Interferon alpha exposure increases the expression of the enzymes belonging to the kynurenine pathway in an in vitro model of human neurons: SH-SY5Y cells

The past two decades have witnessed a burgeoning area of pre-clinical and clinical research linking psychiatric illnesses – particularly major depression (MD) – to activation of the inflammatory immune system. One of the stronger evidence supporting a causal role for inflammation in leading MD comes from reports indicating that depressive symptoms frequently develop in patients undergoing immunotherapy with cytokines, such as interferon (IFN)-α, for the treatment of malignancies or chronic viral infection. Although INF-alpha- induced effects on the brain made of IFN-α a model to study the influence of pro-inflammatory cytokines in the CNS and behavior the molecular mechanisms underlying these effects are far from being fully understood. It has been proposed that IFN-α may contribute to the etiology of MD by inducing indolamine 2,3-dioxygenase (IDO) expression and thus unbalancing in the tryptophan/kynurenine metabolism toward the production of neurotoxic metabolites and\or reducing serotonin (5-HT) availability. IDO catalyzes the initial rate-limiting step in tryptophan degradation along the kynurenine pathway (KP). Kynurenine, the initial product of tryptophan degradation, is further catalysed into neurotoxic end-products through steps catalyzed by kynurenine 3-monooxygenase (KMO) and kynureninase (Kynu). However, Kynurenine can also be catabolised by kynurenine aminotransferase (KAT), into kynurenic acid, a potentially neuroptotective agent. A role for a disturbance in the equilibrium between neurotoxic/ neuropoptective end KP endproducts producing an alteration in the neuroprotective–neurodegenerative balance in the brain of patients with MD, has been proposed in the neurodegeneration hypothesis of depression. Given that we previously demonstrated that IFN-α induces toxic effects in an in vitro model of human neurons (human SH-SY5Y neuroblastoma cells) we were aim to investigate the effects of IFN-α on KP in these cells. Our studies show that IFN-α exposure increased the expression of all the kynurenergic enzymes investigated (IDO, KMO, Kynu and KAT). More particularly strongly induced the expression of IDO mRNA (more than 900 –fold) in SH-SY5Y cells. Similar effects on kynurenergic enzyme expression were also observed when SH-SY5Y cells where differentiated with all-trans retinoic acid (in presence of neurotrophic support and in serum deprived conditions). We also demonstrated that INF-α decreased 5-HT levels whereas increased the kynurenine levels in the medium of both differentiated as well not differentiated SH-SY5Y cells.