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cosmetics
Review
Oxidative Stress and Ageing: The Influence of
Environmental Pollution, Sunlight and Diet on Skin
Khimara Naidoo and Mark A. Birch-Machin *
Dermatological Sciences, Institute of Cellular Medicine, Medical School, Newcastle University,
Newcastle Upon Tyne NE2 4HH, UK; Khimara.Naidoo@ncl.ac.uk
*Correspondence: mark.birch-machin@ncl.ac.uk; Tel.: +44-191-208-5841
Academic Editor: Johanna Maria Gillbro
Received: 25 November 2016; Accepted: 3 January 2017; Published: 10 January 2017
Abstract:
Skin ageing is a complex process that is determined by both intrinsic and extrinsic factors,
which leads to a progressive loss of structure and function. There is extensive evidence indicating
that oxidative stress induced by reactive oxygen species plays an important role in the process of
human skin ageing. Mitochondria are the major source of cellular oxidative stress and are widely
implicated in cutaneous ageing. Extrinsic skin ageing is driven to a large extent by environmental
factors and external stressors such as ultraviolet radiation (UVR), pollution and lifestyle factors which
have been shown to stimulate the production of reactive oxygen species and generate oxidative stress.
The oxidative damage from these exogenous sources can impair skin structure and function, leading
to the phenotypic features of extrinsic skin ageing. The following review highlights the current
evidence surrounding the role of mitochondria and oxidative stress in the ageing process and the
influence of environmental factors such as ultraviolet radiation, pollution and diet on skin ageing.
Keywords:
skin ageing; reactive oxygen species; ROS; oxidative stress; pollution; UVR; sunlight; diet
1. Introduction
Ageing is marked by a progressive functional decline of an organism over time, which leads
to increased susceptibility to age-related diseases, and eventually the death of an organism [
1
].
Skin ageing is a complex process that is determined by both intrinsic and extrinsic factors, which leads
to a progressive loss of structural integrity and physiological function. Intrinsic ageing is largely
genetically determined and occurs as a natural consequence of physiological changes over time [
2
].
Intrinsic ageing is clinically characterised by skin atrophy, prominence of vasculature, loss of elasticity
and fine wrinkles [
3
]. Extrinsic ageing is related to the cumulative effects of environmental factors
such as ultraviolet radiation (UVR), smoking and environmental pollution. Extrinsically aged skin
is characterised by deep wrinkles, rough texture, telengiectasia and irregular pigmentation [
4
].
The severity of extrinsic ageing depends on skin type, with the features being more prominent
in type I or II skin and less noticeable in type III or higher skin [5].
Many studies have been performed to elicit the cause of ageing; however, the exact mechanism
remains unknown. During the past century several theories of ageing have been proposed including the
“wear and tear theory of ageing”, the “antagonistic pleiotropy theory of ageing” and the “disposable
soma theory of ageing”. There have been a large number of studies examining the involvement of
mitochondria in the ageing process. Mitochondria are dynamic double-membrane-bound organelles
found within the cytoplasm of eukaryotic cells. The primary function of mitochondria is the production
of cellular energy, through a process termed oxidative phosphorylation, in the form of adenosine
triphosphate (ATP) [
6
]. Oxygen metabolism in the mitochondria leads to the generation of reactive
oxygen species (ROS). The formation of ROS is a natural consequence of oxygen metabolism and
ROS have integral physiological roles in cell signalling and oxygen homeostasis [
7
]. Mitochondria
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Cosmetics 2017,4, 4 2 of 8
are considered to be the predominant source of intracellular ROS and are believed to contribute 90%
of the ROS generated within the cell [
8
]. In addition to mitochondria, the nicotinamide adenine
dinucleotide phosphate (NADPH) oxidase system has been identified as an important source of
intracellular ROS and a key player in the generation of oxidative stress. NADPH oxidases, also known
as NOX enzymes, act by catalysing the transfer of electrons from NADPH to molecular oxygen to
generate superoxide and other forms of ROS [
9
,
10
]. NOX enzymes can be found in various membranes
including the plasma membrane, the endoplasmic reticulum and the mitochondria [
11
]. They were
initially discovered as the enzyme responsible for producing large amounts of ROS as a first line
of defense against invading pathogens during the immune response; however, it is now thought
that they play roles in other physiological processes including cell signalling [
12
]. Antioxidants are
produced to counteract oxidative stress generated by excess ROS; however, in times of environmental
stress, elevated ROS levels can overwhelm endogenous cellular antioxidant mechanisms [
7
]. This can
lead to an imbalance in tissue oxygen homeostasis, with oxidant effects outweighing antioxidant
effects, and as a consequence the cellular environment becomes oxidatively stressed [
7
]. Free radicals
can cause oxidative damage via a variety of mechanisms. Oxidation of lipids by ROS can damage
cellular structures and result in premature cell death [
13
]. In addition, interaction with nuclear and
mitochondrial nucleic acids results in mutations that predispose them to strand breaks [13].
There is increasing evidence that oxidative stress induced by ROS is a major contributing factor
to the ageing process. The “free radical theory of ageing” proposed by Harman postulated that free
radicals cause cumulative oxidative damage to biological structures, which eventually leads to loss
of cellular function and phenotypic ageing [
14
]. Within this theory Harman included the “vicious
cycle theory of ageing” which summarises how the characteristics of mitochondria can contribute
to ageing [
14
]. Mitochondrial DNA is located in close proximity to the site of ROS production,
making it highly vulnerable to oxidative damage. This is exacerbated by the fact that mitochondria
do not possess efficient DNA repair mechanisms [
13
]. As the integrity of mitochondrial DNA
is essential for mitochondrial function, errors in gene expression may in result in dysfunctional
mitochondrial subunits [
15
]. This leads to a continuous “vicious cycle” where dysfunctional
mitochondria contribute to further ROS production, which can lead to further ROS-mediated oxidative
damage to mitochondria [
8
]. Mitochondrial dysfunction is heavily implicated in the ageing process
and the pathogenesis of age-related diseases such as Alzheimer’s disease (AD) [
16
]. In AD it is believed
that the accumulation of amyloid beta peptide interacts with mitochondria, leading to mitochondrial
dysfunction. The resulting aberrant mitochondrial function leads to increased oxidative stress and
neuronal damage and cognitive decline [
17
]. Recently, a novel concept termed mitohormesis has
been introduced in the field of mitochondria and ageing [
18
,
19
]. Studies have shown that there is a
nonlinear relationship between ROS production and ageing and that different levels of oxidative stress
may have opposite outcomes. Low concentrations have been demonstrated to have a protective effect
by inducing cellular defense mechanisms, whilst higher concentrations promote damage to cellular
structures [
18
]. While there is extensive evidence which links increased levels of oxidative stress with
ageing, mitohormesis introduces the interesting notion that low levels may have the reverse effect and
may actually prevent ageing.
Cellular senescence refers to the irreversible arrest of proliferation, which acts as a
tumour-suppressive mechanism to inhibit cells with potentially cancerous mutations from undergoing
replication [
1
]. The induction of senescence is a stress response which occurs in cells which are
exposed to unfavourable physiological conditions, or those with mutations leading to oncogenic
activation [
13
]. Cells which undergo senescence remain viable but are unable to divide, which is
contrast to biological ageing, where there is progressive decline of an organism which eventually leads
to death [
13
]. Despite these differences between the two processes, there is increasing evidence that
cellular senescence plays a prominent role in ageing [
1
]. Senescent cells accumulate with increasing
age in many organisms including humans and are seen with increased frequency in prematurely
aged skin [
20
]. Furthermore, studies have shown that senescent cells are implicated in the genesis of
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age-related diseases such as AD and atherosclerosis [
1
]. Mitochondrial dysfunction is thought to play
a role in the increased levels of senescent cells seen with advancing age [
21
]. Studies have shown that
mice with knocked-down manganese superoxide, an antioxidant that protects against mitochondrial
oxidative damage, have higher levels of mitochondrial ROS production and an increased number of
senescent cells [
22
]. Interestingly, the mice developed epidermal thinning, an established feature of
aged skin, providing evidence for a causal relationship between mitochondrial dysfunction, cellular
senescence and the phenotypic manifestations of skin ageing [
22
]. Recently, mitochondrial complex
II activity has been implicated in senescence and ageing. Bowman et al. demonstrated that complex
II activity decreases with age in human skin fibroblasts, an effect only observed in senescent cell
populations [
23
]. The authors speculated that the observed decrease in complex II activity contributes
to the ageing process by leading to an increase in ROS which results in mitochondrial dysfunction and
oxidative stress.
2. Environmental Sources of Oxidative Stress
Human skin is continuously exposed to ROS generated from a combination of intrinsic and
extrinsic sources. The skin serves as an interface between the body and the external environment
and is in constant contact with external stressors such as UVR and ambient pollutants. Exposure
to these exogenous factors has been shown to be a major contributing factor to the production of
ROS and the generation of oxidative stress [
24
]. The resultant ROS-mediated oxidative damage from
these sources can impair skin structure and function, leading to the phenotypic features of extrinsic
cutaneous ageing.
2.1. Sunlight
Solar radiation is comprised of UVR, visible light and infrared with relative contributions of
6%, 40% and 54%, respectively. UVR forms can be subdivided into three categories according to
wavelength; UVA (320–400 nm), UVB (280–320 nm) and UVC (100–280 nm) [
7
]. UVR is the primary
environmental factor in the development of extrinsic skin ageing. Excessive exposure to UVR can
result in cellular, genetic and molecular changes in the skin, which can lead to accelerated skin ageing
also known as photoageing. The rate of UVR-induced skin ageing depends on the balance between
the frequency, intensity and duration of exposure and the natural protection by skin pigmentation [
5
].
Both UVA and UVB have been shown to contribute to UVR-induced ageing; however, due to its ability
to penetrate deeper into the dermis, UVA has been shown to cause disproportionately more damage [
5
].
Exposure of skin to UVR is known to stimulate the photochemical generation of ROS, which includes
superoxide anions and hydrogen peroxide [
6
]. Although ROS are continuously produced in the
skin and are involved in physiological processes, there is accumulating evidence for the harmful
effects of high ROS concentrations following exposure to UVR [
5
]. Antioxidants are produced by the
skin to counteract the harmful effects of ROS; however, the higher concentrations of ROS generated
following UVR exposure can induce an imbalance in cellular antioxidant defence systems, leading
to oxidative stress [
6
]. UVB can also be directly absorbed by DNA, leading to direct induction of
damage within cells. Mitochondrial DNA is particularly vulnerable to damage following exposure
to UVR as mitochondria have limited DNA repair mechanisms [
8
]. A recent study looking at the
action spectrum of UVR-induced mitochondrial damage showed that mitochondrial DNA is more
vulnerable to damage at UVR wavelengths >320 nm when compared with nuclear DNA [
25
]. Damage
to mitochondrial DNA can lead to mitochondrial dysfunction which may subsequently increase the
production of ROS. This leads to a continuing vicious cycle whereby the generation of ROS may lead
to further mitochondrial DNA damage [
6
]. This is of particular importance as there is increasing
evidence that alterations in mitochondrial function can adversely impact skin health and lead to skin
disease [
26
]. UVR is therefore able to increase oxidative stress by both direct and indirect means,
either by potent stimulation of ROS or by direct DNA damage [
8
]. This increase in oxidative stress can
impair cellular functions, leading to the phenotypic manifestations of extrinsic skin ageing.
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2.2. Environmental Pollution
Ambient pollution is increasing significantly worldwide and the impact of pollutants on human
health is a growing concern. The majority of ambient pollutants are derived from anthropogenic
sources such as motor vehicle emissions, fossil fuel combustion, forest fires and industrial facilities.
This combination of sources produces a complex mixture of toxic pollutants including particulate
matter and gases such as nitrogen dioxide and ozone [
27
]. Although there is comparatively less research
investigating the cutaneous effects of environmental pollutants, there is growing recognition of the
adverse effects on skin health. Numerous studies have demonstrated that ambient pollutant exposure
is linked with the development of the signs of extrinsic skin ageing [
28
]. A cohort study conducted in
Germany examined the association between ambient pollutant exposure and skin ageing in Caucasian
women. Their results showed that chronic exposure to particulate matter significantly correlated
with signs of extrinsic ageing, in particular pigment spots and more pronounced nasolabial folds [
29
].
The generation of ROS and induction of oxidative stress is implicated as one of the mechanisms
by which particulate matter exert their deleterious effects [
30
]. Particles measuring
<0.1 µm
in
diameter are defined as ultrafine particles and are associated with vehicle exhaust emissions [
31
].
Ultrafine particles are particularly harmful due to their ability to penetrate tissues more easily and
localise in the mitochondria [
32
]. Once absorbed, ultrafine particles are able to induce oxidative
stress and mitochondrial damage [
32
]. Ground-level ozone, a major component of smog, is a
highly reactive environmental pollutant that is capable of inducing oxidative stress in cutaneous
tissues. Ozone mediates its noxious effects through free radical reactions that are achieved either
directly by oxidation of biomolecules to generate ROS or by the production of cytotoxic non-radical
molecules [
33
]. Experimental animal studies have shown that exposure to ozone results in signs
of oxidative stress including significant depletion of cutaneous antioxidants [
34
,
35
]. Furthermore,
it has been demonstrated that topical application of the antioxidants vitamin C and E can prevent the
formation of oxidation products, thus attenuating environmentally induced oxidative damage to the
skin [
36
]. A recent study showed that the application of antioxidant mixtures significantly reduced
ozone-induced oxidative stress in human keratinocytes [
37
]. These findings contribute to the growing
evidence demonstrating that environmental pollution has a detrimental impact on skin health and can
lead to skin ageing [28].
2.3. Diet
Recently there has been considerable interest in the effects of diet on oxidative stress due to the
diet being an important source of exogenously derived antioxidants [
8
]. Strategies such as diet and
exogenous antioxidant supplementation may have a potential role in combating oxidative stress caused
as a result of environmental factors. The highly antioxidant Mediterranean diet, characterised by high
fruit and vegetable intake, has been shown to be associated with increased longevity and reduced
risk of age-related diseases [
38
–
40
]. Studies have demonstrated that adherence to the Mediterranean
diet is significantly associated with lower levels of oxidative stress [
41
,
42
]. Furthermore, it has been
shown that consumption of the Mediterranean diet is able to prevent cellular senescence in human
epithelial cells [
42
]. In contrast, a high-fat diet has been shown to be associated with mitochondrial
dysfunction, increased levels of oxidative stress and accelerated cellular senescence [
43
]. A recent
study showed that insulin resistance secondary to a high-fat diet leads to increased levels of oxidative
stress, which contributes to neurodegeneration in AD [
44
]. Carotenoid substances, found in fruit
and vegetables, as well as vitamins A, C and E, are said to be the most protective and correlate
negatively with levels of oxidative stress [
45
]. A study examining the influence of lifestyle factors on
the level of carotenoid antioxidants beta-carotene and lycopene found that antioxidant levels in the skin
significantly increased with dietary consumption of carotenoid-rich food [
46
]. The results indicated that
dietary supplementation of antioxidants may provide efficient protection against extrinsic skin ageing.
Environmental inducers of ROS, such as UVR exposure, can lead to a reduction in antioxidants
and an increase in levels of oxidative stress [
46
]. There is emerging evidence that antioxidant
Cosmetics 2017,4, 4 5 of 8
supplementation may be able to protect human skin against UVR damage. A study comparing the
effects of mitochondria-localised antioxidants with cellular antioxidants found that tiron, an antioxidant
preferentially localised to the mitochondria, was able to confer complete protection against UVA- and
H
2
O
2
-induced mitochondrial damage [
47
]. Interestingly, the protective effect of tiron was found to be
greater than a range of cellular antioxidants investigated, which included resveratrol and curcumin.
One study showed that lycopene, a carotenoid antioxidant found in red fruit and vegetables, is able
to protect human skin against UVR-induced effects partially mediated by oxidative stress such as
dermal erythema and mitochondrial DNA damage [
48
]. Furthermore, supplementation was associated
with a reduction in UV-induced matrix metalloprotein 1 (MMP-1) expression, a collagenolytic enzyme
which acts as a key regulator in photoageing [
48
]. A recent study demonstrated that oral supplements
containing lycopene-rich tomato nutrient complex and lutein were able to protect human skin against
UVB/A and UVA1 radiation by inhibiting the expression of genes involved in UVR-induced skin
damage [
49
]. Intake of lycopene and lutein was associated with significantly reduced expression of
MMP-1 and heme oxygenase-1, a sensitive marker of oxidative stress [
49
]. Data from these studies
demonstrates that dietary supplementation with carotenoid antioxidants could potentially protect
against the acute and potentially longer-term aspects of photoageing. Despite these scientific studies
providing evidence that antioxidant supplementation could confer significant benefits, some studies
have demonstrated conflicting observations. One study examining life-long supplementation of
vitamin E in mice showed no overall improvement in life span [
50
]. In support of this, a study by
Perez et al. investigated the effect of over- or under-expression of a wide number of genes regulating
antioxidant enzymes and found that only one had an effect on lifespan [
51
]. In addition, clinical data
have failed to demonstrate a beneficial effect with a trial comparing regular sunscreen use with
carotenoid supplementation, showing that beta-carotene had no overall effect on skin ageing [
52
].
A Cochrane review examining the effect of antioxidant supplementation on mortality found that
supplementation with beta-carotene, vitamin E and vitamin A may increase mortality [
53
]. It is clear
from these studies that the link between antioxidant supplementation and ageing is complex and
incompletely understood. Mitohormesis could provide one potential explanation for the observations
discussed above, as according to this hypothesis, moderate amounts of ROS may be physiologically
beneficial whilst excess amounts may have a detrimental effect [
19
,
54
]. Therefore, attempting to
eliminate ROS with antioxidant supplementation might disrupt ROS homeostasis with potentially
harmful side effects. Further studies and trials are required to fully elucidate the balance between ROS
and antioxidants, and how diet and antioxidant supplementation may impact this balance.
3. Conclusions
Extensive evidence indicates that oxidative stress induced by ROS plays an important role in
the process of human skin ageing. Extrinsic skin ageing is driven to a large extent by environmental
factors and external stressors such as UVR environmental pollution, and lifestyle factors have been
shown to stimulate the production of ROS and generate oxidative stress [
24
]. The oxidative damage
from these exogenous sources can impair skin structure and function, leading to the phenotypic
features of extrinsic skin ageing. Many studies have been conducted to elucidate the mechanism of
ageing and from this there is continuing evidence that supports the proposal that mitochondria are
widely implicated in both ageing and senescence. However, the ageing process is not fully understood
and further work is required to understand the molecular processes involved in cutaneous ageing.
This could potentially lead to the development of preventative and therapeutic interventions for
skin ageing.
Author Contributions: Both authors contributed to writing the paper.
Conflicts of Interest: The authors declare no conflict of interest.
Cosmetics 2017,4, 4 6 of 8
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