SUMMARY The mitogen-activated protein kinases (MAPKs) regulate diverse cellular programs by relaying extracellular signals to intracellular responses. In mammals, there are more than a dozen MAPK enzymes that coordinately regulate cell proliferation, differentiation, motility, and survival. The best known are the conventional MAPKs, which include the extracellular signal-regulated kinases 1 and 2 (ERK1/2), c-Jun amino-terminal kinases 1 to 3 (JNK1 to -3), p38 (α, β, γ, and δ), and ERK5 families. There are additional, atypical MAPK enzymes, including ERK3/4, ERK7/8, and Nemo-like kinase (NLK), which have distinct regulation and functions. Together, the MAPKs regulate a large number of substrates, including members of a family of protein Ser/Thr kinases termed MAPK-activated protein kinases (MAPKAPKs). The MAPKAPKs are related enzymes that respond to extracellular stimulation through direct MAPK-dependent activation loop phosphorylation and kinase activation. There are five MAPKAPK subfamilies: the p90 ribosomal S6 kinase (RSK), the mitogen- and stress-activated kinase (MSK), the MAPK-interacting kinase (MNK), the MAPK-activated protein kinase 2/3 (MK2/3), and MK5 (also known as p38-regulated/activated protein kinase [PRAK]). These enzymes have diverse biological functions, including regulation of nucleosome and gene expression, mRNA stability and translation, and cell proliferation and survival. Here we review the mechanisms of MAPKAPK activation by the different MAPKs and discuss their physiological roles based on established substrates and recent discoveries.

Activation and Function of the MAPKs and Their Substrates, the MAPK-Activated Protein Kinases

50 Years of Image Analysis

Replication stress is a complex phenomenon that has serious implications for genome stability, cell survival and human disease. Generation of aberrant replication fork structures containing single-stranded DNA activates the replication stress response, primarily mediated by the kinase ATR (ATM- and Rad3-related). Along with its downstream effectors, ATR stabilizes and helps to restart stalled replication forks, avoiding the generation of DNA damage and genome instability. Understanding this response may be key to diagnosing and treating human diseases caused by defective responses to replication stress.

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Causes and consequences of replication stress.

https://www.cell.com/trends/cell-biology/pdf/S0962-8924(15)00142-7.pdf

Repair Pathway Choices and Consequences at the Double-Strand Break

Genome instability is a hallmark of cancer, and DNA replication is the most vulnerable cellular process that can lead to it. Any condition leading to high levels of DNA damage will result in replication stress, which is a source of genome instability and a feature of pre-cancerous and cancerous cells. Therefore, understanding the molecular basis of replication stress is crucial to the understanding of tumorigenesis. Although a negative aspect of replication stress is its prominent role in tumorigenesis, a positive aspect is that it provides a potential target for cancer therapy. In this Review, we discuss the link between persistent replication stress and tumorigenesis, with the goal of shedding light on the mechanisms underlying the initiation of an oncogenic process, which should open up new possibilities for cancer diagnostics and treatment.

Replication stress and cancer

Oncogene-induced DNA replication stress has been implicated as a driver of tumorigenesis. Many chromosomal rearrangements characteristic of human cancers originate from specific regions of the genome called common fragile sites (CFSs). CFSs are difficult-to-replicate loci that manifest as gaps or breaks on metaphase chromosomes (termed CFS 'expression'), particularly when cells have been exposed to replicative stress. The MUS81-EME1 structure-specific endonuclease promotes the appearance of chromosome gaps or breaks at CFSs following replicative stress. Here we show that entry of cells into mitotic prophase triggers the recruitment of MUS81 to CFSs. The nuclease activity of MUS81 then promotes POLD3-dependent DNA synthesis at CFSs, which serves to minimize chromosome mis-segregation and non-disjunction. We propose that the attempted condensation of incompletely duplicated loci in early mitosis serves as the trigger for completion of DNA replication at CFS loci in human cells. Given that this POLD3-dependent mitotic DNA synthesis is enhanced in aneuploid cancer cells that exhibit intrinsically high levels of chromosomal instability (CIN(+)) and replicative stress, we suggest that targeting this pathway could represent a new therapeutic approach.

Replication stress activates DNA repair synthesis in mitosis

Fragile sites are chromosomal loci with a propensity to form gaps or breaks during early mitosis, and their instability is implicated as being causative in certain neurological disorders and cancers. Recent work has demonstrated that the so-called common fragile sites (CFSs) often impair the faithful disjunction of sister chromatids in mitosis. However, the mechanisms by which CFSs express their fragility, and the cellular factors required to suppress CFS instability, remain largely undefined. Here, we report that the DNA structure-specific nuclease MUS81-EME1 localizes to CFS loci in early mitotic cells, and promotes the cytological appearance of characteristic gaps or breaks observed at CFSs in metaphase chromosomes. These data indicate that CFS breakage is an active, MUS81-EME1-dependent process, and not a result of inadvertent chromatid rupturing during chromosome condensation. Moreover, CFS cleavage by MUS81-EME1 promotes faithful sister chromatid disjunction. Our findings challenge the prevailing view that CFS breakage is a nonspecific process that is detrimental to cells, and indicate that CFS cleavage actually promotes genome stability.

MUS81 promotes common fragile site expression

A comprehensive analysis of the TP53 gene and its protein status was carried out on a panel of 56 colorectal cancer cell lines. This analysis was based on a combination of denaturing HPLC mutation screening of all exons of the p53 gene, sequencing the cDNA, and assessing the function of the p53 protein by assaying the induced expression of phosphorylated p53 and p21 after exposing cells to γ-rays. In a few cases where there was no production of p53 message nor evidence of functional p53 protein, all of the p53 exons were sequenced directly. Thirteen of the 56 cell lines had functional p53, 21 lines had missense mutations (one of which made no detectable protein), 4 lines produced no p53 transcripts, and the remaining 18 lines carried truncating TP53 mutations. Thus, our results showed a relatively high frequency of TP53 mutations (76.8%) in our cell lines, with almost half of the mutations being truncating mutations. This is a rather higher frequency of such mutations than usually reported. Of the 18 cell lines with truncating mutations, 12 had detectable truncated protein based on Western blot analysis, whereas no protein was detected in the remaining 6 cell lines. Our data provide a valuable source of TP 53 mutations for further studies and raise the question of the extent to which truncating mutations may have dominant negative effects, even when no truncated protein can be detected by standard methods.

https://www.pnas.org/content/pnas/103/4/976.full.pdf

Analysis of P53 mutations and their expression in 56 colorectal cancer cell lines

The origins and processing of ultra fine anaphase DNA bridges

The molecular pathogenesis of Barrett's metaplasia (BM) of the esophagus is poorly understood. The change to an intestinal phenotype occurs on a background of esophagitis due to refluxing acid and bile. CDX1, an important regulator of normal intestinal development, was studied as a potential key molecule in the pathogenesis of BM. CDX1 mRNA and protein were universally expressed in all samples of BM tested but not in normal esophageal squamous or gastric body epithelia. This tissue-specific expression was attributable to the methylation status of the CDX1 promoter. Conjugated bile salts and the inflammatory cytokines TNF-α and IL-1β were all found to increase CDX1 mRNA expression in vitro. These effects were primarily mediated by NF-κB signaling but only occurred when the CDX1 promoter was unmethylated or partially methylated. The data suggest that CDX1 is a key molecule linking etiological agents of BM to the development of an intestinal phenotype. Although the initial trigger for CDX1 promoter demethylation is not yet identified, it seems likely that demethylation of its promoter may be the key to the induction and maintenance of CDX1 expression and so of the BM phenotype.

Ying Liu

Papers

Replication stress activates DNA repair synthesis in mitosis

MUS81 promotes common fragile site expression

Analysis of P53 mutations and their expression in 56 colorectal cancer cell lines

The origins and processing of ultra fine anaphase DNA bridges

CDX1 is an important molecular mediator of Barrett's metaplasia