Interferon-alpha exposure increases the expression of enzymes of the kynurenine pathway and induces apoptosis in a model of human neurons

Major depression (MD) is associated with a profound unbalance between the nervous-, the endocrine- and the immune- systems. This suggests the possibility that molecules that regulate the homeostasis of these systems may contribute to the development of MD. For instance, some cytokines, important neuro-endocrineimmuno modulators, have been proposed to have a role in MD as supported by the observation that activation of immune system with therapeutically used cytokines may induce MD. IFN-a is an innate immune cytokine with potent antiviral and anti-proliferative properties that is used to treat viral infections, such as hepatitis C, and certain cancers. Despite therapeutic efficacy in these illnesses, it has been observed that IFN-a exposure may be associated with important side effects including neuropsychological and behavioural changes that overlap with MD. Although IFN-a- induced effects on the brain make IFN-alpha 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 cytokines may contribute to the etiology of MD by inducing indolamine 2,3-dioxygenase (IDO) expression. IDO catalyzes the initial rate-limiting step in tryptophan (TPR) degradation along the kynurenine pathway (KP). Kynurenine, the initial product of TPR 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 neuroprotective agent. A role for a disturbance in the KP in the neuroprotective–neurodegenerative balance in the brain of patients with MD, has been proposed in the neurodegeneration hypothesis of depression. This prompted us to investigate the effects of IFN-a on the expression of the IDO, KMO, Kynu and KAT mRNAs in an in vitro model of human neurons: SH-SY5Y (human neuroblastoma) cells. We also evaluated the IFN-a-induced effect on cell viability and number in these cells. Finally, because cell number results from the balance between cell proliferation and cell elimination, we measured cell proliferation and apoptosis in SH-SY5Y cells after IFN-a exposure. The same experiments were performed in differentiated SH-SY5Y cells with retinoic acid (RA) and Brain- Derived Neurotrophic Factor (BDNF). Our studies show that IFN-a exposure increased the expression of all the kynurenergic enzymes investigated (with an unbalance of the KP toward the synthesis of neurotoxic end-products) and in more particularly strongly induced the expression of IDO mRNA (more than 900-fold) in SH-SY5Y cells. We also demonstrated that IFN-a reduced in a dose- (2, 20, 50 and 100 ng/ml) and time- (24, 48 and 72 hrs) dependent manner the cell number and induces apoptosis in SH-SY5Y cells. Similar results were obtained in SHSY5Y RA/BDNF differentiated cells. Together our results clearly enlighten the cytotoxic effects of IFN-a in this in vitro model of human neurons. Moreover, our findings provide further information on the molecular pathways involved in cytokine-induced effects in the brain and add a piece to the puzzle of what and how these factors or pathways may contribute to the pathogenesis of MD.