"Is hormesis applicable as a pro-healthy aging intervention in mammals and human beings, and how?" Introduction to a special issue of Dose-Response

New considerations on hormetic response against oxidative stress

In order to survive living organisms have developed multiple mechanisms to deal with tough environmental conditions. Hormesis is defined as a process in which exposure to a low dose of a chemical agent or environmental factor that is damaging at higher doses induces an adaptive beneficial effect on the cell or organism. In this paper, we examine several ideas that might be taken into consideration before using hormesis as a therapeutic tool to improve health and life span, and hopefully will open the discussion for new and interesting debates regard hormesis. The first one is to understand that the same stressor or inductor can activate different pathways in a parallel or dual response, which might lead to diverse outcomes. Another idea is related to the mechanisms involved in activating Nrf2, which might be different and have diverse hormetic effects.Last, we discuss mild oxidative stress in association to low-grade chronic inflammation as a stimulating avenue to be explored and the unexpected effects proposed by the obesity paradox theory. All the previous might help to clarify the reasons why centenarians are able to reach the extreme limits of human life span, which could probably be related to the way they deal with homeostasis maintenance, providing an opportunity for hormesis to intervene significantly.

Dissection of the hormetic curve: analysis of components and mechanisms

The relationship between the dose of an effector and the biological response frequently is not described by a linear function and, moreover, in some cases the dose-response relationship may change from positive/adverse to adverse/positive with increasing dose. This complicated relationship is called "hormesis". This paper provides a short analysis of the concept along with a description of used approaches to characterize hormetic relationships. The whole hormetic curve can be divided into three zones: I - a lag-zone where no changes are observed with increasing dose; II - a zone where beneficial/adverse effects are observed, and III - a zone where the effects are opposite to those seen in zone II. Some approaches are proposed to analyze the molecular components involved in the development of the hormetic character of dose-response relationships with the use of specific genetic lines or inhibitors of regulatory pathways. The discussion is then extended to suggest a new parameter (half-width of the hormetic curve at zone II) for quantitative characterization of the hormetic curve. The problems limiting progress in the development of the hormesis concept such as low reproducibility and predictability may be solved, at least partly, by deciphering the molecular mechanisms underlying the hormetic dose-effect relationship.

Possible role of NF-κB in hormesis during ageing

Hormesis in ageing is probably represented by mild stress-induced stimulation of protective mechanisms in cells and organisms resulting in biologically beneficial effects. Mild stress and hormetins may act on bifurcation points in the complex network of cell signaling and transcription factors, often turning homeodynamics to health or survival. Several signaling pathways activated by diverse stimuli and by stress response converge on NF-κB activation, resulting in a regulatory system characterized by high complexity. NF-κB behaves as a chaotic oscillator and it is increasingly recognized that the number of components that impinges upon phenotypic outcomes of signal transduction pathways may be higher than those taken into consideration from canonical pathway representations. NF-κB is closely related to other important upstream signaling networks, creating chaotic oscillators with other receptor-related kinases and targeting hubs for hormesis. The great bulk of natural hormetins acts on these signaling pathways, while NF-κB appears as a key regulatory factor in this context. Due to its tight relationship with main signaling system NF-κB plays a fundamental role in stress response, apoptosis and autophagy and appears to be a possible target for hormesis in ageing.

Age-related and sex-specific differences in proteasome activity in individual Drosophila flies from wild type, longevity-selected and stress resistant strains

We have measured the caspase-like proteasome activity in individual male and female Drosophila flies by using a non-denaturing lysing technique that consistently extracts total protein. The mean proteasome activity in control C1 females was more than two times higher as compared with that in C1 males. However, in longevity-selected LS1 flies the proteasome activity was significantly lower compared to C1 flies, but the sex differences were maintained to some extent. Five other stress resistant lines also had significantly reduced proteasome activity in both sexes. During ageing, there was a progressive decrease in proteasome activity in C1 females, but not in C1 males. This age-related decline in proteasome activity observed in C1 females was not observed in LS1 flies. We conclude that the proteasome activity in control male and female flies is significantly different from each other and that increased lifespan and stress resistance lead to a reduction in proteasome activity and recession of the age-related decline observed in control females.

[Mild stress as a means to modulate aging: from fly to human?]

Hormesis is the phenomenon by which adaptive responses to low doses of otherwise harmful conditions improve the functional ability of organisms. Some mild stresses have beneficial effects on longevity, aging and resistance to strong stresses (heat or cold shocks, infection) in Drosophila flies. Studies on rodents are indeed scarce but mild stress seems to be effective in humans because, for instance, patients suffering from angina have a higher survival when confronted with a heart attack. A few studies, in less tragic situations however, suggest that mild stress could have positive effects in elderly people. Performing more experiments on the effects of mild stress in humans would help to know whether it could be used in therapy or to improve healthspan of elderly.

Time-dependent beneficial effect of chronic polyphenol treatment with catechin on endothelial dysfunction in aging mice

A controlled redox environment is essential for vascular cell maturation and function. During aging, an imbalance occurs, leading to endothelial dysfunction. We hypothesized that, according to the concept of hormesis, exposure to physiologic oxidative stress during the maturation phase of the endothelium will activate protective pathways involved in stress resistance. C57Bl/6 mice were treated with the polyphenol catechin for the last 3 (post-maturation) or 9 months prior study at 12 months of age. Endothelial dysfunction, assessed by acetylcholine-induced dilations of isolated renal arteries, was present at 12 months (P<0.05). Only the 3-month treatment with catechin fully prevented the decline in efficacy and sensitivity to acetylcholine (P<0.05). Splenocytes adhesion to the native endothelium, expression of CD18 and shedding of CD62L and PSGL-1 augmented in 12 months old mice (P<0.05): only 3-month catechin fully normalized adhesion and prevented the expression of adhesion molecules on splenocytes (P<0.05). Aging was associated with vascular gene alterations, which were prevented by 3-month catechin treatment (P<0.05). In contrast, 9-month catechin further increased COX-2, p22(phox) and reduced MnSOD (P<0.05). In conclusion, we demonstrate a pivotal role of cellular redox equilibrium: exposure to physiologic oxidative stress during the maturation phase of the endothelium is essential for its function.

A cold stress applied at various ages can increase resistance to heat and fungal infection in aged Drosophila melanogaster flies

A cold stress applied to young flies can have positive effects on longevity, behavioral aging, and resistance to heat and infection. However, the same mild stress, if applied at older ages, i.e. in frailer flies, could be a strong stress with negative effects. Cold stress was applied at various ages (weeks 1-2, 2-3, 3-4, and 4-5) and its effect on longevity and on resistance at 6 weeks of age to heat or fungal infection was observed. In males, the cold stress had positive effects on longevity and resistance to infection, except when applied at the oldest age. No positive effect on longevity or resistance to infection was detected in cold-stressed females, as already observed in previous experiments using a cold stress at young age only. By contrast, cold stress applied at various ages increased resistance to heat in both sexes. Therefore, a mild stress can have positive effects on longevity and resistance to strong stresses not only when used at a young age, but also at older ages.