Clarifying dioxin's cellular invasion.

Setting acceptable human exposure limits for TCDD -- the most toxic member of the dioxin family -- has spawned controversy since the 1970s, when animal research suggested that even trace quantities of this industrial byproduct might cause cancer. Two new studies now offer clues to the multi-step process whereby TCDD penetrates the nucleus of a cell and affects its genetic material.

The new findings support an emerging theory that the first step in TCDD's toxicity involves its binding to a protein, called a receptor, in the liquid interior of cells. If a certain minimum quantity of this chemical must accumulate before triggering the crucial binding of that receptor, as some scientists suspect, trace amounts of dioxin might have no adverse health effects. If so, identifying that no-effect threshold might allow regulatory agencies to establish a "safe" limit for the pollutant.

A research team led by Oliver Hankinson at the University of California, Los Angeles, has now isolated the biological substance, also a protein, that allows receptor-bound dioxin molecules to successfully invade a cell's nucleus, where its genetic material resides. In the May 17 SCIENCE, the group reports finding that even though the new protein does not bind to TCDD, it must accompany this dioxin or the toxicant will never penetrate the cell nucleus. "We showed that this factor [new protein] is necessary for the receptor-dioxin complex to move into the nucleus," says Hankinson.

His team also showed that without the new protein, TCDD could not turn on a gene for the production of a normally detoxifying enzyme. But with the protein, TCDD spurs the production of this enzyme, a member of the P450 family. P450 enzymes initiate a biochemical process that renders toxic chemicals more soluble in water, so that they can be excreted more readily by the body. But sometimes P450 makes contaminants more toxic instead. TCDD's stimulation of P450 production might therefore indirectly increase levels of other toxicants in the body. But the enzyme does not break down TCDD, leaving researchers uncertain about the direct mechanism through which TCDD cause its toxic effects.

In a second study, Thomas A. Gasiewicz of the University of Rochester (N.Y.) School of Medicine and his colleagues at the Standford University School of Medicine showed that mixtures of TCDD and the receptor protein alone cannot bind to DNA. But when the researchers mixed TCDD and the receptor with fluid taken from ground-up cells, the complex indeed bound to DNA.

These results, reported in the March 19 BIOCHEMISTRY, suggest a second protein is involved in dioxin's action, Gasiewicz says. When his group measured the size of the complexes that did bind DNA, the researchers found they were larger than the receptor and dioxin combined -- further supporting the theory that a second mystery protein was involved.

Gasiewicz and his co-workers have yet to isolate the second protein, but it appears similar to the one now reported by Hankinson's group. In fact, Gasiewicz told SCIENCE NEWS, "We think we could be looking at the same protein."

The two studies support an emerging TCDD-toxicity model, says Linda Birnbaum, director of environmental toxicology at the Environmental Protection Agency's (EPA) Health Effects Research Laboratory in Research Triangle Park, N.C. Earlier this year, dioxin researchers met at Cold Spring Harbor Laboratory on Long Island, N.Y., to iron out a model that might explain why TCDD sometimes appears nontoxic at low levels. The new work "is in support of that model," says Birnbaum.

What's "interesting and exciting" about the two new studies "is . . . that the introduce more complexity" into the model for how TCDD works, Birnbaum observes. Each added level of complexity introduces a point that could require the buildup of certain minimum TCDD levels before triggering the next event in some chain of reactions that ultimately culminates in toxicity. If that is true, she says, TCDD might indeed prove nontoxic at levels below the threshold. Gasiewicz also points out that a need for the second protein may explain why TCDD's apparent toxicity varies so widely among different tissues and species.

In April, following the Cold Spring Harbor meeting, EPA Administrator William K. Reilly directed his agency to conduct a year-long reassessment of TCDD's health risks in light of the new model for this dioxin's actions.

"What EPA is going to do over the next year is work up a receptor-based model for dioxin," says Birnbaum. But, she cautions, "we don't yet know if those risk assessment models are going to be more, or less, restrictive" than current guidelines for dioxin exposure.

Ellen Silbergeld, an environmental toxicologist at the University of Maryland School of Medicine in Baltimore contends that "it would be extremely premature to make global regulatory decisions" based on the assumption that extra receptor-binding steps might mean there is a safe level of TCDD. Silbergeld, who is also an adjunct scientist to the Environmental Defense Fund, says there is no evidence that the dioxin receptor needs a specific accumulation of TCDD before it binds.