Mitotic phosphorylation: breaking the balance of power by a tactical retreat

Give me a break, but not in mitosis: the mitotic DNA damage response marks DNA double-strand breaks with early signaling events

: DNA double-strand breaks (DSBs) are extremely cytotoxic with a single unrepaired DSB being sufficient to induce cell death. A complex signalling cascade, termed the DNA damage response (DDR), is in place to deal with such DNA lesions and maintain genome stability. Recent work by us and others has found that the signalling cascade activated by DSBs in mitosis is truncated, displaying apical, but not downstream, components of the DDR. The E3 Ubiquitin ligases RNF8, RNF168 and BRCA1, along with the DDR mediator 53BP1, are not recruited to DSB sites in mitosis, and activation of downstream checkpoint kinases is also impaired. Here, we show that RNF8 and RNF168 are recruited to DNA damage foci in late mitosis, presumably to prime sites for 53BP1 recruitment in early G1. Interestingly, we show that, although RNF8, RNF168 and 53BP1 are excluded from DSB sites during most of mitosis, they associate with mitotic structures such as the kinetochore, suggesting roles for these DDR factors during mitotic cell division. We discuss these and other recent findings and suggest how these novel data collectively contribute to our understanding of mitosis and how cells deal with DNA damage during this crucial cell cycle stage.

DNA damage signaling in response to double-strand breaks during mitosis

The signaling cascade initiated in response to DNA double-strand breaks (DSBs) has been extensively investigated in interphase cells. Here, we show that mitotic cells treated with DSB-inducing agents activate a "primary" DNA damage response (DDR) comprised of early signaling events, including activation of the protein kinases ataxia telangiectasia mutated (ATM) and DNA-dependent protein kinase (DNA-PK), histone H2AX phosphorylation together with recruitment of mediator of DNA damage checkpoint 1 (MDC1), and the Mre11-Rad50-Nbs1 (MRN) complex to damage sites. However, mitotic cells display no detectable recruitment of the E3 ubiquitin ligases RNF8 and RNF168, or accumulation of 53BP1 and BRCA1, at DSB sites. Accordingly, we found that DNA-damage signaling is attenuated in mitotic cells, with full DDR activation only ensuing when a DSB-containing mitotic cell enters G1. Finally, we present data suggesting that induction of a primary DDR in mitosis is important because transient inactivation of ATM and DNA-PK renders mitotic cells hypersensitive to DSB-inducing agents.

Cell cycle and activation of CK2

Casein kinase 2 (CK2) is a highly conserved and ubiquitous eukaryotic Ser/Thr protein kinase. Genetic, biochemical, and cell biological studies have indicated the involvement of this enzyme in the control of cell proliferation and in signal transduction. The regulation of CK2 is not well defined, and it has been considered a constitutively non-regulated protein kinase. However, we show that CK2 activation occurred during the progression of cell cycle in response to FBS stimuli of G0 arrested cells. Importantly, we show that as the downstream target for CK2, the phosphorylation of eukaryotic translation-initiation factor eIF5 by CK2 may play a critical role in cell cycle progression. We find that eIF5 is associated with CK2 when the kinase activity is at the highest level in vivo, and is phosphorylated at Ser389 and Ser390 by CK2. Expression of eIF5 mutants that lack those phosphorylation sites reveals that these mutants have a dominant-negative effect on phosphorylation of endogenous eIF5, as well as a significant reduction in the formation of the mature complex, in the growth rate, and the expression of cell cycle-regulated proteins. Also, a pool of CK2 translocates into the nuclear fraction following its activation during the progression of the cell cycle. Consistent with these findings, we report that CK2 may be involved in the regulation of cell cycle progression through the phosphorylation of a key molecule for translation initiation and of nuclear substrates upon activation of CK2 by itself.