Characterization of the enhancer-like okadaic acid response element region of the cyclin-dependent kinase 1 (p34cdc2) promoter

NF45 and NF90 Regulate Mitotic Gene Expression by Competing with Staufen-Mediated mRNA Decay

In human cells, the expression of ∼1,000 genes is modulated throughout the cell cycle. Although some of these genes are controlled by specific transcriptional programs, very little is known about their post-transcriptional regulation. Here, we analyze the expression signature associated with all 687 RNA-binding proteins (RBPs) and identify 39 that significantly correlate with cell cycle mRNAs. We find that NF45 and NF90 play essential roles in mitosis, and transcriptome analysis reveals that they are necessary for the expression of a subset of mitotic mRNAs. Using proteomics, we identify protein clusters associated with the NF45-NF90 complex, including components of Staufen-mediated mRNA decay (SMD). We show that depletion of SMD components increases the binding of mitotic mRNAs to the NF45-NF90 complex and rescues cells from mitotic defects. Together, our results indicate that the NF45-NF90 complex plays essential roles in mitosis by competing with the SMD machinery for a common set of mRNAs.

Characterization of HOX gene expression in canine mammary tumour cell lines from spontaneous tumours

Spatial/temporal controls of development are regulated by the homeotic (HOX) gene complex and require integration with oncogenes and tumour suppressors regulating cell cycle exit. Spontaneously derived neoplastic canine mammary carcinoma cell models were investigated to determine if HOX expression profiles were associated with neoplasia as HOX genes promote neoplastic potential in human cancers. Comparative assessment of human and canine breast cancer expression profiles revealed remarkable similarity for all four paralogous HOX gene clusters and several unlinked HOX genes. Five canine HOX genes were overexpressed with expression profiles consistent with oncogene-like character (HOXA1, HOXA13, HOXD4, HOXD9 and SIX1) and three HOX genes with underexpressed profiles (HOXA11, HOXC8 and HOXC9) were also identified as was an apparent nonsense mutation in HOXC6. This data, as well as a comparative analysis of similar data from human breast cancers suggested expression of selected HOX genes in canine mammary carcinoma could be contributing to the neoplastic phenotype.

Human cytomegalovirus UL97 kinase alters the accumulation of CDK1

The UL97 protein kinase is a serine/threonine kinase expressed by human cytomegalovirus (CMV) that phosphorylates ganciclovir. An investigation of the subcellular localization of pUL97 in infected cells indicated that, early in infection, pUL97 localized to focal sites in the nucleus that transitioned to subnuclear compartments and eventually throughout the entire nucleus. When UL97 kinase activity was eliminated with a K355M mutation or pharmacologically inhibited with maribavir, the expansion and redistribution of pUL97 foci within the nucleus was delayed, nuclear reorganization did not occur and assembly complexes in the cytoplasm failed to form normally. As UL97 kinase and its homologues appear to be functionally related to CDK1, a known regulator of nuclear structural organization, the effects of the UL97 kinase on CDK1 were investigated. Expression of CDK1 in infected cells appeared to be induced by UL97 kinase activity at the level of transcription and was not tied to other virus life-cycle events, such as viral DNA replication or virion assembly. These results suggest that, in addition to phosphorylating CDK1 targets, the UL97 kinase modifies G₂/M cell-cycle checkpoint regulators, specifically CDK1, to promote virus replication.

Expression and sequence of canine SIRT2 and p53 genes in canine mammary tumour cells - effects on downstream targets Wip1 and p21/Cip1

We have characterized gene dysfunction in a cellular model of spontaneous canine mammary cancer by investigating specific gene defects in SIRT2 and p53 genes for comparative studies among canine tumour-derived cell lines. These genes and their downstream targets are involved in regulating gene silencing, cell cycle progression and prevention of senescence and apoptosis. Canine SIR2 reverse transcriptase-polymerase chain reaction amplicons were most homologous to human SIRT2 and revealed detectable transcripts in all cell lines. Canine SIRT2 contained non-conserved amino acid substitutions, representing mutations or allelic differences and interspecies differences. Sequence differences between individuals in p53 and SIRT2 were found in two cell lines including a stop codon in p53 and substitutions of conserved cysteine residues in the Zn(2+)-binding motif in SIRT2. Mutations in SIRT2 were coincident with expression of the p53 modulator, Wip1; a failure to activate p21/Cip1 and extended G2/M phase. A third cell line appeared to function normally in these two pathways and likely possesses other defects in proliferation-control genes. This data identify potentially important defects in pathways regulated by p53 and SIRT2 that modulate cell proliferation and integrate development, apoptosis and proliferative lifespan. These genes offer promising therapeutic targets, contributing to the transformed/immortalized phenotype in spontaneous canine mammary cancer.

Alterations in CDK1 expression and nuclear/nucleolar localization following induction in a spontaneous canine mammary cancer model

Transcription of CDK1 is induced as cells re-enter the cell cycle from quiescence and these early cell cycle re-entry events have been modeled by okadaic acid treatment due to its activity on specific enhancer sequences in the human CDK1 promoter. To investigate heterogeneity of control of this mechanism in the context of neoplastic transformation, a cellular model derived from spontaneous canine mammary cancer (CMT) was developed that includes six cell lines derived from different animals. Notable heterogeneity in response to okadaic acid was observed in expression of CDK1 mRNA and protein. In response to okadaic acid treatment, two CMT cell lines exhibited a CDK1 mRNA induction while one cell line exhibited CDK1 mRNA suppression, and three remained unchanged. Despite this variability, three CMT cell lines arrested in S or G2/M phase and five exhibited marked increases in apoptosis. Moderation of some of these differences were observed at the level of CDK1 protein as three of six CMT cell lines exhibited only moderate enhancement in CDK1 protein levels while three remained essentially unchanged. Some additional differences in distribution of CDK1 protein, favoring enhanced nuclear over cytoplasmic CDK1 localization, were observed in treated cells in the form of concentrated nuclear CDK1 labeled foci. Confocal microscopy revealed the presence of brightly labeled punctate foci containing CDK1 protein within nuclei as well as nucleoli in okadaic acid treated non-mitotic cells suggesting a role for this kinase outside the normal G2/mitotic phase of the cell cycle and suggesting a possible new function within the nucleolus.