Fractalkine, sICAM-1 and Kynurenine Pathway in Restrictive Anorexia Nervosa-Exploratory Study.

The link between the kynurenine pathway and immunomodulatory molecules-fractalkine and soluble intercellular adhesion molecule-1 (sICAM-1)-in anorexia nervosa (AN) remains unknown. Fractalkine, sICAM-1, tryptophan (TRP), kynurenine (KYN), neuroprotective kynurenic acid (KYNA), neurotoxic 3-OH-kynurenine (3-OH-KYN), and the expression of mRNA for kynurenine aminotransferases (KAT1-3) were studied in 20 female patients with restrictive AN (mostly drug-free, all during first episode of the disease) and in 24 controls. In AN, serum fractalkine, but not sICAM-1, KYNA, KYN, TRP or 3-OH-KYN, was higher; ratios TRP/KYN, KYN/KYNA, KYN/3-OH-KYN and KYNA/3-OH-KYN were unaltered. The expression of the gene encoding KAT3, but not of genes encoding KAT1 and KAT2 (measured in blood mononuclear cells), was higher in patients with AN. In AN, fractalkine positively correlated with TRP, while sICAM-1 was negatively associated with 3-OH-KYN and positively linked with the ratio KYN/3-OH-KYN. Furthermore, TRP and fractalkine were negatively associated with the body mass index (BMI) in AN. Expression of KAT1, KAT2 and KAT3 did not correlate with fractalkine, sICAM-1 or BMI, either in AN or control. Increased fractalkine may be an independent factor associated with the restrictive type of AN. Excessive physical activity probably underlies increased expression of KAT3 observed among enrolled patients. Further, longitudinal studies on a larger cohort of patients should be aimed to clarify the contribution of fractalkine and KAT3 to the pathogenesis of AN.

Increased expression of kynurenine aminotransferases mRNA in lymphocytes of patients with inflammatory bowel disease.

BACKGROUND: Complex interaction of genetic defects with environmental factors seems to play a substantial role in the pathogenesis of inflammatory bowel disease (IBD). Accumulating data implicate a potential role of disturbed tryptophan metabolism in IBD. Kynurenic acid (KYNA), a derivative of tryptophan (TRP) along the kynurenine (KYN) pathway, displays cytoprotective and immunomodulating properties, whereas 3-OH-KYN is a cytotoxic compound, generating free radicals. METHODS: The expression of lymphocytic mRNA encoding enzymes synthesizing KYNA (KAT I-III) and serum levels of TRP and its metabolites were evaluated in 55 patients with IBD, during remission or relapse [27 patients with ulcerative colitis (UC) and 28 patients with Crohn's disease (CD)] and in 50 control individuals. RESULTS: The increased expression of KAT1 and KAT3 mRNA characterized the entire cohorts of patients with UC and CD, as well as relapse-remission subsets. Expression of KAT2 mRNA was enhanced in patients with UC and in patients with CD in remission. In the entire cohorts of UC or CD, TRP levels were lower, whereas KYN, KYNA and 3-OH-KYN were not altered. When analysed in subsets of patients with UC and CD (active phase-remission), KYNA level was significantly lower during remission than relapse, yet not versus control. Functionally, in the whole groups of patients with UC or CD, the TRP/KYN ratio has been lower than control, whereas KYN/KYNA and KYNA/3-OH-KYN ratios were not altered. The ratio KYN/3-OH-KYN increased approximately two-fold among all patients with CD; furthermore, patients with CD with relapse, manifested a significantly higher KYNA/3-OH-KYN ratio than patients in remission. CONCLUSION: The presented data indicate that IBD is associated with an enhanced expression of genes encoding KYNA biosynthetic enzymes in lymphocytes; however, additional mechanisms appear to influence KYNA levels. Higher metabolic conversion of serum TRP in IBD seems to be followed by the functional shift of KYN pathway towards the arm producing KYNA during exacerbation. We propose that KYNA, possibly via interaction with aryl hydrocarbon receptor or G-protein-coupled orphan receptor 35, may serve as a counter-regulatory mechanism, decreasing cytotoxicity and inflammation in IBD. Further longitudinal studies evaluating the individual dynamics of TRP and KYN pathway in patients with IBD, as well as the nature of precise mechanisms regulating KYNA synthesis, should be helpful in better understanding the processes underlying the observed changes.

Prolonged therapy with antidepressants increases hippocampal level of kynurenic acid and expression of Kat1 and Kat2 genes.

BACKGROUND: Accumulating data suggest an important role of disturbed kynurenine pathway and altered glutamatergic transmission in the pathogenesis of depression. In here, we focused on detailed analyses of kynurenic acid (KYNA) status in vivo following single and 14-day administration of selected tricyclic antidepressant drugs (TCAs) and serotonin selective reuptake inhibitors (SSRIs) in rats. METHODS: The effect of antidepressants on serum and brain KYNA levels, as well as on the activity of kynurenine aminotransferases (KATs I and II) and expression of Kat1 and Kat2 genes mRNA was studied in three brain regions. RESULTS: Chronic, but not acute, application of antidepressants invariably stimulated KYNA production in hippocampus (amitriptyline, imipramine, fluoxetine and citalopram) and sporadically in cortex (amitriptyline, fluoxetine), whereas no change in KYNA level was observed in striatum. Cortical and hippocampal expression of Kat1 and Kat2 genes was increased after chronic, but not single administration of all studied antidepressants. The activity of semi-purified enzymatic proteins, KAT I and II, was not paralleling changes of Kat1 and Kat2 genes. CONCLUSION: Our data indicate that prolonged administration of antidepressants targets expression of KYNA biosynthetic enzymes. Furthermore, post-translational modulation of KATs seems to play an important role in tuning of KYNA synthesis within brain structures. We suggest that consistent increase of hippocampal KYNA levels may represent hallmark of antidepressant activity. Mechanisms governing region- and drug-selective action of antidepressants require further investigations.

New insight into the antidepressants action: modulation of kynurenine pathway by increasing the kynurenic acid/3-hydroxykynurenine ratio.

Altered function of kynurenine pathway has emerged recently as one of the factors contributing to the pathogenesis of depression. Neuroprotective kynurenic acid (KYNA) and neurotoxic 3-hydroxykynurenine (3-HK) are two immediate metabolites of L: -kynurenine. Here, we aimed to assess the hypothesis that antidepressant drugs that may change brain KYNA/3-HK ratio. In primary astroglial cultures, fluoxetine, citalopram, amitriptyline and imipramine (1-10 µM) increased de novo production of KYNA and diminished 3-HK synthesis (24 and 48, but not 2 h). RT-PCR studies revealed that Kat1, Kat2 and kynurenine-3-monooxygenase (Kmo) gene expressions were not altered after 2 h. At 24 h, the expression of Kat1 and Kat2 genes was enhanced by all studied drugs, whereas Kmo expression was diminished by citalopram, fluoxetine and amitriptyline, but not imipramine. After 48 h, the expression of Kat1 and Kat2 was further up-regulated, and Kmo expression was down-regulated by all antidepressants. The ratio KYNA/3-HK was increased by fluoxetine, citalopram, amitriptyline and imipramine in a time-dependent manner-the effect was not observed after 2 h, modest after 24 h and robust after 48 h incubation time. Our findings indicate that the action of antidepressants may involve re-establishing of the beneficial ratio between KYNA and 3-HK. Shift in the kynurenine pathway, observed after prolonged exposure to antidepressant drugs, may partly explain their delayed therapeutic effectiveness.

Novel aspect of ketone action: ß-hydroxybutyrate increases brain synthesis of kynurenic acid in vitro.

Ketone bodies formed during ketogenic diet or non-treated diabetes mellitus may exert neuroprotective and antiepileptic effects. Here, we assessed the influence of ketone body, ß-hydroxybutyrate (BHB) on the brain synthesis of kynurenic acid (KYNA), an endogenous antagonist of glutamatergic and α7-nicotinic receptors. In brain cortical slices and in primary glial cultures, BHB enhanced KYNA production. KT 5270, an inhibitor of protein kinase A, has prevented this action. At hypoglycemia, under pH 7.0 and 7.4, profound (15 mM BHB), but not mild (3 mM) ketosis increased synthesis of KYNA. In paradigm resembling diabetic ketoacidosis in vitro (30 mM glucose, pH 7.0), neither mild nor profound ketosis influenced the production of KYNA. At pH 7.4 and in 30 mM glucose though, both mild and severe ketonemia evoked an increase of KYNA production. The activity of KYNA biosynthetic enzymes, KAT I and KAT II, in cortical homogenate was not altered by BHB (0.05-10.0 mM). However, in cultured glial cells exposed to BHB (10 mM), the activity of KATs increased. This effect was reversed by the co-incubation of cells with KT 5270. Presented data reveal a novel mechanism of action of BHB. Increased synthesis of KYNA in the presence of BHB is most probably mediated by protein kinase A-dependent stimulation of KATs expression/activity leading to an increase of KYNA formation. Ensuing attenuation of the excessive excitatory glutamate-mediated neurotransmission may, at least in part, explain the neuroprotective actions of BHB.

beta-adrenergic enhancement of brain kynurenic acid production mediated via cAMP-related protein kinase A signaling.

The central levels of endogenous tryptophan metabolite kynurenic acid (KYNA), an antagonist of N-methyl-d-aspartate (NMDA) and alpha7-nicotinic receptors, affect glutamatergic and dopaminergic neurotransmission. Here, we demonstrate that selective agonists of beta(1)-receptors (xamoterol and denopamine), beta(2)-receptors (formoterol and albuterol), alpha- and beta-receptors (epinephrine), 8pCPT-cAMP and 8-Br-cAMP (analogues of cAMP) increase the production of KYNA in rat brain cortical slices and in mixed glial cultures. Neither betaxolol, beta(1)-adrenergic antagonist, nor timolol, a non-selective beta(1,2)-adrenergic antagonist has influenced synthesis of KYNA in both paradigms. In contrast, KT5720, a selective inhibitor of protein kinase A (PKA), strongly reduced KYNA formation in cortical slices (2-10 microM) and in glial cultures (100 nM). beta-adrenergic antagonists and KT5720 prevented the beta-adrenoceptor agonists-induced increases of KYNA synthesis. In vivo, beta-adrenergic agonist clenbuterol (0.1-1.0 mg/kg) increased the cortical endogenous level of KYNA; the effect was blocked with propranolol (10 mg/kg). beta-adrenoceptors agonists, cAMP analogues and KT5720 did not affect directly the activity of KAT I or KAT II measured in partially purified cortical homogenate. In contrast, the exposure of intact cultured glial cells to pCPT-cAMP, 8-Br-cAMP and formoterol has lead to an enhanced action of KATs. These findings demonstrate that beta-adrenoceptor-mediated enhancement of KYNA production is a cAMP- and PKA-dependent event. PKA activity appears to be an essential signal affecting KYNA formation. Described here novel mechanism regulating KYNA availability may be of a potential importance, considering that various stimuli, among them clinically used drugs, activate cAMP/PKA pathway, and thus could counteract the central deficits of KYNA.

Clenbuterol enhances the production of kynurenic acid in brain cortical slices and glial cultures.

The effect of a beta(2)-adrenergic agonist, clenbuterol on the production of a glutamate receptor antagonist, kynurenic acid was studied in vitro. Clenbuterol enhanced the production of kynurenic acid in brain cortical slices (0.1-1.0 mM) and in glial cultures (1-50 muM). Timolol, a non-selective beta-adrenergic antagonist prevented this effect. The presented data indicate a novel mechanism of action of beta(2)-adrenoceptor agonists and suggest that an increased formation of the endogenous glutamate receptor antagonist, kynurenic acid could partially contribute to their neuroprotective activity.

Memantine increases brain production of kynurenic acid via protein kinase A-dependent mechanism.

We describe a novel aspect of action of memantine ex vivo, in the brain cortical slices and in vitro, in mixed glial cultures. The drug potently increased the production of kynurenic acid, an endogenous tryptophan metabolite blocking N-methyl-D-aspartate (NMDA) and nicotinic alpha7 receptors. In cortical slices memantine, an open-channel NMDA blocker (100-150 microM), but not the competitive NMDA receptor antagonist, LY235959 increased the production of kynurenic acid. Neither SCH23390, D1 receptor antagonist (50 microM) nor raclopride, D2 receptor antagonist (10 microM) changed the memantine-induced effects. Propranolol (100 microM) has partially reduced its action. Selective cAMP-dependent protein kinase (PKA) inhibitor, KT5720 (1 microM), but not selective protein kinase C (PKC) inhibitor, NPC15437 (30 microM) totally reversed the action of memantine. In mixed glial cultures, 2-24 h incubation with memantine (2-50 microM) enhanced the production of kynurenic acid. Memantine (up to 0.5 mM) has not affected the activity of kynurenic acid biosynthetic enzymes. The obtained data suggest that memantine enhances the production of kynurenic acid in PKA-mediated way. This effect may partially contribute to the therapeutic actions of memantine and be of a potential clinical importance.