Chemical hormesis: its historical foundations as a biological hypothesis

Despite the long history of hormesis-related experimental research no systematic effort to describe its early history has been undertaken. The present paper attempts to reconstruct and assess the early history of such research and to evaluate how advances in related scientific fields affected the course of hormesis-related research. The purpose of this paper is not only to satisfy this gap in current knowledge, but also to provide a foundation for the assessment of how the concept of hormetic dose-response relationships may have affected the nature of the bioassay especially with respect to hazard assessment practices within a modern risk assessment framework.

Radiation hormesis: the demise of a legitimate hypothesis

This paper examines the underlying factors that contributed to the marginalization of radiation hormesis in the early and middle decades of the 20th century. The most critical factor affecting the demise of radiation hormesis was a lack of agreement over how to define the concept of hormesis and quantitatively describe its dose-response features. If radiation hormesis had been defined as a modest overcompensation to a disruption in homeostasis as would have been consistent with the prevailing notion in the area of chemical hormesis, this would have provided the theoretical and practical means to blunt subsequent legitimate criticism of this hypothesis. A second critical factor undermining the radiation hormesis hypothesis was the generally total lack of recognition by radiation scientists of the concept of chemical hormesis which was markedly more advanced, substantiated and generalized than in the radiation domain. The third factor was that major scientific criticism of low dose stimulatory responses was galvanized at the time that the National Research Council (NRC) was organizing a national research agenda on radiation and the hormetic hypothesis was generally excluded from the future planned research opportunities. Furthermore, the criticisms of the leading scientists of the 1930s which undermined the concept of radiation hormesis were limited in scope and highly flawed and then perpetuated over the decades by other 'prestigious' experts who appeared to simply accept the earlier reports. This setting was then linked to a growing fear of radiation as a cause of birth defects, mutation and cancer, factors all reinforced by later concerns over the atomic bomb. Strongly supportive findings on hormetic effects in the 1940s by Soviet scientists were either generally not available to US scientists or disregarded as part of the Cold War mindset without adequate analysis. Finally, a massive, but poorly designed, US Department of Agriculture experiment in the late 1940s to assess the capacity for low dose plant stimulation by radionuclides failed to support the hormetic hypothesis thereby markedly lessening enthusiasm for research and funding in this area. Thus, the combination of a failed understanding of the hormetic hypothesis and its linkage with a strong chemical hormesis database, flawed analyses by prestigious scientists at the critical stage of scientific research development, reinforced by a Cold War mentality led to marginalization of an hypothesis (i.e., radiation hormesis) that had substantial scientific foundations and generalizability.

The marginalization of hormesis

Despite the substantial development and publication of highly reproducible toxicological data, the concept of hormetic dose-response relationships was never integrated into the mainstream of toxicological thought. Review of the historical foundations of the interpretation of the bioassay and assessment of competitive theories of dose-response relationships lead to the conclusion that multiple factors contributed to the marginalization of hormesis during the middle and subsequent decades of the 20th century. These factors include: (a) the close-association of hormesis with homeopathy lead to the hostility of modern medicine toward homeopathy thereby creating a guilt by association framework, and the carry-over influence of that hostility in the judgements of medically-based pharmacologists/ toxicologists toward hormesis; (b) the emphasis of high dose effects linked with a lack of appreciation of the significance of the implications of low dose stimulatory effects; (c) the lack of an evolutionary-based mechanism(s) to account for hormetic effects; and (d) the lack of appropriate scientific advocates to counter aggressive and intellectually powerful critics of the hormetic perspective.

Hormesis: a highly generalizable and reproducible phenomenon with important implications for risk assessment

From a comprehensive search of the literature, the hormesis phenomenon was found to occur over a wide range of chemicals, taxonomic groups, and endpoints. By use of computer searches and extensive cross-referencing, nearly 3000 potentially relevant articles were identified. Evidence of chemical and radiation hormesis was judged to have occurred in approximately 1000 of these by use of a priori criteria. These criteria included study design features (e.g., number of doses, dose range), dose-response relationship, statistical analysis, and reproducibility of results. Numerous biological endpoints were assessed, with growth responses the most prevalent, followed by metabolic effects, reproductive responses, longevity, and cancer. Hormetic responses were generally observed to be of limited magnitude with an average maximum stimulation of 30 to 60 percent over that of the controls. This maximum usually occurred 4- to 5-fold below the NOAEL for a particular endpoint. The present analysis suggests that hormesis is a reproducible and generalizable biological phenomenon and is a fundamental component of many, if not most, dose-response relationships. The relatively infrequent observation of hormesis in the literature is believed to be due primarily to experimental design considerations, especially with respect to the number and range of doses and endpoint selection. Because of regulatory considerations, most toxicologic studies have been carried out at high doses above the low-dose region where the hormesis phenomenon occurs.

Chemical hormesis: its historical foundations as a biological hypothesis

Despite the long history of hormesis-related experimental research, no systematic effort to describe its early history has been undertaken. The present paper attempts to reconstruct and assess the early history of such research and to evaluate how advances in related scientific fields affected the course of hormesis-related research. The purpose of this paper is not only to satisfy this gap in current knowledge but also to provide a foundation for the assessment of how the concept of hormetic dose-response relationships may have affected the nature of the bioassay, especially with respect to hazard assessment practices within a modern risk assessment framework.

The marginalization of hormesis

Despite the substantial development and publication of highly reproducible toxicological data, the concept of hormetic dose-response relationships was never integrated into the mainstream of toxicological thought. Review of the historical foundations of the interpretation of the bioassay and assessment of competitive theories of dose-response relationships lead to the conclusion that multiple factors contributed to the marginalization of hormesis during the middle and subsequent decades of the 20th Century. These factors include the following: (a) the close association of hormesis with homeopathy, which led to the hostility of modern medicine toward homeopathy, thereby creating a guilt-by-association framework, and the carryover influence of that hostility toward hormesis in the judgements of medically based pharmacologists/toxicologists; (b) the emphasis of high-dose effects linked with a lack of appreciation of the significance of the implications of low-dose stimulatory effects; (c) the lack of an evolution-based mechanism(s) to account for hormetic effects; and (d) lack of appropriate scientific advocates to counter aggressive and intellectually powerful critics of the hormetic perspective.

Can the concept of hormesis Be generalized to carcinogenesis?

The concept of hormesis (i.e., low-dose stimulation/high-dose inhibition) has been shown to be widely generalizable with respect to chemical class, animal model, gender, and biological end point. The public health implication of this lack of linearity in the low-dose area of the dose-response curve raises the question of whether low doses of carcinogens will reduce cancer risk. Articles relating to the process of carcinogenesis (i.e., initiation, promotion, tumor development, and progression) were obtained from a recently developed chemical hormesis database and evaluated for their evidence of hormesis. Numerous examples in well-designed studies indicate that U- or J-shaped dose-response relationships exist with respect to various biomarkers of carcinogenesis in different animal models of both sexes. Examples of such J-shaped dose-response relationships in each stage of the process of carcinogenesis were selected for detailed toxicological examination. These results have important implications for both the hazard assessment of carcinogens and cancer risk assessment procedures.

A general classification of U-shaped dose-response relationships in toxicology and their mechanistic foundations

The development of a comprehensive database of chemical hormetic responses (i.e., U- or inverted U-shaped dose-response relationships) using objective a priori study design, statistical and study replication criteria has recently been reported. An assessment of this database reveals the existence of a wide range of hormetic dose-response relationships including those demonstrating a direct stimulation or an overcompensation response to a disruption of homeostasis. These two broad types of hormetic responses are affected by temporal factors and display unique patterns of dose-range stimulation, magnitude of stimulatory response and relationship of the maximum stimulatory response to the NOAEL. A general classification of U-shaped dose-response relationships is proposed to provide a more organized framework to evaluate the highly distinctive and diverse hormetic responses within the context of establishing underlying biological mechanisms and exploring risk assessment implications.

Hormesis as a biological hypothesis

A comprehensive effort was undertaken to identify articles demonstrating chemical hormesis. Nearly 4000 potentially relevant articles were retrieved from preliminary computer database searches by using various key word descriptors and extensive cross-referencing. A priori evaluation criteria were established including study design features (e.g., number of doses, dose range), statistical analysis, and reproducibility of results. Evidence of chemical hormesis was judged to have occurred in approximately 350 of the 4000 studies evaluated. Chemical hormesis was observed in a wide range of taxonomic groups and involved agents representing highly diverse chemical classes, many of potential environmental relevance. Numerous biological end points were assessed; growth responses were the most prevalent, followed by metabolic effects, longevity, reproductive responses, and survival. Hormetic responses were generally observed to be of limited magnitude. The average low-dose maximum stimulation was approximately 50% greater than controls. The hormetic dose-response range was generally limited to about one order of magnitude, with the upper end of the hormetic curve approaching the estimated no observable effect level for the particular end point. Based on the evaluation criteria, high to moderate evidence of hormesis was observed in studies comprised of > 6 doses; with > 3 doses in the hormetic zone. The present analysis suggests that chemical hormesis is a reproducible and relatively common biological phenomenon. A quantitative scheme is presented for future application to the database.

A toxicologically based weight-of-evidence methodology for the relative ranking of chemicals of endocrine disruption potential

A toxicologically based predictive scheme is presented for quantitatively ranking chemical agents with respect to their capacity to ensure endocrine disruption in target species based on short-term bioassays. Criteria providing the predictive framework include: (1) endocrine disruption as a multistage process, (2) phylogenetic considerations, (3) model system, and (4) estrogenic potency. Relative rankings were calculated for 15 environmentally relevant agents reported to have endocrine-disrupting effects. The relative ranking process offers a procedure for assessing the potential of endocrine disruption and for identifying data gaps for specific chemical agents. Although the current scheme is limited to "estrogenic" agents, it is anticipated that future refinements (e.g., incorporation of antiestrogenic potency data) will improve the system.

Potency ranking of methemoglobin-forming agents

This study represents the first systematic attempt to rank methemoglobin-forming agents. It is a quantitative potency ranking study utilizing linear regression analysis of dose-response data for comparative purposes. Six agents that are direct-acting and eight that require bioactivation were tested for their ability to induce methemoglobin formation in Dorset sheep erythrocytes under defined in vitro conditions. The agents were then ranked according to three complementary methods based on the slope of the linear regression, the calculated dose expected to induce a given amount of methemoglobin formation and the calculated percentage methemoglobin response induced by 1 mmol l-1 of the agent. The direct-acting agents, ranked from most to least potent inducers of methemoglobin formation, are: p-dinitrobenzene > o-dinitrobenzene > copper = nitrite > chlorite > chlorate. The ranking from most to least potent inducers of the bioactivated agents are: alpha-naphthol > p-nitroaniline > m-nitroaniline, o-nitroaniline > p-nitrotoluene = aniline > m-nitrotoluene = o-nitrotoluene. The ranking procedures are discussed and issues of interindividual variation and agent-specific sensitivities are addressed.

Decrease in hepatotoxicity by lead exposure is not explained by its mitogenic response

The present research was conducted to evaluate the effect of mitogen pre-exposure on CCl4-induced hepatotoxicity. Male Wistar rats were administered a single i.p. injection of CCl4 (0.3 ml kg-1 in corn oil) 48 h following either a single dose of lead nitrate (0.33 mg kg-1) or distilled water via i.v. injection. Hepatotoxicity, as measured by serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels, was monitored 6, 24, 48, 72 and 120 h after CCl4 exposure. The lead nitrate-pretreated rats displayed markedly lower serum ALT and AST levels at 24, 48 and 72 h than rats pretreated with distilled water. However, treatment with the antimitotic agent colchicine did not alter the lead-induced protection. These findings suggest that the lead-induced protection is not associated with the major mitogenic response of lead, despite its strong temporal association. A critical review of the available toxicological data also argues against the lead protection being a function of its capacity to inhibit cytochrome P-450.

Evaluation of a rodent peroxisome proliferator in two species of freshwater fish: rainbow trout (Onchorynchus mykiss) and Japanese medaka (Oryzias latipes)

Rainbow trout (Onchorynchus mykiss) and Japanese medaka (Oryzias latipes) were exposed to the hypolipidemic drug gemfibrozil, a known rodent peroxisome proliferator. Trout were injected (i.p.) daily for 2 weeks at doses of 0, 46, 87, or 152 mg/kg/day. Medaka were exposed to the nominal concentrations of 0, 1.25, 2.5, or 5 ppm in water for 2 weeks in a static-renewal system. Peroxisome proliferation was assessed by measuring fatty acyl-CoA oxidase (FAO) activity, peroxisomal bifunctional enzyme (PBE) quantity, and changes in liver-to-body weight ratios (LWR). Results indicate that a mild peroxisome proliferative response was observed in rainbow trout (significant increases in FAO activity at all dose levels and in LWR at the highest dose level). Medaka demonstrated a significant increase in PBE at the highest dose level, while nonsignificant increases in FAO activity were observed at the mid- and high-dose levels.

Gap junction-mediated intercellular communication in primary cultures of rainbow trout hepatocytes

Gap junction-mediated intercellular communication was evaluated in primary cultures of rainbow trout hepatocytes by measurement of dye coupling. Donor hepatocytes were microinjected with fluorescent Lucifer yellow CH dye and visualization of dye spread (dye coupling) to adjacent hepatocytes was recorded. A maximum level was reached between 8 and 12 hr which was maintained up to 24 hr. Dye coupling then decreased over the next 48 hr. Cell viability was monitored by the percentage of total LDH released and trypan blue exclusion at each time point. The effect of 12-O-tetradecanoylphorbol-13-acetate (TPA), a known inhibitor of rodent hepatic intercellular communication, on gap junction-mediated intercellular communication was evaluated in 24-hr cultures. A dose-response inhibition was demonstrated with maximum inhibition observed at 3 hr of exposure.

Tissue repair: a critical determinant in CCl4 hepatotoxicity

A single antimitotic dose of colchicine administered 24 hr after a single ip dose of CCl4 profoundly enhanced lethality in the adult Sprague-Dawley male rat from < 10% to over > or = 70% within the next 24 hr. These findings suggest that disruption of normal hepatic cellular repair processes is a critical factor affecting the outcome of CCl4 intoxication.

Improved method for selection of the NOAEL

The paper proposes that the NOAEL be defined as the highest dosage tested that is statistically significantly different from the control group while also being statistically significantly different from the LOAEL. This new definition requires that the NOAEL be defined from two points of reference rather than the current approach (i.e., single point of reference) in which the NOAEL represents only the highest dosage not statistically significantly different from the control group. This proposal is necessary in order to differentiate NOAELs which are statistically distinguishable from the LOAEL. Under the new regime only those satisfying both criteria would be designated a true NOAEL while those satisfying only one criteria (i.e., not statistically significant different from the control group) would be designated a "quasi" NOAEL and handled differently (i.e., via an uncertainty factor) for risk assessment purposes.

A toxicological basis to derive a generic interspecies uncertainty factor

A new method is proposed to derive the size of the interspecies uncertainty factor (UF) that is toxicologically and statistically based. The method involves quantifying interspecies variation in susceptibility to numerous toxic substances via the use of binary interspecies comparisons that are converted to a 95% UF. This interspecies UF represents an estimate of the upper 95% of the population of 95% prediction intervals (PI) for binary interspecies comparisons within four categories of phylogenetic relatedness (species within genus, genera within family, families within order, orders within class). The 95% interspecies UFs range from a low of 10 for species within genus up to 65 for orders within class. Most mammalian toxicology studies involving mice, rats, cats, dogs, gerbils, and rabbits are orders-within-class categories for human risk assessment and would be provided a 65-fold UF. Larger or smaller interspecies UF values could be selected on the level of protection desired.

Comparative response of rainbow trout and rat to the liver mitogen, lead

Lead nitrate and acetate are potent liver mitogens in the Wistar rat. Prior exposure to these agents has been found to alter their susceptibility to hepatotoxins. The present study assessed the capacity of both lead compounds to cause mitogenicity in the liver of adult male and female rainbow trout. Groups treated with a single intravenous or intraperitoneal injection of lead nitrate or lead acetate exhibited no statistically significant (P < 0.05) alterations in liver/body weight ratio nor hepatic DNA content. Results provide evidence of significant interspecies variation in the mitogenic response of rainbow trout and Wistar rats to the mitogenic potential of lead in the liver.

Effect of hypolipidemic drugs gemfibrozil, ciprofibrate, and clofibric acid on peroxisomal beta-oxidation in primary cultures of rainbow trout hepatocytes

Primary cultures of hepatocytes were established from sexually mature male rainbow trout (Oncorhyncus mykiss) and treated with the hypolipidemic drugs gemfibrozil (0.25-1.25 mM), clofibric acid (2.25-3.00 mM), or ciprofibrate (0.25-1.00 mM). Significant dose-related increases in peroxisomal fatty acyl-CoA oxidase (FACO) were seen after exposure for 48 hr to clofibric acid (P < 0.01) and ciprofibrate (P < 0.05) but not gemfibrozil (P = 0.08). Strong correlation was obtained between increased acyl-CoA oxidase activity and the relative amount of peroxisomal bifunctional enzyme (PBE), further supporting evidence of a proliferative effect. These preliminary studies demonstrate that peroxisomal beta-oxidation can be induced in vitro in a primary rainbow trout hepatocyte system.

G2 subpopulation in rat liver induced into mitosis by low-level exposure to carbon tetrachloride: an adaptive response

Recent findings by Mehendale (Med. Hypoth. 33, 289-299, 1990) indicate that prior exposure to chlordecone markedly enhances CCl4-induced lethality. It was established that chlordecone suppressed the capacity of CCl4-induced toxicity to cause an early (i.e., 6 hr after exposure) hepatocellular division which is believed to be a critical tissue response reducing subsequent CCl4-induced hepatotoxicity. Despite the strong evidence presented by Mehendale, occurrence of such an early cellular division has been considered unlikely since most studies indicate that cellular replacement requires from 30-60 hr depending on the agent, dose, and animal species. This paper presents evidence that supports the observations of Mehendale and indicates that the early mitoses were most likely caused by the activation of hepatocytes arrested in the G2 phase of the cell cycle that became activated by CCl4 treatment, induced injury, or both. The concept being put forward here requires additional experimental verification and validation.

Interactive potential of omega-3 fatty acids with clofibrate or DEHP on hepatic peroxisome proliferation in male Wistar rats

The interactive potential of three known peroxisome proliferators, omega-3 fatty acids, clofibrate and di(2-ethylhexylphthalate (DEHP), was evaluated in male weanling Wistar rats for the effect on peroxisomal beta-oxidation. Omega-3 fatty acids were supplied by menhaden oil which was fed in six regimens: low fat (5% w/w), low fat and clofibrate (0.3% w/w) or DEHP (0.25% w/w), high fat (20% w/w), high fat and clofibrate or DEHP in the aforementioned concentrations. Induction of peroxisomal beta-oxidation was measured by changes in liver-to-body weight ratio, fatty acyl-CoA oxidase (FAO) activity, and peroxisomal bifunctional enzyme (PBE) quantity. Analysis of transformed data indicated a less than additive response in FAO activity with no deviation from additivity seen with liver-to-body ratios and PBE.

Possible examples of chemical hormesis in a previously published study

A re-examination of data presented by Daston et al. provided an opportunity to assess the occurrence of chemical hormesis due to the wide dosage range tested and to the large number of treatments within the dosage range. The data revealed numerous examples of possible chemical hormesis for a variety of end-points measured.

The effect of peroxisome proliferators on S-phase synthesis in primary cultures of fish hepatocytes

The ability of seven structurally diverse peroxisome proliferators to induce S-phase synthesis was studied in primary cultures of rainbow trout and medaka hepatocytes. Cells were maintained in serum-free conditions designed to facilitate attachment, viability, and function. Lead nitrate, a well-known and potent rodent liver mitogen, was used in vitro in trout and medaka and in vivo in trout to evaluate the capacity of the species and our system to respond to mitogenic stimuli. S-phase synthesis induced by peroxosome proliferators did not result in any statistically significant increase over control cultures. These in vitro results are supported by in vivo data previously obtained and the present work with lead nitrate.

Ornithine decarboxylase (ODC) activity in the liver of individual medaka (Oryzias latipes) of both sexes

Ornithine decarboxylase (ODC) activity has been determined in the liver of individual medaka (N = 48). The results indicate a mean ODC activity of 1.69 +/- 1.77 nmol 14CO2/hr/mg protein. The fish displayed a large interindividual variation of normal ODC activity with a range of 140fold. However, no sex differences were observed. The findings also indicate that the mean ODC activity in this fish model is approximately 100fold greater than observations in the published literature as well as in our laboratory for commonly employed rodent models.