“The whole is greater than the sum of its parts.”
– Aristotle (384 BC-322 BC)
Aristotle was talking about metaphysics and the emergent theory, but his insight corresponds to an important emerging theory in environmental health: combinations of different chemicals acting together in our bodies can produce larger (or different) effects than would be seen if each chemical were acting independently. In technical terms, this is called “synergism.”
Why does this matter? Through the course of our daily lives, we are all exposed to hundreds of different types of chemicals. Most laboratory toxicity studies, however, only assess the effects of a single compound in a carefully controlled environment. Consequently, the (very limited) data that we have on chemical hazard do not actually reflect real-world exposure situations (ie: co-exposures to mixtures of chemicals). Researchers are beginning to address this deficiency, though, and initial results suggest that Aristotle’s ancient wisdom is eerily relevant to modern-day toxicology.
A recent study published in the journal Toxicological Sciences examined the interaction between polycyclic aromatic hydrocarbons (PAHs) and arsenic. PAHs are organic pollutants that are produced during combustion processes (including from tobacco). Many PAHs, such as benzo[a]pyrene, can cause DNA damage and are known or suspected to cause cancer. Arsenic is a naturally occurring element that can exist in different chemical forms. The inorganic form As+3 can interfere with DNA repair and is linked to skin diseases and cancer. Human exposure to As+3 often occurs through ingestion of contaminated drinking water or rice-based products. Many people around the world are exposed to both PAHs and inorganic arsenic simultaneously, but little is known about how these two chemicals — one that causes DNA damage, and one that interferes with DNA repair – act together in the body.
For this work, researchers examined the effects of As+3 and three specific PAHs (benzo[a]pyrene and two metabolites, BP-Diol and BPDE) separately and together in mouse thymus cells (precursors to T-cells). Because T-cells serve a critical function in the immune system, chemical damage could lead to immune dysfunction.
After chemical treatment, the researchers measured the amount of DNA damage and DNA repair inhibition. At specific combinations of doses (one with As+3 and BP-Diol, and one with As+3 and BPDE), they saw a larger effect from treatment with two chemicals simultaneously than what would have been predicted from treatment with the same chemicals individually.
Next, they measured cell death (specifically, apoptosis) and found that while individual exposures to As+3 and BP-Diol did not increase death, exposure to the compounds together caused a synergistic increase in the percentage of dead cells. One possible explanation for this result is that at low levels of separate exposure, the body can adapt to prevent damage. But perhaps with the two chemicals together, the system is overwhelmed and cannot compensate.
Overall, based on these and other related results in this study, the researchers hypothesized that the As+3 increases the toxicity of certain PAHs through its ability to inhibit DNA repair pathways. As I noted above, PAHs alone can cause DNA damage. With the addition of As+3, which interferes with DNA repair during normal cell cycle replication, cell damage is even greater.
Previous work had documented the existence of similar interactions between PAHs and arsenic, but those studies had used high doses that were not representative of potential human exposures. This study, by contrast, investigated the effects of low-level exposures that are more similar to what we might encounter in the environment.
One important caveat of this work is that the researchers conducted the experiment in isolated mouse thymus cells. In vitro systems (or “test tube experiments”) are increasingly common in toxicology, as the field aims to find alternatives to whole animal testing. However, there are obvious limitations to these models. Not only are mouse cells different from human cells, but these mouse thymus cells are separated from the rest of their system and may not represent how a fully functional organism responds and/or adapts to a toxicant exposure. As follow-up, researchers should test this chemical combination in a relevant animal model to see whether similar results are obtained.
Nevertheless, this study provides important evidence of synergistic effects from low-level exposures to two common environmental contaminants. And these data may be just the tip of the iceberg. What other potential interactions exists between the thousands of other chemicals that we are exposed to over the course of our lives? The challenge with synergistic interactions is that they cannot always be predicted from testing individual chemicals. (I’ve written about this previously, specifically with regard to cancer processes.) It is daunting to think about testing all of the potential combinations that may exist, since our public health agencies are struggling to generate even basic toxicity data on all of these chemicals individually.
I wish we could consult Aristotle on this problem.
One strategy to start to address this challenge could be to prioritize testing combinations of chemicals that – like the pair chosen in the study described here – are most common across the population. Existing biomonitoring efforts, such as the U.S. National Health and Nutrition Examination Study (NHANES), could guide the selection of appropriate mixtures. Testing these highly relevant chemical combinations could provide valuable information that could be immediately translated into risk calculations or regulatory standards.
As Plato, another great ancient thinker said, “the beginning is the most important part of the work.” So, while it is definitely overwhelming to think about tackling the question of chemical combinations, it is crucial that we take first steps to make a start.