Targeted proteomic analysis of 14-3-3 sigma, a p53 effector commonly silenced in cancer

Proteomic identification of molecular targets of gambogic acid: role of stathmin in hepatocellular carcinoma

Gamboge has been developed as an injectable drug for cancer treatment in China. In this study, the inhibition ratio and their IC(50) values of two derivatives from Gamboge in hepatocellular carcinoma (HCC) were determined. Proteomic approach was employed to reveal the target proteins of these two derivatives, gambogic acid (GA), and gambogenic acid (GEA). HCC cells were cultured under varied conditions with the addition of either GA or GEA. Twenty differentially expressed proteins were identified and the four most distinctly expressed proteins were further validated by Western blotting. GA and GEA revealed inhibitory effects on HCC cell proliferation. The expression of cyclin-dependent kinase 4 inhibitor A and guanine nucleotide-binding protein beta subunit 1 were upregulated by both xanthones, whilst the expression of 14-3-3 protein sigma and stathmin 1 (STMN1) were downregulated. Furthermore, overexpression of STMN1 in HCC cells decreased their sensitivity, whilst small interfering RNAs targeting STMN1 enhanced their sensitivity to GA and GEA. In conclusion, our study suggested for the first time that STMN1 might be a major target for GA and GEA in combating HCC. Further investigation may lead to a new generation of anticancer drugs exerting synergistic effect with conventional therapy, thus to promote treatment efficacy.

Differential 14-3-3 affinity capture reveals new downstream targets of phosphatidylinositol 3-kinase signaling

We devised a strategy of 14-3-3 affinity capture and release, isotope differential (d₀/d₄) dimethyl labeling of tryptic digests, and phosphopeptide characterization to identify novel targets of insulin/IGF1/phosphatidylinositol 3-kinase signaling. Notably four known insulin-regulated proteins (PFK-2, PRAS40, AS160, and MYO1C) had high d₀/d₄ values meaning that they were more highly represented among 14-3-3-binding proteins from insulin-stimulated than unstimulated cells. Among novel candidates, insulin receptor substrate 2, the proapoptotic CCDC6, E3 ubiquitin ligase ZNRF2, and signaling adapter SASH1 were confirmed to bind to 14-3-3s in response to IGF1/phosphatidylinositol 3-kinase signaling. Insulin receptor substrate 2, ZNRF2, and SASH1 were also regulated by phorbol ester via p90RSK, whereas CCDC6 and PRAS40 were not. In contrast, the actin-associated protein vasodilator-stimulated phosphoprotein and lipolysis-stimulated lipoprotein receptor, which had low d₀/d₄ scores, bound 14-3-3s irrespective of IGF1 and phorbol ester. Phosphorylated Ser¹⁹ of ZNRF2 (RTRAYpS¹⁹GS), phospho-Ser⁹⁰ of SASH1 (RKRRVpS⁹⁰QD), and phospho- Ser⁴⁹³ of lipolysis-stimulated lipoprotein receptor (RPRARpS⁴⁹³LD) provide one of the 14-3-3-binding sites on each of these proteins. Differential 14-3-3 capture provides a powerful approach to defining downstream regulatory mechanisms for specific signaling pathways.

14-3-3sigma gene silencing during melanoma progression and its role in cell cycle control and cellular senescence

Background

The family of 14-3-3 proteins plays an important role in cancer biology by interfering with intracellular signalling pathways and cell cycle checkpoints. The 14-3-3σ isoform acts as a tumor suppressor and is often inactivated during tumor development.

Results

Here, we demonstrate enhanced CpG methylation of the 14-3-3σ gene in lymph node and cutaneous melanoma metastases compared with primary tumors, associated with dramatically reduced mRNA expression. In line with this, treatment of different metastatic melanoma cell lines with 5-aza-2'-deoxycytidine (5-Aza-CdR), a potent inhibitor of cytosine methylation, significantly induces 14-3-3σ protein expression. Additional treatment with histone deacetylase inhibitor 4-phenylbutyric acid (Pba) further enhances 14-3-3σ expression. Induction of 14-3-3σ expression by 5-Aza-CdR/Pba treatment leads to almost complete inhibition of cell proliferation, with cells predominantly arrested in G2-M. The antiproliferative effect of 5-Aza-CdR/Pba was reversed in 14-3-3σ knockdown cells. Similarly, melanoma cell lines stably overexpressing 14-3-3σ show dramatically reduced cell proliferation rates. Moreover, synchronous 14-3-3σ stably overexpressing cells do not progress through cell cycle, but display a permanent increase in the population of 4n DNA containing cells. Interestingly, overexpression of 14-3-3σ induces senescence of melanoma cells and is involved in melanoma cell senescence under genotoxic stress. Finally, 14-3-3σ knockdown supports migratory capacity of melanoma cells in vitro, while 14-3-3σ overexpression has opposing effects.

Conclusion

Taken together, the present report indicates that epigenetic silencing of 14-3-3σ might contribute to tumor progression in malignant melanoma via loss of cell cycle control, impaired cellular senescence program and support of migratory capacity.

Identification and Distribution of 14.3.3σ (Stratifin) in the Human Cornea

We demonstrate for the first time the expression of 14.3.3sigma, an epithelial cell differentiation marker, in human corneal epithelium. 14.3.3sigma appeared at 30 kDa, pI 4-5, in 2D gels of corneal extracts. We found no significant differences in 14.3.3sigma levels between healthy corneas and corneas from keratoconus, corneal dystrophy, and corneal edema patients. 14.3.3sigma immunofluorescence was observed in the cytoplasm and nucleus of epithelial cells and colocalized with cyclin-B1. 14.3.3sigma was secreted by HCE-2 cells; HCE-2-conditioned medium induced matrix metalloproteinase-1 in cultured keratocytes. In summary, our work presents evidence of 14.3.3sigma expression in corneal epithelium and elaborates over its possible implications in corneal pathologic conditions.

A structural rationale for selective stabilization of anti-tumor interactions of 14-3-3 proteins by cotylenin A

Cotylenin A, a fungal metabolite originally described as a cytokinin-like bioactive substance against plants shows differentiation-inducing and anti-tumor activity in certain human cancers. Here, we present the crystal structure of cotylenin A acting on a 14-3-3 regulatory protein complex. By comparison with the closely related, but non-anticancer agent fusicoccin A, a rationale for the activity of cotylenin A in human cancers is presented. This class of fusicoccane diterpenoids are possible general modulators of 14-3-3 protein-protein interactions. In this regard, specificities for individual 14-3-3/target protein complexes might be achieved by varying the substituent pattern of the diterpene ring system. As the different activities of fusicoccin A and cotylenin A in human cancers suggest, hydroxylation of C12 might be a sufficient determinant of structural specificity.

Proteome-wide identification of novel binding partners to the oncogenic fusion gene protein, NPM-ALK, using tandem affinity purification and mass spectrometry

Nucleophosmin-anaplastic lymphoma kinase (NPM-ALK), an oncogenic fusion gene protein that is characteristically found in a subset of anaplastic large cell lymphomas, promotes tumorigenesis through its functional and physical interactions with various biologically important proteins. The identification of these interacting proteins has proven to be useful to further our understanding of NPM-ALK-mediated tumorigenesis. For the first time, we performed a proteome-wide identification of NPM-ALK-binding proteins using tandem affinity purification and a highly sensitive mass spectrometric technique. Tandem affinity purification is a recently developed method that carries a lower background and higher sensitivity compared with the conventional immunoprecipitation-based protein purification protocols. The NPM-ALK gene was cloned into an HB-tagged vector and expressed in GP293 cells. Three independent experiments were performed and the reproducibility of the data was 68%. The vast majority of the previously reported NPM-ALK-binding proteins were detected. We also identified proteins that are involved in various cellular processes that were not previously described in association with NPM-ALK, such as MCM6 and MSH2 (DNA repair), Nup98 and importin 8 (subcellular protein transport), Stim1 (calcium signaling), 82Fip (RNA regulation), and BAG2 (proteosome degradation). We believe that these data highlight the functional diversity of NPM-ALK and provide new research directions for the study of the biology of this oncoprotein.

TTP at Ser245 phosphorylation by AKT is required for binding to 14-3-3

Transferrin receptor trafficking protein (TTP) is a key molecule for selective internalization of the transferrin receptor (Tf-R) through endocytic protein complexes. To identify the proteins that directly regulate TTP, we performed a yeast two-hybrid analysis and identified 14-3-3, which can modulate the activation state of target proteins. Subsequent analyses demonstrated that TTP directly binds to multiple 14-3-3 isotypes via its Ser(245) residue (Ser(246) in human) and that these proteins are associated at the plasma membrane. Ser(245) was also found to be a substrate for AKT and the resulting Ser(245) phosphorylation induced the TTP-14-3-3 interaction. Exposure to hydrogen peroxide rapidly enhanced this association in an ovarian cell line. These results suggest that TTP Ser(245) is the principal target for the modulation of this protein via the AKT signalling cascade.

Prokaryotic expression, purification of a new tumor-relative protein FAM92A1-289 and its characterization in renal cell carcinoma

In order to study the characterization of a new tumor-relative FAM92A1-289 protein, we first constructed plasmid FAM92A1-pQE30 for fusion expression in Escherichia coli. The recombinant protein FAM92A1-289 was affinity-purified by Ni2+-charged resin and separated by HPLC chromatography with high purity, and it was further identified by electrospray ionization-mass spectrometry. Furthermore, the expression and cell localization of FAM92A1-289 by immunohistochemistry using our self-prepared polyclonal antibody showed it was expressed in cytoplasm of renal carcinoma. FAM92A1-289 mRNA was expressed in 2 of 10 kidney tissues and in 6 of 12 primary renal tumors. FAM92A1-289 can promote cell growth in vitro and in vivo by colony formation and mouse xenograft assay. Our present data indicated FAM92A1-289 is a new tumor-related gene with oncogenic potentials to probably play roles in renal carcinogenesis.

CaMKII-mediated phosphorylation of the myosin motor Myo1c is required for insulin-stimulated GLUT4 translocation in adipocytes

The unconventional myosin Myo1c has been implicated in insulin-regulated GLUT4 translocation to the plasma membrane in adipocytes. We show that Myo1c undergoes insulin-dependent phosphorylation at S701. Phosphorylation was accompanied by enhanced 14-3-3 binding and reduced calmodulin binding. Recombinant CaMKII phosphorylated Myo1c in vitro and siRNA knockdown of CaMKIIdelta abolished insulin-dependent Myo1c phosphorylation in vivo. CaMKII activity was increased upon insulin treatment and the CaMKII inhibitors CN21 and KN-62 or the Ca(2+) chelator BAPTA-AM blocked insulin-dependent Myo1c phosphorylation and insulin-stimulated glucose transport in adipocytes. Myo1c ATPase activity was increased after CaMKII phosphorylation in vitro and after insulin stimulation of CHO/IR/IRS-1 cells. Expression of wild-type Myo1c, but not S701A or ATPase dead mutant K111A, rescued the inhibition of GLUT4 translocation by siRNA-mediated Myo1c knockdown. These data suggest that insulin regulates Myo1c function via CaMKII-dependent phosphorylation, and these events play a role in insulin-regulated GLUT4 trafficking in adipocytes likely involving Myo1c motor activity.

Protein partners of deubiquitinating enzymes

Protein modification by ubiquitin and ubiquitin-like molecules is a critical regulatory process. Like most regulated protein modifications, ubiquitination is reversible. Deubiquitination, the reversal of ubiquitination, is quickly being recognized as an important regulatory strategy. Nearly one hundred human DUBs (deubiquitinating enzymes) in five different gene families oppose the action of several hundred ubiquitin ligases, suggesting that both ubiquitination and its reversal are highly regulated and specific processes. It has long been recognized that ubiquitin ligases are modular enzyme systems that often depend on scaffolds and adaptors to deliver substrates to the catalytically active macromolecular complex. Although many DUBs bind ubiquitin with reasonable affinities (in the nM to microM range), a larger number have little affinity but exhibit robust catalytic capability. Thus it is apparent that these DUBs must acquire their substrates by binding the target protein in a conjugate or by associating with other macromolecular complexes. We would then expect that a study of protein partners of DUBs would reveal a variety of substrates, scaffolds, adaptors and ubiquitin receptors. In the present review we suggest that, like ligases, much of the regulation and specificity of deubiquitination arises from the association of DUBs with these protein partners.

The emerging CK2 interactome: insights into the regulation and functions of CK2

Protein kinase CK2 represents a small family of protein serine/threonine kinases implicated in a variety of biological processes including events relating to cell proliferation and survival. Notably, CK2 displays oncogenic activity in mice and exhibits altered expression in several types of cancer. Accordingly, a detailed understanding of the cellular functions of CK2 and elucidation of the mechanisms by which CK2 is regulated in cells is expected to contribute to understanding its role in tumorigenesis with the prospect of novel approaches to therapy. While CK2 has traditionally been viewed as a tetrameric complex composed of two catalytic and two regulatory subunits, mounting evidence suggests that its subunits may have functions independent of tetrameric CK2 complexes. In mammals, as is the case in the budding yeast Saccharomyces cerevisiae, there are two isozymic forms of CK2, adding additional heterogeneity to the CK2 family. Studies in yeast and in human cells demonstrate that the different forms of CK2 interact with a large number of cellular proteins. To reveal new insights regarding the regulation and functions of different forms of CK2, we have examined the emerging interactomes for each of the CK2 subunits. Analysis of these interactomes for both yeast and human CK2 reinforces the view that this family of enzymes participates in a broad spectrum of cellular events. Furthermore, while there is considerable overlap between the interactomes of the individual CK2 subunits, notable differences in each of the individual interactomes provides additional evidence for functional specialization for the individual forms of CK2.

Proteomic analysis of different temporal expression patterns induced by N-methyl-N'-nitro-N-nitrosoguanidine treatment

We have previously shown that N-methyl- N'-nitro- N-nitrosoguanidine (MNNG), a well-known DNA alkylating agent and carcinogen, can induce multiple cellular responses with dynamic characteristics, including such responses as nontargeted mutations (NTM) at undamaged bases in DNA, up-regulation of low fidelity DNA polymerases, clustering of epidermal growth factor receptor (EGFR) and interference with its downstream signaling pathway. A dose-related analysis also revealed that different concentrations of MNNG can trigger diverse proteome changes associated with different cytotoxic effects. To further understand the dynamic cellular responses and hazardous effects caused by environmental carcinogen, a proteomic time-course study of whole cellular proteins from human amniotic epithelial cells after MNNG treatment was performed. Analysis at three different time points (3, 12 and 24 h after exposure) revealed that the major changes were taking place around 3 and 12 h after exposure. Using MALDI-TOF MS coupled with a micro solid-phase extraction (SPE) device, 90% ( n = 70) differentially expressed proteins were identified. Functional assignment revealed that many important pathways were affected, including the protein biosynthesis pathway and Ran GTPase system. We also carried out a network analysis of these proteins and the data suggest a central role for some key regulators in different pathways.

A novel tandem affinity purification strategy for the efficient isolation and characterisation of native protein complexes

Isolation and dissection of native multiprotein complexes is a central theme in functional genomics. The development of the tandem affinity purification (TAP) tag has enabled an efficient and large-scale purification of native protein complexes. However, the TAP tag features a size of 21 kDa and requires time consuming cleavage. By combining a tandem Strep-tag II with a FLAG-tag we were able to reduce the size of the TAP (SF-TAP) tag to 4.6 kDa. Both moieties have a medium affinity and avidity to their immobilised binding partners. This allows the elution of SF-tagged proteins under native conditions using desthiobiotin in the first step and the FLAG octapeptide in the second step. The SF-TAP protocol represents an efficient, fast and straightforward purification of protein complexes from mammalian cells within 2.5 h. The power of this novel method is demonstrated by the purification of Raf associated protein complexes from HEK293 cells and subsequent analysis of their protein interaction network by dissection of interaction patterns from the Raf binding partners MEK1 and 14-3-3.

HIV-1 Nef binds PACS-2 to assemble a multikinase cascade that triggers major histocompatibility complex class I (MHC-I) down-regulation: analysis using short interfering RNA and knock-out mice

Human immunodeficiency virus, type 1, negative factor (Nef) initiates down-regulation of cell-surface major histocompatibility complex-I (MHC-I) by assembling an Src family kinase (SFK)-ZAP70/Syk-phosphoinositide 3-kinase (PI3K) cascade through the sequential actions of two sites, Nef EEEE(65) and PXXP(75). The internalized MHC-I molecules are then sequestered in endosomal compartments by a process requiring Nef Met(20). How Nef assembles the multikinase cascade to trigger the MHC-I down-regulation pathway is unknown. Here we report that EEEE(65)-dependent binding to the sorting protein PACS-2 targets Nef to the paranuclear region, enabling PXXP(75) to bind and activate a trans-Golgi network (TGN)-localized SFK. This SFK then phosphorylates ZAP-70 to recruit class I PI3K by interaction with the p85 C-terminal Src homology 2 domain. Using splenocytes and embryonic fibroblasts from PACS-2(-/-) mice, we confirm genetically that Nef requires PACS-2 to localize to the paranuclear region and assemble the multikinase cascade. Moreover, genetic loss of PACS-2 or inhibition of class I PI3K prevents Nef-mediated MHC-I down-regulation, demonstrating that short interfering RNA knockdown of PACS-2 phenocopies the gene knock-out. This PACS-2-dependent targeting pathway is not restricted to Nef, because PACS-2 is also required for trafficking of an endocytosed cation-independent mannose 6-phosphate receptor reporter from early endosomes to the TGN. Together, these results demonstrate PACS-2 is required for Nef action and sorting of itinerant membrane cargo in the TGN/endosomal system.

A complex signaling pathway regulates SRp38 phosphorylation and pre-mRNA splicing in response to heat shock

Although pre-mRNA splicing is known to be regulated by cell signaling, the underlying mechanisms are poorly understood. SRp38 is a member of the SR protein family and, when dephosphorylated, functions as a general and potent splicing repressor in response to heat shock. Here we show that SRp38 is dephosphorylated by the phosphatase PP1, which is activated by dissociation of its inhibitors, including NIPP1. PP1 is targeted to SRp38 through direct interaction via its arginine/serine-rich (RS) domain. The specific dephosphorylation of SRp38 and not other SR proteins is determined largely by the low activities of SR protein kinases for it compared to other SR proteins. Finally, we show that 14-3-3 proteins associate with SRp38 and protect it from dephosphorylation under nonstress conditions, but dissociate upon heat shock. Together, our study delineates a complex mechanism involving multiple factors by which a stress signaling pathway regulates protein phosphorylation and, in turn, pre-mRNA splicing.

Regulation of multisite phosphorylation and 14-3-3 binding of AS160 in response to IGF-1, EGF, PMA and AICAR

AS160 (Akt substrate of 160 kDa) mediates insulin-stimulated GLUT4 (glucose transporter 4) translocation, but is widely expressed in insulin-insensitive tissues lacking GLUT4. Having isolated AS160 by 14-3-3-affinity chromatography, we found that binding of AS160 to 14-3-3 isoforms in HEK (human embryonic kidney)-293 cells was induced by IGF-1 (insulin-like growth factor-1), EGF (epidermal growth factor), PMA and, to a lesser extent, AICAR (5-aminoimidazole-4-carboxamide-1-b-D-ribofuranoside). AS160-14-3-3 interactions were stabilized by chemical cross-linking and abolished by dephosphorylation. Eight residues on AS160 (Ser318, Ser341, Thr568, Ser570, Ser588, Thr642, Ser666 and Ser751) were differentially phosphorylated in response to IGF-1, EGF, PMA and AICAR. The binding of 14-3-3 proteins to HA-AS160 (where HA is haemagglutinin) was markedly decreased by mutation of Thr642 and abolished in a Thr642Ala/Ser341Ala double mutant. The AGC (protein kinase A/protein kinase G/protein kinase C-family) kinases RSK1 (p90 ribosomal S6 kinase 1), SGK1 (serum- and glucocorticoid-induced protein kinase 1) and PKB (protein kinase B) displayed distinct signatures of AS160 phosphorylation in vitro: all three kinases phosphorylated Ser318, Ser588 and Thr642; RSK1 also phosphorylated Ser341, Ser751 and to a lesser extent Thr568; and SGK1 phosphorylated Thr568 and Ser751. AMPK (AMP-activated protein kinase) preferentially phosphorylated Ser588, with less phosphorylation of other sites. In cells, the IGF-1-stimulated phosphorylations, and certain EGF-stimulated phosphorylations, were inhibited by PI3K (phosphoinositide 3-kinase) inhibitors, whereas the RSK inhibitor BI-D1870 inhibited the PMA-induced phosphorylations. The expression of LKB1 in HeLa cells and the use of AICAR in HEK-293 cells promoted phosphorylation of Ser588, but only weak Ser341 and Thr642 phosphorylations and binding to 14-3-3s. Paradoxically however, phenformin activated AMPK without promoting AS160 phosphorylation. The IGF-1-induced phosphorylation of the novel phosphorylated Ser666-Pro site was suppressed by AICAR, and by combined mutation of a TOS (mTOR signalling)-like sequence (FEMDI) and rapamycin. Thus, although AS160 is a common target of insulin, IGF-1, EGF, PMA and AICAR, these stimuli induce distinctive patterns of phosphorylation and 14-3-3 binding, mediated by at least four protein kinases.

14-3-3-dependent inhibition of the deubiquitinating activity of UBPY and its cancellation in the M phase

The deubiquitinating enzyme UBPY, also known as USP8, regulates cargo sorting and membrane traffic at early endosomes. Here we demonstrate the regulatory mechanism of the UBPY catalytic activity. We identified 14-3-3 epsilon, gamma, and zeta as UBPY-binding proteins using co-immunoprecipitation followed by mass spectrometric analysis. The 14-3-3 binding of UBPY was inhibited by mutating the consensus 14-3-3-binding motif RSYS(680)SP, by phosphatase treatment, and by competition with the Ser(680)-phosphorylated RSYS(680)SP peptide. Metabolic labeling with [(32)P]orthophosphate and immunoblotting using antibody against the phosphorylated 14-3-3-binding motif showed that Ser(680) is a major phosphorylation site in UBPY. These results indicated that 14-3-3s bind to the region surrounding Ser(680) in a phosphorylation-dependent manner. The mutation at Ser(680) led to enhanced ubiquitin isopeptidase activity of UBPY toward poly-ubiquitin chains and a cellular substrate, epidermal growth factor receptor, in vitro and in vivo. Moreover, addition of 14-3-3epsilon inhibited the UBPY activity in vitro. Finally, UBPY was dephosphorylated at Ser(680) and dissociated from 14-3-3s in the M phase, resulting in enhanced activity of UBPY during cell division. We conclude that UBPY is catalytically inhibited in a phosphorylation-dependent manner by 14-3-3s during the interphase, and this regulation is cancelled in the M phase.

Proteomic analysis of in vivo 14-3-3 interactions in the yeast Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae produces two 14-3-3 proteins, Bmh1 and Bmh2, whose exact functions have remained unclear. Here, we performed a comprehensive proteomic analysis using multistep immunoaffinity purification and mass spectrometry and identified 271 yeast proteins that specifically bind to Bmh1 and -2 in a phosphorylation-dependent manner. The identified proteins have diverse biochemical functions and cellular roles, including cell signaling, metabolism, and cell cycle regulation. Importantly, there are a number of protein subsets that are involved in the regulation of yeast physiology through a variety of cell signaling pathways, including stress-induced transcription, cell division, and chitin synthesis at the cell wall. In fact, we found that a yeast mutant deficient in Bmh1 and -2 had defects in signal-dependent response of the MAPK (Hog1 and Mpk1) cascade and exhibited an abnormal accumulation of chitin at the bud neck. We propose that Bmh1 and -2 are common regulators of many cell signaling modules and pathways mediated by protein phosphorylation and regulate a variety of biological events by coordinately controlling the identified multiplex phosphoprotein components.

14-3-3 sigma increases the transcriptional activity of the androgen receptor in the absence of androgens

The androgen receptor (AR) is a ligand-activated transcription factor that regulates numerous target genes, including prostate-specific antigen (PSA). We examined the ability of each member of the 14-3-3 family to modulate transcription of PSA through the AR. Despite significant homology within the 14-3-3 family we observed differences in the ability of each isoform to alter the transcriptional activity of the AR. Significantly, 14-3-3 sigma activated PSA-luciferase reporters not only at castrate levels of androgens, but also in the complete absence of androgens. 14-3-3 sigma also increased expression of the endogenous PSA gene in the absence of androgens. Knockdown of the AR by siRNA oligonucleotides abolished activation of these reporters by 14-3-3 sigma. These findings may have greatest significance in hormone refractory prostate cancer where the AR may be activated in a ligand-independent manner.

DYRK1A autophosphorylation on serine residue 520 modulates its kinase activity via 14-3-3 binding

Dual-specificity tyrosine-phosphorylated and regulated kinase (DYRK) proteins are an evolutionarily conserved family of protein kinases, with members identified from yeast to humans, that participate in a variety of cellular processes. DYRKs are serine/threonine protein kinases that are activated by autophosphorylation on a tyrosine residue in the activation loop. The family member DYRK1A has been shown to phosphorylate several cytosolic proteins and a number of splicing and transcription factors, including members of the nuclear factor of activated T cells family. In the present study, we show that DYRK1A autophosphorylates, via an intramolecular mechanism, on Ser-520, in the PEST domain of the protein. We also show that phosphorylation of this residue, which we show is subjected to dynamic changes in vivo, mediates the interaction of DYRK1A with 14-3-3beta. A second 14-3-3 binding site is present within the N-terminal of the protein. In the context of the DYRK1A molecule, neither site can act independently of the other. Bacterially produced DYRK1A and the mutant DYRK1A/S520A have similar kinase activities, suggesting that Ser-520 phosphorylation does not affect the intrinsic kinase activity on its own. Instead, we demonstrate that this phosphorylation allows the binding of 14-3-3beta, which in turn stimulates the catalytic activity of DYRK1A. These findings provide evidence for a novel mechanism for the regulation of DYRK1A kinase activity.

Improving sensitivity in shotgun proteomics using a peptide-centric database with reduced complexity: protease cleavage and SCX elution rules from data mining of MS/MS spectra

Correct identification of a peptide sequence from MS/MS data is still a challenging research problem, particularly in proteomic analyses of higher eukaryotes where protein databases are large. The scoring methods of search programs often generate cases where incorrect peptide sequences score higher than correct peptide sequences (referred to as distraction). Because smaller databases yield less distraction and better discrimination between correct and incorrect assignments, we developed a method for editing a peptide-centric database (PC-DB) to remove unlikely sequences and strategies for enabling search programs to utilize this peptide database. Rules for unlikely missed cleavage and nontryptic proteolysis products were identified by data mining 11 849 high-confidence peptide assignments. We also evaluated ion exchange chromatographic behavior as an editing criterion to generate subset databases. When used to search a well-annotated test data set of MS/MS spectra, we found no loss of critical information using PC-DBs, validating the methods for generating and searching against the databases. On the other hand, improved confidence in peptide assignments was achieved for tryptic peptides, measured by changes in DeltaCN and RSP. Decreased distraction was also achieved, consistent with the 3-9-fold decrease in database size. Data mining identified a major class of common nonspecific proteolytic products corresponding to leucine aminopeptidase (LAP) cleavages. Large improvements in identifying LAP products were achieved using the PC-DB approach when compared with conventional searches against protein databases. These results demonstrate that peptide properties can be used to reduce database size, yielding improved accuracy and information capture due to reduced distraction, but with little loss of information compared to conventional protein database searches.

Mass spectrometry-based proteomic analysis of the epitope-tag affinity purified protein complexes in eukaryotes

In recent years, MS has been widely used to study protein complex in eukaryotes. The identification of interacting proteins of a particular target protein may help defining protein-protein interaction and proteins of unknown functions. To isolate protein complexes, high-speed ultracentrifugation, sucrose density-gradient centrifugation, and coimmunoprecipitation have been widely used. However, the probability of getting nonspecific binding is comparatively high. Alternatively, by use of one- or two-step (tandem affinity purification) epitope-tag affinity purification, protein complexes can be isolated by affinity or immunoaffinity columns. These epitope-tags include protein A, hexahistidine (His), c-Myc, hemaglutinin (HA), calmodulin-binding protein, FLAG, maltose-binding protein, Strep, etc. The isolated protein complex can then be subjected to protease (i.e., trypsin) digestion followed by an MS analysis for protein identification. An example, the epitope-tag purification of the Arabidopsis cytosolic ribosomes, is addressed in this article to show the success of the application. Several representative protein complexes in eukaryotes been isolated and characterized by use of this approach are listed. In this review, the comparison among different tag systems, validation of interacting relationship, and choices of MS analysis method are addressed. The successful rate, advantages, limitations, and challenges of the epitope-tag purification are also discussed.

Proteomic analysis in cancer research: potential application in clinical use

The ultimate goal of cancer proteomics is to adapt proteomic technologies for routine use in clinical laboratories for the purpose of diagnostic and prognostic classification of disease states, as well as in evaluating drug toxicity and efficacy. The novel technologies allows researchers to facilitate the comprehensive analyses of genomes, transcriptomes, and proteomes in health and disease. The information that is expected from such technologies may soon exert a dramatic change in cancer research and impact dramatically on the care of cancer patients. Analysis of tumor-specific proteomic profiles may also allow better understanding of tumor development and the identification of novel targets for cancer therapy. The localization of gene products, which is often difficult to deduce from the sequence, can be determined experimentally. Mechanisms, such as regulation of protein function by proteolysis, recycling, and isolation in cell compartments, affect gene products, not genes. Finally, protein-protein interactions and the molecular composition of cellular structures can be determined only at the protein level. The biological variability among patient samples as well as the great dynamic range of biomarker concentrations are currently the main challenges facing efforts to deduce diagnostic patterns that are unique to specific disease states. While several strategies exist to address this problem, we have tried to offer a wide perspective about the current possibilities.

Identification of 14-3-3ε Substrates from Embryonic Murine Brain

Mice deficient in 14-3-3epsilon exhibit abnormal neuronal migration and die perinatally. We report here the first large-scale analysis of 14-3-3 interacting partners from primary animal tissue, identifying from embryonic murine brain 163 14-3-3epsilon interacting proteins and 85 phosphorylation sites on these proteins. Phosphorylation of the deubiquitinating enzyme USP8 at serine 680 was found essential for its interaction with 14-3-3epsilon and for maintaining USP8 in the cytosol.

14-3-3 Mediates phosphorylation-dependent inhibition of the interaction between the ubiquitin E3 ligase Nedd4-2 and epithelial Na+ channels

Although recent studies show that the 14-3-3 protein is a negative regulator of ubiquitin E3 protein ligases, the molecular mechanism remains largely unknown. We previously demonstrated that 14-3-3 specifically binds one of the E3 enzymes, Nedd4-2 (a human gene product of KIAA0439, termed hNedd4-2), which can be phosphorylated by serum glucocorticoid-inducible protein kinase 1 (SGK1); this binding protects the phosphorylated/inactive hNedd4-2 from phosphatase-catalyzed dephosphorylation [Ichimura, T., et al. (2005) J. Biol. Chem. 280, 13187-13194]. Here we report an additional mechanism of 14-3-3-mediated regulation of hNedd4-2. Using surface plasmon resonance spectrometry, we show that 14-3-3 inhibits the interaction between the WW domains of hNedd4-2 and the PY motif of the epithelial Na(+) channel, ENaC. The inhibition was dose-dependent and was dependent on SGK1-catalyzed phosphorylation of Ser468 located between the WW domains. Importantly, a mutant of hNedd4-2, which can be phosphorylated by SGK1 but cannot bind 14-3-3, reduced SGK1-mediated stimulation of the ENaC-induced current in Xenopus laevis oocytes. In addition, 14-3-3 had similar effects on hNedd4-2 that had been phosphorylated by cAMP-dependent protein kinase (PKA). Our results, together with the recent finding on 14-3-3/parkin interactions [Sato, S., et al. (2006) EMBO J. 25, 211-221], suggest that 14-3-3 suppresses ubiquitin E3 ligase activities by inhibiting the formation of the enzyme/substrate complex.

14-3-3 proteins: a historic overview

This chapter includes a historic overview of 14-3-3 proteins with an emphasis on the differences between potentially cancer-relevant isoforms on the genomic, protein and functional level. The focus will therefore be on mammalian 14-3-3s although many important developments in the field have involved Drosophila 14-3-3 proteins for example and the cross-fertilisation from parallel studies on plant 14-3-3 should not be underestimated. In the major part of this review I will attempt to focus on some novel data and aspects of 14-3-3 structure and function, in particular regulation of 14-3-3 isoforms by oncogene-related protein kinase phosphorylation and aspects of 14-3-3 research with which newcomers to the field may be less familiar.

Dynamic 14-3-3/client protein interactions integrate survival and apoptotic pathways

The serine/threonine binding protein, 14-3-3, possesses a diverse array of client proteins. It is involved in the regulation of apoptosis through multiple interactions with proteins of the core mitochondrial machinery, pro-apoptotic transcription factors, and their upstream signaling pathways. 14-3-3 coordinates with survival kinases to inhibit multiple pro-apoptotic molecules. One prominent mechanism for the suppression of apoptosis is through 14-3-3-mediated sequestration of pro-apoptotic client proteins. On the other hand, cellular stresses appear to signal through the inhibition of 14-3-3 function to exert their pro-apoptotic effect. Global inhibition of 14-3-3/client protein interaction induces apoptosis, and stands as an attractive intervention in diseases involving overactive survival signaling pathways. Because dysregulation of 14-3-3 has been associated with poor survival of cancer patients, targeting 14-3-3 may provide a novel therapeutic approach for the treatment of cancer.

14-3-3 proteins as potential oncogenes

14-3-3 proteins are a family of highly conserved cellular proteins that play key roles in the regulation of central physiological pathways. More than 200 14-3-3 target proteins have been identified, including proteins involved in mitogenic and cell survival signaling, cell cycle control and apoptotic cell death. Importantly, the involvement of 14-3-3 proteins in the regulation of various oncogenes and tumor suppressor genes points to a potential role in human cancer. The present review summarizes current findings implicating a 14-3-3 role in cancer while discussing potential mechanisms and points of action of 14-3-3 during cancer development and progression.

Epigenetic silencing of 14-3-3sigma in cancer

The 14-3-3sigma gene is a direct target of the p53 tumor suppressor and its product inhibits cell cycle progression. Recently, a proteomic analysis revealed that 14-3-3sigma regulates additional cellular processes relevant to carcinogenesis, as migration and MAP-kinase signalling. The expression of 14-3-3sigma is down-regulated by CpG methylation in several types of human cancer, among them prostate, lung, breast and several types of skin cancer. The epigenetic inactivation of 14-3-3sigma occurs at an early stage of tumor development and may allow evasion from senescence and promote genomic instability. In the future the detection of CpG methylation of 14-3-3sigma may be used for diagnostic and prognostic purposes.

14-3-3 proteins in cell cycle regulation

Co-ordinated progression through the cell cycle is essential for the maintenance of genomic integrity. Several checkpoint mechanisms guarantee that the next step in cell cycle progression is only entered after error-free completion of the previous phase. Cell cycle deregulation caused by changes in 14-3-3 expression has been implicated in cancer formation. 14-3-3 proteins function at several key points in G(1)/S- and G(2)/M-transition by binding to regulatory proteins and modulating their function. In most cases, the association with 14-3-3 proteins requires a specific phosphorylation of the protein ligand and mediates cell cycle arrest. 14-3-3 binding may lead to cytoplasmic sequestration of the protein ligand but may also have other functional consequences. The 14-3-3sigma gene is induced by p53 and its product inhibits G(2)/M progression by cytoplasmatic sequestration of CDC2-cyclin B complexes. In addition, 14-3-3 proteins have been implicated in the transcriptional regulation of CDK-inhibitors as they modulate the transcription factors p53, FOXO and MIZ1. Effects of 14-3-3 proteins on cell cycle progression and the regulation of 14-3-3 activity during the cell cycle are reviewed in this chapter.

Rapid identification of 14-3-3-binding proteins by protein microarray analysis

The 14-3-3 protein family consists of acidic 30-kDa proteins composed of seven isoforms in mammalian cells, expressed abundantly in neurons and glial cells of the central nervous system (CNS). The 14-3-3 isoforms form a dimer that acts as a molecular adaptor interacting with key signaling components involved in cell proliferation, transformation, and apoptosis. Until present, more than 300 proteins have been identified as 14-3-3-binding partners, although most of previous studies focused on a limited range of 14-3-3-interacting proteins. Here, we studied a comprehensive profile of 14-3-3-binding proteins by analyzing a high-density protein microarray using recombinant human 14-3-3 epsilon protein as a probe. Among 1752 proteins immobilized on the microarray, 20 were identified as 14-3-3 interactors, most of which were previously unreported 14-3-3-binding partners. However, 11 known 14-3-3-binding proteins, including keratin 18 (KRT18) and mitogen-activated protein kinase-activated protein kinase 2 (MAPKAPK2), were not identified as a 14-3-3-binding protein. The specific binding to 14-3-3 of EAP30 subunit of ELL complex (EAP30), dead box polypeptide 54 (DDX54), and src homology three (SH3) and cysteine rich domain (STAC) was verified by immunoprecipitation analysis of the recombinant proteins expressed in HEK293 cells. These results suggest that protein microarray is a powerful tool for rapid and comprehensive profiling of 14-3-3-binding proteins.

A mutation in stratifin is responsible for the repeated epilation (Er) phenotype in mice

Stratifin (Sfn, also called 14-3-3sigma) is highly expressed in differentiating epidermis and mediates cell cycle arrest. Sfn is repressed in cancer, but its function during development is uncharacterized. We identified an insertion mutation in the gene Sfn in repeated epilation (Er) mutant mice by positional cloning. Er/+ mice expressed a truncated Sfn protein, which probably contributes to the defects in Er/Er and Er/+ epidermis and to cancer development in Er/+ mice.

14-3-3 Proteins: Regulators of numerous eukaryotic proteins

14-3-3 proteins form a family of highly conserved proteins capable of binding to more than 200 different mostly phosphorylated proteins. They are present in all eukaryotic organisms investigated, often in multiple isoforms, up to 13 in some plants. 14-3-3 binding partners are involved in almost every cellular process and 14-3-3 proteins play a key role in these processes. 14-3-3 proteins interact with products encoded by oncogenes, with filament forming proteins involved in Alzheimer'ss disease and many other proteins related to human diseases. Disturbance of the interactions with 14-3-3 proteins may lead to diseases like cancer and the neurological Miller-Dieker disease. The molecular consequences of 14-3-3 binding are diverse and only partly understood. Binding of a protein to a 14-3-3 protein may result in stabilization of the active or inactive phosphorylated form of the protein, to a conformational alteration leading to activation or inhibition, to a different subcellular localization or to the interaction with other proteins. Currently genome- and proteome-wide studies are contributing to a wider knowledge of this important family of proteins.

14-3-3 proteins: a number of functions for a numbered protein

Many signal transduction events are orchestrated by specific interactions of proteins mediated through discrete phosphopeptide-binding motifs. Although several phosphospecific-binding domains are now known, 14-3-3s were the first proteins recognized to specifically bind a discrete phosphoserine or phosphothreonine motif. The 14-3-3 proteins are a family of ubiquitously expressed, exclusively eukaryotic proteins with an astonishingly large number of binding partners. Consequently, 14-3-3s modulate an enormous and diverse group of cellular processes. The effects of 14-3-3 proteins on their targets can be broadly defined using three categories: (i) conformational change; (ii) physical occlusion of sequence-specific or structural protein features; and (iii) scaffolding. This review will describe the current state of knowledge on 14-3-3 proteins, highlighting several important advances, and will attempt to provide a framework by which 14-3-3 functions can be understood.

14-3-3 proteins—an update

14-3-3 is a highly conserved acidic protein family, composed of seven isoforms in mammals. 14-3-3 protein can interact with over 200 target proteins by phosphoserine-dependent and phosphoserine-independent manners. Little is known about the consequences of these interactions, and thus are the subjects of ongoing studies. 14-3-3 controls cell cycle, cell growth, differentiation, survival, apoptosis, migration and spreading. Recent studies have revealed new mechanisms and new functions of 14-3-3, giving us more insights on this fascinating and complex family of proteins. Of all the seven isoforms, 14-3-3sigma seems to be directly involved in human cancer. 14-3-3sigma itself is subject to regulation by p53 upon DNA damage and by epigenetic deregulation. Gene silencing of 14-3-3sigma by CpG methylation has been found in many human cancer types. This suggests that therapy-targeting 14-3-3sigma may be beneficial for future cancer treatment.

The crystal structure of the non-liganded 14-3-3σ protein: insights into determinants of isoform specific ligand binding and dimerization

Seven different, but highly conserved 14-3-3 proteins are involved in diverse signaling pathways in human cells. It is unclear how the 14-3-3sigma isoform, a transcriptional target of p53, exerts its inhibitory effect on the cell cycle in the presence of other 14-3-3 isoforms, which are constitutively expressed at high levels. In order to identify structural differences between the 14-3-3 isoforms, we solved the crystal structure of the human 14-3-3sigma protein at a resolution of 2.8 Angstroms and compared it to the known structures of 14-3-3zeta and 14-3-3tau. The global architecture of the 14-3-3sigma fold is similar to the previously determined structures of 14-3-3zeta and 14-3-3t: two 14-3-3sigma molecules form a cup-shaped dimer. Significant differences between these 14-3-3 isoforms were detected adjacent to the amphipathic groove, which mediates the binding to phosphorylated consensus motifs in 14-3-3-ligands. Another specificity determining region is localized between amino-acids 203 to 215. These differences presumably select for the interaction with specific ligands, which may explain the different biological functions of the respective 14-3-3 isoforms. Furthermore, the two 14-3-3sigma molecules forming a dimer differ by the spatial position of the ninth helix, which is shifted to the inside of the ligand interaction surface, thus indicating adaptability of this part of the molecule. In addition, 5 non-conserved residues are located at the interface between two 14-3-3sigma proteins forming a dimer and represent candidate determinants of homo- and hetero-dimerization specificity. The structural differences among the 14-3-3 isoforms described here presumably contribute to isoform-specific interactions and functions.

The role of epigenetic inactivation of 14-3-3σ in human cancer

Cancer cells show characteristic alterations in DNA methylation patterns. Aberrant CpG methylation of specific promoters results in inactivation of tumor suppressor genes and therefore plays an important role in carcinogenesis. The p53-regulated gene 14-3-3sigma undergoes frequent epigenetic silencing in several types of cancer, including carcinoma of the breast, prostate, and skin, suggesting that the loss of 14-3-3sigma expression may be causally involved in tumor progression. Functional studies demonstrated that 14-3-3sigma is involved in cell-cycle control and prevents the accumulation of chromosomal damage. The recent identification of novel 14-3-3sigma-associated proteins by a targeted proteomics approach implies that 14-3-3sigma regulates diverse cellular processes, which may become deregulated after silencing of 14-3-3sigma expression in cancer cells.