Editing Benzo(a)pyrene (section)

== Toxicity ==
[[File:Benzo(a)pyrene numbered.png|thumb|right|Benzo[''a'']pyrene, showing the base [[pyrene]] ring and numbering and ring fusion locations according to [[IUPAC nomenclature of organic chemistry]].]]

=== Nervous system ===
[[Prenatal exposure]] of B''a''P in rats is known to affect learning and memory in rodent models. Pregnant rats eating B''a''P were shown to negatively affect the brain function in the late life of their offspring. At a time when [[synapse]]s are first formed and adjusted in strength by activity, B''a''P diminished [[NMDA]] receptor-dependent nerve cell activity measured as mRNA expression of the NMDA [[NR2B]] receptor subunit.<ref>{{Cite journal|pmc=2752856|year=2008|last1=McCallister|first1=M. M.|title=Prenatal Exposure to Benzo[''a'']pyrene Impairs Later-Life Cortical Neuronal Function|journal=NeuroToxicology|volume=29|issue=5|pages=846–854|last2=Maguire|first2=M|last3=Ramesh|first3=A|last4=Aimin|first4=Q|last5=Liu|first5=S|last6=Khoshbouei|first6=H|last7=Aschner|first7=M|last8=Ebner|first8=F. F.|last9=Hood|first9=D. B.|display-authors=3|doi=10.1016/j.neuro.2008.07.008|pmid=18761371}}.</ref>

=== Immune system ===
B''a''P has an effect on the number of [[white blood cell]]s, inhibiting some of them from differentiating into [[macrophage]]s, the body's first line of defense to fight infections. In 2016, the molecular mechanism was uncovered as damage to the macrophage membrane's lipid raft integrity by decreasing membrane cholesterol at 25%. This means less immunoreceptors [[CD32]] (a member of the Fc family of immunoreceptors) could bind to [[IgG]] and turn the white blood cell into a macrophage. Therefore, macrophage membranes become susceptible to bacterial infections.<ref>{{cite journal | title=Validation of research trajectory 1 of an Exposome framework: Exposure to benzo[''a'']pyrene confers enhanced susceptibility to bacterial infection. | vauthors=Clark RS, Pellom ST, Booker B, Ramesh A, Zhang T, Shanker A, Maguire M, Juarez PD, Patricia MJ, Langston MA, Lichtveld MY, Hood DB |display-authors=3| journal=Environ Research | year=2016 | volume=146 | pages=173–84 | doi=10.1016/j.envres.2015.12.027 | pmid=26765097| pmc=5523512 | bibcode=2016ER....146..173C }}</ref>

=== Reproductive system ===
In experiments with male rats, [[subchronic]] exposure to inhaled B''a''P has been shown to generally reduce the function of testicles and [[epididymis]] with lower sex steroid/testosterone production and sperm production.<ref>{{cite journal | last1 = Ramesh A1 | first1 = Inyang F | last2 = Lunstra | first2 = DD | last3 = Niaz | first3 = MS | last4 = Kopsombut | first4 = P | last5 = Jones | first5 = KM | last6 = Hood | first6 = DB | last7 = Hills | first7 = ER | last8 = Archibong | first8 = AE |display-authors=3| date = Aug 2008 | title = Alteration of fertility endpoints in adult male F-344 rats by subchronic exposure to inhaled benzo(a)pyrene | journal = Exp Toxicol Pathol | volume = 60 | issue = 4–5| pages = 269–80 | doi = 10.1016/j.etp.2008.02.010 | pmc = 3526104 | pmid = 18499416 }}</ref>

=== Carcinogenicity ===
B''a''P's metabolites are [[mutagen]]ic and highly [[carcinogen]]ic, and it is listed as a [[List of IARC Group 1 carcinogens|Group 1 carcinogen]] by the [[International Agency for Research on Cancer|IARC]]. Chemical agents and related occupations, Volume 10, A review of Human Carcinogens, IARC Monographs, Lyon France 2009 <ref>A review of human carcinogens—part F: chemical agents and related occupations</ref>

In June 2016, B''a''P was added as benzo[''def'']chrysene to the [[Registration, Evaluation, Authorisation and Restriction of Chemicals|REACH]] Candidate List of [[Substance of very high concern|Substances of very high concern]] for Authorisation.<ref>{{cite web|last1=European Chemicals Agency|title=ED/21/2016|url=http://echa.europa.eu/documents/10162/4b054c5b-8511-4a30-8ef8-35ab143b4fd0|website=ECHA|access-date=21 June 2016}}</ref>

Numerous studies since the 1970s have documented links between B''a''P and cancers.<ref>{{cite book|chapter-url = https://books.google.com/books?id=Fw_G7Sk382IC&q=Benzo%28a%29pyrene-7%2C8-dihydrodiol-9%2C10-epoxide+cas&pg=PA102|first = W.|last = Kleiböhmer|publisher = [[Elsevier]]|year = 2001|chapter = Polycyclic Aromatic Hydrocarbon (PAH) Metabolites|pages = 99–122|title = Environmental Analysis (Volume 3 of Handbook of Analytical Separations)|isbn = 978-0-08-050576-3}}</ref> It has been more difficult to link cancers to specific B''a''P sources, especially in humans, and difficult to quantify risks posed by various methods of exposure (inhalation or ingestion).<ref>{{cite journal | title=Lung Cancer Risk After Exposure to Polycyclic Aromatic Hydrocarbons: A Review and Meta-Analysis | journal=Environmental Health Perspectives | volume=112 | issue=9 | pages=970–978 | pmc=1247189 | year=2004 | last1=Armstrong | first1=B. | last2=Hutchinson | first2=E. | last3=Unwin | first3=J. | last4=Fletcher | first4=T. | pmid=15198916 | doi=10.1289/ehp.6895 }}</ref> A link between [[vitamin A]] deficiency and [[emphysema]] in [[smoking|smokers]] was described in 2005 to be due to B''a''P, which induces vitamin A deficiency in rats.<ref>{{Cite web| title=Benzopyrene and Vitamin A deficiency | work=Researcher links cigarettes, vitamin A and emphysema | url=http://www.tobacco.org/news/171229.html| access-date=March 5, 2005 }}</ref>

A 1996 study provided molecular evidence linking components in [[tobacco smoke]] to [[lung cancer]]. B''a''P was shown to cause genetic damage in lung cells that was identical to the damage observed in the [[DNA]] of most malignant [[lung]] tumours.<ref>{{Cite journal |last1=Denissenko |first1=M. F. |last2=Pao |first2=A. |last3=Tang |first3=M. |last4=Pfeifer |first4=G. P. |date=1996-10-18 |title=Preferential formation of benzo[a]pyrene adducts at lung cancer mutational hotspots in P53 |journal=Science |volume=274 |issue=5286 |pages=430–432 |doi=10.1126/science.274.5286.430 |pmid=8832894|bibcode=1996Sci...274..430D |s2cid=3589066 }}</ref>

Regular consumption of [[cooked meats]] has been epidemiologically associated with increased levels of [[colon cancer]]<ref>{{cite journal | pmid = 12351160 | volume=506–507 | title=Well-done red meat, metabolic phenotypes and colorectal cancer in Hawaii | date=September 2002 | journal=Mutat. Res. | pages=205–14 | last1 = Le Marchand | first1 = L | last2 = Hankin | first2 = JH | last3 = Pierce | first3 = LM | display-authors = etal | doi=10.1016/s0027-5107(02)00167-7}}</ref> (although this in itself does not ''prove'' carcinogenicity),<ref>{{cite journal | last1 = Truswell | first1 = AS | date = Mar 2002 | title = Meat consumption and cancer of the large bowel | journal = European Journal of Clinical Nutrition | volume = 56 | issue = Suppl 1| pages = S19–24 | pmid = 11965518 | doi=10.1038/sj.ejcn.1601349| s2cid = 23886438 | doi-access =  }}</ref>
A 2005 NCI study found an increased risk of [[colorectal adenoma]]s was associated with B''a''P intake, and more strongly with B''a''P intake from all foods.<ref>{{Cite journal |last1=Sinha |first1=Rashmi |last2=Kulldorff |first2=Martin |last3=Gunter |first3=Marc J. |last4=Strickland |first4=Paul |last5=Rothman |first5=Nathaniel |display-authors=3|date=August 2005 |title=Dietary benzo[a]pyrene intake and risk of colorectal adenoma |url=https://aacrjournals.org/cebp/article/14/8/2030/258146/Dietary-Benzo-a-Pyrene-Intake-and-Risk-of |journal=Cancer Epidemiology, Biomarkers & Prevention |volume=14 |issue=8 |pages=2030–2034 |doi=10.1158/1055-9965.EPI-04-0854 |pmid=16103456|s2cid=33819830 |doi-access=free }}</ref> 

The detoxification enzymes cytochrome P450 [[CYP1A1|1A1 (CYP1A1)]] and cytochrome P450 [[CYP1B1|1B1 (CYP1B1)]] are both protective and necessary for benzo[''a'']pyrene toxicity. Experiments with strains of mice engineered to remove ([[gene knockout|knockout]]) CYP1A1 and CYP1B1 reveal that CYP1A1 primarily acts to protect mammals from low doses of B''a''P, and that removing this protection accumulates large concentrations of B''a''P. Unless CYP1B1 is also knocked out, toxicity results from the [[bioactivation]] of B''a''P to benzo[''a'']pyrene -7,8-dihydrodiol-9,10-epoxide, the ultimate toxic compound.<ref>Data presented by [[Daniel W. Nebert]] in research seminars 2007</ref>{{better source|date=July 2016}}

=== Interaction with DNA ===
[[File:Benzo(a)pyrene metabolism.svg|thumb |center |540px |Metabolism of benzo<nowiki>[</nowiki>''a''<nowiki>]</nowiki>pyrene yielding the [[carcinogenic]] benzo<nowiki>[</nowiki>''a''<nowiki>]</nowiki>pyren-7,8-dihydrodiol-9,10-epoxide.]]
[[File:Benzopyrene DNA adduct 1JDG.png|thumb |right |200px |A [[DNA adduct]] (at center) of benzo[''a'']pyrene, the major [[mutagen]] in [[tobacco smoking|tobacco smoke]].<ref>Created from [http://www.rcsb.org/pdb/cgi/explore.cgi?pdbId=1JDG PDB 1JDG] {{Webarchive|url=https://web.archive.org/web/20080922150848/http://www.rcsb.org/pdb/cgi/explore.cgi?pdbId=1JDG |date=2008-09-22 }}</ref>]]
Properly speaking, B''a''P is a [[procarcinogen]], meaning that its mechanism of [[carcinogenesis]] depends on its enzymatic metabolism to B''a''P [[diol]] [[epoxide]]<ref name="DB-19730201">{{cite thesis |last=Bogdan |first=Dennis Paul|title=Interactions Of Benzopyrene And Nucleic Acids In The Presence Of A Mixed-Function Oxidase System - Ph.D. Dissertation (requires login). |url=https://www.proquest.com/docview/302781013 |id={{ProQuest|302781013}} |isbn=979-8661021359 |date=February 1, 1973 |publisher=[[University at Buffalo|State University of New York at Buffalo]] |access-date=May 9, 2021 }}</ref> It [[Intercalation (biochemistry)|intercalates]] in [[DNA]], and the electrophilic epoxide is attacked by nucleophilic [[guanine]] bases, forming a bulky guanine adduct.<ref name="DB-19730201" />
[[File:DG rxn with BPDE.jpg|DG rxn with BPDE]]

[[X-ray crystallography|X-ray crystallographic]] and [[nuclear magnetic resonance]] structure studies have shown how this binding distorts the DNA<ref>{{Cite journal |last1=Volk |first1=David E. |last2=Thiviyanathan |first2=Varatharasa |last3=Rice |first3=Jeffrey S. |last4=Luxon |first4=Bruce A. |last5=Shah |first5=Jamshed H. |last6=Yagi |first6=Haruhiko |last7=Sayer |first7=Jane M. |last8=Yeh |first8=Herman J. C. |last9=Jerina |first9=Donald M. |last10=Gorenstein |first10=David G. |display-authors=3|date=2003-02-18 |title=Solution structure of a cis-opened (10R)-N6-deoxyadenosine adduct of (9S,10R)-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene in a DNA duplex |journal=Biochemistry |volume=42 |issue=6 |pages=1410–1420 |doi=10.1021/bi026745u |pmid=12578353}}</ref> by perturbing the double-helical DNA structure. This disrupts the normal process of copying DNA and induces mutations, which explains the occurrence of [[cancer]] after exposure. This mechanism of action is similar to that of [[aflatoxin]] which binds to the N7 position of guanine.<ref>{{Cite journal |last1=Eaton |first1=D. L. |last2=Gallagher |first2=E. P. |date=1994 |title=Mechanisms of aflatoxin carcinogenesis |journal=Annual Review of Pharmacology and Toxicology |volume=34 |pages=135–172 |doi=10.1146/annurev.pa.34.040194.001031 |pmid=8042848}}</ref>

There are indications that benzo[''a'']pyrene diol epoxide specifically targets the protective [[p53]] gene.<ref>{{Cite journal |last1=Pfeifer |first1=Gerd P. |last2=Denissenko |first2=Mikhail F. |last3=Olivier |first3=Magali |last4=Tretyakova |first4=Natalia |last5=Hecht |first5=Stephen S. |last6=Hainaut |first6=Pierre |display-authors=3|date=2002-10-21 |title=Tobacco smoke carcinogens, DNA damage and p53 mutations in smoking-associated cancers |journal=Oncogene |volume=21 |issue=48 |pages=7435–7451 |doi=10.1038/sj.onc.1205803 |pmid=12379884|s2cid=6134471 }}</ref> This gene is a [[transcription factor]] that regulates the [[cell cycle]] and hence functions as a [[tumor suppressor]]. By inducing G ([[guanine]]) to T ([[thymidine]]) [[transversion]]s in transversion hotspots within [[p53]], there is a probability that benzo[''a'']pyrene diol epoxide inactivates the tumor suppression ability in certain cells, leading to cancer.

[[(+)-Benzo(a)pyrene-7,8-dihydrodiol-9,10-epoxide|Benzo[''a'']pyrene-7,8-dihydrodiol-9,10-epoxide]] is the carcinogenic product of three enzymatic reactions:<ref>{{cite journal | last1 = Jiang | first1 = Hao | last2 = Gelhaus | first2 = Stacy L. | last3 = Mangal | first3 = Dipti | last4 = Harvey | first4 = Ronald G. | last5 = Blair | first5 = Ian A. | last6 = Penning | first6 = Trevor M. |display-authors=3| year = 2007 | title = Metabolism of Benzo[''a'']pyrene in Human Bronchoalveolar H358 Cells Using Liquid Chromatography-Mass Spectrometry | journal = Chem. Res. Toxicol. | volume = 20 | issue = 9| pages = 1331–1341 | pmc=2423818 | pmid=17702526 | doi=10.1021/tx700107z}}</ref>
# Benzo[''a'']pyrene is first oxidized by [[cytochrome P450]] [[Cytochrome P450, family 1, member A1|1A1]] to form a variety of products, including (+)benzo[''a'']pyrene-7,8-epoxide.<ref>{{cite journal | last1 = Shou | first1 = M | last2 = Gonzalez | first2 = FJ | last3 = Gelboin | first3 = HV |author3-link=Harry Gelboin | date = December 1996 | title = Stereoselective epoxidation and hydration at the K-region of polycyclic aromatic hydrocarbons by cDNA-expressed cytochromes P450 1A1, 1A2, and epoxide hydrolase | journal = Biochemistry | volume = 35 | issue = 49| pages = 15807–13 | doi=10.1021/bi962042z | pmid=8961944}}</ref>
# This product is metabolized by [[epoxide hydrolase]], opening up the [[epoxide]] ring to yield (&minus;)benzo[''a'']pyrene-7,8-dihydrodiol.
# The ultimate carcinogen is formed after another reaction with [[Cytochrome P450 oxidase|cytochrome P450]] 1A1 to yield the (+)benzo[''a'']pyrene-7,8-dihydrodiol-9,10-epoxide. It is this diol epoxide that covalently binds to DNA.

B''a''P induces cytochrome P450 1A1 (CYP1A1) by binding to the AHR ([[aryl hydrocarbon receptor]]) in the cytosol.<ref name="Whitlock">{{Cite journal| last = Whitlock | first = JP Jr. | title = Induction of cytochrome P4501A1 | journal = Annual Review of Pharmacology and Toxicology | volume = 39 | pages = 103–125 |date = April 1999| doi = 10.1146/annurev.pharmtox.39.1.103 | pmid = 10331078}}</ref> Upon binding the transformed receptor translocates to the nucleus where it dimerises with ARNT ([[aryl hydrocarbon receptor nuclear translocator]]) and then binds [[xenobiotic response elements]] (XREs) in DNA located upstream of certain genes. This process increases [[Transcription (genetics)|transcription]] of certain genes, notably ''CYP1A1'', followed by increased CYP1A1 protein production.<ref name="Whitlock" /> This process is similar to induction of CYP1A1 by certain [[polychlorinated biphenyl]]s and [[Polychlorinated dibenzodioxins|dioxins]]. Seemingly, CYP1A1 activity in the intestinal mucosa prevents major amounts of ingested benzo[''a'']pyrene to enter portal blood and systemic circulation.<ref>{{cite journal |last1=Uno |first1=S. |year=2008 |title=Basal and inducible CYP1 mRNA quantitation and protein localization throughout the mouse gastrointestinal tract.|journal=Free Radic Biol Med |volume=44 |issue=4 |pages=570–83|pmid=17997381 |doi= 10.1016/j.freeradbiomed.2007.10.044|last2=Dragin|first2=N |last3=Miller |first3=ML |pmc=2754765|display-authors=etal}}</ref> Intestinal, but not hepatic, expression of CYP1A1 depends on TOLL-like receptor 2 ([[TLR2]]),<ref>{{Cite journal| last1 = Do| first1 = KN |last2=Fink|first2=LN |last3=Jensen |first3=TE |last4=Gautier |first4=L | last5=Parlesak |first5=A |display-authors=3|year=2012 |title = TLR2 controls intestinal carcinogen detoxication by CYP1A1. | journal = PLOS ONE| volume = 7 | issue = 3 | pages = e32309 | doi = 10.1371/journal.pone.0032309 | pmid = 22442665 | pmc=3307708| bibcode = 2012PLoSO...732309D | doi-access = free }}</ref> which is a eukaryotic receptor for bacterial surface structures such as [[lipoteichoic acid]].

Moreover, B''a''P has been found to activate a transposon, [[Retrotransposon|LINE1]], in humans.<ref>{{cite journal | last1 = Stribinskis | first1 = Vilius | last2 = Ramos | first2 = Kenneth S. | year = 2006 | title = Activation of Human Long Interspersed Nuclear Element 1 Retrotransposition by Benzo(a)pyrene, a Ubiquitous Environmental Carcinogen | journal = Cancer Res | volume = 66 | issue = 5| pages = 2616–20 | doi = 10.1158/0008-5472.CAN-05-3478 | pmid = 16510580 | doi-access = free }}</ref>