The effects of lead exposure on the expression of HMGB1 and HO-1 in rats and PC12 cells

Highlights

• We have demonstrated a correlation between in vivo and in vitro exposure to Pb in rats and PC-12 cells.

• Pb exposure up-regulated the expression of HO-1 and HMGB1in rats and PC12 cells.

• The effect of Pb on HMGB1 may through regulate HO-1expression in the PC12 cells.

Abstract

Lead (Pb) is an environmental neurotoxic metal. Chronic exposure to Pb causes deficits of learning and memory in children and spatial learning deficits in developing rats. In this study we investigated the effects of Pb exposure on the expression of HMGB1 and HO-1 in rats and PC12 cells. The animals were randomly divided to three groups: control group; low lead exposure group; high lead exposure group; PC12 cells were divided into 3 groups: 0 μM (control group), 1 μM and 100 μM Pb acetate. The results showed that Pb levels in blood and brain of Pb exposed groups were significantly higher than that of the control group (p < 0.05). The expression of HMGB1 and HO-1 were increased in Pb exposed groups than that of the control group (p < 0.05). Moreover, we found that the up-regulation of HO-1 in Pb exposure environment inhibited the expression of HMGB1.

Introduction

Lead (Pb) has been known as a health hazard in multiple environmental matrices. Pb exposure may occur during different periods of development and it has several functional implications in the central nervous system, resulting in medical conditions such as encephalopathy, cognitive deficits, learning impairment and degenerative diseases (Rosen et al., 2017). The nervous system is vulnerably susceptible to Pb exposure, even at low levels, and the brain’s development is also extremely sensitive to the toxic effects of Pb during the prenatal period (Hoffer et al., 1987). Pb damage in the brain is mainly concentrated on some special areas, such as the forebrain of the cerebral cortex, hippocampus and cerebellum. Additionally, Pb may cross the blood-brain barrier and accumulate in the brain, causing alterations in the function of cerebral enzymes, decreased cognitive ability, and behavioral changes in both human and experimental animals (Ademuyiwa et al., 2007; Cory-Slechta et al., 1992). Researchers from worldwide institutes have been investigating the mechanisms of Pb toxicity, such as cytotoxicity, apoptosis, energy metabolism disorders, oxidative stress and inflammation. In the past few years, a number of studies have provided evidence to verify the important role of exposure to Pb in the pathogenesis of inflammatory processes.

Kasten-Jolly et al. found that the gene expressions of interleukin 6 (IL-6), Interleukin16 (IL-16), Interleukin18 (IL-18) and transforming growth factor β1 (TGF-β1) were higher in frontal cortex, cerebellum, hypothalamus, striatum, hippocampus and substantia nigra when mouse pups were exposed to Pb (Kasten-Jolly et al., 2011). Interleukin 10 (IL-10), as anti-inflammatory cytokine, was significantly decreased in the area of the cerebral cortex, following exposure to 0.1 mM Pb acetate (PbAc) (Kasten-Jolly et al., 2012). Liu et al. observed that proanthocyanidins, a class of naturally occurring anti-inflammatory flavonoid, significantly decreased the levels of tumor necrosis factor-α (TNF-α), interleukin 1β (IL-1β) and cyclooxygenase-2 in the brains of lead-exposed rats (Liu et al., 2014). Proinflammatory cytokines, such as IL-1β, TNF-α, IL-6, mediated the early inflammatory response (Hesse et al., 1988). Whether High-mobility group box-1 (HMGB1), a late mediator of the inflammatory, is involved in the neurotoxicity of lead exposure, there has been no literature at home and abroad.

HMGB1 is highly conserved ubiquitous nuclear protein that is involved in DNA replication, recombination, gene transcription, repair and other processes (Polanska et al., 2012). It has not only intracellular functions, but also many extracellular functions, such as cytokine and chemokine activities, mediated by HMGB1 receptors (Hori et al., 1995). HMGB1 can trigger a lethal inflammatory process by significantly increasing the release of inflammatory cytokines, such as TNF-α, IL-1β, IL-6, and IL-8, and macrophage inflammatory protein (MIP)-1β (Andersson et al., 2000a). Therefore, targeting the release of HMGB1 provides a wide window for clinical intervention against systemic inflammatory diseases.

Recently, Heme oxygenase-1 (HO-1) has been identified with anti-inflammatory, anti-apoptosis, and cyto-protective effect in many diseases (Li et al., 2016). It is a cytoprotective enzyme that cleaves heme at the α-methene bridge to generate carbon monoxide (CO), iron, and biliverdin, which is subsequently reduced by biliverdin reductase to bilirubin (Zhang et al., 2017). HO-1 is expressed in many cell types, including neural cells. The cytoprotection afforded by HO-1 involves multiple mechanisms, including the catabolism of toxic free heme to the antioxidants biliverdin and bilirubin, and the generation of CO which possesses potent anti-apoptotic properties (Han et al., 2017). The induction of HO-1 can inhibit the production of pro-inflammatory cytokines induced by LPS, including TNF-α, IL-1β, and IL-6 in activated macrophages (Guo et al., 2016; Wang et al., 2017). Importantly, HO-1 is known to exhibit cytoprotective, anti-inflammatory, antioxidant and anti-apoptotic activities, making it a promising therapeutic target for the treatment of inflammatory diseases of the nervous system (Lee et al., 2016; Li et al., 2016). It has been reported that HO/CO is involved in the process of LTP, synaptic transmission, learning and memory in the hippocampus in the nervous system (Chen et al., 2013). These findings suggest that HO-1 treatment is considered an effective therapeutic strategy for inflammatory diseases. Recent studies have shown that HO-1 also plays an important regulatory role in HMGB1. Li et al. found that hydrogen can up-regulate HO-1 and inhibit the synthesis of HMGB1, thereby reducing the mortality of animals in the mouse ARDS model (Li et al., 2015). In the present study, we examined the effects of HO-1 and HMGB1 expression in Pb exposed rats and PC12 cells models. Further, we explored whether HO-1 regulated the expression of HMGB1.