Protein kinase Cdc7 supports viral replication by phosphorylating Avibirnavirus VP3 protein

IMPORTANCE

The Avibirnavirus infectious bursal disease virus is still an important agent which largely threatens global poultry farming industry economics. VP3 is a multifunctional scaffold structural protein that is involved in virus morphogenesis and the regulation of diverse cellular signaling pathways. However, little is known about the roles of VP3 phosphorylation during the IBDV life cycle. In this study, we determined that IBDV infection induced the upregulation of Cdc7 expression and phosphorylated the VP3 Ser13 site to promote viral replication. Moreover, we confirmed that the negative charge addition of phosphoserine on VP3 at the S13 site was essential for IBDV proliferation. This study provides novel insight into the molecular mechanisms of VP3 phosphorylation-mediated regulation of IBDV replication.

DDK/Hsk1 phosphorylates and targets fission yeast histone deacetylase Hst4 for degradation to stabilize stalled DNA replication forks

In eukaryotes, paused replication forks are prone to collapse, which leads to genomic instability, a hallmark of cancer. Dbf4-dependent kinase (DDK)/Hsk1^Cdc7 is a conserved replication initiator kinase with conflicting roles in replication stress response. Here, we show that fission yeast DDK/Hsk1 phosphorylates sirtuin, Hst4 upon replication stress at C-terminal serine residues. Phosphorylation of Hst4 by DDK marks it for degradation via the ubiquitin ligase SCF^pof3. Phosphorylation-defective hst4 mutant (4SA-hst4) displays defective recovery from replication stress, faulty fork restart, slow S-phase progression and decreased viability. The highly conserved fork protection complex (FPC) stabilizes stalled replication forks. We found that the recruitment of FPC components, Swi1 and Mcl1 to the chromatin is compromised in the 4SA-hst4 mutant, although whole cell levels increased. These defects are dependent upon H3K56ac and independent of intra S-phase checkpoint activation. Finally, we show conservation of H3K56ac-dependent regulation of Timeless, Tipin, and And-1 in human cells. We propose that degradation of Hst4 via DDK increases H3K56ac, changing the chromatin state in the vicinity of stalled forks facilitating recruitment and function of FPC. Overall, this study identified a crucial role of DDK and FPC in the regulation of replication stress response with implications in cancer therapeutics.

Cdc7 kinase stimulates Aurora B kinase in M-phase

The conserved serine-threonine kinase, Cdc7, plays a crucial role in initiation of DNA replication by facilitating the assembly of an initiation complex. Cdc7 is expressed at a high level and exhibits significant kinase activity not only during S-phase but also during G2/M-phases. A conserved mitotic kinase, Aurora B, is activated during M-phase by association with INCENP, forming the chromosome passenger complex with Borealin and Survivin. We show that Cdc7 phosphorylates and stimulates Aurora B kinase activity in vitro. We identified threonine-236 as a critical phosphorylation site on Aurora B that could be a target of Cdc7 or could be an autophosphorylation site stimulated by Cdc7-mediated phosphorylation elsewhere. We found that threonines at both 232 (that has been identified as an autophosphorylation site) and 236 are essential for the kinase activity of Aurora B. Cdc7 down regulation or inhibition reduced Aurora B activity in vivo and led to retarded M-phase progression. SAC imposed by paclitaxel was dramatically reversed by Cdc7 inhibition, similar to the effect of Aurora B inhibition under the similar situation. Our data show that Cdc7 contributes to M-phase progression and to spindle assembly checkpoint most likely through Aurora B activation.

The functional role of polyamines in eukaryotic cells

Polyamines, consisting of putrescine, spermidine and spermine are essential for normal cell growth and viability in eukaryotic cells. Since polyamines are cations, they interact with DNA, ATP, phospholipids, specific kinds of proteins, and especially with RNA. Consequently, the functions of these acidic compounds and some proteins are modified by polyamines. In this review, the functional modifications of these molecules by polyamines are presented. Structural change of specific mRNAs by polyamines causes the stimulation of the synthesis of several different proteins, which are important for cell growth and viability. eIF5 A, the only known protein containing a spermidine derivative, i.e. hypusine, also functions at the level of translation. Experimental results thus far obtained strongly suggest that the most important function of polyamines is at the level of translation.

Knockdown of ornithine decarboxylase antizyme 1 causes loss of uptake regulation leading to increased N1, N11-bis(ethyl)norspermine (BENSpm) accumulation and toxicity in NCI H157 lung cancer cells

Ornithine decarboxylase antizyme 1 (AZ1) is a major regulatory protein responsible for the regulation and degradation of ornithine decarboxylase (ODC). To better understand the role of AZ1 in polyamine metabolism and in modulating the response to anticancer polyamine analogues, a small interfering RNA strategy was used to create a series of stable clones in human H157 non-small cell lung cancer cells that expressed less than 5-10% of basal AZ1 levels. Antizyme 1 knockdown clones accumulated greater amounts of the polyamine analogue N (1),N (11)-bis(ethyl)norspermine (BENSpm) and were more sensitive to analogue treatment. The possibility of a loss of polyamine uptake regulation in the knockdown clones was confirmed by polyamine uptake analysis. These results are consistent with the hypothesis that AZ1 knockdown leads to dysregulation of polyamine uptake, resulting in increased analogue accumulation and toxicity. Importantly, there appears to be little difference between AZ1 knockdown cells and cells with normal levels of AZ1 with respect to ODC regulation, suggesting that another regulatory protein, potentially AZ2, compensates for the loss of AZ1. The results of these studies are important for the understanding of both the regulation of polyamine homeostasis and in understanding the factors that regulate tumor cell sensitivity to the anti-tumor polyamine analogues.

Mrc1 marks early-firing origins and coordinates timing and efficiency of initiation in fission yeast

How early- and late-firing origins are selected on eukaryotic chromosomes is largely unknown. Here, we show that Mrc1, a conserved factor required for stabilization of stalled replication forks, selectively binds to the early-firing origins in a manner independent of Cdc45 and Hsk1 kinase in the fission yeast Schizosaccharomyces pombe. In mrc1Δ cells (and in swi1Δ cells to some extent), efficiency of firing is stimulated, and its timing is advanced selectively at those origins that are normally bound by Mrc1. In contrast, the late or inefficient origins which are not bound by Mrc1 are not activated in mrc1Δ cells. The enhanced firing and precocious Cdc45 loading at Mrc1-bound early-firing origins are not observed in a checkpoint mutant of mrc1, suggesting that non-checkpoint function is involved in maintaining the normal program of early-firing origins. We propose that prefiring binding of Mrc1 is an important marker of early-firing origins which are precociously activated by the absence of this protein.

DDK phosphorylates checkpoint clamp component Rad9 and promotes its release from damaged chromatin

When inappropriate DNA structures arise, they are sensed by DNA structure-dependent checkpoint pathways and subsequently repaired. Recruitment of checkpoint proteins to such structures precedes recruitment of proteins involved in DNA metabolism. Thus, checkpoints can regulate DNA metabolism. We show that fission yeast Rad9, a 9-1-1 heterotrimeric checkpoint-clamp component, is phosphorylated by Hsk1(Cdc7), the Schizosaccharomyces pombe Dbf4-dependent kinase (DDK) homolog, in response to replication-induced DNA damage. Phosphorylation of Rad9 disrupts its interaction with replication protein A (RPA) and is dependent on 9-1-1 chromatin loading, the Rad9-associated protein Rad4/Cut5(TopBP1), and prior phosphorylation by Rad3(ATR). rad9 mutants defective in DDK phosphorylation show wild-type checkpoint responses but abnormal DNA repair protein foci and decreased viability after replication stress. We propose that Rad9 phosphorylation by DDK releases Rad9 from DNA damage sites to facilitate DNA repair.

Hsk1 kinase and Cdc45 regulate replication stress-induced checkpoint responses in fission yeast

In fission yeast, replication fork arrest activates the replication checkpoint effector kinase Cds1(Chk2/Rad53) through the Rad3(ATR/Mec1)-Mrc1(Claspin) pathway. Hsk1, the Cdc7 homologue of fission yeast required for efficient initiation of DNA replication, is also required for Cds1 activation. Hsk1 kinase activity is required for induction and maintenance of Mrc1 hyperphosphorylation, which is induced by replication fork block and mediated by Rad3. Rad3 kinase activity does not change in an hsk1 temperature-sensitive mutant, and Hsk1 kinase activity is not affected by rad3 mutation. Hsk1 kinase vigorously phosphorylates Mrc1 in vitro, predominantly at non-SQ/TQ sites, but this phosphorylation does not seem to affect the Rad3 action on Mrc1. Interestingly, the replication stress-induced activation of Cds1 and hyperphosphorylation of Mrc1 is almost completely abrogated in an initiation-defective mutant of cdc45, but not in an mcm2 or polε mutant. The results suggest that Hsk1-mediated loading of Cdc45 onto replication origins may play important roles in replication stress-induced checkpoint.

Eukaryotic chromosome DNA replication: where, when, and how?

DNA replication is central to cell proliferation. Studies in the past six decades since the proposal of a semiconservative mode of DNA replication have confirmed the high degree of conservation of the basic machinery of DNA replication from prokaryotes to eukaryotes. However, the need for replication of a substantially longer segment of DNA in coordination with various internal and external signals in eukaryotic cells has led to more complex and versatile regulatory strategies. The replication program in higher eukaryotes is under a dynamic and plastic regulation within a single cell, or within the cell population, or during development. We review here various regulatory mechanisms that control the replication program in eukaryotes and discuss future directions in this dynamic field.

Deregulated Cdc6 inhibits DNA replication and suppresses Cdc7-mediated phosphorylation of Mcm2-7 complex

Mcm2–7 is recruited to eukaryotic origins of DNA replication by origin recognition complex, Cdc6 and Cdt1 thereby licensing the origins. Cdc6 is essential for origin licensing during DNA replication and is readily destabilized from chromatin after Mcm2–7 loading. Here, we show that after origin licensing, deregulation of Cdc6 suppresses DNA replication in Xenopus egg extracts without the involvement of ATM/ATR-dependent checkpoint pathways. DNA replication is arrested specifically after chromatin binding of Cdc7, but before Cdk2-dependent pathways and deregulating Cdc6 after this step does not impair activation of origin firing or elongation. Detailed analyses revealed that Cdc6 deregulation leads to strong suppression of Cdc7-mediated hyperphosphorylation of Mcm4 and subsequent chromatin loading of Cdc45, Sld5 and DNA polymerase α. Mcm2 phosphorylation is also repressed although to a lesser extent. Remarkably, Cdc6 itself does not directly inhibit Cdc7 kinase activity towards Mcm2–4–6–7 in purified systems, rather modulates Mcm2–7 phosphorylation on chromatin context. Taken together, we propose that Cdc6 on chromatin acts as a modulator of Cdc7-mediated phosphorylation of Mcm2–7, and thus destabilization of Cdc6 from chromatin after licensing is a key event ensuring proper transition to the initiation of DNA replication.

Hsk1- and SCF(Pof3)-dependent proteolysis of S. pombe Ams2 ensures histone homeostasis and centromere function

Schizosaccharomyces pombe GATA factor Ams2 is responsible for cell cycle-dependent transcriptional activation of all the core histone genes peaking at G1/S phase. Intriguingly, its own protein level also fluctuates concurrently. Here, we show that Ams2 is ubiquitylated and degraded through the SCF (Skp1-Cdc53/Cullin-1-F-box) ubiquitin ligase, in which F box protein Pof3 binds this protein. Ams2 is phosphorylated at multiple sites, which is required for SCF^Pof3-dependent proteolysis. Hsk1/Cdc7 kinase physically associates with and phosphorylates Ams2. Even mild overexpression of Ams2 induces constitutive histone expression and chromosome instability, and its toxicity is exaggerated when Hsk1 function is compromised. This is partly attributable to abnormal incorporation of canonical H3 into the central CENP-A/Cnp1-rich centromere, thereby reversing specific chromatin structures to apparently normal nucleosomes. We propose that Hsk1 plays a vital role during post S phase in genome stability via SCF^Pof3-mediated degradation of Ams2, thereby maintaining centromere integrity.

MicroRNA expression profile of gastrointestinal stromal tumors is distinguished by 14q loss and anatomic site

MicroRNAs are known to regulate gene expression. Although unique microRNA expression profiles have been reported in several tumors, little is known about microRNA expression profiles in GISTs. To evaluate the relationship between microRNA expression and clinicopathologic findings of GISTs, we analyzed the microRNA expression profiles of GISTs. We used fresh frozen tissues from 20 GISTs and analyzed KIT and PDGFRA mutations and chromosomal loss status. MicroRNA expression was analyzed using a microRNA chip containing 470 microRNAs. Using unsupervised hierarchical clustering analysis, we found four distinct microRNA expression patterns in our 20 GISTs. Six GISTs that did not have 14q loss formed a separate cluster. In the 14 GISTs with 14q loss, 5 small bowel GISTs formed a separate cluster and the remaining 9 GISTs could be divided into two groups according to frequent chromosomal losses and tumor risk. We found 73 microRNAs that were significantly down-regulated in the GISTs with 14q loss; 38 of these microRNAs are encoded on 14q. We also found many microRNAs that were down-regulated in small bowel and high-risk group GISTs. Most of the microRNAs down-regulated in the high-risk group and small bowel GISTs are known to be involved in tumor progression, specifically by stimulating mitogen-activated protein kinase (MAPK) and the cell cycle. The microRNA expression patterns of GISTs are closely related to the status of 14q loss, anatomic site, and tumor risk. These findings suggest that microRNA expression patterns can differentiate several subsets of GISTs.

Modulation of cellular function by polyamines

Polyamines (putrescine, spermidine and spermine) are essential for normal cell growth. The polyamine levels in cells are regulated by biosynthesis, degradation, and transport. Polyamines can modulate the functions of DNA, nucleotide triphosphates, proteins, and especially RNA because most polyamines exist in a polyamine-RNA complex in cells. Thus, the major focus on this review is on the role of polyamines in protein synthesis. In addition, effects of polyamines on B to Z conversion of DNA, transcription, phosphorylation of proteins, cell cycle progression, apoptosis and ion channels, especially NMDA receptors, are outlined. The function of eIF5A is also briefly discussed. Finally, a correlation between acrolein, produced from polyamines by polyamine oxidases, and chronic renal failure or brain stroke is summarized. Increased levels of polyamine oxidases and acrolein are good markers of chronic renal failure and brain stroke.

Drug design with Cdc7 kinase: a potential novel cancer therapy target

Identification of novel molecular targets is critical in development of new and efficient cancer therapies. Kinases are one of the most common drug targets with a potential for cancer therapy. Cell cycle progression is regulated by a number of kinases, some of which are being developed to treat cancer. Cdc7 is a serine-threonine kinase originally discovered in budding yeast, which has been shown to be necessary to initiate the S phase. Inhibition of Cdc7 in cancer cells retards the progression of the S phase, accumulates DNA damage, and induces p53-independent cell death, but the same treatment in normal cells does not significantly affect of less than viability. Low-molecular-weight compounds that inhibit Cdc7 kinase with an IC₅₀ 10 nM have been identified, and shown to be effective in the inhibition of tumor growth in animal models. Thus Cdc7 kinase can be recognized as a novel molecular target for cancer therapy.

The role of polyamines in protein-dependent hypoxic tolerance of Drosophila

Background

Chronic hypoxia is a major component of ischemic diseases such as stroke or myocardial infarction. Drosophila is more tolerant to hypoxia than most mammalian species. It is considered as a useful model organism to identify new mechanisms of hypoxic tolerance. The hypoxic tolerance of flies has previously been reported to be enhanced by low protein diets. This study analyses the mechanisms involved.

Results

Feeding adult Drosophila on a yeast diet dramatically reduced their longevities under chronic hypoxic conditions (5% O₂). Mean and maximum longevities became close to the values observed for starving flies. The action of dietary yeast was mimicked by a whole casein hydrolysate and by anyone of the 20 natural amino acids that compose proteins. It was mimicked by amino acid intermediates of the urea cycle such as L-citrulline and L-ornithine, and by polyamines (putrescine, spermidine and spermine). α-difluoromethylornithine, a specific inhibitor of ornithine decarboxylase, partially protected hypoxic flies from amino acid toxicity but not from polyamine toxicity. N¹-guanyl-1,7 diaminoheptane, a specific inhibitor of eIF5A hypusination, partially relieved the toxicities of both amino acids and polyamines.

Conclusion

Dietary amino acids reduced the longevity of chronically hypoxic flies fed on a sucrose diet. Pharmacological evidence suggests that the synthesis of polyamines and the hypusination of eIF5A contributed to the life-shortening effect of dietary amino acids.