Chapter 2

Ultraviolet radiation : UVA, UVB and UVC

So far in this description I have talked about ultraviolet radiation (UVR) as though it was a homogenous entity. To understand more we now need to refine further our concept of UVR. You will be pleased to note that this does not involve any equations.

Ultraviolet radiation (UVR) is defined as electromagnetic radiation between 400nm and 100nm, but radiation below ~ 290nm is not present in sunlight as it is blocked by the atmosphere. Radiation greater than 400nm ( up to 800nm) is visible radiation (‘light’). For example radiation at 420nm will be visible and appear violet [1], whereas radiation at 360 will be invisible to the human eye [2].

In assessing the physical or chemical (and hence biological activity) of any ultraviolet radiation we need to take note of not only the amount which is measured as a unit of energy Joules (J) or milliJoules (mJ), but also the exact wavelengths. Different wavelengths of ultraviolet radiation have profoundly different biological activities. For instance the biological activity of 1 Joule of ultraviolet radiation at 350nm is wildly different from the biological activity of 1 Joule at 300nm.

One simple way to classify ultraviolet radiation is to split the UVR component of electromagnetic radiation by wavelength into three further subtypes:

This classification is widely used and it is useful,but remember it is slightly artificial. Biological and physical activity is a continuous function of wavelength, and wavebands of (for example) 325 and 365nm are very different in biological activity despite both being classified as UVA. This apparent subtlety will be crucial when we come to discuss the cutaneous response to ultraviolet radiation.

UVR causes erythema because ultraviolet radiation causes DNA damage

Figure 2.1 shows the authors back 24 hours after exposure to a dose of UVB radiation. The striking physical sign is the erythema (redness) due to an increase in cutaneous blood flow. Note that the reaction is a direct one in that it is precisely limited to those areas exposed to the ultraviolet radiation ( lettering templates ‘The Importance of….’ shielded the pale areas). The obvious questions to ask are why, and how does UVR cause erythema, and what is its health significance? And if I ask you to accept on trust a fact I will later justify, why is it that those who develop the most erythema in response to ultraviolet radiation are most at risk of skin cancer?

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Figure 2.2 shows the ability of ultraviolet radiation of various wavelengths to induce erythema (redness) and DNA damage in human skin.The ‘X’ axis refers to the wavelength of ultraviolet radiation and the ‘Y’ axis the amount of redness or DNA damage. Note the the ‘Y’ axis is on a logarithmic scale. This sort of graph is known as an action spectrum and illustrates that to produce the same biological effect as a shorter wavelength, the dose of longer wavelength UVR may need to be at least a thousand times greater or more. The moral is that you cannot define the action of UVR in terms of dose alone (i.e. joules), rather you have in addition to dose to describe which wavelengths are being considered. The second point to note is that the action spectrum for erythema and DNA damage are very similar over several orders of magnitude. This provides our first clue that DNA damage is the cause of erythema— but as so often in clinical science it is a disease that provided the critical insight.

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Xeroderma pigmentosum (XP): inability to repair DNA damage leads to
erythema (and cancer)

It is now commonplace to view cancer (whether of the skin or other organs) as being a genetic disease — that is to say that cancers arise in part due to the accumulation of mutations in cellular DNA. It was not always been so obvious and perhaps the disease that has provided the most insight into this process is that of xeroderma pigmentosum (XP).

Although xeroderma pigmentosum is very rare (1:500,000) it is unforgettable once you have seen a single case. Xeroderma pigmentosum is inherited as an autosomal recessive and usually presents in childhood with gross hypersensitivity to sunshine (photosensitivity). Many such children will develop erythema (sunburn) with minimal amounts of ultraviolet radiation and will frequently go on to develop blisters. Others do not show erythema but develop many freckles at an early age [3] (Figure 2.3). These children are especially prone to develop a range of skin cancers including malignant melanoma and a range of non-melanoma skin cancers that may curtail their life.

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If we were to expose patients with xeroderma pigmentosum to small doses of UVR we can easily demonstrate their sensitivity to UVR. Many develop erythema after doses far too small to affect normal individuals, but they are not sensitive to the visible electromagnetic radiation in sunshine but only the UVR component [4]. We know that this defect is a cellular one because if we culture skin cells from such individuals we can show that they too are abnormally sensitive to UVR. If we directly measure DNA damage in these cells we see it is abnormally high, and that is because the normal process of DNA repair after UVR exposure is retarded. This defect in DNA repair is limited to damage induced by UVR — the cells are able to repair damage from other genotoxic insults such as X-irradiation. Finally, if we fuse XP cells in culture with cells that are known to have normal DNA repair systems, the sensitivity to UVR is corrected in the ‘XP’ cells.

This disease and the correlation between redness and DNA damage shown in figure X allows us to state that UVR is a cause of DNA damage, that erythema is a reflection of DNA damage and that unrepaired DNA damage in the long run is associated with skin cancer. Sunburn diminishes over time as your cells repair the damage they have suffered. Unfortunately, even in normal individuals, some damage remains unrepaired, as the repair processes are not perfect. This means that we can view the acute response of erythema as a proxy or intermediate measure for the chronic consequence, cancer. Put another way erythema is a biomarker for skin cancer and, as we will learn later, there is a body of epidemiological and clinical evidence that consolidates this relation with a history of erythema being a marker of risk for subsequent skin cancer

What part of the UVR in sunshine causes erythema?

Before leaving the subject of the relation between UVR and erythema there is a further point that needs consideration. In earlier sections it was pointed out that the action spectrum for erythema showed that shorter wavelengths were more potent at inducing erythema. What happens in natural sunshine? Is the erythema due to shorter wavelength in the UVB range or other longer wavelengths?

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Figure 2.4 shows both the action spectrum for erythema (as we have seen before) and the amount of ultraviolet radiation by wavelength falling on the earths surface again measured on a logarithmic scale (‘Y’ axis). What is immediately apparent is that most of the UVR in sunlight is in the UVA range with only a small component falling within the UVB range (note once again that the scale is logarithmic). Can we therefore assume that a sunburn due to natural sunshine is mainly due to UVA? No, we cannot. Indeed given how much more potent the shorter wavelengths are at inducing erythema we can say that most (~90-95%) of the sunburn is due to the UVB component of natural sunlight even though on a joule to joule basis the UVA comprises most of the incident radiation.

From erythema to tanning

So far we have explained using clinical examples the relation between UVR and erythema. UVR causes DNA damage, unrepaired DNA damage is associated with erythema, and in the long term DNA damage from UVR leads to skin cancer. There are however a series of other events that occur following exposure to sunshine (UVR) that most of us can recognise from sunny holidays such as peeling, and tanning. How are these changes mediated and what is their function?

Skin treats UVR exposure as a form of damage, and seeks both to repair the current damage and to minimise the chances of damage from future exposure to UVR. Skin therefore adapts to UVR — a process called photoadaptation. The response to UVR shares some features with the skin’s response to other noxious stimuli but also appears to a large extent distinct.

UVR causes both direct damage to DNA and indirect damage to DNA via oxidative products and free radical formation. Direct damage in this context means UVR photons alter the chemical structure of DNA bases directly, whereas indirect damage is via other molecular intermediaries. If the DNA damage is not repaired the nucleotide sequence is not faithfully transmitted at mitosis (i.e. a mutation has occurred).

It makes biological sense for cells that have damaged DNA not to progress through mitosis as otherwise these mutations will be transmitted to daughter cells. So, within hours of exposure to UVR the cells in the proliferative compartment of skin (basal cell layer) show a reduced number of mitoses: the principle is that time is required for the DNA damage to be repaired before going through the cell cycle. During this period there is also a reduction in general transcription and protein production. As DNA repair is completed there follows a reversal of all these parameters with an increase in cell proliferation, transcription and protein biosynthesis during the period of 24-72 hours after UVR exposure. This wave of keratinocyte proliferation leads to a thickening of the skin, and is accompanied by an increase in melanin pigmentation.

The thickening of the epidermis is adaptive because with each layer of cells above the proliferative compartment (whether dead or alive), there is an additional layer of protein that protects cells in the proliferative compartment. What is important to realise here is that all the cells above the proliferative compartment are terminally differentiated and incapable of becoming neoplastic or further cell cycling. Their role in skin can therefore be considered secondary to preserving the genetic integrity of cells in the basal layer or in any stem cell niches. Whilst this thickening of skin is not noticeable clinically, small increases in thickness of the epidermis have a disproportionate effect because protection from thickening is an exponential function of thickness [1].

As well as a wave of proliferation, there is also a wave of cell death starting earlier than the usual granular layer (i.e. above the proliferative wave). Some of this is due to the fact that if cellular damage is such that repair is not possible, then cells undergo programmed cell death. Some cells are visible in skin with a characteristic apoptotic morphology and are termed appropriately enough sunburn cells. However other forms of cell death may also contribute to the wave of death that leads to the premature stratum corneum formation. This wave of death is clinically reflected by the peeling of the upper cell layers that is common several days after sunburn.

Inflammation and tanning

Although, the thickening of skin after UVR is not not visible to the naked eye, other changes are very apparent. The first of these is of course erythema due to vasodilation. In cases of severe UVR exposure, there may be swelling and even blister formation. Blister formation occurs when the cellular insults are so great that the majority of keratinocytes die and lose their desmosomal attachment to each other with the result that gaps open up between cells in the epidermis that are then filled with fluid [2].

Erythema is not present immediately after UVR irradiation but peaks between 8 and 24 hours. With severe sunburn some residual erythema may last over a week. Experimentally it is easy to demonstrate an increase in cutaneous blood flow even when it is not visible to the naked eye. This means that the term ‘sunburn’ is very imprecise as it imposes a binary classification on a continuous state, a state that we might not even be aware of.

Some, but not all of the causal pathways linking UVR and erythema are known. UVR activates a cascade of signalling pathways including the production of some prostaglandins (PGE2), a fact demonstrated by the way drugs such as indomethacin applied topically can partially inhibit UVR erythema. The final vascular response we know is mediated by nitric oxide as inhibiters injected locally can completely abolish the UVR induced erythema.

Erythema is not the only colour change that occurs following irradiation. Melanocytes respond to UVR by increasing the amount of melanin produced and throughout the epidermis there appears to be some spatial rearrangement of melanin in the skin. There are a couple of important points to be made about this process of tanning.

Melanin as has been described earlier is photoprotective, that is it seems to function as a natural sunscreen. Its peak absorption is in the shorter wavelengths and diminishes as we progress through the UVA wavelengths and into the visible spectrum. The ability of melanin to protect against UVR is however influenced not just by the chemical make up of melanin (remembering that melanin describes a group of related polymers rather than a single chemical entity) but by the way it is bundled into melanosomes, how these melanosome are shaped, and how these melanosome are distributed in the cell. This is in part because scattering of UVR depends on the size of particles [3] .

Skin colour changes due to melanin begin hours after exposure and after a single exposure to UVR peaks at one week and may persist for several months [4]. These colour changes reflect an increase in melanin biosynthesis, possibly in the relative amounts of different melanins and the way melanin is packaged throughout the epidermis.

Humans vary in the amount of constitutive pigmentation and these differences reflect not just the amounts of melanin, but the types of melanins, and the way the melanin is packaged into melanosomes. There is a clear relation between the constitutive pigmentation and how the skin responds to UVR. In in explaining why people with different skin colours have different skin cancer rates this relation is important to understand.

Variation in pigmentation and variation in susceptibility to UVR.

Since melanin acts as a sunblock, persons with more of it would be expected to show fewer signs of damage following UVR exposure. This is indeed what we observe. If we irradiate persons with very dark skin (say from equatorial Africa) and people with very pale skin as are traditionally found in Scotland with a standard dose of UVR very different responses will be seen. Those with pale skin will be perhaps 100 times more sensitive in terms of erythema, swelling and blister formation. This simply reflects the fact that the darker skin blocks more of the harmful UVR photons before they can inflict the cellular damage that leads to the inflammatory response.

However, now imagine that we were comparing individuals from Northern Europe with those from Southern Europe. Again, we would see differences, with those with darker skin being more resistant to UVR. However, if we increase the dose of UVR such that we produce the same degree of erythema in all subjects we would find that those with darker skin are able to tan better—that is they are able to increase from their pigmentation level by a larger degree than those with paler skin. This is a difficult concept to grasp and an extreme example may help.

Imagine a red headed person with pale skin. However much they are exposed to UVR it is unlikely that they will be able to increase their pigmentation by very much. However long they spend laying on the beach the upper limit of their melanin production is set very low. By contrast a person with olive skin may be quite resistant to sunburn but if they do get sunburn they are able to tan to a far greater degree than the pale skinned Scottish adult. Those with high levels of constitutive pigment are therefore not only more resistant to the harmful effects of UVR but also better able to adapt to future UVR exposure.

The sun sensitive phenotype

Skin colour is a very good predictor of sensitivity to UVR. Those with darker skin are much more resistant to UVR. Even however within pale skinned Northern Europeans we see differences in sun sensitivity. Those with red hair for instance tend to be the most sensitive and, as a result of exposure to UVR tend to develop large numbers of freckles (Figure 2.5). As we have will see in later chapters freckles are a reflection of UVR damage and a marker of the interaction between sensitivity and sun exposure. As one would imagine, they are therefore biomarkers of cancer risk.

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Why do I still burn at the end of the summer holiday?

Above, I have outlined two mechanisms by which skin is able to (partially) adapt to UVR— skin thickening and pigmentation. We have also referred in earlier sections to the importance of DNA repair in repairing the DNA damage that UVR causes. So why do so many holiday makers return from trips to the sun with sunburn?

Simply because all these adaptive processes have quantitative limits, and also take time to reach their maximum. Earlier, I used the example of the red-headed pale-skinned person. This individual is not only sensitive at the beginning of the holiday but is incapable of increasing melanin pigmentation to all but a minor degree. They can increase their skin thickening but there are quantitative limits to this. If the exposure is increased beyond this mild increase in protection, burning results.

Are all tans equal?

The above makes clear that there are two methods of photoprotection relevant in adapting to UVR; an increase in skin thickness which is invisible to the naked eye, and an increase in melanin pigmentation (including perhaps changes in the relevant amounts of the various melanin polymers and their distribution in the skin).

At the risk of offending the common sense of some readers, fake tans do not play any photoprotective role—they can be viewed as either paints that colour skin or chemicals that alter the colour of endogenous substances such as keratins. ‘Tans’ of this sort do not confer photoprotection. (Figure 2.6). (Of course if a ‘fake tan’ contains sunscreens then the picture is more complicated, as by definition, sunscreens ‘screen out’ UVR photons.)

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There is another uncertainty about different sorts of tans which is harder to answer, and that is whether a tan produced by natural sunshine is different from the tan produced by artificial UVR devices such as commercial sunbeds. There are two things to consider here.

First, for a given person, the action spectrum for tanning [5] appears to follow the action spectrum for erythema except that there is some evidence that longer wavelength UVR (UVA) in some more pigmented people may induce more tanning than would be expected on the basis of erythema alone. Second, it appears that shorter wavelength UVR (UVB) is better at producing skin thickening. It is therefore possible to imagine that different spectral output might produce the same tan but alter skin thickness differently. It may well be that not all UVR induced tans are equal.

Chronic skin changes due to UVR

All the processes described so far are short term. Study of the changes after UVR irradiation over the time course of days or weeks is relatively easy. At the same time simple observation shows that the skin of older people differs from that of younger people and that the greatest differences are between the body sites that are continually exposed and those that are are sun protected. Many of the changes with age in skin are therefore not the result of age per se but reflect to a large degree the fact that older people have had more sun exposure than young people (‘age is a proxy for sun exposure’). The prediction from this is that we would expect to see UVR induced chronic changes at an earlier chronological age in persons with the greatest sun exposure. This is indeed what we do see. Most of the changes we call ‘skin ageing’ we see in fair skinned persons living in southern Africa or some parts of Australia at a much younger age than in similar individuals in Northern Europe.

Types of changes seen in ageing skin.

At a purely clinical level we can usefully describe the changes that occur with chronic UVR
exposure into those that are

Such changes are called photoageing.

Focal lesions would include many benign and malignant neoplasms. Many of these lesions reflect proliferation of clones of cells that have accumulated genetic change. In large part this is what this book is about.

For the generalised changes, think of the changes and loss of collagen that are seen on areas of skin that have received lots of UVR (Figure X). The exact mechanism underpinning the changes in the dermal connective tissue are not precisely understood, but UVR alters the ratio of the generation of new connective tissue to the remodelling of connective tissue that is occurring all the time in normal skin. UVR exposure both alters the crosslinking of collagen molecules as well as the remodelling. At a clinical level we see a loss of tissue of the dermis (‘thin skin’), and a loss of the normal suppleness and elasticity of the dermal compartment seen in young people. Pinch the skin over the back of the hand in individuals of differing ages and notice the difference. (Figure 2.7).

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