Dioxin's other face.
In 1983, scares over TCDD forced several thousand residents of Times Beach, Mo., to permanently flee their tainted community. But "given what we now know about this chemical's toxicity and its effects on human health, it looks as though the [Times Beach] evacuation was unnecessary," Vernon N. Houk -- the scientist at the Centers for Disease Control who originally spearheaded the evacuation -- acknowledged, according to the Times article last August.
Most dioxin researchers now suspect that only very high doses of TCDD -- as occur accidentally or in certain occupational settings -- may increase the risk of cancer in humans. But that redefinition does not necessarily imply that the chemical is harmless at lower doses.
Indeed, this near-ubiquitous contaminant -- a by-product of the paper, wood and herbicide industries and of the incineration of organic solvents -- is gaining a new and nasty reputation among toxicologists: as an "environmental hormone" that subtly disrupts normal physiology in ways not completely understood. More potent than some of the body's natural chemical messengers, TCDD suppresses the immune system of mice at least 100 times more effectively than corticosterone, a hormone known for that effect, dioxin researchers say. In fact, increasing evidence suggests that TCDD's ability to mess with the immune system -- not its carcinogenicity -- may represent its greatest threat to public health.
All this flip-flopping on the chemical's toxicity may puzzle the public, but it has proved no less confusing to dioxin researchers. TCDD's toxic deeds result from a perplexing web of interactions. Unlike most toxicants, dioxin causes an array of biological responses that vary widely according to tissue. For example, TCDD may goad one cell type to reproduce wildly and cause another to deviate from its normal path toward specialization.
Different animal species also vary in their responsiveness to dioxin. It takes several thousand times more TCDD to kill a hamster than it does to kill a guinea pig. Yet the hardy hamster is quite susceptible to TCDD's triggering of increased cellular levels of a P450 enzyme - a protein catalyst that plays a role in detoxifying certain chemicals within the body and rendering others more toxic.
Unfortunately, epidemiologic studies have done little to resolve toxicologists' muddy understanding of dioxin's human hazards. For instance, such studies rarely turn up consistent adverse effects among humans exposed to dioxin -- with the exception of chloracne, the disfiguring skin eruptions associated with acute TCDD exposures.
Consider studies of U.S. troops potentially exposed to Agent Orange, a TCDD-tainted herbicide, while serving in Vietnam. An Air Force study of veterans who had participated in the Ranch Hand defoliation program found indications that these men faced an increased -- though statistically insignificant -- risk of skin, genito-urinary and otopharyngeal cancers and a tendency to develop underactive thyroids and diabetes (SN: 3/3/84, p.132). Another study found an increased incidence of high blood pressure, benign fatty tumors, sensitivity to light, and depression among these veterans and miscarriages among their wives (SN: 11/19/88, p.325). A third study found that Vietnam veterans suffer higher-than-normal rates of non-Hodgkins lymphoma, a deadly cancer of the lymph nodes, but it failed to tie the disease to Agent Orange exposure (SN: 4/14/90, p.236).
"If you think of TCDD as a hormore, it makes it easier to understand these very big differences," asserts Linda S. Birnbaum, director of environmental toxicology at the Environmental Protection Agency's Health Effects Research Laboratory in Research Triangle Park, N.C. A single hormone can induce an array of effects in different tissues and species, she explains.
The environmental hormone theory also helps explain why dioxin appears to induce a variety of cancers rather than a single hallmark type -- such as the rare form of cancer, called mesothelioma, that signals asbestos exposure. Unlike most carcinogens, TCDD does not directly damage DNA in a target organ, notes George W. Lucier of the National Institute of Environmental Health Sciences in Research Triangle Park. However, he explains, dioxin clearly enhances abnormal cell growth and appears to cause cancer by amplifying the diverse activities of other carcinogens.
Two recent epidemiologic studies support the human carcinogenicity of TCDD, at least at fairly high doses. In one, researchers at the National Institute for Occupational Safety and Health examined health records for workers exposed to TCDD at a dozen chemical plants. Overall, the 5,172 workers appeared 15 percent more likely to die from cancer than the general population, Marilyn A. Fingerhut and her co-workers reported in the Jan. 24, 1991 NEW ENGLAND JOURNAL OF MEDICINE. However, records on the 1,520 workers whose exposures began at least 20 years ago -- when plant dioxin levels were typically much higher than today -- showed oine times the normal rate for one particular cancer, soft-tissue sarcoma.
A similar study of 1,583 pesticide-plant workers in Germany showed that, compared with the general population, TCDD-exposed workers experienced a 24 percent higher rate of death from all cancers. Among workers with more than 20 years' exposure, the cancer death rate increased to 87 percent above normal, according to Alfred Manz and his co-workers at the Center for Chemical Workers' Health in Hamburg. However, they reported in the Oct. 19, 1991 LANCET, the increases were not linked to any one particular type of cancer.
On the basis of these and other studies, Birnbaum says, "I really feel that high-dose exposure to dioxin has the potential to cause cancer." However, she adds, "I'm very concerned that much lower exposure to dioxin may result in adverse health effects that are very subtle and difficult to detect."
In an effort to update federal regulatory guidelines for human exposures to dioxin -- now considered a "probable human carcinogen" -- EPA has begun reassessing the scientific data on dioxin. In its draft version of this document, due in June, EPA will focus much greater attention on toxicological data revealing TCDD's reproductive, developmental and immunotoxic effects. This document will also establish TCDD as the first pollutant to be regulated on the basis of toxicity observed at the cellular level.
Now that most dioxin researchers believe a single fundamental mechanism underlies all of TCDD's effects (see box, p. 26), toxicologists such as Lucier can construct a unifying mathematical model to describe how dioxin triggers biological effects in cells and organisms. Others, including Birnbaum and Nancy I. Kerkvliet of oregon State University in Corvallis, will help flesh out the model by collecting specific data on the dose-response relationships between TCDD and its array of biological effects.
"Dioxin is no more and no less potent than it ever was," kerkvliet says. "But understanding the mechanism can now help us better estimate the human risk."
So far, studies in mice suggest that dioxin's immunotoxic punch occurs in extremely low doses and may well be more important than cancer in determining dioxin's primary health risk, adds Birnbaum. At least in animals, some suppression of immunity consistently occurs at TCDD doses lower than or equal to those required for triggering increased production of a P450 enzyme -- previously considered a liver cell's most sensitive response. In fact, Birnbaum's preliminary unpublished data suggested that immunotoxicity in mice could be occurring at TCDD doses 1/15 of that needed to boost levels of this enzyme, she says.
Even though scientists continue to debate whether an excess of this P450 enzyme causes any adverse health effects, "few people will contend that suppression of the immune system is not an adverse health effect," she observers.
To study TCDD's immuntoxicity, researchers generally use mice, whose immune systems model those of humans. In one typical test, EPA toxicologists exposed mice to TCDD, then injected them with a harmless, antibody-stimulating agents -- red blood cells from sheep. An animal's ability to produce antibodies serves as one useful measure of its immunological health. Compared with normal mice, the TCDD-treated antimals produced fewer antibodies to the sheep blood cells, Birnbaum says.
EPA researchers have also measured how well TCDD-treated mice respond to viral infections, such as influenza. Mice pretreated with dioxin readily die after exposure to a quantity of virus that rarely kills healthy mice, Gary R. Burleson of EPA's Research Triangle Park facility and his co-workers reported in the November 1990 JOURNAL OF TOXICOLOGY AND ENVIRONMENTAL HEALTH. Birnhaum's team is now trying to determine the dose-response relationships of these immuno-suppressive effects.
Because "there are so many ways to cause immune suppression," Birnbaum explains, scientists can only speculate as to how TCDD weakens immunity. Indeed, she notes, "there could be multiple mechanisms."
At a minimum, TCDD probably interferes with the normal influences of hormones on the immune system, Kerkvliet posits. She says that it appears TCDD can combine with a particular type of receptor protein inside a cell's fluid interior, and then inappropriately turn on specific genes. Some of the victimized cells may reside in glandular tissues, such as the thymus, where hormones influencing immunity are produced.
Dioxin also appears to act directly on the immune system, says Kerkvliet, who studies TCDD's effects on a group of white blood cells called T-lymphocytes. She and her co-workers were initially confounded when they observed that although TCDD boosts production of T-lymphocytes -- which referee the total immune response -- it still causes an overall decline in the mouse immune system's ability to fight foreign substances, be they viruses or pollutants.
"We think TCDD is turning on certain T-helper cells inappropriately, which then makes the overall immune response suppressed," Kerkvliet now says. This idea fits with a new hypothesis that not all of the specialized T-lymphocytes called T-helper cells "help" strengthen the immune response; some may actually inhibit it, she notes.
For the most part, Kerkvliet believes that dioxin initiates its direct immunotoxic effects by binding to the dioxin protein receptor -- perhaps in the bone marrow, where white blood cells are produced -- and by toying with the normal functioning of genes. Recently, her research group studied how TCDD affects a mouse's production of cytotoxic T-lymphocytes, which destroy cells infected with viral invaders.
The team compared responses in TCDD-treated mice with normal and defective dioxin receptors, and found significantly greater immune suppression in the mice with normal receptors. They also compared the responses of these mice to a variety of polychlorinated biphenyls (PCBs), chemical relatives of dioxin. Immunity suppression indeed correlated with each chemical's ability to bind to the protein receptor, Kerkvliet's group reported in the April 1990 FUNDAMENTAL AND APPLIED TOXICOLOGY. These findings suggest that dioxin's protein receptor plays an important role in its immunotoxicity, they say.
Given the complexity of the immune system, however, not all dioxin researchers are ready to settle on a single receptor-based mechanism to describe all of TCDD's immuno-suppressive effects.
Michael P. Holsapple of the Medical College of Virginia/Virginia Commonwealth University in Richmond has also observed that "when we give dioxin to animals or white blood cells, we see problems with their immune function." However, he adds, "the immune system is probably just a microcosm of the whole complex story for dioxin." He suspects that TCDD may employ different routes of attack depending on the conditions of exposure, he says.
For instance, his team compared the effects of acute versus chronic TCDD exposures on the ability of mice to produce antibodies to sheep red blood cells. After a single acute dose, mice with normal dioxin receptors suffered greater immune suppression than mice who had defective receptors. However, when mice received this same amount of TCDD over a two-week period, both mouse strains showed similar immunosuppressive responses, he and his colleagues report in the January 1992 TOXICOLOGY AND PHARMACOLOGY. Holsapple now theorizes that TCDD's mechanisms may not always involve the receptors and may differ at high and low doses.
Throughout the developed world, humans already experience chronic low-dose exposures do dioxins, primarily through their diet (SN: 7/13/85, p.26). Holsapple and his co-workers suspect that people "exposed to low doses over an extended period of time (i.e. months to years) may be at increased risk to immunotoxic effects by these chemicals through additional and presently unidentified mechanisms."
One such mechanism can be inferred from developing research in the field of endocrinology, Holsapple says. Scientists had assumed that, much like dioxin, all steroid hormones act exclusively through an intracellular protein receptor that helps it target a particular gene (SN: 8/10/91, p.85). But Holsapple points to new evidence suggesting that some steroid hormones -- including progesterone, estrogen and testosterone--can also bind to other receptors on the outside of a cell membrane, where they can regulate the flow of sales into and out of a cell. TCDD might also tinker with a cell's physiology through such a mechanism, he suggests.
In mice, it takes far smaller quantities of TCDD to suppress immunity than it does to unleash most of TCDD's other toxic effects. And white blood cells in both mice and humans respond similarly to TCDD. But to date, there's little evidence to suggest that low-dose exposures to TCDD suppress immunity in humans. Birnbaum, Kerkvliet and Holsapple contend that studies of dioxin-exposed humans have asked the wrong questions.
"If I were to take mice and ask the same [research] questions that are routinely asked of the populations at Times Beach, or in the Ranch Hand study, I would come up with a very nebulous picture [of TCDD's immunotoxicity," says Holsapple. "But when we ask different questions [in mice], we can certainly show very strong effects on the immune response."
Birnbaum is now calling for a study that will determine how well TCDD-exposed people mount an antibody response to a novel antigen. Perhaps a new flu vaccine -- one that uses an influenza strain that hasn't previously infected humans -- can serve the function of the sheep red blood cells given to laboratory mice, she says.
But Kerkvliet says EPA shouldn't hold its breath waiting for the definitive epidemiologic study. It would be next to impossible to prove beyond a doubt that dioxin causes immune suppression in humans, she asserts. Unlike sheltered laboratory mice, people come in contact with many immunity-altering forces -- such as stress, drugs and disease. Regulators should therefore base their limits for safe exposures to dioxin on animal models and on our developing scientific understanding of TCDD's mechanisms of action, she says.
Kerkvliet suspects that most Americans -- who harbor about 30 parts per trillion (ppt) of dioxins in their blood, including about 7 ppt of TCDD -- fall below the range of dioxin exposures that can jeopardize immunity. However, she adds, populations that commonly receive higher doses, such as nursing infants (SN: 4/26/86, p.264), chemical workers and people who consume large quantities of fish, could conceivably experience compromised immunity.
"The fact that you can't clearly show the effects in humans in no way lessens the fact that dioxin is an extremely potent chemical in animals -- potent in terms of immunotoxicity, potent in terms of promoting cancer," says Kerkvliet. "I simply don't believe that humans represent some unique species."
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|Title Annotation:||includes related article on how dioxin affects cells|
|Author:||Schmidt, Karen F.|
|Article Type:||Cover Story|
|Date:||Jan 11, 1992|
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