Introduction

Hormesis is becoming a significant concept in biological disciplines that make use of dose response information. This is supported by the fact that the terms hormesis or hormetic have become more common in the research literature. In the 1980s these terms were cited 10–15 times per year in the Web of Science database. By 2010 these terms were cited over 3,000 times in this database. Furthermore, hormesis has also became integrated into leading textbooks in pharmacology (Calabrese 2009) and toxicology (Eaton and Klaassen 2003; Beck et al. 2007) during the past decade, while five books had been devoted to different aspects of hormesis over the past several years (Le Bourg and Rattan 2008a; Mattson and Calabrese 2010; Sanders 2010; Elliott 2011; Stebbing 2011). Likewise, various professional organizations have had keynoted presentations and special sessions on this topic. Leading journals such as Science published a four page news story on hormesis (Kaiser 2003), introducing the concept to the scientific community, while Nature published a highlighted article on the implications of hormesis for the field of toxicology (Calabrese and Baldwin 2003a). Interest in the concept of hormesis has been the growing since it is a reproducible phenomenon that is very generalizable and has important implications for drug discovery, the clinical trial, therapeutics, as well as in toxicological hazard and risk assessment and risk communication for chemicals, radiation and pharmaceutical agents.

This paper provides a broad overview of the concept of hormesis for biogerontologists. The format of this review employed a large number of relatively short questions and answers. This format has been followed once before in an article directed toward toxicologists and risk assessors (Calabrese 2008a).

What is hormesis?

Hormesis, from the Greek meaning “to excite”, may be defined at the descriptive (what is it?), the mechanistic (how does it work?), and at the evolutionary levels (what is its adaptive significance?) (Calabrese and Baldwin 2002a). At the descriptive level, hormesis is a dose response relationship that is generally characterized as a biphasic dose response with a low dose stimulation and a high dose inhibition. This biphasic dose response has consistent quantitative features including the magnitude of the stimulation, the width of the stimulation and their relationship to the zero equivalent point (i.e., threshold). At the mechanism level, there are numerous proximate mechanisms that may account for specific types of biphasic dose responses. The proximate mechanisms may be either “receptor-or non-receptor” based (Calabrese 2008a). The consistency of the quantitative features of the hormetic dose responses at the cell, organ and organismic level may be mediating the effects of upstream and highly conserved allometric gene clusters that control and direct the allocation of biological resources within complex biological systems (Calabrese 2011). At the evolutionary level, hormesis may be viewed as an adaptive response which mediates cellular stress involved in a plethora of preventive, reparative and signaling activities (Mattson and Calabrese 2010).

Are there other names by which the hormesis concept is known?

Long before Southam and Erhlich (1943) coined the term hormesis, the biphasic dose response was known as the Arndt–Schulz law, named after the two individuals who developed and generalized the concept of a biphasic dose response relationship (Calabrese and Baldwin 2000). Hugo Schultz reported the occurrence of multiple biphasic dose responses in laboratory experiments assessing the effects of chemical disinfectants on yeast metabolism (see Crump 2003 for an English translation of H. Schulz autobiography), presenting these findings in 1884 at a regional medical conference. A year later he generalized these observations with the aid of Professor Rudolph Arndt, leading to the Arndt–Schulz law. Schulz claimed that the biphasic dose response characterized how chemical and physical agents affected biological systems. From the start this concept was closely associated with the medical practice of homeopathy (Crump 2003). This was done intentionally by Schulz in his professional activities as a Professor of Pharmacology at the University of Grieswald in northern Germany. The hormesis concept was also widely known in the early decades of the past century as Hueppe’s rule, after Ferndende Hueppe (Hueppe 1896), a microbiologist of considerable prestige and a protégé of the Nobelist Robert Koch. The findings of Hueppe were similar to those of Schulz except that Heuppe worked with bacteria. In the field of psychological stress biology a similar biphasic dose response was known as the Yerkes–Dodson law, named for the eminent researcher Robert Yerkes (Broadhurst 1957; Calabrese 2007, 2008b). The Yerkes–Dodson law still retains its distinct perspective and focus in the psychology literature. In more recent times, that is, starting in the late 1970s a variety of terms were created that tend to be scientific discipline-specific that also describe the hormetic dose response. Some of these terms include the following: U-shaped dose response, J-shaped dose response, adaptive response, biphasic dose response, inverted U-shaped dose response, pre-conditioning, paradoxical response, parabolic dose response and others. In fact, the use of such an array of terms is also a significant contributor to why the term hormesis is not well known within the pharmaceutical industry. In 2007 a paper was published with 57 co-authors proposing an integrated terminology for biological stress responses based on the hormesis concept (Calabrese et al. 2007). It is thought that such efforts are contributing to the increased use of hormesis within the scientific community.

Does the threshold dose response model have important limitations?

This is an important question because the threshold model acceptance affects how toxicological and pharmacological studies are designed and how acceptable exposure studies are derived from clinical testing procedures. The threshold dose response, however, was never validated during the entire 20th century even though widely accepted by the scientific and regulatory communities. While one cannot prove a negative, substantial efforts have been made by my hormesis study group to find any study (studies) which were designed to validate the threshold model for below threshold responses. None have been found. This model was simply assumed to make accurate predictions of responses in the low dose zone rather than proven to do so. This represents a damaging revelation of a profound error by the pharmacological and toxicological communities as well as the vast regulatory apparatus in industrial countries, such as the numerous environmental protection agencies and the food and drug administration in different countries. It also represents a serious failing of the regulated chemical and pharmaceutical industries, many of which process enormous technical resources. Thus, society is confronted with a situation in which the long revered threshold dose response model upon which our health was secured was found not be have been vetted. Beyond this non-vetting and validating the threshold model during the entire 20th century, the threshold dose response model failed validation testing when finally assessed (Calabrese et al. 2006a, b, 2008). So what are the limitations of the threshold model? Simply put it fails to do what society requires, that is, to make accurate predictions of chemical and drug effects in the critical low dose zone, that is, below the toxicological and pharmacological thresholds.

How were biphasic dose responses interpreted by the field of pharmacology?

The paper of Szabadi (1977) argued that the biphasic dose response was a general one, providing a means by which cells could regulate a broad range of responses to numerous stimuli. Of particular importance was that he identified a general mechanistic strategy by which biphasic dose responses occurred. In this respect, he found that agonists often activated two (or more) receptor subtypes. The receptor subtypes would have differential binding affinity for the agonist. The receptor with the greater binding capacity would have far fewer receptors whereas the receptor with the weaker affinity would have greater capacity, that is, more receptors. These receptor subtypes would lead to either stimulatory or inhibitory pathways. When the cells were exposed to a broad range of agonist concentrations the high affinity receptors would dominate the response at lower concentrations whereas at the higher concentrations the receptor with the greater capacity and lowest affinity would dominate the response. When viewed along a broad concentration response continuum these responses would be biphasic. This type of response has now been shown to be a very general strategy, affecting several dozens of receptor systems, and a vast array of biological responses (Table 1).

Table 1 Representative receptor systems displaying biphasic dose–response relationships (source: Calabrese and Baldwin 2003b)
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How were biphasic dose responses applied to the pharmaceutical field, if they were?

The biphasic dose response has been employed extensively in the pharmaceutical industry in the process of drug development, pre-clinical investigations and the clinical trial for specific classes of drugs. For example, the low dose range of a drug may stimulate a response that is desirable. This is often the case for agents such as anxiolytic (Calabrese 2008c) and antiseizure drugs (Calabrese 2008d). In the case of anxiolytic drugs doses normally increase the amount of time that a rodent will spend in lighted areas, indicating a decrease in anxiety. Based on such rodent studies investigators will select the doses of these agents to be tested in the human clinical trials. In these cases the pharmaceutical industry has typically referred to these dose responses as biphasic responses not relating the observations to the possibility that it may be an hormetic dose response.

Does the hormetic concept relate to homeopathy?

The medical practice of homeopathy has had a long relationship with the hormesis concept, starting the Arndt–Schulz law concept in the mid 1880’s when Hugo Schulz announced that he had discovered the explanatory principle of homeopathy. This situation arose from the following: In the early 1880s it was reported that the homeopathic drug called veratrine was successful in the treatment of a type of gastroenteritis (Bloedau 1884). At about this time the causative bacteria of this condition was identified and cultured. Schulz then tested whether veratrine would kill the causative bacteria in a bioassay. He reported that the drug was unable to affect the bacterial colony growth over a very broad range of concentrations. Based on such data many researchers may have concluded that the drug may not be effective in treating this disease and that the original conclusions were premature or possibly in error. However, Schulz hypothesized that the veratrine was therapeutic but that it did not act via a killing mechanism but possibly by increasing the adaptive capacity of the patient to resist infection. While subsequently studying the effects of chemical disinfectants on the metabolism of yeasts, Schulz observed that essentially all of the agents tested induced a biphasic dose response in the yeasts, stimulating the release of carbon dioxide while at higher doses the release was decreased in a dose dependent manner (Schulz 1887, 1888). The findings were replicated, permitting high confidence in the data. While Schulz did not initially grasp the potential biological significance of these findings, later he developed a far reaching conceptual dose response framework following conversations with Professor Rudolph Arndt. Based on these conversations Schulz believed that he had discovered the underlying principle of how homeopathic drugs act. He thought that the low dose stimulatory response in the yeast represented an adaptive response whereas the higher doses reflected toxicity. He then concluded that the reason why the veratrine was effective was that it had induced an adaptive response which permitted the patients to resist the infection without directly killing the bacteria itself (Schulz 1885). It was this relationship that Schulz proposed and popularized and for which he also became highly criticized.

Recent assessment of the Schulz (1887, 1888) publications by Calabrese and Jonas (2010a) argue that the yeast studies of Schulz were improperly interpreted by Schulz as pertaining to homeopathy. They noted that under most medical situations the patient would seek treatment for a disease condition. That is, the patient is treated with a homeopathic preparation after having been challenged by a disease causing agent. In the case of Schulz there was no prior administered stressor agent or condition. Thus, the experimental system of Schulz lacked potential relevance for most clinical evaluative systems. This was recognized in the 1990s by Wiegant et al. (1997, 1998, 1999), Van Wijk and Wiegant (1997) and Van Wijk et al. (1994) who attempted to create a model homeopathic experimental system in which human liver tumor cells were first treated with powerful stressor agents such as heat or toxic metals and then treated with a dose of one of these agents that was sufficiently low that it did not have a measurable effect in control cells. This dose, which did not affect the controls cells, amplified the adaptive response over what the original stressor exposure did. This led the investigators to conclude that their system provided a biomedical model to study the effects of homeopathic drugs within a traditional biomedical setting. Calabrese and Jonas (2010a, b) concluded that this research method represented a means to assess whether such homeopathic drugs could act via a post-conditioning hormesis process (Calabrese and Jonas 2010b). This framing of how homeopathic drugs may work could be tested, creating a legitimate “point of contact” between homeopathy and traditional biomedical research. This perspective has the potential to create opportunities in which homeopathic drugs will be evaluated within a framework consistent with modern molecular biology research.

From a historical perspective Hugo Schulz rejected the high dilutional aspect of homeopathy (Bohme 1986). His biphasic dose response phenomenon was seen within the context of a traditional biomedical dose response. Schulz linked the low dose stimulation to the low-dilutional “wing” of homeopathy. Such a distinction, which is quite clear from the historical literature, was glossed over or overlooked by leaders in the traditional medical community, thereby misrepresenting the position of Schulz.

Is hormesis a better term than biphasic?

An hormetic dose response is a specific type of biphasic dose response with definable quantitative characteristics relating to the magnitude and width of the low dose stimulatory response. The use of the term hormesis is valuable since it lends specificity to the type of biphasic dose response observed (Calabrese 2008a).

How can one determine whether a biphasic dose response is an example of hormesis?

For a dose response to be designated as hormetic it would have to satisfy the following criteria:

Shape of dose response

  1. 1.

    It would have to be a biphasic dose response; generally, there would be low dose stimulation and a high dose inhibition. There is some uncertainty over whether a dose response which displays an inverted U-shaped with a response approaching but not progressing below the control response at higher dose should be considered as hormetic. There may be evolutionary based reasons for this type of dose response and it may be a specialized form of the hormetic dose response.

  2. 2.

    The dose response would have to display a particular magnitude of stimulation. While there is not fixed criterion, it would appear that responses fourfold or greater than the control should not be considered hormetic but may represent a different type of biological phenomenon.

  3. 3.

    The width of the stimulatory zone does not seem to have any set criteria.

  4. 4.

    There does not have to be a time component, although this may be an essential parameter in specific cases.

Type of endpoints

  1. 5.

    It is not clear whether the hormetic dose response should be limited to a certain set of integrated endpoints. For example, in the case of cell survival there are multiple genes that are activated or inhibited during such a complex process (Wang et al. 2005). The question is whether only the integrated cell survival endpoint should be considered as hormetic or the entire set of up and down regulated gene activities which contributed to creating the integrative response. The opinion offered here is that the hormetic response would be the integrated response.

How does “time” relate to the hormetic dose response?

Time can be a crucial component of the hormetic dose response. This would occur when the hormetic dose response represents an overcompensation to a disruption in homeostasis. This dose–time–response has been widely reported (Calabrese 1999, 2001a). The dose time response was first noted in the data of Schulz concerning the effects of chemical disinfectant on the metabolism of yeasts and confirmed later by Branham (1929) in an extensive replication of the original findings of Schulz (1887, 1888).

It is difficult to study hormesis within a dose–time–framework since it requires the use of many doses, with responses followed over a number of time points. Nonetheless, about 20% of the 9,000 doses in the hormetic data have a time component (Calabrese and Blain 2005).

Do hormetic dose response relationships have distinct quantitative features and if so, why?

When the hormesis data base was created in 1996 there was no clear sense that there were defining quantitative features for this type of dose response relationship. However, after the collection and analysis of about 6,000 dose response relationships satisfying the evaluative criteria for hormesis, it became evident that hormetic dose responses typically (~60% of cases) displayed a modest stimulatory response being only about 30–60% greater than the control group at maximum (Fig. 1). About 20% of dose responses had their maximum response exceeding twofold of the control value. These observations were as important as they were unexpected, as suggesting a possible dose response maximum. These findings would later provide insight into the concepts of a pharmacological ceiling effect and biological plasticity (Calabrese et al. 2010; Calabrese 2005a, 2008e). The so-called 30–60% “rule” has also raised the question of whether dose responses with a maximum response greater than threefold of the control are still examples of hormesis even if they also conform to a biphasic dose response (Calabrese 2005b).

Fig. 1
figure 1

Dose–response curve depicting the quantitative feature of hormesis

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The question may be asked as to why there is such a general consistency in the magnitude of the hormetic stimulation across biological models, at different levels of biological organization from the cell to the organ to the whole organism, across biological endpoint and chemical classes and physical stressor agents. This dose response property has been remarkably conserved from bacteria to humans (Calabrese and Blain 2005, 2009). While this area also remains to be clarified it is possible that the limiting of the maximum stimulatory response is a means by which biological resources are managed, controlled and allocated. Given the plethora of possible dose responses that could be activated within any limited time period, having a generalized capping of resource allocation may offer a survival advantage over the modulation of resource allocation to the specific requirements of each dose response for each specific circumstance.

The width of the stimulatory response was generally observed to be within 10–20 fold of the toxicological or pharmacological threshold (Calabrese and Blain 2005). However, in a low percentage of cases the width of the stimulatory response is considerably broader, reaching and exceeding a factor of 1,000-fold. The factors that contribute to this striking variation in the width of the stimulatory zone are generally unknown. They may, in part, be explained by a unique combination of heterogenicity of sample population and pharmacokinetic characteristics but this remains to be clarified.

Can the hormetic response be optimized?

The answer to this question is that it can. The supporting evidence has been well documented in studies dealing with tumor and non-tumor cell biology (Vichi and Tritton 1989), wound healing (Demidova-Rice et al. 2007) and in plant biology (Belz and Cedergreen 2010). The supportive information for this concept will be summarized in the cases of tumor cell biology and wound healing.

In the 1970’s it was reported that low concentrations of the chemotherapeutic adriamycin (AD) could stimulate cellular growth parameters. In 1972 Wang et al. reported that adriamycin enhanced thymidine incorporation into DNA at low doses at short duration times. Subsequently, Roper and Drewinko (1976) revealed stimulation of DNA synthesis at low AD doses. Several years later Huybrechts et al. (1979) noted increased survival of hematopoietic stem cells at sub-toxic AD concentrations. In each of these cases the stimulatory effects were modest in magnitude (10–50%) while at higher concentrations the drug displayed its well-known cytotoxicity. This initial set of observations led Vichi and Tritton (1989) to provide a systematic evaluation of how varying the experimental conditions may affect the occurrence of stimulatory responses in various tumor and non-tumor cells. Initial experiments revealed that AD induced biphasic concentration responses in multiple biological models for a range of endpoints. These included L1210 cells (survival), S180 cells (survival), HL-60 cells (survival), A431 cells (cell number), 3T3 cells (thymidine incorporation), and mouse fetal heart cells (protein synthesis) at 13–15 days of gestation, a period of non-actively dividing cells. While the above studies were conducted to assess AD-induced cytotoxicity, low dose growth stimulation was an unexpected but consistent finding. The consistency of these so-called “accidental” stimulatory responses encouraged Vichi and Tritton (1989) to explore the broader biological significance of these biphasic dose response relationships.

The magnitude of the AD induced enhanced survival depended on the cell density of the suspension culture. Since cells grown in higher densities provided a greater proliferation response stimulation, this could have been due to the high cell density or because growth high densities result in a depletion of growth factors in the serum containing medium. Follow up experiments were conducted with low density cultures with different fetal serum levels (0, 5, and 10%) and exposure to 10−9 M AD. While growth was inhibited at 10% fetal serum, it was enhanced by 30–50% at the 0–5% fetal serum levels.

Several conditions were identified for stimulation to occur: a low AD concentration, just below that which affects a decrease in proliferation, the medium supporting cell growth must be incomplete (i.e., sub-optimal), and that the drug should have the capacity to induce toxicity. This latter condition emerged from the fact that all cytotoxic anthracyclines enhanced growth at low doses, whereas inactive aglycones did not stimulate growth. The hormetic response and its optimization was dependent on cell density, medium composition and the culture’s history. Vichi and Tritton (1989) concluded that to most reliably obtain growth stimulation it is necessary to grow high density cells in partially exhausted medium.

The concept that hormetic responses are optimized when cells are grown under suboptimal conditions has been subsequently addressed by Demidova-Rice et al. (2007) in experiments on the effects of low level light on dermal wound healing. These investigators reported that wound healing could be most efficiently enhanced in animal models with sub-optimal wound healing capacity. They concluded that it was not possible to enhance the healing capacity of wounds in mouse strains that had a normal high efficiency in wound repair. These observations lead the authors to conclude that these strains are probably responding in an optimized manner.

These collective studies are of some interest since they were well conceived and based on substantial findings. It is unfortunate that the hormesis concept has been systematically deconstructed and reconstructed with respect to experimental protocols in so few systems. Yet, these findings have important theoretical and practical implications if they could be further generalized.

Are there hormetic “principles”?

Based on an assessment of the hormetic dose base a set of nine hormetic principles have been derived. They are summarized in Table 2.

Table 2 Hormetic principles
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Can hormesis occur via a receptor based mechanism?

Hormetic biphasic dose responses commonly occur via receptor based mechanisms, often via a single agonist binding to two different receptor subtypes which control opposing responses (Calabrese and Baldwin 2001a; Szabadi 1977). However, this type of response has also been reported to occur with a single agonist acting on a single receptor but one in which there were two distinct binding sites (Quirk et al. 1986; Quirk and Funder 1988). Of importance is that the quantitative features of hormetic dose responses are similar regardless of the number of receptors involved for either the stimulatory or inhibitory pathways or whether there are two receptor subtypes of two different receptors or whether there are two receptor sights on a single receptor or other series of possible permutations. These observations suggest that there are numerous ways to mediate the regulation and distribution of resources within biological systems of profoundly differing levels of complexity (i.e., cell, organ and whole organism). Numerous examples exist in which hormetic dose responses have been accounted for (Calabrese 2005c, d, 2008k). Despite such options and avenues of mechanistic complexity the quantitative features of the dose response are strikingly similar (Calabrese 2001b, c, d, e, f, g, h, i).

Is hormesis a measure of biological performance?

The dose response is comprised of two components, those responses at doses greater than the threshold (or zero equivalent point) and those responses at doses less than the threshold. Generally, the responses at the doses greater than the threshold dose define a toxic response whereas those responses to doses less than the threshold, that is, responses in the hormetic zone of the dose response, describe a new concept called biological performance. The two areas of the dose response define fundamentally different biological phenomena. Biological performance describes activities that relate to integrated biological responses that are generally adaptive in nature and are critical for survival (Calabrese and Mattson 2011). The types of endpoints considered within the context of biological performance are many with examples being entities such as growth, memory, bone strength, disease resistance and longevity. These endpoints are biological effects that are mediated by complex processes that typically display hormetic dose responses. Substantial published research demonstrates that the quantitative features of their dose responses as affected by either endogenous or exogenous agent are hormetic in nature. These observations have many important implications for pharmaceutical companies since many products are beneficially active in the hormetic zone of the dose response (Mattson and Calabrese 2008). The magnitude of such responses will be constrained by the limitations imposed on maximal hormetic responses (30–60% increase) by biological plasticity. Thus, there are significant biological restrictions imposed on the pharmaceutical industry with respect to discovering, developing and finally achieving the successful approval of pharmaceutical agents.

Are hormetic effects trivial with little practical application?

A large body of evidence reveals that hormetic affects are modest, being in the percentage rather than the fold zone. In about 75–90% of the cases the maximum responses are less than twice that of the control group, while about 60% of the cases have a maximum of about 30–60% greater than the control group (Hoffmann 2009). In fact, for the development of pharmaceutical agents that relate to the concept of enhancing biological performance, this is the range of possible improvement. As the increase is modest, it is more challenging to establish the occurrence of these effects in clinical trials. It is also likely that physicians may wish that a drug would increase biological performance by more than a modest extent as shown with the hormesis perspective. Nonetheless, improvement of a modest degree (30–60%) can profoundly affect patient health and the overall patterns of health seen in populations. In the case of environmental regulations, the hormetic concept can have important implications, relating to both hormesis induced benefits and harmful effects. In addition, the hormetic dose response also represents a fundamental strategy for biological processes, including developmental morphogenes that guide the formation of biological curvatures such as capillary formation, eye shape and a plethora of other similar biological patterns that employ concentration gradient signaling that follow the hormetic dose response (Fosslien 2009). Thus, the hormetic concept, while biologically subtle, is anything but trivial.

In the case of environmental regulation the issue of whether responses at low levels of exposure are “real” or not in human populations is problematic because chemicals are rarely tested at low doses. Regulatory agency testing strategy typically involves exposing animal models to high doses of chemicals and extrapolating via mathematical models to human populations under the assumption that the animal is an appropriate model both qualitatively and quantitatively. The principal human validation of such assumptions involves epidemiological investigations. However, these can be out of temporal sequence from the animal studies by some 20–50 years. Epidemiology also has a weak capacity to detect biological effects, rarely detecting population responses below a risk of about 2–3 times greater than background. Biological changes in the 30–60% range as is the case for hormesis would be difficult for epidemiology to detect. Epidemiology is, therefore, far less likely than toxicological studies to have a major impact on the assessment of possible hormetic dose responses.

What are hormetic stimuli and why?

Drugs, chemicals and physical stressors can be hormetic stimuli. Generally speaking any agent that stresses or mildly damages cells will set in process a series of adaptive responses that will lead to the upregulation various pathways that repair damage and improve biological function and performance. Hormetic stimuli can be activities that are very common in daily lives such as exercise, psychosocial stress, fasting, exposure to dietary constitutions such as polyphenols as well as low levels of ionizing radiation, and toxic chemicals.

How can hormesis affect the study and treatment of human aging?

What are the implications of hormesis for aging? There are many implications of hormesis for aging as well as longevity. Hormesis has the potential to improve the quality of life within the normal lifespan. Hormesis is a measure of optimized biological response. This can be manifested in numerous ways, such as in disease prevent and health promotion. These effects can occur at all biological levels of organization from improving the functioning of protein chaperones, to enhancing DNA repair activity and in the fidelity of the repair, to the strengthening of bones to the protection of neurons from damage from a wide variety of agents. Thus, hormesis is expected to affect the quality of life during a normal lifespan, including the delay of neurodegenerative impairments (Calabrese 2008g, h; Le Bourg and Rattan 2008b). However, the hormesis concept also suggests that the duration of the lifespan can be extended by a modest degree, within the 30–60%. A large number of experiments have enhanced the duration of longevity in a wide range of animal models, including nematodes, insects and rodents. These studies are consistent with the predictions of the hormetic dose response regardless of the biological model, the inducing agent or the proximate explanatory mechanisms (Ina and Sakai 2004, 2005; Kitani et al. 2002, 2005).

Does hormesis explain the biological stress response?

The hormesis concept has long been associated with the study of stress induced changes in animal models and humans. In fact, this was first detailed over a century ago in research designed to assess the relationship of biological-induced stress and how it affects performance of tasks with differential complexities (Yerkes and Dodson 1908). This research provided significant insight to the occurrence of animal and human performance which conformed to the dose response features of what we now call hormesis (Calabrese 2008b). The research in the early decades of the 20th century was lead by Robert Yerkes, after whom the Yerkes Primate Center in Atlanta is named. As a tribute to his career achievements in this area, researchers called this phenomenon the Yerkes–Dodson law, after Yerkes and his student John Dodson (Calabrese 2008b). The Yerkes–Dodson law is a specific type of hormetic dose response that integrates the effects of two independent variables, that is, the degree of stress and task complexity. While the research relating to the Yerkes–Dodson law has been behaviorally oriented, there has been considerable research concerning the molecular basis of biological stresses affecting numerous endpoints and their dose response relationships. Such research has generally displayed the occurrence of hormetic dose responses regardless of biological model, the level of biological organization (i.e. cellular, organ or individual), the type of imposed stress and the endpoints of interest (Kahn and Olsen 2010). Of particular interest in this regard has been research directed to the effects of glucocorticoids on neuronal function (Calabrese 2008j) and disease susceptibility (Roy and Rai 2004).

Can hormetic age-related benefits be achieved by the search for mimetics?

Hormetic effects can be induced by a very broad array of biological and physical agents. Numerous toxic agents as well as ionizing radiation can induce hormetic dose responses, many of which can result in beneficial effects at low doses. While the therapeutic applications of such exposures are most likely quite limited due to other undesirable properties of these agents, it suggests the possibility of finding non-toxic agents that could activate the key pathways of these hormetic agents, in effect, mimicking their effects, thus the concept of hormetic mimetics (Hunt et al. 2011; Mattson and Cheng 2006; Sonneborn 2010).

Are preconditioning/postconditioning responses examples of hormesis?

Preconditioning is a term that was introduced into the lexicon of medicine by researchers at Duke University (Murray et al. 1986) who reported that a prior brief hypoxic stress to dogs profoundly reduced their risk of cardiac damage following a subsequent myocardial infarct within the next 24–48 h. The prior stress induced an adaptive response that lead to the observed protection, a phenomenon that the investigators referred to as “preconditioning”. The preconditioning concept was not new to the realm of toxicology where a prior low dose exposure to a toxic substance could also profoundly protect against the occurrence of toxicity from a subsequent and more massive exposure. This had been reported for agents such as chloroform, carbon tetrachloride and numerous other agents starting in the 1960s (Dambraust and Cornish 1970; Glende 1972; Ugazio et al. 1973). It had also been shown to occur for chemical mutagens, with the first such case published by Samson and Cairns (1977). By 1984 this phenomenon had also been shown to occur for radiation induced mutations (Olivieri et al. 1984). Since researchers in these other biological fields used different terms such as autoprotection, heteroprotection and adaptive response these earlier papers were missed by Murray et al. (1986), thus leading to their creating yet another new term for the same general biological concept. Of relevance to the issue of hormesis is that the pre-conditioning dose can be optimized to affect the most protective or least damaging response to the subsequent massive stress or exposure. Of interest is that the responses of a wide range of pre-conditioning doses will yield demonstrate evidence of an underlying hormetic dose response (Abete et al. 2010). Thus, the pre-conditioning phenomenon is actually another specific type of hormetic dose response, demonstrating the same quantitative features of the hormetic dose response.

With respect to post-conditioning, this occurs when a very low dose of the stressor agent/condition is given following the massive exposure/trauma. The same type of protective response occurs in animal models and patients as is seen within pre-conditioning procedures. Of relevance to the hormetic dose response is that the dose response for post-conditioning also follows the hormesis model. Based on such findings it has now been proposed that pre- and post-conditioning concepts be incorporated into an integrative and consistent biomedical terminology framed within an hormetic context (Calabrese et al. 2007).

Are there age related differences in expressing an hormetic dose response?

A number of investigations have explored this question, with the answer being yes in some cases and no in others. With respect to situations in which no age related differences were found in the occurrence of hormetic doses responses several examples will be cited. No differences in the qualitative and quantitative nature of the hormetic dose response were reported for the effects of bisphosphonates in rat pyramidal neurons (Giuliani et al. 1998) and with respect to serotonin agonist induced changes in NMDA-induced current (Arvanov et al. 1999). Similar hormetic-like biphasic dose responses were reported for the effects of opiates on sheep heart rates (Zhu and Szeto 1989) and breathing movements (Szeto et al. 1988) for adult and fetal sheep.

In contrast to these examples of hormesis occurring independent of age are cases where qualitative differences exist in the capacity to show an hormetic response based on age. For example, a lack of hormetic dose response in neonatal rats is a risk factor in the development of drug-induced convulsions. More specifically, muscinol (GABA agonist) treatment alters the flurothyl clonic seizure threshold in a biphasic manner in adults rats with an intermediate dose (25–100 ng) inducing a dose dependent anticonvulsant action whereas at high doses (800–1,600 ng) it displays a proconvulsant effect. However, 16 day old rats displayed only a dose dependent proconvulsant response at doses (100–200 ng) that are anticonvulsant in adults (Garant et al. 1995; Calabrese and Baldwin 2002b). Similar age related differences in dose response were likewise reported with a second GABA agonist, THIP (4,5,6,7-tetrahydroisoxazolo-(5,4-c)pyridin3-ol). Since the anticonvulsant effects of GABA receptor activation by either agonist did not occur in the young rats, but did occur in adult rats, an age related difference in the GABAergic response of the substantia nigra par reticulum was indicated.

A 1986 study by Toda et al. reported dose responses for age dependent changes in the isolated coronary arteries of beagles exposed to tyramine. Tyramine induced a linear concentration related response in large coronary arteries of infant dogs partially precontracted with prostaglandin F2α. However, in a similar study with 3-month old beagle dogs a threshold dose response was reported. Furthermore, as the dogs aged from 3 months out to 1–3 years, a J-shaped concentration response emerged in which the low concentration of tyramine induced a relaxation while higher concentrations produced a dose dependent contraction.

The age related response to tyramine in the beagle was associated with different ratios and activation of α1-adrenergic receptors as compared to those of β-receptors. The β-adrenergic receptor functions in large coronary arteries for infant beagles would probably be minimal since treatment with high doses of antagonists did not reverse the contraction to a relaxation, while the reversal was seen in the arteries of young beagles.

When was the first experimental report of an increase in lifespan with low doses of X-rays?

The most convincing experiment based on reproducibility, sample size, consideration of confounding variables and overall quality was that of Davey at the General Electric facility in Schenectady, New York in the second decade of the 20th century (Davey 1917, 1919). Davey assessed the effects of a broad range of X-ray doses on the longevity of the Confused Beetle (Tribulium confusum). Preliminary experiments determined the latency period from the time of a single exposure to death using five different doses over an 80-fold dose range along with a concurrent control. The surprising observation that the lower doses enhanced survival over that of the control stimulated a detailed follow-up investigation (Davey 1919).

The preliminary study assessed and eliminated possible confounding due to overcrowding, the presence of NO2 during high voltage connections of the X-ray tubes, effects of air ionizations, humidity and other parameters. Experimental exposure methods were addressed in detail, providing documented assurance of an accurate and reproducible X-ray exposure system. As a further illustration of Davey’s attention to detail he kept technicians blind to the study hypothesis. He also ensured a uniformity of age distribution across treatment and control groups. The sample size was also quite large, employing several thousand beetles. Davey also used regression analyses to assess the data as analysis of variance had only just become available (Fisher 1918).

The follow up study employed both a single exposure and daily exposure protocol depending on the experiments. The follow up studies confirmed the biphasic (i.e. hormetic) nature of the dose response relationship. Despite the impressive and reproducible findings of Davey, it was not until 1957 when Cork reinvestigated this area, employing the identical experimental model but this time using a gamma ray source. Cork (1957) found a similar increase of lifespan at low doses in a well designed study, confirming the original observations.

Can hormesis affect the aging process via immune modulation?

A considerable literature exists demonstrating the occurrence of hormetic dose responses for numerous immune endpoints (Calabrese 2005d). While many of these findings are of basic biological interest, a substantial number of cases have biomedical/clinical applications that can affect the normal aging process, survival, longevity and are part of treatment modalities within clinical settings. For example, low concentrations of estrogen enhance proliferation of immune-competent cells, whereas higher concentrations decreased the proliferative response (Kenny et al. 1976). These authors concluded that physiological levels of estradiol in females may enhance lymphocyte mitosis and may account for the more favorable immunological responsive of females as compared to males in many situations. Likewise, low to moderate consumption of ethanol enhances phagocytosis and chemotaxis of human polymorphonuclear leucocytes (Hallengren and Forsgren 1978). Low doses of resveratrol are also immno-stimulatory within an hormetic context and are associated with numerous endpoints of public health significance including tumor responsiveness, cardiovascular health and host infectivity (Calabrese et al. 2010).

Can hormesis reduce the risk of developing type 2 diabetes mellitus?

People with type 2 diabetes mellitus display an impaired capacity to regulate blood glucose levels along with a predisposition to inflammatory processes. While type 2 diabetes mellitus is a systemic disease that affects multiple organs it has been proposed that underlying defense mechanisms exist that enhance the capacity to resist the detrimental influence of widespread diabetes promoting lifestyle factors. Kolb and Eizirik (2011) proposed that mild stress induced adaptive mechanisms of an hormetic nature and determine the capacity to resist the onset of type 2 diabetes. Amongst the proposed hormetic mechanisms are ones that involve enhanced insulin sensitivity, pancreatic beta cell survival as well as protection from inflammatory responses that lead to mitochondrial dysfunction. Amongst the effector agents that facilitate these adaptive responses would be anti-inflammatory cytokines, anti-oxidant enzymes, anti-apoptotic agents, surtuins, HSPs and chaperone proteins.

How may lifespan be extended via hormesis?

By definition hormesis becomes manifest when a low dose of a stressor agent (or condition) induces an adaptive response that protects against subsequent exposure to a more massive stress. This is an example of pre-conditioning hormesis. This type of hormetic stress that has been observed to extend lifespan includes thermal, oxidative, ionizing radiation, and various chemical agents as demonstrated in yeast, worms and flies (Burton et al. 1988; Hunt et al. 2011; Lithgow et al. 1995; McAlister and Finkelstein 1980).

Hunt et al. (2011) have proposed that these different means of enhancing longevity suggest the existence of overlying stress response pathways that converge with prolongevity pathways. In fact, the close association between stress and aging suggests that activation of focused hormetic mechanisms may extend life and/or slow down aging related functional decrements.

While the hormesis concept provides a theoretical means to extend lifespan and improve the process of aging, Hunt et al. (2011) noted the need to synthesize and assess structural analogues of molecules that act via hormetic mechanisms that would permit an expansion of the beneficial dose response range while minimizing possible undesirable effects at higher concentrations.

Can the hormesis concept be used to reduce the risk of Alzheimer’s disease?

Amyloid Beta (Aβ) originates from the amyloid precursor proteins β and γ following the cleaving actions of secretase. Senile plaques observed in Alzheimer’s disease (AD) patients result from the aggregation of various isoforms (Selkoe 2004). While Aβ-induced neurodegenerative disease in the elderly likely involves multiple contributory mechanisms, it is known that AD and nerve growth factor (NGF) compete for binding to the neurotrophin receptor, p75NTR. During this process the Aβ initiates the apoptosis of neurons (Hashimoto et al. 2004; Perini et al. 2002). Despite its harmful causal connections to AD Arevalo et al. (2009) argues that since Aβ is a normal metabolic product it may have some underlying biological function that has not yet been observed due to a preponderant focus on high dose effects. In fact, as Aβ production is enhanced and/or it is not properly cleared, Aβ tends to accumulate leading to changes that are toxic to neurons (Coulson 2006). In follow up research to this question of Aβ induced low dose response Arevalo et al. (2009) revealed Aβ had a distinctly different effect on mouse hippocampal neurons depending on its concentration. At the low concentration of 20 nM the Aβ decreased the number of dendrites while increasing their length, thereby enhancing GABAergic connectivity. However, at the higher concentration of 800 nM the opposite effect occurred. This biphasic dose response was similar to the dose response pattern of NGF. The Aβ may therefore display neurotrophic functions similar to NGF at low doses, whereas at higher concentrations (40-fold higher) Aβ acts as a NGF antagonist enhancing the development of AD. These findings suggest that the biphasic hormetic dose response concept may provide a useful exploratory role in assessing possible biological functions of Aβ but also its therapeutic applications.

How does autophagy affect hormetic outcomes?

Autophagy is a process that involves the activation of a lysosomal degradation pathway which results in the engulfment of portions of the cytoplasm by double-membraned vesicles that fuse with lysosomes and their degradation via lysosomal hydrolases (Criollo et al. 2010; Kroemer et al. 2010; Madeo et al. 2010; Silver et al. 2010). Autophagy is an adaptive process as it affects the removal of damaged organelles such as leaky mitochondria, thereby reducing the risk of cell damage and cell death (i.e. apoptosis) and may increase the dose response threshold for the induction of mitochrondrial outer membrane permabilization (MOMP) inducing agents. The linkage of autophagy and apoptosis is a close one, with some degree of cross-talk and convergence between these processes. For example, BH3 proteins induce either autophagy or apoptosis depending on the dose, with low doses enhancing autophagy and higher doses apoptosis. Various types of dietary, pharmacological and genetic interventions have enhanced lifespan in experimental models and have also enhanced autophagy (Bjedov et al. 2010; Calabrese et al. 2010; Eisenberg et al. 2009; Madeo et al. 2010; Marino et al. 2011; Morselli et al. 2009, 2010, 2011).

While mechanisms by which autophagy induction may have a positive effect on lifespan remain to be clarified, Martins et al. (2011) recently hypothesized that the inhibition of mTOR, which enhances autophagy, may also reduce the rate of aging via other metabolic processes (Anisimov et al. 2010; Blagosklonny 2010; Demidenko et al. 2010; Korotchkina et al. 2010; Leontieva and Blagosklonny, 2010; Leontieva et al. 2010). Furthermore, autophagy may suppress oncogenesis as well as accelerating clearance of altered proteins such as beta amyloid which cause neurotoxicity and neurodegeneration.

Can hormesis slow vascular aging?

In 1954 Altschul offered the prescient statement: “It has been said that one is as old as one’s arteries. In view of the supreme importance of endothelium in arterial function, I should like to modify…..this statement by saying that one is as old as one’s endothelium”.

This statement suggests that vascular aging is a major factor in the aging process. Healthy aging of the endothelial cells requires both a maturational and maintenance phase. According to Thorin-Trescases and Thorin (2010) reactive chemical species are required for proper maturation in order to adapt to growth processes and hormonal changes. They further assert that the exposure of the endothelium to physiological oxidative stress (i.e. hormesis) during development determines the effectiveness of the body’s adaptive capacity to cope with risk factors for cardiovascular disease and account for vascular longevity.

This interpretation was supported in a series of experiments in which the antioxidant polyphenol catechin was given before or after endothelial maturation or endothelial dysfunction of aged mice. They found that when mice were given antioxidants prior to completion of endothelial maturation it was harmful to endothelial function. In contrast, mice treated with antioxidants after adulthood appeared to have benefited from the maturation of the endothelium initially exposed to low physiological oxidative stress. These findings indicate that low ROS levels during the process of development enhance the subsequent expression of antioxidant defenses during adulthood.

Is the low dose protective effect of alcohol consumption a manifestation of hormesis?

Twenty million Americans have an alcohol dependence. Amongst the plethora of biomedical and social problems associated with such alcohol abuse is epidemiological evidence that excessive and chronic consumption of alcohol is associated with type 2 diabetes as compared with nondrinkers. However, this same epidemiological evidence also revealed that type 2 diabetes is lower in those with light to moderate alcohol consumption than abstainers. The molecular basis of this hormetic dose response has been explored by various groups (Baliunas et al. 2009; Djousse et al. 2009; Goude et al. 2002; Kiechl et al. 1996). Of particular interest are findings of Qu et al. (2011) that ethanol biphasically mediated changes in insulin sensitivity in skeletal muscle cells with low doses enhancing the cellular uptake of glucose by about 30% with higher doses being inhibitory. Mechanistic investigations revealed that enhanced glucose transport activity was associated with changes in AKT ser 473 phosphorylation and GLUT4 (i.e. the final molecule in the insulin signal pathway) expression-each acting biphasically.

Epidemiological studies indicate that the most protective consumption level of alcohol for reducing the risk of type 2 diabetes is 22 and 24 g/day for men and women, respectively, whereas the alcohol consumption becomes associated with the enhanced risk of type 2 diabetes at 60 and 50 g/day for men and women, respectively. The peak of the hormetic zone has a threefold “safety” factor for men but only about a two-fold factor for females.

Is pre- and post-conditioning hormesis for cardio-protection affected by aging?

This question has generated considerable research and discussion (Boengler et al. 2009; Niemann et al. 2010; Shim 2010). Cardiovascular disease is principally one of aging in men and women. However, the majority of investigations that have addressed pre-and post-conditioning for cardio-protection have been conducted with healthy young animals. Despite this research emphasis there has been a substantial number of studies concerning how age affects the prevention of IR-induced injury by ischemic preconditioning (IPC). In general, the findings from rodent studies using middle aged and old animals revealed that much of the benefits are lost with age. This seems to be the case whether the intervention was physiological or pharmacological. These disappointing findings lead others to increase the magnitude of the pre-conditioning stimuli. Such changes have shown improvement in young, middle-aged and aged rats, with statistically significantly decreases in risk occurring in the middle aged group. That is, both the young and the aged rats had about 10–12% less damage with the higher pre-conditioning dose while in the middle aged rats it was increased by about 31% (Schulman et al. 2001). This observation challenged the idea that one protocol fits all age groups in a similar fashion. In fact, changing a postconditioning protocol in 80 week old mice from three cycles of 10/10 s IR to five cycles of 5/5 s resulted in going from no enhanced protection to a fully re-established one (Boengler et al. 2008).

An additional complication and/or opportunity may be seen with activities such as caloric restriction and/or exercise. In the case of exercise, it affects cardioprotection against IR injury. In fact, in aged rat hearts (24 months old) the loss of cardioprotection by IPC was partially restored by exercise and more completely by the combination of exercise and caloric restriction (Abete et al. 2005). These animal model studies have been supported with human investigations concerning the effects of exercise on cardioprotection in IPC in patients in their early 70’s (Abete et al. 2001). Thus, the loss of age related cardioprotection can be reversed via exercise/caloric restriction or both. Further research along these lines is needed to better estimate optimal exercise programs within an IPC protocol that could have significant public health translational value.

How do TOR, aging and hormesis intersect?

Target of rapamycin (TOR) is a nutrient sensor and growth regulator. It has the capacity to activate cell growth pathways while shutting off autophagy and stress resistance. Down-regulation of TOR as may occur during caloric restriction is associated with increased life expectancy in experimental models such as C. elegans and Drosophila. This is possibly the result of a decline in protein synthesis. Blagosklonny (2011) has recently provided a conceptual framework bv which hormetic acting agent/processes may be mediated at least in part via TOR pathway antagonism as seen with agents such as resveratrol, metformin, exercise, health shock, hypoxia as well as in p53 inducing stressors.

How can heat shock proteins modulate aging via hormetic processes?

The heat shock HSP70 family members have a significant role in chaperoning and refolding of denaturing protein, which facilitate the restoration of protein function. Altered proteins can be refolded by the ATP-mediated process. Likewise, proteins with irreparable damage can be degraded in the ATP-mediated pathway. While low doses of stressor agents enhance the synthesis of the HSP70 family and facilitate the adaptive response process, high doses of the stressor agents may overload the system, leading to the uncontrolled degradation of essential regulatory molecules (Sonneborn 2005). The hormetic-like biphasic dose response of the HSP70 family has been widely reported in environmental and biomedical models and from a range of inducing agents (Tukaj et al. 2011; Wang et al. 2010). The induction of such adaptive processes occur over a range of roughly 5–100 fold below the onset of cytotoxicity (Bierkens et al. 1998). This range of HSP70 induction below cytotoxicity is similar to that reported for a very large proportion of hormetic dose responses spanning a large number of biological models, endpoints, and chemical/physical stressor agents (Calabrese and Blain 2011).

Of relevance to the present paper is that aging is associated with a diminished capacity to express HSP, including the HSP70 family (Blake et al. 1991; Heydari et al. 1993; Kregel et al. 1995; Liu et al. 1989; Locke and Tanguay 1996; Naito et al. 2001), in numerous tissues (e.g., heart—Gaia et al. 1995; Kregel et al. 1995; Locke and Tanguay 1996; liver—Heydari et al. 1993; Kregel et al. 1995) following acute heat stress. However, whether the HSP70 family expression is diminished in old animals when exposed to other types of physical stress is an important question that addresses the issue of generalizability. Challenging the assumption of generalizability was the study of Naito et al. (2001) which reported that exercise-induced accumulation of HSP72 in highly oxidative skeletal muscles was comparable between young and old rats. In the Naito et al. (2001) study the young rats displayed a higher induction of HSP72 in fast twitch muscle, usually generally by about 20–25% than in the older rats. Even in this case the capacity of the old rats for HSP72 induction was still markedly enhanced, ranging from 15 to 150% depending on the specific muscle.

While aging therefore is associated with diminished HSP response to heat stress, aged tissue can be responsive to other types of stress. Nitta et al. (1994) reported that even though the induction of HSP72 mRNA is reduced following short-term ischemia in the hearts of old rats, using a more prolonged exposure to ischemia resulted in similar HSP72 induction to that observed in the hearts of young rats. This is similar to the case noted earlier for preconditioning, that is, increasing the preconditioning dose elicited the improved performance in the middle-aged rats compared to the younger rats (Schulman et al. 2001). This suggests that the nature of the response is very sensitive to the experimental protocol. It is quite likely that optimal induction protocol may vary by age, gender, and other parameters and their interactions.

Final perspectives

The hormesis concept has value on multiple levels for the biogerontology community. It is a fundamental dose response theory set in an evolutionary framework, and one that governs the maximum potential of a drug or stressor agent to enhance biological performance such as life span and the quality of the aging process. The hormesis concept provides one of the key “rules” of the aging process, that is, the extent to which such processes can be improved. The hormetic dose response also affects how biogerontologists do their jobs by affecting study design, dose number, dose range, statistical power calculations, all factors that can help make a study better able to address key questions more incisively. Knowledge of hormesis is also important because it helps to clarify the health implications where agents induce harmful responses below the threshold as well.

A principal challenge with the concept of hormesis for biogerontologists is to find practical ways to incorporate it within the educational framework of physicians, pharmacologists, biomedical scientists, and environmental regulators. Based on the dynamics of research publications, the field of biogerontology is far ahead of most areas of medicine concerning the hormesis concept and its applications. This will provide both challenges and opportunities to biogerontologists in their interactions with the medical, pharmaceutical and regulatory communities. Since hormesis is a basic biological concept it will be important for the leaders of these fields to “institutionalize” the teaching of this concept and its applications. Nonetheless, considerable progress is becoming more evident along these lines within the field of toxicology and pharmacology where hormesis is prominent in leading textbooks. It is expected that this concept will become more generally integrated within the formal educational process of these biomedical and therapeutic disciplines over the next few years. Such an educational transformation will provide the foundation for integrating hormetic principles into modern medicine and biomedical research and how they may affect the aging process.