MECHANISMS OF INDUCTION OF SKIN CANCER BY UV RADIATION

Holly Soehnge, Allal Ouhtit and Honnavara N. Ananthaswamy

Department of Immunology, The University of Texas M D Anderson Cancer Center, 1515 Holcombe Blvd., Box 178, Houston, TX 77030

Received 10/20/97 Accepted 10/24/97

3. INTERACTION OF UV RADIATION WITH THE SKIN The nature of the UV spectrum.

Sunlight is composed of a continuous spectrum of electromagnetic radiation that is divided into three main regions of wavelengths: ultraviolet, visible, and infrared. This spectrum is diagrammed in figure 1. UV radiation comprises the wavelengths from 200-400 nm, the span of wavelengths just shorter than those of visible light (400-700 nm). UV radiation is further divided into three sections, each of which have distinct biological effects: UVA (320-400 nm), UVB (280-320 nm), and UVC (200-280 nm). UVC is effectively blocked from reaching the earth's surface through being absorbed by the ozone layer of the atmosphere, although some accidental exposure occurs from man-made sources, such as germicidal lamps. UVA and UVB radiation both reach the earth's surface in amounts sufficient to have important biological consequences from exposure of the skin and eyes. Wavelengths in the UVB region of the solar spectrum are absorbed into the skin, producing erythema, burns, and eventually skin cancer. Although UVA is the predominant component of solar UV radiation to which we are exposed, it is weakly cracinogenic. Recent studies have demonstrated that wavelengths in the UVA region not only cause aging and wrinkling of the skin, but they have also been shown to cause skin cancer in animals when given in high doses over a long period of time (6, 7). Interestingly, UVA radiation has been shown to be involved in the development of melanoma in fish (8, 9).

Figure 1. The ultraviolet (UV) component of the electromagnetic spectrum.

3.2. UV radiation causes DNA damage and mutations.

UV radiation is absorbed by DNA maximally from 245-290 nm (10); UV is able to create mutagenic photoproducts or lesions in DNA between adjacent pyrimidines in the form of dimers. These dimers are of two main types: cyclobutane dimers between adjacent thymine or cytosine residues, and pyrimidine (6-4) pyrimidone photoproducts between adjacent pyrimidine residues. Cyclobutane dimers are formed between the C-4 and C-5 carbon atoms of any two adjacent pyrimidines; the double bonds become saturated to produce a four-membered ring (3). Similarly, (6-4) photoproducts are formed between the 5-prime 6 position and the 3-prime 4 position of two adjacent pyrimidines, most often between T-C and C-C residues (10). Cyclobutane dimers are produced overall three times as often as (6-4) photoproducts, although the ratio depends on the DNA sequence and the chromatin environment (10). Both lesions occur most frequently in runs of tandem pyrimidine residues, which are known as "hot spots" of UV-induced mutations (3). Although both lesions are potentially mutagenic, the cyclobutane dimer is believed to be the major contributor to mutations in mammals (10); the (6-4) photoproducts are repaired much more quickly in mammalian cells (11).

In addition to their mutagenic properties, UV-induced pyrimidine dimers may interfere with other important processes of cell cycle regulation involving DNA. Recently, it has been shown that T-T cyclobutane dimers in promoter sequences can strongly inhibit transcription factor binding (12). In addition, pyrimidine dimers can block transcription elongation when present on the transcribed strand (13), and transcribed sequences, especially of active genes, are repaired more rapidly, while promoter sequences are repaired much more slowly (14, 15). UV-induced photodamage to DNA may therefore be an important source of inhibition of transcription factor binding, which could contribute to its carcinogenic effects. Other photoproducts in addition to cyclobutane dimers and (6-4) photoproducts are produced by UV radiation, but these are produced at much lower frequencies and represent less than one percent of all UV-induced photodamage (3). These lesions include pyrimidine monoadducts, purine dimers and an adjacent A-T photoproduct (10). Little is known about the mutagenicity of these lesions (10).

If not repaired, UV-induced DNA lesions can lead to permanent mutations in the DNA sequence. These mutations are in the form of CT and CCTT transitions, known as UV "signature" mutations. Various deletions, insertions, and multiple base changes also can occur. The "A rule" has been proposed to explain how UV signature mutations arise from the DNA lesions (16). According to the A rule, the DNA polymerase places A residues by default where the correct base(s) is not indicated. A mutation is then created upon DNA replication of the strands containing base pair changes. The T-T cyclobutane dimers should not result in mutations; because A normally is paired with T, no mutation results from insertion of A residues by default opposite the dimer. In the C-C cyclobutane dimer, a CCTT transition occurs; two A residues are placed opposite the dimer by default in the place of two G residues. In (6-4) photoproducts between a pyrimidine and a C residue, the 5-prime residue base pairs correctly, but the 3-prime C residue resembles a non-instructional site (3). A CT mutation occurs because an A residue is placed opposite the C residue by default.

3.3. Mutations can lead to loss of cell cycle control and carcinogenesis

UV radiation is a complete carcinogen; it can act alone as both initiator and promoter in carcinogenesis. UV can also act as a promoter with initiating events inside the cell, such as DNA mutations arising from DNA polymerase incorporation errors, depurination, deamination of 5-methylcytosine, or oxidative damage from free radicals (3). UVA, while a complete although far less efficient carcinogen, can act as an initiator with UVB as promoter in skin carcinogenesis; this was first suggested in studies in which mice were irradiated with combinations of UVA and UVB (17, 18). UVA alone was relatively ineffective as a carcinogen, but it increased the carcinogenic effects of UVB when mice were irradiated with both. UVA also increased the carcinogenic effect of UVB when mice were exposed to it several months after the initial UVB exposure (19, 20); these studies were meant to be analogous to the popular tanning habit in which sunbathing was followed by UVA solaria treatments. Recently, UVA was found to have less of an impact on the cell cycle than UVB radiation in epidermal cells of exposed mice (21), further suggesting differences in carcinogenic effectiveness between UVA and UVB.

Mutations in genomic DNA can lead to carcinogenesis through changes in the function of genes that influence cell growth. The complex series of events in carcinogenesis involves three stages: initiation, promotion, and progression (3). Mutations in the DNA may act as initiating events. These may remain dormant for a number of years, until exposure to a promoting agent occurs. Promoters may or may not be carcinogenic themselves, but can act with the initiating events to cause progression into tumor development. Genes can cooperate to effect carcinogenesis, in that multiple mutations at different loci are required. It has been estimated that between three and seven mutational events are required to transform normal cells into cancer cells, depending on the life span of the cell (3). The transforming mutations are usually in tumor suppressor genes or oncogenes, or other genes that are involved in the regulation of cell proliferation.

3.4. DNA repair and apoptosis are defenses against carcinogenesis

The cells of the skin contain protective mechanisms to prevent DNA damage from UV and other sources from resulting in tumor formation. One of these mechanisms is growth arrest followed by DNA repair, and the other is cell death by apoptosis. Both of these mechanisms prevent the transmission of mutations to daughter cells that can lead to transformation and carcinogenesis. Failure of these pathways can result in abnormal cell proliferation. DNA repair processes are very important in the prevention of skin carcinogenesis. This importance is evidenced by the extreme susceptibility of patients with genetic diseases impairing DNA repair processes to skin cancer development. In XP, an autosomal recessive disorder caused by defects in DNA repair and synthesis, the thymine dimers occurring in DNA from UV exposure of the skin fail to be repaired (29). These defects cause an extreme sensitivity to UV radiation and a high risk of development of non-melanoma skin cancers (3). Similar UV sensitivity and skin cancer risk is seen in Cockayne Syndrome, in which nucleotide excision repair is impaired (30).

Upon DNA damage by UV or another agent, the cell cycle may be arrested at at least two checkpoints: at the G1/S phase before DNA replication, or at the G2/M phase before chromosome segregation (22); UV radiation is known to influence the activities of genes active in cell cycle control and growth arrest (23-27). Upon arrest, DNA repair pathways are activated which repair the damage and return the cell to the normal state. If the damage is too severe and cannot be repaired, apoptotic pathways are activated which kill the cell and prevent the formation of a tumor cell population by loss of cell cycle control. DNA damage induced by UVB has been shown to induce apoptosis in human keratinocytes; recently, it has been shown that high doses of UVB irradiation causes the cells to undergo apoptosis, where low doses result in DNA repair (28). Intermediate doses caused a mixture of cells undergoing apoptosis or repair. These observations correlated with differences in the perinuclear versus nuclear localization of the p53 protein, suggesting that p53 plays a role in both repair of UV-damaged DNA and induction of apoptosis.