Cell Cycle and Cancer Flashcards
Briefly describe the circadian clock in mammals
In mammals, circadian “conductor” is the SCN. The SCN machinery consists of feedback loops relying on clock genes. A main loop involves CLOCK and BMAL1, which heterodimerize and activate the transcription of Period (Per1–2–3) and Cryptochrome (Cry1–2) genes. PER:CRY proteins in turn inhibit their own transcription by direct interaction with CLOCK:BMAL1. This essential loop is modulated by the REV-ERBα–β and RORα–β–γ proteins, which operate a negative and positive feedback, respectively, on Bmal1 transcription. This genetic network is also extensively regulated via post-translational processes.
How is the entire organism synchronised?
The SCN is synchronized/entrained to a sharp 24-h period by environmental parameters (light/dark, feeding cycles, hormones). A rhythmic message integrating environmental informations is then generated by the SCN and redistributed to the entire organism to synchronize physiological functions.
Describe the cell cycle
it consists of two critical phases: the S phase, in which the cell replicates its DNA, and the M phase where it divides (mitosis); they are preceded by growth phases G1 and G2, respectively.
The progression of the cell cycle (transition from one phase to another) relies on transient and sequential activation of cyclin-dependent kinases (CDKs) forming complexes with cyclins (CCN). The activity of these complexes can be restrained at each phase by a set of specific inhibitors such as p15, p21, p27, or WEE1.
How is the timing of cell divison different between unicellular and multicellular organisms?
It is well established that in unicellular organisms such as the cyanobacterium Synechoccus elongates, cell division is timed by a circadian mechanism e.g. to avoid UV-induced DNA damage.
However, in multicellular organisms, it is classically thought that the circadian clock and the cell cycle are two independent intracellular timing devices. Supporting this, Yeom et al., (2010) describes that the circadian clock does not regulate the cell cycle. They tested whether the cell-cycle rhythm was coupled to the circadian system in immortalized rat-1 fibroblasts by monitoring cell-cycle gene promoter-driven luciferase activity. They found that there was no consistent phase relationship between the circadian and cell cycles, and that the cell-cycle rhythm was not temperature-compensated in rat-1 fibroblasts. As temperature compensation is a distinct feature of circadian rhythms these data suggest that the circadian system does not regulate the cell-mitosis rhythm in rat-1 fibroblasts.
However, these findings are inconsistent with numerous studies that suggest that cell mitosis is regulated by the circadian system in mammalian tissues in vivo. As a result this discrepany may result from two possibilities: (i) There is no direct coupling between the circadian rhythm and cell cycle but the timing of cell mitosis is synchronized with the rhythmic host environment, or (ii) coupling between the circadian rhythm and cell cycle exists in normal cells but it is disconnected in immortalized cells.
What are two conceptual ways that the clock may regulate the cell cycle?
There are two general conceptual ways in which clock-cell cycle coupling could occur. One possibility is that the speed of progression, or angular velocity, of the cell cycle could be adjusted directly by the clock, such that the 2 periods become equivalent. Such a coupling mechanism might make sense for proliferative cells where the cell cycle length is close to 24 h, as in many cell types, and coincidentally falls within the “range of entrainment” of the circadian clock. Such 1:1 phase locking has been demonstrated in some mammalian proliferative cells, in particular NIH/3T3 mouse fibroblasts, by imaging both cell cycle progression and circadian clock gene expression rhythms in single cells (Bieler et al., 2014).
However, complexities in this 1:1 coupling are seen when the cellular circadian clock is synchronized by an external stimulus, producing several peaks in cell division (Matsuo et al., 2003). A gating mechanism might be more applicable in cells or tissues where the cell cycle length deviates significantly from 24 h and the duration of the cell cycle cannot be easily altered to match the 24-h period of the circadian clock.
An alternative model is that the timing of specific cell cycle events is restricted by a gatingmechanism, in which the clock imposes a specific circadian checkpoint mechanism and subsequent phase on the cell cycle. (more evidence for this model)
In support of this gating system, it has been shown that S phase is restricted to mostly in evenings and night in mammalian skin, outside of the daylight period, which is important in reducing vulnerability to UV-induced DNA damage, although the mechanism of how it does this is unclear.
Furthermore, it has been shown that p21, an important regulator of G1/S transition, is a clock-controlled gene (Gréchez-Cassiau et al., 2008).
The circadian clock has also been demonstrated to control expression of mitotic cell genes, such as Cyclin B1, Cyclin B2, CDC2 and Per1.
Is the timeless protein involved in coupling the clock to cell cycle?
Furthermore, proteins have been reported to participate in both the circadian and cell cycle circuits. One example is mammalian TIM. It has been found to interact with the circadian protein CRY2, CHK1 and ATR, resulting in regulation of S phase and DNA damage response (Unsal-Kaçmaz et al., 2005). This suggests a coupling mechanism of the human circadian and cell cycle by the timeless protein.
Gap junctions/ confluence?
Furthermore, the circadian clock has been shown to regulate gap junctions between cells, thus affecting cell communication and cell cycle timing.
This is supported by the observation that clock-regulated cell cycle rhythms (e.g. rhythms in S phase) in zebrafish cell lines are observed only in cultures approaching confluence (Tamai et al., 2012)
Evidence that clock gene mutations is sufficient to dyregulate the cell cycle and trigger cancer development.
Per2 mutant and KO (Per2m/m and Per2−/−) mice display tumour-prone phenotypes and deregulation of various cyclins, proto-oncogenes, and tumour suppressors (Fu et al., 2002). Supporting this Halberg et al., (2006) showed that per2 overexpression in lung carcinoma reduces cell division and increases apoptosis. The overexpression of per2 results in slightly reduced levels of C-myc and increased levels of p53.
C-myc promotes entry into S-phase (DNA synthesis) and cell proliferation. p53 is a tumour suppressor gene, which has a pro-apoptotic function. This shows that per2 can act as a tumour suppressor.
However, Cry1/Cry2 KO mice do not show increased cancer development after γ-irradiation (Gauger and Sancar 2005). Additionally, mutation of Bmal1 or Clock does not lead to enhanced cancer development (Kondratov et al., 2006, Antoch et al., 2008). Thus, deregulation of the core clock cannot fully account for the observed phenotype in Per2 mutant mice.
So far, it is unclear as to whether clock gene mutations by itself is sufficient to trigger cancer development. It may also be the case that the observed cancer phenotypes of certain clock gene mutants might stem not from the disrupted circadian rhythms per se, but rather from “clock-unrelated” (pleiotropic) functions of these genes.
Evidence that systemic envirornmental disruption of circadian function imapacts cancer developement?
However systemic environmental disruption of the circadian function may impact on cancer development and cell proliferation.
For instance, circadian disruption by chronic jetlag has been shown to accelerate liver carcinogenesis in mice (Filipski et al., 2009). Furthermore, constant light exposure (LL) which is known to suppress the circadian clock exerted a promoting effect on liver carcinogenesis in rats compared to rats in the LD group (Heilgenberg et al., 1999).
Additionally, tumor-prone mice, expressing a mutated allele of p53 in mammary glands, exhibit higher rates of spontaneous tumors, when exposed to weekly alternating light cycles, suggesting that internal desynchronization and sleep disturbances contribute to de novo cancerogenesis (Van Dycke et al., 2015).
Moreover, enhancing the clock function in tumor cells by means of circadian synchronization (i.e., dexamethasone treatment) impinges on the cell cycle and reduces cellular growth (Kiessling et al., 2017).
Similarly, circadian disruption resulting from shift work in humans has been linked to increase risk of cancer. Particularly, increased rates of breast cancer have been found in night and rotating female shift workers (Bhatti et al., 2012). Due to these findings, the World Health Organization’s International Agency for Research on Cancer (IARC) listed “shiftwork that involves circadian disruption” as a probable carcinogen in 2007.
In addition, circadian clock is known to control cell metabolism and cell senescence. As a result, disruption of circadian clock may lead to cancer due to dysregulated cell metabolism and cell senescence.
