A Common Carcinogen Benzo[a]pyrene Causes Neuronal Death in Mouse via Microglial Activation

Abstract

BackgroundBenzo[a]pyrene (B[a]P) belongs to a class of polycyclic aromatic hydrocarbons that serve as micropollutants in the environment. B[a]P has been reported as a probable carcinogen in humans. Exposure to B[a]P can take place by ingestion of contaminated (especially grilled, roasted or smoked) food or water, or inhalation of polluted air. There are reports available that also suggests neurotoxicity as a result of B[a]P exposure, but the exact mechanism of action is unknown.Methodology/Principal FindingsUsing neuroblastoma cell line and primary cortical neuron culture, we demonstrated that B[a]P has no direct neurotoxic effect. We utilized both in vivo and in vitro systems to demonstrate that B[a]P causes microglial activation. Using microglial cell line and primary microglial culture, we showed for the first time that B[a]P administration results in elevation of reactive oxygen species within the microglia thereby causing depression of antioxidant protein levels; enhanced expression of inducible nitric oxide synthase, that results in increased production of NO from the cells. Synthesis and secretion of proinflammatory cytokines were also elevated within the microglia, possibly via the p38MAP kinase pathway. All these factors contributed to bystander death of neurons, in vitro. When administered to animals, B[a]P was found to cause microglial activation and astrogliosis in the brain with subsequent increase in proinflammatory cytokine levels.Conclusions/SignificanceContrary to earlier published reports we found that B[a]P has no direct neurotoxic activity. However, it kills neurons in a bystander mechanism by activating the immune cells of the brain viz the microglia. For the first time, we have provided conclusive evidence regarding the mechanism by which the micropollutant B[a]P may actually cause damage to the central nervous system. In today's perspective, where rising pollution levels globally are a matter of grave concern, our study throws light on other health hazards that such pollutants may exert.

Neurotoxicity may be an overlooked consequence of benzo[a]pyrene exposure that is relevant to human health risk assessment

“…Mouse, BALB/c ,, <, 4-5 weeks [60] Intraperit. injection 1 4 Elevated level of pro-inflammatory cytokines, activation of microglia and astrogliosis in brain Rat, Wistar <, 3 mo [56] Intraperit.…”

“…BaP-induced increases in malondialdehyde support the notion that BaP metabolism is accompanied by ROS formation in the brain. Furthermore, while microglia are resistant to BaP, the supernatant from a BaP-treated murine microglial cell line (BV-2) is cytotoxic to primary cortical neurons through ''bystander'' mechanisms [60]. This cytotoxicity is thought to be caused by BaPinduced increases in ROS, which up-regulate inducible nitric oxide synthase and p38MAP kinase in the microglia leading to the generation of nitric oxide and pro-inflammatory cytokines by the activated microglia [60].…”

“…Furthermore, while microglia are resistant to BaP, the supernatant from a BaP-treated murine microglial cell line (BV-2) is cytotoxic to primary cortical neurons through ''bystander'' mechanisms [60]. This cytotoxicity is thought to be caused by BaPinduced increases in ROS, which up-regulate inducible nitric oxide synthase and p38MAP kinase in the microglia leading to the generation of nitric oxide and pro-inflammatory cytokines by the activated microglia [60]. BaP-induced increases in ROS can also lead to lipid peroxidation that, in turn, can cause neuronal damage and neurodegeneration [61].…”

An environmental contaminant, benzo(a)pyrene, induces oxidative stress-mediated interleukin-8 production in human keratinocytes via the aryl hydrocarbon receptor signaling pathway

“…Several studies have utilized concentrations (200 nM-10 mM) of BaP as a systemic BaP exposure to human endothelial cell [25], monocyte [25] and primary cortical neuron [26]. To our knowledge, there is no study showing BaP concentration in human skin of smokers.…”

Japanese Encephalitis Virus-Infected Macrophages Induce Neuronal Death

“…Primary cortical neuron culture Cortical neurons were cultured following a published protocol (Dutta et al 2010c). Briefly, cortices of P2 BALB/c mouse pups were dissected aseptically in calciummagnesium-free (CMF)-Tyrode solution following decapitation.…”