DISCUSSION
Bupropion is a common antidepressant that has been prescribed for over 20 years as a sole antidepressant or with other classes of antidepressants. Meanwhile, literature has reported the low response rates to bupropion in the patients with treatment-refractory depression25. Low-grade inflammation is believed to be involved in the poor responsiveness to regular antidepressants1. Moreover, despite the repeatedly mentioned anti-inflammatory effects, some controversies are reported regarding bupropion action including the development of some inflammatory abnormalities such as type 1 hypersensitivity reactions (erythema multiformeis) and serum sickness-like reaction 26,27. Moreover, Eller et al observed elevated levels of IL-8 in response to bupropion 28. Bupropion administration also failed to lower interleukins secretion in monocyte-derived macrophages collected from HIV positive women 29. Therefore, we sought to determine bupropion inflammatory effects and underlying pathways in LPS-stimulated PBMCs. Present results revealed that bupropion might induce inflammatory responses via upregulation of TLR2, TLR4, JAK2, and STAT3 and consequently the pro-inflammatory cytokines gene expression.
In consistence with previous reports 30,31, bupropion inhibited secretion of TNF-ɑ, IL-1-ß by LPS-stimulated PBMCs. Data addressing transcriptional changes in cytokine genes following bupropion administration is almost scarce. Surprisingly, bupropion increased mRNA levels of TNF-ɑ, IL-1-ß, in the present study. However, in an animal study conducted by Helaly et al TNF-ɑ gene expression reported being down-regulated by bupropion 14. Studies concerning changes in IL-17 levels in response to bupropion are limited. In 2013 Kim et al failed to find a significant change in IL-17 levels under bupropion treatment 32. However, an interesting study indicated that a higher baseline IL-17 level is associated with a greater reduction in depression severity in a bupropion-SSRI treatment arm 5. Assessing IL-17A expression at both translational and transcriptional levels, we figured out that bupropion significantly increased the cytokine gene expression whereas it somehow inhibited the protein levels. Th17 cells and its major cytokine product, IL-17A, promote activation of microglia and astrocytes that may contribute to the neuroinflammation, neuronal damage, and consequently lead to depression. In addition to the Th17 cells, CD8+ cells, invariant natural killer T cells, monocytes, neutrophils, and γδ T cells also secrete IL-17A. IL-17A facilitates monocyte penetration through the blood-brain barrier, enhances the blood-brain barrier permeability, and extends the damage by attracting cytokine-producing cells33. Reaching the brain, IL-17A can activate microglia via present IL-17 receptors and indirectly induce the release of cytokines and chemokines that exacerbates the inflammation.
The abnormally high level of pro-inflammatory cytokines in MDD reflects the overactive TLR-mediated signaling. Evidence indicated that both protein and mRNA expression of TLRs including TLR2 and TLR4 are higher in the central nervous system and peripheral blood of depressed patients. Bupropion capability to attenuate the deregulated expression of TLR4 and TLR2 has been suggested by few studies34,35. However, our obtained results concerning TLR4 expression profile differed depending on the drug concentration. Bupropion inhibited TLR4 expression at 25 and 200 μM, though it enhanced TLR4 expression at 100 μM. Bupropion also increased TLR2 gene expression in stimulated PBMCs. Microglia, the immune cells of the brain, also express TLR2 and TLR4 and provide pro-/anti-inflammatory cytokines as mediators for the neuroimmune system. Moreover, microglia showed an enhanced response to cytokines in MDD patients. Inflammation-induced by the activation of TLRs in microglia plays a substantial part in the pathology of depression 36,37. Furthermore, TLR4 mRNA and protein reported being higher in both the periphery and CNS of MDD patients 4. J. Hines et al demonstrated that intraperitoneal injection of Tat-TLR4 interfering peptides improves LPS-induced sickness behavior, an important contributor to the occurrence of depressive-like behavior, via rescuing the microglia morphology changes and cytokine production that are normally induced by LPS 38. More importantly, negative regulators of the TLR signaling pathway have been suggested as potential predictive biomarkers of response to antidepressant treatment.
In search of signaling pathways underlying bupropion immunomodulatory action, Tsai et al reported that bupropion did not affect LPS-induced phospho-p65 and phospho-JNK, and phospho-ERK and phospho-p38 expression in THP-1 cells 7. Besides the well-known role in the peripheral immune responses, a growing body of evidence proposed that the JAK2-3 signaling pathway is associated with neuroinflammation. JAK2-3 signaling and consequent secretion of TNF-α and IL-1β by microglia promote neuronal death and can consequently result in anxiety and depression 11,39. JAK2 and 3 genes stated being over-expressed in both MDD patients and mice model with inflammation- induced depressive behavior 40,41. Blocking JAK2-3 signaling inhibited pro-inflammatory cytokines release and attenuated neuroinflammation caused by activated microglia 42. Moreover, intrathecal injection of a JAK2-3 cascade inhibitor considerably normalized the spinal inflammatory profile caused by peripheral inflammation 43,44. JAK2-STAT3inhibitors have been proposed as a beneficial strategy in the treatment of depression 9,45,46. Therefore, we investigated the gene expression of JAK2 and 3 in LPS-treated PBMCs after bupropion treatment. JAK2 expression was upregulated at 50 and 100 μM concentrations of the drug. 3 expression was also increased considerably at all concentrations of bupropion.
Taken together bupropion might contribute to peripheral and CNS inflammation via modulating TLR2/TLR4 and JAK2/STAT3 signaling pathways, particularly at moderate concentrations. We concluded that lack of expected response to bupropion in MDD patients with inflammatory etiology could be due to this unnoticed mode of action. Therefore, bupropion would better get administrated with anti-inflammatory agents. In confirmation of our results, studies demonstrated that bupropion combination with anti-inflammatory agents yielded a better outcome47,48. Of note, bupropion is an antidepressant with complex mechanisms of action including the recently discovered effect on the serotonin receptors. Dopamine, norepinephrine, acetylcholine, and serotonin each exert a specific immunomodulatory effect. Therefore, more comprehensive studies should get conducted to understand the net immunomodulatory effect of bupropion under both inflammatory and immunologically-balanced status in patients with depressive-like behavior. Our findings also notify the importance of a personalized medication approach in MDD patients regarding bupropion prescription.