Lead (Pb) Exposure Enhances Expression of Factors Associated with Inflammation

Abstract

The human immune system is constantly exposed to xenobiotics and pathogens from the environment. Although the mechanisms underlying their influence have already been at least partially recognized, the effects of some factors, such as lead (Pb), still need to be clarified. The results of many studies indicate that Pb has a negative effect on the immune system, and in our review, we summarize the most recent evidence that Pb can promote inflammatory response. We also discuss possible molecular and biochemical mechanisms of its proinflammatory action, including the influence of Pb on cytokine metabolism (interleukins IL-2, IL-4, IL-8, IL-1b, IL-6), interferon gamma (IFNγ), and tumor necrosis factor alpha (TNF-α); the activity and expression of enzymes involved in the inflammatory process (cyclooxygenases); and the effect on selected acute phase proteins: C-reactive protein (CRP), haptoglobin, and ceruloplasmin. We also discuss the influence of Pb on the immune system cells (T and B lymphocytes, macrophages, Langerhans cells) and the secretion of IgA, IgE, IgG, histamine, and endothelin.

Keywords: cytokines; inflammatory processes; interleukins; lead (Pb).

Figure 1

Model of lead (Pb²⁺)-induced activation of the IL-8 gene. The diagram presents a signaling pathway leading to the induction of the IL-8 gene in human AGS cells (human caucasian gastric adenocarcinoma) after the administration of 0.1 μM Pb(NO₃)₂. Pb²⁺ activates the epidermal growth factor receptor (EGFR) and p42/44 mitogen activated protein (MAP) kinase phosphorylation, which then activates the protein heterodimeric transcriptional factor AP-1 (containing the c-jun protein), resulting in the activation of IL-8 gene expression. The use of EGFR inhibitors (PD153035 and AG1478) resulted in a decrease in IL-8 gene expression, but EGFR inhibitors were unable to fully abolish Pb(NO₃)₂-induced IL-8 gene expression. The use of a MAP kinase kinase (MEK) inhibitor (PD98059) significantly suppressed IL-8 gene expression.

Figure 2

Mechanism of activation and inactivation Nrf 2 by Pb²⁺. In physiological conditions, Nrf2 binds to its inhibitor, Kelch-like ECH-associated protein 1 (Keap1). This is followed by the dimerization and then sequestration of Nrf2 in the cytoplasm. The Keap1–Nrf2 complex connects ubiquitin (via the Cul 3-dependent ligase) and is recognized by the 26S proteosome, where the Nrf2 transcription factor is degraded. After exposure to Pb²⁺, Nrf2 is released from the Keap1–Nrf2 complex, and translocated to the nucleus, where a small Maf protein attaches forming a heterodimer and CREB-binding protein (CBP). The incorporation of the newly formed complex into DNA initiates the transcription of antioxidant response genes and IL-8 gene results in enhanced reactive oxygen species (ROS) and IL-8 synthesis.

Figure 3

The signaling pathway that results in the production of tumor necrosis factor alpha (TNF-α) in macrophages after the stimulation by lead and lipopolysaccharide (LPS). Lead increases the expression of TNF-α in LPS-treated cells via post-transcriptional mechanisms. It thus increases the effect of lipopolysaccharide (LPS) that is uptaken by the CD14 receptor (cluster of differentiation 14) and its co-receptor TLR 4 (Toll-like receptor TLR 4). The diagram shows some of the transmitters potentially mediating the regulation of TNF-α expression. The signal is transmitted via the MyD88 adapter protein (myeloid differentiation primary response gene 88). One of the activated pathways is the MAPK pathway. MEK1/2 (mitogen-activated protein kinase kinase), MAPK (mitogen-activated protein kinases), and activator protein 1 (AP-1) are activated. The NF-κB factor (nuclear factor kappa B) activation pathway is also stimulated by the regulation of IKKs (IκB kinase) activity. The resulting AP-1 and NF-κB lead to enhanced transcription of TNF-α mRNA. PI3K–AKT pathway is also activated. Attachment of LPS to the receptor leads to the phosphorylation of PI3K (phosphoinositide 3-kinase), which then activates AKT (protein kinase B). There is a mutual adjustment between the NF-κB and PI3K–AKT signaling pathways. The activation of the above-mentioned signaling pathways by LPS and Pb results in enhanced TNF-α synthesis.