigen-presenting cells like monocyte-derived macrophages, dendritic cells, fibroblasts and other cells [2–5] (Fig. 2) as well as in humans after injection of IFN-γ .
Although less effective, other cytokines or lipopolysaccharides may also induce IDO [6–8]. An in vivo, enhanced cytokine-induced breakdown of TRP is usually seen in context of a cellular Th1-weighted immune response. Thus, a decrease of the serum TRP concentration is accompanied by increased KYN or other TRP catabolites [6, 9, 10]. Under physiologic conditions, the KYN levels are related to TRP concentrations. A reduced nutritional intake of TRP decreases endogenous TRP levels, and lower KYN blood levels are detected. The KYN/TRP ratio is a reasonable indicator of the TRP breakdown, and provides a better approach than the analysis of the absolute TRP or KYN concentrations. A concomitant immune system activation is essential to refer TRP breakdown to the activation of IDO and not to TDO [11, 12]. Thus, activated IDO is present when KYN/TRP and immune activation parameters, especially IFN-γ are positively correlated. Neopterin, which can be easily measured in body fluids or cell culture supernatants, is another sensitive diagnostic marker to monitor a Th1-type immune activation in humans and animal models .
Elevated neopterin concentration is also an independent marker for CVD and a predictor of future cardiovascular events in patients with coronary artery disease . In patients with stable angina pectoris, systemic markers of IFN-γ activity, plasma neopterin, and plasma KYN/TRP provide similar risk estimates for major coronary events and mortality. Interferon-γ levels were positively correlated with acute atherosclerotic complications , and KYN/TRP levels at admission predicted the later outcome in patients with acute stroke . Obesity and metabolic syndrome (MetS) are important conditions associated with an increased cardiovascular risk. We showed recently that the TRP-KYN metabolism was found to be essentially dysregulated in obese persons in dependence of age and parameters of the MetS .
Blood pressure control has a major influence on the clinical course of CVD. Induction of endotoxemia in mice led to increased endothelial expression of IDO, constitutively expressed in all endothelial cells, with decreased TRP/KYN and reduced blood pressure . Pharmacological inhibition of IDO increased blood pressure in these mice but not in mice deficient in either IDO or IFN-γ which is required for IDO induction . Further, both TRP and KYN dilated preconstricted porcine coronary arteries . Whereas the dilating effect of TRP required the presence of active IDO and an intact endothelium, the effect of KYN was endothelium independent. The KYN induced arterial relaxation was mediated by activation of the adenylate and soluble guanylate cyclase pathways. Kynurenine administration decreased blood pressure in a dose-dependent manner in spontaneously hypertensive rats. Thus, IDO mediated TRP metabolism may be centrally involved in the regulatory mechanisms of the vascular tone . On the other hand, it was shown that an increased IDO activity (measured by KYN/TRP ratio in patients suffering from sepsis) caused a potent inhibition of the inducible nitric oxide synthase (iNOS) . Obviously, there is a complex interaction between the vasodilatators NO and IDO which appear to reciprocally inhibit each other. This interaction may have a major contribution to the microvascular reactivity in serious clinical conditions like myocardial infarction, sepsis and stroke. Specifically, activation of IDO and higher concentrations of several KYN metabolites have been observed post-stroke, where they have been associated with increased mortality and the development of post-stroke cognitive impairment . It sounds inconsistent that despite IDO and KYN act as arterial relaxing factors  they are associated with increased post-stroke mortality and cognitive impairment. A probable explanation is that both IDO and KYN are produced due to inflammation as a kind of feedback mechanism to counteract the vasoconstriction mediated by proinflammatory events. Nevertheless, this beneficial effect is probably too weak to achieve a sustained compensation of the negative effects of local and systemic inflammation in the wake of severe stroke events.
Tryptophan breakdown leads to reduced serotonin availability. Thus, increased TRP catabolism was proposed as a pathway leading to mood dysregulation . Furthermore, syndromal depression and adjustment disorder with depressed mood were proposed as contributors to CVD and myocardial infarction (“broken heart syndrome”) [21, 22]. Moreover, increased cardiovascular comorbidity and adverse cardiovascular eventendpoints have been reported in subjects with depressive mood disorders [23–25]. Reciprocally, evidence exists that an immune-mediated scLGI like seen in CVD, is also involved in the pathophysiology of mood disorders [25, 26], and in turn, episodes of mood disorders can result in elevated inflammation markers [27, 28]. This is confirmed by the observation that a sucessful treatment of depression and mania with antidepressive and antipsychotic drugs leads to a decreased activity of the systemic inflammation seen in these patients [29, 30].
Obesity and MetS are known contributors to increased mortality in patients with bipolar disorders [31–33]. Moreover, obesity worsens the course and prognosis of mood disorders, with longer episodes, shorter inter-episode intervals, and higher rates of suicidality . Proposed underlying mechanisms include a Th1-weighted T-helper cell activation shunting TRP from production of serotonin to KYN and its neuroactive metabolites. Details of the complex involvement of TRP metabolism in serotonin availability, mood alterations, inflammation and CVD are shown in (Fig. 3).
Multiple studies found evidence for TRP depletion as well as IDO activation in mood disorders [35–37]. Consistently, we recently reported that patients with bipolar disorder have elevated KYN and KYN/TRP ratios than mentally healthy control participants . Moreover, concentrations of KYN and KYN/TRP were significantly increased in overweight compared to normal-weight bipolar patients . Other studies demonstrated activated T-cells, presumably of the Th1 subtype, over the whole course of manic-depressive illness . Notably, the catabolic KYN pathway contributes to the neuroprotective–neurodegenerative/neurotoxic changes via production of kynureninic acid and quinolinic acid . Additionally, activation of the KYN pathway promotes generation of ROS and lipid peroxidation essentially involved in the oxidative stress as a consequence of chronic inflammation . These molecular changes, involved in both AS and mood disorders could underlie the bi-directional association between mood disorders and cardiovascular disease, as well as other inflammation-mediated medical conditions .
Considering the IDO-mediated interplay between TRP breakdown, Th1-type immune activation, and neuroendocrine alterations, it appears that mood alterations in CVD subjects are more likely the “egg” laid by a “hen” named cLGI caused by the atherosclerotic vascular burden and an obesogenic lifestyle.
Nevertheless, ascribe the mood alterations in CVD only on scLGI is not rigorous. In addition, the feeling of helplessness or lack of social support can also play important roles [43–47] although the disturbed neurotransmitter environment may again represent an important determinant which increases the susceptibilty for improper handling of such psychologic traits.
Melatonin, being derived from L-tryptophan, is produced by the pineal gland during the biological (“internal”) night driven by the master circadian pacemaker in the hypothalamus, and released in dim light or darkness . Impaired melatonin synthesis has been shown to be associated with age-related conditions including CVD [49–51]. A disrupted maturation of the photoneuroendocrine system caused by a genetic absence or mutation of genes involved in melatonin synthesis (including TRP hydroxylase) has been shown to cause an imbalance in the crosstalk among serotonin, progesterone, catecholamines and intracellular calcium . Hence, an impaired TRP catabolism with successive melatonin deficiency may favor CVD by abnormal hormone levels (e.g. blood pressure increase through water retention by aldosterone) . In addition, evidence is cumulating that melatonin has anti-inflammatory, antioxidant, antihypertensive, and possibly antilipidemic properties . For example, treatment with melatonin ameliorated the symptoms of MetS and decreased blood levels of low-density lipoprotein cholesterol (LDL-C) . Further, melatonin has been shown to significantly suppress the formation of cholesterol and reduce LDL-C in isolated human mononuclear leucocytes . In vitro studies have given evidence for antioxidant actions of melatonin on LDL-C oxidation . In keeping with this, Dominguez-Rodriguez et al.  reported a relationship between nocturnally elevated oxidized serum LDL-C and reduced circulating melatonin levels in patients with acute myocardial infarction. Hence, treatment with melatonin reduces blood pressure in rodents  and humans [55, 60]. Integrating the previously described evidence, it may be possible that decreased melatonin secretion secondary to increased TRP breakdown could represent an additional pathophysiological pathway towards CVD in patients with scLGI of various cause, including obesity.
Pro-inflammatory cytokines like IFN-γ and TNF-α suppress the growth and differentiation of erythroid progenitor cells . Patients with anemia of chronic inflammation had decreased TRP plasma levels positively correlated with a drop in hemoglobin . Thus, IFN-γ induced TRP deprivation may play a central role in the cytokine induced suppression of hematopoieses seen in patients with chronic systemic inflammation. With respect to scLGI in patients in coronary artery disease, anemia is probably “a player” rather than a “bystander” because it worsens the oxygenation of the myocardial tissue already reduced by the mechanical process of coronary obstruction . Additionally, the tachycardia of anemia increases the myocardial oxygen demand. This paves the way for a higher probability of fatal outcomes. Taken together, scLGI, anemia and immune (T-cell) dysfunction, based on an insufficient TRP supply due to IDO activation, may have a robust effect on mortality in CVD patients.
Systemic chronic low grade immune-mediated inflammation in aging induces alterations in two enzymatic pathways, the IDO and the guanosine-triphosphate-cyclohydrolase-1 (GTP-CH1) pathways, both involved in the biosynthesis of monoamines  and potential neuropsychiatric symptoms. KYN/TRP was found significantly higher in nonagenarians compared with controls . In a similar way elevated neopterin levels have been described in nonagenerians and being associated with a shorter residual life span . Further, IDO activity predicted subsequent mortality in nonagenarians . As an important regulatory mechanism in the immune system, increased IDO activity downregulates T cell functions. As aging of the immune system has been previously associated with a decline in T cell function , it seems justified to consider increased IDO activity as an important mechanism involved in the decline of T cell responses in immunosenescence . Thus, accelerated TRP catabolism could be a modifiable step on this causal pathway. As T-cells play an important role in the process of progression of CVD towards MI, a suppressed T cell function is expected to be beneficial rather than detrimental to the incidence of coronary events, conceptually supported by the low MI rates in subjects > 85 years. On the other hand, IDO may have a detrimental role in early AS, particularly in young female adults . In these subjects, IDO activity correlated significantly with several risk factors for AS, i.e. with LDL-cholesterol, BMI, and inversely with HDL-C and triglyceride. Thus, IDO may be involved in the immune regulation of AS in a gender and age dependent way.
Raitala et al. reported that the TRP catabolism appears to be genetically moderated by the IFN-γ gene and may thus be operative in disease conditions associated with certain IFN-γ gene polymorphisms . Specifically, interferon-γ+874(T/A) genotypes are known to have an effect on IFN-γ production. The high producer T allele was associated with an increased IDO activity (i.e. elevated KYN/TRP) in females. Hence, it would be worthwhile to investigate if the previously reported IDO associated increased AS risk factors seen in young females  are associated with the IFN-γ risk alleles .
To achieve a normalization of the TRP metabolism would be an important goal for the treatment of related symptoms in patients suffering from CVD. For instance, administration of the IDO-inhibitor 1-methyl tryptophan  may represent a relevant pharmacological approach to be considered for future investigation. However, IDO is well known for its immunosuppressive properties, and inhibiting it may prove detrimental, as it may boost proinflammatory mechanisms. Likewise, an elevated KYN/TRP ratio might represent a natural response of the human immune system to counteract inflammation. Also anti-inflammatory drugs such as aspirin and salicylic acid  but also statins  may affect the IDO mediated pathways. Specifically, Atorvastatin suppresses INF-γ-induced neopterin formation and TRP depletion in human peripheral blood mononuclear cells and in monocytic cell lines . Moreover, immunosuppressants like rapamycin  counteract IDO induction during the pro-inflammatory response in vitro. The same is true for several natural compounds with anti-inflammatory properties and with immunosuppressant capacity in vitro, e.g., resveratrol . Finally, melatonin seems to have cardioprotective properties via its direct free radical scavenger and its indirect antioxidant activity .
Within this review we generated substantial evidence that the Th1-type cytokine IFN-γ causes increased IDO activity which ultimately decreases serum levels of the essential amino acid TRP. As a scLGI with activation of certain T-cell subsets (Th1, less Th2)  is usually present in patients with CVD, these subjects have an increased IDO activation leading to an increased TRP breakdown with an increased KYN/TRP ratio. Importantly, elevated KYN and KYN/TRP indicate a higher likelihood of cardiovascular fatalities. As shown in our recently published observations , increased production of the proinflammatory adipokine leptin in combination with interleukin-6 (IL-6), represents another link between obesity and early occurrence of mediators of AS. Leptin has also been shown to upregulate Th1-type immunity, and increased abdominal fat, an important source of leptin, leads to increased KYN/TRP .
In prolonged states of scLGI, availability of free serum TRP is persistently diminished, and thus, serotonergic functions are affected. Accumulation of neuroactive KYN metabolites such as quinolinic acid is neurotoxic and may contribute to mood dysregulation, previously implicated in increased likelihood of myocardial infarction. However, the data reviewed in this article suggest that mood disorders appear to more likely be a consequence of scLGI/and an epiphenomenon rather than a mediating step of cardiovascular events.
More importantly for CVD, accelerated catabolism of TRP is centrally involved in the pathogenesis of the anemia of scLGI. IFN-γ suppresses erythroid progenitor cells, and the depletion of TRP decreases hemoglobin production. Anemia caused by scLGI and increased TRP breakdown add to the detrimental effects of pre-existent reduced vascular reserves.
Measurement of TRP, KYN concentrations, and calculation of the KYN/TRYP ratio are important predictors of an unfavourable outcome in patients with CVD. It will be important to investigate if these parameters can provide a basis for more successful and precise biologically grounded therapeutic protocols to further reduce cardiovascular morbidity and mortality.
This work was supported by funding under the european FP7 program “NanoAthero”- NMP4-LA-2012-3099820. We want to thank further Dr. Florian Prueller, Clinical Institute of Medical and Chemical Laboratory Diagnosis, Graz, Austria for the expert technical assistance.
CONFLICT OF INTEREST
The author(s) confirm that this article content has no conflicts of interest.