Hormesis: from mainstream to therapy

This issue of the Journal of Cell Communication and Cell Signaling on hormetic mechanisms represents an important step in the evolution of the hormesis dose response concept. Since its modern resurgence in the late 1970s the widespread occurrence of hormesis has been in search of its underlying mechanisms. The present integrative set of papers builds upon significant recent advances in the elucidation of hormetic mechanisms and provides the reader with a deep and extensive view of the concept of hormesis from a broad range of researcher perspectives and in many biomedical applications.     

Radiation-hormesis phenotypes,the related mechanisms and implications for disease prevention and therapy

Humans are continuously exposed to ionizing radiation throughout lifefrom natural sources that include cosmic, solar, and terrestrial. Much harshernatural radiation and chemical environments existed during our planet’s early years.Mammals survived the harsher environments via evolutionarily-conserved gifts acontinuously evolving system of stress-induced natural protective measures (i.e.,activated natural protection [ANP]). The current protective system is differentiallyactivated by stochastic (i.e., variable) low-radiation-dose thresholds and whenoptimally activated in mammals includes antioxidants, DNA damage repair, p53-relatedapoptosis of severely-damaged cells, reactive-oxygen-species(ROS)/reactive-nitrogen-species (RNS)- and cytokine-regulated auxiliary apoptosisthat selectively removes aberrant cells (e.g., precancerous cells), suppression ofdisease promoting inflammation, and immunity against cancer cells. Theintercellular-signaling-based protective system is regulated at least in part viaepigenetic reprogramming of adaptive-response genes. When the system is optimallyactivated, it protects against cancer and some other diseases, thereby leading tohormetic phenotypes (e.g., reduced disease incidence to below the baseline level;reduced pain from inflammation-related problems). Here, some expressed radiationhormesis phenotypes and related mechanisms are discussed along with theirimplications for disease prevention and therapy.     

The fifth dimension of innate immunity

Innate immunity has evolved as a first line defense against invading pathogens. Cellular and humoral elements of the innate immune system detect infectious parasites, initiate inflammatory resistance reactions and finally contribute to the elimination of the invaders. Repeated attacks by pathogenic agents induce adaptive responses of the innate immune system. Typically, reapplication of pathogens provokes tolerance of the affected organism. However, also stimulatory effects of primary infections on subsequent innate immune responses have been observed. The present overview touches an undervalued aspect in the innate immune response: Its pronounced dependency on pathogen load. In addition to localization and timing of innate immune responses the pathogen dose dependency might be considered as a “fifth dimension of innate immunity”. Experimental results and literature data are presented proposing a hormetic reaction pattern of innate immune cells depending on the dose of pathogens.     

Alteration of cytokine profiles in mice exposed to chronic low-dose ionizing radiation

While a high-dose of ionizing radiation is generally harmful and causes damage to living organisms, a low-dose of radiation has been shown to be beneficial in a variety of animal models. To understand the basis for the effect of low-dose radiation in vivo, we examined the cellular and immunological changes evoked in mice exposed to low-dose radiation at very low (0.7 mGy/h) and low (3.95 mGy/h) dose rate for the total dose of 0.2 and 2 Gy, respectively. Mice exposed to low-dose radiation, either at very low- or low-dose rate, demonstrated normal range of body weight and complete blood counts. Likewise, the number and percentage of peripheral lymphocyte populations, CD4⁺ T, CD8⁺ T, B, or NK cells, stayed unchanged following irradiation. Nonetheless, the sera from these mice exhibited elevated levels of IL-3, IL-4, leptin, MCP-1, MCP-5, MIP-1α, thrombopoietin, and VEGF along with slight reduction of IL-12p70, IL-13, IL-17, and IFN-γ. This pattern of cytokine release suggests the stimulation of innate immunity facilitating myeloid differentiation and activation while suppressing pro-inflammatory responses and promoting differentiation of naïve T cells into T-helper 2, not T-helper 1, types. Collectively, our data highlight the subtle changes of cytokine milieu by chronic low-dose γ-radiation, which may be associated with the functional benefits observed in various experimental models.     

A biological-based model that links genomic instability, bystander effects, and adaptive response

This paper links genomic instability, bystander effects, and adaptive response in mammalian cell communities via a novel biological-based, dose-response model called NEOTRANS3. The model is an extension of the NEOTRANS2 model that addressed stochastic effects (genomic instability, mutations, and neoplastic transformation) associated with brief exposure to low radiation doses. With both models, ionizing radiation produces DNA damage in cells that can be associated with varying degrees of genomic instability. Cells with persistent problematic instability (PPI) are mutants that arise via misrepair of DNA damage. Progeny of PPI cells also have PPI and can undergo spontaneous neoplastic transformation. Unlike NEOTRANS2, with NEOTRANS3 newly induced mutant PPI cells and their neoplastically transformed progeny can be suppressed via our previously introduced protective apoptosis-mediated (PAM) process, which can be activated by low linear energy transfer (LET) radiation. However, with NEOTRANS3 (which like NEOTRANS2 involves cross-talk between nongenomically compromised [e.g., nontransformed, nonmutants] and genomically compromised [e.g., mutants, transformants, etc.] cells), it is assumed that PAM is only activated over a relatively narrow, dose-rate-dependent interval (D(PAM),D(off)); where D(PAM) is a small stochastic activation threshold, and D(off) is the stochastic dose above which PAM does not occur. PAM cooperates with activated normal DNA repair and with activated normal apoptosis in guarding against genomic instability. Normal repair involves both error-free repair and misrepair components. Normal apoptosis and the error-free component of normal repair protect mammals by preventing the occurrence of mutant cells. PAM selectively removes mutant cells arising via the misrepair component of normal repair, selectively removes existing neoplastically transformed cells, and probably selectively removes other genomically compromised cells when it is activated. PAM likely involves multiple pathways to apoptosis, with the selected pathway depending on the type of cell to be removed, its cellular environment, and on the nature of the genomic damage.     

Clonally expanded human airway smooth muscle cells exhibit morphological and functional heterogeneity

Background

Mesenchyme-derived airway cell populations including airway smooth muscle (ASM) cells, fibroblasts and myofibroblasts play key roles in the pathogenesis of airway inflammation and remodeling. Phenotypic and functional characterisation of these cell populations are confounded by their heterogeneity in vitro. It is unclear which mechanisms underlie the creation of these different sub-populations.The study objectives were to investigate whether ASM cells are capable of clonal expansion and if so (i) what proportion possess this capability and (ii) do clonal populations exhibit variation in terms of morphology, phenotype, proliferation rates and pro-relaxant or pro-contractile signaling pathways.

Methods

Early passage human ASM cells were subjected to single-cell cloning and their doubling time was recorded. Immunocytochemistry was performed to assess localization and levels of markers previously reported to be specifically associated with smooth muscle or fibroblasts. Finally functional assays were used to reveal differences between clonal populations specifically assessing mitogen-induced proliferation and pro-relaxant and pro-contractile signaling pathways.

Results

Our studies provide evidence that a high proportion (58%) of single cells present within early passage human ASM cell cultures have the potential to create expanded cell populations. Despite being clonally-originated, morphological heterogeneity was still evident within these clonal populations as assessed by the range in expression of markers associated with smooth muscle cells. Functional diversity was observed between clonal populations with 10 μM isoproterenol-induced cyclic AMP responses ranging from 1.4 - 5.4 fold cf basal and bradykinin-induced inositol phosphate from 1.8 - 5.2 fold cf basal.

Conclusion

In summary we show for the first time that primary human ASM cells are capable of clonal expansion and that the resulting clonal populations themselves exhibit phenotypic plasticity.     

NMDA-induced stimulation of glycolysis in developing hippocampal cell cultures

Developmental changes in energy metabolism of primary hippocampal cell cultures from newborn rats were investigated during the first 3 weeks. These changes were measured by intensity of and number of cells exhibiting NAD(P)H fluorescence in response to NMDA-induced activation of neuronal activity. We observed gradual changes of stimulation-evoked NAD(P)H signaling over the first 3 weeks, such that at day 7 and 16, this stimulation is minimal, while at 5 and 12 days, it is maximal. These results describe a biphasic pattern that was similar to earlier findings from experiments investigating developmental changes in population spike amplitudes or glutamate release in young rats. Inhibition of mitochondrial respiration by KCN revealed that the NMDA-evoked stimulation of energy metabolism is mainly due to increased glycolytic activity.     

Responses to selection on genotypic or phenotypic values in the presence of genes with major effects

Summary Average genotypic responses were compared after selection for genotypic values and for phenotypic values on the basis of single-gene models and multigene models in simulated livestock populations. Single-gene models dealt with single gene control of the genetic differences between animals, while multigene models considered a collection of genes with various magnitudes of effects on a trait. In each case, selection lasted through discrete generations until the fixation of the gene frequencies occurred. Generations to reach fixation were used to compare various models, and the two criteria for selection, for their efficiency in selection. Implications of using these models versus using infinitesimal models for selection in practice are presented.     

The application of a model of glucose and insulin dynamics to explain an observed effect of leptin administration in reversal of developmental programming

A dynamical model describing the glucose-insulin physiological system was applied to an experiment on the administration of the adipokine leptin between neonatal days 3 and 13 to rats whose dams were subject to different levels of nutrition during gestation. The effect of leptin treatment on the glucose-insulin equilibrium point and on the levels of other associated metabolites showed a significant change in direction that depended on the level of prenatal nutrition. Leptin has been shown to affect two factors that affect the equilibrium levels of glucose and insulin, gluconeogenesis and insulin sensitivity. Each effect is described by a parameter in the dynamical model. Mathematical analysis shows that changes in these parameters in the manner promoted by leptin would indeed increase or decrease the glucose-insulin equilibria depending on their initial equilibrium levels which might be induced by the level of prenatal nutrition. This analysis explains the results of the leptin experiment in the context of the dynamics of the glucocorticoid system. It also proposes a physiological mechanism for the expression of plasticity in the organism based on the status of the glucose and insulin equilibria.     

Conditional dispersal under kin competition: extension of the Hamilton-May model to brood size-dependent dispersal

I consider a site-based model with contest competition among siblings, and assume that dispersal is conditional on the number of offspring in the natal site. Evolutionarily stable populations contain threshold dispersal strategies, which retain a certain number of offspring in the natal site and disperse the rest (if the actual number of offspring is less than the threshold, then all offspring are retained). Due to the discrete nature of the strategy set (the threshold must be integer), the ESS may not be unique or may not exist. In the latter case, two neighboring threshold strategies coexist in the evolutionarily stable population. Dispersal first decreases and then increases as a function of dispersal mortality, such that all but one offspring should be dispersed both when dispersal mortality is very small or very high. Population-level dispersal fractions are often similar to the unconditional ESS, but differ strongly when fecundity is small and dispersal mortality is high.     

From heterogeneity to plasticity in adipose tissues: site-specific differences

In mammals, two types of adipose tissues are present, brown (BAT) and white (WAT). WAT itself can be divided into subcutaneous and internal fat deposits. All these tissues have been shown to present a great tissue plasticity, and recent data emphasized on the multiple differentiation potentials obtained from subcutaneous WAT. However, no study has compared the heterogeneity of stroma-vascular fraction (SVF) cells and their differentiation potentials according to the localization of the fat pad. This study clearly demonstrates that WAT and BAT present different antigenic features and differentiation potentials. WAT by contrast to BAT contains a large population of hematopoietic cells composed essentially of macrophages and hematopoietic progenitor cells. In WAT, the non-hematopoietic population is mainly composed of mesenchymal stem cell (MSC)-like but contains also a significant proportion of immature cells, whereas in BAT, the stromal cells do not present the same phenotype. Internal and subcutaneous WAT present some discrete differences in the phenotype of their cell populations. WAT derived SVF cells give rise to osteoblasts, endothelial cells, adipocytes, hematopoietic cells, and cardiomyoblasts only from inguinal cells. By contrast, BAT derived SVF cells display a reduced plasticity. Adipose tissues thus appear as complex tissues composed of different cell subsets according to the location of fat pads. Inguinal WAT appears as the most plastic adipose tissue and represents a potential and suitable source of stem cell, considering its easy sampling as a major advantage for cell therapy.     

Genotoxicity in the eyes of bystander cells

The controversial use of a linear, no threshold extrapolation model for low dose risk assessment has become even more so in light of the recent reports on the bystander phenomenon. The answer to the question as to which of the two phenomena, bystander versus adaptive response, is more important has practical implication in terms of low dose radiation risk assessment. In this review, genotoxicity is used as an endpoint to introduce the two phenomena, provide some insight into the mechanisms of bystander effect and to bridge the two low dose phenomena which operate in opposite directions: the bystander effect tends to exaggerate the effect at low doses, by communicating damage from hit to non-hit cells whereas the adaptive response confers resistance to a subsequent challenging dose by an initial low priming dose.     

Interaction of low-dose irradiation with subsequent mutagenic treatment: role of mitotic delay

Experiments were carried out with human lymphocytes to test whether there was any relation between the changes that conditioning treatment can produce in cell progression or in mitotic delay induced by the challenge dose and the presence of an ‘adaptive response’ (AR). In experiments in which the cells were successively fixed after the challenge dose, the interaction between conditioning treatment and challenge was of the same sign for all the fixation times: therefore it is likely that modifications of the cytogenetic damage in primed cells is not a mere reflection of stage sensitivity. In experiments in which using 1 Gy as conditioning treatment we induced a drastic extension of G₂, we did not observe any AR; therefore, even if conditioning treatment can induce modifications in the cell-cycle phases before and/or after challenge, there is probably no link between these modifications and the presence of an AR.     

Adaptive response in irradiated human lymphocytes: radiobiological and genetical aspects

The adaptive response (AR) in human lymphocytes in different experimental protocols was investigated. The AR was found to be present in cells pre-exposed to 3 cGy of X-rays in G0, G1 and S phase as well as with tritiated water (4 muCi/ml) when the 'challenge' dose was given in G2. There was no AR after prior exposure of the cells in S phase to secondary irradiation from 70 GeV protons. The AR was not observed after preliminary X-irradiation of the lymphocytes in G0 and G1 and 'challenge' irradiation in G1. Cells from 6 patients with Down's syndrome were tested. At least 5 of them did not show the AR. The AR is considered to be a phenomenon of the antimutagenic aftereffect.