Superfund Research Program
Lead-Induced Oxidative Stress in Astroglia
Background: Oxidative stress occurs when the generation of reactive oxygen species (ROS) in a system exceeds the system’s ability to neutralize and eliminate them. ROS include free radicals, reactive anions containing oxygen atoms, or molecules containing oxygen atoms that can either produce free radicals or become chemically-activated by them (e.g., hydroxyl radical, superoxide, hydrogen peroxide, and peroxynitrites). An ROS imbalance can result from deficient antioxidant capacity or an overabundance of ROS from an environmental or behavioral stressor. Excess ROS can damage a cell’s lipids, protein or DNA, inhibiting normal function. For years researchers have known that free radicals can cause cell degeneration, especially in the brain, and oxidative stress has been implicated in a growing list of human diseases, such as Alzheimer's disease, Parkinson's disease and cancer.
Astroglia are star-shaped, non-neuronal cells that provide support and nutrition for neurons. They also play an active role in central nervous system development and function, including enhancement of synaptogenesis, regulation of synaptic plasticity, and induction of neurogenesis and neuronal differentiation. When blood lead levels are elevated, lead accumulates in the brain – primarily in the astroglia, impairing their ability to support and protect neurons. There are no known cellular mechanisms for lead removal.
Lead accumulation in astroglia induces a variety of biochemical responses symptomatic of oxidative stress including changes in intracellular glutathione levels, production of lipid peroxidation byproduct malondialdehyde, decrease of superoxide dismutase-catalase activity, induction of heme oxygenase, and activation of NF-kappaB. Interestingly, lead is a non-redox-reactive heavy metal, that is, lead does not directly induce oxidative stress.
Advances: As part of the Texas A&M University Superfund Basic Research Program, Drs. Ken Ramos and Evelyn Tiffany-Castiglioni have directed a series of studies to investigate the mechanisms that result in lead-induced oxidative stress in astroglia. Their recent work has focused on hypotheses involving copper homeostasis. Unlike lead, copper is a highly redox-reactive metal, and disturbances in copper metabolism will disturb ROS homeostasis and have been related to neurodegenerative diseases including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis.
In earlier work, the researchers demonstrated that lead has no effect on copper intake in neural cells, but does block copper release. In studies designed to examine copper transport in non-neuronal astroglia cells, the researchers treated cultures of astroglial cells with lead, copper or both metals to evaluate the interactions of the metals and the resulting impact on the intracellular environment of astroglial cells. They found that neither lead nor copper alone impacted ROS or copper levels. However, the combined lead and copper treatment significantly increased ROS levels and increased average copper levels.
The researchers then examined the affinities of the two metals to bind to the ATP7A protein, a putative copper-transporter controlling copper release. They found that lead can out-compete copper for the heavy metal binding (HMB) domain of ATP7A. As a result, copper release is prohibited, leading to an accumulation of intracellular copper in lead-exposed astroglia.
Digging deeper, the research team, which included Dr. Yongchang Qian, examined the interactions of lead and GRP78, an endoplasmic reticulum-resident molecular chaperone functioning in protein folding, assembly and trafficking in posttranslational quality control. GRP78 also plays a role in protection against cytotoxicity and apoptosis induced by environmental insults. The researchers showed earlier that lead directly binds to GRP78 in vitro. They designed studies to determine if lead induces the compartmentalized redistribution of GRP78 in living cells and if that reduction of GRP78 levels increases the vulnerability of cells to oxidative stress. They used image analysis of cloned fusion proteins to profile the interaction between GRP78 and lead in live human astrocytoma cells and found that following lead exposure, the cells showed a compartmentalized, non-homogeneous distribution of GRP78 in the cytosol. These results imply that GRP78 plays an indirect role in ROS homeostasis by buffering lead ions intracellularly, and that lead had a direct interaction with GRP78 intracellularly. The researchers believe that lead-induced GRP78 redistribution is a "double-edged sword" because it may initially protect the cells from free lead ions but lower the cell's defense against oxidative stress. Thus, GRP78 depletion may increase the availability of lead to target ATP7A, compromise copper homeostasis, and induce oxidative stress.
Significance: High-level exposures to lead are rare, but low-level lead exposures, especially to small children, are all too common. Lead is a well-known developmental neurotoxicant and lead exposure in children is associated with reduced attention span, reduced IQ scores and increased aggression. Understanding of the mechanism leading to disease or dysfunction is a first, critical step to development of prevention or intervention strategies. This work provides the first direct evidence that lead can disrupt ROS homeostasis in astroglia and insight into the mechanisms of lead-induced oxidative stress.
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To learn more about this research, please refer to the following sources:
- Qian Y, Zheng Y, Ramos KS, Tiffany-Castiglioni E. 2005. GRP78 compartmentalized redistribution in Pb-treated glia: role of GRP78 in lead-induced oxidative stress. Neurotoxicology 26(2):267-275. doi:10.1016/j.neuro.2004.09.002 PMID:15713347
- Qian Y, Zheng Y, Ramos KS, Tiffany-Castiglioni E. 2005. The involvement of copper transporter in lead-induced oxidative stress in astroglia. Neurochem Res 30(4):429-438. PMID:16076012
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