[Cell Cycle 5:6, 565-566, 16 March 2006]; ©2006 Landes Bioscience
Elimination of Keratinocytes Stagnant in S-Phase Through Epidermal Turnover Instead of In Situ Apoptosis Cutting Edge View
Overexposure of the skin to ultraviolet (UV) radiation causes apoptosis (“sunburn cells”) which reduces the risk of gene mutations from replicating overly damaged DNA. Despite the persistence of DNA damage, the DNA repair-deficient murine Xpc skin does not display enhanced apoptosis after UV exposure, but replicating damaged keratinocytes arrest in late-S-phase and transit the epidermis in outward direction retaining expression of the basal cytokeratin 5, without switching to suprabasal cytokeratin 10.1 The skin actively copes with various toxic challenges from our environment. The outermost viable layer of skin, the epidermis, continuously renews itself from its proliferating basal layer in a process referred to as ‘epidermal turnover’. Thus, differentiated and damaged keratinocytes migrate outward and are replenished, ensuring lasting integrity and functionality of the epidermis. Evidently, the germinative basal layer is essential to maintain the epidermal turnover. This layer is, however, subjected to continued genotoxic stress by solar radiation. The basal cells are equipped with defence mechanisms which appear to be highly successful when we consider how well our skin copes with sunbathing. Nevertheless, these mechanisms are imperfect: in the long run skin cancer may arise, notably the most common cancers in fair-skinned people. The interplay between defence mechanisms is likely to be of utmost importance in reducing the risk of skin cancer. Our group is interested in how a defect in one mechanism may evoke compensatory reactions in others. The best known cellular defence mechanisms are (1) DNA repair, (2) cell cycle arrest and (3) apoptosis. In studying compensatory reactions in the latter two as a consequence of certain defects in DNA repair, our group stumbled on a novel 4th mechanism as alternative to in situ apoptosis (Fig. 1): the expulsion of UV- damaged cells that stagnate in late-S-phase from the basal cell layer, and subsequent removal through epidermal turnover.1 (1) Nucleotide Excision repair (NER) is a DNA repair pathway which constitutes an important—if not the most important—line of defence against UV-induced skin cancers, as can be inferred from the dramatically increased risk in Xeroderma pigmentosum (XP) patients who are deficient in NER.2 UV-B radiation (wavelengths 280-315 nm) in sunlight is absorbed by DNA and causes lesions, mainly dimers at di-pyrimidine sites. These DNA lesions block transcription and replication, and are substrates of NER. NER operates through two distinct pathways: global genome repair (GGR) and a specialized repair pathway dubbed transcription coupled repair (TCR). The latter is triggered by stalled polymerases and ensures a quick recovery of transcription. If UV-induced dimers are not adequately removed, they can give rise replication errors and subsequent gene mutations contributing to malignant transformation.3 (2) Cell cycle arrests can effectively aid DNA repair by delaying DNA replication and mitosis, thus allowing extended time for repair. UV radiation has been reported to effectuate arrests in all check points in the cell cycle, but a strong reduction in DNA synthesis and lengthening of S-phase is most dominant in the epidermis in the first hours after UV irradiation.4 (3) Apoptotic cells (“sunburn cells”) can be induced in an overly UV-damaged epidermis. In contrast to mouse strains defective in TCR (Xpa and Csb), Xpc mice with solely a deficiency in GGR display no discernable enhancement of cell cycle or apoptotic responses in the first hours after UV radiation.3,5 Apparently, the persistent damage in nontranscribed DNA goes virtually unnoticed over this period of time. However, from 48 up to 72 hours a gradual build-up of cells with ‘near-4N DNA contents’ is observed. This accumulation of late-S-phase cells6 reaches a maximum at 72–96 hours after UV radiation and then gradually resolves in absence of any sign of apoptosis.5 (4) The ‘traceless’ disappearance of these S-phase-arrested cells might be due to epidermal turnover. Therefore, we investigated whether these late-S-phase-arrested cells could be caught in transit through the epidermis. In flow cytometric analyses of epidermal keratinocytes
and Telomerase Group; Madrid, Spain
*Correspondence to: Frank R. de Gruijl; Department of Dermatology; Leiden University Medical Center; NL-2300 RC Leiden, The Netherlands; Email:
[email protected]
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S-phase arrest, epidermal turnover, XPC, UV radiation, DNA repair, skin
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Previously published onlne as a Cell Cycle E-publication: http://www.landesbioscience.com/journals/cc/abstract.php?id=2562
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Original manuscript submitted: 02/01/06 Manuscript accepted: 02/02/06
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3CNIO (Spanish National Cancer Center); Molecular Oncology Program; Telomeres
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1Department of Dermatology; 2Department of ToxicoGenetics; Leiden University Medical Center; Leiden, The Netherlands
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Frank R. de Gruijl1,* Leon H.F. Mullenders2 Gerdine J. Stout3
www.landesbioscience.com
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Elimination of Keratinocytes Stagnant in S-Phase Through Epidermal Turnover Instead of In Situ Apoptosis
Figure 1. Four protective mechanisms (bottom row of boxes) against replication of UV-damaged DNA, replication errors and ensuing gene mutations.
we established that the arrested cells retained cyto-keratin 5 (K5) expression (of basal keratinocytes) throughout, and did not express cytokeratin 10 (K10), a marker of differentiated keratinocytes (in suprabasal layers). In skin sections we observed considerable numbers of K5+ and K10- cells among the differentiated K10-positive keratinocytes solely in the suprabasal layers of UV-exposed GGR-defective epidermis of Xpc mice. The disappearance of these K5+ K10- cells closely paralleled that of the late-S-phase-arrested cells. Hence, proficient TCR prevents UV-irradiated GGR-defective keratinocytes from entering apoptosis. These cells with persistent DNA damage in their nontranscribed DNA continue to replicate and come to a “grinding halt” in late-S-phase after 2–3 days. Surprisingly, these cells do not enter apoptosis, but instead they are removed in epidermal turnover. This elimination of proliferationarrested cells by epidermal turnover apparently constitutes an alternative in vivo pathway for the removal of potentially mutated cells when apoptosis is not activated. This delayed removal may, however, well leave a substantial time window for accidents of failing arrest and ensuing mutational events. Further Reading 1. 2. 3. 4. 5. 6.
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Stout GJ, Westdijk D, Calkhoven DM, et al. Proc Natl Acad Sci USA 2005; 102:18980-5. Kraemer KH, Lee MM, Scotto J. Arch Dermatol 1987; 123:241-50. de Gruijl FR, van Kranen HJ, Mullenders LH. J Photochem Photobiol B 2001; 63:19-27. de Laat A, Kroon ED, de Gruijl FR. Photochem Photobiol 1997; 65:730-5. van Oosten M, Rebel H, Friedberg EC, et al. Proc Natl Acad Sci USA 2000; 97:11268-73. van Oosten M, Stout GJ, Backendorf C, et al. DNA Repair (Amst) 2005; 4:81-9.
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2006; Vol. 5 Issue 6
