Lung carcinogenesis by tobacco smoke
Stephen S. Hecht
Masonic Cancer Center, University of Minnesota, Minneapolis, MN
Cigarette smoke is a complex mixture of chemicals including multiple genotoxic lung carcinogens. The classic mechanisms of
carcinogen metabolic activation to DNA adducts, leading to miscoding and mutations in critical growth control genes, applies
to this mixture but some aspects are difficult to establish because of the complexity of the exposure. This article discusses
certain features of this mechanism including the role of nicotine and its receptors; lung carcinogens, co-carcinogens and
related substances in cigarette smoke; structurally characterized DNA adducts in the lungs of smokers; the mutational
consequences of DNA adduct formation in smokers’ lungs; and biomarkers of nicotine and carcinogen uptake as related to
lung cancer. While there are still uncertainties which may never be fully resolved, the general mechanisms by which cigarette
smoking causes lung cancer are well understood and provide insights relevant to prevention of lung cancer, the number one
cancer killer in the world, causing 1.37 million deaths per year.
The new century has witnessed a continuing decrease in age-
adjusted lung cancer mortality in the United States, from
55.4 per 100,000 persons in 1999 to 50.7 in 2007, the most
recent year for which data are available (http://wonder.-
cdc.gov/). This encouraging positive trend, which has resulted
almost entirely from decreased cigarette smoking, illustrates
the power of cancer prevention. However, the facts about
lung cancer are still undeniably ugly. It is estimated that
156,940 lung cancer deaths will have occurred in the U.S. in
2011.1Lung cancer accounted for 1.37 million deaths in the
world in 2008 (available at: http://www.who.int/mediacentre/
factsheets/fs297/en/index.html). Ninety percent of this unima-
ginable death toll was caused by cigarette smoking in popula-
tions with prolonged use.2An estimated 1 billion men and
250 million women in the world are smokers. Male smoking
prevalence is particularly high in parts of eastern Europe and
Asia, while female smoking prevalence is highest in some
parts of eastern Europe.3Apparently, the now common
knowledge of the relationship between smoking and lung
cancer cannot fully counteract the marketing prowess of the
tobacco industry and the addictive power of nicotine. Still,
there are some hopeful signs on the horizon with the passing
of the Framework Convention on Tobacco Control by the
World Health Organization and the Family Smoking Preven-
tion and Tobacco Control Act by the U.S. government, the
latter giving the U.S. Food and Drug Administration the
power to regulate tobacco products.
A better understanding of mechanisms of lung carcino-
genesis by cigarette smoke is likely to provide new insights
on cancer prevention. This article will present a mechanistic
framework for understanding how cigarette smoking causes
lung cancer, focusing on genotoxic carcinogens and their
DNA addition products (commonly called adducts). Specific
aspects of this mechanism will be discussed, with attention to
some more recent developments or to gaps in the literature.
The pathways by which tobacco products cause lung cancer
essentially recapitulate established mechanisms of carcinogen-
esis by individual compounds, which were elucidated in clas-
sic studies during the late 20th century. It is the complexity
of tobacco carcinogenesis due to the presence of multiple
carcinogens and toxicants, however, which continues to
challenge investigators to identify specific mechanisms that
fully explain the ways in which smoking causes lung cancer.
Overall Mechanism of Lung Carcinogenesis by
Figure 1 presents the current version of a mechanistic frame-
work for understanding how cigarette smoking causes lung
cancer. This figure has evolved somewhat since its first pre-
sentation which featured only its current central track,4but
the central track is still the main player. Each box and hori-
zontal arrow in the central track represents a significant event
or combination of events that drive the process toward can-
cer, while vertical arrows are protective. Details of these path-
ways have been discussed in the 2010 U.S. Surgeon General
Report entitled ‘‘How Tobacco Smoke Causes Disease’’ avail-
able at: http://www.surgeongeneral.gov/library/tobaccosmoke/
report/index.html and an overview is presented here.
People start smoking at a relatively young age, generally
as teen-agers, before they fully appreciate the addictive power
of cigarettes and general issues of mortality. They become
Key words: tobacco smoke, carcinogens, DNA adducts, nicotine
Grant sponsor: U.S. National Cancer Institute; Grant numbers:
CA-81301, CA-92025, and CA-138338
History: Received 5 Apr 2012; Accepted 9 Aug 2012; Online 4 Sep
Correspondence to: Stephen S. Hecht, Masonic Cancer Center,
University of Minnesota, MMC 806, 420 Delaware St. SE,
Minneapolis, MN 55455, USA, Tel.: 612-624-7604, Fax:
612-626-5135, E-mail: email@example.com
Special Section Paper
Int. J. Cancer: 131, 2724–2732 (2012) V
C 2012 UICC
International Journal of Cancer
addicted to nicotine and cannot break the cigarette habit,
leading to years of smoking despite their best intentions. Nic-
otine, perhaps in concert with other tobacco smoke constitu-
ents, is highly addictive. Nicotine is not a carcinogen. But
each puff of each cigarette results in the delivery of a mixture
of carcinogens and toxicants along with nicotine. Over 5,000
compounds have been identified in cigarette smoke, including
73 compounds which are considered carcinogenic to either
laboratory animals or humans by the International Agency
for Research on Cancer.5,6
Most cigarette smoke carcinogens are substrates for drug
metabolizing enzymes such as the cytochromes P450, gluta-
thione S-transferases, and UDP-glucuronosyl transferases
which catalyze their conversion to more water soluble forms
that are detoxified and can be readily excreted. But during
this process, reactive intermediates such as carbocations or
epoxides are produced and these electrophilic compounds
can react with nucleophilic sites in DNA such as the nitrogen
or oxygen atoms of deoxyguanosine and other DNA bases.
The result is the formation of DNA adducts which are criti-
cal in the carcinogenic process. We know that DNA adduc-
tion is important because evolution has dictated the develop-
ment of DNA repair enzymes that can fix damaged DNA.
People with rare syndromes in which DNA repair is defi-
cient, such as Xeroderma pigmentosum, are highly prone to
cancer development. If the DNA adducts persist unrepaired,
they can cause miscoding during DNA replication as bypass
polymerases catalyze the insertion of the wrong base opposite
the adduct. The result is a permanent mutation. If this muta-
tion occurs in a critical region of an oncogene such as KRAS
or a tumor suppressor gene such as TP53, the result is unde-
niably loss of normal cellular growth control mechanisms
and development of cancer. Multiple recent studies using
next generation sequencing methods demonstrate the pres-
ence of thousands of mutations in the lungs of smokers,
including in critical growth regulatory genes, most frequently
KRAS and TP53, but others as well.7–10
Some tobacco smoke constituents such as nicotine and
tobacco-specific nitrosamines bind directly to cellular recep-
tors without a metabolic activation process. This can lead to
activation of Akt, PKA and other pathways which can con-
tribute to the carcinogenic process.11Furthermore, cigarette
smoke contains compounds that can induce inflammation
resulting in enhanced pneumocyte proliferation,12as well as
co-carcinogens, tumor promoters, inducers of oxidative dam-
age and gene promoter methylation, all processes which
undoubtedly contribute to lung cancer development. In the
following, some specific aspects of this general mechanism
are discussed in more detail.
Nicotine is Not a Carcinogen
Nicotine (Fig. 1, first box, central track) is the reason that
people cannot stop smoking, but it is not the cause of lung
cancer. There has been a steady stream of recent studies
(reviewed in Refs. 13 and 14). The results of these studies
have shown among other effects increased cell proliferation,
inhibition of apoptosis, stimulation of cancer cell growth, and
enhancement or inhibition of angiogenesis. Nicotine was also
reported to promote tumor growth and metastasis in cancer
xenograft models. Some lung tumors were induced in ham-
sters treated with nicotine under hypoxic conditions. Collec-
tively, these results have conveyed a message: nicotine may
be a carcinogen, tumor promoter, or co-carcinogen, and it
has been suggested to be the ‘‘estrogen of lung cancer.’’ This
is despite numerous negative carcinogenicity studies of nico-
tine in laboratory animals. The question of nicotine’s possible
carcinogenicity is highly relevant to mechanisms of carcino-
genesis by cigarette smoking and to nicotine replacement
therapy, a smoking cessation aid used by millions of former
Two recent studies have squarely addressed this issue. In
one, Murphy et al. treated A/J mice with nicotine in the
drinking water (0.44 lmol/ml) resulting in a daily dose to
the mouse on a mg/kg basis which was much higher than
that experienced by a smoker.13The A/J mouse is extremely
susceptible to lung tumor induction by carcinogens. Many
compounds, including some which were ultimately shown to
Figure 1. Mechanistic framework for understanding how cigarette smoking causes lung cancer. All events can occur chronically since a
smoker typically uses multiple cigarettes per day for many years.
Special Section Paper
Int. J. Cancer: 131, 2724–2732 (2012) V
C 2012 UICC
smoke inhalation in Syrian golden hamsters. J
Natl Cancer Inst 1973;51:1781–832.
59. Pfeifer GP, Denissenko MF, Olivier M, et al.
Tobacco smoke carcinogens, DNA damage and
p53 mutations in smoking-associated cancers.
60. United States Department of Health and Human
Services. How tobacco smoke causes disease: the
biology and behavioral basis for smoking-
attributable disease: a report of the surgeon
general. Washington, D.C.: U.S. Department of
Health and Human Services, 2010. Chapter 5.
61. Munnia A, Bonassi S, Verna A, et al. Bronchial
malondialdehyde DNA adducts, tobacco smoking,
and lung cancer. Free Radic Biol Med 2006;41:
62. Anna L, Kovacs K, Gyorffy E, et al. Smoking-
related O4-ethylthymidine formation in human
lung tissue and comparisons with bulky DNA
adducts. Mutagenesis 2011;26:523–7.
63. Chou PH, Kageyama S, Matsuda S, et al.
Detection of lipid peroxidation-induced DNA
adducts caused by 4-oxo-2(E)-nonenal and 4-
oxo-2(E)-hexenal in human autopsy tissues.
Chem Res Toxicol 2010;23:1442–8.
64. Boysen G, Hecht SS. Analysis of DNA and
protein adducts of benzo[a]pyrene in human
tissues using structure-specific methods. Mutat
65. Rojas M, Alexandrov K, Cascorbi I, et al. High
benzo[a]pyrene diol-epoxide DNA adduct levels
in lung and blood cells from individuals with
combined CYP1A1 MspI/MspI-GSTM1*0/*0
genotypes. Pharmacogenetics 1998;8:109–18.
66. Pfeifer GP, Besaratinia A. Mutational spectra of
human cancer. Hum Genet 2009;125:
67. Kucab JE, Phillips DH, Arlt VM. Linking
environmental carcinogen exposure to TP53
mutations in human tumours using the human
TP53 knock-in (Hupki) mouse model. FEBS J
68. Smith LE, Denissenko MF, Bennett WP, et al.
Targeting of lung cancer mutational hotspots by
polycyclic aromatic hydrocarbons. J Natl Cancer
69. Denissenko MF, Pao A, Tang M, et al.
Preferential formation of benzo[a]pyrene adducts
at lung cancer mutational hot spots in P53.
70. Tretyakova NT, Matter B, Jones R, et al.
Formation of benzo[a]pyrene diol epoxide-DNA
adducts at specific guanines within K-ras and p53
gene sequences: stable isotope-labeling mass
spectrometry approach. Biochemistry 2002;41:
71. Matter B, Wang G, Jones R, et al. Formation of
diastereomeric benzo[a]pyrene diol epoxide-
guanine adducts in p53 gene-derived DNA
sequences. Chem Res Toxicol 2004;17:731–41.
72. Feng Z, Hu W, Hu Y, et al. Acrolein is a major
cigarette-related lung cancer agent: preferential
binding at p53 mutational hotspots and
inhibition of DNA repair. Proc Natl Acad Sci
73. Zhang S, Villalta PW, Wang M, et al. Detection
and quantitation of acrolein-derived 1,N2-
propanodeoxyguanosine adducts in human lung
by liquid chromatography-electrospray
ionization-tandem mass spectrometry. Chem Res
74. Zhang S, Balbo S, Wang M, et al. Analysis of
adducts in human leukocyte DNA from smokers
and nonsmokers. Chem Res Toxicol 2011;24:
75. Hecht SS, Yuan J-M, Hatsukami DK. Applying
tobacco carcinogen and toxicant biomarkers in
product regulation and cancer prevention. Chem
Res Toxicol 2010;23:1001–8.
76. International Agency for Research on Cancer.
Dry cleaning, some chlorinated solvents and
other industrial chemicals. IARC monographs on
the evaluation of carcinogenic risks to humans,
vol. 63. Lyon, France: IARC, 1995. 393–407.
77. DeMarini DM. Genotoxicity of tobacco smoke
and tobacco smoke condensate: a review. Mutat
78. de Waard F, Kemmeren JM, van Ginkel LA,
et al. Urinary cotinine and lung cancer risk in a
female cohort. Br J Cancer 1995;72:784–7.
79. Boffetta P, Clark S, Shen M, et al. Serum cotinine
level as predictor of lung cancer risk. Cancer
Epidemiol Biomarkers Prev 2006;15:1184–8.
80. Yuan JM, Koh WP, Murphy SE, et al. Urinary
levels of tobacco-specific nitrosamine metabolites
in relation to lung cancer development in two
prospective cohorts of cigarette smokers. Cancer
81. Hecht SS. Human urinary carcinogen metabolites:
biomarkers for investigating tobacco and cancer.
82. Church TR, Anderson KE, Caporaso NE, et al. A
prospectively measured serum biomarker for a
tobacco-specific carcinogen and lung cancer in
smokers. Cancer Epidemiol Biomarkers Prev 2009;
83. Hecht SS, Chen M, Yagi H, et al. r-1,t-2,3,c-4-
human urine: a potential biomarker for assessing
polycyclic aromatic hydrocarbon metabolic
activation. Cancer Epidemiol Biomarkers Prev
84. Hecht SS, Carmella SG, Villalta PW, et al.
Analysis of phenanthrene and benzo[a]pyrene
tetraol enantiomers in human urine: relevance to
the bay region diol epoxide hypothesis of
benzo[a]pyrene carcinogenesis and to biomarker
studies. Chem Res Toxicol 2010;23:900–9.
85. Spitz MR, Etzel CJ, Dong Q, et al. An expanded
risk prediction model for lung cancer. Cancer
Prev Res 2008;1:250–4.
86. Cassidy A, Myles JP, van Tongeren M, et al. The
LLP risk model: an individual risk prediction
model for lung cancer. Br J Cancer 2008;98:
87. Bach PB, Kattan MW, Thornquist MD, et al.
Variations in lung cancer risk among smokers.
J Natl Cancer Inst 2003;95:470–8.
88. Etzel CJ, Kachroo S, Liu M, et al. Development
and validation of a lung cancer risk prediction
model for African-Americans. Cancer Prev Res
(Phila Pa) 2008;1:255–65.
89. Cronin KA, Gail MH, Zou Z, et al. Validation of
a model of lung cancer risk prediction among
smokers. J Natl Cancer Inst 2006;98:637–40.
90. Tammemagi CM, Pinsky PF, Caporaso NE, et al.
Lung cancer risk prediction: prostate, lung,
colorectal and ovarian cancer screening trial
models and validation. J Natl Cancer Inst 2011;
91. Etzel CJ, Bach PB. Estimating individual risk for
lung cancer. Semin Respir Crit Care Med 2011;32:
Special Section Paper
Tobacco smoke carcinogens
Int. J. Cancer: 131, 2724–2732 (2012) V
C 2012 UICC