Daniel W. Nebert Discusses Cancer Causing Polycyclic Aromatic Hydrocarbons
Emerging Research Front Commentary, December 2010
 (Commentary for Oct. 2010 [late entry]) |
Article: Role of aryl hydrocarbon receptor-mediated induction of the CYP1 enzymes in environmental toxicity and cancer
Authors: Nebert, DW;Dalton, TP;Okey,
AB;Gonzalez, FJ |
Daniel W. Nebert talks with ScienceWatch.com and answers a few questions about this month's Emerging Research Front paper in the field of Biology & Biochemistry.
Why do you think your paper is highly
cited?
This 2004 mini-Review in J. Biol. Chem. was simply a snapshot-in-time, summarizing much of the data that my laboratory and many other labs have contributed to this topic for the previous 36 years: since 1968 when I was a postdoctoral fellow in the National Cancer Institute.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
Although this field includes a series of exciting discoveries and uses many new methodologies over the past 36 years, this mini-Review represents a synthesis of all that knowledge in this rapidly moving field.
Would you summarize the significance of your paper
in layman's terms?
Polycyclic aromatic hydrocarbons (PAHs) are environmental pollutants that can be found anywhere. PAHs are present as unwanted byproducts of combustion: cigarette smoke, gas- and diesel-engine exhaust, urban smog, barbecued and charcoal-grilled food, creosote-soaked wood, burning of incense and resins, and coal-tar-oil-and-coke distillation ovens in the petroleum industry.
If we are constantly eating, inhaling, and exposing our skin to PAHs, how does our body handle this foreign material? How do plants and bacteria handle PAHs? These questions are particularly important because we know that PAHs can cause cancer. This topic might have extra relevance in 2010 because of the massive oil spill in the Gulf of Mexico.
This paper therefore summarizes, as of early 2004, many of the major details concerning the properties of three genes and the PAH-metabolizing enzymes they encode, plus the receptor that regulates these three genes. We highlight the roles of these three genes and receptor in causing toxicity and cancer in lab animals, and how these findings might be related to human disease including cancer.
My three coauthors include colleagues who have spent time in my lab and have made major contributions specifically to the P450 field: Allan B. Okey (sabbatical 1978-79), Frank J. Gonzalez (postdoctoral fellow 1982-84), and Timothy P. Dalton (postdoctoral fellow, then Research Assistant Professor 1997-2006). When speaking of "the Nebert lab," I should emphasize that without the help of the 97 postdocs and grad students who have passed through my lab from 1970 to 2010, this exciting work could not have been accomplished.
How did you become involved in this research, and
how would you describe the particular challenges, setbacks, and
successes that you've encountered along the way?
"...perhaps a better understanding of 'the AHR/CYP1 axis' could lead to preventive measures by way of identifying novel drug targets."
As a postdoctoral fellow, I was among the first to open up this entire research field. I showed that hamster fetal secondary cell cultures exhibited no detectable enzyme activity for breaking down PAHs; however, in the presence of one or another PAH added to the cell culture medium, a mysterious enzyme activity (which we named "aryl hydrocarbon hydroxylase"; AHH) became strikingly increased (100- or 200-fold) during the next 6 to 24 hours in response to this "foreign signal".
During the next few years, multiple labs as well as our own determined in laboratory animals that AHH activity is required for the metabolic activation of "chemically inert" PAHs to form reactive oxygenated intermediates capable of binding to DNA and proteins and associated with increased risk of mutations, cancer, birth defects, and other forms of toxicity and oxidative stress.
My lab showed in 1969 that AHH activity was a member of a particular class of enzymes called "cytochromes P450;" AHH specifically reflected what we initially called "cytochrome P1-450," which in the late 1980s became officially named "CYP1A1". A second evolutionarily-related gene was first called "P-448" and now has the name "CYP1A2." Whereas CYP1A2 can metabolize PAHs to a small extent, N-arylamines are the principle substrates for CYP1A2.
Although our lab between 1975 and 1980 had published several lines of evidence in mouse and rabbit, showing that P1-450 and P-448 were enzymes derived from separate genes, it was not until we had the cDNA sequences in hand (1981-84) that most competitors were convinced that we had been correct all along.
In 1979 my lab showed that fish have the CYP1A1, but not the CYP1A2, gene—suggesting that CYP1A2 is a more recent gene arising from CYP1A1 as the result of gene duplication. As evidence of my devotion to this research, my automobile's license plate is "CYP1A1" whereas my wife's is "CYP1A2."
In the 1980s my lab was the first to clone and sequence the mouse, and then the human, CYP1A1 and CYP1A2 genes, and even the fish (trout) CYP1A1 gene. Around 1992, the CYP1B1 gene was identified in mouse, rat, and human by other labs; that enzyme was shown also to exist in most tissues of the body and to metabolize PAHs.
The Gonzalez lab generated Cyp1a2 and Cyp1b1 "knockout" mouse lines in 1995 and 1999, respectively, whereas my lab reported the creation of Cyp1a2 and Cyp1a1 knockout mouse lines in 1996 and 2001, respectively. Our labs together reported the Ahr knockout mouse line in 1995. Together, and in combination, these knockout mouse lines have been exceedingly helpful in delineating the roles of each of these genes in various types of PAH- and N-arylamine-induced toxicity and cancer—and highlights of such studies are included in the 2004 review.
In the late 1970s, virtually all other labs in the field believed they could explain everything with six or fewer P450 genes in the mouse, rat or human. At a cytochrome P450 symposium in Ireland In 1978, I proposed that we would discover dozens, if not hundreds, of P450 genes in each species of animals and plants; it took many years for the laughter and ridicule to subside.
We now know that the mouse genome has 102 Cyp functional protein-coding genes, the human has 57 CYP genes, and plants such as rice and the black poplar tree have several hundred P450 genes. In fact, even some bacteria have been found to have 20 or more Cyp genes. At latest count, David Nelson's web page has categorized more than 12,000 P450 genes and he expects to surpass 18,000 before year's end. This explosion in numbers of new CYP genes is of course the reflection of numerous new genomes being sequenced now, and reported practically every week.
In 1974, in collaboration with Alan Poland who had visited my lab, we showed evidence for a receptor that regulates AHH inducibility; we named it "aryl hydrocarbon receptor" (AHR). At first, this finding was viewed as "impossible" by many—how could a receptor already exist in an animal (also a human), to "receive" a "signal" from PAHs and other foreign chemicals and then proceed to regulate many genes? This seems especially impossible, if the animal had never before seen a PAH molecule.
"PAHs are present as unwanted byproducts of cigarette smoke, gas- and diesel-engine exhaust, urban smog, barbecued and charcoal-grilled food, creosote-soaked wood, burning of incense and resins, and coal-tar-oil-and-coke distillation ovens in the petroleum industry."
After the AHR gene was cloned in 1992 (in mouse, then in human), we now realize that this gene dates back to the fly and the worm, i.e., the AHR gene has been on this planet more than 500 million years and carries out crucial life functions such as development of the nervous system, embryogenesis, cell cycle regulation, and programmed cell death.
During the past several decades, it has become increasingly clear that every foreign toxicant simply utilizes receptors, transporters, and enzymes that Mother Nature has already invented for normal physiological functions; this has now become a basic tenet in the field of Toxicology.
Finally, this 2004 J. Biol. Chem. review summarizes the importance of the knockout mouse lines in showing how PAHs are taken up by the body and how these enzymes both activate (to form dangerous intermediates) and detoxify these foreign chemicals to innocuous products. Extrapolations of these lab animal data to human populations remain as the challenge that lies ahead.
Where do you see your research leading in the
future?
We know for certain, in laboratory animals, that CYP1A1 and CYP1B1 metabolize PAHs, that CYP1A2 metabolizes N-arylamines, that all three enzymes can both enhance metabolic activation to form reactive intermediates or detoxify these incoming foreign chemicals (depending on the route-of-administration and the target organ), and that AHR is pivotal in its role of regulating these three enzymes.
We also know in many cases that these three enzymes sometimes can enhance lab animals' risk of particular types of cancer and toxicity. Clearly, humans are exposed daily to all forms of PAHs and arylamines. To establish in human populations a firm link between these four genes, and pathways in which these genes participate, is the largest goal in the future.
Do you foresee any social or political
implications for your research?
I see social, but not political, implications. In 1964 the US Surgeon General Luther Terry announced the clear association of cigarette smoking with increased risk of lung cancer; yet, more than four decades later, there are more smokers today than back in the 1960s. It is also clear that some people can smoke heavily for many decades without developing any type of cancer, while others who smoke much less wind up with cancer of the lung, head-and-neck, pancreas, esophagus, or urinary bladder (all are sites shown, via innumerable epidemiological studies, to be related to cigarette smoking).
This fact convincingly implies that there are "genetically susceptible" and "genetically resistant" individuals. It is reasonable to assume that heavy exposures to gas- and diesel-engine exhaust, urban smog, barbecued and charcoal-grilled food, creosote-soaked wood, burning of incense and resins, and coal-tar-oil-and-coke distillation in the petroleum industry would result in environmental cancers more so in "susceptible" than in "resistant" people.
Even the risk of these environmental exposures in causing birth defects is likely to be different between "susceptible" and "resistant" pregnant women. Thus, although it would be idealistic to think that we could identify susceptible individuals and convince them not to smoke or to avoid these above-mentioned exposures or occupations, this approach is probably not realistic.
Instead, perhaps a better understanding of "the AHR/CYP1 axis" could lead
to preventive measures by way of identifying novel drug targets. When one
considers free choices in society, however, these issues immediately become
far more complicated than basic science studies of AHR-regulated
CYP1A1/1A2/1B1 induction in the laboratory animal.
Daniel W Nebert, M.D.
Professor, Department of Environmental Medicine, and Center for
Environmental Genetics (CEG) at College of Medicine
Professor, Department of Pediatrics & Molecular Developmental Biology,
Division of Human Genetics at Children's Hospital
University of Cincinnati Medical Center
Cincinnati, OH, USA
KEYWORDS: LUNG-CANCER; HYDROXYLASE-ACTIVITY; AH RECEPTOR; DETERMINES SUSCEPTIBILITY; JAPANESE POPULATION; CYTOCHROMES P450; DEFICIENT MICE; GENE; MOUSE; POLYMORPHISMS.