Large-scale identification of novel mitosis-specific phosphoproteins

Multiphosphorylation and cellular localization of poly(rC) binding protein 1 during mitosis in hela cell

OBJECTIVE

To monitor the phosphorylation modifications and cellular localization of poly(rC)-binding protein-1 (PCBP1) during the cell cycle progression of Hela cells.

RESULT

Hela cells highly synchronized at five different phases from interphase to mitosis were obtained. Using mitotic phosphoprotein-specific monoclonal antibody MPM-2, the exclusive occurrences of multiphosphorylation statuses of PCBP1 in mitosis were confirmed by a series of spots with increasing acidic pI (isoelectric point) in two rounds of 2D western blotting on the same membrane, and a visible molecular mass shift that can be eliminated by the treatment with λ phosphatase in 1D western blotting. Immnuofluorescence revealed the localization shift of PCBP1 during cell cycle, with accumulations in nucleus as a patch pattern in interphase, and a dispersive distribution without the area of the condensed chromosomes during mitosis.

CONCLUSIONS

These observations of mitosis-specific multiphosphorylations and localization shifts of PCBP1 suggest that the versatile PCBP1 was regulatable in a phosphorylation modification- and temporospatial-dependent manner in mitotic regulatory networks.

Proteomic identification of Hsp70 as a new Plk1 substrate in arsenic trioxide-induced mitotically arrested cells

We previously demonstrated that when arsenic trioxide (ATO)-induced mitotically arrested HeLa S3 cells (AIMACs) were treated with staurosporine (SSP) the cells rapidly exited mitosis. To better define the cellular targets and the underlying mechanisms of AIMACs, we applied 2-D DIGE followed by LC-MS/MS analysis and showed that SSP induced a significant change in the phosphoproteome of AIMACs. Among the proteins whose phosphorylation was modulated by SSP, we identified Hsp70, Rad 23B, and eukaryotic translation initiation factor 4B as potentially new substrates of polo-like kinase 1 (Plk1), an essential serine/threonine kinase with versatile mitotic functions. Since Hsp70 is a stress protein responsible for ATO treatment, we further identified Thr(13) , Ser(362) , Ser(631) , and Ser(633) on Hsp70 intracellularly phosphorylated in AIMACs by combining TiO(2) phospho-peptides enrichment and MS/MS analysis. Using antibody specifically against phosph-Ser(631) Hsp70 and further aided by expression of kinase-dead Plk1 and pharmacological inhibition of Plk1, we concluded that Ser(631) on Hsp70 is phosphorylated by Plk1 in AIMACs. By immnuofluorescent staining, we found the colocalization of Hsp70 and Plk1 in AIMACs but not in interphase cells. In addition, Plk1-mediated phosphorylation of Hsp70 prevented AIMACs from mitotic death. Our results reveal that Hsp70 is a novel substrate of Plk1 and that its phosphorylation contributes to attenuation of ATO-induced mitotic abnormalities.

Proteomic identification of proteins involved in the anticancer activities of oridonin in HepG2 cells

Oridonin is the main bioactive constituent of the Chinese medicinal herb Isodon rubescens and has been shown to have anti-neoplastic effects against a number of cancers in vitro and in vivo. Here we report the proteomic identification of proteins involved in the anticancer properties of oridonin in hepatocarcinoma HepG2 cells. Cell viability assay showed that oridonin dose-dependently inhibited cell growth with an IC(50) of 41.77μM. Treatment with oridonin at 44μM for 24h induced apoptosis and G2/M cell cycle arrest, which were associated with nine differentially expressed proteins identified by proteomic analysis. The proteomic expression patterns of Hsp70.1, Sti1 and hnRNP-E1 were confirmed by quantitative real-time PCR and/or immunoblotting. Eight of the nine identified proteins are shown, for the first time, to be involved in the anticancer activities of oridonin. Up-regulation of Hsp70.1, STRAP, TCTP, Sti1 and PPase, as well as the down-regulation of hnRNP-E1 could be responsible for the apoptotic and G2/M-arresting effects of oridonin observed in this study. Up-regulation of HP1 beta and GlyRS might contribute to inhibitory effects of oridonin on telomerase and tyrosine kinase, respectively. These findings shed new insights into the molecular mechanisms underlying the anticancer properties of oridonin in liver cancer cells.

The decision to enter mitosis: feedback and redundancy in the mitotic entry network

The decision to enter mitosis is mediated by a network of proteins that regulate activation of the cyclin B–Cdk1 complex. Within this network, several positive feedback loops can amplify cyclin B–Cdk1 activation to ensure complete commitment to a mitotic state once the decision to enter mitosis has been made. However, evidence is accumulating that several components of the feedback loops are redundant for cyclin B–Cdk1 activation during normal cell division. Nonetheless, defined feedback loops become essential to promote mitotic entry when normal cell cycle progression is perturbed. Recent data has demonstrated that at least three Plk1-dependent feedback loops exist that enhance cyclin B–Cdk1 activation at different levels. In this review, we discuss the role of various feedback loops that regulate cyclin B–Cdk1 activation under different conditions, the timing of their activation, and the possible identity of the elusive trigger that controls mitotic entry in human cells.

p90 ribosomal S6 kinase and p70 ribosomal S6 kinase link phosphorylation of the eukaryotic chaperonin containing TCP-1 to growth factor, insulin, and nutrient signaling

Chaperonin containing TCP-1 (CCT) is a large multisubunit complex that mediates protein folding in eukaryotic cells. CCT participates in the folding of newly synthesized polypeptides, including actin, tubulin, and several cell cycle regulators; therefore, CCT plays an important role in cytoskeletal organization and cell division. Here we identify the chaperonin CCT as a novel physiological substrate for p90 ribosomal S6 kinase (RSK) and p70 ribosomal S6 kinase (S6K). RSK phosphorylates the beta subunit of CCT in response to tumor promoters or growth factors that activate the Ras-mitogen-activated protein kinase (MAPK) pathway. CCTbeta Ser-260 was identified as the RSK site by mass spectrometry and confirmed by site-directed mutagenesis. RSK-dependent Ser-260 phosphorylation was sensitive to the MEK inhibitor UO126 and the RSK inhibitor BID-1870. Insulin weakly activates RSK but strongly activates the phosphoinositide 3-kinase (PI3K)-mammalian target of rapamycin (mTOR) pathway and utilizes S6K to regulate CCTbeta phosphorylation. Thus, the Ras-MAPK and PI3K-mTOR pathways converge on CCTbeta Ser-260 phosphorylation in response to multiple agonists in various mammalian cells. We also show that RNA interference-mediated knockdown of endogenous CCTbeta causes impaired cell proliferation that can be rescued with ectopically expressed murine CCTbeta wild-type or phosphomimetic mutant S260D, but not the phosphorylation-deficient mutant S260A. Although the molecular mechanism of CCTbeta regulation remains unclear, our findings demonstrate a link between oncogene and growth factor signaling and chaperonin CCT-mediated cellular activities.

An integrated comparative phosphoproteomic and bioinformatic approach reveals a novel class of MPM-2 motifs upregulated in EGFRvIII-expressing glioblastoma cells

Glioblastoma (GBM, WHO grade IV) is an aggressively proliferative and invasive brain tumor that carries a poor clinical prognosis with a median survival of 9 to 12 months. In a prior phosphoproteomic study performed in the U87MG glioblastoma cell line, we identified tyrosine phosphorylation events that are regulated as a result of titrating EGFRvIII, a constitutively active mutant of the epidermal growth factor receptor (EGFR) associated with poor prognosis in GBM patients. In the present study, we have used the phosphoserine/phosphothreonine-specific antibody MPM-2 (mitotic protein monoclonal #2) to quantify serine/threonine phosphorylation events in the same cell lines. By employing a bioinformatic tool to identify amino acid sequence motifs regulated in response to increasing oncogene levels, a set of previously undescribed MPM-2 epitope sequence motifs orthogonal to the canonical "pS/pT-P" motif was identified. These motifs contain acidic amino acids in combinations of the -5, -2, +1, +3, and +5 positions relative to the phosphorylated amino acid. Phosphopeptides containing these motifs are upregulated in cells expressing EGFRvIII, raising the possibility of a general role for a previously unrecognized acidophilic kinase (e.g. casein kinase II (CK2)) in cell proliferation downstream of EGFR signaling.