Disability-related inequalities in the prevalence of loneliness across the lifespan: trends from Australia, 2003 to 2020.

BACKGROUND: Experiencing loneliness can be distressing and increasing evidence indicates that being lonely is associated with poor physical and mental health outcomes. Cross-sectional studies have demonstrated that people with disability have increased risk of experiencing loneliness compared to people without disability. However, we do not know if these inequalities have changed over time. This study investigated the prevalence of loneliness for people with disability in Australia annually from 2003 to 2020 to examine whether disability-related inequalities in loneliness have changed over time, and disaggregated results for subgroups of people with disability by age group, sex, and disability group. METHODS: We used annual data (2003-2020) from the Household, Income and Labour Dynamics in Australia Survey. Loneliness was measured by a single question assessing the subjective experience of loneliness. For each wave, we calculated population-weighted age-standardised estimates of the proportion of people experiencing loneliness for people with and without disability. We then calculated the absolute and relative inequalities in loneliness between people with and without disability for each wave. Analyses were stratified by 10-year age groups, sex, and disability group (sensory or speech, physical, intellectual or learning, psychological, brain injury or stroke, other). RESULTS: From 2003 to 2020, the prevalence of loneliness was greater for people with disability, such that people with disability were 1.5 to 1.9 times more likely to experience loneliness than people without disability. While the prevalence of loneliness decreased for people without disability between 2003 and 2020, the prevalence of loneliness did not decrease for people with disability during this period. Inequalities in loneliness were more substantial for people with intellectual or learning disabilities, psychological disability, and brain injury or stroke. CONCLUSION: This study confirms that people with disability have increased risk of loneliness compared to people without disability. We add to the existing evidence by demonstrating that disability-related inequalities in loneliness have persisted for two decades in Australia without improvement. Our findings indicate that addressing inequalities in loneliness for people with disability is a critical public health concern given that loneliness is associated with a wide range of poor health outcomes.

Disability and loneliness in the United Kingdom: cross-sectional and longitudinal analyses of trends and transitions.

BACKGROUND: Loneliness can have a detrimental impact on health, yet little is known about the association between disability and loneliness. METHODS: Secondary analysis of three waves of data collected between 2017 and 2020 by the UK's annual household panel study, Understanding Society. Direct age-standardisation was used to compare the prevalence of loneliness at each wave and the persistence of loneliness across all three waves for participants with/without disabilities aged 16-65 years. Transitional probabilities for the stability of loneliness, the stability of non-loneliness, the onset of loneliness and the offset of loneliness between consecutive waves were also estimated. RESULTS: At each wave, the prevalence of loneliness was significantly higher among respondents with disabilities than respondents without disabilities; these inequalities persisted with no evidence of change over time. The prevalence of persistent loneliness was 46% for respondents with disabilities compared with 22% for respondents without disabilities. Risk factors for the likelihood of persistent loneliness included disability, financial stress, not living as a couple, living in rented accommodation, being female and not being employed. The probability of the onset and stability of loneliness between successive waves were markedly higher for people with disabilities compared with people without disabilities. CONCLUSION: Adults with disabilities were more likely to experience loneliness, become lonely and remain lonely over time than their peers. Policies and interventions aimed at reducing loneliness should ensure that they are accessible and effective for people with disabilities. Further research is needed to explore the health outcomes of persistent loneliness among people with/without disabilities.

Health and wellbeing outcomes associated with loneliness for people with disability: a scoping review.

BACKGROUND: Loneliness is a significant public health concern due to its detrimental impact on health and wellbeing. Despite people with disability reporting higher levels of loneliness than the general population, there has been little research into how this is affecting their health and wellbeing. In light of this, the aim of our study was to scope both the existing evidence about the health and wellbeing outcomes associated with loneliness for people with disability, as well as the conceptual frameworks and measures utilised in this field of research. METHODS: To conduct this scoping review, we followed the methodology outlined by JBI and searched MEDLINE, Scopus, Informit, Embase, and Web of Science for peer-reviewed, English-language articles published between 1 January 2000 and 8 February 2023. Two independent reviewers completed screening, full-text review and data extraction, with consensus sought at each stage. Data were analysed using content analysis and presented both numerically and narratively. RESULTS: Out of the initial 1602 publications identified in the scoping review, only nine were included after duplicate removal, title and abstract screening, and full-text review. This limited number of studies, with the earliest study one published in 2015, represents a key finding. Eight of the nine studies were quantitative, and all were conducted in high income countries. Most of these studies utilised a version of the University of Los Angles Loneliness Scale to measure loneliness and addressed specific impairment groups. Notably, most of the studies identified associations between loneliness and health and wellbeing outcomes for people with disability. CONCLUSIONS: This scoping review highlights the current scarcity of studies examining the effect that loneliness has on the health and wellbeing outcomes of people with disability. As most of the reviewed studies relied on loneliness measures designed for individuals without disability, they potentially overlook the unique life experiences of people with disability. Given that loneliness is an international public health concern, it is imperative that people with disability are not left behind or overlooked in efforts to address the impact of loneliness on health and wellbeing.

Trends in mental health inequalities for people with disability, Australia 2003 to 2020.

OBJECTIVE: Cross-sectional studies have demonstrated that people with disability have substantial inequalities in mental health compared to people without disability. However, it is not known if these inequalities have changed over time. This study compared the mental health of people with and without disability annually from 2003 to 2020 to investigate time trends in disability-related mental health inequalities. METHODS: We use annual data (2003-2020) of the Household, Income and Labour Dynamics in Australia Survey. Mental health was measured using the five-item Mental Health Inventory. For each wave, we calculated population-weighted age-standardised estimates of mean Mental Health Inventory scores for people with and without disability and calculated the mean difference in Mental Health Inventory score to determine inequalities. Analyses were stratified by age, sex and disability group (sensory or speech, physical, intellectual or learning, psychological, brain injury or stroke, other). RESULTS: From 2003 to 2020, people with disability had worse mental health than people without disability, with average Mental Health Inventory scores 9.8 to 12.1 points lower than for people without disability. For both people with and without disability, Mental Health Inventory scores decreased, indicating worsening mental health, reaching the lowest point for both groups in 2020. For some subpopulations, including young females and people with intellectual disability, brain injury or stroke, mental health inequalities worsened. CONCLUSION: This paper confirms that people with disability experience worse mental health than people without disability. We add to previous findings by demonstrating that disability-related inequalities in mental health have been sustained for a long period and are worsening in some subpopulations.

Disability-related inequalities in health and well-being are mediated by barriers to participation faced by people with disability. A causal mediation analysis.

Large inequalities in health and well-being exist between people with and without disability, in part due to poor socio-economic circumstances, and potentially also related to societal factors including issues associated with accessibility and participation. To better understand the contribution of societal factors, we used a unique longitudinal survey of disability in Great Britain to quantify the extent to which barriers to participation contribute to poorer health and well-being. We used data from 2354 individuals who participated in three waves of the Life Opportunities Survey between 2009 and 2014 and compared five health and well-being outcomes (self-rated health, anxiousness, life satisfaction, life worth, happiness) between adults who acquired an impairment and those who remained disability-free. Causal mediation analysis was conducted to quantify how much of the effect of disability acquisition on each outcome was explained by barriers to participation in employment, economic life, transport, community, leisure and civic activities, social contact, and accessibility. People who recently acquired a disability had poorer health and well-being compared to people with no disability. Barriers to participation explained 15% of inequalities in self-rated health, 28% for anxiousness, 32% for life satisfaction, 37% for life worth, and 70% for happiness. A substantial proportion of the inequalities in health and well-being experienced by people with recently acquired disability were socially produced, driven by barriers to participation in different life domains. Furthermore, there was evidence that barriers to participation mediated the effect of well-being measured to a greater extent than the more clinically aligned measures, self-reported health and anxiousness. These findings highlight modifiable factors amenable to public health interventions that could lead to substantial improvements in health and well-being for people with disability.

Young people in Australian residential aged care: evaluating trends from 2008 to 2018.

Objective Over the past decade, various programs and reforms have targeted the issue of people aged <65 years living in 'permanent' residential aged care (PRAC). As context for ongoing policy discourse, the aim of this study was to evaluate trends in rates of young people entering and leaving PRAC from 2008 to 2018. Methods Counts of people aged <65 years entering, remaining in and exiting PRAC were obtained from the National Aged Care Data Clearinghouse. Age standardisation was used to control for changes in the age and size of the Australian population. Annual age-standardised rates of admissions (subtracting transfers) and exits to the community were calculated. Linear regression models tested for a sustained increase or decrease in age-standardised rates nationally and within state and age subgroups. Results Notwithstanding year-to-year variation, neither admissions (subtracting transfers) nor exits to the community showed statistically significant increasing or decreasing trends in the national age-standardised rates. Admission rates varied by age and state. Conclusions Many more young people are admitted to PRAC each year than return to community living, with no sustained change between 2008 and 2018 at the national level. Age standardisation is crucial for evaluating systemic population-level change regarding younger people in PRAC. What is known about the topic? As at June 2018, over 6000 people aged <65 years still live in PRAC in Australia. Previous research has demonstrated that this cohort experiences a much poorer quality of life on average than people of similar age and disability who reside in other community settings. Various strategies for improving outcomes have been trialled, many aiming to reduce the number of younger people living in aged care; the National Disability Insurance Scheme (NDIS) also has this among its aims. What does this paper add? This paper reports trends in the number of young people entering and exiting aged care, after statistically controlling for changes due to population growth and aging. The paper highlights that national admission rates did not increase or decrease in a sustained manner, and that most of those admitted never return to community living before turning 65 years of age. What are the implications for practitioners? Programs and policies aimed at reducing the number of young people in aged care must grapple with the scale of the issue and its apparent resistance to amelioration over the past 10 years. The results of this study provide a benchmark against which to judge the future impact of the NDIS.

Uptake and Toxicity of Hemin and Iron in Cultured Mouse Astrocytes.

Hemin is a breakdown product of the blood protein, hemoglobin and is responsible for much of the secondary damage caused following a hemorrhagic stroke. Hemin is toxic to cultured astrocytes and it is thought that this toxicity is due to iron that is liberated when hemin is degraded. However, free iron applied to astrocytes is not toxic and the reason for this discrepancy is unknown. The present study exposed primary astrocyte cultures from neonatal mice to hemin-iron (25 µM hemin) or non-hemin iron (25 µM ferric ammonium citrate; FAC) for 12 or 24 h. Perls' and Turnbull's staining, as well as measures of cell viability and iron accumulation, were used to assess the valency, solubility and distribution of iron within cells. While cells accumulated similar amounts of iron from both sources, hemin was shown to be highly toxic to astrocytes, whereas FAC was not. Iron released by the degradation of hemin was present in both valencies (Fe(2+) and Fe(3+)), was mostly soluble and did not induce ferritin expression in most cells, whereas non-hemin iron (from FAC) was present in astrocytes almost exclusively as insoluble Fe(3+) and it induced widespread ferritin expression. These results show that the cellular mechanisms for processing hemin-iron and non-hemin iron are very different. The data suggest that hemin-iron has a greater potential to damage astrocytes by participating in unregulated redox reactions.

Inhibition of Astrocytic Glutamine Synthetase by Lead is Associated with a Slowed Clearance of Hydrogen Peroxide by the Glutathione System.

Lead intoxication in humans is characterized by cognitive impairments, particularly in the domain of memory, where evidence indicates that glutamatergic neurotransmission may be impacted. Animal and cell culture studies have shown that lead decreases the expression and activity of glutamine synthetase (GS) in astrocytes, yet the basis of this effect is uncertain. To investigate the mechanism responsible, the present study exposed primary astrocyte cultures to a range of concentrations of lead acetate (0-330 µM) for up to 24 h. GS activity was significantly reduced in cells following 24 h incubation with 100 or 330 µM lead acetate. However, no reduction in GS activity was detected when astrocytic lysates were co-incubated with lead acetate, suggesting that the mechanism is not due to a direct interaction and involves intact cells. Since GS is highly sensitive to oxidative stress, the capacity of lead to inhibit the clearance of hydrogen peroxide (H2O2) was investigated. It was found that exposure to lead significantly diminished the capacity of astrocytes to degrade H2O2, and that this was due to a reduction in the effectiveness of the glutathione system, rather than to catalase. These results suggest that the inhibition of GS activity in lead poisoning is a consequence of slowed H2O2 clearance, and supports the glutathione pathway as a primary therapeutic target.

Phenanthrolines protect astrocytes from hemin without chelating iron.

Hemin, the degradation product of hemoglobin, contributes to the neurodegeneration that occurs in the weeks following a hemorrhagic stroke. The breakdown of hemin in cells releases redox-active iron that can facilitate the production of toxic hydroxyl radicals. The present study used 3-week old primary cultures of mouse astrocytes to compare the toxicity of 33 µM hemin in the presence of the iron chelator 1,10-phenanthroline or its non-chelating analogue, 4,7-phenanthroline. This concentration of hemin killed approximately 75 % of astrocytes within 24 h. Both isoforms of phenanthroline significantly decreased the toxicity of hemin, with the non-chelating analogue providing complete protection at concentrations of 33 µM and above. The decrease in toxicity was associated with less cellular accumulation of hemin. Approximately 90 % of the hemin accumulated was not degraded, irrespective of treatment condition. These observations indicate that chelatable iron is not the cause of hemin toxicity. Cell-free experiments demonstrated that hemin can inactivate a molar excess of hydrogen peroxide (H2O2), and that the rate of inactivation is halved in the presence of either isoform of phenanthroline. We conclude that phenanthrolines may protect astrocytes by limiting hemin uptake and by impairing the capacity of intact hemin to interact with endogenous H2O2.

The metabolism and toxicity of hemin in astrocytes.

Hemin is cytotoxic, and contributes to the brain damage that accompanies hemorrhagic stroke. In order to better understand the basis of hemin toxicity in astrocytes, the present study quantified hemin metabolism and compared it to the pattern of cell death. Heme oxygenase-1 (HO-1) expression was first evident after 2 h incubation with hemin, with maximal expression being observed by 24 h. Despite the induction of HO-1, it was found that the proportion of hemin metabolized by astrocytes remained fairly constant throughout the 24 h period, with 70-80% of intracellular hemin remaining intact. A period of cell loss began after 2 h exposure to hemin, which gradually increased in severity to reach a maximum by 24 h. This cell loss could not be attenuated by the iron chelator, 1,10-phenanthroline, or by several antioxidant compounds (Trolox, N-acetyl-L-cysteine and N-tert-butyl-α-phenylnitrone), indicating that the mechanism of hemin toxicity does not involve iron. While these results make it unlikely that hemin toxicity is due to interactions with endogenous H(2)O(2), hemin toxicity was increased in the presence of supraphysiological levels of H(2)O(2) and this increase was ameliorated by PHEN, indicating that the iron released from hemin can be toxic under some pathological conditions. However, when H(2)O(2) is present at physiological levels, the toxicity of hemin appears to be caused by other mechanisms that may involve bilirubin and carbon monoxide in this model system.

Uptake, metabolism and toxicity of hemin in cultured neurons.

Following hemorrhagic stroke, red blood cells lyse and release neurotoxic hemin into the interstitial space. The present study investigates whether neurons can accumulate and metabolize hemin. We demonstrate that cultured neurons express the heme carrier protein 1 (HCP1), and that this transporter appears to contribute to the time- and concentration-dependent accumulation of hemin by neurons. Although exposure of neurons to hemin stimulates the synthesis of the iron storage protein ferritin, approximately 80% of the hemin accumulated by neurons remains intact. Within 24h of incubation, substantial neurotoxicity was observed that was not attenuated by the cell permeable, selective ferrous iron chelator, 1,10-phenanthroline. These results demonstrate that while neurons efficiently accumulate hemin they slowly degrade it, and they support the conclusion that intact hemin is more neurotoxic than the iron released from the breakdown of hemin. Further investigations are required to determine the basis of this neurotoxicity.

Accumulation of non-transferrin-bound iron by neurons, astrocytes, and microglia.

Neurodegenerative conditions such as Alzheimer's disease, Parkinson's disease, and hemorrhagic stroke are associated with increased levels of non-transferrin-bound iron (NTBI) in the brain, which can promote Fenton chemistry. While all types of brain cells can take up NTBI, their efficiency of accumulation and capacity to withstand iron-mediated toxicity has not been directly compared. The present study assessed NTBI accumulation in cultures enriched in neurons, astrocytes, or microglia after exposure to ferric ammonium citrate (FAC). Microglia were found to be the most efficient in accumulating iron, followed by astrocytes, and then neurons. Exposure to 100 µM FAC for 24 h increased the specific iron content of cultured neurons, astrocytes, and microglial cells by 30-, 80-, and 100-fold, respectively. All cell types accumulated iron against the concentration gradient, resulting in intracellular iron concentrations that were several orders of magnitude higher than the extracellular iron concentrations. Accumulation of these large amounts of iron did not affect the viability of the cell cultures, indicating a high resistance to iron-mediated toxicity. These findings show that neurons, astrocytes and microglia cultured from neonatal mice all have the capacity to accumulate and safely store large quantities of iron, but that glial cells do this more efficiently than neurons. It is concluded that neurodegenerative conditions involving iron-mediated toxicity may be due to a failure of iron transport or storage mechanisms, rather than to the presence of high levels of NTBI.

Two routes of iron accumulation in astrocytes: ascorbate-dependent ferrous iron uptake via the divalent metal transporter (DMT1) plus an independent route for ferric iron.

Astrocytes are central to iron and ascorbate homoeostasis within the brain. Although NTBI (non-transferrin-bound iron) may be a major form of iron imported by astrocytes in vivo, the mechanisms responsible remain unclear. The present study examines NTBI uptake by cultured astrocytes and the involvement of ascorbate and DMT1 (divalent metal transporter 1). We demonstrate that iron accumulation by ascorbate-deficient astrocytes is insensitive to both membrane-impermeant Fe(II) chelators and to the addition of the ferroxidase caeruloplasmin. However, when astrocytes are ascorbate-replete, as occurs in vivo, their rate of iron accumulation is doubled. The acquisition of this additional iron depends on effluxed ascorbate and can be blocked by the DMT1 inhibitor ferristatin/NSC306711. Furthermore, the calcein-accessible component of intracellular labile iron, which appears during iron uptake, appears to consist of only Fe(III) in ascorbate-deficient astrocytes, whereas that of ascorbate-replete astrocytes comprises both valencies. Our data suggest that an Fe(III)-uptake pathway predominates when astrocytes are ascorbate-deficient, but that in ascorbate-replete astrocytes, at least half of the accumulated iron is initially reduced by effluxed ascorbate and then imported by DMT1. These results suggest that ascorbate is intimately involved in iron accumulation by astrocytes, and is thus an important contributor to iron homoeostasis in the mammalian brain.

Astrocytes retain their antioxidant capacity into advanced old age.

Oxidative stress has been implicated in the progression of ageing and in many age-related neurodegenerative conditions. Astrocytes play a major role in the antioxidant protection of the brain, yet little is known about how the antioxidant defenses of astrocytes change across the lifespan. This study assessed the antioxidant capacity and glutathione metabolism of astrocytes cultured from the brains of neonatal (<24 h old), mature (12-month-old), old (25-month-old), and senescent (31-month-old) C57BL/6J mice. When exposed to 100 microM hydrogen peroxide, mature, old, and senescent astrocytes cleared the peroxide approximately 30% more slowly than neonatal astrocytes. This difference persisted when catalase was inhibited by 3-aminotriazole, but was abolished when glutathione was depleted by application of buthionine sulfoximine, suggesting a deficit in the glutathione system. Correspondingly, the specific glutathione reductase activity of mature, old, and senescent astrocytes was approximately 30% lower than that of neonatal cultures, whereas no age-related change was observed in the specific activities of glutathione peroxidase, catalase, or in total antioxidant capacity. In addition, the specific rate of glutathione export was almost identical in mature, old, and senescent astrocytes, but was more than double that of neonatal astrocytes. These results indicate that the antioxidant capacity and glutathione metabolism of astrocytes are preserved from mature adulthood into senescence. It is concluded that the oxidative stress seen in ageing brains is likely due to factors extrinsic to astrocytes, rather than to an impairment of the antioxidative functions of astrocytes.

Histidine, cystine, glutamine, and threonine collectively protect astrocytes from the toxicity of zinc.

In epilepsy, traumatic brain injury, and ischemic stroke, toxic levels of zinc released from neurons contribute to the brain damage associated with these disorders. Zinc causes oxidative stress by increasing the generation of reactive oxygen species and by inhibiting glutathione reductase (GR). This study investigated whether naturally occurring amino acids can protect cultured mouse astrocytes from zinc. Astrocytes incubated for 24h with 33 microM zinc acetate in a minimal medium displayed a loss of GR activity, a depletion of total glutathione, and a loss of total antioxidant capacity. These changes were accompanied by extensive cell death. Four amino acids (200 microM L-histidine, 201 microM L-cystine, 4mM L-glutamine, 798 microM L-threonine), which are a subset of the 15 present in Dulbecco's modified Eagle medium, were found to collectively prevent zinc toxicity. Histidine was the most protective, followed by cystine, glutamine, and threonine. It is proposed that each of these amino acids protects against different aspects of zinc toxicity by chelating zinc, by serving as precursors for glutathione, or by converting to tricarboxylic acid cycle intermediates. It is possible that these 4 amino acids contribute in vivo to the protection of brain cells from the toxic effects of zinc.

Synergistic accumulation of iron and zinc by cultured astrocytes.

Iron and zinc are essential for normal brain function, yet the mechanisms used by astrocytes to scavenge non-transferrin-bound iron (NTBI) and zinc are not well understood. Ischaemic stroke, traumatic brain injury and Alzheimer's disease are associated with perturbations in the metabolism of NTBI and zinc, suggesting that these two metals may collectively contribute to pathology. The present study has investigated the accumulation of NTBI and zinc by rat primary astrocyte cultures. It was found that astrocytes express mRNA for both divalent metal transporter 1 (DMT1) and Zip14, indicating the potential for these transporters to contribute to the accumulation of NTBI and zinc by these cells. Astrocytes were found to accumulate iron from ferric chloride in a time- and dose-dependent manner, and the rate of accumulation was strongly stimulated by co-incubation with zinc acetate. In addition, cultured astrocytes rapidly accumulated zinc from zinc acetate, and this accumulation was stimulated by co-incubation with ferric chloride. Because a synergistic stimulation of iron and zinc accumulation is inconsistent with the known properties of DMT1 and Zip14, the present results suggest that additional mechanisms assist astrocytes to scavenge iron and zinc when they are present together in the extracellular compartment. These mechanisms may be involved in disorders that involve elevations in the extracellular concentrations of these metal ions.

Effects of carboxylic acids on the uptake of non-transferrin-bound iron by astrocytes.

The concentrations of non-transferrin-bound iron are elevated in the brain during pathological conditions such as stroke and Alzheimer's disease. Astrocytes are specialised for sequestering this iron, however little is known about the mechanisms involved. Carboxylates, such as citrate, have been reported to facilitate iron uptake by intestinal cells. Citrate binds iron and limits its redox activity. The presence of high citrate concentrations in the interstitial fluid of the brain suggests that citrate may be an important ligand for iron transport by astrocytes. This study investigates whether iron accumulation by cultured rat astrocytes is facilitated by citrate or other carboxylates. Contrary to expectations, citrate, tartrate and malate were found to block iron accumulation in a concentration-dependent manner; alpha-ketoglutarate had limited effects, while fumarate, succinate and glutarate had no effect. This blockade was not due to an inhibition of ferric reductase activity. Instead, it appeared to be related to the capacity of these carboxylates to bind iron, since phosphate, which also binds iron, diminished the capacity of citrate, tartrate and malate to block the cellular accumulation of iron. These findings raise the possibility that citrate may have therapeutic potential in the management of neurodegenerative conditions that involve cellular iron overload.

Uptake of ferrous iron by cultured rat astrocytes.

Astrocytes are considered to play an important role in iron homeostasis of the brain, yet the mechanisms involved in the uptake of iron into astrocytes remain elusive. To investigate the uptake of iron into astrocytes, we have applied ferric ammonium citrate (FAC) to rat astrocyte-rich primary cultures. These cultures express the mRNAs of two membrane-bound ferric reductases, Dcytb and SDR2, and reduce extracellular ferric iron (100 muM) with a rate of 3.2 +/- 0.4 nmol/(hr x mg). This reduction rate is substantially lower than the rate of cellular iron accumulation from 100 muM FAC [24.7 +/- 8.9 nmol/(hr x mg)], which suggests that iron accumulation from FAC does at best partially depend on extracellular ferric reduction. Nonetheless, when the iron in FAC was almost completely reduced by an excess of exogenous ascorbate, astrocytes accumulated iron in a time- and concentration-dependent manner with specific iron accumulation rates that increased linearly for concentrations of up to 100 muM ferrous iron. This accumulation was attenuated by lowering the incubation temperature, by the presence of ferrous iron chelators, or by lowering the pH from 7.4 to 6.8. These data indicate that, in addition to the DMT1-mediated uptake of ferrous iron, astrocytes can accumulate ferric and ferrous iron by mechanisms that are independent of DMT1 or transferrin.

The putative heme transporter HCP1 is expressed in cultured astrocytes and contributes to the uptake of hemin.

Hemin, which is toxic to brain cells, has been reported to be taken up by cultured astrocytes; however, the mechanism of uptake is currently unknown. The present study investigated the mechanism of hemin uptake by rat primary astrocyte cultures. In medium containing 10% fetal calf serum, cultured astrocytes failed to accumulate significant amounts of heme-iron, while in serum-free medium the accumulation of heme-iron was found to be time- and concentration-dependent. After 6 h of incubation with 24 muM hemin, cells contained 36.2 +/- 2.4 nmol heme-iron/mg protein, which was 21% of the applied hemin. These results suggest that the accumulation of hemin in astrocytes does not require serum proteins such as hemopexin. A potential mechanism of hemin uptake in astrocytes involves the heme carrier protein 1 (HCP1), which is reported to mediate hemin uptake into intestinal cells. RT-PCR analysis revealed that astrocyte cultures contained HCP1 mRNA, and immunocytochemical staining and Western blot analysis confirmed the expression of HCP1 protein in cultured astrocytes. The functionality of HCP1 in astrocytes was demonstrated by incubating cells with zinc protoporphyrin IX (ZnPPIX), which is known to be transported into cells via HCP1, and ZnPPIX autofluorescence was detected in HCP1-positive astrocytes. In addition, ZnPPIX was found to attenuate the accumulation of heme-iron by astrocytes. These results are the first to demonstrate that cultured astrocytes contain functional HCP1 and that this transporter contributes to hemin uptake by astrocytes. HCP1 may therefore provide a new target for reducing hemin-related toxicity in brain cells.

Hemin toxicity: a preventable source of brain damage following hemorrhagic stroke.

Hemorrhagic stroke is a common cause of permanent brain damage, with a significant amount of the damage occurring in the weeks following a stroke. This secondary damage is partly due to the toxic effects of hemin, a breakdown product of hemoglobin. The serum proteins hemopexin and albumin can bind hemin, but these natural defenses are insufficient to cope with the extremely high amounts of hemin (10 mM) that can potentially be liberated from hemoglobin in a hematoma. The present review discusses how hemin gets into brain cells, and examines the multiple routes through which hemin can be toxic. These include the release of redox-active iron, the depletion of cellular stores of NADPH and glutathione, the production of superoxide and hydroxyl radicals, and the peroxidation of membrane lipids. Important gaps are revealed in contemporary knowledge about the metabolism of hemin by brain cells, particularly regarding how hemin interacts with hydrogen peroxide. Strategies currently being developed for the reduction of hemin toxicity after hemorrhagic stroke include chelation therapy, antioxidant therapy and the modulation of heme oxygenase activity. Future strategies may be directed at preventing the uptake of hemin into brain cells to limit the opportunity for toxic interactions.