**7. Shared regulatory pathway for circadian clock and DNA repair in zebrafish**

Although solar light has several beneficial uses, including the regulation of circadian clocks, the UV component of solar light is harmful to living cells because it produces cytotoxic and mutagenic lesions in DNA called cyclobutane pyrimidine dimers (CPDs) and pyrimidine (6-4) pyrimidone photoproducts [(6-4) photoproducts] (**Figure 2**) [51, 52]. Natural selective pressure has forced the development of a self-defense system mediated by photoreactivation. Photoreactivation is the light-dependent DNA repair mechanism mediated by DNA photolyases (PHRs), which bind to and repair the UV-induced DNA damage using visible light as an energy source [53]. Two classes of PHRs have been identified, one specific for CPDs and the other specific for (6-4) photoproducts. Importantly, both the induction of PHRs in response to light and subsequent light-dependent repair of DNA by PHRs are essential for a successful photoreactivation in zebrafish cells [42, 54, 55]. Notably, the expression of the zebrafish *Phr* repairing (6-4) photoproduct (*z64Phr*) is regulated by the same light-induced MAPK cascades as those controlling the expression of the clock genes *zCry1a* and *zPer2* (**Figure 2**) [49]. The light-induced ERK activation triggers the expression of *z64Phr*, whereas the light-induced p38 activation inhibits it. Thus, the light-dependent DNA repair and regulation of the circadian clock are governed by shared regulatory pathways. Both CRYs and PHRs belong to the DNA photolyase/cryptochrome protein family and have highly similar amino acid sequences [42, 55, 56]. Evolutionary studies have shown that the animal CRY proteins functionally diverged first from the CPD photolyase and then further

**19**

*A Molecular Link between the Circadian Clock, DNA Damage Responses, and Oncogene…*

**8. Cellular responses to photooxidative stress are the candidate** 

Cellular reactive oxygen species (ROS) were originally thought to solely act as toxic metabolites because they react with components of DNA, proteins, and lipids and exert oxidative stress. However, ROS are also ideally suited as signaling molecules because they are small and can easily diffuse to short distances within a cell [60]. In addition, mechanisms for ROS production and the rapid removal (such as via catalase) are present in almost all cell types [61]. Much evidence has accumulated indicating significant roles of ROS in circadian clock controls that have resulted in the functional coupling of the circadian clock and DDR. For example, in *Drosophila*, a genome-wide screen identified several redox molecules as essential for the light entrainment of the circadian clock [62]. Similarly, a study in mammals showed that changes in reduced NADPH and NADH levels altered the affinity of the NPAS2:BMAL1 complex for its target DNA *in vitro* [63]. Thus, redox state may be an important determinant of circadian oscillations in mammalian cells. Nuclear factor erythroid-derived 2-like 2 (NRF2) is one of the components involved in the major cellular antioxidant defense pathways [64]. It induces a transcriptional program that maintains cellular redox balance and protects cells from oxidative insults. Importantly, it has been reported in mouse that cellular clock generates circadian rhythm in NRF2 level, which is essential in regulating the rhythmic expression of antioxidant genes involved in glutathione redox homeostasis in the lung [65].

In zebrafish, the transcriptional induction of *zCry1a* and *zPer2* genes has been proposed to be required for the light entrainment of cellular clocks [45, 66, 67]. The lightdependent transcription of *zCry1a* and *zPer2* is controlled through the production and removal of cellular ROS [66, 68]. The light-induced ROS stimulate the intracellular ERK signaling pathway and transduce photic signals to the transactivation of *zCry1a* and *zPer2* (**Figure 2**). Importantly, light increases the intracellular catalase activity by increasing the expression *of catalase*, an event that occurs after the maximum expression of the *zCry1a* and *zPer2* genes has been reached. This increased catalase activity diminishes the light-induced cellular ROS levels, resulting in decreased expression levels of *zCry1a* and *zPer2* genes. These findings provide evidence that ROS induced by light are the second messenger coupling photoreception to the entrainment of the circadian clock in zebrafish and further indicate that cellular responses to photooxi-

**9. The light entrainment of the circadian clock in zebrafish would reflect** 

It is conceivable that the development of circadian clocks is one way to segregate daytime from nighttime processes with light-dark cycles acting as selective forces

dative stress would be the evolutionary origin of circadian clocks.

**a cellular response to photooxidative stress**

**evolutionary origin of circadian clocks**

to generate 64PHR [57]. These facts, together with the observation that *zCry1a* and *z64Phr* share regulatory pathways, strongly indicate an evolutionary link between the circadian clock and DNA repair. Importantly, evolutionary links functionally coupling the circadian clock and DNA repair also have been reported in other organisms. For example, *Neurospora* PRD-4, an orthologue of mammalian Chk2, transduces stress signals into the core circadian clock machinery, contributing the regulation of circadian clock [58]. Additionally, in the diatom *Phaeodactylum tricornutum*, *Phaeodactylum tricornutum* cryptochrome/photolyase family1 (PtCPF1), a novel cryptochrome/photolyase family member, not only repairs UV-induced DNA damage but also acts as a transcriptional repressor of the circadian clock [59].

*DOI: http://dx.doi.org/10.5772/intechopen.81063*

*A Molecular Link between the Circadian Clock, DNA Damage Responses, and Oncogene… DOI: http://dx.doi.org/10.5772/intechopen.81063*

to generate 64PHR [57]. These facts, together with the observation that *zCry1a* and *z64Phr* share regulatory pathways, strongly indicate an evolutionary link between the circadian clock and DNA repair. Importantly, evolutionary links functionally coupling the circadian clock and DNA repair also have been reported in other organisms. For example, *Neurospora* PRD-4, an orthologue of mammalian Chk2, transduces stress signals into the core circadian clock machinery, contributing the regulation of circadian clock [58]. Additionally, in the diatom *Phaeodactylum tricornutum*, *Phaeodactylum tricornutum* cryptochrome/photolyase family1 (PtCPF1), a novel cryptochrome/photolyase family member, not only repairs UV-induced DNA damage but also acts as a transcriptional repressor of the circadian clock [59].
