Wavelengths of both ultraviolet A (UVA 320-400nm) and ultraviolet B (UVB 280-320nm) radiation have been implicated as carcinogens (cancer causing agents), though their methods of action are distinct. The two wavelengths of radiation are able to penetrate to different depths of the skin and hence affect different cells in the epidermis and dermis: UVB radiation is mainly absorbed by epidermal components such as proteins or deoxyribonucleic acid (DNA), whereas UVA radiation penetrates deeply into the skin and reaches the lower epidermis and dermal fibroblasts. The extent of their effect also varies, with UVB being described as the most carcinogenic among all types of solar radiation. UVB radiation’s main deleterious effect is DNA damage caused by its direct interaction with the molecule, while UVA radiation’s toxicity mainly comes from oxidative damage to skin cell components.
DNA lesions or photoproducts
UVB radiation directly damages the genetic information (DNA) within skin cells, causing specific DNA lesions or DNA photoproducts. If not repaired properly, these lesions can lead to the development of skin cancer.
DNA encodes the genetic information which provides instructions for the structure and function of living organisms. Within a DNA molecule are separate sections known as genes, the units of heredity, which each confer a trait or set of traits and determine how the cells operate. Cancer cells contain genetic abnormalities in DNA, which affect their growth, replication and ability to survive and invade surrounding tissue.
DNA is a helical molecule composed of two strands; it resembles a ladder twisted into a spiral. The sides of the ladder are composed of sugar and phosphate and are described as the “backbone”. The rungs of the ladder are made up of four nitrogenous bases: the purines, adenine (A) and guanine (G), and the pyrimidines, thymine (T) and cytosine (C).
Damage occurs when the chemical bonds within a DNA molecule are altered. A photon of UVB radiation penetrates the cell and is absorbed by the bond between the bases which it then breaks. The unbonded base then interacts with adjacent bases on the same DNA strand to create new bonds and form dimers, a type of molecular lesion.
The effects of UVB on DNA are mostly caused by the formation of these dimers (photoproducts) between two adjacent pyrimidines (cytosine and/or thymine) on the same DNA strand. The two major DNA lesions induced by UVB radiation are CPDs (cyclobutane pyrimidine dimers) and 6-4PPs (pyrimidine (6-4) pyrimidone photoproducts).
Figure 3. The most common DNA photoproduct, CPD, resulting from the formation of bonds between two neightbouring pyrimidine bases on the same DNA strand.
These reactions occur hundreds of times within seconds of exposing skin to sunlight (approximately 80,000 dimers per cell within one hour of sunlight exposure), though most of this damage is temporary. The human body has inbuilt systems of DNA repair, known as “nucleotide excision repair” (NER) and “base excision repair” (BER), to recognise and eliminate such changes. Almost immediately, the section surrounding the flawed segment is excised and replaced by the correct sequence.
Occasionally such errors are left unrepaired or incorrectly repaired and the damage becomes permanent, this is called a mutation. Sometimes the change is benign and the sequence can still be read correctly and the cell functions properly. On occasion, the lesions within the DNA interfere with the proper translation of a gene, altering or impairing its function. Accumulation of mutations in key genes due to chronic exposure to UV can lead to the development of skin cancer. It is these UV light-induced genetic changes or lesions, also called UV signature mutations, which can cause skin cells to become cancerous.
Genes involved in skin cancer development
Cancer is multifactorial, but there are three main kinds of genes that when affected by mutations can induce cancer cell transformation: oncogenes, tumour suppressor genes and DNA repair genes. Oncogenes are growth regulators of normal cell division. When oncogenes are activated by a mutation, they enable the cancerous cells to grow or replicate excessively. Oncogenes can also give cancer cells the ability to invade new tissue where normal cells would face biological obstacles to crossing these margins. Alternatively, they enable the cells to evade apoptosis (programmed cell death) which would normally occur if they were damaged. Tumour suppressor genes are negative regulators of the cell cycle that maintain normal growth control. Tumour suppressor genes tend to be down regulated and/or inactivated by mutations that may induce cancer transformation.
It is suggested that a series of genetic changes need to occur for a healthy skin cell to be converted into a cancerous one which can grow vertically beyond the epidermis. Initially, a cell mass must proliferate from a daughter cell. Secondly, normal epidermal cells have a limited number of divisions (this phenomenon is known as cell senescence), so there needs to be a change which enables the melanocyte or keratinocyte to divide infinitely, as cancer cells do. Finally, when a normal cell is damaged or mutated it is usually programmed to die (a process called apoptosis) in order to prevent this damage from being replicated. For an invasive skin cancer to form, a genetic change must also occur which allows the skin cells to evade apoptosis and survive despite their genetic damage. A plethora of genes and their products are involved in each of these biological pathways, some are better understood than others.
The DNA mutations resulting from unrepaired or misrepaired pyrimidine dimers frequently arise in the p53 tumor suppressor gene in skin cancers and impede its protective function. The protein produced from the p53 gene in a healthy cell pauses the cell cycle so that DNA damage can be repaired prior to the cell’s replication. Failing this, it has an important role in the pathway leading to apoptosis. UV-specific p53 mutations have been reported in 50% of human basal cell carcinoma (BCC) and in over 90% of squamous cell carcinoma (SCC) (the most common types of skin cancer), making them the mutations most frequently found in skin cancer patients.
UV mutations in the PTCH gene have been identified as an important player in BCC development. Allegedly a tumour suppressor, the PTCH gene regulates basal cell propagation, so mutations within this gene usually produce uncontrolled growth of the keratinocytes.
Photoproducts resulting from UV signature mutations are rare in melanoma patients, suggesting that there are other forms of UV damage incurred by skin cells which can contribute to the development of malignant tumours. As well as the primary DNA damage, UV irradiation may also induce oxidative damage or ‘oxidative stress’.
Oxidation is a chemical reaction between oxygen (O2) and other molecules. It is a natural process which occurs constantly in the human body, particularly in mitochondrial cellular respiration which produces energy from the food we eat. Oxidation reactions in cells produce low levels of a variety of reactive oxidants called reactive oxygen species (ROS). ROS may include superoxide anion radicals, hydrogen peroxide (H2O2), hydroxyl radicals or hypochlorous acid (HOCl). In addition to their intracellular generation through cellular respiration, ROS can also be triggered by a number of external agents such as environmental pollutants or UV radiation. While low levels of ROS have physiological functions, high levels are toxic for cells. Increased levels of ROS are reduced by cellular antioxidants such as glutathione, vitamin E, vitamin C and specific enzymes. Antioxidants are molecules which neutralize and stabilize the ROS, protecting cells against these highly reactive species. An accumulation of ROS can damage lipids, proteins and DNA within cells by altering their chemical structure. Disruption of the balance between ROS generation and the cellular antioxidant defenses create what is called ‘oxidative stress’ and causes cellular damage.
UVA radiation generates ROS which induce DNA and cellular damage by an indirect mechanism.There is also evidence that UVB radiation may induce some oxidative damage to cells, in addition to its direct damaging effect on DNA. This occurs by activation of a photosensitizer (e.g. porphyrins, riboflavin, quinones), a substance within the cell which absorbs UV radiation and instigates a reaction which then generates the ROS. The resulting damage may include breaks in DNA strands and binding of proteins to the DNA. Oxidation of components of the DNA - especially the nitrogenous bases - can also create mutations; one example is the 8 oxo-deoxyguanosine (8-oxo-dG) lesion formed in guanine bases which is involved in the carcinogenic process.
This type of DNA damage, following the action of the intermediate ROS, is termed ‘indirect’, whereas mutations resulting from the action of UV radiation within the DNA molecule itself are called ‘direct’ DNA damage. ROS may also activate elements within cells called transcription factors which can promote cell proliferation and/or cell death.
There is some evidence to suggest that oxidative DNA damage, caused by UV radiation, may be responsible for mutations in genes which could contribute to melanoma formation. Two such genes are the BRAF oncogene and the CDKN2A gene which encodes a tumour suppressing protein. The latter pathway is vital for cell senescence; this pathway is inactivated in over 80% of melanomas. Mutations in the BRAF gene activate it, allowing the affected melanocytes to multiply uninhibited; this gene has been shown to be mutated in approximately 70% of all malignant melanoma cases.
UVB vs. UVA radiation
UVB radiation has several effects at a molecular level which, combined, can contribute to skin cancer development. Along with the production of photoproducts (CPDs and 6-4PPs) and suppressing the immune reaction, UVB may increase the expression of certain genes through signaling within the cell; this may lead to tumour promotion. On the other hand, current knowledge suggests that UVA radiation primarily damages cells through oxidative stress.
Several studies indicate that both UVB and UVA can impair a person’s immunity, decreasing the level of immune surveillance for malignant cells and altering the cell’s capacity to repair damaged DNA. UV-induced immunosupression can be localised, by the inhibition of Langerhans cells that usually recognise abnormal cells within the epidermis. Immunosupression can also be systemic; immunosuppressive mediators (such as cytokines) are released by keratinocytes, they enter the circulation and affect the whole body. Such ineffectiveness may allow for skin cancer to develop. It is thought that this hindrance may be triggered by prior DNA damage.
Some chemicals may intensify the impairments caused by UV light by binding to the molecular lesions in DNA, further distorting its structure and increasing the cell’s carcinogenic potential. Chemicals which can form these bulky adducts include: N-acetoxy-N-acteyl aminofluorene (AAAF), benzo(a)pyrene, aflatoxin, photoactivated psoralens and cis-platinum.
Some drugs can interact with UV radiation and visible light to trigger skin damage, or photosensitivity reactions. These drugs are commonly called photoreactive or photosensiting drugs and include drugs such as antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs), diuretics, neuroleptics or psoralens. In general, photoreactive drugs have the ability to damage DNA and cellular components via generation of ROS and oxidative stress. In the case of psoralen-induced phototoxicity, the drug interacts directly with DNA, binding it irreversibly and creating adducts which alter the molecule's shape.
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