Sang Y. Lee

Increased incidence of the Hfe mutation in amyotrophic lateral sclerosis and related cellular consequences

The etiology of amyotrophic lateral sclerosis (ALS) is unknown. The presence of mutations in the superoxide dismutase gene (SOD1) has led to theories regarding a role for oxidative stress in the pathogenesis of this disease. A primary cause of oxidative stress is perturbations in cellular iron homeostasis. Cellular iron mismanagement and oxidative stress are associated with a number of neurodegenerative diseases. One mechanism by which cells fail to properly regulate their iron status is through a mutation in the Hfe gene. Mutations in the Hfe gene are associated with the iron overload disease, hemochromatosis. In the current study, 31% of patients with sporadic ALS carried a mutation in the Hfe gene, compared to only 14% of patients without identifiable neuromuscular disease, or with neuromuscular diseases other than ALS (p<0.005). To determine the cellular consequences of carrying an Hfe mutation, a human neuronal cell line was transfected with genes carrying the Hfe mutation. The presence of the Hfe mutation disrupted expression of tubulin and actin at the protein levels potentially consistent with the disruption of axonal transport seen in ALS and was also associated with a decrease in CuZnSOD1 expression. These data provide compelling evidence for a role for the Hfe mutation in etiopathogenesis of ALS and warrant further investigation.

17β-Estradiol activates ICI 182,780-sensitive estrogen receptors and cyclic GMP-dependent thioredoxin expression for neuroprotection

Clinical studies suggest that estrogen may improve cognition in Alzheimers patients. Basic experiments demonstrate that 17β‐estradiol protects against neurodegeneration in both cell and animal models. In the present study, a human SH‐SY5Y cell model was used to investigate molecular mechanisms underlying the receptor‐mediated neuroprotection of physiological concentrations of 17β‐estradiol. 17β‐Estradiol (<10 nM) concomitantly increased neuronal nitric oxide synthase (NOS1) expression and cell viability. 17β‐Estradiol‐induced neuroprotection was blocked by the receptor antagonist ICI 182,780, also prevented by inhibitors of NOS1 (7‐nitroindazole), guanylyl cyclase (LY 83,583), and cGMP‐dependent protein kinase (PKG) (Rp‐8‐pCPT‐cGMPs). In addition to the expression of NOS1 and MnSOD, 17β‐estradiol increased the expression of the redox protein thioredoxin (Trx), which was blocked by the inhibition of either cGMP formation or PKG activity. The expression of heme oxygenase 2 and brain‐derived neurotrophic factor was not altered. Estrogen receptor‐enhanced cell viability against oxidative stress may be linked to Trx expression because the Trx reductase inhibitor, 5,5′‐dithio‐bis(2‐nitrobenzoic acid) significantly reduced the cytoprotective effect of 17β‐estradiol. Furthermore, Trx (1 μM) inhibited lipid peroxidation, proapoptotic caspase‐3, and cell death during oxidative stress caused by serum deprivation. We conclude that cGMP‐dependent expression of Trx–the redox protein with potent antioxidative and antiapoptotic properties–may play a pivotal role in estrogen‐induced neuroprotection.

HFE mutations and Alzheimer's disease

An imbalance in brain iron status has been established in Alzheimers Disease (AD). This iron imbalance can impact plaque formation, amyloid processing, and expression of and response to inflammatory agents. In a more general sense, a deregulation of iron homeostasis underlies the generation of reactive oxygen species leading to oxidative damage. Thus, loss of iron homeostasis can be central to the pathogenic events in AD. Recently a number of studies have begun to investigate the frequency of mutations in the HFE gene in AD. Mutations in the HFE gene occur more frequently in Caucasians than any other mutation. The two most common mutations of HFE are the C282Y (2% of the total population) and the H63D (9%. Mutations in this gene are associated with loss of iron homeostasis, alterations in inflammatory responses and in its most severe form, a clinical disorder known as Hemochromatosis. The C282Y mutation is more frequently associated with Hemochromatosis and the frequency of the H63D mutation is receiving increasing attention in neurodegenerative disorders. This review summarizes the data on HFE mutations in neurodegenerative disorders and what is known about HFE in the brain and the cell biology underlying the HFE mutation.

Consequences of expressing mutants of the hemochromatosis gene (HFE) into a human neuronal cell line lacking endogenous HFE

HFE mutations have traditionally been associated with the iron overload disorder known as hemochromatosis. Recently, it has become clear that the two most common mutations in the HFE gene, H63D and C282Y, may be genetic modifiers for risk of neurodegenerative disorders and cancer, respectively. We developed human neuroblastoma stable cell lines that express either wild‐type (WT) or mutant HFE to determine the cellular consequences of the mutant forms of HFE. The presence of the C282Y mutation was associated with relatively higher labile iron pool and iron regulatory protein activity than WT or H63D HFE. Targeted gene arrays revealed that the signal transduction pathway was up‐regulated in the C282Y cells. H63D cells had higher levels of lipid peroxidation, protein oxidation, and lower mitochondrial membrane potential, suggesting higher baseline stress. This cell line was also more vulnerable to exposure to oxidative stress agents and more responsive to iron chelation than the C282Y cells. These data demonstrate that the different mutations in the HFE gene have unique effects on the cells and provide insights into how the different mutations may have different clinical consequences. The results also raise multiple novel questions for future study about the function of the HFE protein.—Lee, S. Y., Patton, S. M., Henderson, R. J., Connor, J. R. Consequences of expressing mutants of the hemochromatosis gene (HFE) into a human neuronal cell line lacking endogenous HFE. FASEB J. 21, 564–576 (2007)

Induction of antioxidative and antiapoptotic thioredoxin supports neuroprotective hypothesis of estrogen.

The original neuroprotective hypothesis of estrogen was based on the gender difference in brain response to the ischemia-reperfusion injury. Additional clinical reports also suggest that estrogen may improve cognition in patients with Alzheimer disease. 17β-Estradiol is the most potent endogenous ligand of estrogen, which protects against neurodegeneration in both cell and animal models. Estrogen-mediated neuroprotection is probably mediated by both receptor-dependent and -independent mechanisms. Binding of estrogen such as 17β-estradiol to estrogen receptors (ERs) activates the homodimers of ER-DNA and its binding to estrogen response elements in the promoter region of genes such as neuronal nitric oxide synthase (NOS1) for regulating gene expression in target brain cells. In addition to the induction of NOS1, estrogen increases the expression of antiapoptotic protein such as bcl-2. Furthermore, our recent observations provide new molecular biologic and pharmacologic evidence suggesting that physiologic concentrations of 17β-estradiol (<10 nM) activate ERs (ERβ >ERα) and upregulate a cyclic guanosine 5′-monophosphate (cGMP)-dependent thioredoxin (Trx) and MnSOD expression following the induction of NOS1 in human brain-derived SH-SY5Y cells. We thus proposed that the estrogen-mediated gene induction of Trx plays a pivotal role in the promotion of neuroprotection because Trx is a multifunctional antioxidative and antiapoptotic protein. For managing progressive neurodegeneration such as Alzheimer dementia, our estrogen proposal of the signaling pathway of cGMP-dependent protein kinase (PKG) in mediating estrogen-induced cytoprotective genes thus fosters research and development of the new estrogen ligands devoid of female hormonal side effects such as carcinogenesis.

Influence of HFE variants and cellular iron on monocyte chemoattractant protein-1

BackgroundPolymorphisms in the MHC class 1-like gene known as HFE have been proposed as genetic modifiers of neurodegenerative diseases that include neuroinflammation as part of the disease process. Variants of HFE are relatively common in the general population and are most commonly associated with iron overload, but can promote subclinical cellular iron loading even in the absence of clinically identified disease. The effects of the variants as well as the resulting cellular iron dyshomeostasis potentially impact a number of disease-associated pathways. We tested the hypothesis that the two most common HFE variants, H63D and C282Y, would affect cellular secretion of cytokines and trophic factors.MethodsWe screened a panel of cytokines and trophic factors using a multiplexed immunoassay in human neuroblastoma SH-SY5Y cells expressing different variants of HFE. The influence of cellular iron secretion on the potent chemokine monocyte chemoattractant protein-1 (MCP-1) was assessed using ferric ammonium citrate and the iron chelator, desferroxamine. Additionally, an antioxidant, Trolox, and an anti-inflammatory, minocycline, were tested for their effects on MCP-1 secretion in the presence of HFE variants.ResultsExpression of the HFE variants altered the labile iron pool in SH-SY5Y cells. Of the panel of cytokines and trophic factors analyzed, only the release of MCP-1 was affected by the HFE variants. We further examined the relationship between iron and MCP-1 and found MCP-1 secretion tightly associated with intracellular iron status. A potential direct effect of HFE is considered because, despite having similar levels of intracellular iron, the association between HFE genotype and MCP-1 expression was different for the H63D and C282Y HFE variants. Moreover, HFE genotype was a factor in the effect of minocycline, a multifaceted antibiotic used in treating a number of neurologic conditions associated with inflammation, on MCP-1 secretion.ConclusionOur results demonstrate that HFE polymorphisms influence the synthesis and release of MCP-1. The mechanism of action involves cellular iron status but it appears there could be additional influences such as ER stress. Finally, these data demonstrate a pharmacogenetic effect of HFE polymorphisms on the ability of minocycline to inhibit MCP-1 secretion.

Temozolomide resistance in glioblastoma multiforme

Temozolomide (TMZ) is an oral alkylating agent used to treat glioblastoma multiforme (GBM) and astrocytomas. However, at least 50% of TMZ treated patients do not respond to TMZ. This is due primarily to the over-expression of O6-methylguanine methyltransferase (MGMT) and/or lack of a DNA repair pathway in GBM cells. Multiple GBM cell lines are known to contain TMZ resistant cells and several acquired TMZ resistant GBM cell lines have been developed for use in experiments designed to define the mechanism of TMZ resistance and the testing of potential therapeutics. However, the characteristics of intrinsic and adaptive TMZ resistant GBM cells have not been systemically compared. This article reviews the characteristics and mechanisms of TMZ resistance in natural and adapted TMZ resistant GBM cell lines. It also summarizes potential treatment options for TMZ resistant GBMs.

Mutant HFE H63D Protein Is Associated with Prolonged Endoplasmic Reticulum Stress and Increased Neuronal Vulnerability

A specific polymorphism in the hemochromatosis (HFE) gene, H63D, is over-represented in neurodegenerative disorders such as amyotrophic lateral sclerosis and Alzheimer disease. Mutations of HFE are best known as being associated with cellular iron overload, but the mechanism by which HFE H63D might increase the risk of neuron degeneration is unclear. Here, using an inducible expression cell model developed from a human neuronal cell line SH-SY5Y, we reported that the presence of the HFE H63D protein activated the unfolded protein response (UPR). This response was followed by a persistent endoplasmic reticulum (ER) stress, as the signals of UPR sensors attenuated and followed by up-regulation of caspase-3 cleavage and activity. Our in vitro findings were recapitulated in a transgenic mouse model carrying Hfe H67D, the mouse equivalent of the human H63D mutation. In this model, UPR activation was detected in the lumbar spinal cord at 6 months then declined at 12 months in association with increased caspase-3 cleavage. Moreover, upon the prolonged ER stress, the number of cells expressing HFE H63D in early apoptosis was increased moderately. Cell proliferation was decreased without increased cell death. Additionally, despite increased iron level in cells carrying HFE H63D, it appeared that ER stress was not responsive to the change of cellular iron status. Overall, our studies indicate that the HFE H63D mutant protein is associated with prolonged ER stress and chronically increased neuronal vulnerability.

HFE polymorphisms affect cellular glutamate regulation

HFE gene variants are relatively common genetic variants in Caucasians. The H63D HFE genetic variant has been repeatedly associated with a number of neurodegenerative diseases. We developed neuroblastoma cell lines expressing different HFE polymorphisms to explore the mechanisms behind these associations. Here we tested the hypothesis that cells with the H63D variant have a phenotype that promotes glutamate toxicity. In support of this hypothesis, expression of H63D HFE is associated with increased calcium-induced glutamate secretion and decreased cellular glutamate uptake. The polymorphism-associated changes in glutamate secretion were mimicked by altering cellular iron. Additionally, intracellular calcium is altered in a genotype-specific manner which could further impact glutamate secretion. HFE-dependent effects on glutamate uptake were confirmed in astrocytoma cell lines with endogenous expression of HFE. The ability of minocycline and the antioxidant Trolox to increase glutamate uptake differed by HFE genotype and implicate oxidative stress in glutamate regulation. This study demonstrates HFE cellular effects that extend beyond iron regulation, and suggests that H63D HFE may promote glutamate toxicity.

H63D mutation in hemochromatosis alters cholesterol metabolism and induces memory impairment

The H63D variant of the hemochromatosis (HFE) gene, when expressed in carriers of the apolipoprotein E4 allele, is implicated as a risk factor for earlier onset of Alzheimers disease (AD). We tested the hypothesis that like expression of apolipoprotein E4, expression of H63D-HFE disrupts cholesterol metabolism contributing to an increase in neurodegeneration and memory deficits. Analysis of SH-SY5Y human neuroblastoma cells transfected to stably express either wild type- (WT) or H63D-HFE indicated about a 50% reduction in cholesterol content in cells expressing H63D-HFE. This was accompanied by a significant decrease in expression of 3-hydroxy-3-methyl-glutaryl-CoA reductase, and a significant increase in expression of cholesterol 24-hydroxylase. Consistent with these studies, H67D-HFE (orthologous to human H63D-HFE) knock-in mice, showed a greater age dependent decline in brain cholesterol than WT-HFE animals and changes in expression of proteins regulating cholesterol metabolism. Brains of aged H67D-HFE mice also exhibited a significant decrease in expression of synapse proteins and a significant increase in caspase-3 expression relative to WT-HFE controls. H67D-HFE mice also had a greater reduction in brain volume and poorer recognition and spatial memory than WT-HFE mice, symptoms associated with AD. These results indicate that the alterations in cholesterol metabolism associated with expression of H63D-HFE may contribute to the development of AD.