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.