Partial characterization of the MPM-2 phosphoepitope

Epigenetic modifications and p21-cyclin B1 nexus in anticancer effect of histone deacetylase inhibitors in combination with silibinin on non-small cell lung cancer cells

There is a renewed focus on targeted therapy against epigenetic events that are altered during the pathogenesis of lung cancer. However, the use of epigenomic modifiers as monotherapy lacks efficacy; thus, there is a need to develop safe and effective drug combinatorial regimens, which reverse epigenetic modifications and exhibit profound anticancer activity. Based on these perspectives, we evaluated, for the first time, the efficacy and associated mechanisms of a novel combinatorial regimen of histone deacetylase inhibitors (HDACi)-trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA)-with silibinin (a flavonolignan with established pre-clinical anti-lung cancer efficacy) against non-small cell lung cancer (NSCLC). Silibinin inhibited HDAC activity and decreased HDAC1-3 levels in NSCLC cells, leading to an overall increase in global histone acetylation states of histones H3 and H4. Combinations of HDCAi with silibinin synergistically augmented the cytotoxic effects of these single agents, which was associated with a dramatic increase in p21 (Cdkn1a). Subsequent ChIP assay indicated increased acetylated histone H3 and H4 levels on p21 promoter region, resulting in its increased transcription. The enhanced p21 levels promoted proteasomal degradation of cyclin B1, the limited supply of which halts the progression of cells into mitosis. Indeed, the resultant biological effect was a significant G 2/M arrest by the combination treatment, followed by apoptotic cell death. Similar epigenetic modulations were observed in vivo, together with a marked reduction in xenograft growth. These findings are both novel and highly significant in establishing that HDACi with silibinin would be safe and effective to suppress NSCLC growth.

Phosphoregulation of STIM1 leads to exclusion of the endoplasmic reticulum from the mitotic spindle

The endoplasmic reticulum (ER) undergoes significant reorganization between interphase and mitosis, but the underlying mechanisms are unknown. Stromal interaction molecule 1 (STIM1) is an ER Ca(2+) sensor that activates store-operated Ca(2+) entry (SOCE) and also functions in ER morphogenesis through its interaction with the microtubule +TIP protein end binding 1 (EB1). We previously demonstrated that phosphorylation of STIM1 during mitosis suppresses SOCE. We now show that STIM1 phosphorylation is a major regulatory mechanism that excludes ER from the mitotic spindle. In mitotic HeLa cells, the ER forms concentric sheets largely excluded from the mitotic spindle. We show that STIM1 dissociates from EB1 in mitosis and localizes to the concentric ER sheets. However, a nonphosphorylatable STIM1 mutant (STIM1(10A)) colocalized extensively with EB1 and drove ER mislocalization by pulling ER tubules into the spindle. This effect was rescued by mutating the EB1 interaction site of STIM1(10A), demonstrating that aberrant association of STIM1(10A) with EB1 is responsible for the ER mislocalization. A STIM1 phosphomimetic exhibited significantly impaired +TIP tracking in interphase but was ineffective at inhibiting SOCE, suggesting different mechanisms of regulation of these two STIM1 functions by phosphorylation. Thus, ER spindle exclusion and ER-dependent Ca(2+) signaling during mitosis require multimodal STIM1 regulation by phosphorylation.

Regulation of store-operated calcium entry during cell division

Store-operate Ca2+ channels gate Ca2+ entry into the cytoplasm in response to the depletion of Ca2+ from endoplasmic reticulum Ca2+ stores. The major molecular components of store-operated Ca2+ entry are STIM (stromal-interacting molecule) 1 (and in some instances STIM2) that serves as the endoplasmic reticulum Ca2+ sensor, and Orai (Orai1, Orai2 and Orai3) which function as pore-forming subunits of the store-operated channel. It has been known for some time that store-operated Ca2+ entry is shut down during cell division. Recent work has revealed complex mechanisms regulating the functions and locations of both STIM1 and Orai1 in dividing cells.

Actin behavior in bulk cytoplasm is cell cycle regulated in early vertebrate embryos

The mechanical properties of cells change as they proceed through the cell cycle, primarily owing to regulation of actin and myosin II. Most models for cell mechanics focus on actomyosin in the cortex and ignore possible roles in bulk cytoplasm. We explored cell cycle regulation of bulk cytoplasmic actomyosin in Xenopus egg extracts, which is almost undiluted cytoplasm from unfertilized eggs. We observed dramatic gelation-contraction of actomyosin in mitotic (M phase) extract where Cdk1 activity is high, but not in interphase (I-phase) extract. In spread droplets, M-phase extract exhibited regular, periodic pulses of gelation-contraction a few minutes apart that continued for many minutes. Comparing actin nucleation, disassembly and myosin II activity between M-phase and I-phase extracts, we conclude that regulation of nucleation is likely to be the most important for cell cycle regulation. We then imaged F-actin in early zebrafish blastomeres using a GFP-Utrophin probe. Polymerization in bulk cytoplasm around vesicles increased dramatically during mitosis, consistent with enhanced nucleation. We conclude that F-actin polymerization in bulk cytoplasm is cell cycle regulated in early vertebrate embryos and discuss possible biological functions of this regulation.

Phosphorylation of STIM1 underlies suppression of store-operated calcium entry during mitosis

Store-operated Ca(2+) entry (SOCE) and Ca(2+) release-activated Ca(2+) currents (I(crac)) are strongly suppressed during cell division, the only known physiological situation in which Ca(2+) store depletion is uncoupled from the activation of Ca(2+) influx [corrected]. We found that the endoplasmic reticulum (ER) Ca(2+) sensor STIM1 failed to rearrange into near-plasma membrane puncta in mitotic cells, a critical step in the SOCE-activation pathway. We also found that STIM1 from mitotic cells is recognized by the phospho-specific MPM-2 antibody, suggesting that STIM1 is phosphorylated during mitosis. Removal of ten MPM-2 recognition sites by truncation at amino acid 482 abolished MPM-2 recognition of mitotic STIM1, and significantly rescued STIM1 rearrangement and SOCE response in mitosis. We identified Ser 486 and Ser 668 as mitosis-specific phosphorylation sites, and STIM1 containing mutations of these sites to alanine also significantly rescued mitotic SOCE. Therefore, phosphorylation of STIM1 at Ser 486 and Ser 668, and possibly other sites, underlies suppression of SOCE during mitosis.

Regulation of inositol 1,4,5-trisphosphate receptor type 1 function during oocyte maturation by MPM-2 phosphorylation

Egg activation and further embryo development require a sperm-induced intracellular Ca(2+) signal at the time of fertilization. Prior to fertilization, the egg's Ca(2+) machinery is therefore optimized. To this end, during oocyte maturation, the sensitivity, i.e. the Ca(2+) releasing ability, of the inositol 1,4,5-trisphosphate receptor type 1 (IP(3)R1), which is responsible for most of this Ca(2+) release, markedly increases. In this study, the recently discovered specific Polo-like kinase (Plk) inhibitor BI2536 was used to investigate the role of Plk1 in this process. BI2536 inactivates Plk1 in oocytes at the early stages of maturation and significantly decreases IP(3)R1 phosphorylation at an MPM-2 epitope at this stage. Moreover, this decrease in Plk1-dependent MPM-2 phosphorylation significantly lowers IP(3)R1 sensitivity. Finally, using in vitro phosphorylation techniques we identified T(2656) as a major Plk1 site on IP(3)R1. We therefore propose that the initial increase in IP(3)R1 sensitivity during oocyte maturation is underpinned by IP(3)R1 phosphorylation at an MPM-2 epitope(s).

Phosphorylation of IP3R1 and the regulation of [Ca2+]i responses at fertilization: a role for the MAP kinase pathway

A sperm-induced intracellular Ca2+ signal ([Ca2+]i) underlies the initiation of embryo development in most species studied to date. The inositol 1,4,5 trisphosphate receptor type 1 (IP3R1) in mammals, or its homologue in other species, is thought to mediate the majority of this Ca2+ release. IP3R1-mediated Ca2+ release is regulated during oocyte maturation such that it reaches maximal effectiveness at the time of fertilization, which, in mammalian eggs, occurs at the metaphase stage of the second meiosis (MII). Consistent with this, the [Ca2+]i oscillations associated with fertilization in these species occur most prominently during the MII stage. In this study, we have examined the molecular underpinnings of IP3R1 function in eggs. Using mouse and Xenopus eggs, we show that IP3R1 is phosphorylated during both maturation and the first cell cycle at a MPM2-detectable epitope(s), which is known to be a target of kinases controlling the cell cycle. In vitro phosphorylation studies reveal that MAPK/ERK2, one of the M-phase kinases, phosphorylates IP3R1 at at least one highly conserved site, and that its mutation abrogates IP3R1 phosphorylation in this domain. Our studies also found that activation of the MAPK/ERK pathway is required for the IP3R1 MPM2 reactivity observed in mouse eggs, and that eggs deprived of the MAPK/ERK pathway during maturation fail to mount normal [Ca2+]i oscillations in response to agonists and show compromised IP3R1 function. These findings identify IP3R1 phosphorylation by M-phase kinases as a regulatory mechanism of IP3R1 function in eggs that serves to optimize [Ca2+]i release at fertilization.

Molecular alterations after Polo-like kinase 1 mRNA suppression versus pharmacologic inhibition in cancer cells

Multiple roles within mitosis have been assigned to Polo-like kinase 1 (Plk1), making it an attractive candidate for mitotic targeting of cancer cells. We have employed chimeric antisense oligonucleotides to investigate the molecular alterations after targeted interference with Plk1 in RKO human colon adenocarcinoma and PC3 prostate cancer cells. Suppression of Plk1 mRNA resulted in a dramatic increase of the mitotic index followed by the onset of apoptosis. Mitotically arrested cells displayed randomly separated condensed chromosomes and the occurrence of multiple spindle poles with well-formed asters. Induction of apoptosis was strictly dependent on cell cycle progression: Genetically engineered RKO cells with inducible expression of the cyclin-dependent kinase inhibitor p27(Kip1) were completely refractory to Plk1 depletion-induced apoptosis when they were arrested in the G1 phase of the cell cycle. Various mitotic markers, including MPM-2, cdc25c, cyclin B1, or phosphorylated histone H3, were investigated to explore the molecular consequences of Plk1 depletion. Whereas most marker proteins showed similar alterations compared with treatment with paclitaxel, cdc25c was fully phosphorylated solely in paclitaxel-treated cells but only partially phosphorylated in Plk1-depleted cells, although both treatments caused a profound mitotic arrest. This differential phosphorylation of cdc25c was used to test whether a pharmacologic inhibitor of Plk1 would exert the same cellular effects as interference with Plk1 on a mRNA level. It was found that the differential electrophoretic mobility of cdc25c can serve as a reliable molecular marker to track inhibition of Plk1 by small-molecule inhibitors within a cell.

Histone hyperacetylation in mitosis prevents sister chromatid separation and produces chromosome segregation defects

Posttranslational modifications of core histones contribute to driving changes in chromatin conformation and compaction. Herein, we investigated the role of histone deacetylation on the mitotic process by inhibiting histone deacetylases shortly before mitosis in human primary fibroblasts. Cells entering mitosis with hyperacetylated histones displayed altered chromatin conformation associated with decreased reactivity to the anti-Ser 10 phospho H3 antibody, increased recruitment of protein phosphatase 1-delta on mitotic chromosomes, and depletion of heterochromatin protein 1 from the centromeric heterochromatin. Inhibition of histone deacetylation before mitosis produced defective chromosome condensation and impaired mitotic progression in living cells, suggesting that improper chromosome condensation may induce mitotic checkpoint activation. In situ hybridization analysis on anaphase cells demonstrated the presence of chromatin bridges, which were caused by persisting cohesion along sister chromatid arms after centromere separation. Thus, the presence of hyperacetylated chromatin during mitosis impairs proper chromosome condensation during the pre-anaphase stages, resulting in poor sister chromatid resolution. Lagging chromosomes consisting of single or paired sisters were also induced by the presence of hyperacetylated histones, indicating that the less constrained centromeric organization associated with heterochromatin protein 1 depletion may promote the attachment of kinetochores to microtubules coming from both poles.

Overexpression of simian virus 40 small-T antigen blocks centrosome function and mitotic progression in human fibroblasts

Recombinant adenoviruses that express high levels of the simian virus 40 (SV40) small-t (ST) antigen have been used to study the requirement for ST to drive cell cycle proliferation of confluent human diploid fibroblasts. This occurs when either large-T (LT) antigen or serum is added to provide a second signal. While cells readily completed S phase in these experiments, they were found to accumulate with 4N DNA content. Cellular and nuclear morphology, as well as the biochemical status of cyclin B complexes, showed that these cells entered mitosis but were blocked prior to mitotic metaphase. The defect appears to reflect an inability of cells overexpressing ST to form organized centrosomes that duplicate and separate normally during the cell cycle and, therefore, the absence of a mitotic spindle. The ability of ST to bind protein phosphatase 2A was required for this pattern, suggesting that altered phosphorylation of key centrosomal components may occur when ST is overexpressed. Although the possible significance of ST effects on the centrosome cycle is not fully understood, these findings suggest that ST could influence chromosomal instability patterns that are a hallmark of SV40-transformed cells and LT expression.

Kinetics of endomitosis in primary murine megakaryocytes

Megakaryocytes (MKs) develop from diploid progenitor cells via successive rounds of DNA synthesis in the absence of cell division, a process termed endomitosis (EnM). While the mechanism underlying EnM is not known, studies in yeast and leukemic cell lines have suggested that it may be due to reduced levels of cyclin B1 or cdc2, leading to a decrease in mitotic kinase activity. Using flow cytometry to study EnM highly purified marrow-derived MK precursors, we found that: (1) on average, 36% of 8N-32N MKs expressed abundant cyclin B during G2/M. The percentage of cells in G2/M decreased in >64N MKs, suggesting the limit of EnM, (2) the level of cyclin B per G2/M MK increased linearly with ploidy, (3) cyclin B expression oscillated normally in polyploid MKs, (4) MPM-2, a phosphoepitope created by the action of mitotic kinases and specific to M-phase cells, was expressed in a significant fraction of polyploid MKs, and (5) there was an apparent increase of cyclin B in G1-phase in polyploid MKs. This study provides the first qualitative kinetic data regarding the cell cycle status of MKs within individual ploidy classes. It also demonstrates the feasibility of using anti-cyclin B antibody and flow cytometry to resolve G1 from G2/M populations in polyploid MKs. Finally, these findings establish that neither a relative nor absolute deficiency of mitotic kinase components is responsible for EnM, suggesting that the departure from normal cell division kinetics seen in polyploid MKs is likely due to alterations in other cell cycle regulators.

Mitotic phosphorylation of DNA topoisomerase II alpha by protein kinase CK2 creates the MPM-2 phosphoepitope on Ser-1469

DNA topoisomerase II alpha is required for chromatin condensation during prophase. This process is temporally linked with the appearance of mitosis-specific phosphorylation sites on topoisomerase IIalpha including one recognized by the MPM-2 monoclonal antibody. We now report that the ability of mitotic extracts to create the MPM-2 epitope on human topoisomerase II alpha is abolished by immunodepletion of protein kinase CK2. Furthermore, the MPM-2 phosphoepitope on topoisomerase II alpha can be generated by purified CK2. Phosphorylation of C-truncated topoisomerase II alpha mutant proteins conclusively shows, that the MPM-2 epitope is present in the last 163 amino acids. Use of peptides containing all conserved CK2 consensus sites in this region indicates that only the peptide containing Arg-1466 to Ala-1485 is able to compete with topoisomerase II alpha for binding of the MPM-2 antibody. Replacement of Ser-1469 with Ala abolishes the ability of the phosphorylated peptide to bind to the MPM-2 antibody while a peptide containing phosphorylated Ser-1469 binds tightly. Surprisingly, the MPM-2 phosphoepitope influences neither the catalytic activity of topoisomerase II alpha nor its ability to form molecular complexes with CK2 in vitro. In conclusion, we have identified protein kinase CK2 as a new MPM-2 kinase able to phosphorylate an important mitotic protein, topoisomerase II alpha, on Ser-1469.

Cell cycle-dependent phosphorylation of centrosomes: localization of phosphopeptide specific antibodies to the centrosome

The microtubule nucleation capacity of the centrosome increases dramatically as cells progress from interphase into mitosis. The increase in nucleation capacity of the centrosome correlates with the cell cycle-dependent localization of the mitotic protein monoclonal-2 (MPM-2) phosphoepitope-specific antibody to the mitotic centrosome. Therefore, the phosphorylation state of centrosomal components may regulate the microtubule nucleation capacity of this organelle during mitosis. Neither the identity of the MPM-2 kinase(s) nor all of the MPM-2-reactive phosphoproteins associated with the centrosome have been fully elucidated. Only recently have the characteristics of the MPM-2 epitope site been defined, and we used this information to prepare polyclonal antibodies against synthetic phosphopeptides containing potential MPM-2 epitopes derived from the sequences of two MPM-2-reactive proteins, topoisomerase II, and microtubule associated protein 1B (MAP1B). We demonstrate that these phosphopeptide-specific antibodies also localize to the centrosome in a cell cycle-dependent fashion. Thus, polyclonal antibodies have been generated against defined phosphopeptides that reiterate many of the immunofluorescence staining properties exhibited by the MPM-2 antibody. These new phosphopeptide-specific antibodies will provide additional probes to examine the phosphorylation of centrosomal components and the functional consequences of their phosphorylation during mitosis.

The disappearance of cyclins A and B and the increase in activity of the G(2)/M-phase cellular kinase cdc2 in herpes simplex virus 1-infected cells require expression of the alpha22/U(S)1.5 and U(L)13 viral genes

In uninfected cells the G(2)/M transition is regulated by cyclin kinase complex containing cdc2 and, initially, cyclin A, followed by cyclin B. cdc2 is downregulated through phosphorylation by wee-1 and myt-1 and upregulated by cdc-25C phosphatase. We have examined the accumulation and activities of these proteins in cells infected with wild type and mutants of herpes simplex virus 1. The results were as follows. (i) Cyclin A and B levels were reduced beginning 4 h after infection and were undetectable at 12 to 16 h after infection. (ii) cdc2 protein also decreased in amount but was detectable at all times after infection. In addition, a fraction of cdc2 protein from infected cells exhibited altered electrophoretic mobility in denaturing gels. (iii) The levels of cdk7 or myt-1 proteins remained relatively constant throughout infection, whereas the level of wee-1 was significantly decreased. (iv) cdc-25C formed novel bands characterized by slower electrophoretic mobility that disappeared after treatment with phosphatase. In addition, one phosphatase-sensitive band reacted with MPM-2 antibody that recognizes a phosphoepitope phosphorylated exclusively in M phase. (v) cdc2 accumulating in infected cells exhibited kinase activity. The activity of cdc2 was higher in infected cell lysates than those of corresponding proteins present in lysates of mock-infected cells even though cyclins A and B were not detectable in lysates of infected cells. (vi) The decrease in the levels of cyclins A and B, the increase in activity of cdc2, and the hyperphosphorylation of cdc-25C were mediated by U(L)13 and alpha22/U(S)1.5 gene products. In light of its normal functions, the activated cdc2 kinase may play a role in the changes in the morphology of the infected cell. These results are consistent with the accruing evidence that herpes simplex virus scavenges the cell for useful cell cycle proteins and subverts them for its own use.

A hyperphosphorylated form of RNA polymerase II is the major interphase antigen of the phosphoprotein antibody MPM-2 and interacts with the peptidyl-prolyl isomerase Pin1

The monoclonal antibody MPM-2 recognizes a subset of M phase phosphoproteins in a phosphorylation-dependent manner. It is believed that phosphorylation at MPM-2 antigenic sites could regulate mitotic events since most of the MPM-2 antigens identified to date have M phase functions. In addition, many of these proteins are substrates of the mitotic regulator Pin1, a peptidyl-prolyl isomerase which is present throughout the cell cycle and which is thought to alter its mitotic targets by changing their conformation. In interphase cells, most MPM-2 reactivity is confined to nuclear speckles. We report here that a hyperphosphorylated form of the RNA polymerase II largest subunit is the major MPM-2 interphase antigen. These findings were made possible by the availability of another monoclonal antibody, CC-3, that was previously used to identify a 255 kDa nuclear matrix protein associated with spliceosomal components as a hyperphosphorylated form of the RNA polymerase II largest subunit. MPM-2 recognizes a phosphoepitope of the large subunit that becomes hyperphosphorylated upon heat shock in contrast to the phosphoepitope defined by CC-3, whose reactivity is diminished by the heat treatment. Therefore, these two antibodies may discriminate between distinct functional forms of RNA polymerase II. We also show that RNA polymerase II large subunit interacts with Pin1 in HeLa cells. Pin1 may thus regulate transcriptional and post-transcriptional events by catalyzing phosphorylation-dependent conformational changes of the large RNA polymerase II subunit.

Recombinant, replication-defective adenovirus gene transfer vectors induce cell cycle dysregulation and inappropriate expression of cyclin proteins

First-generation adenovirus (Ad) vectors that had been rendered replication defective by removal of the E1 region of the viral genome (DeltaE1) or lacking the Ad E3 region in addition to E1 sequences (DeltaE1DeltaE3) induced G2 cell cycle arrest and inhibited traverse across G1/S in primary and immortalized human bronchial epithelial cells. Cell cycle arrest was independent of the cDNA contained in the expression cassette and was associated with the inappropriate expression and increase in cyclin A, cyclin B1, cyclin D, and cyclin-dependent kinase p34(cdc2) protein levels. In some instances, infection with DeltaE1 or DeltaE1 DeltaE3 Ad vectors produced aneuploid DNA histogram patterns and induced polyploidization as a result of successive rounds of cell division without mitosis. Cell cycle arrest was absent in cells infected with a second-generation DeltaE1Ad vector in which all of the early region E4 except the sixth open reading frame was also deleted. Consequently, E4 viral gene products present in DeltaE1 or DeltaE1 DeltaE3 Ad vectors induce G2 growth arrest, which may pose new and unintended consequences for human gene transfer and gene therapy.

Cell cycle control of chorion gene amplification

Over-replication of two clusters of chorion genes in Drosophila ovarian follicle cells is essential for rapid eggshell biosynthesis. The relationship of this amplification to the follicle cell cycles has remained unclear. To investigate the regulation of amplification, we developed a technique to detect amplifying chorion genes in individual follicle cells using BrdU incorporation and FISH. Amplification occurs in two developmental phases. One of the gene clusters begins to amplify periodically during S phases of follicle cell endocycles. Subsequently, after endocycles have ceased, both clusters amplify continuously during the remainder of oogenesis. In contrast to the early phase, late amplification commences synchronously among follicle cells. The pattern of Cyclin E expression mirrors these two phases. We present evidence that Cyclin E is required positively for amplification. We suggest that Cyclin E also acts negatively to inhibit refiring of most origins within a cycle, and that specific factors at chorion origins allow them to escape this negative rereplication control. Our findings suggest that chorion amplification is a model for understanding metazoan replicons and the controls that restrict replication to once per cell cycle.