Intracellular mechanisms of hydroquinone toxicity on endotoxin-activated neutrophils.

Circulating neutrophils promptly react to different substances in the blood and orchestrate the beginning of the innate inflammatory response. We have shown that in vivo exposure to hydroquinone (HQ), the most oxidative compound of cigarette smoke and a toxic benzene metabolite, affects circulating neutrophils, making them unresponsive to a subsequent bacterial infection. In order to understand the action of toxic molecular mechanisms on neutrophil functions, in vitro HQ actions on pro-inflammatory mediator secretions evoked by Escherichia coli lipopolysaccharide (LPS) were investigated. Neutrophils from male Wistar rats were cultured with vehicle or HQ (5 or 10 µM; 2 h) and subsequently incubated with LPS (5 µg/ml; 18 h). Hydroquinone treatment impaired LPS-induced nitric oxide (NO), tumour necrosis factor α (TNF-α), interleukin (IL)-1ß and IL-6 secretions by neutrophils. The toxic effect was not dependent on cell death, reduced expression of the LPS receptor or toll-like receptor-4 (TLR-4) or cell priming, as HQ did not induce reactive oxygen species generation or ß(2)integrin membrane expression. The action of toxic mechanisms on cytokine secretion was dependent on reduced gene synthesis, which may be due to decreased nuclear factor κB (NF-κB) nuclear translocation. Conversely, this intracellular pathway was not involved in impaired NO production because HQ treatments only affected inducible nitric oxide synthase protein expression and activity, suggesting posttranscriptional and/or posttranslational mechanisms of action. Altogether, our data show that HQ alters the action of different LPS-activated pathways on neutrophils, which may contribute to the impaired triggering of the host innate immune reaction detected during in vivo HQ exposure.

Hydroquinone stimulates inflammatory functions in microvascular endothelial cells via NF-κB nuclear activation.

Hydroquinone impairs several leucocyte cell functions, which alter the immune response. Although endothelial cell functions are important for the development of immune responses, hydroquinone actions on endothelial cell have not been shown. Therefore, the effect of hydroquinone exposure (10 or 100 µM for 2 hr) on primary culture of microvascular endothelial cells (PMECs) obtained from the cremaster muscle of Wistar rats incubated in the presence or absence of lipopolysaccharide (LPS, 2 µg/mL) was investigated. Hydroquinone treatment induced the membrane expression of cell adhesion molecules (CAMs) from the immunoglobulin superfamilies ICAM-1 (intercellular), VCAM-1(vascular) and PECAM-1 (platelet endothelial) and induced the secretion of cytokines interleukin-1ß (IL-1ß) and tumour necrosis factor-α (TNF-α). The effects were dependent on transcriptional modifications because enhanced CAM mRNA expression as well as both cytokines and nuclear factor κB (NF-κB) nuclear activation was found. These effects may be due to the direct action of hydroquinone rather than its quinone metabolites, because endothelial cells do not present myeloperoxidase enzyme and hydroquinone incubation did not induce the expression of cytochrome P450 2E1 (CYP2E1) or prostaglandin H synthase 1. In addition, the incubation of endothelial cells with benzoquinone (10 µM, 2 hr) impaired PECAM-1 expression and did not modify NF-κB nuclear activation. Taken together, the data herein presented reveal that hydroquinone evokes pro-inflammatory properties in endothelial cells that are triggered by the enhancement of NF-κB nuclear translocation-dependent gene transcription.