International Journal of Cancer Research and Prevention ISSN: 1554-1134
Volume 4, Number 3 © 2011 Nova Science Publishers, Inc.
Skin Cancer, Free Radicals and Antioxidants
B. Poljsak, U. Glavan, and R. Dahmane
Laboratory for Oxidative Stress Research, Faculty of Health Sciences,
University of Ljubljana, Slovenia
Human skin is constantly directly exposed to the air, solar radiation, other environmental
pollutants or other mechanical and chemical insults, which are capable of inducing the
generation of free radicals as well as reactive oxygen species (ROS) of our own metabolism.
Extrinsic skin damage develops due to several factors: ionizing radiation, severe physical and
psychological stress, alcohol intake, poor nutrition, overeating, environmental pollution, and
exposure to UV radiation (UVR). It is estimated that among all these environmental factors,
UVR contributes up to 80%. UV-induced generation of ROS in the skin develops oxidative
stress, when their formation exceeds the antioxidant defence ability of the target cell. The
primary mechanism by which UVR initiates molecular responses in human skin is via
photochemical generation of ROS mainly formation of superoxide anion (O2-˙), hydrogen
peroxide (H2O2), hydroxyl radical (OH˙), and singlet oxygen (1O2). Oxidative
phosphorylation in the mitochondria is an important energy-producing process for eukaryotic
cells, but this process can also result in producing potentially cell-damaging side products, e.g.
free radicals and other ROS. The only protection of our skin is its endogenous protection
(melanin and enzymatic antioxidants) and antioxidants we consumed with the food (vitamin
A, C, E, etc.). Dietary antioxidants thus play a major role in maintaining the homeostasis of
the oxidative balance. Vitamin C (ascorbic acid), vitamin E (tocopherol), beta-carotene and
other micronutrients such as carotenoids, polyphenols and selenium have been evaluated as
antioxidant constituents in the human diet. The most important strategy to reduce the risk of
sun UVR damage is to avoid the sun exposure and the use of sunscreens. The next step is the
use of exogenous antioxidants orally or by topical application and interventions in preventing
oxidative stress and in enhanced DNA repair.
Human skin is naked and is constantly directly exposed to the air, solar radiation, other
environmental pollutants or other mechanical and chemical insults, which are capable of
inducing the generation of free radicals as well as reactive oxygen species (ROS) of our own
metabolism. Reactive oxygen species are usually of little harm if intracellular mechanisms
that reduce their damaging effects work properly. Most important mechanisms include
antioxidative enzymatic and non-enzymatic defences as well as repair processes. But the
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B. Poljsak, U. Glavan, and R. Dahmane 194
problem arises with age, when endogenous antioxidative mechanisms and repair processes do
not work anymore in the effective way.
The identification of free radical reactions as initiators and promoters of the cancer
process implies that interventions aimed at limiting or inhibiting these factors should be able
to reduce the rate of cancer incidence. There still remains the answer regarding controversial
data on the use of synthetic antioxidants in cancer prevention and cancer treatment.
1. Free Radicals and Oxidative Stress
A free radical is a chemical species possessing an unpaired electron (Cheeseman and
Slater, 1993). It can also be considered a fragment of a molecule. Excess generation of free
radicals may overwhelm natural cellular antioxidant defences, leading to oxidation and
further cellular functional impairment. In reality, the oxidative damage potential is greater,
and thus there is a constant small amount of toxic free radical formation, which escapes the
defense of the cell. Oxidative stress is proportional to the concentration of free radicals which
depends on both processes (formation and quenching). The degree of oxidative stress
experienced by a cell will be a function of the activity of free radical generating reactions on
one hand, and the activity of the free radical scavenging system on the other.
UV-induced generation of ROS in the skin develops oxidative stress, when their
formation exceeds the antioxidant defence ability of the target cell (Katiyar and Mukhtar,
2001). Although the skin possesses an elaborate antioxidant defence system to deal with UV-
induced oxidative stress and immunotoxicity, excessive and chronic exposure to UV light can
overwhelm the cutaneous antioxidant and immune response capacity, leading to oxidative
damage and immunotoxicity, premature skin aging, and skin cancer. Acute exposure to UV
irradiation depletes the catalase activity in the skin and increases protein oxidation (Sander et
2. Oxidative Damage
Oxidative damage to both nuclear and mitochondrial DNA has detrimental effects,
leading to uncontrolled cell proliferation or accelerated cell death (Evans et al., 2004).
Furthermore, redox modification of transcriptional factors leads to the activation or
inactivation of signalling pathways, which will subsequently produce changes in gene
expression profiles (Martindale et al., 2002), including those affecting cellular proliferation,
differentiation, senescence and death (Kregel and Zhang, 2007). Damage to human skin due
to ultraviolet light from the sun (photoaging) and damage occurring as a consequence of the
passage of time (chronologic or natural aging) were considered to be distinct entities. The
findings of the study performed by Varani et al., (2000) indicate that naturally aged sun-
protected skin and photoaged skin share important molecular features including connective
tissue damage, elevated matrix metalloproteinase levels, and reduced collagen production.
The intrinsic (genetically determined) and the extrinsic (UV- and toxic exposure mediated)
skin damage processes are thus overlapped and are strongly related to the increased
generation of free radicals in the skin.
Skin Cancer, Free Radicals and Antioxidants 195
3. Beneficial Role of ROS
Oxidant agents, including reactive oxygen species (ROS), and reactive nitrogen species
(RNS) are recognized to play a dual role as both malefic and beneficial species, being
sometimes compared with fire, which is dangerous, but nonetheless useful to humans (De
Magalhaes and Church, 2006). The "two-faced" character of ROS is substantiated by growing
body of evidence that ROS within cells act as secondary messengers in intracellular signalling
cascades, which induce and maintain the oncogenic phenotype of cancer cells, however, ROS
can also induce cellular senescence and apoptosis and can therefore function as anti-
tumorigenic species. Low amounts of these ROS are important for cellular-signalling
pathways. In general, the balance between the production and scavenging of ROS leads to
homeostasis (Wittgen and van Kempen, 2007). But it seems that there is always a bit more
free radicals produced leading to constant oxidative stress and cell damage which
accumulates with time.
4. Extrinsic Skin Damage
Extrinsic skin damage develops due to several factors: ionizing radiation, severe physical
and psychological stress, alcohol intake, poor nutrition, overeating, environmental pollution,
and exposure to UV radiation (UVR). It is estimated that among all these environmental
factors, UV radiation contributes up to 80%. UV radiation is the most important
environmental factor in the development of skin cancer and skin aging (Poljsak, 2010).
4.1. UVR and ROS formation
UVR increases the ROS formation in the skin cells. The primary mechanism by which
UVR initiates molecular responses in human skin is via photochemical generation of ROS
mainly formation of superoxide anion (O2-˙), hydrogen peroxide (H2O2), hydroxyl radical
(OH˙), and singlet oxygen (1O2) (Hanson and Clegg, 2002). UVR penetrates the skin, reaches
the cells and is absorbed by DNA, leading to the formation of photoproducts that inactivate
the functions of DNA. UVA radiation acts mostly indirectly through the generation of ROS,
producing high amounts of singled oxygen which can further initiate lipid peroxidation,
oxidation of proteins or generation of DNA strand breaks (Scharffetter-Kochanek et al.,
2000). UVB action is mostly by direct interaction with DNA via the induction of DNA
damage. The epidermis and dermis are both affected by UVB, but the dermis is also affected
to a significant extent by UVA. UVA radiation constitutes an oxidant stress that involves the
generation of active species including singlet oxygen and hydroxyl radicals. Hydrogen
peroxide can be generated by UVA irradiation of tryptophan (McCormick et al., 1976), and
superoxide can be produced by UVA irradiation of NADH and NADPH (Cunningham et al.,
1985). The skin-damaging effects of UVA appear to result from type II, oxygen-mediated
photodynamic reactions in which UVA or near-UV radiation in the presence of certain
photosensitizing chromophores (e.g., riboflavin, porphyrins, nicotinamide adenine
dinucleotide phosphate (NADPH), etc.) leads to the formation of reactive oxygen species
B. Poljsak, U. Glavan, and R. Dahmane 196
(1O2, O2.-, .OH) (Dalle Carbonare and Pathak, 1992). As well as causing permanent genetic
changes involving protooncogenes and tumour suppressor genes, ROS activate cytoplasmic
signal transduction pathways that are related to growth differentiation, senescence,
transformation and tissue degradation (Scharffetter-Kochanek et al., 1997).
4.2. UV-Induced Skin Damage
According to Pattison and Davies (2006) UV radiation can mediate damage via two
different mechanisms: (a) direct absorption of the incident light by the cellular components,
resulting in excited state formation and subsequent chemical reaction, and (b)
photosensitization mechanisms, where the light is absorbed by endogenous (or exogenous)
sensitizers that are excited to their triplet states. The excited photosensitisers can induce
cellular damage by two mechanisms: (a) electron transfer and hydrogen abstraction processes
to yield free radicals (Type I); or (b) energy transfer with O2 to yield the reactive excited
state, singlet oxygen (Type II) (Pattison and Davies, 2006). Oxidation of DNA can produce
different types of DNA damage: strand breaks, sister chromatid exchange, DNA-protein
crosslinks, sugar damage, abasic sites, and base modifications. Cell death, chromosome
changes, mutation and morphological transformations are observed after UV exposure of
prokaryotic and eukaryotic cells. Numerous types of UV induced DNA damage have now
been recognized that include stand breaks (single and double), cyclobutane-type pyrimidine
dimers, 6-4 pyo photoproducts and the corresponding Dewar isomer, thymine glycols, 8-
hydroxy guanine, and many more. Additionally, the specific lesions in DNA which can be
induced by UVA radiation include pyrimidine dimmers, single-strand breaks (both not
thought to be the critical lesions in UVA radiation-induced cellular lethality), and, perhaps
more importantly, DNA protein crosslinks (Peak et al., 1987; Rosenstein and Ducore 1983;
Peak et al., 1988; Peak et al., 1985). The number of different DNA modifications that OH˙ is
capable of producing appears to be over 100 (Hutchinson, 1985). In addition, DNA-protein
cross-links are produced during UV exposure. Larger scale genetic alterations include
chromosome breakage, sister chromatid exchanges and chromatid aberrations. Although
partial UV action spectra are now available for many of these lesions, the most studied have
been the different types of pyrimidine dimers (International programme on chemical safety,
Environmental health criteria 160).
Besides oxidation of nuclear DNA, UVR can induce also oxidative damage to
mitochondrial DNA (mtDNA). It has been suggested that sunlight passing through the skin
can even cause DNA damage in white cells circulating through skin capillaries (Yang et al.,
2004) but the greatest damage is within the skin cells, including the damage to dermal
mitochondrial DNA (Wang et al., 2004). Singlet oxygen produced by UVA light has been
shown to cause strand breaks in the mitochondrial DNA, which has resulted in mtDNA
deletions. Mitochondrial DNA is believed to be the most critical target of endogenous ROS
production since it lies in the inner mitochondrial membrane, in close proximity to the
electron transport chain where the most free radicals are formed. In the past it was believed
that mitochondria lack DNA repair capacity but this is not true. However, it is true that
mitochondria do not remove UV induced DNA damage which might be important in
photodamage and skin cancer formation. There have been observed greater accumulation of
Skin Cancer, Free Radicals and Antioxidants 197
mtDNA found in sun exposed skin compared to protected skin (Berneburg et al., 1999; Birch-
Machin et al., 1998). The most frequent mutation is a 4,977-base pair deletion also called the
common deletion, which is increased in photoaged skin.
5. Intrinsic Skin Damage
The changes in our skin cells occur partially as the result of cumulative endogenous
damage due to the continuous formation of reactive oxygen species (ROS), which are
generated by oxidative cellular metabolism. Oxidative phosphorylation in the mitochondria is
an important energy-producing process for eukaryotic cells, but this process can also result in
producing potentially cell-damaging side products, e.g. free radicals and other ROS. Oxygen
is the final proton acceptor in this cascade of electron/proton transfer and results in harmless
water. The electron transfer, however, is not completely efficient and oxygen is not totally
reduced to water. It is estimated that approximately 1-3% oxygen is reduced to superoxide
instead to water.
There are two main sources of ROS: mitochondrial sources (which play the principal role
in aging) and non-mitochondrial sources (which have different, sometimes specific, roles
especially in the pathogenesis of age-related diseases). Most estimates suggest that the
majority of intracellular ROS production is derived from mitochondria. Mitochondrial
sources are represented by the electron transport chain and the nitric oxide synthase reaction.
The rate of mitochondrial respiration is responsible for the rate of generation of ROS. Fenton
reaction is an example of the non-mitochondrial source of ROS. The H2O2 degrading Fenton
reaction is catalyzed by the free iron bivalent ions and leads to the generation of OH˙. It
should be taken into account that body's content of iron increases with age (Koster and
Sluiter, 1995; Vercellotti, 1996). Sources of H2O2 could be mitochondria superoxide
dismutase reaction, peroxisomes (acyl-CoA oxidase reaction) and amyloid β of senile plaques
(superoxide dismutase-like reactions) (Rottkamp et al., 2001). Sources of superoxide (O2-˙)
are mitochondria, microsomes which contain the cytochrome P450 enzymes, the respiratory
burst of phagocytic cells and others. Estimates of how much oxygen reacts directly to
generate free radicals vary (Speakman, 2003). However, typically cited values are around
1.5–5% of the total consumed oxygen (Beckman and Ames, 1998b; Casteilla et al., 2001).
These estimates have been questioned by Hansford et al. (1997) and Staniek and Nohl (1999,
2000), which suggested that H2O2 production rates were less than 1% of consumed O2. Yet,
even if we accept a conservative value of 0.15%, this still represents a substantial amount of
free radicals (Speakman, 2003). Also the skin cells are constantly exposed to ROS and
oxidative stress from exogenous and endogenous sources. It has been found that in aged rat
skin the oxidized lipid phosphatidylcholine hydroperoxide (PCOOH) increases from 3.46
±1.02 μmol/PC mol at 6 months to 7.14 ±1.63 μmol/PC mol at 24 months. The free 7-hydro-
peroxycholesterol (ChOOH) content also increased from 22.83 ±3.97 at 6 month to 42.58 ±
16.59 μmol/ free Ch mol at 24 months. The TBARS (ThioBarbituric Acid Reactive
Substances, harmful substances formed by lipid peroxidation, and detected by the TBARS
assay, using thiobarbituric acid as a reagent) content increases from 4.71 ± 1.53 nmol/ mg
protein at 6 months to 11.10 ± 2.05 nmol/ mg protein at 30 months. The oxidized DNA in rat
skin also increases with age and reaches the level of 2.04 ± 0.27 8-oxoG/ 105 dG at 30 months
B. Poljsak, U. Glavan, and R. Dahmane 198
of age compared to 1.67 ± 0.16 8-oxoG/ 105 dG at 6 months of age. Results suggest that
chronic accumulation of oxidative damage occurs also in skin cells with age (Sivonova et al.,
2007; Tahara et al., 2001; Lasch et al., 1997)
The energy demand of skin cells comes from three sources: mitochondrial oxidative
phosphorylation, glycolysis and creatine/phosphocreatine system. All three major energy
sources are affected by intrinsic and extrinsic skin aging and offer potential entry points for
intervention strategies to decelerate the skin aging process (Blatt et al., 2010). Due to
impaired mitochondria with age, less energy is produced by mitochondrial oxidative
phosphorylation although the number of mitochondria does not change with age. Higher
energy demand needs higher energy production via non-mitochondrial pathways, such as
glycolysis. With advancing age energy production is mostly anaerobic. Primary keratinocytes
derived from old donors show a higher glucose uptake and the increased lactate production
which indicates a suboptimal utilization of glucose and a shift in metabolism towards an
increased glycolysis (Blatt et al., 2010).
Mostly skin tissues engage in, and derive energy using aerobic glycolysis. Despite the
presence of oxygen there is preferential conversion of glucose to lactate via the glycolytic
cycle (Krebs, 1972; Philpott and Kealey, 1991). This results in the production of substantial
amounts of lactate, which is carried to the liver by the bloodstream and converted back to
glucose (the Cory cycle). Skin tissues have a strong preference for the metabolism of glucose
rather than fatty acids or ketone bodies, though alternative citric acid cycle intermediates such
as glutamine are also actively utilized (Williams et al., 1993). Interestingly, of the relatively
small amount of oxygen that is metabolized by skin the majority is supplied to the epidermis
and upper dermis by diffusion from the atmosphere (Stucker et al., 2000). ROS and RNS are
constitutively produced also by endogenous sources in most cell types, including epidermal
keratinocytes and dermal fibroblasts (Fuchs, 1992; Darr and Fridovich, 1994). In addition to
stimulated ROS/RNS production by resident epidermal and dermal cells, these species as well
as reactive halogen species (RHS) can be produced and released into skin by invading
macrophages as well as polymorphonuclear and eosinophilic leukocytes.
6. Skin Antioxidant Defenses
Although the skin possesses an elaborate antioxidant defense system to deal with
oxidative stress, excessive and chronic exposure to UV light or cigarette smoke can
overwhelm the cutaneous antioxidant and immune response capacity, leading to oxidative
damage and immunotoxicity, premature skin aging, and skin cancer.
A biological antioxidant has been defined as any substance that when present at low
concentrations compared to those of an oxidizable substrate, significantly delays or prevents
oxidation of that substrate (Halliwell and Gutterigde, 1999). Antioxidant functions are
associated with lowering oxidative stress, DNA damage, malignant transformation, and other
parameters of cell damage in vitro as well as epidemiologically with lowered incidence of
certain types of cancer and degenerative diseases. Antioxidants attenuate the damaging effects
of ROS and can impair and/or reverse many of the events that contribute to epidermal toxicity
and disease. However, increased or prolonged free radical action can overwhelm ROS
defense mechanisms, contributing to the development of cutaneous diseases, disorders and
Skin Cancer, Free Radicals and Antioxidants 199
skin aging. The two main categories of antioxidant defences are those whose role is to prevent
the generation of ROS, and those that intercept any radicals that are generated (Cheeseman
and Slater, 1993). The defence system exists in aqueous and membrane compartments of cells
and can be enzymatic and non-enzymatic. A second category of natural antioxidants are
repair processes, which remove the damaged biomolecules before they accumulate to cause
altered cell metabolism or viability (Cheeseman and Slater, 1993).
The skin is equipped with a network of protective antioxidants. They include enzymatic
antioxidants such as glutathione peroxidase, superoxide dismutase and catalase, and
nonenzymatic low-molecular-weight antioxidants such as vitamin E isoforms, vitamin C,
glutathione (GSH), uric acid, and ubiquinol (Shindo et al., 1993).Various other components
present in skin are potent antioxidants including ascorbate, uric acid, carotenoids and
sulphydrils. Water-soluble antioxidants in plasma include glucose, pyruvate, uric acid,
ascorbic acid, bilirubin and glutathione. Lipid soluble anti-oxidants include alpha-tocopherol,
ubiquinol-10, lycopene, ß-carotene, lutein, zeaxanthin and alpha-carotene. In general, the
outer part of the skin, the epidermis, contains higher concentrations of antioxidants than the
dermis (Shindo et al., 1994a,b,c). In the lipophilic phase, α-tocopherol is the most prominent
antioxidant, while vitamin C and GSH have the highest abundance in the cytosol. On molar
basis, hydrophilic non-enzymatic antioxidants including L-ascorbic acid, GSH and uric acid
appear to be the predominant antioxidants in human skin (Thiele et al., 2006). Their overall
dermal and epidermal concentration are more than 10- to 100-fold greater than those found
for vitamin E or ubiquinol.
The antioxidant capacity of the human epidermis is far greater than that of dermis. This
was demonstrated in the study by Shindo et al., (1994a,b,c) where enzymic and non-enzymic
antioxidants in human epidermis and dermis from six healthy volunteers undergoing surgical
procedures was measured. A similar study was done by Shindo et al. (1993) where enzymic
and non-enzymic antioxidants in epidermis and dermis of hairless mice were compared.
Catalase, glutathione peroxidase, and glutathione reductase were higher in epidermis than
dermis. Lipophilic antioxidants (alpha-tocopherol, ubiquinol 9, and ubiquinone 9) and
hydrophilic antioxidants (ascorbic acid, dehydroascorbic acid, and glutathione) were also
higher in epidermis than in dermis. The stratum corneum (SC) was found to contain both
hydrophilic and lipophilic antioxidants. Vitamins C and E (both αγ and α-tocopherol) as well
as GSH and uric acid were found to be present in the SC (Weber et al., 1999; Thiele et al.,
1998). Surprisingly, they were not distributed evenly, but in gradient fashion, with low
concentrations on the outer layers and increasing concentrations toward the deeper layers of
the SC. This phenomenon may be explained by the fact that O2 partial pressure is higher in
the upper SC, which already causes a mild oxidative stress resulting in the partial depletion of
All the major antioxidant enzymes are present in skin but their role in protecting cells
against oxidative damage generated by UV radiation has not been elucidated. In response to
the attack of reactive oxygen species, the skin has developed a complex antioxidant defence
system including among others the manganese-superoxide dismutase (MnSOD). The study of
Poswig et al. (1999) revealed that adaptive antioxidant response of manganese-superoxide
dismutase following repetitive UVA irradiation can be induced. The authors provide evidence
for the increasing induction of MnSOD upon repetitive UVA irradiation that may contribute
to the effective adaptive UVA response of the skin during light hardening in phototherapy.
The study of Fuchs et al., (1989a,b) on mouse skin showed that acute UV exposures lead also
B. Poljsak, U. Glavan, and R. Dahmane 200
to changes in glutathione reductase and catalase activity in mouse skin but insignificant
changes in superoxide dismutase and glutathione peroxidase (Fuchs et al., 1989a,b). The
study of Sander et al. (2002) confirmed that chronic and acute photodamage is mediated by
depleted antioxidant enzyme expression and increased oxidative protein modifications.
DNA Repair Systems
Generation of ROS and the activity of antioxidant defence appear more or less balanced
in vivo. In fact, as already mentioned, the balance may be slightly tipped in favor of the ROS
so that there is continuous low-level oxidative damage in the human body. This creates a need
for a second category of endogenous antioxidant defence system, which removes or repairs
damaged biomolecules before they can accumulate and before their presence results in altered
cell metabolism. DNA is the most critical target for damage by UVA, UVB and UVC
radiations. Measurable DNA damage is induced in human skin cells in vivo after exposures to
UVA, UVB and UVC radiation, including doses in the range commonly experienced by
humans. A number of different DNA repair mechanisms have been established (Freifelder
1987), the best known being photoreactivation, excision repair, postreplication repair and sos
repair. Most of the DNA damage after a single exposure is repaired within 24 h. The
majorities of DNA lesions are repaired by BER (Base Excision Repair), NER (Nucleotide
Excision Repair), and MMR (Mismatch Repair) (Norbury and Hickson, 2001). DNA repair
capacity has been found to decrease with age. For example, decrease in the level of proteins
that participate in nucleotide excision repair was reported to occur for aged dermal fibroblasts
(Goukassian et al., 2000). The aging and survival of endothelial cells are linked to molecular
mechanisms that control cell proliferation, quiescence, apoptosis and senescence.
Hormesis effect activates the synthesis of melanin and antioxidant protection and
damaged lipids are cleaved and replaced. Irreparably cells are removed by apoptosis (Yarosh,
2003). However, these repair mechanisms are not 100% effective. The damaged components
are not always completely repaired. The problem arises in the cases of intensive acute sun
exposure or in the cases of chronical sun exposures over longer decades which manifests as
skin photoaging. Despite the fact that biological species, including man, are exposed to
potentially harmful levels of solar UVR, mechanisms have evolved to protect cells and to
repair damaged molecules
Apoptosis is a cellular end point of the stress response. Apoptosis removes damaged cells
from UV-irradiated tissues. If the cell damage cannot be repaired before the next cell division
the cell rather decides to “commit a suicide” than to spread mutations to its daughter cells.
Activation of apoptosis is associated with generation of reactive oxygen species. Superoxide
is produced by mitochondria isolated from apoptotic cells due to a switch from the normal 4-
electron reduction of O2 to a 1-electron reduction when cytochrome c is released from
mitochondria. Apoptosis is stimulated through release of mitochondrial cytochrome c, which
results in activation of death protease (caspase-3) and increased free radical generation due to
uncoupled respiration (Cai and Jones, 1998; Cai et al., 1998). Antioxidant treatment could
sometimes possess anti-apoptotic properties and for this reason antioxidants should be
consumed before exposure to the factors that increase oxidative stress and not after. The
genes that control apoptosis in the epidermis, such as the bcl-2 gene, are disregulated during
Skin Cancer, Free Radicals and Antioxidants 201
aging. The decreased efficiency of apoptosis may contribute to chronological aging and
extrinsic skin aging (Rocquet and Bonte, 2002). Differentiation, proliferation, and cell death
are coordinated tightly within the epidermis. Alterations within keratinocytes that disrupt
these processes are believed to contribute to the development of nonmelanoma skin cancers.
8. ROS and Cancer
Increasing evidence has implicated a role for free radicals and oxidative stress in all three
stages of the carcinogenic process. Radicals may be involved in the initiation step, either in
the oxidative activation of a procarcinogen to its carcinogenic form or in the binding of the
carcinogenic species to DNA, or both (Guyton and Kensler, 1993; Trush and Kensler, 1991;
Pryor, 1997), thus making oxidative stress an important cofactor for carcinogen activation.
Promotion always involves radicals, at least to some extent (Cerutti, 1985; Troll and Wiesner,
1985; Marks and Fu¨rstenberger, 1985; Kensler and Taffe, 1986; Crawford et al., 1988; Sun,
1990; Cheng et al., 1992; Agarwal and Mukhtar, 1993; Feig et al., 1994; Kensler et al., 1995;
Slaga, 1998), while their role in progression is controversial (Pryor, 1997).
8.1. Skin Cancer
The target organ of UV radiation is the skin. Sun exposure is the major known
environmental factor associated with the development of skin cancer of all types. Skin cancer
is a malignant growth on the skin which can have many causes. There are various types of
skin cancer. One main class is formed by the cutaneous melanocytes - melanoma. The other
main types are basal cell carcinomas and squamous cell carcinomas, cancers of the epithelial
cells. These carcinomas of the skin (basal cell and squamous cell carcinomas) are sometimes,
collectively, called "non-melanoma skin cancers". UV exposure appears to promote the
induction of skin cancer by two mechanisms. The first involves direct mutagenesis of
epidermal DNA, which promotes the induction of neoplasia. The second is associated with
immune suppression, which allows the developing tumor to escape immune surveillance and
grow progressively (Katiyar and Mukhtar, 2001). It has been proposed that if unrepaired
damage occurs to regulatory genes (e.g. tumour suppressor genes), this may be involved in
the process of carcinogenesis. In this context mutations to and activation of genes may be
important. Other responses likely to result from UV exposure of cells include increased
cellular proliferation, which could have a tumor promoting effect on genetically altered cells,
as well as changes in components of the immune system present in the skin (International
programme on chemical safety, Environmental health criteria 160).
8.2 The Importance of Antioxidants in Decreasing ROS Formation
and Skin Cancer Prevention
It is estimated that ¾ of sun exposure is non-intentional. Our skin is exposed to majority
of UV-radiation when we are outdoor working, walking, etc. and not when we are
B. Poljsak, U. Glavan, and R. Dahmane 202
intentionally exposed to the sun on the beach. At this time we also do not use sun-creams
with UVA/UVB protection. The only protection of our skin is its endogenous protection
(melanin and enzymatic antioxidants) and antioxidants we consumed with the food (vitamin
A, C, E, etc.). Dietary antioxidants thus play a major role in maintaining the homeostasis of
the oxidative balance. Vitamin C (ascorbic acid), vitamin E (tocopherol), beta-carotene and
other micronutrients such as carotenoids, polyphenols and selenium have been evaluated as
antioxidant constituents in the human diet. UVR exposure affects the skin antioxidants.
Ascorbate, GSH, SOD, catalase and ubiquinol are depleted in UV-B exposed skin, both
dermis and epidermis. Levels of electron paramagnetic resonance (EPR)-detectable ascorbyl
radical rise on UV exposure of skin. Studies of cultured skin cells and murine skin in vivo
have indicated that UVR-induced damage involves the generation of reactive oxygen species
and depletion of endogenous antioxidant systems (McArdle, et al., 2002). For example, the
study by Shindo et al. (1993) where enzymatic and non-enzymiatic antioxidants in epidermis
and dermis and their responses to ultraviolet light of hairless mice were compared. After
irradiation epidermal and dermal catalase and superoxide dismutase activities were greatly
decreased. α-Tocopherol, ubiquinol 9, ubiquinone 9, ascorbic acid, dehydroascorbic acid, and
reduced glutathione decreased in both epidermis and dermis by 26-93%. Oxidized glutathione
showed a slight, non-significant increase (Shindo et al., 1993). Many other studies confirmed
that acute exposure of human skin to UVR in vivo leads to oxidation of cellular biomolecules
that could be prevented by prior antioxidant treatment. There have been many studies
performed where different antioxidants or combinations of antioxidants and different
phytochemicals were tested in order to find evidence against ROS induced damage.
8.3. Vitamin C
Oral vitamin C supplements (500 mg/day) were taken by 12 volunteers for 8 weeks
resulting in significant rises in plasma and skin vitamin C content (McArdle et al., 2002).
Supplementation had no effect on the UVR-induced erythemal response. The skin
malonaldehyde content was reduced by vitamin C supplementation, but surprisingly,
reductions in the skin content of total glutathione and protein thiols were also seen. Authors
speculate that this apparently paradoxical effect could be due to regulation of total reductant
capacity by skin cells, such that vitamin C may have been replacing other reductants in these
Ascorbic Acid was a photoprotectant in clinical human UV studies at doses well above
the minimal erythema dose (MED). An opaque cream containing 5% Ascorbic Acid did not
induce dermal sensitization in 103 human subjects. A product containing 10% Ascorbic Acid
was non-irritant in a 4-day minicumulative patch assay on human skin and a facial treatment
containing 10% Ascorbic Acid was not a contact sensitizer in a maximization assay on 26
humans (McArdle, 2002). Many other studies have found that vitamin C can increase
collagen production, protect against damage from UVA and UVB rays, correct pigmentation
problems, and improve inflammatory skin conditions (Poljsak, 2011).
Skin Cancer, Free Radicals and Antioxidants 203
8.4. Vitamin E
Skin exposure to UV and ozone alone and in combination resulted in a significant
potentiation of the UV-induced vitamin E depletion (Packer and Valacchi, 2002), which
means that vitamin E is efficiently quenching ROS during O3 and UVR skin exposure.
Depletion of SC vitamin E is one of the earliest oxidative stress markers in human skin
exposed to UVR and other environmental stress (Thiele 2001). One study showed that the
number of sunburn to cells was decreased by treatment with the antioxidant tocopherol, and
may result from both direct protection from free radicals and indirect protection by means of
increased epidermal thickness. (Ritter et al., 1997). Additionally, Packer et al., (2001) showed
that vitamin E has skin barrier-stabilizing properties. In a study by Werninghaus and
coworkers (1994), a relatively small group of 12 healthy volunteers received 295 mg (400 IU)
-tocopherol acetate or a placebo daily for 6 months along with their regular diet. Mean
MEDs were similar in both groups before supplementation, but increased in some subjects
and decreased in others after supplementation. Plasma concentrations of α-tocopherol
increased during the study, but no parallel increase was detected in the skin. A study revealed
that topical application of alpha-tocopherol inhibits ultraviolet (UV) B-induced
photocarcinogenesis and DNA photodamage in C3H mice in vivo. This study also suggests
that incorporation of tocopherol compounds into sunscreen products confers protection
against procarcinogenic DNA photodamage and that cellular uptake and distribution of
tocopherol compounds is necessary for their optimal photoprotection (McVean and Liebler,
1999). Vitamin E provides protection against UV-induced skin photodamage through a
combination of antioxidant and UV absorptive properties. Topical application of alpha-
tocopherol on mouse skin inhibits the formation of cyclobutane pyrimidine photoproducts.
However, topically applied alpha-tocopherol is rapidly depleted by UVB radiation in a dose-
dependent manner (Krol et al., 2000).
β-carotene is a major constituent of commercially available products administered for
systemic photoprotection. β-carotene supplements are frequently used as so-called oral sun
protectants, but studies proving a protective effect of oral treatment with β-carotene against
skin responses to sun exposure are scarce and conflicting results have been reported (Stahl et
al., 2006). Studies on the systemic use of β-carotene provide evidence that 15-30 mg/d over a
period of about 10-12 wk produces a protective effect against UV-induced erythema. Similar
effects have been attributed to mixtures of carotenoids or after long-term intake of dietary
products rich in carotenoids. Supplementation with carotenoids contributes to basal protection
of the skin but is not sufficient to obtain complete protection against severe UV irradiation
(Stahl and Krutmann 2006). Studies showed that the efficacy of β-carotene in systemic
photoprotection depends on the duration of treatment and on the dose (Stahl, 2000). For
successful intervention, treatment with carotenoids is needed for a period of at least ten weeks
(Sies and Stahl, 2004). A study by Stahl et al., (2000) was performed where carotenoids and
tocopherols antioxidant effect was investigated against scavenging reactive oxygen species
generated during photooxidative stress. It was investigated whether antioxidant oral
B. Poljsak, U. Glavan, and R. Dahmane 204
supplementation may protect the skin from ultraviolet light-induced erythema. The
antioxidants used in this study provided protection against erythema in humans and may be
useful for diminishing sensitivity to ultraviolet light. Heinrich et al., (2003) additionally
compared the erythema-protective effect of beta-carotene (24 mg/d from an algal source) to
that of 24 mg/d of a carotenoid mix consisting of the three main dietary carotenoids, beta-
carotene, lutein and lycopene (8 mg/d each). In a placebo-controlled, parallel study design,
volunteers with skin type II (n = 12 in each group) received beta-carotene, the carotenoid mix
or placebo for 12 weeks. Serum beta-carotene concentration increased three- to fourfold in the
beta-carotene group, whereas in the mixed carotenoid group, the serum concentration of each
of the three carotenoids increased one- to threefold. No changes occurred in the control group.
The intensity of erythema 24 h after irradiation was diminished in both groups that received
carotenoids and was significantly lower than baseline after 12 wk of supplementation. Long-
term supplementation for 12 weeks with 24 mg/d of a carotenoid mix supplying similar
amounts of beta-carotene, lutein and lycopene ameliorates UV-induced erythema in humans.
According to the authors, the effect is comparable to daily treatment with 24 mg of beta-
carotene alone.Carotenoids have been shown to inhibit UV-induced epidermal damage and
tumor formation in mouse models (Mathews-Roth and Krinsky, 1987). The use of sunscreens
on the skin can prevent sunburn but whether long-term use can prevent skin cancer is not
known. Also, there is evidence that oral beta-carotene supplementation lowers skin-cancer
rates in animals, but there is limited evidence of its effect in human beings (Green et al.,
1999). In a community-based randomized trial performed by Green et al., (1999) with a 2 by
2 factorial design, individuals were assigned to four treatment groups: daily application of a
sun protection factor 15-plus sunscreen to the head, neck, arms, and hands, and beta-carotene
supplementation (30 mg per day); sunscreen plus placebo tablets; beta-carotene only; or
placebo only. The endpoints after 4.5 years of follow-up were the incidence of basal-cell and
squamous-cell carcinomas both in terms of people treated for newly diagnosed disease and in
terms of the numbers of tumours that occurred. There were no significant differences in the
incidence of first new skin cancers between groups randomly assigned daily sunscreen and no
daily sunscreen. Similarly, there was no significant difference between the beta-carotene and
placebo groups in incidence of either cancer. In terms of the number of tumours, there was no
effect on incidence of basal-cell carcinoma by sunscreen use or by beta-carotene but the
incidence of squamous-cell carcinoma was significantly lower in the sunscreen group than in
the no daily sunscreen group (1115 vs. 1832 per 100,000). The authors concluded that there
was no harmful effect of daily use of sunscreen in this medium-term study. Cutaneous
squamous-cell carcinoma, but not basal-cell carcinoma seems to be amenable to prevention
through the routine use of sunscreen by adults for 4.5 years. There was no beneficial or
harmful effect on the rates of either type of skin cancer, as a result of beta-carotene
supplementation (Green et al. 1999).
A randomized, placebo-controlled clinical trial on the efficacy of oral β-carotene (50
mg/day over 5 years) in prevention of skin cancer in patients with recent nonmelanoma skin
cancer showed no significant effect of β-carotene on either number or time of occurrence of
new nonmelanoma skin cancer (Greenberg et al., 1990). In a separate trial among healthy
men, 12 years of supplementation with β-carotene (50 mg on alternate days) produced no
reduction of the incidence of malignant neoplasms, including nonmelanoma skin cancer
(Hennekenset al., 1996). It must be pointed out that these intervention trials were conducted
with patients whose skin cancer was primarily UV-induced and it remains to be seen whether
Skin Cancer, Free Radicals and Antioxidants 205
antioxidants are clinically effective in prevention of cutaneous chemocarcinogenesis (Fuch et
Another study investigated the effects of oral vitamin E and beta-carotene
supplementation on ultraviolet radiation-induced oxidative stress in human skin (McArdle et
al., 2004). Results revealed that vitamin E or beta-carotene supplementation had no effect on
skin sensitivity to UVR. Although vitamin E supplements significantly reduced the skin
malondialdehyde concentration, neither supplement affected other measures of UVR-induced
oxidative stress in human skin, which suggested no photoprotection of supplementation.
In a study by Wolf et al (1988), 23 healthy volunteers received 150 mg of an oral
carotenoid preparation containing 60 mg ß-carotene and 90 mg canthaxanthin daily for 4 wk.
No differences in MEDs were shown in a comparison of values before and after carotenoid
supplementation. Concentrations in serum increased during treatment, but concentrations in
the skin were not reported. Additionally, no effects of ß-carotene were detected when UV
irradiation–induced unscheduled DNA synthesis was investigated, suggesting that carotenoids
were not protective against DNA lesions repairable by excision repair.
Although the photoprotective effects of beta-carotene are thought to originate from its
antioxidant properties, some studies documented pro-oxidant effects of beta-carotene.
A study was done to compare the effects of dietary administration of a vitamin A drug
(13-cis-retinoic acid) to the natural form of vitamin A (retinyl palmitate).
Female mice were administered a chemical carcinogen to evaluate the incidence and
severity on mouse skin tumour promotion. The results showed that retinyl palmitate inhibited
the number and weight of tumours, whereas 13-cis-retinoic acid resulted in a decrease in
weight, but not in number of tumours promoted (Gensler et al., 1987).
In another study, tumours were chemically induced in a group of Swiss mice over a 23-
week period. The topical application of 13-cis-retinoic acid was compared to natural vitamin
A (retinyl palmitate). This study showed that both retinyl palmitate and 13-cis-retinoic acid
inhibited the development of skin papillomas and also had a marked effect on skin cancers
(Abdel-Galil et al., 1984).
8.7. Coenzyme Q10
It was recently reported that Coenzyme Q10 protects against oxidative stress-induced cell
death and enhances the synthesis of basement membrane components in dermal and
epidermal cells (Muta-Takada et al., 2009).
Coenzyme Q10 (CoQ10) was reported to reduce ROS production and DNA damage
triggered by UVA irradiation in human keratinocytes in vitro. Further, CoQ10 was shown to
reduce UVA-induced MMPs in cultured human dermal fibroblasts (Inui et al., 2008). It was
reported that it is considered that CoQ10 appears to have also a cutaneous healing effect in
vivo (Choi et al., 2009).
B. Poljsak, U. Glavan, and R. Dahmane 206
In cell culture models using human skin cells, it has been clearly shown that glutathione
depletion leads to a large sensitization to UVA (334 nm, 365 nm) and near-visible (405 nm)
wavelengths as well as to radiation in the UVB (302 nm, 313 nm) (Tyrrell and Pidoux,
There is a direct correlation between the levels of sensitisation and cellular glutathione
content. Additional evidence that glutathione is a photoprotective agent in skin cells is
derived from experiments which have demonstrated that glutathione levels in both dermis and
epidermis are depleted by UVA treatment (Connor and Wheeler, 1987).
8.9. Green Tea
In vitro and in vivo animal and human studies suggest that green tea polyphenols are
photoprotective in nature, and can be used as pharmacological agents for the prevention of
solar UVB light-induced skin disorders including photoaging, melanoma and nonmelanoma
skin cancers after more clinical trials in humans. Topical treatment or oral consumption of
green tea polyphenols (GTP) inhibits chemical carcinogen- or UV radiation-induced skin
carcinogenesis in different laboratory animal models. Topical application of GTP and EGCG
prior to exposure of UVB protects against UVB-induced local as well as systemic immune
suppression in laboratory animals, which was associated with the inhibition of UVB-induced
infiltration of inflammatory leukocytes (Katiyar, 2003).
Another study of Vayalil et al., (2003) demonstrated that topical application of green tea
polyphenols reduced UVB-induced oxidation of lipids and proteins and depletion of
Other protective effects include the reduced production of ROS and lipid peroxidation
products, a reduced depletion of Langerhans cells and of endogenous antioxidant systems as
reported by Afaq and Mukhtar (2002).
9. Pro-Oxidant Effects of Antioxidants
Antioxidants that are reducing agents can also act as pro-oxidants. Antioxidants, which
are reducing agents, are capable of reacting with molecular oxygen (e.g. ascorbic acid) and
will generate superoxide radicals under aerobic conditions. This will dismutate to H2O2 that
can enter cells and react with superoxide or reduced metal ions to form highly damaging
hydroxyl radicals. The presence of redox cycling metal ions with antioxidants might result in
a synergistic effect, resulting in increased free radical formation or the so called pro-oxidant
effect. For example, vitamin C or glutathione have antioxidant activity when they reduce
oxidizing substances such as hydrogen peroxide, however, they can also reduce metal ions
which leads to the generation of free radicals through the Fenton reaction. Both vitamin C and
E possess pro-oxidant properties, at least in vitro, depending on their concentration, the
existence of regenerating co-antioxidants and traces of metal ions.
Skin Cancer, Free Radicals and Antioxidants 207
The results of epidemiologic studies where people were treated with synthetic
antioxidants are inconclusive and contradictory: from the proven beneficial effect, proven no
difference, to the proven harmful effect of synthetic antioxidant supplements. None of the
major clinical studies using mortality or morbidity as an end point has found positive effects
of supplementation with antioxidants such as vitamin C, vitamin E or β-carotene.
There are several possible explanations for the potential negative effect of antioxidant
supplements. Reactive oxygen species in moderate concentrations are essential mediators of
defense against unwanted cells. Thus, if administration of antioxidant supplements decreases
free radicals, (it may interfere with essential defensive mechanisms for ridding the organism
of damaged cells, including those that are precancerous and cancerous (Salganik, 2001).
Thus, antioxidant supplements may actually cause some harm (Vivekananthan et al.,
2003; Bjelakovic et al., 2004a; Bjelakovic et al., 2004b; Miller et al., 2005; Bjelakovic et al.,
2007; ; Caraballoso et al., 2003). Our diets typically contain safe levels of vitamins, but high-
level antioxidant supplements could potentially upset an important physiologic balance
(Vivekananthan et al., 2003; Bjelakovic et al., 2004a; Bjelakovic et al., 2004b; Miller et al.,
2005; Bjelakovic et al., 2007; Caraballoso et al., 2003). Additionally, consuming antioxidant
molecules such as polyphenols and vitamin E will produce changes in other parts of the
metabolism, and these other non-antioxidant effects may be the real reason for their
importance in human nutrition (Azzi, 2007; Aggarwal and Shishodia, 2006) and their positive
effect on aging and chronical degenerative diseases prevention. There seems to be an effect
between exogenous antioxidants that tends to depress endogenous antioxidant levels.
Changing the level of one antioxidant causes a compensatory change in others, while the
overall antioxidant capacity remains unaffected. Dosing cells with exogenous antioxidants
might decrease the rate of synthesis or uptake of endogenous antioxidants, so that the total
“cell antioxidant potential” remains unaltered.
Thus, the key to the future success of dietary antioxidant supplementation should be the
suppression of oxidative damage without disruption of the well-integrated antioxidant defense
network. Increasing cellular viability with antioxidants prior to toxic compound-induced
toxicity (e.g. Cr(VI), UV-radiation, ionizing radiation) might not always be beneficial
(Poljsak et al., 2006). Carcinogen-induced growth arrest and apoptosis are at the molecular
decision point between carcinogen toxicity and carcinogen carcinogenesis (Carlisle, 2000).
When normal growing cells come in contact with carcinogens, they may respond by
undergoing growth arrest, apoptosis and necrosis.
A population of genetically damaged cells may also emerge, which exhibits either
intrinsic or induced resistance to apoptosis (Carlisle 2000). Such cells may be predisposed to
neoplasia as a result of their altered growth/death ratio, disrupted cell cycle control, or
genomic instability. This, however, raises the question of whether decreasing carcinogen
toxicity with antioxidants might actually increase the incidence of cancer by allowing the
inappropriate survival of genetically damaged cells. This hypothesis was recently confirmed
also by the study of Schafer et al. (2009) which revealed an unanticipated mechanism for cell
survival in altered matrix environments by antioxidant restoration of ATP generation.
Antioxidant activity may promote the survival of pre-initiated tumor cells in unnatural matrix
environments, and thus enhance malignancy.
At low glutathione concentrations, UVB-induced mtDNA deletions have been prevented,
but at high levels of glutathione, when it acts as an electron donor the pro-oxidative properties
reveal and the mtDNA deletions return (Ji et al., 2006). A number of experimental studies
B. Poljsak, U. Glavan, and R. Dahmane 208
indicate protective effects of beta-carotene against acute and chronic manifestations of skin
photodamage. However, most clinical studies have failed to convincingly demonstrate its
beneficial effects so far. Nevertheless, intake of oral β-carotene supplements before sun
exposure has been recommended on a population-wide basis. Studies on skin cells in culture
have revealed that beta-carotene acts not only as an antioxidant but also has unexpected
prooxidant properties (Biesalski and Obermueller-Jevic, 2001). Lycopene was reported to
enhance UVA-induced oxidative stress in C3H cells, and authors of the study suggest that
under UVA irradiation, lycopene may produce also oxidative products that are responsible for
the prooxidant effects (Yeh et al., 2005).
Skin DNA molecules are constantly “bombarded” by ROS originating from endogenous
processes as well as from environmental agents and from radiation sources. Damaged DNA is
being constantly repaired by many cellular repair systems. If the frequency of damaging
events exceeds the repair capacity, damaged DNA is not repaired in time and can pass to
daughter cells and thus trigger tumor initiation and progression process. Although DNA
damage due to ROS is not a rare event since it is estimated that human cell sustains an
average of 105 oxidative hits per day due to cellular oxidative metabolism (Fraga et al., 1991),
DNA is functionally very stable, so that the incidence of cancer is much lower than one
would expect, taking into account the high frequency of oxidative hits. Nevertheless,
avoidance of excessive cumulative and sporadic sun exposure is important in reducing the
risk of skin cancer and skin aging. Additionally, antioxidants might act by enhancing the
DNA enzyme repair systems through a post-transcriptional gene regulation of transcription
factors (Xanthoudakis et al., 1992; Hirota et al., 1997; Schenk et al., 1994). The repair
capacity of human skin cells therefore directly relates to the probability of initiation of the
carcinogenesis process and eventually tumor formation. Cellular antioxidant defense
mechanisms are therefore crucial for the prevention or removal of the damage caused by the
oxidizing component of UV radiation. Evidence is accumulating that dietary changes and
special nutrients may help to reduce oxidative stress, free radical formation and thereby slow
down the skin damage process. Foods rich in antioxidants and other phytochemicals, such as
fruits, vegetables, wine and green tea help protect against oxidative damage and free radical
attack of all body cells including the skin. The primary treatment of photoaging is
photoprotection but secondary treatment could be achieved with the use of antioxidants and
some novel compounds such as polyphenols. Exogenous antioxidants like vitamin C, E, and
many others cannot be synthesized by the human body and must be taken up by the diet. They
have been shown to prevent exogenous free radical formation (e.g. UVA and UVB). They
could also possess beneficial effects in endogenous ROS prevention. Antioxidants can
regulate the transfer of electrons or quench free radicals escaping from electron transport
chain. Since the effectiveness of endogenous antioxidant system is diminished during aging,
the exogenous supplementation of antioxidants might be a protective strategy against age-
associated skin oxidative damage. It can be concluded that oxidative stress is a problem of
skin cells and endogenous as well as exogenous antioxidants could play an important role in
decreasing it. However, it is important to pre-treat the skin with antioxidants before sun
Skin Cancer, Free Radicals and Antioxidants 209
exposure. Animal and human studies have convincingly demonstrated pronounced photo-
protective effects of 'natural' and synthetic antioxidants when applied topically before UVR
exposure. No significant protective effect of melatonin or the vitamins when applied alone or
in combination were obtained when antioxidants were applied after UVR exposure. UVR-
induced skin damage is a rapid event, and antioxidants possibly prevent such damage only
when present in relevant concentration at the site of action beginning and during oxidative
stress (Dreher et al., 1999). Treatment of the skin with antioxidants after the damage was
caused by UVR might cause additional harmful effects on cell cycle control and apoptosis
process. The photoprotective effects of antioxidants are significant when applied in distinct
mixtures in appropriate vehicles.
The most important strategy to reduce the risk of sun UV radiation damage is to avoid the
sun exposure and the use of sunscreens. The next step is the use of exogenous antioxidants
orally or by topical application and interventions in preventing oxidative stress and in
enhanced DNA repair. The laboratory studies conducted in animal models suggest that many
plant compounds have the ability to protect the skin from the adverse effects of UV radiation,
including the risk of skin cancers. It is suggested that antioxidants may favourably
supplement sunscreens protection, and may be useful for skin diseases associated with solar
UV radiation-induced inflammation, oxidative stress and DNA damage. At this point, it
should be stressed that extrapolation of in vitro data to the in vivo situation is often difficult.
Moreover, in vivo studies of the effects of nutrients on human skin have mainly focused on
indirect measures of skin function after supplementation. Many more placebo-controlled
human studies are required to support food and supplement product claims regarding skin
beneficial effects. The controversy in beneficial vs. harmful synthetic antioxidant properties
may also reflect a misinterpretation of epidemiology. Fruits, grains and vegetables contain
multiple components that might exert protective effects against disease. It could be any or any
combination of those factors that is a true protective agent. For example, high plasma
ascorbate levels or high ascorbate intake could simply be a marker of a good diet rather than a
true protective factor (Rietjens et al., 2001). Antioxidants may thus have dichotomous
activities with respect to tumorigenesis, namely, suppressing tumorigenesis by preventing
oxidative damage to DNA (Gao, 2007; Narayanan, 2006) and promoting tumorigenesis by
allowing survival of cells that are metabolically impaired (e.g. in altered matrix
Besides the compounds mentioned in our review, many recent studies showed potentially
interesting effects of some naturally occurring, less well investigated compounds that may
improve skin conditions. This area of research is constantly emerging and new antioxidants
are reported. Nevertheless, endogenous skin protection with the use of selected antioxidants
or plant extracts contributes to the protection of sensitive dermal target sites beyond those
reached with sunscreens and especially because of lifelong exposure to sunlight which mainly
occurs under everyday circumstances, when no topical protection is applied. According to
Stahl et al., (2006) endogenous photoprotection is complementary to topical photoprotection,
and these two forms of prevention clearly should be considered mutually exclusive. We have
to realize that the use of synthetic vitamin supplements is not an alternative to regular
consumption of fruit and vegetables. It is probable that many antioxidants are still
undiscovered; furthermore the combination of antioxidants in fruit and vegetables causes their
reciprocal regeneration and consecutively intensifies their defense from free radicals. Given
B. Poljsak, U. Glavan, and R. Dahmane 210
our complex genetic and physiological make-up, it is important to directly assess the role of
oxidative stress in human cancer processes.
Abdel-Galil, AM; Wrba H; El-Mofty MM. (1984). Prevention of 3-methylcholanthrene-
induced skin tumors in mice by simultaneous application of 13-cis-retinoic acid and
retinyl palmitate (vitamin A palmitate). Exp. Pathol., 25(2), 97-102.
Afaq, F; Mukhtar, H. (2002). Photochemoprevention by botanical antioxidants. Skin
Pharmacol. Appl. Skin Physiol, 15, 297-306.
Agarwal, R; Mukhtar, H. Oxidative stress in skin chemical carcinogenesis. In: Fuchs J,
Packer L, eds. Oxidative Stress in Dermatology. New York: Marcel Dekker, Inc, 1993;
Aggarwal, BB; Shishodia, S. (2006). Molecular targets of dietary agents for prevention and
therapy of cancer.” Biochem. Pharmacol., 71, 1397–421.
Azzi, A. (2007). "Molecular mechanism of alpha-tocopherol action.” Free Radic. Biol. Med.,
Babior, BM; Woodman, RD. (1990). Chronic granulomatous disease. Semin. Hematol, 27,
Beckman, KB; Ames, BN. (1998). The free radical theory of aging matures. Physiol. Rev, 78,
Berneburg, M; Grether-Beck, S; Kürten, V; Ruzicka, T; Briviba, K; Sies, H; Krutmann, J.
(1999). Singlet oxygen mediates the UVA-induced generation of the photoaging-
associated mitochondrial common deletion. J. Biol. Chem., 274(22), 15345-9.
Biesalski, HK; Obermueller-Jevic, UC. (2001). UV light, beta-carotene and human skin--
beneficial and potentially harmful effects. Arch. Biochem. Biophys., 389(1), 1-6.
Birch-Machin, MA; Tindall, M; Turner, R; Haldane, F; Rees, JL. (1998). Mitochondrial DNA
deletions in human skin reflect photo- rather than chronologic aging. J. Invest. Dermatol.,
Bjelakovic, G; Nikolova, D; Gluud, L; Simonetti, R; Gluud, C. (2007). "Mortality in
Randomized Trials of Antioxidant Supplements for Primary and Secondary Prevention:
Systematic Review and Meta-analysis.” Jama, 8, 842–57.
Bjelakovic, G; Nikolova, D; Simonetti, RG; Gluud, C. (2004). Antioxidant supplements for
prevention of gastrointestinal cancers: a systematic review and meta-analysis. Lancet
2004, 36, 1219-28.
Blatt, T; Wenck, H; Wittern, KP. Alterations of energy metabolism in cutaneous aging. In:
Textbook of aging skin. Farage, MA; Miller, KW; Maibach, HI. (Eds). Springer-Verlag,
Cai, J; Jones, DP. (1998). Superoxide in apoptosis. Mitochondrial generation triggered by
cytochrome c loss. J. Biol. Chem., 273(19), 11401-4.
Cai, J; Yang, J; Jones, DP. (1998). Mitochondrial control of apoptosis: the role of cytochrome
c. Biochim. Biophys Acta., 1366(1-2), 139-49.
Caraballoso, M; Sacristan, M; Serra, C; Bonfill, X. (2003). Drugs for preventing lung cancer
in healthy people. Cochrane Database Syst. Rev, 2. CD002141.
Skin Cancer, Free Radicals and Antioxidants 211
Carlisle, DL; Pritchard, DE; Singh, J; Owens, BM; Blankenship, LJ; Orenstein, JM; Patierno,
SR. (2000). Apoptosis and P53 induction in human lung fibroblasts exposed to
chromium(VI): effect of ascorbate and tocopherol. Toxicol. Sci., 55, 60-68.
Castiella, L; Rigoulet, M; Penicaud, L. (2001). Mitochondrial ROS metabolism: modulation
by uncoupling proteins. Iubmb. Life, 52, 181–188.
Cerutti, PA. (1985). Prooxidant states and promotion. Science, 227, 375–381.
Cheeseman, KH; Slater, TF. (1993). An introduction to free radical biochemistry. Br. Med.
Bull, 49, 481-493.
Cheng, KC; Cahill, DS; Kasa, H; Nishimura, S; Loeb, LA. (1992). 8-Hydroxyguanosine, an
abundant form of oxidative DNA damage, causes G-T and A-C substitutions. J. Biol.
Chem., 267, 166–172.
Choi, BS; Song, HS; Kim, HR; Park, TW; Kim, TD; Cho, BJ; Kim, CJ; Sim, SS. (2009).
Effect of coenzyme Q10 on cutaneous healing in skin-incised mice. Arch. Pharm. Res.,
Connor, MJ; Wheeler, LA. (1987). Depletion of cutaneous glutathione by ultraviolet
radiation. Photochem. Photobiol., 46(2), 239-45.
Crawford, D; Zbinden, I; Amstad, P; Cerutti, P. (1988). Oxidant stress induces the proto-
oncogenes cfos and c-myc in mouse epidermal cells. Oncogene, 3, 27–32.
Cunningham, ML; Johnson, JS; Giovanazzi, SM; Peak, MJ. (1985). Photosensitized
production of superoxide anion by monochromatic (290–405 nm) ultraviolet irradiation
of NADH and NADPH coenzymes. Photochem. Photobiol., 42, 125–128.
Dalle Carbonare, M; Pathak, MA. (1992). Skin photosensitizing agents and the role of
reactive oxygen species in photoaging. J. Photochem. Photobiol B., 14(1-2), 105-24.
Darr, D; Fridovich, I. (1994). Free radicals in cutaneous biology. J. Invest. Dermatol, 102,
De Magalhaes, JP; Church, GM. (2006). Cells discover fire: employing reactive oxygen
species in development and consequences for aging. Exp. Gerontol., 41, 1-10.
Dreher, F; Denig, N; Gabard, B; Schwindt, DA; Maibach, HI. (1999). Effect of topical
antioxidants on UV-induced erythema formation when administered after exposure.
Dermatology., 198(1), 52-5.
Evans, MD; Dizdaroglu, M; Cooke, MS. (2004). Oxidative DNA damage and disease:
induction, repair and significance. Mutat. Res., 567, 1–61.
Feig, DI; Reid, TM; Loeb, LA. (1994). Reactive oxygen species in tumorigenesis. Cancer
Res., 54(7), 18902–18904.
Fraga, CG; Motchnik, PA; Shigenaga, MK; Helbock, HJ; Jacob, RA; Ames, BN. (1991).
Ascorbic acid protects against endogenous oxidative DNA damage in human sperm.
Proc. Natl. Acad. Sci. US., 88(24), 11003-6.
Freifelder, D. (1987). Molecular Biology 2nd edn. Boston: Jones and Bartlett, pp 277-92.
Fuch, J; Podda, M; Zollner, T. Redox Modulation and Oxidative Stress in
Dermatotoxicology. In: Environmental stressors in health and disease (Ed. Fuchs, J;
Packer, L.). Marcel Dekker, Inc., 2001.
Fuchs, J. (ed.) Oxidative Injury in Dermatopathology. Heidelberg: Springer Verlag, 1992.
Fuchs, J; Huflejt, M; Rothfuss, L; Carcamero, G; Packer, L. (1989a). Impairment of enzymic
and nonenzymic antioxidants in skin by UVB irradiation. J. Invest. Dermatol., 93, 769-
B. Poljsak, U. Glavan, and R. Dahmane 212
Fuchs, J; Huflejt, ME; Rothfuss, LM; Wilson, DS; Carano, G; Packer, L. (1989b). Acute
effects on near ultraviolet and visible light on the cutaneous antioxidant defense system.
Photochem Photobiol., 50(6), 739–744.
Gao, P. (2007). HIF-dependent antitumorigenic effect of antioxidants in vivo. Cancer Cell,
Gensler, HL; Watson, RR; Moriguchi, S; Bowden, GT. (1987). Effects of dietary retinyl
palmitate or 13-cis-retinoic acid on the promotion of tumors in mouse skin. Cancer Res.,
Goukassian, D; Gad, F; Yaar, M; Eller, MS; Nehal, US; Gilchrest, BA. (2000). Mechanisms
and implications of age-associated decrease in DNA repair capacity. FASEB J., 14, 1325-
Green, A; Williams, G; Neale, R; Hart, V; Leslie, D; Parsons, P; Marks, GC; Gaffney, P;
Battistutta, D; Frost, C; Lang, C; Russell, A. (1999). Daily sunscreen application and
betacarotene supplementation in prevention of basal-cell and squamous-cell carcinomas
of the skin: a randomised controlled trial. Lancet., 354(9180), 723-9.
Greenberg, ER; Baron, JA; Stukel, TA; Stevens, NM; Mandel, JS; Spencer, SK; Elias, PM;
Lowe, N; Nierenberg, DW; Bayrd, G; Vance, JC; Freeman, DH; Clendenning, WE;
Kwan, T; and the alpha-tocopherol, beta-carotene cancer prevention study group. (1990).
A clinical trial of betacarotene to prevent basal-cell and squamous-cell cancer on the skin.
N. Engl. J. Med, 323, 789–795.
Guyton, KZ; Kensler, TW. (1993). Oxidative mechanisms in carcinogenesis. Br. Med. Bull,
Halliwell, B; Gutteridge, J. (1999). Free radicals in biology and medicine (3nd edn). Oxford:
Clarendon Press, 1999.
Hansford, RG; Hogue, BA; Mildaziene, V. (1997). Dependence of H2O2 formation by rat
heart mitochondria on substrate availability and donor age. J. Bioenerg. Biomembr.,
Hanson, K; Clegg, R. (2002). Observation and quantification of UV-induced reactive oxygen
species in ex vivo human skin. Photochem. Photobiol, 76, 57-63.
Heinrich, U; Gärtner, C; Wiebusch, M; Eichler, O; Sies, H; Tronnier, H; Stahl, W. (2003).
Supplementation with beta-carotene or a similar amount of mixed carotenoids protects
humans from UV-induced erythema., 133(1), 98-101.
Hennekens, CH; Buring, JE; Manson, JE; Stampfer, M; Rosner, B; Cook, NR; Belanger, C;
LaMotte, F; Gaziano, JM; Ridker, PM. (1996). Lack of effect of long-term
supplementation with beta carotene on the incidence of malignant neoplasms and
cardiovascular disease. N. Engl. J. Med., 334, 1145–1149.
Hirota, K; Matsui, M; Iwata, S; Nishiyama, A; Mori, K; Yodoi, J. (1997). AP-1
transcriptional activity is regulated by a direct association between thioredoxin and Ref-1.
Proc. Natl. Acad. Sci. U S A., 94(8), 3633-8.
Hutchinson, F. (1985). Chemical changes induced in DNA by ionizing radiation. Prog.
Nucleic. Acid. Res. Mol. Biol,. 32, 115-154.
International programme on chemical safety, Environmental health criteria 160, Ultraviolet
radiation (EHC 160, 1994, 2nd edition), accessed:
Skin Cancer, Free Radicals and Antioxidants 213
Inui, M; Ooe, M; Fujii, K; Matsunaka, H; Yoshida, M; Ichihashi, M. (2008). Mechanisms of
inhibitory effects of CoQ10 on UVB-induced wrinkle formation in vitro and in vivo.
Biofactors., 32(1-4), 237-43.
Katiyar, SK. (2003). Skin photoprotection by green tea: antioxidant and immunomodulatory
effects. Curr. Drug Targets Immune. Endocr. Metabol. Disord., 3(3), 234-42.
Katiyar, SK; Mukhtar, H. (2001). Green tea polyphenol (-)-epigallocatechin-3-gallate
treatment to mouse skin prevents UVB-induced infiltration of leukocytes, depletion of
antigen-presenting cells, and oxidative stress. J. Leukoc. Biol., 69(5), 719-26.
Kensler, T; Guyton, K; Egner, P; McCarthy, T; Lesko, S; Akman, S. (1995). Role of reactive
intermediates in tumor promotion and progression. Prog. Clin. Biol. Res., 391, 103–116.
Kensler, TW; Taffe, BG. (1986). Free radicals and tumor promotion. Adv. Free Rad. Biol.
Med., 2, 347–387.
Koster, JF; Sluiter, W. (1995). Is increased tissue ferritin a risk factor for atherosclerosis and
ischaemic heart disease? Br. Heart J, 73, 208-9.
Krebs, HA. (1972). Some aspects of the regulation of fuel supply in omnivorous animals.
Adv. Enzyme. Regul., 10, 397-420.
Kregel, KC; Zhang, HJ. (2007). An integrated view of oxidative stress in aging: Basic
mechanisms, functional effects and pathological considerations. Am. J. Physiol. Regul.
Integr. Physiol, 292, 18-36.
Krol, ES; Kramer-Stickland, KA; Liebler, DC. (2000). Photoprotective actions of topically
applied vitamin E. Drug Metab. Rev., 32(3-4), 413-20.
Lasch, J; Schonfelder, U; Walke, M; Zellmer, S; Beckert, D. (1997). Oxidative damage of
human skin lipids. Dependence of lipid peroxidation on sterol concentration. Biochim.
Biophys. Acta., 1349, 171-181.
Marks, F; Fu¨rstenberger, G. Tumor promotion in skin: Are active oxygen species involved?
In: Sies H, ed. Oxidative Stress. London: Academic Press, Inc, 1985, 437–475.
Martindale, JL; Holbrook, NJ. (2002). Cellular response to oxidative stress: signaling for
suicide and survival. J. Cell Physiol., 192, 1–15.
Mathews-Roth, MM; Krinsky, NI. (1987). Carotenoids affect development of UV-B induced
skin cancer. Photochem. Photobiol, 46, 507-509.
McArdle, F; Rhodes, LE; Parslew, R; Jack, CI; Friedmann, PS; Jackson, MJ. (2002). UVR-
induced oxidative stress in human skin in vivo: effects of oral vitamin C supplementation.
Free Radic. Biol. Med., 33(10), 1355-62.
McArdle, F; Rhodes, LE; Parslew, RA; Close, GL; Jack, CI; Friedmann, PS; Jackson, MJ.
(20049. Effects of oral vitamin E and beta-carotene supplementation on ultraviolet
radiation-induced oxidative stress in human skin. Am. J. Clin. Nutr., 80(5), 1270-5.
McCormick, JP; Fisher, JR; Pachlatko, JP; Eisenstark, A. (1976). Characterization of a cell
lethal product from the photooxidation of tryptophan: hydrogen peroxide. Science, 198,
McVean, M; Liebler, DC. (1999). Prevention of DNA photodamage by vitamin E compounds
and sunscreens: roles of ultraviolet absorbance and cellular uptake. Mol. Carcinog.,
Miller, ER; Pastor-Barriuso, R; Dalal, D; Riemersma, R; Appel, LJ; Guallar, E. (2005). Meta-
Analysis: High-Dosage Vitamin E Supplementation May Increase All-Cause Mortality.
Annals of Internal Medicine, 142, 37-46.
B. Poljsak, U. Glavan, and R. Dahmane 214
Muta-Takada, K; Terada, T; Yamanishi, H; Ashida, Y; Inomata, S; Nishiyama, T; Amano, S.
(2009). Coenzyme Q10 protects against oxidative stress-induced cell death and enhances
the synthesis of basement membrane components in dermal and epidermal cells.
Biofactors., 35(5), 435-41.
Narayanan, BA. (2006). Chemopreventive agents alters global gene expression pattern:
predicting their mode of action and targets. Curr. Cancer Drug Targets, 6, 711–727.
Norbury, CJ; Hickson, ID. (2001). Cellular responses to DNA damage. Annu. Rev.
Pharmacol. Toxicol., 41, 367-401.
Packer, L; Valacchi, G. (2002). Antioxidants and the response of skin to oxidative stress:
vitamin E as a key indicator. Skin Pharmacol. Appl. Skin. Physiol., 15(5), 282-90.
Packer, L; Weber, SU; Rimbach, G. (2001). Molecular aspects of alpha-tocotrienol
antioxidant action in cell sinaling. J. Nutr., 131, 369S-373S.
Pattison, DI; Davies, MJ. (2006). Actions of ultraviolet light on cellular structures. EXS., (96),
Peak, JG; Peak, MJ; Sikorski, RA; Jones, RA. (1988). Induction of DNA-protein crosslinks in
human cells by ultraviolet and visible radiations: action spectrum. Photochem. Photobiol,
Peak, MJ; Peak, JG; Carnes, BA. (1987). Induction of direct and indirect single-strand breaks
in human cell DNA by far- and near-ultraviolet radiations: action spectrum and
mechanisms. Photochem. Photobiol, 45, 381–387.
Peak, MJ; Peak, JG; Jones, CA. (1985). Different (direct and indirect) mechanisms for the
induction of DNA-protein crosslinks in human cells by far- and near ultraviolet radiations
(290 and 405 nm). Photochem. Photobiol., 42, 141–146.
Philpott, MP; Kealey, T. (1991). Metabolic studies on isolated hair follicles: hair follicles
engage in aerobic glycolysis and do not demonstrate the glucose fatty acid cycle. J.
Invest. Dermatol., 96(6), 875-9.
Poljsak, B. Decreasing Oxidative Stress and Retarding the Aging Process, Nova
Poljsak, B; Gazdag, Z; Pesti, M; Jenko-Brinovec, Š; Belagyi, J; Plesnicar, S; Raspor, P.
(2006). Pro-oxidative versus antioxidative reactions between trolox and Cr(VI) : the role
of H2O2. Environ. Toxicol. Pharmacol. 22, 15-19.
Poljsak, B; Pesti, M; Jamnik, P; Raspor P. (2011). Impact of environmental pollutants on
oxidation-reduction processes in the cell environment. In: Dr. Jerome Nriagu (ed).
Encyclopedia of Environmental Health. Elsevier.
Poswig, A; Wenk, J; Brenneisen, P; Wlaschek, M; Hommel, C; Quel, G; Faisst, K;
Dissemond, J; Briviba, K; Krieg, T; Scharffetter-Kochanek, K. (1999). Adaptive
antioxidant response of manganese-superoxide dismutase following repetitive UVA
irradiation. J. Invest. Dermatol. 112(1), 13-8.
Pryor, WA. (1997). Cigarette smoke radicals and the role of free radicals in chemical
carcinogenicity. Environ. Health Perspect., 105(4), 875–882.
Rietjens, I; Boersma, M; de Haan, L. (2001). The pro-oxidant chemistry of the natural
antioxidants vitamin C, vitamin E, carotenoids and flavonoids. Environ Toxicol.
Pharmacol, 11, 321-333.
Ritter, EF; Axelrod, M; Minn, KW; Eades, E; Rudner, AM; Serafin, D; Klitzman, B. (1997).
Modulation of ultraviolet light-induced epidermal damage: beneficial effects of
tocopherol. Plast. Reconstr. Surg. 100(4), 973-80.
Skin Cancer, Free Radicals and Antioxidants 215
Rocquet, C; Bonté, F. (2002). Molecular aspects of skin ageing - recent data. Acta
dermatologica Alpina, Pannonica ed. Adriatica, 11(3), 71–94.
Rosenstein, BS; Ducore, JM. (1983). Induction of DNA strand breaks in normal human
fibroblasts exposed to monochromatic ultraviolet and visible wavelengths in the 240–546
nm range. Photochem Photobiol, 38, 51–55.
Rottkamp, CA; Raina, AK; Zhu, X; Gaier, E; Bush, AI; Atwood, CS. (2001). Redox-active
iron mediates amyloid-beta toxicity. Free Radic. Biol. Med., 30, 447-50.
Salganik RI. The benefits and hazards of antioxidants: controlling apoptosis and other
protective mechanisms in cancer patients and the human population. J. Am. Coll Nutr.
2001 Oct;20(5 Suppl):464S-472S; discussion 473S-475S.
Sander, CS; Chang, H; Salzmann, S; Müller, CS; Ekanayake-Mudiyanselage, S; Elsner, P;
Thiele, JJ. (2002). Photoaging is associated with protein oxidation in human skin in vivo.
J Invest Dermatol, 118(4), 618-25.
Schafer, ZT. (2009). Antioxidant and oncogene rescue of metabolic defects caused by loss of
matirx attachment. Nature, 461, 109-113.
Scharffetter-Kochanek, K; Brenneisen, P; Wenk, J; Herrmann, G; Ma, W; Kuhr, L; Meewes,
C; Wlaschek, M. (2000). Photoaging of the skin from phenotype to mechanisms. Exp.
Gerontol., 35(3), 307-16.
Scharffetter-Kochanek, K; Wlaschek, M; Brenneisen, P; Schauen, M; Blaudschun, R; Wenk,
J. (1997). UV-induced reactive oxygen species in photocarcinogenesis and photoaging.
Biol. Chem., 378(11), 1247-57.
Schenk, H; Klein, M; Erdbrügger, W; Dröge, W; Schulze-Osthoff, K. (1994). Distinct effects
of thioredoxin and antioxidants on the activation of transcription factors NF-kappa B and
AP-1. Proc. Natl. Acad. Sci. US., 91(5), 1672-6.
Shindo, Y; Witt, E; Han, D; Epstein, W; Packer, L. (1994b). Enzymic and non-enzymic
antioxidants in epidermis and dermis of human skin. J. Invest. Dermatol, 102, 122–124.
Shindo, Y; Witt, E; Han, D; Tzeng, B; Aziz, T; Nguyen, L; Packer, L. (1994a). Recovery of
antioxidants and reduction in lipid hydroperoxides in murine epidermis and dermis after
acute ultraviolet radiation exposure. Photodermatol. Photoimmunol. Photomed, 10, 183–
Shindo, Y; Witt, E; Han, D; Tzeng, B; Aziz, T; Nguyen, L; Packer, L. (1994c). Recovery of
antioxidants and reduction in lipid hydroperoxides in murine epidermis and dermis after
acute ultraviolet radiation exposure. Photodermatol Photoimmunol Photomed, 10(5),
Shindo, Y; Witt, E; Packer, L. (1993). Antioxidant defense mechanisms in murine epidermis
and dermis and their responses to ultraviolet light. J. Invest. Dermatol, 100, 260–265.
Sies, H; Stahl, W. (2004). Carotenoids and UV protection. Photochem Photobiol Sci., 3(8),
Sivonova, M; Tatarkova, Z; Durackova, Z; et al. (2007). Relation between antioxidant
potential and oxidative damage to lipids, proteins and DNA in aged rats. Physiol. Res,
Slaga, TJ. (1998). Tumor promotion and-or enhancement models. Int. J. Toxicol, 17(3), 109–
Speakman, JR; van Acker, A; Herper, EJ. (2003). Age-related changes in the metabolism and
body composition of three dog breeds and their relationship to life expectancy. Aging
cell, 2, 265-275.
B. Poljsak, U. Glavan, and R. Dahmane 216
Stahl, W; Heinrich, U; Jungmann, H; Sies, H; Tronnier, H; (2000). Carotenoids and
carotenoids plus vitamin E protect against ultraviolet light-induced erythema in humans.
Am. J. Clin. Nutr., 71(3), 795-8.
Stahl, W; Krutmann, J.(2006). Systemic photoprotection through carotenoids. Hautarzt.,
Stahl, W; Mukhtar, H; Afaq, F; Sies, H. Vitamins and polyphenols in systemic
photoprotection. In: Skin aging, Gilchrest B, Krutmann J (Eds.). Springer-Verlag, Berlin,
Staniek, K; Nohl, H. (1999). H2O2 detection from intact mitochondria as a measure for one-
electron reduction of dioxygen requires a non-invasive assay system. Biochim. Biophys.
Acta., 1413, 70–80.
Staniek, K; Nohl, H. (2000). Are mitochondria a permanent source of reactive oxygen
species? Biochim. Biophys. Acta., 1460, 268–75.
Stücker, M; Struk, PA; Hoffmann, K; Schulze, L; Röchling, A; Lübbers, DW. (2000). The
transepidermal oxygen flux from the environment is in balance with the capillary oxygen
supply. J. Invest. Dermatol., 114(3), 533-40.
Sun, Y. (1990). Free radicals, antioxidant enzymes and carcinogenesis. Free Rad. Biol. Med.,
Tahara, S; Matsuo, M; Kaneko, T. (2001). Age related changes in oxidative damage to lipids
and DNA in rat skin. Mech. Ageing Dev, 122, 415-426.
Thiele, J. (2001). Oxidative targets in the stratum corneum. A new basis for antioxidative
strategies. Skin Pharmacol. Appl. Skin Physiol, 1, 87-91.
Thiele, J; Barland, CO; Ghadially, R; Elias, P. Permeability and antioxidant barriers in aged
skin. In: Skin aging, Gilchrest, B; Krutmann, J. (Eds.). Springer-Verlag, Berlin, 2006.
Thiele, J; Traber, MG; Packer, L. (1998). Depletion of human stratum corneum vitamin E: an
early and sensitive in vivo marker of UV induced photo-oxidation. J. Invest. Dermatol,
Troll, W; Wiesner, R. (1985). The role of oxygen radicals as a possible mechanism of tumor
promotion. Ann. Rev. Pharmacol. Toxicol, 25, 509–528.
Trush, MA; Kensler, TW. (1991). An overview of the relationship between oxidative stress
and chemical carcinogenesis. Free Rad. Biol. Med., 10, 201–209.
Tyrrell, RM; Pidoux, M. (1986). Endogenous glutathione protects human skin fibroblasts
against the cytotoxic action of UVB, UVA and near-visible radiations. Photochem.
Photobiol, 44, 561-564.
Tyrrell, RM; Pidoux, M: (1988). Correlation between endogenous glutathione content and
sensitivity of cultured human skin cells to radiation at defined wavelengths in the solar
UV range. Photochem. Photobiol, 47, 405-412.
Varani, J; Warner, RL; Gharaee-Kermani, M; Phan, SH; Kang, S; Chung, JH; Wang, ZQ;
Datta, SC; Fisher, GJ; Voorhees, JJ. (2000). Vitamin A antagonizes decreased cell
growth and elevated collagen-degrading matrix metalloproteinases and stimulates
collagen accumulation in naturally aged human skin. J. Invest. Dermatol., 114(3), 480-6.
Vayalil, PK; Elmets, CA; Katiyar, SK. ( 2003). Treatment of green tea polyphenols in
hydrophilic cream prevents UVB-induced oxidation of lipids and proteins, depletion of
antioxidant enzymes and phosphorylation of MAPK proteins in SKH-1 hairless mouse
skin. Carcinogenesis., 24(5), 927-36.
Skin Cancer, Free Radicals and Antioxidants 217
Vercellotti, GM. (1996). A balanced budget-evaluating the iron economy. Clin. Chem., 42,
Vivekananthan, DP; Penn, MS; Sapp, SK; Hsu, A; Topol, EJ. (2003). Use of antioxidant
vitamins for the prevention of cardiovascular disease: meta-analysis of randomised trials.
Lancet, 361, 2017–23.
Wang X, Liang J, Koike T, Sun H, Ichikawa T, Kitajima S, Morimoto M, Shikama H,
Watanabe T, Sasaguri Y, Fan J. (2004). Overexpression of human MMP-12 enhances the
development of inflammatory arthritis in transgenic rabbits. Am. J. Pathol., 165, 1375.
Weber, SU; Thiele, JJ; Cross, CE; Packer, L. (1999). Vitamin C, uric acid, and glutathione
gradients in murine stratum corneum and their susceptibility to ozone exposure. J. Invest.
Dermatol, 113, 1128–1132.
Werninghaus, K; Meydani, M; Bhawan, J; Magolis, R; Blumberg, JB; Gilchrest, BA. (1994).
Evaluation of the photoprotective effect of oral vitamin E supplementation. Arch
Dermatol, 130, 1257–61.
Williams, R; Philpott, MP; Kealey, T. (1993). Metabolism of freshly isolated human hair
follicles capable of hair elongation: a glutaminolytic, aerobic glycolytic tissue. J. Invest.
Dermatol., 100(6), 834-40.
Wittgen, HG; van Kempen, LC. (2007). Reactive oxygen species in melanoma and its
therapeutic implications. Melanoma Res., 17(6), 400-9.
Wolf, C; Steiner, A; Honigsmann, H. (1988). Do oral carotenoids protect human skin against
ultraviolet erythema, psoralen phototoxicity, and ultraviolet-induced DNA-damage? J.
Invest Dermatol, 90, 55–7.
Xanthoudakis, S; Miao, G; Wang, F; Pan, YC; Curran, T. (1992). Redox activation of Fos-
Jun DNA binding activity is mediated by a DNA repair enzyme. EMBO J., 11(9), 3323-
Yang, S. et al. (2004). Expression of Nox4 in osteoclasts. J. Cell. Biochem. 92, 238.
Yarosh, D. UV-DNA repair enzymes and liposomes. In: Clinical reviews in oxidative stress
and aging. Cutler, RG and Rodriguez, H (Eds). World Scientific, 2003.