Common ancestors: chronic progressive diseases have the same pathogenesis

Towards a unifying, systems biology understanding of large-scale cellular death and destruction caused by poorly liganded iron: Parkinson's, Huntington's, Alzheimer's, prions, bactericides, chemical toxicology and others as examples

Exposure to a variety of toxins and/or infectious agents leads to disease, degeneration and death, often characterised by circumstances in which cells or tissues do not merely die and cease to function but may be more or less entirely obliterated. It is then legitimate to ask the question as to whether, despite the many kinds of agent involved, there may be at least some unifying mechanisms of such cell death and destruction. I summarise the evidence that in a great many cases, one underlying mechanism, providing major stresses of this type, entails continuing and autocatalytic production (based on positive feedback mechanisms) of hydroxyl radicals via Fenton chemistry involving poorly liganded iron, leading to cell death via apoptosis (probably including via pathways induced by changes in the NF-κB system). While every pathway is in some sense connected to every other one, I highlight the literature evidence suggesting that the degenerative effects of many diseases and toxicological insults converge on iron dysregulation. This highlights specifically the role of iron metabolism, and the detailed speciation of iron, in chemical and other toxicology, and has significant implications for the use of iron chelating substances (probably in partnership with appropriate anti-oxidants) as nutritional or therapeutic agents in inhibiting both the progression of these mainly degenerative diseases and the sequelae of both chronic and acute toxin exposure. The complexity of biochemical networks, especially those involving autocatalytic behaviour and positive feedbacks, means that multiple interventions (e.g. of iron chelators plus antioxidants) are likely to prove most effective. A variety of systems biology approaches, that I summarise, can predict both the mechanisms involved in these cell death pathways and the optimal sites of action for nutritional or pharmacological interventions.

Iron behaving badly: inappropriate iron chelation as a major contributor to the aetiology of vascular and other progressive inflammatory and degenerative diseases

BACKGROUND

The production of peroxide and superoxide is an inevitable consequence of aerobic metabolism, and while these particular 'reactive oxygen species' (ROSs) can exhibit a number of biological effects, they are not of themselves excessively reactive and thus they are not especially damaging at physiological concentrations. However, their reactions with poorly liganded iron species can lead to the catalytic production of the very reactive and dangerous hydroxyl radical, which is exceptionally damaging, and a major cause of chronic inflammation.

REVIEW

We review the considerable and wide-ranging evidence for the involvement of this combination of (su)peroxide and poorly liganded iron in a large number of physiological and indeed pathological processes and inflammatory disorders, especially those involving the progressive degradation of cellular and organismal performance. These diseases share a great many similarities and thus might be considered to have a common cause (i.e. iron-catalysed free radical and especially hydroxyl radical generation).The studies reviewed include those focused on a series of cardiovascular, metabolic and neurological diseases, where iron can be found at the sites of plaques and lesions, as well as studies showing the significance of iron to aging and longevity. The effective chelation of iron by natural or synthetic ligands is thus of major physiological (and potentially therapeutic) importance. As systems properties, we need to recognise that physiological observables have multiple molecular causes, and studying them in isolation leads to inconsistent patterns of apparent causality when it is the simultaneous combination of multiple factors that is responsible.This explains, for instance, the decidedly mixed effects of antioxidants that have been observed, since in some circumstances (especially the presence of poorly liganded iron) molecules that are nominally antioxidants can actually act as pro-oxidants. The reduction of redox stress thus requires suitable levels of both antioxidants and effective iron chelators. Some polyphenolic antioxidants may serve both roles.Understanding the exact speciation and liganding of iron in all its states is thus crucial to separating its various pro- and anti-inflammatory activities. Redox stress, innate immunity and pro- (and some anti-)inflammatory cytokines are linked in particular via signalling pathways involving NF-kappaB and p38, with the oxidative roles of iron here seemingly involved upstream of the IkappaB kinase (IKK) reaction. In a number of cases it is possible to identify mechanisms by which ROSs and poorly liganded iron act synergistically and autocatalytically, leading to 'runaway' reactions that are hard to control unless one tackles multiple sites of action simultaneously. Some molecules such as statins and erythropoietin, not traditionally associated with anti-inflammatory activity, do indeed have 'pleiotropic' anti-inflammatory effects that may be of benefit here.

CONCLUSION

Overall we argue, by synthesising a widely dispersed literature, that the role of poorly liganded iron has been rather underappreciated in the past, and that in combination with peroxide and superoxide its activity underpins the behaviour of a great many physiological processes that degrade over time. Understanding these requires an integrative, systems-level approach that may lead to novel therapeutic targets.

Hyperinsulinemia and cognitive decline in a middle-aged cohort

OBJECTIVE

Determining modifiable risks factors for cognitive decline and dementia are a public health priority as we seek to prevent dementia. Type 2 diabetes and related disorders such as hyperinsulinemia increase with aging and are increasing in the U.S. population. Our objective was to determine whether hyperinsulinemia is associated with cognitive decline among middle-aged adults without type 2 diabetes, dementia, or stroke in the Atherosclerosis Risk in Communities (ARIC) cohort.

RESEARCH DESIGN AND METHODS

Middle-aged adults (aged 45-64 years at baseline) in the ARIC cohort had fasting insulin and glucose assessed between 1987 and 1989. Subjects with dementia, type 2 diabetes, or stroke at baseline were excluded from analysis. Three tests of cognitive function available at baseline and 6 years later were delayed word recall (DWR), digit symbol subtest (DSS), and first letter word fluency (WF). Cross-sectional comparisons and linear regression models were computed for cognitive tests at baseline and change in cognitive test scores to determine whether cognitive function was associated with two measures of insulin resistance, fasting insulin and homeostasis model assessment (HOMA). Linear regression models controlled for age, sex, race, marital status, education level, smoking status, alcohol use, depression, hypertension, and hyperlipidemia.

RESULTS

In unadjusted and adjusted analyses, hyperinsulinemia based on fasting insulin and HOMA at baseline was associated with significantly lower baseline DWR, DSS, and WF scores and a greater decline over 6 years in DWR and WF.

CONCLUSIONS

Insulin resistance is a potentially modifiable midlife risk factor for cognitive decline and dementia.

Association studies of transforming growth factor-beta 1 and Alzheimer's disease

Substantial laboratory evidence suggests Transforming Growth Factor-beta1 (TGFB1) is linked to Alzheimer's Disease (AD) pathology. The purpose of the study was to estimate the genetic association of TGFB1 with AD while controlling for apolipoprotein E4 allele (APOE4) status, the only well-established genetic risk factor for AD. Two study populations were genotyped for the TGFB1-509 and +869 single nucleotide polymorphisms (SNPs) that have been associated with TGFB1 levels. Constituting these populations were 203 families from the NIMH AD Genetic Initiative with at least two affected siblings and a normal sibling, and a population of 126 African-American (AA) AD cases versus 93 age matched controls. Results from family-based analyses showed a significant association between the TGFB1 -509 SNP and AD for the entire set of 203 families (P = 0.007), and a subset of these families without a homozygous APOE4 family member (P = 0.026). Results from family-based analyses on the TGFB1 +869 SNP were not significant in the 203 families. While associations for the main effects of the TGFB1 +869 and -509 SNP with AD in the AA case-control study were also not significant, results did indicate that TGFB1 may function as an effect modifier of APOE4 risk. Specifically, the odds of AD associated with having at least one APOE4 allele increased in an additive fashion with one or two copies of the higher producer allele when stratified by TGFB1 -509 genotype and by TGFB1 +869 genotype. Results support a role for TGFB1 in AD pathogenesis.

Diabetes and brain aging: epidemiologic evidence

Increasingly, data from epidemiologic studies suggest diabetes is a risk factor in old age for brain aging, including cognitive impairment and dementia. These associations may reflect a direct effect on the brain of hyperglycemia, or the effects of the diabetes-associated comorbidities of hypertension, dyslipidemia, or hyperinsulinemia. Epidemiologic data on diabetes and brain aging are reviewed. A brief overview is also given of the physiologic mechanisms supporting the epidemiologic data.