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Skin cancer is the most common form of cancer in the United States. The main cause of this cancer is DNA damage induced by the UV component of sunlight. In humans and mice, UV damage is removed by the nucleotide excision repair system. Here, we report that a rate-limiting subunit of excision repair, the xeroderma pigmentosum group A (XPA) protein, and the excision repair rate exhibit daily rhythmicity in mouse skin, with a minimum in the morning and a maximum in the afternoon/evening. In parallel with the rhythmicity of repair rate, we find that mice exposed to UV radiation (UVR) at 4:00 AM display a decreased latency and about a fivefold increased multiplicity of skin cancer (invasive squamous cell carcinoma) than mice exposed to UVR at 4:00 PM. We conclude that time of day of exposure to UVR is a contributing factor to its carcinogenicity in mice, and possibly in humans.circadian clock | cryptochrome | sunbathing | tanning salons S kin cancer is the most common form of cancer in the United States. With over 1.3 million new cases each year, it constitutes nearly 40% of all diagnosed cancers (1). Moreover, because of changes in lifestyle and the environment, the incidence of skin cancer is steadily increasing (2). The main causative agent of skin cancer is the UV component of sunlight. UV radiation (UVR) produces two major lesions in DNA, the cyclobutane pyrimidine dimer (CPD) and the (6-4) photoproduct [(6-4) PP], both of which are mutagenic and carcinogenic in animal model systems and are thought to be the primary cause of skin cancer in humans (3-7).In mice and humans, nucleotide excision repair is the sole repair system for removing CPDs and (6-4) PPs from DNA. As a consequence, humans with hereditary mutations in excision repair genes suffer from xeroderma pigmentosum, a syndrome characterized by a nearly 5,000-fold increase in skin cancer in sunlight-exposed areas of the afflicted individuals (8). Excision repair involves photoproduct removal by dual incisions bracketing the lesion, removal of the damage in the form of a 24-to 32-nt-long oligomer, filling in the resulting single-stranded gap, and sealing by ligase (9). The dual incision is carried out by six excision repair factors: RPA, xeroderma pigmentosum group A (XPA), XPC, TFIIH, XPG, and XPF-ERCC1 (10). Recently, in a study that analyzed liver and brain tissues from mice, it was found that XPA, a critical protein involved in damage recognition and a rate-limiting factor in excision repair, is controlled by the core molecular circadian clock (11, 12). As a consequence, excision repair activity exhibited circadian rhythmicity in these organs, increasing during the day to reach a maximum at 4-6:00 PM and decreasing during the night to a minimum at 4-6:00 AM.Here we analyzed the expression pattern of XPA and excision repair activity in mouse skin. We found that protein and repair activity exhibit a circadian rhythm similar to that found in the liver and brain. To determine whether this rhythmicity affected UV-induced skin cancer development we exposed a...

UV-induced DNA damage stalls DNA replication forks and activates the intra-S checkpoint to inhibit replicon initiation. In response to stalled replication forks, ATR phosphorylates and activates the transducer kinase Chk1 through interactions with the mediator proteins TopBP1, Claspin, and Timeless (Tim). Murine Tim recently was shown to form a complex with Tim-interacting protein (Tipin), and a similar complex was shown to exist in human cells. Knockdown of Tipin using small interfering RNA reduced the expression of Tim and reversed the intra-S checkpoint response to UVC. Tipin interacted with replication protein A (RPA) and RPA-coated DNA, and RPA promoted the loading of Tipin onto RPA-free DNA. Immunofluorescence analysis of spread DNA fibers showed that treating HeLa cells with 2.5 J/m2 UVC not only inhibited the initiation of new replicons but also reduced the rate of chain elongation at active replication forks. The depletion of Tim and Tipin reversed the UV-induced inhibition of replicon initiation but affected the rate of DNA synthesis at replication forks in different ways. In undamaged cells depleted of Tim, the apparent rate of replication fork progression was 52% of the control. In contrast, Tipin depletion had little or no effect on fork progression in unirradiated cells but significantly attenuated the UV-induced inhibition of DNA chain elongation. Together, these findings indicate that the Tim-Tipin complex mediates the UV-induced intra-S checkpoint, Tim is needed to maintain DNA replication fork movement in the absence of damage, Tipin interacts with RPA on DNA and, in UV-damaged cells, Tipin slows DNA chain elongation in active replicons.

The Timeless protein is essential for circadian rhythm in Drosophila. The Timeless orthologue in mice is essential for viability and appears to be required for the maintenance of a robust circadian rhythm as well. We have found that the human Timeless protein interacts with both the circadian clock protein cryptochrome 2 and with the cell cycle checkpoint proteins Chk1 and the ATR-ATRIP complex and plays an important role in the DNA damage checkpoint response. Down-regulation of Timeless in human cells seriously compromises replication and intra-S checkpoints, indicating an intimate connection between the circadian cycle and the DNA damage checkpoints that is in part mediated by the Timeless protein.The circadian and cell cycles are two global regulatory systems that have pervasive effects on organismal and cellular physiology. Circadian rhythm is the oscillation in the physiology and behavior of organisms with a 24-h periodicity (17,33,40). The rhythm consists of light and dark phases which coincide with the phases of the solar day. Cell cycle checkpoints are regulatory pathways that ensure completion of biochemical reactions unique to each phase of the cell cycle (G 1 , S, G 2 , and M in proliferating mammalian cells) prior to initiation of subsequent phases (26,30,35,41). While these two regulatory systems involve distinct mechanisms, there is some evidence that these cycles are linked. Most mammalian diploid cells exhibit an approximately 24-h cell cycle period, and the circadian clock has been implicated in regulation of the phases of cell division (3). The emerging field of chronotherapy aims to coordinate the time of delivery of chemotherapeutic drugs with the circadian and cell cycles so as to minimize side effects while optimizing therapeutic efficacy (4).Although a few recent studies have shown that some cell proliferation and cell cycle checkpoint genes in mammals (such as c-myc, Wee1, and cyclin D1) are first-and second-order clock-controlled genes (10, 23), the circadian cycle-cell cycle connection remains ill-defined. Here we present evidence that the mammalian Timeless (Tim) protein (18, 36), which appears to be required for a robust circadian rhythm (1), is also a core component of the cell cycle checkpoint system, suggesting a possibly more intimate and direct connection between the circadian cycle and cell cycle checkpoints in mammals.Despite its initial identification as a homologue of the Drosophila clock protein Tim, the closest phylogenetic relatives of the mammalian Tim protein are actually cell cycle-related proteins: budding yeast Tof1 (9, 32), fission yeast Swi1 (29,19), Caenorhabditis elegans TIM-1 (5), and Drosophila Tim-2/Timeout (dTim2/dTimeout) (2). Tof1 and Swi1 have been implicated in DNA damage checkpoint activation as mediators, and Swi1 plays an additional role in preventing replication fork collapse (29). TIM-1 is essential for chromosome cohesion in C. elegans, and Timeless null mutation results in embryonic lethality in both C. elegans (5) and mice (12). Based on these find...

Inhibition of replicon initiation is a stereotypic DNA damage response mediated through S checkpointmechanisms not yet fully understood. Studies were undertaken to elucidate the function of checkpoint proteins in the inhibition of replicon initiation following irradiation with 254 nm UV light (UVC) of diploid human fibroblasts immortalized by the ectopic expression of telomerase. Velocity sedimentation analysis of nascent DNA molecules revealed a 50% inhibition of replicon initiation when normal human fibroblasts were treated with a low dose of UVC (1 J/m 2 ). Ataxia telangiectasia (AT), Nijmegen breakage syndrome (NBS), and AT-like disorder fibroblasts, which lack an S checkpoint response when exposed to ionizing radiation, responded normally when exposed to UVC and inhibited replicon initiation. Pretreatment of normal and AT fibroblasts with caffeine or UCN-01, inhibitors of ATR (AT mutated and Rad3 related) and Chk1, respectively, abolished the S checkpoint response to UVC. Moreover, overexpression of kinase-inactive ATR in U2OS cells severely attenuated UVC-induced Chk1 phosphorylation and reversed the UVC-induced inhibition of replicon initiation, as did overexpression of kinase-inactive Chk1. Taken together, these data suggest that the UVC-induced S checkpoint response of inhibition of replicon initiation is mediated by ATR signaling through Chk-1 and is independent of ATM, Nbs1, and Mre11.Accurate replication and segregation of the human genome depends on interactions between cell cycle checkpoints and pathways of DNA repair. Cell cycle checkpoints are biochemical surveillance pathways that slow or arrest progression through the cell cycle, pending completion of essential events and/or repair of DNA damage. DNA damage checkpoints minimize the probability of replicating and segregating damaged DNA and therefore reduce the frequencies of mutations and chromosomal aberrations that are induced by genotoxic stress.

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Recent microarray studies have identified distinct subtypes of breast tumors that arise from different cell types and that show statistically significant differences in patient outcome. To gain insight into these differences, we identified in vitro and in vivo changes in gene expression induced by chemotherapeutics. We treated two cell lines derived from basal epithelium (immortalized human mammary epithelial cells) and two lines derived from luminal epithelium (MCF-7 and ZR-75-1) with chemotherapeutics used in the treatment of breast cancer and assayed for changes in gene expression using DNA microarrays. Treatment doses for doxorubicin and 5-fluorouracil were selected to cause comparable cytotoxicity across all four cell lines. The dominant expression response in each of the cell lines was a general stress response; however, distinct expression patterns were observed. Both cell types induced DNA damageresponse genes such as p21 waf1 , but the response in the luminal cells showed higher fold changes and included more p53-regulated genes. Luminal cell lines repressed a large number of cell cycle-regulated genes and other genes involved in cellular proliferation, whereas the basal cell lines did not. Instead, the basal cell lines repressed genes that were involved in differentiation. These in vitro responses were compared with expression responses in breast tumors sampled before and after treatment with doxorubicin or 5-fluorouracil/mitomycin C. The in vivo data corroborated the cell-type-specific responses to chemotherapeutics observed in vitro, including the induction of p21 waf1 . Similarities between in vivo and in vitro responses help to identify important response mechanisms to chemotherapeutics.

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